Geological Society, London, Special Publications
A review of fine-grained sediment origins, characteristics,
transport and deposition
D. S. Gorsline
Geological Society, London, Special Publications 1984; v. 15; p. 17-34
doi:10.1144/GSL.SP.1984.015.01.02
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A review of fine-grained sediment origins, characteristics,
transport and deposition
D.S. Gorsline
SUMMARY: Fine-grained sediments and sedimentary rocks make up as much as 75% of the
present and past sedimentary records. River discharge is the largest single source of
fine-grained material, followed by biological, volcanic and aeolian sources. The composition,
texture and bulk properties that characterize different sediment facies are controlled primarily
by climate, tectonics, sediment supply, oceanic dispersal systems and biological activity.
Fine sediments are transported into deep water by low-concentration nepheloid plumes,
turbidity currents of a range of concentrations, flows and mass-movements. Mass-movement
may be the major process delivering fine-grained sediment to the deep-sea floor over long time
periods. Abyssal plains, the end of the passive margin transport system, are more affected by
turbidity currents. Much bottom current reworking of sediments occurs at all depths in the
ocean with results that may be very subtle. In shallower waters, balances between supply and
dispersal may allow for the accumulation of mud belts on shelves and much fine sediments
passes through such systems. Organic matter concentrations in fine sediment may be more a
function of accumulation rates of organic versus terrigenous matter than local biological
productivity.
Fine-grained sediments require the application of methodology and results of a broad range
of scientific disciplines. Effective understanding of the fine record will require interaction
between several sciences.
The theme of this symposium is fine-grained
sedimentation in deep water. Viewed from global
sedimentation perspectives, the oceans are the
ultimate sink for world sedimentation. Although
continental fine sedimentary environments are
not discussed here, some papers on these topics
have been included in a complementary symposium edited by Reinhard Hesse (Sedimentary
Geology, in press).
Effective understanding of the oceanic fine
sedimentation system requires cooperative
studies by geologists, biologists, chemists and
oceanographers. Therefore, the introductory discussion will, in part, be aimed at non-geologists.
On the other hand, since many geologists are not
familiar with oceanography, I will include some
brief discussion of ocean circulation features that
affect the distribution and deposition of fine
sediments.
This paper complements the paper in this
volume by Stow & Piper which discusses deepwater fine-grained sedimentary facies. I will
briefly touch upon facies and some special
sedimentary features, but primarily from an
oceanographer's view of the environmental
conditions and characteristics rather than specific discussion of sedimentary structures and textures.
Readers interested in the oceanic eddies and
rings noted later should see the symposium
volume by Robinson (1983).
Fine sediment mass, source and
characteristics
Mass and Source
The stratigraphic importance of fine-grained sediments is evident from conservative estimates of the
proportion of silts and clays and their lithified
equivalents in the geologic record. Recent authors
(Potter et al. 1980; Blatt 1982) estimate that
70-75% of sediments and sedimentary rocks are
fine particulate aggregates, including fine biogenic
sediments and rocks. Years ago, Clarke (1924)
noted that average shales contain one-third clay
minerals, one-third quartz and one-third
organics, carbonates and other minor minerals.
The primary source of fine deposits is river
discharge; and, secondarily, volcanic explosive
eruptions, aeolian transport and biological production. There are other interesting sources such
as chemical precipitates and meteoritic debris
which contribute very minor amounts of material
to the overall budget. Recent estimates for the
bulk contribution of the major sources in present
times give river discharge as 1.5-2 • 101~tons yr - l
(Garrels & Mackenzie 1971; Milliman & Meade
1983); aeolian transport, exclusive of volcanic
ejecta, as approximately 107 tons yr-1 (based on
data from Prospero & Bonatti 1969; Windom
1969; Pewe 1981; Prospero 1981); the contribution of explosive volcaniclastics as approximately
17
I8
D.S. Gorsline
107 tons yr -1 (estimated from eruption sizes,
Huang et al. 1979; Smith 1979); and fine-grained
biogenic sediment sources of the order of 109 tons
yr-1, based on stratigraphic estimates and ocean
carbonate budgets (Broecker 1974; Blatt 1982).
River discharge
The bulk of the annual river discharge comes
from the world's 20 largest rivers, with the three
largest Asian rivers contributing almost onequarter (Milliman & Meade 1983). Because of the
limited number of large rivers, areas of major
accumulation in the oceans are relatively few in
number. For example, the few giant submarine
fans are mostly tributary to the largest Asian
rivers (Curray & Moore 1971).
River load is primarily a function of drainage
area for given climatic regions and river drainage
areas are, to a first order, a function of tectonics.
Inman & Nordstrom (1971) have shown that the
major rivers drain into trailing edge or marginal
seas due to the displacement of continental
divides towards the collision edge of continental
plates.
As Strakhov (1970) has discussed, the amounts
of sediment discharged can be related on a global
scale to latitude which, in turn, controls the
factors of temperature, rainfall, vegetation and
evapo-transpiration. Undoubtedly some of these
relationships were different for pre-Middle
Palaeozoic time, but since most of the sedimentary record is younger than this (Garrels &
Mackenzie 1971) we need not consider a nonvegetated world.
Mid-latitude streams, or streams draining high
plateaus in the case of some of the large Asian
rivers, deliver the largest amounts of sediment per
discharge area (Milliman & Meade 1983). The
major rivers deliver very large loads of relatively
fine grain-size. As noted earlier, these tend to
debouch from the trailing edge of the cratons.
Gibbs (1967) has shown the importance of the
highlands in the headwaters of big rivers in
determining the characteristics of the river particulate load.
Undoubtedly the variation in land-sea surface
area with time has produced matching variations
in river load delivered. It is also likely that there is
a minimum land area necessary to provide significant sediment discharge.
Volcaniclastic contribution
Volcanic explosive eruptions tend to be associated with the andesitic and dacitic volcanoes of
active margins and plate convergences and with
large silicic eruptions derived from major batho-
liths (Smith 1979). Rate of plate subduction may
also be a factor. Variable proportions of the
erupted material are ejected as local low altitude
air-fall and/or as high altitude injection into the
stratosphere or high troposphere (Sheridan
1979). The deposits resulting from these events
are strongly influenced by the local winds and
upper atmosphere circulation, respectively.
Although initially from point sources, the high
atmospheric injections are mixed and dispersed
so that the resulting air-fall deposits are globally
distributed. Thus the general distribution of this
fraction is essentially the same as the global
distribution of dust from other sources (Windom
1969).
The ash-falls of this large scale and their
depositional record form an intriguing source of
data regarding the periodicity of major volcanic
events. Many authors have considered the periodicity of volcanism in the oceans and globally
(e.g. Stille 1924; Umbgrove 1947; Huang et al.
