Boggs S. Principles of Sedimentology and Stratigraphy

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356

Chapter 10 I Siliciclastic Marine Environments

settling. The geographic distribution of these organisms in surface waters is affected

by nutrients and prevailing ocean currents. After the tiny shells settle onto the

ocean floor, they may be retransported by turbidity currents or contour currents.

These skeletal materials form extensive biogenic deposits, or oozes, in some areas

of the modern ocean floor. They are also important contributors to ancient deep

ocean sediments, particularly in Jurassic and younger rocks, forming thick de

posits of chalk, radiolarian chert, and diatomite (Chapters 6 and 7). Fine-size sili

ciclastic particles (e.g., clay minerals, quartz, feldspars) transported from land by

surface plumes, wind, or floating ice may settle along with organic remains to

form mixed biogenic and siliciclastic sediment referred to as hemipelagic deposi

(Fig. 10.15).

Explosive Vo lcanism

Vo lcanism within and along the margins of marine basins may contribute impor

tant quantities of sediment to both the shelf and deeper water, particularly near

volcanic arcs. Volcanic ash, lapilli, and bombs can be ejected both subaerially and

subaqueously. Coarse material ejected subaerially tends to be deposited by air fall

close to the eruption column on all sides of the vent. If strong prevailing winds are

blowing during eruption, fine ash will be carried downwind for considerable dis

tances before settling. Pyroclastic particles ejected beneath the sea as well as air

fall particles can be dispersed still more widely within the ocean basin by various

transport processes. Pumice may even be dispersed to some extent by floating on

the ocean surface.

Tu rbidity Currents and Other Mass-Tr ansport Processes

As indicated in Figure 10.14, a variety of mass-flow processes can take place within

the deep ocean. Catastrophic or surge-type, high-velocity turbidity currents gener

ated on the shelf or upper slope are probably the single most important mechanism

for transporting sands and gravels to deeper water through submarine channels

(e.g., Noark and Piper, 1991).  passive margins and in back-arc basins, the de

posits of these flows spread out from the mouths of the canyons onto the deep

seafloor to form deep-sea fans (see Fig. 10.16), and they contribute in part to build

ing of the continental rise. In the fore-arc region of active margins, submarine

canyons discharge turbidites into fore-arc basins on the slope or into deep-sea

trenches where they may spread out along the canyon axis. Down-canyon grain

flow of beach sands swept into the heads of nearshore submarine canyons during

storms and submarine debris flows may also be locally important. Under some

conditions, submarine debris flows may be transformed downslope into turbidity

currents (e.g., Piper, Cochonat, and Morrison, 1999). In addition to these process

es, other mass-transport processes, such as creep, gliding (sliding), and slumping,

appear to be responsible for large-scale en masse retransport of sediments on

oversteepened contental slopes and ridge slopes. Some of these slump masses can

be of enormous size-up to 300 m thick and 100  long (e.g., Saxov and Nieuwen

huis, 1982; Bouma, Normark, and Baes, 1985; Stow, Reading, and Collinson,

1996). Also, some sediment can be retransported off oceanic ridges and seamounts

after accumulating there by pelagic rainout or volcanic processes.

Principal Kinds of Modern Deep-Sea Sediments

Little agreement exists regarding the classification of deep-sea sediments. Sug

gested classifications range from those that are largely genetic (Shepard, 1973;

Berger, 1974) to those that are largely descriptive (Dean, Leinen, and Stow, 1985;

Pickering, Hiscott, and Hein, 1989). No entirely satisfactory scheme that gives due

regard to both genesis and descriptive properties of all kinds of deep-sea sediments

BARRIER

R 10.16

10.3 The Oceanic (Deep-Water) Environment

357

MID-FAN CHANNELIZED

SUPRAFAN lOBE

10 �50 km

Depositional model for a point-source sand-rich submarine fan. [From Reading, H. G., and

M. Richards, 1994, Tu rbidite systems in deep-water basin margins classified by grain size

and feeder system: American Association Petroleum Geologists Bull., v. 78, Fig. 7, p. 805,

reproduced by permission.]

