The Ichnofabric Concept - Trace Fossils as Indicators of Sedimentary Environments - page 140

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 ii

¼

1 (no bioturbation) to ii

¼

5 (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 BI 6 (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 the BI approach because it seems intuitive that zero
bioturbation should have an index value of zero. But other workers prefer the ii
approach 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 outcrops
and 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 BPBI

¼ 0 (no bioturbation) to BI ¼

¼ 0 (no horizon-

tal burrows) to BPBI 5 (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

¼