1975; Ninkovitch & Donn 1975; Vogt 1979;
Kennett 1981 ).
In deep ocean distal pelagic sediments, the
lithic contribution is probably almost entirely
from wind-borne sources and oceanic volcanic
input. This was probably also true in the past (e.g.
Lindstrom 1974, for Ordovician pelagites).
Aeolian contribution
Non-volcanic aeolian transport to the oceans
tends to be from arid lands or from large outwash
systems. The geographic arrangement of these
types of areas and the major wind-belts determines whether material is transported to the deep
ocean. Windom (1969) examined the dust falls
deposited in ice of the polar caps and large
glaciers and compared these values with estimates
of the wind-borne contribution in lower latitude
deep-sea sediments as determined by applying the
composition data derived from the ice cap deposits to the components observed in the oceanic
accumulations. He noted that the influx is
affected by latitude, distance from source and
climatic factors. (See also Rex & Goldberg 1958;
Rex et al. 1969.)
Loess is a major continental fine deposit in
periglacial areas and downwind from arid
regions. These deposits are eroded by the major
stream systems draining the continental interiors.
In the present mass budget, a large portion of the
silt fraction may come from erosion of these
glacial deposits.
Analysis of the influence of arid regions
upwind of ocean areas has been summarized by
Pewe ( 1981), Prospero (1981) and earlier workers
(see Rapp 1974). Windom (1969) states that
A review o f f i n e - g r a i n e d sediment
aeolian input contributes between 10 and 75~o of
the nonbiogenic fraction of deep ocean sediments
depending on distance from source, latitude,
dilution and effectiveness of the tracers used.
Heath et al. (1973) have shown the shift in the
pattern of wind-borne quartz in deposits in
subtropical latitudes in the eastern Pacific over
late Pleistocene time. On a shorter time-scale, the
dust plumes from the Sahara into the Atlantic
shift in latitude with the seasonal shift in windbelts. Prospero (1981) has noted the generally low
windborne concentrations in the distant ocean
areas outside the arid belts.
Biogenic contributions
Biogenic sediments are a world unto themselves.
Many pelagic tests, capsules, plates and spines are
silt size and smaller, and so primary biological
productivity is an important fine particulate
source. Bioerosion is an important source of fine
lime particulates (see Futterer 1974; Hein & Risk
1975).
Accumulation ofbiogenic sediments represents
an intricate balance between production and
solution (Berger 1974). Variations in biogenic
content and in the stratigraphic appearance of
biogenic-influenced sedimentation are a function
of dilution of biogenic production by terrigenous
influx. Ginsburg & James (1974) have discussed
the importance of biogenic contribution to ocean
margins other than the tropics, and note that
although diluted by terrigenous influx, the middle
and higher latitude contribution represents a
major addition to the tropical biogenic source
that is most generally used as the basis for
biogenic budgets.
Biological production of fine particulates is, of
course, related to areas of high productivity. The
older view was that these were essentially in areas
of major upwelling along eastern ocean boundaries, the eastern equatorial system and major
ocean water mass convergences as in the Circumpolar Antarctic Convergence.
More recent work has shown that western
boundary currents can generate zones of quite
high productivity (Dunstan & Atkinson 1976;
Janowitz & Pietrafesa 1980; Blanton et al. 1981;)
and that the central gyres of the surface ocean
circulation are also much more productive than
was thought a few years ago (Shulenberg & Reid
1981; Jenkins 1982; Martinez et al. 1983).
Parrish (1983) has introduced an interesting
but preliminary multicomponent model in which
the spectrum of organic-rich sediments and sedimentary rocks, cherts, chalks, phosphate rocks
and glauconite are related to a range of oceanographic factors and mass sediment budgets from
19
land. Certainly, as noted earlier, the older simple
view of geographically restricted ocean upwelling
cannot be simply applied to palaeogeographic
reconstructions. Organic content alone is not
simply a function of original organic production
but must include consideration of lithic dilution.
Composition and bulk properties
Clays
Clay is defined texturally as all material finer than
4 /~m (Udden 1914; Wentworth 1922). In this
discussion, clay will be used in its mineralogical
sense with a grain-size usually less than 10 #m and
typically less than 2 #m (referring to discrete
grains and not aggregates).
Mineralogically, clay is composed of a family
of silicate minerals with sheet structures similar to
the phyllosilicate micas. The clays represent the
stable product of the weathering of feldspars and
micas at earth surface conditions. Clays possess
the property of ion exchange to a greater or lesser
extent depending on the clay species and this
property reflects the unsatisfied bonds on and
within the sheet structures. This property is an
important one in that it provides a mechanism for
transfer of organics and metal ions and also is the
primary factor in the characteristic aggregation
of clay grains in ocean environments (Gibbs 1981;
Gibbs & Heitzel 1982). Due to the charges on
their surfaces, clays have cohesion, which is a
basic factor influencing the strength of fine
sediments of high clay content.
Clay mineralogy reflects climate and relief, and
secondarily lithology. Several authors (e.g.
Loughnan 1969) have shown that a specific
feldspathic rock can deliver a wide variety of clay
species depending on such factors as temperature,
rate of soil drainage, and chemical environment.
For large areas with heterogeneous lithologies,
climate is the main control. Storage time increases
with increasing drainage area and this also influences composition and texture. Changes of
global climate therefore exert a strong influence
on fine sediment characteristics; the proportion of
silt to clay and of silt +clay to coarser grain sizes.
As has been shown by clay mineralogists
(Loughnan 1969) and by large scale oceanic
studies (Biscaye 1965; Griffin & Goldberg 1968)
there is a primary latitudinal variation in
dominant clay species that reflects climate. In
general, tropical clays tend to be laterites or
kaolinites; mid-latitudes produce montmorillonites and illites; and polar latitudes produce
illites and chlorites.
20
D.S. Gorsline
Silts
Bulk properties
Texturally, silts range between 4 and 63 /~m in
grain-size, although both silt and sand are part of
a continuous spectrum of particle sizes. Silt is a
particularly diagnostic grain-size class for studies
of deposition and reworking by bottom currents
and of general directions of fine sediment transport (e.g. McCave 1982).
Silts are typically heterogeneous mixtures of
detrital primary minerals of which quartz is most
common, and feldspars and ferromagnesian
minerals are common accessories. Silt origin has
been attributed to glacial grinding (see Smalley
1966; Kuenen 1969), volcaniclastic eruptions
(Smith 1979) and weathering processes in soils in
tropical and mid-latitude regions (Pye & Sperling
1983).
