The fabrics of deep-sea detrital muds and mudstones: A scanning electron microscope study

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Content THE FABRICS OF DEEP-SEA DETRITAL MUDS
AND MUDSTONES: A SCANNING ELECTRON MICROSCOPE STUDY
by
Suzanne Reynolds
A Dissertation Presented to the
FACULTY OF THE GRADUATE SCHOOL
UNIVERSITY OF SOUTHERN CALIFORNIA
In Partial Fulfillment of the
Requirements for the Degree
DOCTOR OF PHILOSOPHY
(Geological Sciences)
May 1988
UMI Number: DP28581
All rights reserved
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a note will indicate the deletion.
UM I
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UMI DP28581
Published by ProQuest LLC (2014). Copyright in the Dissertation held by the Author.
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ACKNOWLEDGEMENTS
After four years of dissertation research, one finds
a great number of debts to be acknowledged. The majority
of these are to my advisor, Donn Gorsline, whose
unflagging optimism and resources have held me up during
many a sagging moment.
This research could not have been accomplished
without the use of facilities at the Chevron Oilfield
Research Co. My personal thanks go to Will Schweller, Jeff
Warner, Barbara Sweany, Allen White, Carol Meyer, Alan
Reed, Chris Grant, and June Gidman of this company.
At the USC Center for Electron Microscopy, Alicia
Thompson, Jack Worrall, and Robert Bils all aided in the
early stages of my analysis.
Every member of the USC Sedimentology Laboratory has
lent their support to me at one time or another. In
particular, my deepest gratitude goes to Rick Herrera,
Susan Concha, Chuck Savrda, and Shannon Fitzgerald.
The USC faculty have also lent me a great deal of
support and valued guidance. Special thanks go to David
Bottjer, Robert Douglas, Robert Osborne, A1 Fischer, Gil
Jones, and Ron Kolpack.
Communications with other workers in the microfabric
field also contributed to this study. Among these are Fran
Hein, Gerhard Oertel, Dick Bennett, and Bill Bryant.
Samples from the California Borderland were obtained
ii
UNIVERSITY OF SOUTHERN CAUFORNIA
THE GRADUATE SCHOOL
UNIVERSITY PARK
LOS ANGELES, CAUFORNIA 90089
This dissertation, written by
under the direction of h.c,.<... Dissertation
Committee, and approved by all its members,
has been presented to and accepted by The
Graduate School, in partial fulfillm ent of re
quirements for the degree of
ft.D.
< k t
>88
..
D O C T O R O F P H ILO S O P H Y
Dean of Graduate Studies
DISSERTATION COMMITTEE
during the course of a National Science Foundation grant
to Dr. Gorsline. Other recent muds and the sediment trap
samples were obtained through a Department of Energy grant
to Dr. Gorsline. I have also received grants to support
this work from the Geological Society of America, the
Department of Geological Sciences at USC, the Achievement
Rewards for College Scientists, and Mobil Oil Corporation.
Fellowship stipends were provided by the Tyler Prize to
the Edison Company, and by the John Stauffer Dissertation
Merit Program. Special thanks go to Rosanne Dutton for
helping me obtain these fellowships.
But most importantly, I need to thank my family for
their support and encouragement throughout the long years
of my education. And to my husband, Azzam, who stood by me
throughout, I hold an infinite debt of gratitude.
Thanks to all, I am done.
iii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS...........................................ii
LIST OF FIGURES............................................vi
ABSTRACT......................................................
PREFACE.................................................... i
INTRODUCTION.................................................1
Purpose of Research...................................... 1
The Nature of the Clay-Water System....................2
Controls on Microfabric..................................5
Previous Classifications of Microfabrics............. 11
METHODS..................................................... 17
Experiments With Sample Preparation for the SEM.....17
Routine Preparation of SEM Samples....................25
X-Rad iogr aphy............................................26
Thin-section Preparation............................... 26
Backscatt ered Electron Microscopy.....................27
Grain Size Analysis..................................... 28
X-ray Diffraction....................................... 29
Stereo SEM Microscopy...................................29
RESULTS, SANTA MONICA BASIN.............................. 31
Sediment Trap Material..................................32
Basin Floor Sediments...................................41
RESULTS, LOS ANGELES BASIN............................... 66
General Characterization of Inglewood Core...........66
Silt Microfabric.........................................77
Clay Microfabric.........................................98
iv
RESULTS, VENTURA BASIN...................................118
Silt Microfabric........................................118
Clay Microfabric........................................126
RESULTS, NIOBRARA FORMATION............................. 136
DISCUSSION ................................................143
Origin and Significance of Bioflocs..................143
Origin and Significance of P-chem Floes............. 145
The Effect of Sediment Type on Microfabric..........147
The Effect of Depositional Process on Microfabric••148
The Effect of Organisms on Microfabric.............. 150
The Effect of Consolidation on Microfabric..........151
The Effect of Diagenesis on Microfabric............. 153
Origin of Preferred Orientation...................... 154
CONCLUSIONS................................................158
REFERENCES.................................................162
v
LIST OF FIGURES
Figure Page
1. Sample location map.....................................3
2. Previous clay microfabric models................... 12
3. SE image of sediment trap material..................33
4. SE image of cylindrical fecal pellet................34
5. SE image of spherical biofloc........................35
6. SE image of clay cloaking of bioflocs.............. 36
7. SE image of physico-chemical floe...................38
8. SE image of organic floes............................ 39
9. SE image of small organic floes..................... 40
10. X-radiograph of box core USC/DOE-21................. 42
11. BSE image of pelagic mud............................. 44
12. BSE image of pelagic mud............................. 45
13. BSE image of bioturbated mud.........................47
14. BSE image of bioturbated mud.........................48
15. BSE image of turbidite mud...........................49
16. BSE image of turbidite mud...........................51
17. X-radiograph of boxcore USC/DOE-17.................. 53
18. SE image of bioturbated mud..........................54
19. SE image of bioturbated mud..........................56
20. SE image of burrow fill.............................. 57
21. SE image of cylindrical pellet...................... 58
22. SE image of turbidite mud............................ 59
23. SE image of turbidite mud............................ 61
vi
24. SE image of pelagic mud..............................62
25. SE image of pelagic mud..............................63
26. Early diagenetic nontronite......................... 63
27. Photograph of Inglewood 899'.........................68
28. Photograph of Inglewood 906'.........................69
29. Photograph of Inglewood 2097'........................70
30. Photograph of Inglewood 2128'........................71
31. SE and EDS analysis of biotite...................... 73
32. SE and EDS analysis of chlorite..................... 74
33. SE and EDS analysis of smectite..................... 75
34. SE and EDS analysis of illite........................76
35. Thin section image of bioturbated mudstone........78
36. Thin section image of bioturbated sandy mudstone..79
37. Thin section image of meniscate backfill.......... 80
38. Thin section image of turbidite mudstone.......... 82
39. Thin section image of Chondrites................... 83
40. BSE image of bioturbated mudstone IW906 .............85
41. BSE image of mudstone IW906 .......................... 86
42. BSE image of turbidite IW906 ......................... 87
43. BSE image of turbidite IW906 ......................... 88
44. BSE image of turbidite IW906 ......................... 89
45. BSE image of Chondrites in turbidite IW906 ......... 91
46. BSE image inside Chond r i t e ......................... 92
47. BSE image outside Chondrites........................93
48. BSE image of bioturbated mudstone IW2097 ........... 94
49. BSE image of turbidite mudstone IW2097 ..............95
vii
50. BSE image of turbidite mudstone IW2097 ..............96
51. BSE image of sandy mudstone IW2128..................97
52. BSE image of sandy mudstone IW2128..................99
53. BSE image of sandy mudstone IW2128.................100
54. BSE image of sandy mudstone IW2128.................101
55. SE image of bioturbated mudstone IW899 ............ 103
56. SE image of bioturbated mudstone IW902 ............ 105
57. SE image of bioturbated mudstone IW902 ............ 106
58. SE image of bioturbated mudstone IW906 ............ 107
59. SE image of turbidite mudstone IW906 .............. 109
60. SE image of bioturbated mudstone IW2097 ........... Ill
61. SE image of turbidite mudstone IW2097 ............. 1 12
62. SE image of bioturbated mudstone IW2097 .......... 113
63. SE image of turbidite mudstone IW2097 ............. 1 14
64. SE image of sandy mudstone IW2128..................116
65. SE image of sandy mudstone IW2128..................117
66. Photograph of Ventura Basin mudstones............. 119
67. Thin section image of pelagic mudstone ............. 121
68. Thin section image of turbidite mudstone ...........122
69. BSE image of pelagic mudstone.......................123
70. BSE image of pelagic mudstone.......................124
71. BSE image of pelagic mudstone.......................125
72. BSE image of turbidite mudstone ....................127
73. BSE image of turbidite mudstone ....................128
74. BSE image of turbidite mudstone ....................129
75. BSE image of turbidite mudstone ....................130
viii
76. SE image of pelagic mudstone ......................132
77. Nontronite in pelagic mudstone .....................133
78. SE image of turbidite mudstone .....................134
79. Alteration of feldspar to clay in pelagite........ 135
80. Photographs of Niobrara samples................... 137
A. Chondrit e s in black shale
B. Zoophycus in calcareous shale
81. SE image of Niobrara black shale (stereo)......... 138
82. SE image of Niobrara Chondrites (stereo)...........139
83. SE image of Niobrara meniscate backfill............141
84. SE image of Niobrara meniscate backfill............142
ix
ABSTRACT
The purpose of this research was to develop a method
of examining detrital marine mud(stone)s whereby more
information about their depositional history could be
obtained. The methods of analysis for this study were: x-
radiography; thin-section petrography; and scanning
electron microscopy. Recent muds from Santa Monica Basin,
and closely analogous mudstones from the Los Angeles and
Ventura Basins, were the focus of this study.
Most of the previous research on clay microfabric
focussed on defining the clay fabric present in sediments,
without regard to differences in the environment of
deposition. Several idealized models were developed,
based either on theoretical considerations or on
examination of laborat ory-sedimented clay. These include:
cardhouse , bookhouse, honeycombe, and turbostratic , for
example. However, when other researchers attempted to
utilize these models to describe fabrics of natural
sediments, they found that natural fabrics were too
diverse and heterogeneous to be described by a single
idealized model.
In order to isolate different factors within the
environment of deposition, the following facies were
specified for this research: unbioturbated pelagic
mud(stone)s; unbioturbated turbiditic mud(stone)s; and
bioturbated mud(stone)s. The method of attack was to study
x
the fabric at increasingly smaller scales, thus beginning
with x-radiography for a rough definition of structure;
following with thin-section petrography and backscattered
electron (BSE) microscopy to define the "silt
microfabric;1 1 and concluding with stereo-secondary
electron imaging in order to define the clay microfabric.
"Silt microfabric" refers to the fabrics seen in BSE
images at magnifications of 100-500X. In these images,
silt particles, along with their distribution and
orientations across the sample, are the dominant fabric
feature. The benefit of these images over conventional
thin-section micrographs is that they more distinctly
delineate fine silts and micas, which are important
constituents of the fabric.
Turbidite muds display an even distribution of silt
particles across the sample. Grain size decreases upwards.
Micas and platy silts display preferential orientations in
the lower portions of the mud; this is interpreted to
result from deposition during a waning current. Upwards in
the deposit, micas display a random orientation, resulting
from deposition from suspension.
Bioturbated muds display lateral segregations of silt
and clay, resulting in a mottled or swirly fabric.
Orientations of micas are random. Specific
ichnostructures, including meniscate backfill structures
and microburrows, are commonly observed.
xi
Pelagic muds display fine laminations in x-
radiographs and thin-section micrographs. BSE images show
that some of these laminations are textural, the result of
thin gravity flows of sediment similar to turbidity
currents. These may represent turbid plumes associated
with wet year floods. Organic material also forms
discontinuous laminae.
In pelagic mudstones, BSE images show layers rich in
pyrite and foram tests alternating with layers poor in
these constituents. Some silt-sized fecal pellets are
readily identified. Randomly interspersed with these
layers are thin mud turbidites, which display the
characteristic even distribution of silts, and fining
upwards of thicker turbidites. Some of these also contain
fecal pellets in their upper portions.
Consolidation has the effect of increasing the
density of packing of silts, and increasing the
preferential orientation of micas in all sediment types.
The basic building blocks of the clay fabric in
recent muds are floes 2-200 microns in diameter. "P-chem
floes" are thought to result from the electrical
attraction of clay particles. These have an open fabric
of clays in dominantly edge/face (EF) clay particle
contacts. Floes are roughly equidimensional, but
boundaries between floes are generally poorly defined.
"Bio-flocs" are thought to result from biological
agglomeration during ingestion by benthic and pelagic
organisms. These are more densely-packed floes containing
mostly face/face (FF) or low-angle EF clay particle
contacts; organism tests are a common constituent. The
outer surface of these floes may exhibit clay cloaking in
an 1 1 onionskin" fabric. Floes are equidimensional, usually
ovoid, and individual floes are well-defined.
Bioturbated sediments near the sediment/water
interface contain abundant bio-flocs, silts, micas, and
tests. Large voids ( > 5 microns) are abundant. Some areas
contain floes more densely packed together, occasionally
exhibiting surfaces of flat-flying clays in FF particle
contacts. These areas could be either large composite
pellets or burrows. Below 15 cm burial depth, large pores
disappear and boundaries between floes become more
blurred. With increasing burial depth (1000') these
bioflocs are squeezed between silt particles to form
welded packets of clays which exhibit abundant clay faces.
At burial depths of 2000', consolidation and diagenesis
changes these packets to intensely welded clays which are
crenulate and intergrown.
Turbidite muds near the sediment/water interface are
dominantly composed of p-chem floes. Silts are enmeshed
within a continuously flocculated mass of dominantly high-
angle EF particle contacts. Small pores (1 micron) are
abundant, and pore size is relatively uniform. As burial
xiii
depth increases, high-angle EF clay particle contacts
change to low-angle contacts, and silt particles begin to
influence the fabric of clays in their immediate vicinity.
At burial depths of 1000', low-angle EF contacts change to
very low angle EF and edge-on FF domains. Some preferred
orientation of clays is developed in thin lamina as they
wrap around silts in adjacent lamina. Individual clay
particles are still discernible and clay edges dominate
over clay faces as the dominant visible side of clays. At
burial depths of 2000', diagnesis and consolidation have
produced densely welded zones of crenulate, intergrown
clays virtually indistinguishable from nearby bioturbated
mud s tones.
Pelagic muds have a more heterogeneous fabric,
containing both bio-flocs and p-chem floes in various
proportions, along with abundant organism tests. Small
porosity is high, but not as uniform as in turbidite muds;
large pores are rare. With increasing burial depth,
bioflocs become more densely welded and intergrown with
other fabric features, making individual packets more
difficult to discern. The p-chem floes reorient to
moderately well-developed preferred orientation with very
low angle EF and edge-on FF particle contacts. Silicate
tests dissolve and diagenesis creates a crenulate,
intergrown fabric.
The dominant controls on microfabric, as seen in this
xiv