has yet been devised. Two broad classes of deep-sea sediment, terrigenous and

pelagic, are often mentioned; however, these two terms are difficult to define pre

cisely. Te rrigenous deposits include gravel, sand, and mud derived from land and

transported within the more proximate parts of the deep ocean by a variety of

processes (e.g., turbidity currents, contour currents, ice rafting). Some pelagic de

posits (clays) are also derived from land but are deposited by slow settling  the

more distal parts of the ocean; others consist of pelagic biogenic remains that rain

do from near-surface waters. A third minor category of deeper-water sedi

ments is shallow-water carbonate sediments (Chapter 11} that have been retrans

ported from the shelf into deeper water (allochthonous deep-water carbonates).

The principal nds of deep-sea sediments are summarized in Ta ble 10.1; addi

tional details are discussed below.

Te igenous Sediments

A wide variety of deep-sea siliciclastic sediments are grouped under this heading,

including, as shown in Ta ble 10.1, hemipelagic muds, turbidites and other sedi

ment gravity-flow deposits, contourites, glacial-marine sediments, and slumps

and slides. These deposits are all composed mainly of siliciclastic materials, which

may range in size from gravel to clay. Some deposits exhibit well-developed bed

ding and may or may not display vertical size grading. Others have a disorga

zed fabric with poorly developed bedding. For convenience of discussion, these

terrigenous deposits are grouped below under mainly genetic subheadings that

reflect their principal mode of transport and deposition.

Hemipelagic Muds. Hemipelagic ("half-pelagic") deposits are difficult to define

precisely. According to Stow and Piper (1984a), they are muddy deposits that

358

Chapter 10 I Siliciclastic Marine Environments

Hemipelagic mud-mixtures of terrigenous mud and biogenic remains; deposited from

nepheloid plumes and by suspension settling and pelac rain-out

Turbidites-graded gravel/sand/mud; deposited by turbidity currents

Contourites-sandy or muddy sediments deposited and/or reworked by contour cur

rents

Glacial-marine sediments-Gravel, sand, and mud deposited by ice rafting

Slump and slide deposits-Te rrigenous or pelagic deposits emplaced downslope by

mass-wasting processes

Pelagic clay- >2/3 siliciclastic day; deposited by suspension settling and authigenic for

mation of day minerals

Oozes- >2/3 planktonic biogenic remains; deposited by pelagic rain-out

Calcareousominantly C4C03 biogenic remains

Siliceous-dominantly Si02 biogenic remains

Allochthonous deo-sea carbonates

Shallow-water carbonates emplaced downslope by storms or sediment gravity ows

contain more than 5 percent biogenic remains and a tergenous component of

more than 40 percent silt, although some geologists may find this definition too re

strictive. They are deposited under very low current velocities (e.g., by suspension

settling), and they are probably the principal kind of sediment deposited on conti

nental slopes and in fore-arc basins on slopes. Much of the compendium volume

on fine-grained sediments and deep-water processes and facies edited by Stow

and Piper (1984b) is devoted to discussion of hemipelagic sediments.

Hemipelagic muds range in color from gray to green and, more rarel to red

dish brown. Te xtures range from clay to silty, sandy clay.  addion to biogenic re

mains, they are commonly composed of fine terrigenous quartz, feldspar, micas,

and day minerals and/ or volcanogenic sediments such as ash, fine pumice, and

palagonite. Volcanic ash or glass may be intermixed with other hemipelagic sedi

ment or concentrated into distinct tephra layers that range in thickness from less

than 1 em to more than 25 em (d. Boggs, 1984). Granules and pebbles of pumice may

occur as isolated fragmts, pockets, or distinct layers associated with hemipelagic

muds. Hemipelagic muds may contain the remains of siliceous orgasms, partu

larly diatoms, and calcareous organisms such as foraminifers and nannofossils, as

well as fine lime muds swept o carbonate platforms into deeper water.

