Middleton G.V. (Ed.) Encyclopedia of Sediments and Sedimentary Rocks

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BIOGENIC SCDIMCNTARY STRUCTURES

General Traits

Common to

Brackish

Water

Ichnofacies

kolilhos^CyUndrichnus

Assoc.

nch

simple,

veftiail hitrnnvs are

common to estiiarine

sands and

sand-dominated

heterolithic beds

typicalJortm:

Skolithos.

Cylindrichnus &

.Atvnicolites

Figure B17 General brackish waler Irace fossil assemblaf^c. (Moditicd from Pomborlon ct .il.,

20021.

reported from a wide range of freshwater-terrestrial environ-

ments, in both ancienl and recent settings (Ekdale ctitl.. 1984).

At present, nonmarine environments have been characterized

by ihi'cc main ichnofacies: the Scoyetiiit. the Mfrniui. and the

Icrniitichiuis ichnofacies. Frey clal. (19^4) concluded that the

Scoyenia ichnofacies remains a valid concept within appro-

priate limits and suggests deposition on the shore of ephemeral

lakes and the overbank of sluggish rivers. The Scoyenia

assemblage is characterized by: (1) small horizontal, lined,

backfilled feeding btirrows; (2) curved to tortuous feeding

burrows; (3) sinuous crawling traces: (4) vertical, nnlined.

cylindrical to irregular shafts: and (5) vertebrate and arthro-

pod tracks and trails. Buatois and Mangano (ly^J.^) concluded

that the Mcruiia ichnofaeies typifies unconsolidated, fine-

grained, permanent subaqueous substrates, and well-oxyge-

nated, low-energy lake bottoms. The Mcrtnia assemblage is

characterized by: (I) dominance of horizontal to sub-liorizon-

lal grazing and feeding (races produced by mobile deposit

feeders: (2) subordinate occurrence of locomotion traces: O)

generally high to moderate diversity and abundance: and (4)

low abundance of specialized grazing structures. Smith ciat.

(1993) named the Termitichnus ichnofacies as a distinct

terrestrial assemblage characterizing paleosols. The Termitich-

nits ichnofacies is characterized by: (I) vertebrate tracks and

trails:

(2) low diversity and abundance:

(?•}

dominated by bee,

beetle, and termite nests; (4) few feeding structure; and (5)

rhizoliths. coprolites. and leaf miners. Prospects for the

recognition of additional archetypal nonmarine ichnofacies

remain encouraging. For example, Fkdale ct id. (19S4) and

Frey and Pemberton (1987) noted that distinct suites of traee

fossils characterize eolian dunes, fluvial, paleosol, and lake

environments, amonu others.

76 BIOCFNIC SFDIMFNTARY STRUCTURES

Brackish water ichnofacies

Brackish marginal marine environments are widespread in the

modern, and are being documented with more frequency from

the roek record. The general brackish water iraee fossil

assemblage (Figure B18) rellects inherently iluctuating envir-

onmenlal parameters (salinity, exposure, sedimentation rates,

temperature, and shifting substrates). These faetors suggest

that the benthie biota of braekish water environments is indeed

unique and in contrast to Bromley and Asgaard (1991) the

influenec of salinity gradients can hardly be divorced from the

ichnocoenosc concept. For the same reasons that distinct non-

marine associations are now being reeognized. unique brackish

water assemblages arc also widespread. In summary, brackish

water environments tend to be eharacterized by: (I) an

impoverished marine suite of benthie organisms; (2) traee-

making infaunal animals are more abundant than epifaunal

animals; (3) soft bodied organisms are more prevalent than

shelled organisms; (4) some animals show a distinet size

reduction; (5) some animals are bathymetrically displaced; (6)

a higher percentage of the assemblage of benthie organisms

are trophie generalists whose netivities result in morphologi-

cally simple burrows; and (7) although diversity is reduced,

high individual densities may be attained (Pemberton el al.,

1992).

