Chapter 5

Developments in Sedimentology, Vol. 64. http://dx.doi.org/10.1016/B978-0-444-5381# 2012 Elsevier B.V. All rights reserved.

The Ichnofabric Concept

Allan A. Ekdale,*,1 Richard G. Bromley† and Dirk Knaust‡

*Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah, USA,†Geological Museum—SNM, Copenhagen, Denmark, ‡Statoil ASA, Stavanger, Norway1Corresponding author: e-mail: a.ekdale@utah.edu

1. INTRODUCTION

Most people would agree that ichnology is the study of traces and trace fossils,

which of course is quite true, at least in part. But ichnology includes much more

than just identifying and interpreting trace fossils. Ichnology broadly encom-

passes the preservable and preserved effects that active organisms (animals,

plants, bacteria, etc.) produce in the substrate (both unconsolidated and lithi-

fied). It includes analytical approaches to understanding the processes of bio-

turbation and bioerosion, as well as the products of those processes.

While recognition and description of identifiable trace fossils are of para-

mount importance in ichnological studies, the taphonomic overprint and pres-

ervation mode of trace fossils also hold great value for interpreting the post-

depositional history of sedimentary units. Even more broadly, recognition of

characteristic ways in which the texture and fabric of a sediment has been

affected by organism activity offers significant sedimentological, paleoecolog-

ical and even stratigraphical information. This is the arena of ichnofabric.

2. EARLY DEVELOPMENT OF THE CONCEPT

The fabric of a bolt of fine cloth, or an ornate oriental carpet, or an intricately

constructed tapestry is composed of many multihued, multitextured, inter-

twined threads that create an exquisite whole. A single thread does not a tapestry

make. Yet, the omission of a single thread may alter the appearance and possi-

bly even the overall character of the fabric, and removal of one thread may

cause the entire fabric to unravel. The fabric of a sedimentary deposit is not only

composed of many different threads (involving the mineralogy, size, shape, ori-

entation, distribution, etc. of grains and matrix) but also constructed by the com-

plex interplay of numerous physical and biological processes. Fabrics are not

woven instantaneously, and so the fabric of a sedimentary deposit (especially

3-0.00005-8

139

PART I History, Concepts, and Methods140

one developed on a sedimentary surface) may reflect substantial changes with

the passage of time. Ignoring any of the many threads that are woven together in

a sedimentary deposit will yield an incomplete, or possibly misleading, view of

the geological history and paleoenvironmental significance of that deposit.

Beginning only about three decades ago, the ichnofabric approach is a rel-

atively young direction in ichnology, although its roots can be traced back a few

decades further to the neoichnological studies of the German scientists Rudolf

Richter, Wilhelm Schafer, and Hans-Erich Reineck at the Senckenberg Institute

in Wilhelmshaven (see Baucon et al., 2012). Based on experiments and obser-

vations in the North Sea, Schafer (1956) noticed that benthic organisms modify

the sediment in many different ways, which can lead to total sediment homo-

genization (Fossitexturae deformativae) with subsequent overprinting and pres-ervation of discrete burrows (Fossitexturae figurativae). Reineck (1958)

applied these features to delineate the amount of reworking in tidal deposits,

and subsequently documented biologically generated sediment fabrics in com-

bination with other primary sedimentary features based on box-core samples

from the North Sea shelf Reineck (1963). He also developed a semiquantitative

scheme for estimating relative amounts of bioturbation in such samples, a

method that has subsequently been adapted in ichnofabric analysis (see discus-

sion of the bioturbation index in Knaust, 2012). Refinement of the box-core

sampling procedure allowed for the collection and detailed description of bio-

genic sedimentary fabrics in modern deposits (Reineck et al., 1967). In the

1970s, this methodology was expanded and refined by the use of X-ray radio-

graphy to carry out neoichnological studies, especially along the Georgia coast

of North America (Howard and Elders, 1970; Howard and Frey, 1975).