study, are bioturbation, depositional process, grain size,
consolidation and diagenesis. The results of this research
may be applicable to many other disciplines; however, much
remains to be done.
xv
PREFACE
"Do not be surprised, therefore, Most Serene
Prince, if, for a whole year's time, and, what is
more, almost daily, I have said that the
investigation for which the teeth of the shark had
furnished an opportunity, was very near an end. For
having once or twice seen regions where shells and
other similar deposits of the sea are dug up, when I
observed that those lands were sediments of the
turbid sea and that an estimate could be formed of
how often the sea had been turbid in each place, I
not only over-hastily fancied, but also dauntlessly
informed others, that a complete investigation on the
spot was the work of a very short time. But
thereafter, while I was examining more carefully the
details of both places and bodies, these day by day
presented points of doubt to me as they followed one
another in indissoluble connection, so that I saw
myself again and again brought back to the starting
place, as it were, when I thought I was nearest the
goal. I might compare these doubts to the heads of
the Lernean Hydra, since when one of them had been
got rid of, numberless others were born; at any rate,
I saw that I was wandering about in a sort of
labyrinth, where the nearer one approaches the exit,
the wider circuits does one tread."
Nicolaus Steno (1638-1687)
xvi
INTRODUCTION
Purpose of Research
Mudstones constitute about 60% of the known
sedimentary section (Potter et al., 1980), but papers
dealing specifically with these sediments are rare. Muds
contain very few clues as to their origin and depositional
history, so they are cursorily described and subsequently
ignored, whereas the more unusual sand layers found
interbedded with them are studied with a great deal of
enthusiasm and detail. One reason for our lack of
knowledge concerning mudstones may be a result of the
inability to visually discern their individual components.
In other words, fine-grained rocks demand a fine scale of
investigation. One method of fine-scaled examination is
scanning electron microscopy (SEM), which is capable of
showing structural details at magnifications from 10X to
100,000X.
This study utilized various SEM techniques to search
for distinctive microfabrics which could be correlated to
environmental factors and used as clues to unravel the
depositional history of the mudstones. The first step was
the development of a descriptive classification of
microfabrics. Following that, these microfabrics were
analyzed in terms of depositional process and other
factors in the environment of deposition. Both recent muds
and mudstones from contemporary and ancient basins of the
1
California Continental Borderland were studied (Fig, 1);
all of these samples represent deep-sea detrital
deposition. In addition, a few samples from the Cretaceous
Niobrara Formation were studied.
The Nature of the Clay-Water System
The behavior of clays in suspension is different from
that of any other mineral material. This unusual behavior
results from a combination of the unique crystal structure
of clay minerals and from their typically small size.
Clay minerals are composed of sheet-like arrangements
of two types of layers. The tetrahedral layer contains
silica atoms which are tetrahedrally coordinated to four
oxygens. Octahedral layers are composed of cations,
generally Al, Mg, and Fe, arranged so that each metal is
surrounded by six oxygen or hydroxyl atoms. Differences
between clay minerals arise from the type of cation in the
octahedral layer, substitution of aluminum for silicon in
the tetrahedral layer, the presence of interlayer
molecules or cations, and stacking differences. This
crystal structure results in minerals which are typically
of a flaky morphology. Clay minerals break preferentially
between layers, so that the atoms on the exposed face
typically are oxygen or hydroxyl groups. Therefore, the
face of the clay flake has a net negative charge. The
edges of the clay flakes generally have a positive charge
due to exposed silicon and aluminum atoms.
2
o f
34 30
34
1 19
Ventura
Basin
\
\
\
Santa Paula Creek I
/
Section
LOCATION
Los Angeles
Basin
USC/DO E-17^
_ '
}
vjJSC/DOE-21 >
Santa Monica'
Basin
Inglewood Corej
Figure 1. Sample location map.
3
The typically small size of most clay minerals
(usually less than 5 microns) makes them classifiable as
colloids. Colloidal materials display unique properties
resulting from their small size (van Olphen, 1963).
Colloids tend to not settle from a suspension, because the
downward settling tendency induced by their weight is
small compared to the randomly-oriented forces derived
from the Brownian motion of the fluid. Van der Waals
attractive body forces, which are a negligible influence
on larger objects, become important for colloidal
particles. However, these forces are strong only when two
colloidal particles approach each other closely.
In a dilute electrolyte solution (such as fresh
water), a diffuse ionic cloud develops around the clay
particle (Bennett and Hulbert, 1986). On the clay face,
negative charges attract positive ions. The opposite is
true of the clay edge. So the ionic cloud consists of a
positive face charge and a negative edge charge. This
leads to edge-face flocculation in fresh water. However,
the net repulsive force between particles is still high,
and particles cannot approach each other closely enough
for van der Waals attraction to occur. But in a
concentrated electrolyte solution such as seawater, the
ionic cloud is condensed, and particles approach each
other more closely: van der Waals attraction becomes
important. Thus face-face flocculation can occur in
4
saltwat er.
Other chemical effects are also important. A fluid
with a low dielectric constant will compress the ionic
cloud and cause face/face flocculation. An acidic
environment increases the positive edge charge, increasing
the edge-face flocculation tendency.
Another significant factor in clay behavior is the
nature of water directly adjacent to the clay particle
(Barden and Sides, 1970). This water, called bound water,
adheres tightly to the clay particle. The thickness of the
layer is not known with certainty. It exhibits properties
different from normal water, and is considered to have a
quasicrystalline structure. It has high viscosity and may
be capable of viscous creep. It may also have high
strength when subjected to normal loading, but low shear
strength. In high water content muds, the clay particles
themselves may not be in actual contact with each other;
instead, bound water is present between the contacts. This
bound water should not be considered part of the pore
water. It is not expelled from the sediment except during
the later stages of compaction. Other polar molecules,
especially organic molecules, may behave in the same way
as bound water.
Controls on Microfabric
The microfabric of muds and mudrocks is dependent on
many different variables, including sediment type,
5
biological, physical and chemical characteristics of the
environment of deposition, compaction and diagenesis. All
of these different factors will affect the resulting
microfabric in various ways; unraveling these different
clues in order to achieve a complete definition of the
sedimentary history of any given mudrock may be
impos s ible.
Sediment type is a major control of microfabric.
Grain size characteristics may have subtle and complex
effects on microfabric. A positive correlation between the
percent of clay and silt and the degree of preferred
orientation of the clay particles has been found (Gipson,
1966; Curtis et al . , 1980). A wide range of clay sizes may
lead to a more varied fabric (Bennett et al . , 1981). An
order of magnitude difference in grain size can affect
compaction by as much as an order of magnitude difference
in depth of burial (Meade, 1966). Coarse, equigranular
silt may impede the development of preferred orientation
(Torresan and Schwab, 1987). Clay mineralogy is also
important since different minerals have different
morphologies, characteristic sizes, and flocculation
potentials. The type of floes formed by different minerals
may also be different (Gibbs, 1983; Gipson, 1966). The
presence of organic carbon and carbonate will affect both
the initial microfabric and also the changes in
microfabric during consolidation (Meade, 1966; Rashid and
6
Brown, 1975 ; Bryant et al • , 1974). Organic material can
either cause flocculation of clays (Santoro and Stotzky,
1967) or dispersion (Moon and Hurst, 1984). Where plant
residues dominate, they can determine clay fabric (Bennett
and Hulbert, 1986). Large organic molecules can also block
pore spaces (Pusch, 1973).
The process by which the sediment is deposited also
plays a role in determining the resulting microfabric.
Clays may be deposited in a nonflocculated state from a
low concentration suspension, since these clay particles
will have little interaction, whereas high particle
concentrations lead to increased flocculation (O'Brien,
1970). A turbulent flow may increase flocculation since it
induces increased particle contacts; however, it may also
create an upper size limit of floes (Gibbs, 1983). O'Brien
(1970) was able to differentiate between turbiditic and
hemipelagic mudstones on the basis of microfabric; Hein
(1985) has noticed liquefaction structures (dewatering
tubes) on the microscale in recent marine sediments.
O'Brien (1987) found that turbiditic mudstones had random
orientations of particles, whereas pelagic mudstones had
preferred orientations. However, Torresan and Schwab
(1987) found no correlation between fabric type and
depositional environment in unbioturbated recent muds.
Hemipelagic clays may be deposited in the form of fecal
pellets of pelagic organisms; these pellets may be
7
preserved at depth. Primary sedimentary structures,
stratification and grading induced by the depositional
process may also be reflected in microfabric.
The chemical characteristics of the environment of
deposition is another determining factor of clay
microfabric, as discussed in the previous section, since
water chemistry determines the morphology of floes (Gibbs,
1983; Osipov and Sokolov, 1978). These relationships may
become more complex as the pore fluid chemistry changes
through compaction and diagenesis (von Engelhardt and
Gaida, 1963; Chilingarian et al., 1973). However, some
workers believe that after deposition, clay fabric is very
little affected by changing pore fluid chemistry (Bennett
and Hulbert, 1986).
Diagenesis involves changing the components of the
sediment; thus it will have a tremendous influence on the
resulting microfabric of mudrocks. Sholkovitz (1973) has
studied the shallow geochemistry of anoxic basin sediments
with regard to early diagenesis. The surficial (upper 1 m)
zone is characterized by the decomposition of organic
material, the reduction of sulfate and precipitation of
iron oxides and calcium carbonate, the dissolution of
silica shells, and the uptake of magnesium by clays. Later
diagenesis may involve both dissolution, crystallization,
and recrystallization of minerals and other components of
the sediment; since conditions vary from locale to locale,
8
no general scheme is possible (de Segonzac, 1970).
Consolidation also affects the microfabric of
mudrocks. According to the Terzaghi theory of primary
consolidation (1925), the load applied to saturated
sediments is initially taken up only by the pore water. As
the pore water migrates upward to the region of lower
water pressure, the load is transferred to the solid
skeleton, with a resulting volume decrease and soil
framework displacement. The type and amount of
displacement may be dependent on the rate at which the
load is applied to the sediment. In the natural
environment, the load on a sediment layer is provided by
the deposited mass of soil itself and varies with
sedimentation rate. Higher rates of deposition lead to
higher water contents retained at depth; high water
contents are an important factor in developing preferred
orientation of clays during compaction. The water acts as
a lubricant, allowing particles to reorient themselves
more easily (O'Brien, 1964; Meade, 1966). Besides inducing
preferred orientation of particles, consolidation may also
amalgamate single clays into domains (Ingles, 1968; Bowles
et al., 1969; Bennett et al., 1981; Faas and Crockett,
1983). This could be a result of either electrochemical
forces or an increased concentration of clays (Moon and
Hurst, 1984).
Secondary consolidation occurs after the excess water
9
pressure has dissipated and primary consolidation is
complete; the soil structure has readjusted to the new
stress state. However, even under constant effective
pressure, some soils continue to show a slow deformation,
or creep. Barden (1965) has attributed this creep to the
presence of bound water on clay particles; it is
considered quasicrystalline and may exhibit viscous flow
under constant effective stress. In areas of slow
sedimentation rates, secondary consolidation may be more
important than primary consolidation; this could result in
different types of microfabric.
Organism activity in the environment of deposition
also plays an important role in the determination of
microfabric. In aerobic and dysaerobic environments, it is
probable that most detritus will pass through an
intestinal tract at least once (Myers, 1977). This
activity produces highly compacted material; these feces
may be preserved at depth. Destruction of clay minerals
(Pryor, 1975), depletion of organic carbon (Gordon, 1966)
and minor dissolution of carbonate (Hammond, 1981) may
occur during digestion. In addition, some organisms have
selective feeding habits and can alter grain size
distributions (Rhoads and Stanley, 1964). Bacterial
secretions serve to bind the sediment and help preserve
fabric; permeability is decreased (Mitchell and Nevo,
1964) and shear strength increased (Rhoads and Boyer,
1 0
1982; Myers, 1977). Locomotion causes local deformation of
sediment around the organism while burrowing; the fabric
of the burrow fill is also determined by the organism
(Rhoads, 1970). Liquefaction incurred during bivalve
burrowing may cause dilation and change porosity and
compaction (McMaster, 1967; Rhoads and Boyer, 1982).
Polychaete activity can also change porosity (Featherstone
and Risk, 1977). O'Brien (1987) found that, in Paleozoic
and Mesozoic mudstones, bioturbated clays had random
orientations compared to unbioturbated rocks.
Previous Classifications of Microfabrics
Selected illustrations of previous microfabric models
are shown in figure 2. The first worker to develop a
theoretical classification of clay microfabric was
Terzaghi (1925). He suggested that marine muds were most
likely to be arranged in a 1 1 honeycombe" pattern in which
clay particles form chain-like arrangements of floes into
which silt particles are enmeshed. This fabric type was
also favored by Casagrande (1932).
Lambe (1958) proposed three major types of
microfabric: 1) salt flocculated, where particles are
generally arranged with edge-face (EF) contacts, with some
face-face (FF) and edge-edge (EE) contacts also occurring;
2) non-salt flocculated, with exclusively EF contacts; and
3) dispersed, with FF contacts dominant and minor EF
contacts. The EF-dominated structure was termed
1 1
clay particle
t
domain
cardhouse bookhouse
dispersed
I
turbostratic
honeycombe
Figure 2. Previous clay microfabric models.
Cardhouse fabric has dominant EF particle contacts; after
consolidation, this is the dispersed fabric. FF contacts
form domains, and EF contacts of domains form the
bookhouse structure; after consolidation, this forms the
turbostratic fabric. Honeycombe is an artifact of freeze
drying.
1 1 cardhouse" by later workers. Tan ( 1959) favored the
cardhouse arrangement and discussed various possible
explanations for EF bonding, including Coulomb attractive
forces, nonpolar van der Waals forces, cations, and
hydrogen bridging.
Sloane and Kell (1966) defined the term "bookhouse",
meaning a cardhouse type structure wherein domains are
arranged with dominantly EF contacts. Smalley and Cabrera
(1969) introduced the term "stepped face-face" to describe
particle orientations in compacted kaolinite. Aylmore and
Quirk (1960) introduced the term "domains" to describe
particles aggregated in FF contacts. They also described a
"turbostratic" structure in which domains are turbulently
arranged within a compacted clay.
van Olphen (1963) combined these schools of thought
into a complete classification of possible clay
interactions in suspension: 1) single particles in
dispersion; 2) domains in dispersion; 3) single particles
in EF contacts; 4) single particles in EE contacts; 5)
domains in EF contacts; 6) domains in EE contacts; and 7)
domains in EF and EE contacts.
During the 1960s and 1970s, emphasis was placed on
determining which of these theoretical particle
interactions is dominant in freshly deposited mud.
Rosenqvist (1962) produced TEM pictures of quickclays
showing a structure similar to cardhouse. Pusch (1966)
1 3
described EF and EE contacts from recent muds, von
Engelhardt and Gaida (1963) also favored a cardhouse
arrangement on a theoretical basis. Pusch (1966) and
Bowles (1968) show EM pictures resembling honeycombe
structures. Bowles et al. (1969) found domains in
compacted clays. Barden and Sides (1970) found no evidence
for a cardhouse structure; only domains were observed.
O'Brien (1971) discovered long twisted chains of stepped
FF and EF contacts resembling the honeycombe structure in