Many pelagic biologic tests, capsules, spines
and plates are silt size, and so primary biological
production produces much silt as well as clay size
particulates (Nahon & Trompette 1982). Silts,
because of the small particle size, have large grain
surface areas and so weathering will probably
rapidly remove metastable or unstable minerals
leaving quartz as the dominant mineral. If, as has
been suggested by Pye & Sperling (1983), much
silt is formed by chemical fracturing of strained
quartz in the soil profile, then silt initially begins
with a high quartz content.
Hydrodynamically, silts can be defined as
particles that are moved primarily in suspension,
while sands are moved in traction or saltation.
The boundary is a broad one but probably lies
somewhere between 30 and 100/~m (Inman 1949).
In most rivers, fine sand is moved as suspension
load at times of flood (Brownlie & Taylor 1981).
In the ocean, the levels of turbulence and shear
stress are lower and particles larger than about 30
~m generally move as bedload. Under strong
wave surge in coastal and inner shelf regions,
sands are rarely suspended more than a few tens
of centimetres above the bottom (Cook & Gorsline 1972). Wildharber (1966) found that the
typical suspension load a few kilometres off
southern California was already limited to particles with a coarsest diameter of 30/~m, and most
were below 20 Ftm.
Since fine sediments of silt size move in suspension, they will tend to preserve their original grain
shape and this may be a means of pinpointing
source in a given sedimentary system. Ehrlich and
his associates (e.g. Ehrlich & Weinberg 1970;
Ehrlich et al. 1980) have pioneered use of Fourier
statistical methods to analyse ~grain shapes as a
means of making source determinations. Silt
grain shape analysis is a promising area for
research.
There are three important bulk properties of
fine-grained sediments: (a) the sediment strength,
which is a function of the cohesive properties of
clay particles, internal grain friction and grain
packing; (b) the bulk density, which is a function
of water content, sediment composition and void
ratio; and (c) sediment fabric, or the stacking
arrangement of clay particles and aggregates.
The behaviour of fine-grained sediments on
slopes depends on these three bulk properties, in
addition to the slope angle and sediment load.
The water content (or bulk density) appears
particularly important (Field 1981b; Almagor
1982; Bennett & Nelson 1983). Thus cohesive
sediments with high water contents (low bulk
densities) may fail even on low gradients (Lewis
1971; Coleman 1976; Coleman & Garrison 1977);
and cohesive sediments with low water content
(high bulk density) may be stable on relatively
high gradient slopes. To a first approximation,
for a given textural type, the water content is
directly related to rate of accumulation, thus it is
evident that rapidly deposited sediments on
slopes may be metastable or unstable and subject
to mass failure.
On a smaller scale, several workers have examined the microstructure of clay fabrics, and
related these both to the depositional mode and
to bulk properties (Bennett et al. 1981; Hein, in
press). Bennett and his co-workers have examined the arrangement of clay flakes, domains
(stacks of clay flakes) and other aggregate forms
(pellets) in sediments of various bulk densities. As
a rule, the lower density sediments have more
open packing with larger voids; flakes and
domains are arranged in loose chains. In denser
sediments, the structures are flatter and more
collapsed and voids are flat and lenticular. Hein
and her associates are presently examining the
three dimensional packing of fine grains in sediments from a variety of environments in the
California Borderland. Preliminary work has
shown that the bulk properties of the various
facies and depositional environments are different. It will be interesting to see if the flake fabrics
show the influence of creep or mass failure of
more advanced stages of movement. O'Brien et
al. (1980) have examined microfabric in older
sediments and sedimentary rocks and see systematic arrangements even in quite compact and
lithified sedimentary deposits.
Cycles in fine sedimentation
Hemipelagic deposits may preserve almost complete records of depositional history because they
A review of fine-grained sediment
usually (not exclusively) accumulate in areas of
low energy. They can therefore record the results
of cyclic forces driving deposition.
These cyclic periods range from seconds to
millions of years (e.g. Fischer & Arthur 1977;
Hesse & Chough 1980). Common cycles include
seasonal variations, larger term climatic variations ranging from decades to tens of thousands
of years, tectonic cycles which can overlap the
longer period climatic cycles and pass on to
hundreds of millions of years. On a regional basis,
local uplifts or depressions, stream capture and
channel avulsion are examples of other quasicyclic driving forces. Cycles with periods of 103
yrs and more are usually preserved in bioturbated
sediments, but may require careful analysis for
their recognition.
Resolution of cyclic sedimentation is limited by
accumulation rates and bioturbation; records of
periods of up to a thousand years or so (depending on accumulation rates) are usually destroyed
by bioturbation except where anoxic bottom
waters exclude benthos (e.g. Emery & Hulsemann
1962; Soutar & Crill 1977; Malouta et al. 1982;
Savrda et al. 1983). Bed thickness is another
factor since bioturbation is often most intense
only in the top 5-10 cm (Nittrouer & Sternberg
1981) of the accumulating fine sediment, and
thick rapidly deposited units may never be entirely bioturbated even in well aerated benthic
environments (Savrda et al. 1983). Variations in
oxygen content of bottom waters can be rapid
events geologically and where accumulation rates
are slow, the anoxic accumulations may be
bioturbated during the following period of oxygenation. Benthic burrowing activity can be
surprisingly intense even at oxygen levels of less
than 0.5 ml/L (Savrda et al. 1983).
Dean & Arthur (in press) and Arthur et al. (this
volume) have described interactions between
cycles in the Cretaceous black shales of the deep
Atlantic that influence the timing and magnitude
of reduction of the sediments and the degree of
bioturbation. The change from dysaerobic to
anaerobic conditions is apparently triggered by
the shorter frequency climatic cycles at times
when the longer climatic or tectonic cycles bring
the environments close to anoxia. These features
represent cyclic periods of 10 4 yrs and more.
Human influence on sediment discharges
One major problem in the use of recent deposits
and mass budgets of contemporary rivers to
develop analogs for ancient deposits is the pervasive influence of man's activities on erosion
(Meade 1969, 1982). Most workers agree that this
effect has been to increase sediment discharges
21
from rivers as a result of overgrazing and other
agricultural practices, and the much more recent
discharge of wastes and dredge spoil on a large
scale (National Research Council 1976). Conversely, as in southern California, many rivers
have been truncated in drainage area by flood
control dams and spreading basins which trap
sediments (Brownlie & Taylor 1981). Estimates of
effects of human activities vary by an order of
magnitude, but work on the sedimentary accumulations in contemporary offshore southern
California basins suggests a two- to three-fold
increase in accumulation rates since late Pleistocene time (Nardin 1981; Schwalbach 1982) which
may reflect the initiation of human influences
superimposed upon the natural climatic and
sea-level changes which have also occurred during that time span.