Marine soft ground ichnofacies

The Psilonichmis ichnofacies is associated with supralittoral/

upper littoral, moderate to low-energy marine and/or eolian

conditions typically found in beach to backshore to dune

environments. The comments by Bromley and Asgaard (1991)

notwithstanding, the Fsilonichinis iehnofacies was founded on

fossil e.xamplcs (Frey and Pemberton. 1987) and is no more

theoretical than any other reeiirrent iehnofacies; rather, the

modern ichnocoenoses were emphasized to show the richness

that one might reasonably expect to have existed for various

ancient ichnofaunas. Furthermore, one of the major tenets of

ichnofaeies reconstruction is that the namebearer need not be

present in every occurrence of the iehnofacies; thus, just as

Crti-iana is rare in post-Paleozoic oeeurrenees of the Ciuzi-

anii ieiinofaeies. Psilonichmis may well be absent in pre-

iVleso7oic occurrences of the Psilonichmis ichnolacies.

The Skolilhos ichnofaeies (Figure BI9) is generally asso-

ciated with high-energy, sandy, shallow-marine environments.

The traee fossils are characterized by: (1) predotninantly

vertical, cylindrical or U-shaped burrows; (2) few horizontal

structures; (3) few structures produced by mobile organisms;

(4) low diversity, although individual forms may be abundant;

and (5) mostly dwelling burrows eonstrueted by suspension

feeders or passive earnivores.

The Cruziana iehnofacies (Figure B2()) usually is associated

with infralittoral/shallow circalittoral marine substrates below

minimum wave base and above maximum wave base. The

trace fossils are characterized by: (I) high diversity; (2) low

individual densities; (3) a mixed association of vertical,

inclined, and horizontal structures; (4) presenee of struetures

produced by mobile organisms; (?) ihey are dominated by

feeding and grazing struetures eonstrueted by deposit feeders

some of whieh show quite eomplex behavior patterns resulting

in complex trace fossils (Pemberton cial.. 1992).

The Zoophycos ichnofaeies ideally is found in circalittoral to

bathyai. quiet-water marine tnuds or muddy sands, below

maximum wave base to fairly deep water in areas free of

turbidity Hows and subject to oxygen deficiencies. The trace

fossils are eharaeterized by: (1) low diversity, though

individual traces may be abundant; (2) grazing and feeding

structures produced by deposit feeders; and (3) horizontal to

gently inclined spreiten struetures (i.e., reptitious parellel or

concentric burrows or traces). However, the ichnofacies also

may occur in restricted intracoastal settings, particularly in

Paleozoic sequences; the ichnogenus Zoophycos possibly

represents greater depths of burrowing in Mesozoic and

Cenozoie deposits than in Paleozoic deposits (Pemberton

et al,,, 1992), hence the character of the Zoophycos ichno-

facies may vary from one part of the stratigraphic column lo

the next.

The Ncrciics ichnofacies typically is associated uiih bathyai/

abyssal, low-energy, oxygenated marine environments subject

to periodic turbidity flows. The trace fossils are characterized

by; (I) high diversity but low abundanee: (2) complex

horizontal grazing traces and patterned feeding/dwelling

structures; (3) numerous crawling/grazing traces and sinuous

feeal castings; and (4) structures produced by deposit feeders,

scavengers, or possibly harvesters (Ekdale ei al.. 1984). As

presently understood, the Nereites ichnofacies is restricted

primarily to flyseh or turbidite sequences; sediments in the

great expanses of sealloor beyond intluence of turbidity flows

eonsist chiefly of bioturbate texiures rather than discrete

traees hence there is no well-preserved record of specific

ichnocoenoses.

Substrate controlled ichnofacies

The remaining three ichnofacies represent speeialized,

substrate-controlled tracemakers and. environtnentally. are

very general in scope. The Glossifungites iehnofacies develops

in firm but unlithified substrates (i.e.. dewatercd muds) the

Trypaiiiies develops in fully eetnented substrates, and the Ter-

edoliics ichnotacies forms in woody substrates. Such horizons

may be critical markers in the evolving concept of sequence

stratigraphy (see Suhslraie-ConirolledIchnojacies),

Evaluation of the models

These archetypical models, particularly the marine ones, have

proven to be valuable indicators of general environmental

conditions. For instance, except for a capping layer of planar

bedforms, climbing ripples, or antidunes. primary physical

sedimentary structures of fluvial point bars may be strikingly

like those of estuarine point bars. However, biogenic

sedimentary structures are very different in the two settings.