laboratory sedimented clays. Sides and Barden (1971)
discussed the lack of good evidence for cardhouse or
dispersed fabrics and concluded that salt-flocculated and
turbostratic fabrics are the most reasonable fabrics.
Likewise, Moon (1972) argued against the possibility of
single-particle interactions and proposed that most clay
is sedimented in the form of domains. Barden (1972) found
bookhouse and turbostratic structures. Zabawa (1978)
observed EE, FF, and EF contacts in clays from filtered
suspensions. Laboratory experiments with artificially
sedimented clays (Osipov and Sokolov, 1978) found that
kaolinite sedimented in fresh water had a bookhouse
structure, whereas in salt water it had a honeycombe
fabric. Likewise, illite sedimented in fresh water had a
cardhouse structure and in salt water a honeycombe
structure. Montmorillonite sedimented in fresh water
exhibited small foliated microaggregates with complex
1 4
interactions; in salt water these microaggregates were
arranged in a distinct honeycombe fabric. However, all
natural samples had a honeycombe structure when sedimented
in fresh or salt water (Osipov and Sokolov, 1978). Hyne et
al . ( 1979 ) found both honeycombe and cardhouse
arrangements in recent delta sediments. Bennett et al.
(1981) found an open arrangement of floes and interlinking
chains in a DSDP Cretaceous red clay sample, while recent
delta sediments displayed domains arranged with a random
orientation.
Another controversial topic in the microfabric field
is the origin of preferred orientation. Most experiments
with laboratory consolidation have shown that preferred
orientation is developed within a few kg/cm of pressure
(Martin, 1965; Meade, 1966; Bowles et al., 1969). However,
studies of natural shales have produced more conflicting
results. Some show a positive correlation of preferred
orientation with depth of burial (Kaarsberg, 1959; Oertel
and Curtis, 1972; Bennett et al., 1981) while other
studies find no correlation (White, 1961; Beall, 1964;
Gipson, 1966; Meade, 1966; Odom, 1967). It is apparent
that factors other than compaction may also be responsible
for the development of preferred orientation and that
random orientation may be preserved under consolidation.
Most studies to date have used the above descriptors
(random orientation, preferred orientation, bookhouse,
1 5
cardhouse, domain, turbostratic, etc...) as the sole means
of describing the fabric. However, it is apparent that
much more information is available through a more complete
description, Collins and McGown (1974) attempted such a
description. Their categories included: 1) description of
voids; 2) the presence of silt and sand and their
distribution and interaction with clay particles; 3) the
ease with which individual particles can be discerned; 4)
distribution and dominance of any one type of clay
particle interaction (EE,EF,FF) or structure
(turbostratic, cardhouse, bookhouse); 5) the presence of
distinct aggregations of chemical, mechanical, or
biological origin; and 6) the presence of larger, distinct
packets of sediment, and the type of connections between
such packe t s•
Sergeyev et al, (1980) developed a more descriptive
classification of clays. These categories include:
honeycombe, skeletal, matrix, turbulent, laminar, domain,
pseudoglobular, and sponge.
Previous workers have also focussed attention on
methods of sample preparation for the SEM (Barden and
Sides, 1971; Gillott, 1973 ; Boyde , 1978 ; Tovey and Wong,
1978), These are discussed in more detail in the next
section •
1 6
METHODS
Experiments with Sample Preparation for the SEM
Because of the high-vacuum conditions of the scanning
electron microscope, all samples must be dehydrated before
viewing. Dehydration can cause serious disruption of
microfabric; the high surface tension of water can exert
strong stresses on the individual particles and cause
reorientation. Therefore, the initial step of this
research was experimentation with several methods of
dehydration, including air, oven, substitution, critical
point, and freeze drying. All of these techniques
represent means of reducing the surface tension of the
pore fluid, thus reducing the risk of structural
rearrangement during drying. Overall, critical point
drying proved to be the best technique.
The simplest method of dehydration is air drying;
however, the large amount of volumetric shrinkage caused
during normal air drying is sufficient to prove that this
method is unsatisfactory. Oven drying is a slight
improvement over air drying, since heating decreases the
surface tension of water. In this study, both air and
oven dried samples exhibited a large amount of shrinkage.
The dried samples were dense and difficult to fracture.
Viewed under the SEM, only large domains and packets of
domains were visible. Single particles were not
identifiable and packing was dense. These characteristics
1 7
suggest that the fabric was altered by air drying.
Another method of reducing surface tension is
replacement of the original pore fluid with one
characterized by a low surface tension. This new fluid is
usually an organic solvent such as ethanol. Substitution-
dried samples were less dense than air-dried samples, but
shrinkage still occurred, suggesting that the fabric was
disrupted. As discussed below, replacement of pore fluid
by an organic solvent can also possibly disrupt fabric, so
the marginal benefit of substitution drying is outweighed
by the potential problems of the method.
Freeze drying is more accurately termed sublimation
drying. In this method, the water passes directly from
solid to vapor without a liquid phase, thus eliminating
surface tension. Small (1 x 1 x 0.4 cm) samples were
placed in LabTek TissueTek plastic sieve-like capsules and
immersed directly in liquid nitrogen for several minutes.
This ultrarapid freezing should eliminate ice-crystal
formation and its associated structural disruption. After
freezing, the samples were quickly transferred to a small
dessicator attached to the sublimator apparatus which
provided a very high vacuum. Sublimation was complete
within six hours. Little volumetric shrinkage occurred
with this method. When viewed under the SEM, these
samples exhibited a prominent honeycombe fabric of
flocculated domains and individual particles.
18
Critical point drying is based on the theory that
above a critical point in temperature and pressure, the
phase boundary between liquid and gas disappears (Cohen,
1979). Since the critical point of water is at high
temperature and pressure, pore fluid is usually replaced
with liquid carbon dioxide, using ethanol as an
intermediate fluid. Ethanol substitution was accomplished
by soaking the samples for one day each in the following
concentrations of ethanol: 10, 25, 50, 75, 100, 100, 100%.
After substitution was complete, the samples were placed
in a boat filled with ethanol inside a pressure bomb at
15-20* C; the bomb was then filled approximately halfway
(to cover samples) with liquid CO2 ; the pressure was
increased to 825 psi over a 2-hour period. Pressure
changes should be accomplished as slowly as possible in
order to reduce the chances of pressure differentials
within the sample; faster rates of pressure change led to
obvious cracking within the sample. The samples were
then allowed to soak in the CO2 for 3 hours, flushing
hourly with fresh C02* Then the temperature and pressure
were raised to 4 2* C and 1200 psi (above the critical
point) by heating over a period of 2-4 hours. The
meniscus disappeared at this point, indicating that the
gas/liquid phase boundary was no longer present. Then the
pressure was lowered to atmospheric, keeping the
temperature constant, over a 1-2 hour period and the
1 9
samples were dry when removed.
The potential problem with critical point drying is
that, whenever the pore fluid of a sample is altered, the
new fluid should have other chemical characteristics
similar to the original pore fluid so that changes in
attractive and repulsive forces between clay particles do
not result in structural rearrangement (flocculation or
dispersal). This factor is most important for high-water
content samples in which particles are more mobile. For
example, both ethanol and liquid carbon dioxide have a
lower dielectric constant than water, meaning that
increased attraction between particles could occur (van
Olphen, 1963). This could lead to increased flocculation
and fabric disruption.
For this study, experiments were conducted in which
mud was suspended in various fluids and the amount of
flocculation was visually determined. Ethanol in >30%
concentration exhibited a much higher ability to
flocculate mud than did seawater. Treatment of ethanol
with peptizers (sodium hexametaphosphate, sodium oxalate,
magnesium chloride, lithium chloride, sodium carbonate,
sodium hydroxide, and ammonium hydroxide) to decrease this
flocculation was not successful. These ant iflocculating
agents guard against edge-face floculation caused by
particle electrical edge charges. However, a low
dielectric constant fluid such as ethanol suppresses the
20
depositional processes also validates the belief that
these CPD fabrics are the natural fabrics.
Other workers agree with these conclusions. Based on
measurements of shrinkage and specific surface area, Tovey
and Wong (1978) concluded that oven and substitution
drying had only minimal advantages over air drying,
whereas critical point drying was shown to give much
greater improvement. Both Tovey and Wong (1978) and
Gillott (1973) concluded that freeze drying and critical
point drying were the best techniques, with a slight
preference given to critical point drying. The main
objection to freeze drying was the possibility of
formation of ice crystals. However, Boyde (1978) preferred
freeze drying over critical point drying because of
shrinkage due to replacement of pore fluid with ethanol
during critical point drying.
After a good method of dehydrating the samples was
found, it was discovered that the high water content
samples were extremely friable and disintegrated at a
touch. Thus experiments were conducted with various
impregnating and hardening agents to enhance sample
integrity. Impregnation with polyester resin (see methods
of thin section preparation) removed the topographic
relief necessary for SEM viewing. Attempts to etch back
the resin and reproduce topography— using acetone, a
variety of acids, and methylene chloride— were
21
micrographs shows that most of these samples which display
a pronounced honeycombe structure were prepared by freeze-
drying (Bowles, 1968; O'Brien, 1971; Chen, Banin and
Schnitzer, 1976; Osipov and Sokolov, 1978; Bohlke and
Bennett, 1980; Sergeyev et al., 1980). On the other hand,
Delage and Lefebvre (1984) and Tovey and Wong (1978) have
published micrographs of freeze-dried samples which do not
exhibit a pronounced honeycombe structure. Their procedure
involved rapid freeze-drying by immersion in Freon or
propane which had been cooled by liquid nitrogen. Samples
were kept at far below freezing temperatures during
sublimation. Their methods are probably preferable to
older methods of freeze-drying, but caution should be used
in interpreting fabrics of any freeze-dried clays.
Although critical-point method appears to be
preferable to freeze-drying, the problem of theoretically
increased flocculation due to changing pore fluid still
remains. Two specific types of floes described in this
study, the biofloc and the p-chem floe, could possibly be
artifacts of the drying process. However, both of these
floe types were also observed in freeze-dried samples
(although slightly distorted by the honeycombe texture).
Since the pore fluid was not altered during freeze-drying,
this suggests that these are not artifacts of the critical
point method. Furthermore, the ability to correlate
different floe types with different depositional and post-
22
total repulsive forces between clay particles
allowing attractive body forces (van der Waals') to become
important. The peptizers are not effective against this
for a fluid having low surface tension and a dielectric
constant similar to water, but hydrocyanide seemed to be
the only close candidate. Most scientists would object to
regular laboratory use of such lethal chemical.
On the opposing side, Bennett and Hulbert (1986) have
propounded the theory of chemical irreversibility of
fabric. This theory states that, once sediment is
deposited, the fabric does not change as a result of
changing pore fluids. The validity of this theory can be
tested by examining the fabric of critical-point-dried
recent sediments.
Little volumetric shrinkage occured with the CPD
method. When viewed under the SEM, none of the samples
exhibited the prominent honeycombe structure of the
freeze-dried sediment, even though samples for both
methods originated from the same sediment type. Instead,
the samples showed a variety of delicate flocculent
structures the description of which forms the basis of
this thesis. Upon consideration of the freeze-dried
samples, it became apparent that small, 5-micron ice
crystals had formed, causing the prominent honeycombe
structure. An evaluation of previously-published
type of attraction. A search was conducted
23
unsuccessful. Since lacquer is more easily dissolved than
resin, samples were also impregnated with this material,
but lacquer has a high volatility and tends to shrink upon
drying, thus disrupting fabric. Shellac, white glue, and
hair styling gel produced similar poor results.
Impregnation with gelatine was accomplished by soaking in
a concentrated solution under low heat followed by drying
in a dessicator (Barden and Sides, 1971). Impregnation
was poor and shrinkage cracks were visible. Gelatin is a
colloidal protein; the molecules attach themselves to the
clay particles in a manner similar to bound water (van
Olphen, 1963). Since these molecules tend to form a
regular pattern during gelation, it seems possible that
fabric disruption could occur in high water content
samples during this process. Furthermore, there could be
difficulty in distinguishing between gelatine and clay
particles under the SEM.
In order to expose a fresh surface, fracturing of
the sample after dehydration usually provided good
results, except when the sample was too friable. The
fractured surface may be cleaned electrostatically
(Hulbert and Bennett, 1975) by rubbing a glass wand with a
polyester cloth, then passing the wand over the sample.
This did not work particularly well in this study. A
peeling technique has been described by Wong and Tovey,
(1975), where the sample is cut with a razor blade, and
24
then quick-drying epoxy is applied to the surface and
peeled away, removing 1-2 microns of the disturbed surface
material. The problem with this method was that the epoxy
retained most of the coarser particles, resulting in an
unrepresentative particle size distribution on both sides.
However, the epoxy side, as with peels used on macroscopic
samples, did highlight the sedimentary fabric in silty
material. Freeze-fracturing, in which the sample is
frozen, fractured, and sublimated, did not work well in
this study since frosting will occur within several
seconds on exposure to air. This frosting significantly
disrupted the fabric to several microns depth in the
s amp1e.
Routine Preparation of SEM Samples
Recent sediment samples were obtained using a Reineck
box corer. The box was subsampled using a 4"x4" square
cylinder. This subsample was allowed to dewater slightly
prior to extrusion and further subsampling. Since the
sample remained saturated with water, little fabric
disturbance was caused by this dewatering except increased
compaction. Final subsampling was accomplished using a
taut thin wire and/or dissection blades.
Recent sediments and the core from the Los Angeles
Basin were critical point dried following the procedures
outlined above. All of the samples taken from subaerial
outcrops (Pico, Point Loma, and Niobrara Formations) had
25
already been air-dried due to exposure to the natural
environment. Samples were fractured to expose a fresh
surface, and attached to SEM stubs using silver paint.
Once dry, the stubs were held upside down and gently blown
clean. They were baked in a vacuum oven at 60°C for 1
hour and then sputter-coated with gold/palladium.
X-Radiography
Vertical slabs of the core, 2 cm thick, were cut. X-
radiographs were made of these slabs using a 45kV Hewlett-
Packard oven-type x-ray machine.
Thin—section Preparation
Wet samples of appropriate size for thin-section work
were cut from the x-radiography slab. The pore water was
progressively replaced with acetone by soaking the samples
in 25, 50, 75, 95, and 100% acetone, increasing the
concentration daily. Three changes of pure acetone were
made. Then the samples were soaked in a 1:1 mixture of
acetone and polyester resin (Standard Brands marine
resin), with the amount of hardener reduced by 2/3. The
height of the mixture in the container must be double the
height of the sample to allow for the decrease in height
as the acetone evaporated. Samples were left at room
temperature and hardening occurred within 2-3 weeks.
Impregnation was excellent and thin sections were cut and
ground following normal procedures.
26
Backseattered Electron Microscopy
Back-scattered electron imaging (BSE) has had limited
use in the investigation of shales (Krinsley et al•, 1983;
Pye and Krinsley, 1984) because high-resolution images
have only recently been made possible due to the
development of improved backscatter electron detectors.
In normal scanning electron microscopy, the primary
electron beam interacts with the target material and