Hemipelagic muds are poorly laminated to massive, and generally are m

erately to highly bioturbated. In general, they are deposited closer to shore than

are pelagic muds. They are widely distributed on continental slopes of volcanic

arcs, such as those of the Western Pacific They also occur in back-arc basins, 

inner trench walls, and on the tops of some rises. Hemipelagic muds are probably

deposited

mainly from nepheloid layers and plumes. Deposition may be aided 

some settings by planktonic organisms that aggregate fine sediment into fecal pel

lets that settle rapidly.

idite

The general characteriscs of turbidites are described in Chapter 4. Tur

bidites

may occur in the lower reaches of submarine canyons and farther seaward in

10.3 The Oceanic (Deep-Water) Environment

359

deep-sea channels, but most are deposited in broad, cone-shaped fans. These tur-

bidite fans, or submarine fans, spread outward on the seafloor from the mouths of

canyons. Where submarine canyons are closely spaced along the slope, the fans at

the base of the slope may coalesce to build a broad, gently sloping continental rise.

On active margins where a trench is present, turbidites commonly occur on the

trench floor throughout the length of the trench owing to deposition from turbidi-

ty currents flowing longitudinally through the trench. Most turbidites are com-

posed

of sands, silty sands, or gravelly sands interbedded with pelagic clays. They

are commonly characterized by normal size grading and may or may not display

complete Bouma sequences (see Fig. 2.8). Figure 10.17 is an example of a sediment

core composed of turbidites. As discussed in Chapter 2, many h1rbidites lack ei-

ther the basal (A unit) or upper (D unit) part of the Bouma sequence, or both. Sole

markings, such as flute casts, groove casts, and load casts, are common on the base

of many rbidite sequences.  addition to sandy turbidites, mud turbidites occur

many parts of the modern ocean floor. These turbidites are composed of nor-

mally graded silt and clay that may be either laminated or massive and that com-

monly lack extensive bioturbation. The depositional processes that generate

fine-grained turbidites and the characteristics of these deposits are described in

detail in Bouma and Stone (2000). A comparison of the characteristics of mud-rich

and sand-rich turbidites is provided by Bouma (2000). Tu rbidites are widely dis-

tributed in the modern ocean on passive margins and in both the back-arc and the

fore-arc regions of active margins.

Because the main depositional environments of hubidites are submarine

fans, and most modern turbidites occur in such fans, geologists have displayed

Figure 10.17

Graded volcaniclastic tur

bidite with Bouma divi

sions marked, from an

Ocean Drilling Program

(ODP) Leg 127 core of

Miocene sediments in the

japan Sea back-arc basin.

[Photograph courtesy of

Ocean Drilling Program,

Texas A & M University.]

360

Chapter 10 I Siliciclastic Marine Environments

Figure 10.18

Depositional model for a point

source gravel-rich submarine fan.

[From

Reading, H. G., and M.

Richards, 1994, Turbidite systems

in deep-water basin margins clas

sified by grain size and feeder sys

tem: American Association

Petroleum Geologists Bull., v. 78,

Fig. 9, p. 807, reproduced by per

mission.]

Figure 10.19

Depositional model for a

point-source mud/sand-rich

submarine fan. [From Read

ing, H. G., and M. Richards,

1994, Turbidite systems in

deep-water basin margins

classified by grain size and

feeder system: American Asso

ciation Petroleum Geologists

Bull., v. 78, Fig. 5, p. 803, re

produced by permission.)

CHENIERS. BARRIERS

considerable interest  developing models for submarine fa n sedimentation (e.g.,

Normark, 1978; Wa lker, 1978, 1992; Multi, 1985). Reading nd chards (1994)

point out ,that submarine fail ystems may migate from pot sources or linear

sources and can include gravel-rich, sand-rich, mud/sand-rich, and mud-rich sys

tems. They suggest that sand-rich systems are characterized by braided channels

and channelized sediment lobes (Fig. 10.16). Gravel-rich systems display chutes

but lack distinct sediment lobes (Fig. 10.18). Mud/sand-rich systems are charac

terized by channel levees, and distinct depositional lobes (Fig. 10.19); mud-rich

systems may contain sheetlike deposits in addition to channel levees and lobes.