Also in the 1970s, trace fossils and complex sedimentary fabrics resulting

from burrowing were beginning to be recognized and documented in thoroughly

bioturbated sediment in deep-sea cores (Berger et al., 1979; Chamberlain, 1975;

Ekdale, 1977, 1978; Ekdale and Berger, 1978; Van der Lingen, 1973; Warme

et al., 1973). Accompanying those descriptive investigations of ichnological

features in cores were analytical attempts to quantify the nature and extent of

vertical mixing of pelagic sediment by burrowing organisms in the deep sea

(Berger and Heath, 1968; Guinasso and Schink, 1975), because that aspect of

ichnology has direct implications for the sharpness (or fuzziness) of biostrati-

graphic horizons based on microfossils. Observations in deep-sea box cores

led to the interpretation of a three-tiered vertical stratification (mixed layer, tran-

sition layer, and historical layer) of burrow emplacement in deep-sea pelagic

deposits (Berger et al., 1979; Ekdale et al., 1984b). Wetzel (1981, 1983, 1984,

1985) interpreted the ecological and stratigraphical significance of biogenic sed-

imentary structures in box cores of modern deep-sea sediments, and he outlined

their characteristic position in vertical tiers within the sediment.

Following the aforementioned seminal investigations, the concept of ichno-fabric per sewas born in the 1980s and grew out of a need to decipher a meaning

from the results of bioturbation and/or bioerosion in situations where

Chapter 5 The Ichnofabric Concept 141

individually distinct and identifiable trace fossils cannot be observed. The

parallel concept of ichnofacies, introduced by Seilacher (1964, 1967), is an

extension of the biofacies approach by recognizing recurrent associations of

ichnotaxa that represent particular paleoenvironments or specific sets of envi-

ronmental conditions, such as bathymetry, salinity, substrate consistency, etc.

(MacEachern et al., 2012). In contrast to ichnofacies, ichnofabric extends

beyond a simple listing of common associations of ichnotaxa by highlighting

the broader effects of organism behavior on the substrate itself.

In the first formal use of the term “ichnofabric” in a refereed publication,

Ekdale and Bromley (1983) exemplified the concept by illustrating in great

detail the ichnofabric of the 15-cm-thick Kjølby Gard Marl, a thoroughly bio-

turbated marly chalk layer in the uppermost Cretaceous of western Denmark.

They wrote that ichnofabric includes “those aspects of the texture and internal

structure of the bed resulting from all phases of bioturbation” (Ekdale and

Bromley, 1983: 110). In the glossary of an SEPM short course text on ichnology,

they further defined ichnofabric as “all aspects of the texture and internal struc-

ture of a sediment that result from bioturbation and bioerosion at all

scales; includes both bioturbation fabric and bioerosion fabric” (Ekdale et al.,

1984a: 308).

From that point, the practical application of the ichnofabric concept ramified

in several different, complementary directions. Some ichnofabrics may be

thought of as simple ichnofabrics in cases where they contain just one type

of trace fossil. In certain cases, the ichnofabric consists of a single ichnotaxon

superimposed on primary stratification with portions of the original sedimen-

tary laminae still discernible behind the trace fossils (Fig. 1). In other cases,

the entire bed is totally bioturbated with only a single ichnotaxon evident in

FIGURE 1 Monoichnospecific ichnofabrics in incompletely bioturbated sediment, as depicted in

these two examples, often reflect high-energy depositional environments, such as intertidal settings

(as in A) or storm-related deposits (as in B). In such situations, the interplay of sedimentation, bio-

turbation, and erosional processes is directly reflected in the resultant ichnofabrics. (A) Skolithosichnofabric in the Watson Ranch Quartzite, Lower Ordovician, Confusion Range, Millard County,

Utah. (B) Ophiomorpha ichnofabric alternating with low-angle, cross-stratified beds in a “lam-

scram” succession (after Ekdale, 1985a) in the Blackhawk Formation, Upper Cretaceous, Wasatch

Plateau, Carbon County, Utah.

PART I History, Concepts, and Methods142

the resultant ichnofabric (Fig. 2). Monoichnospecific ichnofabrics can yield

important insights concerning the response of benthic organisms to sedimentary

dynamics in high-energy depositional settings (de Gibert and Goldring, 2007;

Droser and Bottjer, 1989; Nara, 1997, 2002).