transfers energy to it. This leads to the generation of
secondary electrons (SE), which are collected by the SE
detector and used to image the sample. Since the energy
level of the secondary electrons is a function of the
angle of incidence between the primary beam and the target
material, the image produced in the SE mode reflects
topography•
Some of the primary electrons, however, interact with
the target material with little energy loss. In this case,
the primary electrons are scattered through the material
and eventually are reflected back out as backscattered
electrons (BSE). The energy level of these electrons is a
function of the atomic number of the target material,
along with the angle of incidence. If topography of the
specimen is minimized, the grey-level intensity of the
image is almost wholly a function of atomic number
cont ras t .
To prepare samples for BSE, thin (or thick) sections
27
were polished on a lapping wheel, finishing with 600 grit
polishing paper. Further polishing using a <5 micron
alumina grit in an aqueous solution, and felt paper, did
not result in a smoother surface. The alumina gouged out
the resin and clay and left quartz, feldspar, and
carbonate "islands." The 600 grit paper did leave some
long gouges across the surface of the sample, which were
visible in the BSE image, but these did not significantly
interfere with interpretation of the image. Samples were
very thinly coated with gold/palladium. Normal coatings
with metal decrease image quality significantly. This
problem can be avoided by coating with carbon, although
the method is more time-consuming. Best images were
obtained using a 15 mm working distance and small spot
size.
Grain Size Analysis
Samples were digested in 50% acetone, and subjected
to sonic disaggregation. Wet-sieving removed the >63
micron fraction, which was analyzed using the USC settling
tube. The mud fraction was split into <63 micron and <38
micron splits, ammonium hydroxide was added as a peptizer
and they were again subjected to sonic disaggregation.
Using a Model TAII Coulter Counter, total counts of
100,000 particles of the <38 micron split, and 50,000
particles of the <63 micron split were made. Population
counts were recalculated as weight percents.
28
X-Ray Diffraction
The <38 micron split from grain size analysis samples
was analyzed by x-ray diffraction in order to determine
clay mineralogy. A thick suspension of the mud was
sedimented on a glass slide to form a thin film. Three
slides of each sample were made. One sample was glycolated
to differentiate chlorite and smectite, another was heated
at 600 C for 1 hour to differentiate chlorite and
kaolinite. The third sample was run untreated. Copper k-
alpha radiation, 40 kV, 20 mA Norelco wide-angle
goniometers, were used.
Stereo SEM Microscopy
Stereo SEM photographs are obtained by taking an
image, tilting the specimen through 6 degrees, and taking
another image of the area. The sample may need to be
translated during tilting so that the centers of both
photographs are exactly the same. Angles of offset less
than 5 degrees creates a flat topography, and angles
greater than 10 degrees produce a fisheye effect. If the
image goes out of focus during tilting, refocussing should
be accomplished by raising or lowering the z-axis, not
with the focus knob, in order to maintain magnification.
These images can be viewed using stereo lenses
following the techniques for aerial photography. Also,
they can be projected onto a screen for 3-D presentation.
To do this, 35 mm slides are made from the positive or
29
negative images. These slides must be pin-registered
during mounting so that two points in the center of each
image coincide. These slides are projected simultaneously
on a silver-lenticular projection screen using two
projectors. The two images should be aligned as much as
possible, "Test-pattern" slides may be used to prealign
the projectors. Polarizing filters are placed over the
projector lenses such that the two directions of
polarization are at right angles to each other. Northwest
and northeast are preferable to north and east. Then, the
audience wears polaroid spectacles such that the right eye
has the same direction of polarization as the right
projector and vice versa.
30
RESULTS, SANTA MONICA BASIN
Santa Monica Basin, an inner basin of the California
Continental Borderland, has been described in detail by
Malouta et al. (1981). It lies directly off the narrow
continental shelf near Los Angeles. Maximum water depth is
960 m; below 800 m the water is anoxic. Maximum detrital
input is from Hueneme Canyon to the north. A submarine fan
complex dominates the northwest margin of the basin; the
southeastern margin has less well-developed fans and mass
movements. Sedimentation rates vary from 40-80 cm/1000
years. The central basin floor has surface sediments
characterized by a mean grain size in the fine silt range,
with 10-15% calcium carbonate and 8-12% organic material
by weight. Sediment is bioturbated to 800 m water depth;
below 800 m, bottom waters are anoxic and primary laminae
are preserved. Turbidite layers are identified in the
laminated zone by grey color, Bouma sequence sedimentary
structures, and well-sorted, fining-upwards grain size
characteristics (Reynolds, 1986). The clays present in
these sediments are dominantly illite, montmorillonite,
kaolinite and chlorite; clay mineralogy does not vary
significantly between grey turbidite layers and green
hemipelagic or bioturbated sediments (Fleischer, 1970).
Water contents of surficial material range from 50-70% of
total volume•
31
Sediment Trap Material
These samples were obtained from a Soutar sediment
trap, suspended at 850 m water depth (100 m above the
central basin floor). The trap was deployed between
November 1987 and February 1988, which is the wet season
in these latitudes. Samples were prepared in the normal
way for SEM analysis.
Several distinct types of particle aggregates were
identified (Fig. 3). One type of aggregate is generally
cylindrical to ovoid in shape, about 50 microns in
diameter and 100-300 microns long (Fig. 4). These are
interpreted as fecal pellets. The interior of these
pellets contain very densely packed clays, silts, shells,
and organic detritus. The outer surface is fairly smooth,
with occasional amorphous organic coatings.
A second type of aggregate is subequant to
irregularly shaped, 20-100 microns in diameter (Fig. 5).
The interior contains clay, silt, shells, and organic
matter in loose packing. Individual clays are organized
into floes 2-10 microns in diameter. These have about
equal amounts of EF and FF particle contacts. Internal
void space is low and voids are generally less than 1
micron in diameter. These aggregates are usually present
as distinct packages, even when amassed together.
Sometimes clays cloak the outer surface of the floe (Fig.
6). These are the dominant floe type within bioturbated
32
Figure 3. SE image of sediment trap material (Stereo).
Note densely packed football to cylindrical shaped fecal
pellets; loosely packed spherical fecal pellets; and small
floes attached to long organic strands. Notice that there
are no individual clay particles.
33
Figure 4. SE image of cylindrical fecal pellet (Stereo).
These are typically 50 microns in diameter and 100-300
microns long. Outer surface is typically smooth.
Note dense packing of detrital material, typical of most
cylindrical pellets. This dense packing is retained after
sediment burial.
34
\
I
>
Figure 5. SE image of spherical biofloc (Stereo).
Sometimes irregularly shaped. These are typically 20-100
microns in diameter. Note loose packing of detrital and
organic material. Internally, clays are again agglomerated
into smaller floes and domains 2-10 microns in diameter.
35
Figure 6. SE image of clay cloaking of bioflocs (Stereo)
sediments, as discussed later. They are interpreted to be
agglomerated through ingestion by organisms; hence, in
this study, they are referred to as "bio-flocs" .
Another floe type is composed of domains and single
particles in low to high angle EF contacts (Fig. 7). Some
EE and FF contacts, or even smaller individual floes,
occur within these aggregates. The outer surface of the
floe is poorly defined; when amassed together, floes tend
to blend in with each other. Many voids are present within
the floe and average 1 micron in diameter. These floes are
reminiscent of, but not equivalent to, the cardhouse
structure. The cause of their agglomeration is the purely
physico-chemical attraction between clay particles; hence,
they will be referred to as "p-chem floes" in this study.
Small floes are also found attached together or onto
long strands of organic material (Fig.8). These strands of
organic material include micro-organismal or algal spines,
or filamentous bacteria (Fig. 9). Sticky organic compounds
on the outer surfaces of the strands are probably
responsible for holding these aggregates together.
However, it is difficult to discern whether these floes
formed within the water column or after deposition in the
sediment trap. If they did form in the water column, these
could be an important mode of pelagic sedimentation.
Other particles present in the sediment trap include
solitary silt and sand particles and organism tests.
37
Figure 7. SE image of physico-chemical floe (Stereo).
These are large clay minerals or mica. Note dominance of
EF particle contacts, and abundant interior void space.
These are agglomerated as a result of electrical forces;
clay edges tend to have positive charges and faces have
negative charges.
38
Figure 8. SE image of organic floes (Stereo).
These are bioflocs and other aggregate types attached onto
long strands of algal material. Bacterial processes may
also play a role in this type of agglomeration.
39
>
)
Figure 9 . SE image of small organic floes (Stereo).
These are aggregated through sticky organic compounds or
filamentous bacteria.
40
Basin Floor Sediments
Station USC/DOE-21 was cored on the central basin
floor within the anoxic zone (Fig. 1). Looking at a
freshly cut surface of this core, the upper 14 cm
displayed green/black zebra-stripe laminations, which
disappeared rapidly upon exposure to oxygen. From 14-21
cm, the sediment was homogeneously green, and from 21-28
cm it was grey. The x-radiograph of this core (Fig. 10)
shows that the upper 14 cm is finely laminated,
characteristic of pelagic deposition in the absence of
bioturbation. From 14-21 cm, the sediment is bioturbated;
individual horizontal burrows are clearly visible at 18
cm. Below 21 cm there is a marked increase in the opacity
of the sample to x-rays. Textural laminations are present
at 27 cm, and some individual burrows can be seen in the
upper part of the bed.
BSE images of the laminated pelagic zone shows
generally fine grain size with relatively little silt. No
regular alternation of a two-layer system was immediately
apparent, as the macroscopic examination would seem to
suggest. Instead, it appears that lamination results from
irregularly spaced thin stringers of sediment which has a
composition markedly different from the background
material. These stringers are of two types: organic and
textural. Textural stringers are less than 1 mm thick, and
may be composed of silt overlain by clay, or of a silt
41
Figure 10. X-radiograph of boxcore USC/D0E-21.
0-14 cm: Pelagic mud. Thin laminations preserved due
anoxic conditions.
14-21 cm: Bioturbated mud. Note horizontal burrows.
21-28 cm: Turbidite mud. Note laminations near base,
burrows near top.
layer alone, or of a clay layer alone (Fig, 11), Organic
laminations are irregular and discontinuous and contain
little detrital material (Fig. 12). On a larger scale,
bands of sediment containing more foraminifera shells can
be differentiated from areas without foraminifera shells.
These sediment relations in the pelagic zone lead to
some interesting interpretations. As evidenced by the
sediment trap, most material arrives on the basin floor in
the form of fecal pellets and large organic aggregates.
Since these pellets contain silt particles in random
distribution, these sediments will also contain random
distributions of silts. Production of pellets takes place
continuously, but the rate increases dramatically during
the spring organic bloom. Thus we expect a background
matrix representing fecal pellets and organic aggregates,
with more rapid deposition of this material, along with
abundant organism tests, during the spring bloom. No other
difference in microfabric will be seen in bloom and non
bloom time, since the depositional process— pelleting— is
the same. Occasional deposition of larger organic bodies
is probably responsible for the randomly interspersed
organic laminae.
Also occasionally, this rain of pellets will be
interrupted by gravity-flow deposition of detrital
material, resulting in their textural laminations. These
thin gravity flows probably result from high detrital
43
Figure 11. BSE image of pelagic mud.
Note silt and clay laminations. These are caused by
influxes of detrital material through dilute sediment
gravity flows. They probably occur during major winter
s t o rms.
44
Figure 12. BSE image of pelagic mud.
Note discontinuous organic laminations (black areas).
These probably result from deposition of large organic
bodies; they are preserved due to lack of bioturbation.
runoff during major winter storms. Alternatively, they
could represent deposition from nepheloid plumes in the
water column. In either case, microfabric of these thin
layers should be different from that of the background
pelagic material, and should be similar to that of thicker
turbidites•
The bioturbated mud differs from the pelagic in
having a higher silt content. This silt is not present in
thin horizontal lamina, but is dispersed irregularly
through the sample in a mottled texture (Fig. 13). The
bioturbated areas represent material that was originally
deposited through either pelagic or turbiditic processes
or both. Bioturbation has disrupted the characteris tic
silt microfabric of the original processes, and produced a
textural mottling. Mottling is the result of particle size
selection by organisms during particle feeding. Some of
the mottling resembles the meniscate-backfi11 structure
(Fig. 14). Parts of the sediment which may escape
bioturbation may retain the clay fabric of the original
depositional process; most, however, will be altered due
to ingestion by deposit-feeding organisms.
The turbidite mud also has a higher silt content, but
lacks the mottled structure of the bioturbated mud. At the
bottom of the bed, coarse silts and micas are abundant and
some micas show preferred orientation (Fig. 15). Upwards
in the deposit, silt size decreases and micas are in
46
;
)
Figure 13. BSE image of bioturbated mud.
Note textural mottling caused by lateral segregation of
silt and clay. Also note random orientation of micas. This
is a typical fabric caused by bioturbation.
47
\
Figure 14. BSE image of bioturbated mud.
Note meniscate backfill structure (U-shaped distribution
of silts). This results from detritivorous organisms
(typically heart urchins) burrowing through the sediment
Selective ingestion of clays results in textural
laminations in a meniscate pattern.
48
Figure 15. BSE image of turbidite mud.
This is near the base of the deposit. Coarse silt is
abundant, but most grains are floating in the clay matrix.
Note preferred orientation of micas, occurring as a result
of deposition during a waning current.
random orientation— some are even vertical (Fig. 16). The
clay microfabric of these sediments will reflect the
physical-chemical processes of flocculation and
depos it ion•
These relationships suggest that the preferred
orientation in the basal section is a result of
depositional process— this portion was deposited while the
current was still moving, acting to orient the micas
parallel to the depositional surface. Alternatively, the
preferred orientation in the basal portion could be a
result of differential compaction occurring rapidly after
deposition. The basal section would have had a different
compaction history on the microscale, since: 1) it
initially carried more load than the upper portion of the
turbidite; 2) this load was applied rapidly, while the mud
still had a high water content; and 3) the presence of
coarser silt particles and less clay could have aided
drainage and allowed compaction to proceed more rapidly.
Previous laboratory studies have shown that all of these
factors could lead to increased reorientation of grains.
However, the first hypothesis, that this is a depositional
fabric, is preferred because: 1) only the very basal
section has PO; 2) the sections immediately above this
display random orientation, but similar grain sizes; and
3) similar relationships persist in deeply buried
turbidites, as discussed in later sections.
50
Figure 16. BSE image of turbidite mud.
This is near the top of the deposit. Grain size is finer
here as compared to the base of the deposit. Note random
orientation of micas, occurring as a result of quiet
deposition from suspension. White dendritic patches are
artifacts.
51
Station USC/DOE-17 was cored on the lower slope of
Santa Monica Basin at 750 m water depth. The x-radiograph
shows a mottled fabric characteristic of bioturbation
(Fig, 17), Silt microfabric is similar to that of the
bioturbated section of core USC/DOE-21.
The most characteristic fabric element of bioturbated
muds is the biofloc (Fig, 18), Bioflocs are round to
ovoid, 10-50 microns in diameter, and exhibit medium-dense
packing. Intrafloe porosity is less than 1 micron in
diameter and is moderately high. Clay particle contacts
