Walker J. Facies models, Canada, 1992

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INTRODUCTION

Trace fossils contribute a unique blend

of sedimentological and paleontolog-

ical information about depositional en-

vironments. They record the behaviour

of active, in situ organisms to a much

greater extent than body fossils.

Ich-

nology,

the study of trace fossils, is

therefore concerned with the behav-

ioral record of benthic organisms, as

dictated or modified by environmental

constraints.

Biogenic structures appear in many

forms (Frey and Pemberton,

1985), but

this chapter is concerned mainly with

tracks, trails, burrows, and borings. In

order to define the facies implications

of these biogenic structures, it is first

necessary to discuss their behavioral

(ethological)

significance. The two

main objectives of the chapter are

describe and interpret nine associa-

tions of trace fossils

(ichnofacies),

and

2) to show how these ichnofacies can

be used in stratigraphic reconstruc-

tions, and in defining allostratigraphic

bounding discontinuities.

FACTORS INFLUENCING THE

CLASSIFICATION OF TRACE

FOSSILS

Special classification schemes have

been developed for trace fossils, be-

cause they represent

behaviour

rather

than actual body remains. Historically,

trace fossils have been classified in de-

scriptive, preservational, taxonomic,

and ethological terms. Of these, the

ethological scheme is by far the most

important, because the behavioral

record of benthic organisms is dictated

and modified not only by genetic pre-

adaptations, but also by prevailing envi-

ronmental parameters.

Ekdale

et a/.

(1984) recognized

seven basic categories of behaviour;

resting traces

(cubichnia),

locomotion

traces

(repichnia),

dwelling structures

(domichnia),

grazing traces

(paschnia),

feeding burrows

(fodinichnia),

farming

Trace Fossil Facies Models: Environmental

and Allostratigraphic Significance

S. George Pemberton and James A. MacEachern, Department of Geology,

University of Alberta, Edmonton, Alberta T6G 2E3

Robert

Frey, Department of Geology, University of Georgia,

Athens, Georgia 30602, U.S.A.

systems

(agrichnia),

and escape traces

(fugichnia).

Ekdale (1985) added pre-

dation traces

(praedichnia),

and Frey

et a/.

(1987) emphasized the impor-

tance of equilibria

(fugichnia)

to all

other behavioral patterns (Fig. 1).

These fundamental behavioral pat-

terns are genetically controlled, but are

not phylogenetically restricted. The

basic ethological categories have

mostly persisted throughout the Pha-

nerozoic. Individual tracemakers have

evolved, but basic benthic behaviour

essentially has not. For example, de-

posit feeders are preadapted to low-

energy environments where deposited

foodstuffs are most abundant; they do

not fare well in turbulent water settings.

The opposite is true for suspension

feeders. Similarly, locomotion traces

can be preserved only in fine-grained,

low-energy quiescent environments

such as lagoons. This ability to discern

behavioral trends of benthic organisms

represented in the rock record greatly

facilitates environmental interpretations.

THE ICHNOFACIES CONCEPT

Just as various physical sedimentary

structures can be grouped together to

define facies (Chapter

I), so individual

ichnofossils can be grouped into

ich-

nofacies.

Experience has shown that

there is a limited number of groups or

associations of ichnofossils. This

grouping, developed by Adolf Seilacher

(1 967) in the 1950s and

1960% was

originally based on the concept that

many of the parameters controlling the

distribution of tracemakers tend to

change progressively with increased

water depth (Fig. 2).

Because of the geological value of

this bathymetric relationship (Fig. 2),

the ichnofacies sequence devised by

Seilacher soon came to be regarded

almost exclusively (albeit erroneously)

as a relative paleobathometer. Today

the ichnofacies remain valuable in envi-

ronmental reconstructions, but paleo-

bathymetry is only one aspect of the

modern ichnofacies concept (Frey

al.,

1990).

Agrlchnla Praedlchnia

Pseudlchnla

[Ix-

Figure

Behavioral classification

trace fossils. With environmental fluctuation, virtually

all traces are intergradational with the

fugichnia

escape or equilibrium structures.

Extended stress may lead to death throes and the

taphichnia,

some of which might be

confused with inorganic structures resembling trace fossils, the

pseudichnia.

Adapted from

Frey

a/.

987).

48 PEMBERTON, MACEACHERN, FREY

The ichnofacies concept is closely

related to the concept of

ichnocoe-

noses.

An ichnocoenose (sometimes

spelled ichnocoenosis) is an associa-

tion of contemporaneous, environmen-

tally related traces, somewhat analo-

gous to a community of organisms. An

ichnofacies

is the preserved record of

that ichnocoenose (Frey and

Pem-

berton, 1985, p. 90-94, their table

2).

Seilacher's ichnofacies, therefore, are

merely distinctive, recurrent, archetypal

associations of traces.

In practice the term ichnocoenose

has been applied in different ways by

different workers (Bromley, 1990, p.

181 -1 82). The concept clearly applies

not only to traces in modern settings

but also to their ancient counterparts

in the rock record (Radwanski and

Roniewicz, 1970; Frey and Pemberton,

1987). Modern ichnocoenoses (Dorjes

and Hertweck, 1975) are studied as a

basis for the interpretation of fossil

equivalents.

lchnofacies and reconstructed

ichno-

coenoses are part of the total aspect of

the rock. Isolated bored shells or clasts,

for example, do not in themselves con-

stitute the

Trypanites

ichnofacies.

Rather, they must conform to Walther's

Law where there should be some sem-

blance of stratification, lateral conti-

nuity, and vertical succession. It follows

that interpretations of ichnofaunas are

improved substantially when the traces

are studied in the context of the host

rocks, the physical sedimentary struc-

tures, and the stratigraphic position

within an overall facies succession.