Meade (1982) has noted that after such events
as intensive agricultural alteration of a drainage
area, the resulting sediment is stored within the
drainage system and released over time periods of
as much as centuries.
Bourrouilh (pers. comm. 1983) has noted that
the construction of the Aswan Dam on the Nile
has generated major problems over the Nile delta.
These effects include the influence on natural
fertilization of the agricultural lands by cutting
off the annual deposition of silts, and changes in
habitats of economically important fish species in
the areas off the mouth of the Nile.
Controls on fine sediment
deposition
Tectonics
The tectonic setting of the source area or depositional site exerts a first order control on finegrained sedimentation. It affects the rates of uplift
and denudation drainage patterns and volumes of
river discharge, coastal plain and shelf widths,
margin gradients, gross sediment budgets, the
morphology of receiving basins and local sealevel changes. The relative activity, or style and
frequency of seismicity and faulting, is also of
primary importance to sedimentation. It may
exceed and mask the effects of the other controls
or, in areas of low tectonic activity, play a
secondary role.
Supply
As is true for all sedimentary systems, the interaction of supply and dispersal energy can produce a
variety of fine sedimentary deposits, facies and
structures for given tectonic and sea-level condi-
22
D.S.
Gorsline
tions (Sloss 1962). As we have seen, the principle
fine sediment supply is from river contributions.
Initially this is deposited almost entirely in the
continental margin as a result of capture, filtering
and pelleting processes that have been reviewed.
Where this supply is constrained by climatic,
oceanographic or bathymetric barriers, the
secondary sources become dominant as for biogenie contributions of either carbonate or siliceous composition (Ginsburg & James 1974).
Oceanic dispersal systems
Dispersal is achieved by different dominant processes or agents as we progress from shelf to deep
ocean floor. Shelves lie within the mixed surface
layer of the oceans where strong wave, tide and
wind stress currents are active. Although part of
the ocean wave spectrum, internal waves should
be specifically mentioned here. These have their
largest development at density discontinuities in
the ocean water column. Cacchione & Southard
(1974) have discussed these wave forms and Swift
(1973) has discussed their influence on shelves.
Slopes receive energy from gravity, large-scale
eddy interaction with the substrate and slope
currents. Deep ocean floors are affected by tidal
currents and large-scale deep slow circulations
and possibly by transient stronger flows perhaps
associated with large-scale rings and eddies.
Where topography reduces the flow cross sections
as over gaps in mid-ocean ridges or over and
around seamounts, the tidally driven and large
water mass circulations are accelerated and can
produce subtle to strong substrate effects as noted
earlier. In all environments transient effects can
be important as in the passage of a turbidity
current, or a mass failure or storm effects in
shallower waters.
Sea-level
As has been noted by Rona (1973), Pitman (1978)
and Watts (1982), sea-level position relative to the
shelf edge is a major factor in the delivery of
continental sediments to the deep ocean. At high
sea-levels, decreased stream relief, broad shelves
and possibly slower atmospheric and ocean circulation rates (in the case of climatically driven
sea-level changes) tend to produce conditions of
trapping of sediments on the submerged craton
edge. When sea-level falls to or below the shelf
edge, delivery is almost entirely to the slopes and
the deep-sea floor. Sea-level can be both tectonically and climatically driven (Morner 1974;
Sclater et al. 1977) and the oscillations over time
will show significant spikes in power spectral
analysis that correspond to the two major driving
factors. The tectonic signal will be of the order of
106 yr and the climatic signal will be of the order
of 104-105 yr (the Milankovitch cycles).
The onset of late Tertiary continental glaciation and the resulting rapid sea-level changes, has
been a major factor in world sediment budgets.
Laine (1980) has shown that much of the sediment in the North Atlantic abyssal plains probably was delivered during the glacial epochs. This
would have required very high sediment discharges from periglacial streams. Emery &
Uchupi (1972) have noted the concentration of
submarine canyons along the edge of the northeast Atlantic margin of the US and Canada
matches the area and latitude of the Pleistocene
ice extent. This indicates that at low glacial
sea-levels, large quantities of glacial outwash
were dumped at the shelf edge and redeposited
downslope by turbidity currents and other massmovements.
Biological productivity
Biological production occurs over large areas of
ocean surface waters where nutrient recharging is
active (Eppley & Peterson 1979), therefore the
sedimentation patterns will reflect these areal
inputs rather than point or linear sources. This
differentiation or classification is of interest
because it will affect the concentration of particles
in the water column and thereby control the
accumulation rates on the underlying substrates.
Plumes and jets associated with areas of coastal
upwellings approximate point sources, and
regional coastal upwellings are typically linear
sources (Gorsline 1978). Large-scale eddies and
rings usually move over trajectories of considerable distance and thus their sedimentation effects
are integrated over large areas (Swallow 1976).
An exception may occur in the California Current
System where the seasonally developed large
eddies appear to remain essentially fixed in
position (Koblinsky et al. 1983; Simpson et al.
1983). If this pattern has been characteristic of the
past few thousand years of roughly stable sealevel position and ocean circulation, then there
should be some record of their existence preserved in the sediments. Such eddies can be areas
of relatively high productivity (Chelton et al.
1982; Haury 1983).
Biological pelletization
Planktonic organisms use several means to filter
particulates from the water column, aggregate
them and then produce large pellets that sink
rapidly to the sea floor (e.g. Osterberg et al. 1964;
McCave 1975). The processes include filtering,
A review of fine-grained sediment
digestion and excretion (e.g. Osterberg et al. 1964;
Frankenberg & Smith 1967); capture on films or
extended nets (Gilmer 1972; Bruland & Silver
1981; Silver & Bruland 1981); generation of
particulate organic matter from dissolution of
bubbles in sea-water (Johnson & Cooke 1980);
and large-scale pellets from fish feeding (Robison
& Bailey 1981).
Some small fraction of fine, dispersed grains do
get through to the deep open ocean, but it is
probably less than 5% of the total sediment
injected from the continents. This is supported by
the slow accumulation rate of deep pelagic nonbiogenic sediments. These contain mainly oceanic
volcanic material and aeolian material, so the
transfer of fine particulates through the coastal
and margin systems must be very small.
Porter & Robbins (1978) have recognized
preserved pellets in older shales and mudstones
and point out their close similarity to contemporary copepod faecal pellets.