include EE, EF, and FF. Organic material, such as diatom
and coccolith tests, are abundant within the floes. Some
floes have a smooth outer surface. The morphology of these
particles, along with their consistent appearance in
bioturbated sediments and in the sediment trap, leads to
the conclusion that they represent the fecal pellets of
detritivorous meiobenthos and microbenthos. Some of these
pellets may have been partially destroyed by bacterial
degradation, although no bacteria were identified in these
images•
Near the sediment/water interface, the muds contain
silts, micas, and bioflocs in short chains separated by
abundant voids 10-30 microns in diameter. Some areas
between floes and between chains are filled with
flocculated clays displaying a majority of EF particle
contact s •
52
Figure 17. X-radiograph of boxcore USC/DOE-17.
This sample is from the lower slopes of Santa Monica Basin
in an aerobic environment, resulting in a completely
bioturbated sediment. Some thin branching burrows are
visible at the top, otherwise the fabric is diffusely
bioturbated.
53
\
Figure 18. SE image of bioturbated mud (Stereo).
USC/DOE-17, 5 cm below the sediment/water interface.
Note agglomerations (bioflocs or pellets) 10-15 microns in
diameter. Internally, clays are moderately to densely
packed in random orientation. Large porosity is abundant.
Micas, silts, and tests also present.
54
Increased consolidation at 20 cm depth leads to
denser packing of bioflocs, silts, and flocculated clays
(Fig. 19). At this depth, individual bioflocs are less
well-defined because they are coming into closer contact
with each other and with zones of physico-chemical
flocculated clays. Compaction leads to a reduction of
large pore space; a moderate number of pores in the 5-10
micron range are present, but most of the remaining
porosity is intra-floc pores in the 1ess-than-1-micron
range•
Another important fabric feature in these samples are
areas 100-200 microns in diameter which are more densely
packed than surrounding material (Fig. 20). Some of these
areas have negative relief. These are interpreted as
burrow- fill cross-sections. Other areas of densely-packed
clays could be the densely-packed cylindrical fecal
pellets seen in the sediment trap (Fig. 21).
Unbioturbated turbidite muds are composed of
continuously-flocculated clays with dominantly EF particle
contacts; individual p-chem floes within this mass can
occasionally be distinguished (Fig. 22). The clay fabric
is very open and 1-micron pores are abundant. Large pores
are scarce, mostly occurring between grain-grain contacts
of coarser silt particles. However, near the sediment-
water interface, larger grains float in the continuous
matrix of flocculated clays. Organism tests are rare.
55
Figure 19. SE image of bioturbated mud (Stereo).
USC/DOE-17, 20 cm below the sediment/water interface.
Pellets and bioflocs have been brought closer together,
making it difficult to define individual floes. Large
porosity is gone, small interfloe porosity still high.
56
Figure 20. SE image of burrow fill.
The burrow fill is oval-shaped, 100 microns in diameter,
located in the center of the image. It appears darker than
surrounding material because it is more densely packed.
The surface of this material is smooth due to FF
amalgamation of clays as a result of organism packing.
57
Figure 21. SE image of cylindrical pellet.
This is a feature similar to the densely-packed pellets
seen in the sediment trap material (Fig. 3). This sample
was rapidly-freeze-dried; note the abundant void space
about 3 microns in diameter characteristic of honeycombe
fabric; this is an artifact of freeze-drying.
58
Figure 22. SE image of turbidite mud (Stereo).
USC/DOE-21, 7 cm below sediment/water interface. Note
continuously-flocculated fabric of clays in dominantly EF
high-angle contacts. Abundant void space, most voids about
1 micron in diameter. Note silts floating in matrix. Some
individual p-chem floes can be subtly discerned.
59
At 25 cm burial depth, packing of the clays is
denser, caused by high-angle EF contacts changing to low-
angle EF contacts (Fig. 23). This decreases the amount of
void space overall, and also the average pore size. Coarse
silts and sands are playing a more active role in altering
the fabric of clays. Rather than passively floating in the
clay matrix as above, they are locally squeezing and
bending the clays immediately surrounding them.
Organization of clays into p-chem floes is less apparent
at this burial depth.
Anoxic laminated sediments display a wider variety of
clay fabrics. Some areas display continuously EF
flocculated clays, similar to turbidite muds. These
represent the thin textural laminations caused by dilute
gravity flows. Other areas are abundant in organism tests
and bioflocs (Fig. 24). These latter areas are similar to
bioturbated sediments, differing only by the lack of large
(5-10 micron) porosity. The fabric of these pelagic
sediments at 2 cm burial depth is more similar to that of
bioturbated sediments at 15 cm burial depth. Essentially,
they show the result of undisturbed consolidation.
Other areas within these anoxic laminated sediments
display subequal amounts of p-chem, bioflocs, and organism
tests. Less commonly, high concentrations of organism
tests are found, representing a spring-bloom deposit (Fig.
25) .
60
Figure 23. SE image of turbidite mud (Stereo).
USC/DOE-21, 26 cm below sediment/water interface. Still a
continuous1y-f1occu1 ated mass of EF contacts.
Consolidation has decreased the angle of contact between
individual clay particles. Small pores are still abundant,
silts are beginning to influence local clay fabric.
I
61
Figure 24. SE image of pelagic mud (Stereo).
USC/DOE-21, 2 cm below the sediment/water interface.
Contains tightly packed 1-5 micron bioflocs, and more
loosely packed 5-10 micron bioflocs, silts and tests. Most
pores are smaller than 1 micron— otherwise, it appears
very similar to bioturbated sediments.
62
Figure 25. SE image of pelagic mud (Stereo).
USC/DOE-21, 2 cm below sediment/water interface.
This is a thin lamina containing almost exclusively
co c eoliths. This layer was probably deposited during a
period of upwelling, which increases nutrient
concentrations and thus pelagic populations.
63
Rare lamina containing abundant globular-shaped floes
of crenulate, intergrown clays are also found (Fig. 26).
The morphology of these clays suggests that they are
diagenetic. They are found within 3 cm of the sediment
surface. EDS chemical analysis shows high amounts of iron,
and high silica relative to alumina. Potassium and calcium
are present along with trace amounts of titanium and
magnesium. Eluvial clays with very similar morphology have
been imaged by Sergeyev et al. (1980) and identified as
nontronite. The chemical analyses are consistent with this
identification. Also, geochemists working with these
sediments suggest that early diagenetic formation of
nontronite is consistent with stoichiometric studies of
pore water geochemistry (Bret Leslie, pers. comm., 1988).
64
Au
Mg
KeV
Figure 26. Early diagenetic nontronite.
USC/DOE-21, 3 cm below sediment/water interface.
SE image and EDS analysis (dot shows location).
These clays tend to occur in high concentrations in thin
1aminae.
65
RESULTS, LOS ANGELES BASIN
General Characterization of Inglewood Core
The Los Angeles Basin is one of the filled inner
basins of the California Continental Borderland, The
general geologic nature has been summarized by Yerkes et
al . ( 1965). At depth within the basin lie Pliocene deep
marine deposits which are roughly equivalent to modern
deposition in the Santa Monica Basin. A continuous core
was drilled through these strata in 1986 (Gidman et al,
1987) by Chevron Oilfield Research. They recovered 1400'
of Pliocene Deep marine fan deposits. These are
interpreted to have been deposited by turbidity currents
on lobes and overbank areas (Schweller et al., 1987).
Three broad groups of mudstones were identified in
these cores: 1) bioturbated mudstones— brownish green muds
having no apparent sedimentary or bioturbate structures;
2) bioturbated sandy mudstones— brownish green muds having
circular to ovoid, unlined, sand-filled burrows; and
3) turbidite mudstones— grey muds having no apparent
sedimentary or bioturbate structures, found in association
with a basal sandstone layer. The top of this material was
usually bioturbated. This mud type was rare, probably
because bioturbation was intense and only the thickest
deposition of mud, or very frequent turbidity current
activity, caused these to remain unbioturbated.
The descriptions of the studied intervals are as
66
follows: 899 .5-900 . 0 ' (Depth below well-head) Bioturbated
mudstone with no obvious structures; 902 .0-9 0 3 . 0 '
Bioturbated mudstone with no obvious structures (Fig, 27);
906 , 0-906 . 6 ' Turbidite mudstone with thin turbidite
sandstone, interbedded with bioturbated mudstone (Fig,
28); 2097 .0-2097 . 5 ' An upper 2 cm of turbidite mudstone
with a thin basal sandstone, underlain by bioturbated
mudstone (Fig, 29); 2128.0-2129.0' Bioturbated sandy
mudstone with sand-filled, large diameter burrows (Fig,
30) .
Water contents ranged from 24% at 900' to 17% at
200', The turbidite mudstones were softer and less dense
than the bioturbated mudstones. Density and hardness of
the mudstones increased with burial depth.
A general description of the composition of these
samples was made by inspections of thin-sections. The silt
particles were generally quartz with subordinate feldspar.
Chlorite was abundant, and usually coarse-grained. Matrix
clays were highly birefringent and probably belong to the
smectite group of clays. Foraminifera and other tests were
absent in the turbidite muds, but common in the other
samples. All of the samples contained between 5-10%
refractory organic material in close association with
pyr i t e •
X-ray diffraction was used to determine clay
mineralogy; quantification of the mineralogy was not
67
Figure 27. Photograph of Inglewood 899'.
Macrofabric is fairly homogeneous at this scale.
Figure 28. Photograph of Inglewood 906".
Note rough laminations of grey/green mudstone. The green
is bioturbated hemipelagic. The grey represents thin mud
turbidites. Some portions of these turbidites have been
reworked, but most portions remain unbioturbated , probably
as a result of frequent turbidity currents.
69
Figure 29. Photograph of Inglewood 2097".
The topmost 2 cm of this sample is a grey, unbioturbated
turbidite mudstone. Below this is green bioturbated
mudstone. A thin sandy turbidite occurs at about 6 cm.
Figure 30. Photograph of Inglewood 2 12 8".
Bioturbated sandy mudstone. These were originally
deposited as turbidites and pelagic settling, but
bioturbation has completely reworked the fabric. Note
sand-filled burrows.
possible. All of the samples contained chlorite, illite,
and smectite. Kaolinite was a minor constituent in some
samples. The lack of a clear-cut distinction between the
mineralogies of the bioturbated mudstones and the
unbioturbated turbidite mudstones was surprising, since
their colors and other characteristics were so different.
However, these color differences could arise from organic
content •
The EDS was used to identify particular particles
imaged in the SEM. Most of the silt particles were quartz
and feldspar. Pyrite was common, usually in a framboidal
habit. Tourmaline was a minor constituent. Carbonate and
siliceous tests were present, usually showing signs of
mechanical disaggregation and/or chemical dissolution. The
coarse micas included biotite (Fig. 31) and chlorite (Fig.
32). Smaller clay minerals generally have a variable
composition which is typical of smectite (Fig. 33). Illite
was present (Fig. 34), but no kaolinite was identified
with the EDS •
The mean grain sizes of all samples varied from 6 to
7 phi. The textural parameters were similar between
samples and variable within a given sample. The mean was
particularly variable in the sandy mudstones. The standard
deviations (sorting) ranged from 1.35-1.70 phi. In
general, the turbidites were better sorted than the
bioturbated mudstones, although there was some overlap.
72
20.0KU 22mm t018.36|it 01.04KX 026465
KEV-10/CHANNEL
Figure 31. SE and EDS analysis of biotite.
Inglewood 899' bioturbated mudstone. Dot indicates
location of analysis.
73
Si
Au s
Ca
Fe
KEV—10/CHANNEL 2
8 6 4
Figure 32. SE and EDS analysis of chlorite.
Inglewood 902 ' bioturbated mudstone. Dot shows location of
analysi s.
74
20 OKU 21mm T014.20jiT 01 37KX 026771
4------------------------- 1 ---------------------f
1 KEV-10/CHANNEL 3
Figure 33. SE and EDS analysis of smectite.
Inglewood 899^ bioturbated mudstone. Dot shows location of
analysi s.
75
KEV—10/CHANNEL
Figure 34. SE and EDS analysis of illite.
Inglewood 8 9 9^ bioturbated mudstone. Dot sbows location of
analys is.
76
The bioturbated mudstones are composed of moderately
to poorly sorted silts and clayey silts. The bioturbated
sandy mudstones are composed of poorly sorted clayey silts
and sandy silts. The turbidite mudstones are composed of
moderately sorted clayey silts.
Silt Microfabric
Petroscopic examination of the specimens shows that
the spatial distribution (across the sample) of the
various size classes was very distinctive for a given
mudstone type. This refers to the amount of variation in
the grain size distribution through a vertical or
horizontal transect, caused by a segregation of sand,
silt, and clay•
The bioturbated mudstones displayed a slight spatial
heterogeneity of grain sizes (Fig. 35). Silt and clay were
distributed fairly evenly over each small sample.
In contrast, the sandy mudstones (bioturbated) had a
high degree of spatial segregation of grain sizes (Fig.
36). Sand was generally restricted to burrow fillings;
outside the burrows, silt and clay were segregated in
mottled or swirling patterns. One striking example of this
(Fig. 37) is a pattern interpreted as a meniscate backfill
ichnostructure (as described in a later section, this type
of burrow fill is commonly seen in electron micrographs).
The meniscae of the burrow fill are outlined by
preferential placement of coarse silt particles. Based on
77
1
Figure 35. Thin section image of bioturbated mudstone.
Inglewood 2097", magnification = 5X.
Note segregation of silt and clay resulting in a mottled
texture. Irregular-shaped clay-filled burrow towards upper
left of photograph, possible fecal pellets towards bottom
of micrograph. Abundant organic material in matrix.
78
Figure 36. Thin section image of bioturbated sandy
mudstone. Inglewood 2128", magnification = 10X, up is to
the top. Note extreme spatial heterogeneity caused by
silt- or clay-filled burrows. This is a typical fabric of
bioturbated turbidites.
79
Figure 37. Thin section image of meniscate backfill.
Inglewood 212 8", magnification = lOx. The U- or chevron
shaped distributions of silt and clay are typical of
Zoophycus, thought to be caused by backfilling of burrows
aTs det r itivorous organisms eat their way through the
sediment, selectively feeding on clays (Rhoads and
Stanley, 19 64).
80
the similarity between this pattern and the pattern of
grain size segregation elsewhere in the sample, it is
probable that most of this mottling is also caused by
burrowing and feeding activities.
The turbiditic mudstones did not display any
pervasive segregation of grain sizes (Fig. 38). Silt
becomes progressively finer upwards in the sample. Near
the top of the sample, smal1-diameter clay-filled burrows
were present (Fig. 39).
Similar textures are seen in BSE images, with the
difference that in these images, the presence and
orientation of micas, and the effect of these micas on the
fabric, is more clearly illuminated. The BSE image shows
a truer representation of the overall structure of the
material; that is, that clays are a matrix material
between a suprastrueture of enmeshed micas and silts. In
these images, the very bright spots are pyrite. Shell
material is also quite bright, probably due to the
inclusion of heavy metals. The white lathes are chlorite.
Equant grey minerals are mostly quartz. Dark grey, diffuse
areas are clays, and black areas are voids and/or organic
material (the impregnating agent is a hydrocarbon).
At 906^, the bioturbated muds display this spatial
segregation of grain sizes, exhibited as lumps of clay-
rich zones separated by zones of densely-packed silts
(Fig. 40). Some preferred orientation of micas is
81
Figure 38. Thin section image of turbidite mudstone.
Inglewood 906', magnification = 10X. Note the fairly
homogeneous spatial distribution of clays and silts. The
lighter color of the matrix material here indicates a
lower organic content.
Figure 39. Thin section image of Chondrit es.
Inglewood 2097", magnification = 5X, up is to the upper
right corner. This is the top of a turbidite which has
been minimally bioturbated, as evidenced by clay-filled
burrows. Most of the deposit, however, retains the
character of unbioturbated turbidites.
83
Figure 40. BSE image of bioturbated mudstone IW906.
Note lateral segregation of silts and clay resulting in a
mottled texture. Part of a meniscate backfill structure is
in the center of the micrograph.
84
apparent, especially right below the contact with the
overlying turbidite (Fig. 41). Note that at this scale,
the lower contact of the turbidite is not sharp. Silts
are being pushed into the underlying, more clayey deposit.
This relationship could be a result of depositional
process or of consolidation.
The lower portion of the turbidite at 906" shows
densely packed micas and coarse equigranular silts. Micas
are wrapped around the silts as a result of consolidation.