RECURRING ARCHETYPAL

ICHNOFACIES

Nine recurring ichnofacies have been

recognized, each named for a repre-

sentative ichnogenus, namely

Scoy-

enia, Glossifungites, Trypanites, Tere-

dolites, Psilonichnus, Skolithos,

Cruziana, Zoophycos,

and

Nereites.

These ichnofacies (Fig. 2) reflect

adaptations of tracemaking organisms

to environmental factors such as sub-

strate consistency, food supply, hydro-

dynamic energy, and salinity and

oxygen levels (Frey and Pemberton,

1984; Frey

et al.,

1990).

Figure

Recurring marine ichnofacies, placed in a representative, but not exclusive, suite of environmental gradients. Local physical, chem-

ical, and biological factors ultimately determine which traces occur at which sites. Typical trace fossils include

Caulostrepsis,

Entobia,

echinoid borings, unnamed,

Trypanites,

5,6)

Gastrochaenolites

or related ichnogenera,

Diplocraterion,

Psilonichnus,

Skolithos,

10)

Diplocraterion,

1 1)

Thalassinoides,

12)

Arenicolites,

13)

Ophiomorpha,

14)

PhycodeB,

15)

Rhizocorallium,

16)

Teichichnus,

17)

Crossopodia,

18)

Asteriacites,

19)

Zoophycos,

20)

Lorenzinia,

)

Zoophycos,

22)

Paleodictyon,

23)

Taphrhelminthopsis,

24)

Helminthoida,

25)

Spirorhaphe,

26)

Cosmorhaphe.

Modified from Crimes

(1975);

Frey and Seilacher

(1980).

Bathymetric terms are from

Ager

(1963),

mainly from the Treatise on Marine Ecology and Paleoecology. Not to scale.

ORGANIZATION OF

THE DESCRIPTIVE PART OF

THIS CHAPTER

Nonmarine ichnofacies are described

first, but most (other than the Scoyenia

assemblage) are in need of further

study. Marine softground ichnofacies

are described under two subheadings,

nearshore marine and coastal

(Psilo-

nichnus, Skolithos, Cruziana), and

open marine and deep marine

(Zoo-

phycos and Nereites). Substrate-

controlled ichnofacies, associated with

firmgrounds (Glossifungites),

wood-

Figure

Characteristic types

biogenic

structures in nonmarine environments.

Structures typical of modern and ancient soil

zones;

bee burrows,

wasp burrows,

beetle burrows, 4) vertebrats burrows, and

insect bite traces in leaves.

Structures

representative of the Scoyenia ichnofacies,

which typifies the shore zone of ephemeral

lakes and the

overbank of rivers;

Scoyenia,

Ancorichnus, 3) Cruziana, and

Skolithos.

Structures typical of shallow

lacustrine settings;

oligochaete burrows

(e.g., Tubifex),

sphaerid and unionid

bivalve burrows, and 3) chironomid larvae

burrows. Modified after Ekdale

eta/. (1984).

4. TRACE FOSSILS 49

grounds (Teredolites), and hardgrounds

(Trypanites), are described on the basis

of substrate type and consistency.

Most of these ichnofacies form in soft

to fairly soft substrates. Representative

occurrences are described below.

However, members of particular

ichno-

facies can appear in other settings, de-

pending on the correct set of charac-

teristic environmental parameters. For

example, from the standpoint of the

ethological requirements of the

trace-

making organisms, certain intertidal

back-barrier environments are not all

that different from certain

subtidal fore-

barrier environments. They may contain

virtually identical suites of trace fossils.

NONMARINE ICHNOFACIES

Scoyenia

ichnofacies, and

the ichnology of

nonmarine environments

At present, the only recognized named

ichnofacies for nonmarine environ-

ments is the Scoyenia ichnofacies (Fig.

36). Frey et a/. (1 984) concluded that

the Scoyenia ichnofacies remains a

valid concept within appropriate limits

and is suggestive of deposition in the

shore zone of ephemeral lakes and

along the

overbank of sluggish rivers.

Other distinctive assemblages charac-

terizing nonmarine aquatic environ-

ments remain unnamed, because of a

paucity of ancient examples. For in-

stance, modern lacustrine environ-

ments are characterized by traces

generated by

oligochaete annelids, am-

phipod crustaceans, sphaerid and

unionid bivalves, and insect larvae

(most notably midge larvae or

chirono-

mids; Fig. 3C). Unfortunately, ancient

examples have not been widely docu-

mented.

The nonmarine realm includes both

terrestrial and aquatic environments.

Contrary to popular misconception, the

Scoyenia ichnofacies is only one of

many nonmarine ichnofacies (Frey et

a/., 1984; Bromley and Asgaard, 1991

;

Gierlowski-Kordesch, 1991

Although

the ichnological record of most

nonma-

rine environments is indeed meagre, in

some cases distinct suites of trace

fossils are present that may prove

useful as diagnostic tools in

paleoenvi-

ronmental interpretation. Useful sum-

maries of nonmarine ichnology, from

both practical and philosophical points

of view, were given by Ekdale et al.

(1984), Miller (1984), Maples and

Archer (1

989), Bromley (1 990), and

Bromley and Asgaard (1 991).

Prospects for the recognition of addi-

tional archetypal nonmarine ichnofacies

remain encouraging. For example,

Ekdale et

a/. (1984) and Frey and

Pemberton (1987) noted that distinct

suites of trace fossils characterize

aeolian dunes and

fluvial, paleosol, and

lake environments, among others. The

envirgnments and deposits essentially

devoid of water include eolian settings

(Chapter 8) and paleosols. Most eolian

deposits have a meagre trace fossil

record, but reptile and mammal

track-

ways, scorpionid, millepede, and

isopod trails, and a variety of other non-

descript surface trails and burrows

made by dune-dwelling invertebrates

can be present, as described by

Ahlbrandt et

a/. (1 978). They concluded

that modern eolian deposits are domi-

nated by only three distinctive types of

biogenic structures (Fig. 4),

1) vertical

unlined shafts constructed by numerous

insects

(e.g., sand wasps, crickets, bee-

tle larvae) and arachnids, 2) horizontal

meniscate burrows produced by crane

fly larvae, and 3) footprints attributed to

various reptiles, mammals, and inverte-

brates that commonly crawl or walk

across the dunes.