Bioturbation
In the deep oceans of the present world, bioturbation is almost ubiquitous except in very restricted
basins where biological oxidation demands are
high or where sills intercept low oxygen water
from the Intermediate Water masses of the world
oceans (e.g. California Borderland, Cariaco
Trough). The degree of benthic reworking of deep
floor substrates is related to the oxygen content of
the bottom waters. Bioturbation is limited only
by the lowest oxygen content (less than 0.5 ml/L)
and some organisms (e.g. nematodes) can continue to live in near-anoxic conditions. These do
not completely bioturbate the sediment but leave
characteristic fine networks of tiny burrows in the
otherwise unreworked primary sedimentary
laminations. In the ancient oceans, times of deep
ocean anoxic conditions are not uncommon and
have been noted above and in other papers in this
volume.
Dilution
In the southern California borderland basins, the
trapping effect of the inner basins screens out
much of the tcrrigenous supply and so the outer
basins are zones of relatively slow sediment
accumulation and biogcnic fractions approach
half the volume of the fine hemipelagic sediments.
Cores in the central and outer basins reveal that
the carbonate content decreases during times of
glacially lowered sea-level due to augmentcd
influx of detritals from exposed banks or the
direct delivery of river load to the exposed shelf
edge on the adjacent continent. Use of dating
23
methods shows that in actuality, both biogenic
accumulation rates and terrigenous rates increased at times of lower sea-level corresponding
to times of increased ocean circulation rate. The
biogenics increased by perhaps twofold while the
terrigenous output sometimes increased almost
an order of magnitude (Gorsline & Prensky 1975;
Gorsline 1981). Similar patterns have been
observed in the deep ocean (e.g. Broecker et al.
1958).
Processes of fine sediment
transfer to deep water
The discussion will consider the large-scale primary processes of transfer of particulates resulting from water motion and the secondary transfer
processes that are related primarily to massmovement processes. Stow & Piper (this volume)
discuss sedimentary structures characteristic of
various transport processes which complements
and extends this discussion. The transport processes include low concentration (less than 10
/agm L-1) nepheloid plumes, high concentration
plumes at or near the mouths of large rivers (more
than 10 mg L-1), resuspension of fine sediments
by strong bottom currents, turbidity currents,
flow as matrix of debris flows (high concentration
flows; Lowe 1976, 1979, 1982), and by massmovement of various types (Nardin et al. 1979a).
These processes can be grouped into those
operating throughout the water column, which
are generally continuous although with seasonal
and longer term variations, and those that occur
along the bottom, which are usually discontinuous and discrete events including turbidity
currents and other mass-movements (Gorsline &
Emery 1959).
The continuous processes usually involve large
volumes but contain low concentrations per unit
volume and thus produce relatively slow accumulation rates at any single locality. The discontinuous processes involve smaller water volumes,
but because of their high concentrations and high
input rates, produce much larger accumulation
rates at a given point. Viscous boundary layer
effects are likely to be preserved as sedimentary
structures in the latter (Hesse & Chough 1980;
Stow & Bowen 1980); the influence of boundary
layer processes on the low concentration continuous depositional processes may appear as
regional textural gradients in the surficial sediments (McCave & Swift 1976).
Plumes and nepheloid-layers
The discharges from the world's large rivers
typically exit from the seaward mouths of deltaic
24
D.S.
distributaries as turbid plumes (Meade et al.
1975; Coleman 1976; Gibbs 1976; Coleman et al.
1981). Seasonal variations may be very important
and long term climatic cycles that affect run-off
will exert a major effect. The turbid plume
interacts with the shelf circulation. In some
instances, the season of flooding may be contemporaneous with shelf current patterns that
are different from those of other seasons. This
is the case off the Oregon-Washington coast
where the floods typically occur at times when the
shelf circulation moves coastal waters to the
north in contrast to the mean annual flow to the
south.
Discharge via deltaic and estuarine systems is
typically as flocculates resulting from ion
exchange between the clays and saline ocean
waters. High particle concentrations and biological factors facilitate the flocculating process.
Combinations of tidal exchange and salinity
gradient processes tend to hold the suspended fine
particulates in the head of the estuaries, producing turbidity maxima in or near the zone of
maximum salinity gradient (Postma 1967). Tidal
effects interacting with river flow can alter this
position to some degree (Allen et al. 1980). When
fine particulates enter the ocean via rivers, coastal
erosion, or airborne infall, they slowly fall
through the water column. Since aggregation
occurs by various processes, the initial particle
size distribution is rapidly altered (see Kranck
1975; Paffenhofer et al. 1979). Depending on the
bulk density of the aggregates, they may fall much
faster than the discrete fine particles. It should be
remembered that biologically aggregated particles are attractive sites for bacteriological colonization. Bacterial degradation of biogenic pellets
can be well advanced after a few days. As these
particles fall, therefore, they are increasingly
broken down and become less dense in bulk with
concommitant decrease in settling velocity
(Pomeroy & Diebel 1980; Pomeroy, in press).
Secondary aggregation near-bottom can be effective when near-bottom pelagic populations of
pelagic filter feeders are present as in the example
of benthopelagic holothuroids in deep waters of
California Borderland Basins (Carney, pers.
comm. 1983).
The fall velocity depends on water viscosity
also (viscosity varies inversely with temperature).
In deeper colder waters, the velocity will decrease.
In addition, the ocean is a density stratified
system with many density discontinuities. Each of
these tends to slow particle fall because of
turbulence at the boundary. As a result, particle
concentration in the ocean is non-uniform with
depth as was noted by Kalle (1939) and as further
discussed by Jerlov (1953~ 1968). Their work,
Gorsline
based primarily on optical methods, has been
verified by later workers using both optical and
direct sampling methods (see Eittrem et al. 1969;
Drake 1972). Rodolfo (1964, 1970) used water
sampling and filtration in continental margin
waters to define these zonations. He noted a
surface turbid layer most strongly developed near
the coast and river sources (Manheim et al. 1972;
Meade et al. 1975), mid-water layers, and a
bottom layer. These zones are typically layers
with particle concentrations of/~gm/L and maximum concentrations of a few mg/L near the
months of rivers, and are commonly known as
nepheloid-layers (Eittrem et al. 1969; Ewing &
Connary 1970). They are now recognized to be an
important pathway for much of the fine open
ocean particulate transport. The bottom nepheloid-layer may be up to 2000 m in thickness
although the highest concentration is typically
within a few tens of metres of the bottom.
Larger aggregates probably pass rapidly
through these nepheloid-layers to the bottom,
although bacterial degradation of pellets falling
through the water column can increase the layer
concentration (Honjo et al. 1982; Pomeroy, in
press). In the Pacific (Ewing & Connary 1970), the
bottom nepheloid-layers appear to be moved in
large gyres within the major deep basins of that
ocean and draw much of their lithic fraction from
the western continental margins of North America. Variations in thickness may in part result
from bottom erosion. Recent work in the eastern
tropical Pacific (Lonsdale 1976; Heath et al. 1976)
suggests that winnowing occurs in narrow passages between basins, with the subsequent transfer
of suspended particulate material to the bottom
nepheloid-layers of the deeper basins. Thus, some
of the fine material deposited in one depression
may have its source from an adjacent basin or sill.