Many grain-grain contacts are apparent, but some floating
grains are also observed (Fig. 42). Higher up in the
deposit, well-developed PO of micas is observed. As
discussed earlier, this is probably a result of
depositional process, not consolidation (Fig. 43). In
contrast, the upper portions of the turbidite displays
less PO. Clay-rich areas here may represents floes or
fecal pellets (Fig. 44).
Near the top of this sample, Chondrites burrows are
present as l-2mm diameter blebs (Fig. 45). In the thin-
sections, the burrow fill is darker and finer-grained, but
the mineralogical differences between the burrow fill and
the surrounding area is not clear. In the BSE image, it is
clear that the interior of the burrow has a darker grey
level because it contains less silt and mica, and more
clay and organic material. A comparison between high
magnification BSE images of burrow fill (Fig. 46) and of
85
Figure 41. BSE image of mudstone IW906.
The lower portion of the micrograph is bioturbated
mudstone, overlain by silty turbidite. Note large
foraminifera tests in the turbidite. The contact is not
very sharp at this scale; silt particles are sinking into
the underlying mud. The turbidite has little matrix.
86
Figure 42. BSE image of turbidite IW906.
This is the base of the turbidite. Note densely packed
structure of coarse silts and micas. Many grain-grain
contacts are present.
87
Figure 43. BSE image of turbidite IW906.
This is near the base of the turbidite, probably Bouma
Division D (laminated). Preferred orientation of micas is
well-developed , as a result of original depositional
preferred orientation due to deposition during a waning
current, enhanced by consolidation.
88
Figure 44. BSE image of turbidite IW906.
This is near the top of the turbidite. Note that micas are
more randomly oriented as compared to the lower laminated
portion of the turbidite. Some spatial segregation of
clays and silts is present, suggesting some bioturbation .
89
the surrounding material (Fig, 47) show that the clay
fabrics are roughly similar. Clays occur singly or in
face/face amalgamated packets. These particles and packets
have an overall random orientation with respect to
horizontal, but within a small area, particles appear to
have a consistent orientation with respect to each other.
This type of fabric is reminiscent of the turbostratic
fabric defined by Aylmore and Quirk (1960).
At 2097', the bioturbated mudstone does not display
well-developed preferred orientation (Fig. 48). Floating
silt grains are apparent. The spatial segregation of grain
sizes is not as apparent here, but clays are more densely
packed in some areas. At the base of the mud turbidite,
preferred orientation is well-developed; even a hint of
imbrication is noticed (Fig. 49). Nearer the top,
preferred orientaation is still fairly well-developed, but
some micas at high angles to horizontal are present (Fig.
50). Overall, the difference in preferred orientation
between the upper and lower portions of this turbidite is
less than that observed at 906'. This is probably because
increased consolidation has increased the amount of PO
overall. Packing in all samples is dense and homogeneous.
In the bioturbated sandy mudstone, some preferred
orientation of mica particles is apparent (Fig. 51).
However, there are less micas here than in the turbidite
mud, so their net contribution to the overall structure of
90
Figure 45. BSE image of Chond ri t e s in turbidite IW906.
This is the top of a turbidite (note fairly random
orientation of micas) containing a well-defined clay-
filled burrow. This is similar to other structures seen in
thin section images.
91
Figure 46. BSE image inside Chond ri te s .
This is a higher-magnification image of the fabric inside
the clay-filled burrow of Fig. 45. In comparison to the
following image, it can be seen that the clay fill is
darker because it contains less silt than surrounding
material, and probably more organic material also.
92
Figure 47. BSE image outside Chond r i t e s.
This is a higher-magnification image of the fabric of the
unbioturbated turbidite of Fig. 45. In comparison to the
previous image, it can be seen that this area contains
much more silt.
93
Figure 48. BSE image of bioturbated mudstone IW2097.
There is some spatial segregation of silts and clays here
as indicated by grey-level intensity of matrix material.
Note also that micas are in random orientation; typically,
bioturbated muds allow for less reorientation of micas
during consolidation, compared to turbidites.
94
Figure 49. BSE image of turbidite mudstone IW2097.
This is located near the base of the deposit. Note
abundance of pyrite. Preferred orientation of micas is
very well-developed, due to a combination of original
depositional fabric and reorientation of micas during
consolidation .
35
Figure 50. BSE image of turbidite mudstone IW2097. This is
located near the top of the deposit. Preferred orientation
of micas is well-developed here, due to reorientation
during consolidation. However, it is less-well-developed
compared to the base of the deposit.
96
Figure 51. BSE image of sandy mudstone IW2128.
Note prominent foraminifera tests. Some preferred
orientation of micas is present, but is not as well-
developed as in nearby unbioturbated mudstones. Some
textural mottling is apparent.
the sample is less* As in thin sections, spatial
segregation of silt and clay particles occurs. The other
obvious difference between this mudstone and the turbidite
is the presence of microscopic tests.
Since these portions of the mud probably represent
bioturbated hemipelagic and turbiditic deposits, it was
interesting to find some instances of preferred
orientation of micas which was similar to that found in
the unbioturbated intervals (Fig. 52). Perhaps these
represent an area which was relatively less bioturbated.
Overall, preferred orientation is better-developed in
the bioturbated muds of 2128' than in those at 900'.
However, the PO is patchily distributed; the direction of
PO changes from area to area (Fig. 53).
At higher magnifications, a randomly oriented
silt/clay fabric, similar to the turbidite sample, is
apparent (Fig. 54). However, there are some subtle
differences. Fewer clay edges can be seen in this sample.
Clayey areas have a diffuse intensity, suggesting that we
are seeing more face-on clay particles, or that the
packets of clays are more tightly welded together. The
apparent void ratio is also lower (less black spaces).
Clay Microfabric
Samples from 899-902' are near the upper boundary of
the cored section. They are bioturbated mudstones composed
of brownish-green, dense, moderately to poorly sorted
98
Figure 52. BSE image of sandy mudstone IW2128.
Note abundance sand and generally coarse-grained nature.
Well-developed preferred orientation of micas occurs. This
is probably reflective of original depositional fabric —
hence, this area must have been only minimally
bioturbated.
99
Figure 53. BSE image of sandy mudstone IW2128.
Some preferred orientation of mica is also present here,
but the direction varies across the sample. This is
actually a meniscate backfill structure.
1 00
Figure 54. BSE image of sandy mudstone IW2128.
This is a high-magnification view of a generally muddy
area. Note dominance of silt, and dense, diffuse fabric of
clays. Very few clay edges are visible, suggesting a
random clay fabric.
1 01
silts and clayey silts. No sedimentary structures are
apparent. Depositional processes probably include pelagic
settling, low-density turbiditic and nepheloid layer flow;
however, bioturbation has destroyed any structures typical
of these sedimentary processes. Thus, the microfabric is
more a result of the bioturbation than of the depositional
processes .
The overall fabric of these rocks (899-902') is one
of small packets of clay 5-100 microns in diameter, wedged
between coarser silt particles (Fig. 55). Clay packing
within the packets is dense; FF clay particle contacts are
dominant, with minor occurrences of low-angle EF particle
contacts. Some packets are so densely welded together that
individual clays cannot be discerned. Some internal
preferred orientation is found in some packets; however,
the overall orientation is random and caused by the local
influence of silt particles. The large proportion of clay
faces, relative to edges, seen in vertical sections, is
evidence of the large number of clays having a vertical
orientation, and hence a very random structure. Packets
probably represent the bioflocs found in bioturbated
sediments. Their shapes have been distorted because of
high local pressures produced by silt particles. Since the
clay particles within the bioflocs were pre—compacted in
random orientation, overburden pressure was not very
effective in reorienting individual clays to horizontal,
1 02
Figure 55. S E image of bioturbated mudstone IW899
(Stereo). Subtle packages of clay, 5-100 microns in
diameter, are wedged between silt particles. Many clay
faces are apparent, suggesting that many clays have a
vertical orientatione. Some packets are so densely welded
that individual particles cannot be discerned.
1 03
resulting in a highly random compacted fabric.
In sample 902', a 2 cm diameter, unlined burrow
having an ovoid morphology (Pianoli t e s) was apparent. It
seemed to be filled with material similar to the rest of
the sample, but of a lighter color. Samples from this
burrow displayed an unusual fabric type, which is herein
termed face/face amalgamation (Fig. 56). This type of
amalgamation is not rare of itself, but in this sample, it
formed a fairly continuous, flat surface of clay faces
(Fig. 57). This surface could possibly represent a burrow
wall, where movement of the organism caused a
reorientation of clay particles and aided in the
preservation of this fabric. However, caution should be
used in interpreting face/face amalgamation as resulting
from bioturbation. This fabric can also be produced by the
"plastering" of clays against larger silt and sand
particles•
The sampled interval at 906' contains unbioturbated
turbiditic mudstone interbedded with bioturbated mudstone.
The turbidite mudstone is grey, soft, and composed of
moderately sorted clayey silts. The tops of the mudstones
are bioturbated. The bioturbated mudstones are similar to
the samples from 899' and 902'.
The clay fabric of the bioturbated mudstones at 906'
(Fig. 58) is similar to that at 899-902'. The clay fabric
of the turbiditic mudstone at 906', however, is subtly
1 04
Figure 56. SE image of bioturbated mudstone IW902.
This is a vertical section taken through a Planoli te s
burrow. Note the smooth surface caused by FF amalgamation
of clays. This amalgamation probably occurred as the
organism moved against the burrow wall.
1 05
Figure 57. SE image of bioturbated mudstone IW902.
This is a higher-magnification image of the surface of FF
amalgamated clays. Surfaces of this type are common
throughout bioturbated samples, but are easily confused
with fabrics of clays which have been plastered against a
silt particle.
1 06
Figure 58. SE image of bioturbated mudstone IW906
(Stereo), Subtle packets of clay, representing compressed
bioflocs, are wedged between silts. Abundant clay faces
are present, suggesting a highly random fabric. Pre-
compaction of pellets hinders reorientation of clays
during consolidation.
1 07
different (Fig, 59), Here, clays are present as distinct
lamina 5-10 microns thick and fairly laterally continuous.
Within the laminae, clays are fairly densely packed but
individual clays can usually be discerned. Low angle EF
contacts are dominant. True preferrred orientation is not
present, but many particles are subhorizontal and wrap
around the silts in a fold-belt pattern. As opposed to the
bioturbated mudstones, more clay edges than faces are
visible. In silt-rich laminae, clays are wedged between
the silts, with poor development of preferred orientation,
as a result of local stresses. These clays are also more
densely welded together. Most silts are cloaked with
clays. Porosity is low, but probably marginally higher
than the bioturbated mudstone. Pores are less than 1
micron; iniv ’•connection is low.
This fabric can easily be correlated with that of
recent turbidite muds. The high-angle EF contacts have
been transformed to low-angle EF contacts and FF contacts.
As voids decreased during compaction, silt particles drew
closer together, and exert a very strong local influence
on clay fabric.
The sampled interval at 2097' contains 2 cm of
unbioturbated turbidite mudstone underlain by bioturbated
mudstone. The lithologies are similar to those described
above for 906".
Low-magnification images show that the major
1 08
Figure 59. SE image of turbidite mudstone IW906 (Stereo).
Clays are densely packed in dominantly very-low-angle EF
and FF contacts. More clay edges are visible than faces,
suggesting that most clays are oriented at least
subhorizontal. Clays wrap around silts in a fold-belt
fashion. Some areas around silts are densely welded.
1 09
microfabric difference between these two lithologies is
preferred orientation of micas (Figs, 60, 61). However,
the clay fabrics of these mudstones at 2097' are markedly
different from those at 906', and fairly similar to each
other (Figs. 62, 63). In both lithologies, silt particles
have drawn very close together and now exert the dominant
influence on local clay fabric. All clays are present as
packets or lamina between coarser silts. Clays are densely
welded together and individual particles are almost
impossible to discern. This intergrowth lends a crenulate
texture to the fabric and is a result of diagenesis.
Porosity is vanishingly low; pores are on the scale of
nannometers•
The sampled interval at 2128' contains dense,
brownish green mudstones having large-diameter (~2 cm),
sand-filled, unlined burrows. Grain sizes range from
poorly sorted clayey silts to sandy silts. Thin sections
display a distinct spatial segregation of silt and clay.
This leads to a textural mottling caused by the
segregation of silt, clay, and sand. The presumed
depositional history is that of deposition by turbiditic
currents and by pelagic settling, followed by bioturbation
which destroyed the sedimentary stratification and
resulted in a mottled fabric. However, no fabric
correlatable to the original depositional process, or to
bioturbation, was found. Instead, clays are massively
1 1 0
Figure 60. SE image of bioturbated mudstone IW2097.
Clays are present as packets in-between silts. Note random
orientation of micas.
1 1 1
Figure 61. SE image of turbidite mudstone IW2097.
Clays are present as thin laminae wedged between micas and
silts. Note the preferred orientation of mica.
1 1 2
Figure 62. SE image of bioturbated mudstone IW2097
(Stereo). Clays are present as massive, crenulate
intergrowths. Individual clays are almost impossible to
differentiate. This texture results from diagenesis.
1 1 3
Figure 63. SE image of turbidite mudstone IW2097 (Stereo).
Clays are massively intergrown with a crenulate texture,
making identification of individual clay particles nearly
impossible. This fabric is virtually indistinguishable
from that of bioturbated mudstones at this depth; both are
a result of diagenesis.
1 1 4
intergrown and squeezed between silt and sand particles in
a fabric similar to that of 2097' (Fig. 64). Some semi
circular areas of very densely-welded clays were found
(Fig. 65); these may represent larger fecal pellets. Since
these pellets were originally more compact than the
surrounding clays , they remain more densely packed than
matrix clays after consolidation.
1 1 5
Figure 6 4. SE image of sandy mudstone IW2128 (Stereo).
Note massively intergrown clays squeezed between silts,
the lower right is a more open floe; perhaps this is an
original floe preserved between grain—grain contacts of
coarse silts.
Figure 65. S F image of sandy mudstone IW2128.
A circular area of densely intergrown clays; this could
represent a fecal pellet or a burrow fill.
1 1 7
RESULTS, VENTURA BASIN
The Plio-Pleistocene Pico Formation crops out (Fig.
1) in Santa Paula Creek near the city of Santa Paula,
California (Johnson, 1978). It is comprised of deep-sea
fan deposits of the Ventura Basin, and includes thick
sequences of mudstones deposited under anoxic conditions.
Exposures of these mudstones are excellent— the rocks are
soft enough to be scraped with a knife, and sedimentary
structures are quite prominent. Close analysis revealed a
clear-cut distinction between pelagic and turbiditic
mudstones (Fig. 66). The pelagic mudstones were dense,
mahogany-red, and finely laminated. The turbidite
mudstones were softer, grey to tan, and contained textural
grading and lamination characteristic of mud turbidites.
Since these samples were taken from an outcrop
section, water contents were not determined, but an
estimate is about 20% with an error of 5%. Grain size was
measured with Coulter Counter. The turbidites were clayey
silts to sandy silts, with mean grain size in the medium
to fine silt range. The pelagites were clayey silts with a
mean in the fine silt range. X-ray diffraction shows
chlorite, montmorillonite, and illite in both types of
strata.
Silt Microfabric
Thin section examination revealed that the pelagites
contained a high percentage of organic material and
1 1 8
Figure 66. Photograph of Ventura Basin mudstones.
Section from Santa Paula Creek, near the dam. Turbidite
sandstones are yellow; turbidite mudstones are grey,
sometimes laminated; pelagic mudstones are reddish-brown,
laminated, and speckled with foraminifera tests.
1 1 9
foraminifera tests (Fig. 67). They contained about 85%
matrix material which ranged from dark brown to light