Perhaps the most misunderstood aspect of these recurrent

ichnolacies is their use in paleobathymetry. Although some

workers have been complacent in this aspect of environmental

reconstruction (cf. Frey ctal,. 1990). various iehnologists have

long and persistently emphasized that local sets of environ-

mental factors are most important in controlling the distribu-

tion of traeemakers. whether or not these parameters oeeur at

speeilic water depths

Nevertheless, many kinds of environmental paiameters do

tend to change progressively with water depth and distance

from shore (grain size, energy levels, food resources, etc.). and

these gradients affect corresponding ehanges in the distribu-

tion of physical and biogenic sedimentary struetures. To

I^IOGFNIC

SEDIMFNTAKY STKUCTURFS 77

Seilacher's

Concept

of Recurring Ichnofacies

TRACE FOSSILS ^ BEHAVIOR W ENVIRONMENT

Trace fossils are a

manifestation

of behavior

which

can be modified by the

environment.

ECOLOGICAL

CONTROLS

The distribution and behavior of benthic organisms is limited by a number of interrelated

ecological controls, including:

Sedimentation Rate

Substrate Coherence

Salinity

Oxygen Level

5. Turbidity

6. Light

7. Temperature

8. Water Energy

Rocky

Coast Peat or

Xylic

Substrate

Distribution of

Common

Marine Ichnofacies

Typical trace fossils include." 1) Caulostrepsis; 2) Entohia; 3) echinoid borings; 4) Trypanites; 5) Teredolites;

6) Thalassinoides; 7. 8) Ga.strochaenolites or related genera; 9) Diplocraterion (Glossifungites); 10) Skolithos;

11.12) Psilonichnus; 13) Macanopsis; 14) Skolithos: 15) Diplocraterion; 16) Arenicolites; 17) Ophiomorpha;

18) Phycodes: 19) RhizocoralUum; 20) Teichichnus; 21) Planolites; 22) Asteriacites; 23) Zoophycos; 24) Lorenzinia;

25) Zoophycos; 26) Paieodictyon; 27) Taphrheiminthopsis; 28) Helminthoida; 29) Cosmorhaphe; 30) Spirorhaphe.

Figure B18 The concfpi of recurring ithnotaties (Modified ,ifter Pemberton et ,il., 1992).

BIOGENIC SEDIMENTARY STRLICTURES

Association

Skolithos

Ichnofacies

irenicolites

variabilis

'Skolithos linearis

Figure B19 The different expressions of the Skolithos ichnofacies (Modified from Pemberlon w ^/.,

20021.

thai extent, trace fossil associations arc indeed useful in

paleobathymetry.

Equally important is the long temporal duration of most

kinds of trace fossils. These basic benthic behavioral

patterns arc more nearly like stable ecologic niches than

individualistic records of particular animal species (Frey and

Scilachcr. 1980). As long as the functional niche remains

advantageous under given environmental conditions, many

different animal species, over long intervals of geologic time.

may be expected to exploit it: their preserved traces are

strikingly similar and have equivalent significance. Hence,

although we conveniently speak of the "SkolUhos animar" as

the architect for a particular kind of dwelling structure,

numerous dilTercnt animal species actually were involved. The

longevity of recurrent ichnofacies thereby exceeds the long-

evity ol recurrent biofacies by a considerable margin, and are

correspondingly more useful as archetypical models not only

for environmental interpretation but also for comparisons of

depositionat environments of widely differing ages.