In any event, a conservative estimate of the total
suspended load in ocean water at any given
moment is of the order of 10 9 tons (Lal 1977;
calculations by Gorsline). McCave (1982) has
noted that the fine particulates must pass into the
benthic boundary layer at the base of flows. As
the bottom shear increases these small scale
processes become increasingly important at given
concentrations. When this layer is developed the
processes within the layer are both erosional and
depositional on a micro scale. Thus effects of even
weak bottom flow may be subtlely recorded in the
sediments deposited.
Resuspension by bottom currents
Bottom currents can resuspend and sort fine
deposits and particles at all depths in the ocean
(Heezen et al. 1966). High velocity flows are able
A review of.fine-grained sediment
to shape large-scale bedforms and act as an agent
of major fine sediment winnowing (Lonsdale &
Malfait 1974). More subtle textural and grain
orientation properties of the sediments can reveal
data about flow direction and winnowing in the
silt and clay particle size ranges by lower velocity
currents (Johnson et al. 1977; Ledbetter & Ellwood 1980).
Drake & Gorsline (1973) have reported observations in California submarine canyons that
suggest periodic reworking of canyon sediments.
Their studies were made at the same time as
near-bottom current-metre measurements by
Shepard and his associates (Shepard et al.
1979), which show that near-bottom tidally
generated water motions can resuspend fine
sediment and move it progressively up or down
canyon.
Karl (1976, 1981) has given numerous examples of the reworking of shelf substrates by
bottom currents in shallow shelf waters off
southern California. His work also supports the
laboratory studies by Cacchione & Southard
(1974) in which the strong water motions are
associated with shoaling internal waves. These
typically concentrate at the base of the thermocline. Work by J. D. Smith and his colleagues at
the University of Washington has modelled the
influence of bottom current shear on fine sediments on shelves and predicted the depth of
reworking and the resulting textural changes
(e.g. Jumars et al. 1981; Nowell et al. 1981;).
Nittrouer & Sternberg (1981) have also noted the
reworking of fine mid-shelf mud belts associated
with the Columbia River discharge and the
progressive winnowing of fines over a period
of decades (also see Baker 1976). Larsen (1982)
has discussed the influence of seaward directed
dispersive flow generated by wave groups and
longer waves working on the coast as a
mechanism for reworking and seaward transfer
of fine sediments.
McCave (1982) and McCave et al. (1982) have
reviewed the results of the high energy benthic
boundary layer experiment (HEBBLE) studies on
the deep continental rise off the Atlantic margin
of the US and showed the influence of transient
strong bottom currents on the substrate.
Yingst & Aller (1982) have described the effects
of bioturbation on the same substrates. Thus
both biologic and physical processes may overprint to produce a final sedimentary depositional
structure assemblage. The large scale eddies and
rings associated with Gulf Stream turbulence can
reach to the deep-ocean floor, at least at inception
of the eddy or ring, and may be the source of the
deep water transient flow events noted in the
HEBBLE area.
25
Turbidity currents
Turbidity currents are turbulent, relatively high
concentration flows (Nardin et al. 1979b; Middleton & Bouma 1973) that require a high mud
content to carry the sands and gravels characteristic of the early phases of this mechanism. As the
coarser fractions settle out leaving distinctive
structures and facies as a record of flow conditions (Bouma 1962), the remaining fine suspensates are laid down as distal turbidites on the
lower slope or fan and abyssal floor, or in
overbank deposits away from the conducting
channels (Walker 1967; Haner 1971). These fine
turbidites are typically finely laminated, graded,
homogeneous or combinations of these features
(Piper 1978; Stow 1979; Stow & Bowen 1980;
Hesse & Chough 1980; Thornton 1981a).
As contemporary turbidite basin deposits have
been more closely studied, it is evident that
thicker layers (up to several metres thick) are also
common. Stanley (1981) and Stanley & Maldonado (1981) have called these massive layers
unifites. They apparently represent rapid deposition from large muddy turbidity flows generated
by slumping of unstable slope sediments. Thornton (1981 a) has described smaller examples from
Santa Barbara Basin in the California Borderland, that contain transported microfauna from
shallower slope depths.
Since the recognition of the subtle grading and
occasional faint lamination requires X-radiography and detailed textural analysis, these have
probably been missed in the stratigraphic record.
It is likely that turbidity current-deposited fine
sediments are much more common in shales than
previously supposed. Bioturbation may erase the
thin-bedded forms, but the thicker units (more
than 10 cm) will probably survive biologic stirring.
Debris-flows and other high concentration flows
In a theoretical sequence, canyons store sediment
from shelf and coastal sources and periodically
release masses of the stored sediment in response
to a variety of triggering forces (see Gorsline
1980). The sequence of processes that results is
slides and slumps, progressing to high concentration flows and debris-flows and then turbidity
currents. Changes in slope, concentration, channelization and textural composition produce a
variety of facies and sedimentary structures (e.g.
Mutti & Ricci Lucchi 1978; Nardin et al. 1979;
Hein 1982, in press). Deposition of sands and
gravels from high concentration flows and of
pebbly muds from debris-flows is probably rapid.
As the mass velocity slows to some critical level,
the coarse mass 'freezes' as a slug.
26
D.S.
Gorsline
mud belts. The shift in position will be the
response to the interaction of the two main
Slope sediments are strongly affected by mass- variables (supply and dispersal) at given sea-level
movements (Dott 1963; Emery & Uchupi 1972; positions.
Inner shelf mud belts are exemplified by the
Jacobi 1976; Embley 1976; Haner & Gorsline
1978; Nardin et al. 1979; Damuth 1980). This is Amazon muds that deposit along the Surinam
recorded in the geologic record and in acoustic coast (Wells & Coleman 1981). These form
seismic profiles in the contemporary oceans by aggrading belts that cause a general progradation
contorted and sheared bedding, and by smaller of the coast. High mud supply is the basic factor
scale evidence of displacement based on displaced (also see Augustinas 1978). The muds are of low
faunal remains and textural anomalies (Douglas bulk density initially and actually damp out
1981). The magnitude of the material lost from a incoming wave energy. The cusps of mud accregiven slope or palaeoslope surface in a sedimen- tionary deposits have wave lengths of about 100
tary section can be determined by study of the km.