brown. The light brown areas were highly birefringent
under crossed polars and represent clay material, possibly
smectite. The dark brown areas did not change color under
crossed nicols and represented organic matter.
Petroliferous organic matter was also present as blebs and
stringers, about 5% of the total composition. The
laminations arise from varying percentages of organic
material v s . fine silt. Silts were quartz and feldspar.
Turbidite mudstones had less clay and more fine silt
than the pelagites (Fig. 68). Matrix material was light
brown in polarized light and highly birefringent under
crossed nicols. Organic material was present as
petroliferous blebs, about 3%. Silt particles were quartz
and feldspar, with minor shells. Micas were chlorite,
biotite, and muscovite.
BSE images showed that pelagic and nepheloid-layer
deposits can be distinguished by the presence of numerous
pyrite cubes and framboids scattered evenly throughout the
pelagic layers (Fig. 69). However, these make the
interpretation of other fabric elements in the pelagic
layers difficult. Although thin sections show thin mm
laminae, BSE images don't reflect this small-scale
lamination. Some very-fine-scale laminae, due to organic
material (Fig. 70) and pyrite (Fig. 71) were observed.
1 20
Figure 67. Thin section image of pelagic mudstones.
Fine stratification results from organic and textural
laminations. Coarse silts are distributed fairly evenly
across the sample.
Figure 68. Thin section image of turbidite mudstones.
Silts are fine-grained, texturally well-sorted, and
distributed evenly across the sample.
1 22
Figure 69. BSE image of pelagic mudstone.
Note the abundance of pyrite cubes scattered evenly
throughout the sample. Coarse silts are also present,
the dominance of pyrite makes observation of silt
microfabric difficult.
but
1 23
Figure 70. BSE image of pelagic mudstone.
Again, pyrite is the dominant textural element. A thin
stringer of organic material appears as a black lamination
across the sample.
1 24
Figure 71. BSE image of pelagic mudstone.
A thin laminae of pyrite appears.
1 25
In the nepheloid/turbidite layers (Figs, 72, 73), the
preferred orientation relationships seen in the Santa
Barbara and Los Angeles Basins samples were not observed
here. This is probably due to the lack of coarse-grained
micas in the Santa Paula Creek samples compared to the
previous ones.
The upper portion of one nepheloid/turbidite layer
contained numerous clay-rich ovals about 0.5 mm in
diameter (Figs. 74, 75). The shape and size of these are
consistent with copepod fecal pellets which are found in
abundance in recent anoxic basins. These pellets, however,
are much largerr than the bioflocs found in recent
sediments.
Clay Microfabric
Diagnesis has turned most clays into crenulate,
intergrown masses similar to the fabric seen in the Los
Angeles Basin. Organism tests, particularly diatoms and
coccoliths, are not present, suggesting dissolution.
Alteration of feldspar to chlorite has been documented.
The clay fabric of the pelagic zone is different from
that of other mudrocks examined for this study because it
has a low abundance of silt. Thin sections show a high
abundance of organic material (kerogen ?) which is not
easily discerned in the SE images. The morphology of this
material must be flaky, similar to clays. Some subtle
packets of clay, 3-10 microns in diameter, may be
1 26
Figure 72. BSE image of turbidite mudstone.
This is near the base of the deposit. Note dominance of
coarse silts and textural lamination. Mica is less
abundant here than in Los Angeles Basin samples, but some
preferred orientation of micas can be detected. Grain-
grain contacts dominate.
127
Figure 73. BSE image of turbidite mudstone.
This is near the top of the deposit. Silts are finer and
mica is more abundant. Preferred orientation of mica is
mo derate.
1 28
Figure 74. BSE image of turbidite mudstone.
This is near the top of a thin turbidite. Note ovoid,
clay-rich (dark) areas which are interpreted to represent
fecal pellets. These could be pelagic fecal pellets (if
this is a nepheloid-layer flow) or benthic fecal pellets
(if this was a slump-generated turbidity current).
1 29
Figure 75. BSE image of turbidite mudstone.
This is a higher-magnifi cation image of the fecal pellets
in Fig. 74. Note that they are ovoid, clay-filled—
characteristics typical of fecal pellets found in recent
s ed iment s.
1 30
discerned (Fig. 76). These dominantly display clay faces
and are roughly correlatable to packets seen in Los
Angeles Basin bioturbated mudstones. Other areas display
low-angle EF contacts with fair internal preferred
orientation, which may be roughly correlatable with
fabrics seen in turbidite mudstones of the Los Angeles
Basin. The presence of both fabric types is consistent
with fabric relationships in recent pelagic muds, where
sediments occur both as bioflocs and p-chem floes.
The pelagic muds did occasionally display an unusual
fabric consisting of cotton-boll shaped, crenulate masses
of particles (Fig. 77). These were sometimes associated
with pyrite. Their morphology and occurrence is similar to
the diagenetic nontronite seen in Santa Monica Basin
anoxic sediments. EDS analysis was not undertaken.
The muddy turbidites differ most significantly from
the pelagic layers by the presence of coarser silt
particles cloaked with clay (Fig. 78). Diagnesis and
consolidation has created a crenulate, intergrown clay
fabric which is difficult to differentiate from that of
the pelagic zone. Some alteration of feldspar to clay was
noted (Fig. 79). The only significant difference is the
lack of subtle packaging of clays in the turbidites. Both
clay fabrics are similar to those found at 2000' in the
Los Angeles Basin.
1 31
Figure 7 6. S E image of pelagic mudstone (Stereo).
Clays are crenulate and intergrown, suggesting strong
diagenesis. Some subtle packets, showing dominantly clay
faces, may represent biofloes. Other areas of low-angle
EF, FF, edge-on clays may represent p-chem floes. Note no
tests; organic material difficult to identify.
1 32
Figure 77. Nontronite in pelagic mudstone.
Pyrite framboids dominate on the left side of the image.
"Cotton-boll"-shaped clays dominate on the right side. The
occurrence and morphology of these clays is similar to the
nontronite identified in pelagic muds of Santa Monica
Basin (Fig. 26).
1 33
Figure 7 8. SE image of turbidite mudstone (Stereo).
Clays are crenulate and intergrown, suggesting strong
diagenesis. Fabric is dissimilar to that of turbidite
mudstones of Los Angeles Basin; here, more clay faces are
present compared to edges.
1 34
Figure 79. Alteration of feldspar to clay in pelagite
SE image and EDS analysis (dot shows location).
ICE V—10/CHANNEL
1 35
RESULTS, NIOBRARA FORMATION
The Niobrara samples used in this study were collected
by C.E. Savrda, and details of stratigraphy, depositional
history and sampling locations are given in Savrda (1986).
These mudstones accumulated through pelagic deposition in
the Cretaceous anoxic Denver Basin, resulting in black
laminated shales; brief periods of oxygenation allowed for
limited bioturbation. This is apparent in the form of
isolated Chondrites burrows in some samples (Fig. 80A) .
These burrows, 1-3 mm in diameter, occur in vertical
sections as isolated horizontal to subhorizontal blebs.
They are thought to be produced by deposit-feeding
organisms (Savrda, 1986).
The microfabric inside the Chondrites and in the
surrounding black shale was studied in the SEM. The clays
in the black shale matrix (Fig. 81) are crenulate and
intergrown, suggesting diagenesis, but they are not
massively intergrown as in the deep Los Angeles Basin
samples. Clay edges dominate over faces, and a moderate
degree of preferred orientation is developed. Inside the
burrows, some areas have a clay fabric similar to that of
the black shale. However, other areas contain more
massively-intergrown clays exhibiting abundant clay faces
(Fig. 82). This means that the fabric inside the burrows
is more random and compact. This structure may result from
initial random compaction within fecal pellets.
1 36
Figure 80. Photographs of Niobrara samples.
A. Chondrites in black shale.
B. Zoophycus in calcareous shale.
1 37
Figure 81. SE image of Niobrara black shale (stereo).
Clays are crenulate and intergrown, but not massive,
suggesting low-level diagenesis. Clay edges dominate over
faces, and a subhorizontal preferred orientation is
moderately well-developed.
1 38
Figure 82. SE image of Niobrara Chondrites (stereo).
Clays are crenulate and intergrown to massively
intergrown. Abundant clay faces suggests highly random
fabric, which could result from initial random packing
within fecal pellets.
1 39
Other sections of the Niobrara contain bioturbated,
calcareous shales (Fig, 80B), One sample containing well-
preserved Zoophycus burrows was imaged in the SEM, These
are horizontal to shallowly inclined, 2-8 mm diameter
burrows with well-developed internal meniscate laminae.
The burrows are probably the result of deposit-feeding
polychaetes and were actively filled during burrowing
(Savrda , 19 86),
Because this sample is calcareous, the microfabric is
quite dissimilar to the terrigenous samples. Clays are
flaky to intergrown with a spongy texture. Although the
internal meniscae of the Zoophycus were previously thought
to result from pellet/clay laminations (Savrda, 1986),they
actually result from silt/clay lamination. The arcuate
pattern of the meniscae are outlined by silt
par tides (Fig • 83) neatly stacked against each other in a
brick and mortar style, with clay acting as the mortar
between the silts (Fig. 84). This type of structure is
quite similar to that seen in thin sections of the
Inglewood 2128' sample.
1 40
Figure 83. SE image of Niobrara meniscate backfill.
The arcuate distribution of silt and sand particles is
responsible for the meniscate backfill structure seen in
Fig. 8 0 B .
141
Figure 84. SE image of Niobrara meniscate backfill.
In a higlier-magnification image of the sand particles, it
can be seen that sands are packed within an arcuate line,
with clay present as "mortar" between the "bricks."
1 42
DISCUSSION
Origin and Significance of Bioflocs
Several other studies of the microfabric of recent
sediments have published micrographs of features similar
to the bioflocs described in this study. Collins and
McGown (1974) described "regular aggregations", some of
which can easily be recognized as bioflocs, although
others may have a different origin. These aggregations
were found in a variety of modern environments. Exact
correlation between these features is difficult, since
their air-drying technique probably created some
artifacts•
A study of unpublished micrographs taken by Fran Hein
(pers. comm., 1985) also reveals structures similar to
bioflocs. These were not recognized in Hein (1985) as a
distinct fabric type, probably because that study focussed
on sediments more deeply buried than this study, and the
distinction between floe types becomes very difficult to
make below about 0.5 m burial depth. These bioflocs were
noted in pelagic and bioturbated sediments.
More recently, Torresan and Schwab (1987) described
"equidimensional floes" in muddy sand samples from
Shelikof Strait, Alaska. It was thought that
equidimensional floes were probably present in all
samples, but had been preferentially preserved in-between
the grain-grain contacts of the muddy sand facies
143
(Torresan, pers, comm,, 1988), The morphology of these
floes is very similar to the bioflocs of this study.
The composition of bioflocs is one important clue to
their origin. In this study, they usually contained
abundant organic material, mostly organism tests, in
addition to detrital matter. The presence of organism
tests suggests that these bioflocs were not agglomerated
purely as a result of physico-chemical flocculation, since
tests do not have the unusual electrostatic character of
clays. Oppositely, all of the p-chem floes identified in
this study contained only clays. The dominance of detrital
material in some fecal pellets has been noted by other
workers (Harding, 1974 ; Poulet , 1976 ; Turner, 1977 ; Honj o,
1980; and Syvitski and Lewis, 1980) and the relative
abindance of mineral matter is mostly a function of its
concentration in the water column (Poulet, 1976).
The morphology of these floes is another important
clue, Zabawa (1978) differentiated pellets from
electrochemical floes on the basis of shape and internal
packing. Cylindrical and spherical shapes are typical of
previously-identified pellets (Zabawa, 1978; Dunbar and
Berger, 1981), Pellets also tend to be more densely packed
and contain fewer internal voids as compared to electro
chemical floes (Zabawa, 1978), Internal particle contacts
are also different; electro-chemical floes have abundant
internal voids as a result of dominantly EF and EE clay
1 44
particle interactions (Zabawa, 1978).
Some of the very small, loosely packed bioflocs may
not be fecal pellets, but instead may be held together by
either bacterial processes or organic films. These are
particularly in evidence within the sediment trap
material. Several studies have shown that bacterial webs
are capable of agglomerating detritus (Paerl 1973, 1974).
The mode of occurrence of bioflocs supports the
interpretation of fecal pellets. They have only been found
in bioturbated sediments and in pelagic sediments outside
turbidity current deposits. These were the two
environments in this study where one could expect to see
an abundance of fecal pellets.
Origin and Significance of P-chem Floes
Several other studies of natural sediments have
described structures which are similar to the p-chem floes
of this study. Collins and McGown (1974) found very low-
angle EF structures in marine and estuarine sediments, and
FF, EF, EE fabrics in marine, estuarine, and lacustrine
deposits. All of these fabrics contained more FF domains
than the p-chem floes of this study. These domains could
be an artifact of air-drying. Nothing resembling p-chem
floes was found in aeolian deposits. Water salinity seemed
to have no effect on the fabric of these floes.
Estuary sediments from the St. Lawrence River,
however, contained only domains in FF, low-angle EF clay
1 45
fabrics (Delage and LeFebvre, 1984).
Bennett et al. (1981) found large EF floes in deep-
sea red clay deposits. These floes were well-defined, but
it is difficult to determine from their TEM images if they
are p-chem or bioflocs. In the Mississippi Delta, they
found sediments containing dominantly domains with low to
high angle EF contacts.
O'Brien (1980) found EF continuous1y-flocculated
clays in turbidite sediments. This fabric was very similar
to that of turbidites in this study.
Hein (1985) shows micrographs of sediment containing
p-chem floes, but does not show the continuously-
flocculated fabric of turbidites as shown in this study.
Torresan and Schwab (1987) found abundant p-chem
floes in their samples. These had fabrics ranging from
high to low angle EF contacts, and occurred within all of
their textural categories. However, specific depositional
processes of this sediment was not determined.
It appears that these floes are formed in water-borne
sediment as a result of electrostatic attraction between
clays. EF particle contacts dominate because of
negatively-charged faces and positively-charged edges, low
to high angles of contact are possible and are probably
dependent on overburden pressure.
One variable appears to be the amount of domains vs.
single particles. This could be partly controlled by water
1 46
salinity (van Olphen, 1963). This could explain the greter
number of domains within p-chem floes in the estuarine
sediments studied by Delage and Lefebvre, 1984). Another
control may be clay mineralogy. Torresan (pers. comm.
1988) felt that his clay-size particles did not
agglomerate in domains because many of them were non-clay
minerals ground to clay size by glacial action. Also,
certain clay minerals may simply form domains more readily
than others. The concentration of clays in suspension may
also control domain formation, since higher concentrations
cause more particle collisions and more potential
agglomeration. This could explain the dominance of domains
in Mississippi Delta sediments where detrital input is
very high (Bennett et al., 1981).
The Effect of Sediment Type on Microfabric
Sediment grain size is a very important determinant
of fabric, particularly in consolidated sediments. If
coarse silt is abundant in a sample, these grains will
largely determine the fabric of surrounding clays. Since
silts are incompressible relative to the clay matrix,
during consolidation silts reorient and sink into the
matrix, squeezing the clays around them. This causes
dramatic changes in clay fabric during consolidation and
helps to obliterate the differences in clay fabric which
are seen in unconsolidated sediments.
The effect of clay mineralogy was not specifically
1 47
analyzed in this study, except to determine that differing
clay mineralogy was not the cause of the different clay
fabrics defined herein- However, clay mineralogy probably
does affect flocculation styles; thus, p-chem flocculation
in other geographic areas may have different
characteristics if clay mineralogy is different.
The presence, and occasional dominance, of organism
tests does affect the visual look of the microfabric, but
does not have a strong effect on the type of agglomeration
of clays. However, the dissolution of these tests during
diagenesis will yield abundant ions which could play an
important role in the diagenesis of clays. The effect of
organic carbon content on clay microfabric was not fully
