Ichnofabric and Siliciclastic Depositional Systems: Integration for Sequence Stratigraphic Analysis

BOTTJER, DAVID J., University of Southern California, Los Angeles, CA, and MARY L. DROSER, University of California, Riverside, CA

Much previous research on biogenic sedimentary structures has established how ichnofacies (assemblages of discrete trace fossils) vary within marine depositional systems. However, studies aimed at understanding the distribution of ichnofabric (sedimentary rock fabric resulting from biogenic reworking) have only recently been attempted. Because ichnofabric can be recorded using a semi-quantitative series of ichnofabric indices (ii), its distribution in marine sedimentary rocks can be easily recorded through vertical sequence analysis. Thicknesses of strata recording different ichnofabric indices (from no reworking [ii1] to complete reworking [ii6]) can be logged from stratigraphic sections or cores. These data are best displayed in histograms as percent of each ii recorded from the tot l thickness measured. These ichnofabric histograms ("ichnograms") show variable but distinctive distributions for genetic units such as facies within systems tracts of siliciclastic depositional sequences. An average ichnofabric index for any genetic sedimentary unit can also be computed from the data used to construct ichnograms. Because skeletal fossils are typically much less commonly preserved in siliciclastic than carbonate depositional systems, such ichnofabric analyses have the potential of providing an important new line of evidence for depositional systems and sequence stratigraphic analysis of siliciclastic strata. In petroleum exploration results from completing analyses of ichnofabric distribution could provide important information including: (1) systems tracts with fine-gra ned facies that have relatively low ichnofabric values are potential source beds; and (2) petroleum reservoirs that occur in coarse episodically deposited beds are more likely to form in systems tracts with facies that have low rather than high ichnofabric values.

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)