The purpose for recognizing these recurrent ichnofacies must

not be overlooked. Interpreted in terms of the original trace

(bssil assemblage, these iehnofacies are merely archetypal

facies models with which the local ichnofacies may be

compared. The archetypes are intended to supplement, not

supplant, local ichnofacies designations, some of which are

quite distinctive.

The idealized ichnofacies succession works well in most

"iiormar" situations (Krey and Pemberton. 1987). including

distributions according to salinity gradients (cf. Bromley and

Asgaard. 1991); yet one should not be surprised to find

nearshore assemblages in offshore sediments, and vice versa,

for example, if these accumulated under conditions otherwise

like those preferred by the tracemaking organisms. The basic

consideration rests not with such inanimate backdrops as

water depth or distance from shore, or some particular tectonic

or physiographic setting, but rather with such innate, dynamic

controlling factors as substrate consistency, hydraulic energy.

rates of deposition, turbidity, oxygen and salinity levels, toxic

BIOCCNIC SEDIMENTARY STRUCTDRFS

CRUZIANA ICHNOFACIES

Figure B20 SchenLJlic diagram of

typit.il Mesozoic example of the

Cruz'hin.i

ichnofacies (Modified from Pcniherton

cl.?/.,

H){M).

substances, the quality and quantity of available food, and the

ecologic or ichnologic prowess of tracemakers themselves.

Finally, ihc models should not be divorced from associated

patterns of bioturbatioii. Numerous local ieiinofaeies. parti-

cularly those representing low-energy conditions cind slow

rates of deposition, are set in a eomplex biogcnie fabric.

Several generations of burrows may be discernible via their

crossctittinjj relationships, showing that the same volume of

sediment passed repeatedly through various styles of rework-

ing. In environinental reeonstruction, such ichnologic fabrics

may be equally as important as the individual, named trace

lossils (Frey and Pemberton. 19S4).

Paleoenvironmenta! significance of trace fossils

I he coneept of lu net lonal morphology,

basic premise employed

by ecologisls and palcoeeologisls in en\ironinenlal reconstruc-

tion, is equally applicable to iehnology. In faet, trace fossils are

unique in that they represent not only the morphology and

ethology of the tracemaking organism but also the physical

characteristics of the substrate; they are elosely linked to tlie

environmenlal eonditioiis prevailing at the time of their

eonstruetion. Frey and Seilacher (I'JSd) re-emphasized that sueh

variables as bathymetry, temperature, volumes of sediment

deposited or eroded, aeration of water and sediment, and

substrate coherence and stability, bave a profound effect on

resultant traee fossil distributions and morphology, and hence

can be used in the determination of original biological.

ethnological, and sedimcntologicai conditions.

The applieation of iehnology to paleoenvironmental analy-

sis goes far beyond the mere establishment of gross or

archetypal ichnolacies. For instance, shallou-water, eoastal

marine environments comprise a multitude of sedimentologieal

regimes, which are subject to fluctuations in many physical and

ecological parameters (see Coastal Sedimcniary Facies). In

order fully to eomprehend the depositional history of such

zones in tbe rock reeord. it is imperative to have some reliable

means of differentiating subtle ehanges in these parameters.

Detailed investigations ol' many of these coastal tnarine zones

in Georgia have shown the value of utilizing biogenic

sedimentary struetures (in eoneert with physieal sedimentary

structures) in delineating them (Frey and Pemberton, 1987).

The application of these studies in deeiphering paleoenviron-

tnents has also proven invaluable. For example, the shoreface

consists of a seaward-sloping sediment wedge extending from

the low tide mark, generally to the fairwealher (minimum)

wave base, corresponding to approximately l()-2()m of water

depth. The shorefaee setting is dominated by wave energy and.

as a result of decreasing uave interaction uith the substrate in

a seaward direetion. shows a pronouneed basinward fming.