described by this study. This failure was due to the
difficulty in identifying non-shell organic material in
the SEM. The mode of occurrence of organics is not well
understood; it may oe present as organic molecules
adsorbed onto clay surfaces, or as discrete flakes. In
either case, it would be nearly impossible to
differentiate from clays. The presence of organic carbon
material could have a significant impact on fabric changes
during consolidation, as discussed later.
The Effect of Depositional Process on Microfabric
Depositional process plays an important role in
determining silt microfabric. In turbidite sediments,
deposition from a waning current results in horizontal
1 48
orientation of platy silts and micas; quiet settling from
suspension results in an initially random orientation of
these particles* No difference in clay fabric as a result
of these slight differences in depositional process was
noted* In pelagic sedimentation, unusual sedimentation
events result in small-scale laminae of detrital or
organic material*
Since pelagic deposition was found to be largely a
biological process of filtering and pelletization,
followed by downward drift of pellets, the only purely
physical depositional process studied here was gravity-
driven sediment flows* These ranged from medium to low-
density fine-grained turbidity currents* No difference in
fabric was found, suggesting that the two end-members of
this process were operating under the same basic
mechanism, and that the concentration of clays in
suspension, at least within this range, did not affect the
amount of domains formed* Higher-density flows were not
studied, and these may cause more domain formation.
It is emphasized that no preferred orientation of
clays occurred in any type of recent sediments studied;
thus questioning O'Brien's (1987) contention that
preferred orientation of clays in shales is a result of
original depositional fabric.
1 49
The Effect of Organisms on Microfabric
The dominant effect of organisms on silt microfabric
is disruption of orginal depositional fabric and creation
of textural mottling. Occasionally, distinct
ichnostructures (Chondrites and Zoophycus) have been
observed in minimally-bioturbated sediments. However,
complete reworking tends to disrupt fabric completely and
individual traces cannot be detected. The mottling
typically results from segregation of silt and clay, which
probably arises from grain size selection by organisms.
Also, bioturbation randomizes the preferential orientation
caused by turbidity current deposition. Reorientation of
micas during consolidation occurs to a lesser degree than
in adjacent turbidites; perhaps this is a result of a pre
compacted orientation inside pellets.
Clay microfabric is dramatically changed as a result
of ingestion and defecation, by either pelagic or benthic
organisms. Individual bioflocs, or fecal pellets, are
formed, usually with distinctive external morphologies.
Internally, void space is moderate to low. Cylindrical
pellets are generally more densely packed than spherical
pellets. Smaller floes are sometimes present within the
more loosely packed pellets. Domains are abundant within
pellets; EF particle interactions are less abundant.
Exteriors of pellets sometimes exhibit clay cloaking in an
"onionskin" fashion.
1 50
Locomotion of organisms through the sediment also
changes the clay fabric in a variety of styles. Burrowing
by some organisms, probably non-detritivores, creates
large, open porosity. Other organisms eat their way
through the sediment, defecating and pushing sediment
behind them. This results in a densely compacted burrow
fill. Other organisms may occupy a burrow for an extended
time; movement of the body against the burrow wall may
create a sort of slickensides, where a surface of FF
amalgamated clays is produced.
Bacterial activity may also play a role in
determining clay fabric, although their influence is
difficult to define. Some of the small floes in the
sediment trap material may have been held together by
bacterial webs (Paerl, 1973, 1974) but it was difficult to
identify specific microbes within the SE images. Bacteria
also play a role in the early destruction of bioflocs,
since they attack the organic coatings which bind the
floes together.
The Effect of Consolidation on Microfabric
The dominant effect of consolidation on silt
microfabric is to densify the fabric, bringing silt
particles more closely together. It also tends to rotate
micas and other platy silts towards a horizontal direction
of PO. This process of reorientation is more effective in
unbioturbated, as compared to bioturbated, muds.
1 51
Consolidation also increases the preferred
orientation of refractory organic material. In the shallow
Los Angeles Basin samples, refractory organic material had
an equant shape; at depth, these are linear, with long
axes in the horizontal direction.
Consolidation affects the clay microfabric of
turbidite sediments by changing high-angle EF contacts to
low-angle EF contacts to FF contacts. After deep burial,
most clays are oriented in the sub-horizontal direction,
even if preferred orientation is only poorly developed.
This causes a dominance of clay edges, rather than faces,
to be visible in vertical sections. The degree and
direction of PO of clays is dependent on their proximity
and relation to local silts. These silts, with increasing
burial depth, tend to squeeze clays around themselves.
Only in a clay-rich zone could preferred orientation
become well-developed.
Consolidation initially affects bioturbated sediments
through reduction of large pores. This process brings
bioflocs closer together and makes it difficult to
distinguish the boundaries between individual floes. As
consolidation progresses, floes are squeezed together and
between silts such that individual packets can only be
subtly discerned. Since clays have been pre-consolidated
(within the intestinal tract) in random orientation,
overburden pressure is not as effective in rotating them
1 52
towards horizontal. The large proportion of clay faces,
relative to edges, seen in vertical section, is evidence
of the large number of clays having a vertical orientation
and hence a very random structure.
Consolidation of pelagic sediments containing
abundant bioflocs is generally the same as for bioturbated
sediments, with the exception that very large pores are
not present in pelagic sediments. Pelagic sediments
deposited by dilute gravity flows experience changes
during consolidation which are more similar to those in
turbidite muds.
The effect of sedimentation rate on the rate of
consolidation, and on resultant differences in fabric, was
not adequately defined by this study. Secondary
consolidation caused by the presence of organic material
may enhance the development of preferred orientation in
clays, as discussed later.
The Effect of Diagenesis on Microfabric
The first evidence for diagenesis was the
neoformation of nontronite (?) in the anoxic sediments of
Santa Monica Basin. Other than this, no morphological
changes in detrital clays were seen in the recent muds.
In the mudstones, diagenesis had the effect of
creating massive to crenulate intergrowths of clay. The
exact mechanism of diagenesis— either dissolution of
detrital clays followed by growth of new clays from
1 53
solution, or the in situ crystallographic transformation
of clays, was not determined. The latter mechanism is
preferred, because morphological changes occurred
gradually, affecting all clays similarly. Deteriorating
clays, or small growths of new authigenic clays, were not
detected •
Diagenesis also had the effect of dissolving organic
tests, particularly in pelagic sediments where they formed
a significant fraction. Growth of pyrite crystals was
ubiquitous in all sediments. Some transformation of
feldspar to chlorite was noted in the Ventura Basin,
Diagenetic changes should also affect the organic
material within these sediments, but such changes were not
detected due to the difficulty in identifying organic
material in the SEM, Changes in organic material,
especially hydrocarbon generation and expulsion, could in
turn have a significant effect on clay fabric. This could
be a fruitful area of study.
Origin of Preferred Orientation
Previous workers have suggested that preferred
orientation results from pelagic settling of clays
(O'Brien et al., 1980), from the lack of bioturbation
(Byers, 1974), through mechanical consolidation (Martin,
1965; Meade, 1966; Bowles et al,, 1969), or through the
growth of phyllosilicates under pressure (Siever, 1966),
No preferred orientation of clays was found in any
1 54
samples of recent sediment (unbioturbated pelagic and
turbiditic, or bioturbated), so preferred orientation
cannot be a result of depositional process or the lack of
bioturbation alone. Consolidation alone also does not
produce the perfect parallelism seen in some shales. The
best preferred orientation observed was in samples which
had undergone strong diagenesis, suggesting that preferred
orientation is developed during diagenesis. Thus, we must
look to factors within the environment of diagenesis to
determine why some diagenetic mudstones have preferred
orientation and others do not.
The salient factors are: original clay fabric; grain
size, clay mineralogy; organic content; pore water
chemistry and its interaction with solid-phase chemistry;
and overburden pressure. As this and other studies have
shown, a high silt-content mudstone will not develop
preferred orientation (Folk, 1962; Gipson, 1966; Curtis et
al . , 1980). Also, the very finest clays (von Engelhardt
and Gaida, 1963) such as montmorillonite (Beall, 1964) do
not develop pronounced preferred orientation. Illite is
the mineral which shows the best development of preferred
orientation (Gipson, 1966). Laboratory sedimentation of
clays has shown that dispersed kaolinite only develops
moderate preferred orientation after consolidation;
dispersed illite develops nearly perfect preferred
orientation after consolidation; and montmorillonite is
1 55
almost impossible to disperse (Sides and Barden, 1971).
High organic carbon content can lead to increased
preferred orientation (Ingram, 1953; Odom, 1967; and
Byers, 1974). The effect of chemistry and chemical changes
during diagenesis, and their effect on preferred
orientation, has not been studied in detail.
These factors are probably sufficient to explain why
diagenesis causes preferred orientation in some mudstones
and not in others, without having to invoke factors in the
depositional environment. For example, Byers (1974) used
the lack of bioturbation to explain preferred orientation
in black shales, and random orientation in mudstones. But
the black shales that he studied had much higher organic
carbon and less silt compared to the mudstone. These
factors, in the environment of diagenesis, could have
affected the development of preferred orientation. O'Brien
(1970) found better preferred orientation of clays in
black illitic shales as compared to organic-poor
kaolinitic and illitic shales. Although he recognized that
the amount of organic carbon was probably the most
important factor, he suggested that the labile organics
led to a deposition of clays in the dispersed condition
and therefore the original muds had a preferential
orientation of clays. However, the refractory organic
carbon probably played a more important role in the
environment of consolidation and diagenesis, because this
1 56
more compressive material would allow for greater
realignment of clay particles. Clay mineralogy may have
also played a role since illites develop preferred
orientation better than kaolinite.
1 57
CONCLUSIONS
The results of this research have, hopefully, been
presented in a straightforward manner; however, they were
certainly not achieved that way. At least one cumulative
year was wasted chasing down blind alleys, and doing
things backwards. Therefore, it may be useful to discuss
some of the reasons for the eventual (fair) success of
this study, in order to ease the way for future students
of microfabric.
The most frustrating problem in microfabric is not
being able to discern distinctive differences in
microfabric. This problem mostly results from an improper
scale of observation. Most microfabrics are
"characteristic" over only a small range in
magnifications; at magnifications above or below that
"characteristic" range, several different microfabrics may
appear to be very similar. Thus, samples must be observed
over a wide variety of scales. It is most beneficial to
begin with the largest scale of observation, and work
towards the higher-magnifications . So one should begin by
visual examination of the sample, followed by x-
radiography to enhance macrofabric features. Next, thin
sections should be examined; these sections may then be
imaged in the BSE mode for enhanced resolution of silt
microfabric. Finally, SE-SEM, at ranges of 1000-5000X,
should be used to image clay microfabric. The importance
1 58
of stereo micrography in examining these small-scale
fabrics cannot be overemphasized.
The type of samples chosen for microfabric analysis
is also of critical importance- Firstly, the environment
of deposition should be well-understood. Once distinctive
microfabrics are observed, one is at a loss to explain
their significance if small-scale depositional processes
are not known. Secondly, it is most beneficial if studies
begin with freshly-deposited sediment, because fabrics are
most distinctive, and correlation with specific processes
is made more easily, in muds compared to mudstones. Then
one should work towards deeper burial depths, preferably
within one basin, but at least within closely analogous
depositional environments. Many microfabric studies have
examined only ancient sediments; but the correlation
between their microfabrics and processes in the
environment of deposition is tenuous at best.
Admittedly, microfabric studies tend to be tedious
and oftentimes frustrating. However, when they are
successful, they can yield a wealth of information. For
example, the information gained in this study could be
useful in many areas of research. For sedimentology, new
concepts of pelagic sedimentation have been explored; in
particular the enigma of very thin turbidites— the origin
and mechanics of such flows should be further studied. For
stratigraphy, new tools for paleoenvironmental
1 59
reconstruction have been developed. For petroleum geology,
the porosity characteristics of and structure of recent
analogues to hydrocarbon source rocks should help in
understanding the generation and expulsion of hydrocarbons
from such rocks. For geochemistry, new effects of early
and late diagenesis have been discovered. For metamorphic
petrology, the early development of shale cleavage has
been outlined. For micropaleontology, the processes
affecting organismal tests after deposition will aid in
understanding differential preservation. For civil
enginering, the structures outlined in this study may be
used to explain soil behavior. For biology, some aspects
of the pelagic and benthic food chain may be illuminated
through examination of fecal pellets and bioflocs. And
finally, for anyone working in the recent deep-sea
environment, there is finally a picture of what their
research area really looks like.
While this study has answered a few very basic
questions, its main benefit has been to open up a field of
questioning which will require the combined efforts of
many scientists to resolve. A few of the more pressing
problems are: the effects of other depositional processes
(besides pelagic, turbiditic, and bioturbation) on
microfabric; the effect of other factors in the
environment of deposition on microfabric (water chemistry,
sedimentation rates, pore fluid chemistry, etc...); fabric
1 60
changes during shallow burial of 1-500 m; the nature of
organic material and its effect on fabric; and the origin
of preferred orientation in shales.
In 1980, Potter, Maynard, and Pryor stated that "the
present attitude of many sedimentologists towards shales
is well exemplified by their typical representation of
shales in vertical profiles as solid black— a
structureless, uninteresting matrix." In 1988, this study
has shown that muds have a fascinating variety of
structures which can be catalogued and which do yield
important information about the environment of deposition.
1 61
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Asset Metadata

Creator Reynolds, Suzanne (author)

Core Title The fabrics of deep-sea detrital muds and mudstones: A scanning electron microscope study

Contributor Digitized by ProQuest (provenance)

Degree Doctor of Philosophy

Degree Program Geological Sciences

Publisher University of Southern California (original), University of Southern California. Libraries (digital)

Tag Marine Geology,OAI-PMH Harvest

Language English

Permanent Link (DOI) https://doi.org/10.25549/usctheses-c29-351544

Unique identifier UC11218898

Identifier DP28581.pdf (filename),usctheses-c29-351544 (legacy record id)

Legacy Identifier DP28581.pdf

Dmrecord 351544

Document Type Dissertation

Rights Reynolds, Suzanne

Type texts

Source University of Southern California (contributing entity), University of Southern California Dissertations and Theses (collection)

Access Conditions The author retains rights to his/her dissertation, thesis or other graduate work according to U.S. copyright law. Electronic access is being provided by the USC Libraries in agreement with the au...

Repository Name University of Southern California Digital Library

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