Paleosols also have a meagre trace

fossil record (Fig. 3A) which includes 1)

invertebrate burrows that tend to be

dominated by the larval cells of dung

beetles (Retallack,

1984),

vertical

unlined shafts possibly created by small

insects and arachnids,

indentations

on fossil plant leaves interpreted as

insect bite traces or leaf mines, 4) ver-

tebrate structures, including tracks,

burrows, and coprolites from a variety

of mammals and reptiles, and

rhi-

zoliths representing the traces of roots.

.Other nonmarine deposits are formed

in running water (rivers, streams,

wave-

swept beaches) and standing water

(ponds, pools and deep lakes) environ-

ments. The ichnofossils of these envi-

ronments were summarized recently

by Ekdale et al.

(1984), Frey et al.

(1 984), Miller (1 984), Gray (1 988),

Pollard (1 988), Maples and Archer

(1989), Bromley (1990), and Bromley

and Asgaard (1 991).

Even though sediments of modern

nonmarine environments are subjected

to intense bioturbation, most ancient

examples are characterized by a

sparse ichnofossil assemblage. This

50 PEMBERTON, MACEACHERN, FREY

difference is probably due to numerous

evolutionary and taphonomic factors,

including

1) the fact that most organism

groups originated in marine environ-

ments, 2) subsequently, representa-

tives of some groups adapted to

brackish and freshwater conditions, 3)

diverse Tertiary nonmarine ichnofossil

suites are related to the evolution of an-

giosperms (seed-bearing plants) in the

Late Cretaceous, 4) angiosperm devel-

opment led to an evolutionary explosion

in the insects, 5) with the increase in

insects came a corresponding increase

in terrestrial and nonmarine crus-

taceans, and 6) most biogenic struc-

tures generated by nonmarine organ-

isms are surficial and therefore

ephemeral. These trends are reflected

in examples documented from the

Cretaceous-Tertiary of the Western

lnterior of North America (Table 1).

Such trends must be considered when

evaluating the use of trace fossils in de-

lineating marine to nonmarine transi-

tions. Pre-Cretaceous occurrences, for

instance, may not bear close resem-

blance to later ones and must be evalu-

ated with care.

The trace fossils are characterized by

1) small horizontal, lined, backfilled

feeding burrows,

curved to tortuous

unlined feeding burrows,

sinuous

crawling traces, 4) vertical cylindrical to

irregular shafts, and 5) tracks and trails.

The invertebrates are mostly deposit

feeders or predators, whereas the ver-

tebrates are predators, herbivores or

grovellers. Invertebrate diversity is very

low, yet some traces may be abundant.

GENERAL CHARACTERISTICS OF

MARINE

CLASTIC SOFTGROUND

ICHNOFACIES

Softground ichnofacies tend to be dif-

ferentiated one from another by vari-

ables that typically are depth related.

The

Zoophycos

and

Nereites

assem-

blages are more characteristic of deep

water environments, whereas the

Psilonichnus, Skolithos,

and

Cruziana

ichnofacies

are represented in near-

shore marine or coastal environments.

For example, in the Cretaceous of the

Western lnterior of North America, the

interdeltaic marine shoreface can be

zoned ichnologically (Fig. 5). This

zonation is based on the way in which

relative energy levels influence the

distribution of food resources.

Recent

summaries of the ichnology of

marine shoreface environments have

been presented by Saunders and

Pemberton

(1986), Frey and Pemberton

(1987), Frey and Howard (1990), and

Bromley

(1 990). Details of the effect of

storms on the distrbution of trace fossils

in shoreface settings were given by

Pemberton and Frey

(1984), Vossler

and Pemberton

(1988a), and Frey

(1990). The ichnological distinction be-

tween river-dominated delta-front envi-

ronments and the interdeltaic shore-

face was summarized recently by

Moslow and Pemberton

(1 988).

The

Zoophycos

and especially the

Nereites

ichnofacies tend to charac-

terize deep water environments, in-

cluding outer shelf, slope, and bathyal

to abyssal settings (Fig. 2), as dis-

cussed by Crimes

et a/.

(1 981), Ekdale

et a/.

(1 984), McCann and Pickerill

(1 988), Pickerill and Harland (1 988),

and Crimes and Crossley (1991).

NEARSHORE MARINE AND

COASTAL

ICHNOFACIES

Marginal marine environments, in-

cluding the intertidal zone, shallow

lagoons, estuaries, and delta plat-

forms, characteristically display steep

salinity gradients resulting from varia-

tions in

amounts of freshwater input

from rivers and runoff from land, 2)

rainfall, 3) evaporation, 4) tidal range

and salinity content in adjacent open-

ocean coastal waters, 5) morphology

of the coastal area, and

differences

in wind direction and velocity (Dorjes

and Howard, 1975). Such salinity fluc-

tuations (combined with corresponding

Table

Evolution of nonmarine trace fossil suites from the Early Cretaceous to Tertiary;

examples are from Western lnterior Basin.

AGE Early Cretaceous Late Cretaceous Tertiary

TYPES OF

Skolithos, Planolites, Celliforma, Edapichnium,

TRACE FOSSILS

planolites Conichnus, Scaphichnium,

Lockeia, Macanopsis, Ichnogyrus,

lmbrichnus

Planolites,

Thalassinoides, Taenidium,

?Palaeophycus, ?Ophiomorpha,

Skolithos, Skolithos, Pallichnus

Psammichnites

PROBABLE worms, insects worms, insects, worms, oligochaetes,

PRODUCERS bivalves, bivalves, spiders,

crustaceans insects, beetles, bees,

wasps, crustaceans,

crabs, mammals

Crane

Iiy

larva

burrow.

lber

beelk

larva

~~w

Sand

wasp

Crane

Iiy

Urva

burrow,

Figure

Schematic representation of characteristic organism traces in modern eolianites.