Often, however, the situations are not so simple, as in the creation of compos-ite ichnofabrics by the superimposition of different (successive) suites of bio-

genic structures. Commonly this occurs when a vertically tiered arrangement

of different types of burrows occurs in unconsolidated sediment (Bromley and

Ekdale, 1986a). That idea was expanded upon by Wetzel and Aigner (1986),

who likened a composite ichnofabric to a measuring stick for understanding

the magnitude and sequence of depositional and erosional events reflected in

a sedimentary deposit. Ekdale and Bromley (1991) further exemplified the

concept of composite ichnofabrics by illustrating the tiered structure of ichno-

coenoses in Late Cretaceous pelagic chalks of Denmark, where “detailed ichno-

fabric analysis reveals over a dozen successive burrowing episodes in the

Danish chalk, testifying to the fact that the original ooze passed through guts

of perhaps hundreds of organisms before it lithified” (Ekdale and Bromley,

1991: 232).

Another, slightly different application of the ichnofabric concept involved

the development of ichnofabric indices to describe the overall intensity of bio-

turbation that had affected a sedimentary deposit (Droser and Bottjer, 1986,

1989; see Knaust, 2012). This approach followed earlier (pre-“ichnofabric”)

attempts to categorize bioturbated sediment on the basis of how much sediment

had been disturbed by organisms (Reineck, 1967), as well as the simple recog-

nition that sediment had been burrowed without resulting in preservation of

FIGURE 2 Thalassinoides ichnofabrics. In both cases, the intense burrowing by infaunal shrimps

has completely churned the sediment, yielding a totally bioturbated—but monoichnospecific—

ichnofabric (ii¼5; BI¼6). (A) Modern Thalassinoides ichnofabric, produced by burrowing deca-

pod crustaceans (Upogebia sp.) in high-intertidal sediment at Estero Morua, a macrotidal flat on the

coast of the northern Gulf of California near Puerto Penasco, Sonora, Mexico. (B) Thalassinoides

ichnofabric in Pleistocene intertidal rock in a coastal outcrop in the same area as (A).

Chapter 5 The Ichnofabric Concept 143

individually distinguishable burrows, sometimes referred to as “bioturbate

texture” (Frey, 1973).

The role of ichnofabric analysis in understanding paleo-oxygen conditions

within the sediment has been explored and outlined by a large number of

workers (Bromley and Ekdale, 1984a; Ekdale, 1985b; Ekdale and Mason,

1988; Leszczynski, 1991; Savrda and Bottjer, 1986, 1989a,b, 1994; Savrda

and Ozalas, 1993; Wignall, 1991, 1993). This application of ichnofabric re-

cognition is very important for several reasons, including the enhancement of

our understanding of paleoecological adaptations of certain benthic organ-

isms to low-oxygen environments, tectonic development of silled marine

basins, and generation and accumulation of hydrocarbons in organic-rich sedi-

mentary units. Currently, there is a strong economic emphasis on exploration

for unconventional hydrocarbon resources in black shale strata, which fre-

quently exhibit characteristic, Chondrites-dominated, oxygen-influenced

ichnofabrics (Bromley and Ekdale, 1984a; Savrda and Bottjer, 1989a;

Schieber, 2003).

Trace-fossil taphonomy contributes to the development of ichnofabrics.

Bromley and Ekdale (1984b, 1986a,b) recognized the role of ichnofabric in

influencing the early diagenetic preservation of trace fossils, particularly in

the case of burrow flints in pelagic chalk deposits (Bromley and Ekdale,

1984b, 1986a) and also in early stage pressure-solution seams in fine-grained

carbonates (Ekdale and Bromley, 1988).

In 1990, Pollard and Ekdale convened the first international symposium

with ichnofabric as the focal point at the International Sedimentological Con-

gress in Nottingham, England (Ekdale and Pollard, 1991). Participants pre-

sented a diverse array of seminal ideas of how ichnofabric approaches can

aid in describing the biogenic aspects of various sedimentological situations

and solving particular sedimentological problems. A dedicated theme issue

dealing with ichnofabrics in the journal Palaios (1991, volume 6, number 3)

stemmed directly from this symposium. Several papers in this issue expanded

upon the description of local ichnofabrics to demonstrate their use in regional

studies, such as in stratigraphic correlation (Mortimore and Pomerol, 1991),

basin dynamics (Droser and Bottjer, 1991), and petroleum exploration

(Bockelie, 1991).

