THE PRESENT IS THE KEY TO THE PAST...AS LONG AS THE PAST LOOKED LIKE THE PRESENT: BAYHEAD DELTAS AND BACK-BARRIER SYSTEMS IN TIME AND SPACE (Invited Presentation)

A survey of modern and ancient back-barrier deposits reveals a diversity of depositional styles, processes, lithotypes, and ichnocoenoses. The modern Galveston-Trinity Bay (GTB) is a classic example of a microtidal, humid subtropical, siliciclastic back- barrier complex and has been “the model” for back barrier systems for decades. While some ancient systems are similar (e.g., Paleocene Ferris and Hanna formations in southern WY), others differ. These examples provide a range of depositional models for different tectonic, climatic, and tidal settings.

The Campanian Neslen Formation (NF) of the Book Cliffs, UT, accumulated in a low-accommodation setting with humid, subtropical climate like GTB and contains bioturbated, sandy-to-muddy heterolithic deposits indicating micro-to-mesotidal conditions in the Cretaceous epeiric sea. Bayhead delta deposits contain inclined heterolithic stratification (IHS) recording the interplay of variable fluvial discharge and tidal modification. In contrast to GTB, highly heterolithic distributary channel deposits, also dominated by IHS, are associated with delta deposits. The NF is a good model for bayhead delta deposits of mud-rich, tidal, back-barrier systems in epeiric seas.

The Oxfordian Windy Hill Member of the Morrison Fm, southern WY was deposited as intertidal flats and tidal creeks behind transgressive barrier bars and coastal sand dunes in a shallow, epeiric sea surrounded by semiarid fluvial and eolian systems. Clay is virtually absent, and the dominant lithology is very fine-grained, ripple-laminated to cross bedded, sheetlike sandstone with some silt. Major down-cutting of fluvial channels to create valleys like those on the modern Texas coast did not occur because there was no shelf-slope break in the basin to serve as a nick point origin for landward propagation of incision.

The Pennsylvanian (Desmoinesian) Upper Strawn Group deposited along the Eastern Shelf of the Permian Basin, TX, contains strata of mixed carbonate/clastic systems. Bayhead deltas and associated back-barrier deposits preserve the interfingering of clastic feeder systems and ramp carbonates emplaced on a shallow-dipping, tidal shelf during high-frequency, high-amplitude sea-level fluctuations, providing a unique perspective on characteristics of mixed tidal systems.