The shorefaee is elassically div ided into three subenvironments

(from seaward to landward): the lower, the middle and the

upper shoreface. Tbe boundaries between these are not always

eiearly defined (Reiiison. 1984). Although the large-scale

BIOCENIC SEDIMENTARY STRUCTURES

Shoreface Model

Cretaceous Western Interior

Ichnological Assemblages

Psilonichnus

Ichnofacies

Skolithos Ichnofacies

Cruziana Ichnofacies

Backshore

Foreshore

Upper

Shoreface

Middle

Shoreface

Lower

Shoreface

Transition

Upper

Offshore

Lower

Offshore

Zoophycos Ichnofacies

Psilonichnus

Ichnofacies

Mucaronichnits

Assemblage

Skolithos

Ichnofacies

High

Tide

Low

Tide

Proximal

Cruziana

Archetypal

Cruziana

Fair-weather

wave base

Distal

Cruziitmi

Storm

_ivave base

S Zoophycos

^ Ichnofacies

Dominant Processes

Subordinate Processes

Minor Processes

Dominant Behaviors

Subordinate Behaviors

Minor Behaviors

* Many tube dwellers are pa.ssive earnivores rather than suspension feeders.

Fair-weather suites are subenvirunniental indicators, not event suites

Figure B21 Shoreface model showing the distribution uf Ichnological assemblages based on examples from Ihe Cretaceous of the Western

Interior of North America (Modified after Pemberton ef

.1/.,

1992).

regional context may vary, the specific subenvironments of

the shorefaee are not signifieantly different whether they

occur as part of a strandplain. barrier island or wave/storm-

dominated delta (free from interference from active distribu-

tary channels). The shoreface grades distally into offshore

units and landward into foreshore deposits. A complete

shorefaee progradational suceession reflects offshore to fore-

shore environments: eonsequently. these adjaeent environ-

ments bear inclusion in any discussion of shorefaee deposits

per se. There are several exeellent outerop examples of

Cretaceous shoreface deposits in the Western Interior Seaway.

The integration of the sedimentology and iehnolugy within

Crevasse Splay

Resriciec

Brackish Bay

Delta Front

^^Ulracklsh

Bay Model

Cot^monly recurring ichnofacies

Figure B22 Schem<ilic

et ,il

of typittit rwurring Irate fossil jssembltij;es in hr.u kish water li.iy-like environments iModified from Pemberton

82 BIOCENIC SEDIMi^NTARY STRUCTURES

these deposits affords the opportunity to ehanieterize the faeies

and facies suceessions (Figure B2!). and to explain the

observed facies variability.

Hudson elal. (1995) recently stressed that Jurassie benthie

mollusean assetnblages in the Great Estuarine Group of Great

Britain are controlled largely by salinity with different genera

or groups of organisms characterizing a spectrum of tnainly

brackish water environments. As pointed out by Pemberton

and Wightman (1992) salinity gradients also iittluenee the

nature of the ichnofauna. Although some behavioral patterns

may occur on each side of the salinity transition, distinct

ichnoeoenoses are present in freshwater, braekish water and

fully marine environments. Recently, marginal marine envir-

onments (including tidal channels, estuaries, bays, shallow

lagoons, delta plains, etc.) have been recognized with more

frequency in the rock record. Such environments character-

istically display steep salinity gradients, which, when combined

with corresponding changes in tetnperature. turbulence.

exposure, and oxygen levels, result in a physiologically

stressful environment for numerous groups of organisms.

Brackish water bay-like environments include bays, sounds.

lagoons, inter-distributary bays, and the central basins of

wave-dominated estuaries to name just a few (Figure B22).

One of iehnologys greatest strengths, the bridging of

sedimentology and paleontology, in some respects can be its

greatest liability. Sedimenlologists tend to tise a striet

uniformitarian approaeh to paleoenvironmental interpretation

and rely heavily on modern analogues. Paleontologists, on the

other hand, must temper their observations in the light of

organic evolution. Although trace fossils can be considered as

biogenie sedimentary structures and are difficult to classify

phylogenetically. they are constructed by biological entities

and are thus subjected at least to some degree to evolutionary

trends. For example, oeeurrenees of well-developed terrestrial

trace fossil assemblages are much more prevalent in post-

Cretaceous rocks. This development corresponds to the

evolutionary explosion of the insects, brought on by the

diversification of the angiosperms in the Late Cretaceous.