Linear-source submarine fan systems differ from the point-source models shown

in Figures 10.16, 10.18, and 10.19 in that they form coalescing lobes along the base

of a slope. Miall (1999a) and Galloway (1998) further discuss fans and other slope

and base-of-slope depositional systems.

10.3 e Oceanic (Deep-Water) Environment

361

Modem submarine fans occur in many parts of e world's ocean. The largest

of these fans include e Amazon fan in the Sou Atlantic (330,000 km

surface

aa; Manley and Flood, 1988); the Rhone fan (70,000 km

) in the Mediterranean

Sea (Droz and Bellaiche, 1985); the Indus fan (1,100,000 km

) o e coast of Pak

istan and India (Kalla and Coumes, 1985); the Laurentia! fan (300,000 km

) off the

Grand Banks of New Foundland (Piper, Stow, and Normark, 1984); and the Mis

sissippi fan (300,000 km

) in the Gulf of Mexico (Weimer, 1989). Pickering et al.

(1995) discuss addional examples.

Attempts to relate the characteristics of modem fans to ancient turbidite sys

tems are complicated by problems of scale (many features on modern fans are

much larger than corresponding features in ancient fan systems) and by the na

ture of the features themselves (e.g., the abundance of channel-levee systems and

e scarcity of depositional lobes in modem fans). For example, the presence of

thickening- and thinning-upward successions in ancient turbidites has commonly

been attributed respectively to progradation of fan lobes and gradual channel fill

ing and abandonment. This interpretation is difficult to reconcile with the scarcity

of lobes in modem fans (Walker, 1992).

Contourites. Coring of sediments on continental rises has shown that in addition

to turbidites, rise deposits also include sediments interpreted to be the deposits of

contour currents. Several different countourite facies are recoized, such as

muddy, silty, sandy, and gravel-rich (Stow et al., 1998). A composite contourite fa

cies model is illustrated in Figure 10.20. This model shows overall negative grading

Lithology

Mean

and

grain size

structure

4 8 16 32  Lm

Sandy silt

Mottled silt

and mud

Mud



� Bioturbation

---- Gradational

contact

Bioturbated

Laminated

- -cro -Ta ;it8

Bioturbated

-t motes,--

lenses, and irregular

horizontal alignment

_

�

�£ __ _

Silt mottles, lenses,

and irregular layers

Bioturbated

Contacts variable

Bioturbated

� Discontinuous silt lenses

� Sharp (irregular)

contact

Figure 10.20

Composite contourite facies model showing grain

size variations and sedimentary structures through a

mud-silt-sand contourite succession. [From Stow

et al., 1998, Fossil contourites: A critical review: Sedi

mentary Geology, v. 105, Fig. 2, p. 12. Reproduced

by permission of Elsevier Science.]

362

Chapter 10 I Siliciclastic Marine Environments

Figure 10.21

Distribution and dominant

types of deep-sea sediments

in the modern ocean. (From

Davies, T. A., and D. S.

Gorsline, 1976, Oceanic sedi

ments and sedimentary

processes, in Riley, 1. P., and

R. Chester (eds.), Chemical

oceanography, v. 5, 2nd ed.,

Fig. 24.7, p. 26, reprinted by

permission of Academic

Press, Orlando, Fla.]

(coarsening upward) from muddy through silty to sandy contourites followed up

section by positive grading (fining upward) back through silty to muddy con

tourite facies. Muddy contourites tend to be homogeneous, poorly bedded, and

highly bioturbated . Silty contourites are also bioturbated and commonly display a

mottled appearance. Sandy contourites occur as thin, irregular layers within the

finer grained facies and are also commonly bioturbated, although some primary

horizontal and cross-lamination may be preserved. Contourites may have either

sharp or gradational bed contacts. The composition of contourites can include

both terrigenous constituents and biogenic components. Pickering, Hiscott, and

Hein (1989, p. 219-245); Stow, Reading, and Collinson (1996); and Stow et al.