Also as a result of the symposium in Nottingham, an ongoing series of Inter-

national Ichnofabric Workshops (IIW) was initiated in order to bring together

diverse views for advancement of ichnofabric investigations. The 1st IIW was

held in Norway in 1991, the 2nd in the United States (Utah) in 1993, the 3rd in

Denmark in 1995, the 4th in the Bahamas in 1997, the 5th in England in 1999,

the 6th in Venezuela in 2001, the 7th in Switzerland in 2003, the 8th in New

Zealand in 2005, the 9th in Canada in 2007, the 10th in China in 2009, and

the 11th in Spain in 2011. These workshops have grown steadily in size and

diversity over time, and all indications are that they will continue on a regular

basis for many years to come.

PART I History, Concepts, and Methods144

3. A CONTROVERSIAL CONCEPT?

The ichnofabric concept has not been without controversy. Just as there

are varying approaches to the application of the concept, there have been

differing opinions about the validity and/or appropriateness of the concept.

For example, Frey and Pemberton (1990, 1991) even objected to the word itself,

arguing that “ichnofabric” is an inappropriate term in an etymological sense

and that it is simply synonymous with the previously proposed term “bioturbate

texture”. Ekdale et al. (1991) responded in defense, arguing that there is no

etymological problem with the word and that the “ichnofabric” concept encom-

passes far more ichnological aspects than can be described as “bioturbate

texture”. In the long run, “ichnofabric” has survived and is currently in wide-

spread use in the literature, whereas “bioturbate texture” is employed in-

frequently.

Another controversy has been the employment of three competing index

schemes to describe the intensity of bioturbation that has occurred in a sediment

(Knaust, 2012). Droser and Bottjer (1986) introduced the rank scale of “ichno-

fabric indices” (ii), which spans from ii¼1 (no bioturbation) to ii¼5 (com-

pletely bioturbated). They created a visual scale for various facies, including

shallow-marine carbonates, Skolithos-rich sandstones,Ophiomorpha-rich sand-stones, and pelagic deposits (Droser, 1991). This method has proven useful in

field observations because simple ii flash cards can be carried easily and used

for visual assessment of the ii of successive beds in outcrops and cores.

As an alternative, some other workers (Taylor and Goldring, 1993; Taylor

et al., 2003) have championed the use of “bioturbation indices” (BI), which span

a scale of BI¼0 (no bioturbation) to BI¼6 (complete bioturbation). Although

the BI method is very similar to the ii method, confusion arises, because the

index numbers are slightly different in the amount of bioturbation that they re-

present. Someworkers prefer theBI approach because it seems intuitive that zero

bioturbation should have an index value of zero. But other workers prefer the iiapproach because assigning the first level (no bioturbation) with the integer one

allows for numerical manipulations of the data that avoid the problem of a zero

in a denominator. Thus, ii data can be used to summarize intensity of bioturba-

tion in vertical successions via statistical techniques.

Since both the ii and BI approaches are applied in vertical faces in outcropsand cores, a third index was proposed by Miller and Smail (1997) to assess the

amount of burrowing on a horizontal plane. They developed the “bedding plane

bioturbation index” (BPBI) with a scale extending from BPBI¼0 (no horizon-

tal burrows) to BPBI¼5 (full coverage of a bedding plane by horizontal bur-

rows). Like the ii method of Droser and Bottjer (1986, 1989), Miller and

Smail (1997) created BPBI flash cards for easy use in the field. In order to

ensure objectivity, the categories of successive index numbers were constructed

using an image analysis program that calculates coverage (in %) of a surface by

burrows. Some workers employ BPBI with success, but other workers have not

Chapter 5 The Ichnofabric Concept 145

embraced the scheme, because it offers little of sedimentological significance,

since it addresses situations with no appreciable vertical (stratigraphic) mixing.