Prior to this time terrestrial substrates may not have been as

extensively bioturbated due to a paucity of tracetnakers.

Likewise, patterned grazing traces, which characterize deep-sea

environments, show a trend toward more eomple.\ organiza-

tion through most of the Phaiierozoie. This trend may be

related to the evolution of more effieient foraging strategies

(Seiiaeher. 1986). For these reasons, paleoenvirontnental

interpretations based on trace fossils must be considered not

in striet uniformitarian terms, but rather, in actualistic ones.

Equally important, unique quantitative environmental in-

dieators are indeed rare in the geologieal reeord. and ichnology is

no exception (Frey and Seilacher. 1980). However, traee fossils

can supply a wealth of environmetUal information that eannot

be obtained in any other way and v\'hieh should not be ignored.

Their potential usefulness is accentuated when fully integrated

with other (chemieal. physical, and biological) lines of evidence.

Combined studies of physieal and biogenic sedimentary

structures constitute a powerful approach to facies analysis.

Summary

Iehnology and its significance to sedimentai'y geology is based

on the following concepts:

(a) Biogenic structures represent the activity of soft-bodied

organisms that are not generally preserved. Such

organisms (including many entire phyla) are commonly

the dominant component of the biomass of tnany

environments.

(b) Biogenie struetures are eommonly enhaneed by diagenesis

and ean be used in horizons where physical sedimentary

struetures have been masked. For example, in oil sands

deposits bitumen staining obliterates mosi physical struc-

tures,

but due to the coneeutration of clay minerals, it

enhances the visibility of biogenie structures.

not eontain any other fossils. In many silieiektstie regimes,

diagenesis dissolves most of the shelly fauna and trace

fossils represent the only clue as to the original biogenic

component of the unit.

(d) Biogenic structures can be used for the paleoecological

reconstruction of depositional environments.

(e) Biogenic struetures are sensitive to fluctuations in sedi-

mentary dynamics and are important in recognizing event

beds and distinet sedimentation patterns.

(0 Biogenie struetures are sensitive indieators of substrate eohe-

renee and substrate-controlled iehnofaeies are emerging as

important elements in genetie stratigraphie paradigtns.

(g) Biogenie struetures ordinarily eannot be transported and

therefore represent the original environmental position of

the trace making anitnal.

(h) Biogenic structures are sensitive to changes in certain

ecological paratiieters that are otherwise difficult to

ascertain sueh as salinity and oxygen levels.

(i) An integrated approach utilizing physieal, chemical.

biological, and ichnological lines of evidence constitutes

a powerful tool for facies Interpretation.

S. George Pemberton

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Kuiseb River. Namibia. SedimenkiryGeology. 85: 579 599.

Cross-references

Aman/GressK (1S14 1S65)

f-acies Models

Johannes Waltlier( 1860 1937)

Rudolf Richter (ISSI 1957) and the Senckcnbcrg Laboratory

Substi ale-Controlled Ichnofacies

Taphonomy: Sedimenlological Implications of Fossil Preservation

BLACK SHALES

HIack Shitlcs iirc line grained, gcncriilly orgatiic carboii-rich

scilimcntary rocks that primarily consist of a mixture of clay

iiiincriils. quartz silt, organic particles (mostly plaiiktonic algae

atid plant debris), and kerogen. They may also contain

viiriablc amounts of disseminated finely crystalline calcite

and dolomite, as well as phosphate (eommonly as concretions).

Most black shales arc found in marine seditncnts (Potter elal..

1980).

but they can also form prominent deposits in lacustrine

successions (Boliacs elal.. 2(K)()). Their black color is due to

Iwo constituents: (I) the contained organic matter, and

(2) finely disseminated pyrite. The reducing conditions

indicated by the latter have long led geologists to believe that

aneient black shales required anoxie bottom waters for their

formation, and were a typical deposit of the distal, deepest

portions of sedimentary basins (via comparisons with the

abyssal Black Sea where earbonaceous muds currently

accumulate).