Modified after Ahlbrandt

et a/.

(1 978) and Ekdale

et al.

(1 984).

4. TRACE FOSSILS

changes in temperature, turbulence,

oxygen content, etc.) result in a physio-

logically stressful environment for nu-

merous organism groups.

It is commonly held that most inver-

tebrates originated in marine environ-

ments (Whitfield, 1976) and represen-

tatives of some groups subsequently

adapted to brackish and freshwater

conditions. The physiological mecha-

nisms involved in the adaptive process

centre on the capability of the organ-

isms to control osmotic flooding

(osmo-

regulation) and the ionic concentration

of body fluids (ionic regulation) due to

lower salt concentrations (Croghan,

1983). The extent to which these adap-

tations occur in different organism

groups is highly variable and thus their

SHOREFACE

MODEL

Psilonichnus

&&shore

lchnofacies

Skolithos

lchnofacies

Lower

Transltbn

Minimum

wave

base

Cruziana

lchnofacies

m%e

Maximum

Many

tube

dwellers are

passive

wave

base

carnivores rather than suspension feeders.

Figure

ldealised shoreface model for

ichnofacies, based mainly on facies obser-

vations in the Cretaceous interior Seaway.

For organism behaviour or feeding strategy,

dashed intervals reflect a presence,

whereas solid lines indicate a dominance.

Deposit-feeding strategies are character-

istic of lower offshore to lower shoreface

settings; associated grazing and foraging

tracemakers typical of lower offshore set-

tings mostly supplant the suspension

feeders and passive carnivores of shallower

settings, reflecting increased water depths

and decreased energy levels, among other

parameters (Frey

eta/.,

1990).

distribution in relation to salinity gradi-

ents shows marked differences. There-

fore, based on physiological criteria,

freshwater and fully saline faunas con-

stitute somewhat stable end members.

The freshwater fauna is impoverished

and is dominated by relatively few

groups, most notably branchiopods

(freshwater crustaceans) and insects

(Croghan, 1983). By sharp contrast, the

brackish water fauna is characterized

by opportunistic euryhaline (tolerant to

wide ranges of salinities) species and

consists of numerous elements, in-

cluding

1) a freshwater component con-

sisting of euryhaline species restricted

to low salinity, 2) a marine component

comprising euryhaline species, 3) a

brackish component consisting of

species that penetrate neither fresh nor

fully saline waters, and 4) a migratory

component comprising species that

spend only a portion of their life cycles

in brackish water (Perkins, 1974).

Investigations on modern brackish

water environments indicate that al-

though their biotic component is highly

variable, the following generalizations

may have significant ecological and pa-

leoecological ramifications. l) Brackish

waters are generally reduced in

species numbers with respect to both

freshwater and fully saline water

(Dorjes and Howard, 1975). 2)

Whereas the number of freshwater

species decreases rapidly even with

slight increases in salinity, the reduc-

tion of marine species is more gradual.

Therefore, the brackish water fauna

can be considered more as an impov-

erished marine assemblage than a true

mixture of freshwater and marine ele-

ments. 3) With reduced salinities, the

marine macrofauna decreases more

rapidly than the microfauna

(Remane

and Schieper, 1971). 4) lnfaunal organ-

isms are more abundant in low-salinity

waters than are epifaunal organisms

(Sanders et

a/., 1965). 5) With de-

creasing salinity, the reduction of

species in groups forming a calcareous

skeleton is greater than in their coun-

terparts lacking such a skeleton

(Remane and Schlieper, 1971). 6)

Many marine organisms undergo a

size reduction when subjected to less

saline water (Milne, 1940). 7) There is

a distinct shift in the bathymetric range

of marine organisms when subjected to

salinity reductions

(Remane and

Schieper, 1971). Thus the influence of

salinity gradients can hardly be di-

vorced from the ichnocoenose concept

as suggested'by Bromley and Asgaard

(1991).

Such trends reflect a stressful envi-

ronment, and successful colonization

depends (aside from physiological con-

straints) on the ability of the organisms

to develop an effective adaptive stra-

tegy. Sediments of estuaries and other

brackish water environments are effec-

tive at dampening the influence of

salinity fluctuations within the substrate

(Sanders et

a/., 1965). Therefore, the

deep

infaunal habitat serves as a

refuge to buffer the organism against

rapid and extreme salinity variations.

As a result, burrowing organisms are

able to penetrate a greater distance up

an estuary than epifaunal (animals

living on the substrate surface) organ-

isms, which are subjected to fluctuating

salinities of the overlying water column

(Alexander et

a/., 1935).

Although an unfortunate paucity of

studies deals specifically with biogenic

structures in modern brackish water en-

vironments, work by Howard and Frey

(1973) on estuaries of the Georgia

coast indicates that

1) the diversity and

abundance of biogenic structures in-

creases seaward,

distinct biogenic

structures and bioturbate textures are

also more diverse and abundant along

the margins of the estuaries than in the

deeper channels,

distinct burrows

and dwelling tubes characterize most

muds whereas sandy muds or muddy

sands may exhibit both distinct burrows

and various types of bioturbate tex-

tures, and 4) coarser sediments tend to

lack biogenic sedimentary structures. In

addition, they also recognized that the

entire suite of estuarine biogenic struc-

tures generally consists of both vertical

and horizontal burrows or burrow

systems and thus does not fall conve-

niently into any of Seilacher's (1967)

universal ichnofacies. Instead, the as-

semblage tends to be composed of a

mixture of structures typical of both the

Skolithos and Cruziana ichnofacies.

