THE IMPORTANCE OF INTEGRATING SEDIMENTOLOGY AND ICHNOLOGY TO ACCURATELY INTERPRET THE SPATIAL-TEMPORAL RELATIONSHIP OF MIXED CARBONATE-CLASTIC COASTAL-SHELFAL DEPOSITIONAL SYSTEMS, PENNSYLVANIAN (DESMOINESIAN) STRAWN GROUP, TEXAS

Coastal-shelfal deposits of the Strawn Group found along the Eastern Shelf of the Permian Basin and Fort Worth Basin, Texas, are those of mixed carbonate-clastic systems that commonly experienced tidal modification. The complexity of facies, facies relationships, and stratal stacking in these mixed-tidal systems requires careful examination to interpret depositional environments and identify the paleo-position along a shallow-dipping shelf to ramp setting. This complexity and variability also applies to the ichnologic assemblage, which varies greatly over space and time.

An excellent example of the usefulness, and true necessity, of careful examination of both the facies and ichnology of deposits is found in the Pennsylvanian (Desmoinesian) Upper Strawn Group. Because of the juxtaposition of deltaic clastic point sources and crinoid-rich carbonate communities, delta-lobe switching, the presence of a low-angle extensive shelf, and high-frequency high-amplitude sea-level fluctuations, the accurate interpretation of paleoenvironments, paleoenvironmental evolution, and tracking the location of shorelines over time can be extremely difficult.

We provide an example of Upper Strawn Group deposits from Stonewall and King counties, Texas, that contain interbedded and juxtaposed quartz-rich, carbonate-rich, and sandy-muddy heterolithic deposits that, in the past, have proven difficult to interpret. We highlight intervals of the stratigraphy using core imagery that differ slightly in composition, sedimentary structures, and the type, diversity, and intensity of bioturbation. Close examination of the vertical and lateral variability of facies and ichnology allowed for the identification of bayhead delta to tidal deltaic depositional systems, back-barrier and bay deposits, shorefaces, and reef-marginal environments that interfinger with clastic deposits. The interplay of eustatic variability, tides, and fluctuations in discharge and location of clastic point sources are likely responsible for the evolution of depositional systems and paleoenvironments recorded in the Upper Strawn Group stratigraphy.