A third controversial topic in the realm of ichnofabric involves differing

ideas of the role of biogenic activity in influencing, or even controlling, early

diagenetic processes in the sediment. Virtually, all ichnologists agree that dia-

genetic features should be shunned in ichnotaxonomic and paleoethological

studies of trace fossils. However, sedimentary rocks displaying an ichnofabric

also commonly display a diagenetic fabric, which may have developed along a

pathway outlined by burrows and other ichnological attributes of the original

sediment. A diagenetically induced ichnofabric may include differential

cementation and/or mineral replacement of burrow walls or burrow-fill sedi-

ment. The geological record is replete with nodular fabrics and mineralized bur-

rows, where the resultant diagenetic fabric reflects and enhances the

ichnofabric, particularly in carbonate sediments (Bromley, 1967; Bromley

and Ekdale, 1986b; Kennedy and Garrison, 1975; Knaust et al., 2012). Caution

is warranted, of course, because some types of non-biogenic features resulting

from diagenetic processes (including compaction and dissolution phenomena)

may be mistaken for biologically generated ichnofabrics.

4. IMPORTANCE OF ICHNOFABRIC

Ichnofabric, at its center, pertains to the potential interpretations one can make

from the mode and style of preservation of trace fossils and other evidence of

bioturbation and bioerosion. In recent years, ichnofabric has been compared and

contrasted with ichnofacies and ichnocoenoses by some workers, sometimes

confusing their definitions and applications. In a way, they (ichnofabrics, ich-

nofacies, and ichnocoenoses) simply are three different approaches to interpret-

ing the same thing. In a strict sense, ichnofacies analysis is centered on

interpreting physical aspects of the depositional environment (e.g., bathymetry,

salinity, oxygenation, substrate character, etc.) based on the association of the

identifiable trace fossils that occur. Someworkers apply the ichnofacies concept

more broadly to include ichnological aspects that go beyond the recognition of

trace fossils (MacEachern et al., 2007; Pemberton et al., 2004), but the principal

objective of ichnofacies analysis nevertheless has a distinctly paleoenviron-

mental focus, whereas the scope of ichnofabric analysis is much broader

(Buatois and Mangano, 2011; Gerard and Bromley, 2008; Gingras et al.,

2011; McIlroy, 2007, 2008).

In a strict sense, ichnocoenosis studies are centered on interpreting paleo-

ecological aspects of the benthic community based on the wide range of results

of their burrowing and bioeroding activities. The ichnoguild approach, cham-

pioned by Bromley (1996), contributes significantly to such studies. Ichnoguild

offers a combined view of the microhabitat within the substrate, especially in

terms of infaunal tiering (Bromley and Ekdale, 1986a) or ecological stratifica-

tion (Seilacher, 1978), and the ecological niche (i.e., functional role within the

PART I History, Concepts, and Methods146

community). In a sense, an ichnoguild is an amalgam of the ichnological clues

to what type of food a group of organisms ate in the context of where and how

those organisms obtained their food. Thus, the ichnoguild concept lies at the

heart of analyzing ichnocoenoses.

Although there are obvious overlaps of ichnofabrics with ichnofacies and

ichnocoenoses, there is one important area of ichnology that is solely the

domain of ichnofabric, namely, trace-fossil diagenesis. Diagenetic processes

have nothing to do with ichnocoenoses and are not appropriate concerns of ich-

nofacies, but diagenetic processes are influenced by—and in some cases con-

trolled by—the sediment fabric, including those aspects of the sediment fabric

that are biogenic in origin (Knaust et al., 2012).

A very important application of ichnofabric studies involves the effects of

reworking on sediment porosity and permeability. As burrowers move through

the sediment, grain packing typically is disturbed, cohesiveness is reduced, and

porosity may be increased. Of course, a change in permeability naturally results,

especially if clay redistribution inside the burrow reduces the continuity that

would be provided in cleaner sediment. In some cases, the mean (and/or the

maximum) grain size is decreased by means of the ingestion, digestion, and

egestion activities of sediment feeders. In situations where the sediment consis-

tency is firm, burrow-fill material generally is more loosely packed and more

porous than the surrounding host material; so, the burrows serve as permeability

conduits (Cunningham et al., 2012; Gingras et al., 1999, 2004, 2012). This

effect is clearly manifested where burrows are preferentially cemented (or alter-

natively uncemented) and/or mineralized (dolomitized or chertified) relative to

the host rock (Fig. 3).