This assemblage is characterized by

burrows that, if preserved, would

consist of Skolithos, Monocraterion,

Thalassinoides, Ophiomorpha,

Pla-

nolites, and Palaeophycus (Howard

and Frey, 1973). Burrows typical of

freshwater deposits

(i.e., Scoyenia

and insect perturbations) were not ob-

served. Similarly, although sediments

52 PEMBERTON, MACEACHERN, FREY

and physical sedimentary structures in

tidal stream bars are very similar to

those in terrestrial bars, biogenic struc-

tures in the two are very different (Frey

and Pemberton, 1984).

Although brackish marginal-marine

environments are widespread in the

modern realm, relatively few have been

documented from the rock record.

Notable exceptions, in which the authors

mention ichnofossils, include parts of the

following units; the

Devonian-Missis-

sippian Price Formation of the central

Appalachians

(Bjerstedt, 1987), the

Middle Jurassic Great Estuarine Group

(Hudson,

1980), the Upper Jurassic of

Portugal (Fursich,

1981), the Lower

Cretaceous

Wealden Group of Great

Britain (Stewart,

1981), the Lower

Cretaceous Fall River Formation of

Wyoming (Campbell and Oaks,

1973),

the Grand Rapids Formation of Alberta

(Beynon

eta/., 1988), the McMurray

Formation of Alberta (Pemberton et a/.,

1982), various members in the Mannville

Group of Alberta (Wightman et

a/.,

1987), the Upper Cretaceous Fox Hills

Formation of Wyoming (Land,

1972),

and the Eocene Bagshot Beds of

southern England (Goldring

a/., 1978).

All these units contain well-developed

suites of biogenic structures which, in

most instances, have been used as evi-

dence of at least some saline influence.

The general assemblage reflects inher-

ently fluctuating environmental parame-

ters and is characterized by 1) low

diversity, 2) forms typically found in

marine environments, 3) structures con-

structed by trophic generalists, 4) suites

that are commonly dominated by a

single ichnogenus (Fig.

6),

and 5) ver-

tical and horizontal ichnofossils that are

common to both the

Skolithos and

Cruziana ichnofacies (Fig. 7).

Psilonichnus

ichnofacies

The Psilonichnus ichnofacies (Fig. 8)

represents

mixture of marine, quasi-

marine, and nonmarine conditions. Typ-

ical environments include the backshore

of the beach, dunes,

washover fans, and

supratidal flats. Frey and Pemberton

(1 987) noted that such environments are

subject to extreme variations in energy

levels, sediment types, and physical and

biogenic sedimentary structures. The

environments may also be strongly af-

fected by torrential rains and storm

surges. Marine processes generally

dominate during spring tides and storms,

whereas eolian processes predominate

during neap tides and nonstorm periods.

Because of their topographic position

Figure

Brackish water trace fossil ichnocoenoses are commonly characterized by

rnonospecific associations. A) Teichichnus-dominated association from offshore.

Thebaud

core.

Gyrolithes-dominated association from the Waseca Formation (core A10-34-47-

23W3, 489.5 rn) of the GoldenCake Field, Saskatch-

few such substrates are available to

benthic marine animals. The only persis-

tent, notable exceptions are amphibious

crabs of the family Ocypodidae (ghost

crabs), which include both scavengers

and

surficial deposit feeders. These

animals typically excavate

J-,

Y-,

or U-

shaped dwelling burrows, some with

bulbous basal cells, referable to the

trace fossil

Psilonichnus (Fig. 8). Other

biogenic structures are generated by es-

sentially terrestrial organisms and

include 1) the vertical

shafts of insects

and spiders,

the horizontal tunnels

formed by the crawling and foraging of

Figure

Association of trace fossils that

are typically found in both marine and

brackish water environments.

Skolithos,

2) Planolites, 3) Palaeophycus, 4)

Teich-

ichnus,

Chondrites,

Rosselia,

Gy-

rolithes. Modified after Wightrnan et a/.

987).

TRACE FOSSILS 53

insects and tetrapods, and 3) the

ephemeral tracks, trails, and fecal pel-

lets of insects, reptiles, birds, and

mammals; these are mostly predators

or herbivores. The

Psilonichnus ichno-

facies includes plant-root penetrations.

The types of plants able to exploit

these substrates range from intertidal

halophytes (plants tolerant of saltwater)

on the distal margins of some

washover

fans, to maritime or terrestrial grasses,

weeds, vines, shrubs, bushes, and

trees on dunes.

To the extent that ocypodid crab

burrows may appear in the uppermost

foreshore or the upper part of estuarine

point bars, the Psilonichnus ichnofacies

may slightly overlap the

Skolithos ichno-

facies; nevertheless, the boundary be-

tween these two ichnofacies is normally

distinct. Because of its potentially large

numbers of terrestrial traces, however,

the

Psilonichnus ichnofacies may be

broadly intergradational not only with

the Scoyenia ichnofacies but also with

several other, as yet unnamed

nonma-

rine ichnocoenoses.

The naming of the Psilonichnus

ich-

nofacies has caused some problems. It

was founded on fossil examples (Frey

and Pemberton, 1987, p. 336) but the

modern ichnocoenoses were empha-

sized to show the richness that one

might reasonably expect to have existed

for various ancient ichnofaunas. This

follows one of the major tenets of

ich-

nofacies reconstruction, namely that

the namebearer need not be present in

every occurrence of the ichnofacies.

Thus just as

Cruziana is rare in post-

Paleozoic occurrences of the Cruziana

ichnofacies, Psilonichnus may well be

absent in pre-Mesozoic occurrences of

the Psilonichnus ichnofacies.