Mid-shelf mud belts (of the order of 100 km
bulk properties of the section. If a zone is
overcompacted as compared to immediately long) have been studied offthe Washington coast,
overlying beds, or if surface sediments are over- the New England shelf and off the Eel-Klamath
compacted, estimates can be made of the unload- Rivers of northern California (Emery & Uchupi
ing that has occurred (Booth 1979). Almagor 1972; Baker 1976; Field 1981a, respectively). All
(1976) has shown that a regional slope profile can three areas represent dynamic accumulations
be drastically modified by large-scale mass-move- where seasonal flood discharge of fine particuments (also see Emery & Uchupi 1972; Seibold & lates is greater than the capacity of shelf dispersal
Hinz 1973). Mass-movement is a major factor processes. Offthe Columbia and the Eel-Klamath
volumetrically in continental slope transport and Rivers, the flood stages come at times when the
shelf current set is to the north and thus both mud
is particularly characteristic of fine sediments.
A variety of triggering forces can generate belts trend north from the source river mouths.
mass-movement including earthquake shocks, The fines are constantly recycling through the
sudden or rapid loading, undermining or under- deposits with periods of a few decades (Nittrouer
cutting, and gas charging. Several writers have et al. 1979; Nittrouer & Sternberg 1981). The
considered the influence of benthic sediment Washington deposit is graded with depth as a
reworking on sediment strength. Richardson & result of the reworking of the accumulating silts
Young (1980) have found that bioturbation can and clays over time and the basal, somewhat
either increase or decrease strength, whereas coarser particles probably represent an equilibDrake (1976) suggested that bioturbation de- rium residuum of the process. The east coast belt
creases cohesion and strength. Modelling studies probably has its source as the fine sediments of
of the erodibility of substrates on shelves and the the Bay of Fundy and Gulf of Maine rather than a
effects of burrowing organisms by Nowell et al. discrete river source. This deposit appears to be in
(1981) and Jumars et al. (1981) showed the dynamic equilibrium.
On many shelves, small mud patches are found
theoretical effects on strength of a range of
biological influences, and predict the resulting of scales an order of magnitude or more smaller
substrate sensitivity, erosion and redeposition in than the mid-shelf mud belts discussed above. On
response to strong near-bottom fluid stress as the west coast of the US the shelf systems are
probably sufficiently close to dynamic equilibrium
during storms.
so that major changes in shelf sediment character
can occur when the systems are perturbed. An
example is the effect of the 1969-70 floods in the
Depositional sites for fine
southern California area which delivered the
sediments in the ocean
largest sediment discharges since 1938 (Curtis et
al. 1973). These discharges overwhelmed the
Shelves
dispersal processes and the result was a mud
Fine sediments can form appreciable deposits on deposit on the shelf that was over 30 cm thick
shelves and shallow marine platforms when the inshore (Kolpack 1971; Drake et al. 1972) and
supply exceeds the dispersal rate (Sloss 1962; required almost two years of normal wave reworkMcCave t972). Stanley et al. (1983) have dis- ing to remove. This shelf is normally sandy with an
cussed the mudline as a textural marker that outer silty sand zone and occasional compact mud
demarcates the positions of mud belts on shelves. patches a few hundred square metres in area. The
Several scenarios have been examined: coastal reworking of the shelf deposits had a secondary
mud belts, mid-shelf mud belts and shelf edge effect on the adjacent basin sedimentation which
Mass-movement: slumps and slides
A review o f f i n e - g r a i n e d s e d i m e n t
has been discussed by Fleischer (1970b), Kolpack
(1971) and Drake (1972).
Outer shelf mud belts are also common. Off
southern California, in the San Pedro area,
Gorsline & Grant (1972) have described an outer
shelf belt of very fine silty sand that is bioturbated
and appears to be accumulating. Recent work by
Drake et al. (in press) gives dynamic support to
this since they show that the seasonal shift in the
threshold depth for wave stirring of fine sediments essentially bounds the outer shelf fine silty
belt. That some reworking by low velocity currents does occur is shown in profiles of water
turbidity collected over a multi-year period by
Karl (1976) in which increased turbidity at and
above the bottom is seen at the depth of intersection of the local seasonal thermocline and the
substrate. This may be the result of breaking
internal waves striking the shelf at those depths.
The dispersion of wave energy seaward and
return flow generated by build-up against the
coast by onshore wind stresses can be the sources
of offshore net motion of fine particulates once
they have been stirred into the water column by
storm wave surge of tidal currents (Larsen 1982;
Drake et al., in press).
Slopes
Fine sediments moving off the shelf are typically
deposited on the adjacent slopes. Both along
slope currents (Drake, pers. comm.) and biological pelletization tend to favour their accumulation on the slope.
Haner & Gorsline (1979) have reported that the
slopes with most mass-movement activity off
southern California are those that are in the path
of turbid plumes passing seaward off promontories and from gyres centred over broad shelf
segments (also see Davis 1980; Thornton 1981b).
Those that are not so affected are typically the
areas of highest stability, least mass-movement
and slowest sediment accumulation rate. Similar
convergences of nepheloid plumes and slopes
along other coasts must determine the sites of
highest accumulation rates and least stability.
The extreme end member of the possible series
of slope sedimentation types is the slope off the
mouth of a major delta distributary. In these
areas (e.g. Mississippi Delta, Coleman 1976) the
slope sediments are deposited on gentle gradients
at very high rates and creep and fluid failures are
common. As discussed earlier, mass-movement
and gravity influence dominates this depositional
environment.
Where sediment accumulations are large, the
scale of the mass-movements also increases as
witnessed by the large-scale failures on the Atlan-
27
tic margin of the US described by Emery &
Uchupi (1972) in their comprehensive review of
that margin. Such mass-movements must contribute a large amount of sediment to continental
rises on passive margins and to trench floors on
active margins. In the California Borderland, a
transform margin, the basins of the northern
inner borderland in the zone of highest fine
sedimentation (Gorsline 1981; Schwalbach 1982)
typically have slopes that are dominated by
mass-movement features (Gorsline 1978; Nardin
et al. 1979a).
Deep sea and basin floors
The floors of marginal basins, trenches and
abyssal plains are the end of the slope gradient
from continental platform to deep ocean floor
and, as such, represent the ultimate sediment sink
in the oceans. Reworking and sediment transfer
can still take place, particularly in gaps and sills in
seamounts and oceanic ridges (Lonsdale & Malfait 1974; Chamley 1975; Diester-Haas 1975). In
small marginal basins, sea-level position is an
important control on sedimentation rate in
general (Rona 1973; Pitman 1978; Watts 1982).