The Black Sea is a stratified, silled basin, in which a lower

marine water body is overlain by a layer of brackish water due

large input of freshwater frotn rivers. Beeause of density

contrasts a halocline is developed at a depth of approxirnately

20(1

m, severely restricting vertieal advection of oxygen-rich

surface waters. Oxygen demand by decaying and descending

organic matter exceeds oxygen replenishment by \crtical

advection. rendering the water column beneath the haloeline

anoxie and facilitating the accumulation of organic-rich muds.

These observations have suggested to many geologists that

organic earbon enriehment in the bottom sediments was

basically a result of enhanced preservation (Wignall. 1994).

Although the Black Sea model dominated the study of blaek

shales for many years, it is of limited use vvhen studying

ancient black shales. A major difference between the Black Sea

and practically all black shales in the rock reeord is that the

substantial depth (-200()m) of the former, in conjunction

with a stable halocline. greatly restriets eireulation, whereas

ancient black shales accumulated in much shallower epicontj-

nental seas with a much larger width/depth ratio. Thus, on a

geologic timescale at least, stortns should have caused frequent

mi\ing of the water column. With continued research into the

origin of black shales, many occurrences that were initially

interpreted according to the Black Sea model, are now in the

process of re-evaluation.

Evolving views of the origin of the Late Devonian

Chattanooga Shale of the eastern US may serve as an example

for these changing perspectives in blaek shale deposition. On

the basis of well developed line lamination and apparently little

evidence of benthie life, it was iititially thought of as a very

good representative of a basin with a stratilied water column,

and enhanced preservation as the primary cause for black

shale accumulation. Recent research, however, has uncovered

abundant evidence for wave reworking and intermittent

erosion (Figure B23) of black shale (Schieber cial., 1998).

The observation that erosion surfaees of the type illustrated in

Figure B2? can be traced for hundreds of kilometers actually

forms the basis for a sequence stratigraphic inierpretation of

this succession (Schieber. 1998). Careful examination ol'shale

fabrics has also revealed SLibtlc but nonetheless widespread

evidence (bioturbatioii features triostly) for benlhic coloniza-

tion of the sealloor. The high organic carbon contents in the

Chattanooga Shale (up to 20 per cent) initially suggest that

preservation due to oxygen restriction may have been mainly

what controlled its accumulation. Yet. sedimentary and

bioturbation features suggest that anoxia, if they indeed

occurred, were intertnittent. due to frequent water cokmin

mixing by storms or seasonal temperature variations. In the

case of the Chattanooga Shale, therefore, surface productivity

probably was also an important factor for organic carbon

enrichment (Schieber t'/i//.. 1998).

Although the "deep anoxie basin" model has enjoyed

prominenee for many years, when we look at blaek shales

more closely we find increasingly that they can actually form in

a wide range of depositional settings. Black shales do not

constitute a large proportion of the sedimentary rock column,

but they are of great eeonomic interest as the main source rock

for hydrocarbon production, as well as containing unusually

high metal concentrations (Potter ei al.. 1980). Fstimates

suggest that tnorc than 90 pereent of the world's recoverable

oil.

and gas reserves were generated from blaek shales

(Klemme and Lllmishek. 1991). Equally significant is the fact

that the latter are restricted to six stratigriiphic intervals that

represent a mere one third of Phanero/oic time (Silurian.

Upper Devonian-Tournaisian. Pennsylvanian-Lower Permian.

Upper Jurassic. Middle Cretaceous. Oligocene-Miocenc).