Skolithos ichnofacies

The Skolithos ichnofacies (Fig.

is in-

dicative of relatively high levels of wave

or current energy, and typically is de-

veloped in slightly muddy to clean,

well-sorted, loose or shifting particulate

substrates, Increasing energy levels

enhance physical reworking, thus oblit-

erating the biogenic structures and pre-

serving physical sedimentary struc-

tures. Abrupt changes in rates of deposi-

tion, erosion, and physical reworking of

sediments are frequent. Such conditions

commonly occur on the foreshore and

shoreface of beaches, bars, and spits,

but the same conditions sometimes

occur on estuarine point bars, tidal deltas

and submarine fans. As dictated by fun-

damental relationships between water

agitation, sediment transport and animal

distribution, most tracemakers found

here are suspension feeders. Substrates

serve mainly as an anchoring medium.

The organisms typically construct deeply

penetrating, more or less permanent

domiciles (Fig. 9). Depth of burrowing in

the intertidal zone is controlled in part by

tidal range and height of the low water

table. During low tide, moist sediments at

depth help buffer the organisms against

desiccation and salinity or temperature

shock, and also help provide respiratory

water. In both intertidal and high-energy

subtidal settings, deep burrowing is one

means of escaping the instability of the

ever-shifting substrate surface.

The trace fossils are characterized by

predominantly vertical, cylindrical or U-

shaped burrows,

protrusive and retru-

sive spreiten in some U-burrows, which

develop in response to substrate

aggra-

dation or degradation,

few horizontal

structures,

few structures produced by

mobile organisms, 5) low diversity, al-

though individual forms may be abun-

dant, 6) mostly dwelling burrows

constructed, by suspension feeders or

passive carnivores, and 7) vertebrate

traces, particularly in low-energy inter-

tidal settings.

The Skolithos ichnofacies ordinarily

grades landward into supratidal or ter-

restrial zones and seaward into the

Cruziana ichnofacies. The landward

boundary tends to be more abrupt than

the seaward boundary. With reduced

energy, it may also grade into intertidal

or shallow

subtidal extensions of the

Cruziana ichnofacies. Mixed Skolithos-

Cruziana associations are common in

both recent (Howard and Frey, 1973)

and ancient settings.

Finally, the

Skolithos ichnofacies may

appear in slightly to substantially deeper

water deposits wherever energy levels,

food supplies, and hydrographic and

substrate characteristics are suitable

(Crimes

a/., 1981). Potential examples

include submarine canyons, deep sea

fans; and bathyal slopes swept by strong

contour currents. Therefore, as empha-

sized previously,

paleobathymetric inter-

pretations cannot be based solely on

checklists of trace fossil names. Eval-

uation of associated physical sedimen-

tary structures, stratigraphic position, and

other evidence is essential, even in

normal beach-to-offshore sequences.

Skolithos lchnofacies

Not

scale

Figure

Trace fossil association charac-

teristic of the

Psilonichnus ichnofacies. 1)

Figure

Trace fossil association characteristic

the Skolithos ichnofacies. After Frey and

Psilonichnus,

Macanopsis.

Pemberton

984).

Ophiomorpha,

Diplocraterion,

Skolithos,

Moncraterion.

54 PEMBERTON, MACEACHERN, FREY

Cruziana

ichnofacies

The Cruziana ichnofacies (Fig. 10) is

most characteristic of subtidal, poorly

sorted and unconsolidated substrates.

Conditions typically range from mod-

erate energy levels in shallow waters

below fairweather wave base but

above storm wave base, to low energy

levels in deeper, quieter waters. The

ichnofacies is also found in the littoral

to sublittoral parts of some estuaries,

bays, lagoons and tidal flats. Sediment

deposition is negligible to appreciable,

but is not necessarily rapid. Sediment

textures and bedding styles exhibit

considerable diversity. They include 1)

thinly bedded, well-sorted silts and

sands, 2) discrete mud and shell lay-

ers, 3) interbedded muddy and clean

silts and sands, and 4) extremely

poorly sorted beds derived from any of

the above through intense bioturbation.

With reduced but not negligible

energy levels, food supplies consist of

both suspended and deposited com-

ponents. Either fraction may predomi-

nate locally, or the two may be inter-

mixed. Characteristic organisms there-

fore include both suspension and

deposit feeders, as well as mobile car-

nivores and scavengers. Because of

lowered energy and abrupt shifts in

temperature and salinity levels, bur-

rows tend to be constructed horizon-

tally rather than vertically, although

scattered vertical or steeply inclined

burrows occur. Profusions of burrows

may be present at stable, low-energy

sites. Trails of epibenthic (living on the

seafloor) and endobenthic (living within

the substrate) foragers also may be

common and reflect the abundance, di-

versity, and accessibility of food.

In shallow waters, periodic scour by

storm waves, and renewed deposition

after the storm may incorporate storm

layers within

sequence of otherwise

low-energy deposits (Pemberton and

Frey, 1984). Development of hum-

mocky cross stratification may involve

the introduction of new sediment as

well as the reworking of previously de-

posited sediment. Any of these condi-

tions may yield burrow truncations and

escape structures. Increased energy

and allied parameters thus represent a

temporary excursion of Skolithos-type

conditions into a Cruziana-type set-

ting. However, this overall bedding

style differs from that of the main

Skolithos ichnofacies, in which stratifi-

cation features, substrate scour, bur-

row truncations, and escape structures

are contained entirely within sequences

of high-energy deposits. Furthermore,

the storm layers eventually are over-

printed with Cruziana-type traces.

The trace fossils are characterized

by 1) a mixed association of vertical,

inclined, and horizontal structures,

presence of traces constructed by mo-

bile organisms, 3) generally high diver-

sity and abundance, and 4) mostly

feeding and grazing structures con-

structed by deposit feeders, except

where crawling traces are predominant

(as in some Paleozoic nearshore set-

tings). Inclined U-burrows mostly have

protrusive spreiten indicating feeding

swaths (soft-sediment Rhizocorallium),

and forms of Ophiomorpha and

Thalassinoides have irregularly inclined

to horizontal components.