FIGURE 3 These two Paleozoic examples display elite burrows that have been accentuated by

preferential mineralization of the latest (and presumably) deepest burrows. (A) “Thalassinoides”

(or probably Balanoglossites) ichnofabric illustrating preferentially dolomitized burrows in the

Marjum Formation, Middle Cambrian, House Range, Millard County, Utah. (B) Thalassinoides

ichnofabric illustrating preferentially silicified (chertified), large Thalassinoides burrows in the

Great Blue Limestone, Mississippian (Lower Carboniferous), South Lakeside Mountains, Tooele

County, Utah.

FIGURE 4 Ichnofabrics in vertically slabbed cores from wells in the marginal-marine Cook For-

mation (Lower Jurassic), offshore Norway (Block 34/2 and 35/10). (A) Laminated siltstone with

sparse Phycosiphon (P), sharply eroded at its top and overlain by coarse-grained sandstone with

rip-up clasts (c). This erosion surface occurs regionally and is associated with deeply penetrating,

sand-filled, uncompacted burrows (b) indicative of a firmground omission surface. (B) Complex,

intensely bioturbated ichnofabric, including Siphonichnus (Si), Palaeophycus (Pa), Teichichnus

(T),Rosselia (R),Phycosiphon (P), and Schaubcylindrichnus (S). This interval is interpreted as a lowershoreface deposit. (C) Heterolithic, ripple-laminated sandstone with monoichnospecific occurrence of

minute Siphonichnus as indicative of a restricted environment (e.g., decreased salinity) on a tidal flat.

(D) Sandstone with the complex trace fossil Hillichnus, probably resulting from the deep-infaunal

activity of a tellinacean bivalve.

Chapter 5 The Ichnofabric Concept 147

A B

TT

PP

P

S

SS

P

PC

C

T

T

T

T

T

P

C D

FIGURE 5 Ichnofabrics in vertically slabbed cores from wells in the Fram Field area, offshore

Norway (Block 35/11), containing Upper Jurassic (Oxfordian) shelf turbidite deposits. (A) Gravel,

deposited on channelized fans, with abundant sand-filled Thalassinoides (T) and an thickly lined

Palaeophycus ichnoclast (P) in a deep-tier colonization. (B) Proximal turbidite deposit, with internal

subdivision that can be recognized by multiple colonization surfaces (arrows). Cylindrichnus (C)

occurring as a discrete trace fossil, while the upper sand layer shows a mottled fabric (mixed zone).

The muddy burrow fills and drapes include Palaeophycus (P). (C) Thin-bedded turbidite deposit,

dominated by Scolicia (S), interbedded with intensely bioturbated heterolithic shelf sandstone con-

taining minute Planolites, Palaeophycus, and Phycosiphon. (D) Completely bioturbated sandy clay-

stone with Phycosiphon (P), Schaubcylindrichnus (S), and Teichichnus (T).

PART I History, Concepts, and Methods148

FIGURE 6 Ichnofabrics in slabbed cores from wells in the Vøring Basin, offshore Norway (Block

6706/12 and 6707/10), containing deep-marine fan deposits of the Santonian Kvitnos Formation

(B, C) and the Campanian Nise Formation (A, D). (A) Top of a sandy turbidite unit with complete

(upper part) to moderate (lower part) bioturbation, consisting of monoichnospecific, irregularly

branched, walled, and meniscate burrows indicative of cf. Ophiomorpha (or Keckia) (45� deviatedwell). (B) Highly bioturbated (mottled) heterolithic sandstone with relict microbial mats (laminated

chips, l), discrete spreiten burrows (s, some of which preserve the cast of their producer as a bright

spot, p), Palaeophycus (Pa), and Planolites (Pl). (C) Highly bioturbated sandstone with fragmented

microbial mats (laminated chips), fractured carbonate concretion (lower right), and calcitic bioclasts

(bright spots). (D) Thinly bedded sandstone to siltstone with low bioturbation consisting of

Taenidium (thick backfilled burrow) and Zoophycos (thin spreiten) (45� deviated well).