(1998) provide further discussion of contourites.

As discussed in a preceding section ("Contour Currents"), recent work has

shown that modern contour currents develop velocities high enough to stir up

and transport huge quantities of fine sediments during what is called abyssal or

benthic "storms" (Hollister and Nowell, 1991). It thus appears that transport and

deposition of sediment by contour currents may be even more important than pre

viously believed. Contourites have been reported in the modern ocean from nu

merous places, including the continental rise of easte North America, the

continental margin off northwest Britain, and the southern Brazil margin; ancient

contourites have been described from many continents (e.g., Stow et al., 1998).

Glacial-Marine Sediments. Sediments ice-rafted to deep water are typically poor

ly sorted gravelly sands or gravelly muds that show crude to well-developed

stratification. The coarse fraction may include angular, faceted, and striated peb

bles. Significant areas of the modern ocean floor at high latitudes are covered by

these glacial-marine sediments, particularly the subpolar North Atlantic, the cir

cum-Antarctic, some parts of the Arctic Ocean, the North Pacific, and the Norwe

gian Sea (Fig. 10.21). Glacial-marine deposits are discussed in further detail under

"Glacial Systems" in Chapter 9.

Slump and Slide Deposits. These deposits consist of pviously sedimented pelagic

or terrigenous deposits that have been emplaced downslope owing to mass-move

ment processes. During the transport press, consistency of the slump masses is

� Calcareous sediments

,•::. ·· Silics sediments

··.·:·

Dee p-sea clay

:;:�:; rigens sediments

 Glacial sediments

Blank • ean mgin sediment

10.3 The Oceanic {Deep-Water) Environment

363

disturbed, resulting in faulted, contorted, and chaotic bedding and inteal struc-

ture. Studies of the ocean floor with sidescan sonar and bottom d sub-bottom ac-

coustical and seismic profiling show that slump and slide deposits are particularly

common on continental slopes with high rates of deposition, such as off the Missis-

sippi and Rhone deltas, and on slopes with glacial-marine deposits (e.g., Nardin,

Edwards, and Govsline, 1979; Schwab, Lee, and Tw ichell, 1993). ese deposits con-

sist of slides, in which failure took place elastically and only minor inteal defor-

mation of strata occurred, and of various kinds of failed masses that were emplaced

plastically and are characterized by different degrees of inteal deformation. Em-

placement of sediment along some modem lower continental slopes by mass-

transport processes appears to be the dominant sediment transport process.

Pelagic Sediments

e term pelagic sediment has been defined in dierent ways, but it is generally

taken to mean sediment deposited far from land influence by slow settling of parti

cles suspended in the water column. Pelagic sediments may be composed dominant

ly of day-size particles of terrigenous or volcanogenic origin, or they may contain

significant amots of silt- to sand-size planktonic biogenic remai. Pelagic days

a siciclastic muds that contain clay minerals, zeotes, iron oxides, and windblown

dust or ash. ey are commonly red to red-brown owing to oxidation by oxygen

bearing deep waters in areas of very slow sedimentation. These days cover vast aas

of the deeper par of the ocean below about 4500 m (Fig. 10.21). Pelagic sediments

that contain significant quantities of biogenic remains are called oozes, as mentioned.

Little agreement exis with regard to the amount of biogenic remains required to

qualify a sediment as an ooze. In Table 10.1, I suggest that oozes have more than two

irds biogenic components. Oozes composed predonantly of CaC03 tests are

calcareous oozes; those composed mainly of siliceous tests are siceous oozes. Cal

careous oozes are dominated by the tests of foramifers and nannofossils such as

coccoliths, but may they clude somewhat larger fossils such as peopods, which

a planktonic molluscs. Calcareous oozes are widespread in the modem deep ocean

at depths shallower than about 4500 m, e calci carbonate compensation depth

(Chapter 6), particularly in the Atlanc Ocean (Fig. 10.21). In deeper ocean basins

such as e Pacific, they may occur on the shallow tops of ridges and rises. Lithified

equivalen of calcareous oozes are called chalks (limestones).