OPEN MARINE AND

DEEP MARINE

ICHNOFACIES

Zoophycos

ichnofacies

Of all recurrent marine ichnofacies,

this assemblage is most debated and

least understood. The ichnogenus

Zoophycos has an extremely broad

paleobathymetric range, hence its

designation as namebearer for a sup-

posedly depth-related ichnofacies has

long been controversial. In popular

bathymetric schemes, the Zoophycos

ichnofacies (Fig.

11) typically is por-

trayed as an intermediary between the

Cruziana and Nereites ichnofacies, at

a position corresponding more or less

to the continental slope. More specifi-

cally, the original designation placed it

in areas below storm wave base but

free of turbidity currents, within the

broad depositional gradient that

defines the outer shelf-to-slope transi-

tion (Seilacher, 1967).

As re-evaluated recently (Frey and

Seilacher,

1980), one of the major envi-

ronmental controls represented by the

ichnofacies is lowered oxygen levels

associated with abundant organic ma-

terial in quiet water settings. To some

extent, these conditions do occur in the

broad area across the shelf-slope

break, and hence the popularized

bathymetric placement of the ichnofa-

cies (Fig. 2) is suitable. However, such

reducing conditions replete with a domi-

nance of Zoophycos, are perhaps even

better known in shallower water, epeiric

deposits (Marintsch and Finks, 1982).

Considering the above characteris-

tics of the ichnofacies, together with

the widespread distribution of indivi-

Cruziana

lchnofacies

Not to

scale

Figure

Trace fossil association characteristic of the Cruziana ichnofacies. After Frey

and Pemberton (1984). 1) Asteriacites,

Cruziana,

Rhizocorallium,

Aulichnites,

Thalassinoides,

Chondrites,

Teichichnus, 8) Arenicolites, 9) Rossella,

10)

Planolites.

dual specimens of Zoophycos in both

shallow- and deep-water deposits (Frey

a/.,

1990), we speculate that the

Zoophycos animal was broadly adap-

ted in most ecologic respects. It toler-

ated not only a considerable range of

water depths but also numerous sub-

strate types, variable food resources,

and different energy and oxygen levels.

Its traces therefore appear in the

Cruziana through Nereites ichnofacies

(Crimes et

a/., 1981). Indeed, its toler-

ance may be its most distinguishing en-

vironmental characteristic. The animal

was able to compete successfully with

diverse tracemakers under the condi-

tions associated with the Cruziana and

Nereites ichnofacies, but few other

animals were able to compete with it

under the restricted conditions outlined

above.

Because of its singular prominence in

many restricted settings, the associa-

tion seems to warrant its own ichnofa-

cies designation. Conversely, the less

restrictive the environment at a given

site, the less distinctive the ichnofacies

as a separate entity. In numerous

places, the ichnofacies is hard to recog-

nize in the transition from the Cruziana

to the Nereites ichnofacies, especially

on unstable ancient slopes originally

subjected to turbidity flows or contour

currents.

The exact environmental implica-

tions of the Zoophycos ichnofacies

and its variants have not yet been de-

termined. The most important factors

in the distribution of the animal, in ad-

dition to its own opportunism, evidently

include water depth, depth of bur-

rowing, sediment cohesiveness, and

interstitial or bottom-water oxygen

levels. Stressed quiet water environ-

4. TRACE FOSSILS

ments, particularly those exhibiting

anoxia, seem to be the primary com-

mon denominator, even though the

animal itself was cosmopolitan.

The ichnofacies may also occur in

restricted intracoastal settings, particu-

larly in Paleozoic sequences. The

ichnogenus Zoophycos possibly repre-

sents greater paleobathymetry in

Mesozoic and Cenozoic deposits than

in Paleozoic deposits (Frey and Pem-

berton, 1984, p. 201 -202). The charac-

ter of the Zoophycos ichnofacies may

therefore vary from one part of the

stratigraphic column to the next.

The trace fossils are characterized

by 1) low diversity, though individual

traces may be abundant, 2) simple to

moderately complex, efficiently exe-

cuted grazing and feeding structures

produced by deposit feeders, and

horizontal to gently inclined spreiten

structures distributed in delicate

sheets, ribbons, lobes or spirals (flat-

tened forms of Zoophycos or, in pelitic

sediments, Phycosiphon).

Nereites ichnofacies

The Nereites ichnofacies occurs in

bathyal to abyssal quiet but oxygenated

waters, commonly influenced by tur-

bidity currents. Thus the bathymetric

implications of the Nereites ichnofacies

(Fig. 12) are less equivocal than those

of any other recurrent ichnofacies.

Although numerous trace fossils other-

wise typical of shallow water deposits

occasionally range into deep sea de-

posits, the reverse is not ordinarily true.

The depth- and energy-related vari-

ables seem to be more important influ-

ences than turbidity current deposition

(Crimes et

a/., 1981). For example, the

trace assemblage persists today on

distal abyssal plains essentially beyond

reach of turbidity currents but is absent

among well-developed, shallow water

turbidite sequences.

Nevertheless, most Nereites associa-

tions studied to date occur in

turbidites,

probably because the stratigraphic

record of deep water deposits is mainly

from subsiding basins rather than the

broad abyssal plain.