Because organic carbon results from photosynthesis by

plants and algae, each atom of carbon buried implies a

molecule of oxygen added to the atmosphere. Carbon burial is

BLACK SHALES

Figure B23 Erosion surfaces in the ChatLinoogd Shale (entire exposure consists of black shale). Picture shows truncation of black shale packages

by erosion surfaces (dashed white lines marked wilh \jr^e white arrows), and illustrates how succeeding black shale packages are draped

conformably over these prosion surfaces. Small white arrows point out truncation o! shale beds by erosion siirfaco. In places erosion has removed

in excess of

m ot black shale befween successive block shale packages (at a total Chaltanooga thickness of less than 10m).

linked to other global biogeochcmical cycles and is a critical

variable in our attempts to understand the history and

evolution of the oceans and atmosphere, as well as global

climate change. Over geologic time, carbon burial probably

was responsible for a gradual rise in atmospheric oxygen levels.

Also,

by reducing the greenhouse effect it can lead to lower

global temperatures and even iee ages (Berner. 1997). In that

context, considering that the geological record is punctuated

by global episodes of widespread black shale formation

(Klemme and Ulmishek. 1991). understanding what cornbina-

tions of variables are required to produce a "black shale

world"

is of considerable significance.

Three factors control organic matter accumulation in

sediments: (1) organic matter input: |2) organic matter decay:

and i?) dilution oforgauic matter by other ingredients. Factor

I. organic tiiattcr input, has two components, dctrilal organic

matter washed in by rivers and organie matler from primary

production in the water column. The latter is strongly

dependent on nutrient availability. Factor 2 relates to break-

down of organic matter, mostly by microbes. Its efficiency and

the types of bacteria involved depend on oxygen availability in

the water column and the sediment. Factor 3. dilution, has a

terrigenous component (fluvial and eolian contributions), and

an intrabasinal component of skeletal remains

(e.g..

coccoliths,

radiolaria, foraminlfera. diatoms, etc.). The fluvial cotnponent

is linked to climate and relief in the source region, and to sea

level fluctuations. Because the magnitude of this component

rises and falls with continental runotT, it is directly linked to

the nutrient input that impacts primary production. Relief and

sediment supply are linked to tectonism. which in addition can

affect subsidence rates, basin gcotnctry, and water depth

(Schieber

etal..

I99S). Because each factor is a function of

several other variables that are in part cross-linked to other

factors, determining the magnitude of each of these factors in

the rock record is far from trivial. Obviously, black shale

formation is a multifacetcd problem and depending on the

interaction of the variables, there are multiple scenarios that

are conducive to black shale formation (Tyson and Pearson,

1991). Microbial mats can also promote accumulation, and

preservation of organic-rich sediments in a wide range of

environments (Schieber, 1999). a currently under appreciated

fact (see Microbially IndmetlSeilinwulary Siruclures).

Black shales are not only common in thick, basinai.

successions, but also occur as thin tongues within basin-

margin successions. The latter are thought to owe their

existence to high stands of sea level when terrigenous input

was at a minimum, and as such may mark maximum flooding

surfaces (Creaney and Passey, 1993). Whether these shallow

water black shale tongues reflect deposition beneath a

nearshorc zone of oxygen restriction, or simply are the outer

edge of an expanding anoxie water body that tisually only

occupies the basin center (Wignall. 1994). is still being debated.

Nearshore oxygen restriction could, for example, be produced

by salinity stratiiication due to freshwater plumes, or by high

nearshore productivity fueled by nutrients in continental

runoff. Alternatively, rapid sea-level rise could have caused

expansion of central anoxie water bodies into shallow

nearshore waters. Recent efforts to integrate black shales into

a sequence stratigraphic context suggest that at the onset of

transgression, black shale deposition may commence in the

basin interior, that nearshore black shale deposition is possible

early in transgressions, and that black shales related to

maximum flooding can form during high stands of sea level

(Wignall.

1994: Schieber

elal..

1998). Sea level variations may

also have lead to conditions with recurring water column

mixing events and recycling ol" key nutrients (N, P) to surface

waters, making possible a '"productivity-anoxia feedback"

mechanism to maintain high rates of carbon burial (Ingall

i'lal..

1993).

Black shales are the end product of the complex interplay

among a range of geologic variables and processes, and there

are no easy answers and no "one size tits all" rnodels. The

seemingly drab and uniform nature of many of these rocks

poses a considerable challenge and requires us to extract