This was also true of the Atlantic in Late Neogene
times (Laine 1980) when glacial influx was a
major source for the construction of the western
abyssal plains and the North American slope and
rise areas. In Pacific type oceans with active
margins, the deep basins are starved and receive
mainly hemipelagic sediments plus locally derived turbidity current and mass-movement
deposits. In marginal basins, trenches and Atlantic type abyssal plains, turbidity current deposition is a major factor together with continuous
nepheloid-layer transport.
Bennetts & Pilkey (1976), Bornhold & Pilkey
(1971) and Malouta et al. (1981) have described
the area and volumes of a number of recent
turbidites in Atlantic abyssal plains and marginal
basin regimes. Elmore et al. (1979) described a
turbidite from Hatteras Abyssal Plain that
extended over a distance of at least 500 km. All of
these turbidites involve volumes of sediment that
range from 108 to 101~m 3, and that represent very
large initial sediment accumulations in the source
areas, although the resulting deposits are thin
turbidites. Thus the important characteristic of
this environment is an area which is large compared to bed thicknesses that are relatively small
(centimetre range).
As Ewing & Connary (1970) have stated, the
nepheloid plumes in deep ocean basin plains are
controlled by the deep-water circulation. As a
result of earth rotation and tidal influences these
deep circulations are gyres and thus suspension
28
D.S. Gorsline
transport is not directly to the basin centre but
rather slope-parallel with much longer travel
paths than a simple downslope flow over the
gentle basin slopes. Such circulations could lead
to radial transfer of suspension load to the low
velocity centre of the basin-centred gyre with
slightly higher sedimentation rates in mid-basin.
Some limited evidence of this has been accumulated for small margin basins (Malouta et al.
1981). Isopach maps of Holocene basin sedimentation in the basins of the California Borderland
show similar thicker accumulations in mid-basin
rather than in the areas of associated fans
(Schwalbach 1982). This reflects the low level of
depositional activity in fans in the present time of
high sea-level and also the probable influence of
deep-basin water circulation on nepheloid transport. Data reported by Drake (1972) for Santa
Barbara Basin in the borderland also suggests the
presence of gyral flow in the deep-basin water as
well as some possible long wave effects perhaps
resulting from tidal resonance. Large-scale rings
and eddies associated with large ocean boundary
current turbulence and instability may affect the
deep-sea floor. As shown in data from Koblinsky
et al. (1983) the rings have diameters of 100-200
km and extend to depths of at least 1500 m (the
maximum sampling depth) in the California
Current System. The Gulf Stream eddies and
rings may extend to the sea floor at times of
formation and since many of these progress into
the western north Atlantic, they may have an
important effect on deep-sea floor substrates. The
effect may be subtle and probably requires careful
textural analysis together with grain orientation
studies and represents an interesting problem for
study.
Fine-grained biogenic sediments are an important facies in pelagic sediments of the contemporary deep-sea floors. They occur in areas of high
productivity, low terrigenous influx and relatively
slow accumulation rate compared to ancient
biogenous sediments (less than 5 cm/1000 yr
versus 15 cm/1000 yr; Hakansson et al. 1974). The
chalks and diatomites which are widely distributed in some parts of the geologic record appear
to have been shelf or platform deposits (Hakansson et al. 1974; Surlyk & Birkelund 1979; Bottjer,
pers. comm.). For example, the extensive chalks
of the Cretaceous were deposited in water depths
of less than 200-300 m. Diatomites represent
times of high nutrient supply whereas the chalks
formed at times of relatively reduced nutrient
supply. Both are formed in starved environments
where other factors have screened out the continental detrital contribution. Analysis of chalk
stratigraphy has produced evidence for cycles
with periods of the order of 104 yr that may match
the climatic cycles resulting from inequalities of
solar insolation at high latitudes related to variation in the Earth's rotation, axial wobble and
precession (Milankovitch cycles).
Conclusions
It is obvious when looking at the broad spectrum
of fine-grained sediment research, that it is an
area of sedimentary petrology that requires interactions with several other disciplines. At least
since late Palaeozoic time, most fine terrigenous
particles have probably been worked upon by
organisms either in transit or after deposition.
Since fine sediments move in suspension for at
least part of their transport history, oceanography is a necessary consideration in any palaeogeographic reconstruction of ancient depositional environments. Since fine sediments are
associated with organic matter, partly due to
similar transport pathways and also due to
absorption of organics on clay flake surfaces, fine
sediments are hosts to biotas that feed on this
food source. Biological activity, including bacterial processes, alters the chemical environment
after deposition and is a major factor in diagenetic change. Thus, microbiology and marine biology are important data sources for the stratigrapher and sedimentologist. Bioturbation can erase
the primary physical structures that diagnose
transport processes and so require very detailed
analysis of mudstone and shale sections to see the
evidence of textural or compositional differences
that signal different processes such as pelagic
infall versus fine turbidite deposition.
It is evident that associations of organic matter
and fine sediment are not restricted to a given
ocean condition; e.g. upwelling in the classic
sense. Rather, the organic content may be more a
function of dilution by terrigenous input rather
than high productivity (Gorsline 1981).
The work to date and much of the work
reported in this volume shows that the fine
sedimentary record must be extensively restudied.
Much of the shale record may actually be composed of small scale mass-movement (creep) and
fine turbidites. Reworking may be much more
extensive and it will require careful and very
detailed analysis to define small hiatuses. Micropalaeontologists (e.g. Douglas 1981) are discovering that microfaunal remains are much more
mobile than might be thought in the absence of
evidence of large-scale turbidity current flow or
large mass-movements. Thus many supposed
hemipelagic records may contain mixed fossil
assemblages in what may appear to be pelagic
A review of fine-grained sediment
infall a c c u m u l a t i o n with no evidence of lateral
transport.
W h e n fine sediment depositional records are
examined in detail, cyclicity is evident at a variety
o f scales. This is an i m p o r t a n t research area in
which m u c h w o r k needs to be done. W h e n fine
sediments are deposited in anoxic environments,
the fine detail of cycles as small as seasons can
often be deciphered from the preserved primary
structures.
The bulk of the geologic record is fine-grained
sediment. Therefore most of geologic history is in
this part of the section. We have only begun to
read this record.
29
ACKNOWLEDGEMENTS; M u c h of the research u p o n
which this discussion is based has been sponsored
by various grants over the past 10 years from the
N a t i o n a l Science F o u n d a t i o n . Their continued
support is most gratefully acknowledged. Drs. R.
Bourrouilh, R. D u t t o n , R. Douglas, D. D r a k e
and S. T h o r n t o n all read one or more versions of
this paper and their c o m m e n t s were most helpful.
Drs. D. Stanley, J. Syvitski and D. Stow all read
earlier versions and also m a d e useful comments.
The degree of incompleteness or u n d e r s t a n d i n g is
solely the responsibility of the author.
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