FIGURE 7 Ichnofabrics in slabbedwell cores from theOsebergField, offshoreNorway (Block30/9),

with deep-marine, redeposited Shetland Chalk (Upper Maastrichtian–Danian). The wavy to nodular

fabric is enhanced by the abundant occurrence of dissolution seams. UV light (A, B) and white

light (C, D) (�60� deviated well). (A) The overall degree of bioturbation is high (approx. 85%), and

the ichnofabric comprises a monoichnospecific suite of branched and partly lined networks with a

meniscate or passive fill assigned to Thalassinoides/Ophiomorpha, probably produced by burrowing

shrimp. Two colonization phases can be recognized: (1) predeformation ichnofabric consisting of

relatively homogeneous, oil-stained chalk (yellow color) of the background sediment, and (2) post-

deformation ichnofabric resulting from subsequent massive colonization, which led to the destruction

and in-situ brecciation of the primary sedimentary fabric and the incorporation of gray mud.

(B)Higher up in the succession, sediment destruction during the second colonization phase is increased

and contributes to subsequent sediment reworking, which again leads to considerably reduced res-

ervoir properties. (C) Mass-transport complex consisting of debrite units (D) with well-rounded

chalk intraclasts (some with bioerosion traces, arrows) and larger fragments of semiconsolidated

rocks. The rock fragment in the upper portion of the core shows a gradual transition from original soft

(S) to firm (F) sediment and preserves a diffuse ichnofabric with Planolites, Palaeophycus, andChondrites from the pelagic background. The debris-flow deposit is weakly bioturbated (�15%)

and only comprises discreteThalassinoides/Ophiomorpha filledwith graymud and partly incorporated

chalk clasts. (D) Detail of (C), showing a Thalassinoides burrow with an active (meniscate) fill in

the debrite and softground (left and middle portion of the image), gradually changing to a passive

fill in the firmground (right portion of the image).

Chapter 5 The Ichnofabric Concept 151

This post-depositional aspect of ichnofabric obviously is of considerable

interest to exploration geologists and hydrogeologists because it demonstrates

the direct influence that biogenic processes have on the migration and pool-

ing of fluids in porous media. Since most Phanerozoic sedimentary rocks

exhibit an ichnofabric of one sort or another, it follows that ichnofabric data

must be a component in the characterization and evaluation of oil, gas, and

water reservoirs. This is especially true when examining drill cores for

hydrocarbon exploration. Ichnofabrics are commonly described in core

(Bockelie, 1991; Gerard and Bromley, 2008; Knaust, 2009, 2010; Martin

and Pollard, 1996; McIlroy, 2007; Taylor and Goldring, 1993), whereas

specific ichnotaxa are not always recognized with ease and identified with

certainty in core.

Given the expense of recovering drill cores from the subsurface, and in the

light of the invaluable information they can provide to the exploration or res-

ervoir geologist, ichnological analysis is a crucial part in the description and

interpretation of well cores (see Knaust, 2012). A few selected examples of

common ichnofabrics in typical marine reservoirs from offshore Norway illus-

trate common ichnofabrics and their appearance in slabbed cores and demon-

strate their value for reservoir characterization (Figs. 4–7).

5. CONCLUSIONS

The ichnofabric concept and application of the techniques of ichnofabric anal-

ysis that derive from the concept have proven to be robust and resilient. Sedi-

mentologically, ichnofabric can be used as a way to assess the degree or

intensity of bioturbation. It is also a direct reflection of sediment consistency

and sedimentary dynamics within the depositional environment, and as such

it may be a crucial tool in interpreting event sedimentation. Geochemically,

the dynamics of interstitial oxygen concentrations will be reflected in different

ichnofabrics. Paleoecologically, ichnofabric provides a direct reflection of the

endobenthic community, including the vertical tiering structure of the ichno-

coenosis. Taphonomically, ichnofabric ostensibly contributes to the diagenetic

fabric that results from lithification of a burrowed sediment. Stratigraphically,

recurrent ichnofabrics can be used effectively for correlation in certain situa-

tions within sedimentary basins. Practically, the analysis of ichnofabrics can

be used as a powerful describer of reservoirs.

ACKNOWLEDGMENTS

The authors’ ideas about ichnofabrics have benefited considerably from the shared thoughts

and insights of many astute colleagues over the past three decades. Much of the early work on

ichnofabrics by A. A. E. was supported by research grants from the National Science Foun-

dation. D. K. is grateful to Statoil for permission to publish the data presented in Figs. 4–7. The

review of Jean Gerard greatly improved this chapter.

PART I History, Concepts, and Methods152

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