Sequence Stratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 181

7. Slope/outer ramp : Grainstone, wackestone, and mudstone with low biotur-
bation intensities and moderate ichnodiversity. Cylindrichnus dominates,
accompanied by Skolithos and Asterosoma .
8. Basin/deeper intrashelf : Laminated mudstone/marlstone with thin wacke-
stone and grainstone layers, weakly bioturbated, and displaying a moderate
ichnodiversity.
Small-scale sequences of higher frequency, serving as building blocks of the
long-period sequences, were identified, based on changing lithofacies, bioturba-
tion characteristics, trace-fossil assemblages, and tiering patterns. These peritidal
carbonate sequences (parasequences) can be hard to differentiate based purely on
sedimentological data, but are evident when ichnological criteria are employed.
The meter-scale, shallowing-upward sequences are composed of subtidal to
supratidal sedimentary units and are bounded at their tops by MRSs and TSEs.
Finally, major stratigraphic surfaces, such as SUs, have the potential to bemapped
and traced regionally, based on their trace-fossil contents ( Fig. 2A and B ).
A special aspect of the Khuff Formation is the influence of the end-Permian
mass extinction on the evolution of the carbonate platform. This not only had
an impact on the carbonate-building organisms but also significantly controlled
the abundance and distribution of ichnotaxa and the resulting bioturbation
( Knaust, 2010 ).
4. CONCLUSIONS
The strong facies control implicit in ethologically based trace-fossil analysis
allows ichnology to be applied to a wide range of sequence-stratigraphic prob-
lems. The applications of ichnology to genetic stratigraphy are twofold. The
most obvious use lies in the identification of omission suites (e.g., substrate-
controlled suites attributable to the Glossifungites , Teredolites , and Trypanites
ichnofacies, as well as palimpsest softground assemblages), which demarcate
erosional discontinuities that correspond to stratigraphically significant sur-
faces. Unfortunately, insufficient numbers of case studies exist to indicate
whether there are suites unique to particular discontinuity types. At present,
ichnological omission, on its own, only helps to identify the presence of a stra-
tigraphic break and to help identify the surface's position in a stratal succession.
The second use is facies based. It relies on interpreting organism behaviors,
as preserved in trace fossils, to help identify the depositional environments
above and below a stratigraphic break, and in using Walther's Law to map their
paleogeographic distributions. Juxtaposed softground suites that do not reflect
originally adjacent depositional environments (i.e., successions of suites that
contravene Walther's Law) are highly effective in assisting in the sequence-
stratigraphic assessment of the ancient record. Indeed, the integration of ichno-
logical and sedimentological datasets can be crucial for the reliable delineation
and interpretation of facies and facies associations at the local scale.