Siliceous oozes are particularly abdant in the modern ocean at high lati

tudes in a belt more than 200  wide stretching across the ocean (Fig. 10.21 ). They

occur also in some equatorial regions of upwelling where nutrients are abundant

and productivity of siliceous organisms is high. Siliceous oozes are composed pri

marily of the remains of diatoms and radiolarians but may include other siliceous

organis such as silicoflagellates and sponge spicules. Diatom oozes occur main

ly in high-latude areas and along some continental margins, whereas radiolarian

oozes are more characteristic of equatorial areas. As discussed in Chapter 7,

siliceous oozes are modified and transformed during burial into bedded cherts.

Planktonic sediments that settle onto steep slopes, such as those of seamounts

and ridges, may be retransported to adjacent basins by turbidity currents or

slumping and sliding. Exceent discussions of pelagic environments and sedi

ments are given in Jenkyns (1986); Kennett (1982); Scholle, Arthur, and Ekdale

(1983); Stow and Piper (1984b); and Stow, Reading, and Collinson (1996).

Chemical Sediments

Chemical processes in the ocean can operate to modify sediments deposited by

other processes, such as dissolution of calcareous oozes; however, chemical

processes may also form some new sediment. Clay minerals, zeolites, ferroman

ganese crusts and nodules, and phosphates are formed in minor amounts by these

authigenic chemical processes.

364

Chapter 10 I Silic iclastic Marine Envi ronments

Ancient Deep-Sea Sediments

As mentioned, deep-sea sedimentary deposits other than turbidites are not as

abundant in the rock record as shallow-water sediments because the potential for

preservation and uplift of these sediments above sea level is much less. Nonethe

less, they are known from stratigraphic units of most ages. Typically, deep-sea

sedimentary rocks consist predominantly of sHiciclastic turbidite sandstones,

shales, and conglomerates; pelagic and hemipel&gic shales, which may be associ

ated with bedded cherts formed by recrystallization of siliceous oozes; chalks d

marls (lithHied, clayey pelagic calcareous oozes); limestone breccias (slope de

posits); and carbonate turbidites (Chapter 11). Except for turbidites and carbonate

breccias, which may be very coarse grained deposits, deep-sea deposits are distin

guished in general by their fine grain size. Other than turbidites, most deep-sea

deposits do not show vertical facies successions that change upward  any fixed

order. Physical sedimentary structures in ancient deep-sea sediments consist p

dominantly of thin, horizontal laminations, although rippled bedding and graded

bedding are common in turbidites, and cross-lamination occurs in some con

tourites. The bedding of many deep-sea deposits is well developed, even, and lat

erally persistent.

Colors of deep-water sediments are typically dark gray to black; red pelagic

shales are much rarer. Deep-water muds may be well bioturbated or essentially

nonbioturbated; they are commonly characterized by distinctive deep-water trace

fossil associations. Fine-grained, deep-water sediments are characterized also by the

presence of much greater concentrations of planktonic organisms than occur  shal

low-water sedents. These organisms include diatoms, radiolarians, foraminifers,

coccoliths, and, in older rocks, graptolites and ammonites. Deep-water sedimen

tary rocks occur in extensive tabular- or blanket-shaped deposits and may be

underlain by ocean crustal rocks such as submarine basalts and ophiolite assem

blages consisting of serpentinized peridotite, dunite, gabbros, sheeted dikes,

and pillow lavas.

Figure 10.22

;-� . : ;

. ·

. · .

.

�

-�

�



Rhythmically bedded turbidites in the Ca nning Formation (early Te rtiary), Arctic National

Wildlife Refuge, Alaska. Note the large, low-angle truncation in the middle of the outcrop.

[Photograph by D. W. Houseknecht, U.S. Geological Survey Open File Report 98-34,

1999, The oil and gas resource potential of the Arctic National Wildlife Refuge 1 002 Area,

Alaska.]

· -