Animals exploiting lower bathyal to

abyssal environments have two major

concerns,

scarcity of food, relative to

more abundant supplies in shallower

settings, and 2) periodic disruption by

strong downcanyon bottom currents or

actual sediment-gravity flows. Over

long spans of geologic time, the overall

community ultimately developed two

component parts; preturbidite and

post-turbidite. The preturbidite resident

association is characteristic of quiet,

normal conditions and is dominant

wherever the substrate is free of the in-

fluence of turbidity currents. It tends

to be overwhelmed or eliminated by

severe erosion or turbulence, however,

and is replaced by the post-turbidite

association immediately after a tur-

bidity current. Later, as conditions re-

vert to a normal, low-energy setting,

the preturbidite association gradually

re-establishes itself. Preturbidite

animals thus comprise a stable, persis-

tent community well adapted to quiet

conditions, derived mainly from original

early-Paleozoic colonizers of the deep-

sea floor. In contrast, post-turbidite

animals represent a more oppor-

tunistic, less stable community better

adapted to turbidite colonization,

derived mainly from subsequent evolu-

tionary immigrants from shallower

waters (Frey and Seilacher, 1980).

Zoophycos lchnofacies

Not

scale

Figure

Trace fossil association characteristic of the Zoophycos ichnofacies. After Frey and Pernberton (1984).

Phycosiphon,

Zoophycos,

Spirophyton.

PEMBERTON, MACEACHERN, FREY

In addition to pre- and post-turbidite

associations, numerous turbidites dis-

play ichnologic gradients down deposi-

tional dips (Crimes et

a/., 1981). Where

strong bottom currents issue from sub-

marine canyons or flow along fan chan-

nels, components of the Skolithos

ichnofacies may be present. Otherwise,

proximal parts of turbidites may be char-

acterized by rosetted or radiating traces,

and the gently meandering forms of

Scolicia. Medial parts may be indicated

by spiralled or tightly meandering traces

(Figs. 12-1, 12-6 and 12-7). Distal parts

are typified by patterned networks (Figs.

12-2, 12-4, and 12-5) although other

traces are generally present. Zoophycos

is common locally in various settings, but

it tends to be multilobed and in places is

more complex than in the Zoophycos

ichnofacies.

Finally, the Nereites ichnofacies has

been recognized in cores of ancient, un-

consolidated, well-bedded fine sedi-

ments, including distal turbidites and

pelagic rhythmites of the present ocean

basins. However, the association, if pre-

sent, tends not to be preserved on great

expanses of abyssal plain, where sedi-

mentation and bioturbation are more or

less constant rather than episodic.

The trace fossils are characterized

1) high diversity but low abundance,

2) complex horizontal grazing traces

and patterned

feedingldwelling struc-

tures reflecting highly organized effi-

cient behaviour, 3) spreite are typically

nearly planar, although Zoophycos

forms are spiralled, multilobed and

complex, 4) numerous crawling and/or

grazing traces and sinuous fecal cast-

ings (Helrninthoida, Cosrnorhaphe) that

are mostly intrastratal, 5) structures

produced by deposit feeders and scav-

engers, and 6) possible structures as-

sociated with trapping or farming

microbes within essentially permanent

open domiciles (Paleodictyon, Mega-

grapton; Ekdale et

a/., 1984).

As presently understood, the Nereites

ichnofacies is restricted primarily to

tur-

bidite successions. Sediments on the

basin plains beyond the influence of tur-

bidity currents are mostly characterized

by bioturbate textures rather than dis-

crete traces (Frey and Wheatcroft, 1989).

SUBSTRATE-CONTROLLED

ICHNOFACIES

The ichnofacies described above are

characteristic of soft substrates, in non-

marine, marginal marine and open

marine, and deep marine settings. How-

ever, there are three ichnofacies related

to firm, hard and woody substrates.

Glossifungites

ichnofacies

The Glossifungites ichnofacies is envi-

ronmentally wide ranging, but only de-

velops in

fin unlithified substrates such

as dewatered muds. Dewatering results

from burial and the substrates are made

available to tracemakers if exhumed by

later erosion (Pemberton and Frey,

1985). Exhumation can occur in shallow

water environments as a result of coas-

tal erosion, or as a result of submarine

channels cutting through previously de-

posited sediments. Such horizons com-

monly form important bounding

discontinuities (Chapter

1).

The traces are characterized by 1)

vertical, cylindrical,

U-

or tear-shaped

pseudo-borings, or sparsely to densely

branching dwelling burrows, or mixtures

of borings and burrows, 2) protrusive

spreiten in some burrows that develop

mostly through

anidal growth (fan-

shaped

Rhizocorallium and Diplocra-

terion [formerly Glossifungites]), 3)

animals that leave the burrow to feed

(e.g., crabs), as well as suspension

feeders,

low diversity, yet individual

structures may be abundant.

Glossi-

fungites excavations tend to avoid ob-

structions within the substrate, demon-

strating a preference for firm, rather

than hard, substrates.

Trypanites

lchnofacies

This is also a specialized ichnofacies

that characterizes fully lithified marine

substrates such as hardgrounds, reefs,

rocky coasts, beachrock, and omission

surfaces. The ichnofacies may develop

even on igneous substrates (Fischer,

1981), and the collective volume of bio-

eroded sediments may be substantial.

(Torunski, 1 979).

The traces are characterized by

cylindrical to vase-, tear- or U-shaped

to irregular domiciles of organisms

living within the borings,

borings ori-

ented perpendicular to the substrate, or

consisting of shallow anastomosing

systems of borings (sponges, bry-

ozoans), 3) borings excavated by sus-

pension feeders or passive carni-

vores,

raspings and gnawings of

algal grazers and similar organisms

(mainly

chitons, limpets and echi-

noids), 5) moderately low diversity, al-

though borings and scrapings of

individual ichnogenera may be abun-

dant. In contrast to the Glossifungites

ichnofacies, the walls of the borings

cut through hard parts of the substrate

(larger grains, shells) rather than

skirting around them.

Nereites lchnofacies

Not

scale

Figure

Trace fossil association characteristic of the Nereites ichnofacies. After Frey

and Pemberton (1984).

Spirorhaphe,

Uroheiminthoida,

Lorenzinia,

Megagrapton,

Paleodictyon,

Nereites,

Cosmorhaphe.