TAPHONOMIC CONTROLS ON THE PRESERVATION OF VERTEBRATE TRACKS IN EPHEMERAL-BRAIDED RIVER DEPOSITS OF THE MIDDLE AND UPPER MEMBERS OF THE MISSISSIPPIAN MAUCH FORMATION, EASTERN PENNSYLVANIA

The middle member of the Mississippian/Visean Mauch Chunk Formation that crops out in eastern Pennsylvania has produced a diverse assemblages of both invertebrate and vertebrate trace fossils that span a critical juncture in the evolution of terrestrial ecosystems. Analysis of the facies succession from the middle to upper member indicates that these deposits developed in an ephemeral-braided stream shifting to a more perennial setting. Within this depositional system, the presence or absence of the mudstone-draped ripple facies is the factor that controls the taphonomic filtering of this important paleoecosystem.

Fining-upwards, stacked channel sequences characterize the middle member. Vertically, the channel fills are characterized by: 1) scoured base, 2) either medium-grained, trough cross-bedded or structureless sandstones with intense bioturbation, 3) intermittently occurring parallel laminated sandstones, 4) ripple deposits with various climb angles commonly preserving bedforms with mudstone drapes increasing in abundance upwards and the preservation of rain drop impressions, mud cracks, roots, as well as vertebrate footprints including Palaeosauropus and Batrachichnus, and 5) mudstones with pedogenic Bk horizons. This vertical facies succession is best interpreted as the product of an ephemeral-braided stream system. In the upper member, the coarsest sediment size is gravel. Channel-fill bases are deeply cut and filled with several upward-fining sequences that are characterized by massive, clast-supported or trough cross-bedded conglomerate, overlain by trough cross-bedded sandstone that fines upward. Tops of smaller-scale fining-upward cycles may be overlain by mudstone. The channel sequences are capped with massive mudstones that lack, or have weakly developed, Bk horizons. Vertebrate tracks are not present where the mudstone-draped ripple facies are not present or well developed. This depositional system is best interpreted as a higher-gradient braided stream system with mud sedimentation restricted to overbank flooding events. Our study indicates that terrestrial mudstone-draped sandstone should be explored extensively to discover ichnofossils that fill gaps in our understanding of vertebrate and invertebrate evolution.