A NEW PERMIAN VERTEBRATE TRACKWAY SITE IN THE DUNKARD GROUP OF THE APPALACHIAN BASIN

A new tetrapod trackway site in the Appalachian Basin has yielded at least five ichnogenera within the Marietta Sandstone member of the Washington Formation, a part of the Pennsylvanian to Permian Dunkard Group. Preliminary identification of specimens that have numerous consecutive prints forming trackways includes the ichnogenera Limnopus, Dromopus, Dimetropus, Batrachichnus, and Laoporus. Numerous stray prints that are not part of distinct trackway sets are also present, although difficult to ascribe to a genus. The tracks are convex casts on the bases of successive layers of thinly bedded, fine grained sandstone that are discontinuous and each being up to several centimeters thick. The sandstone is interbedded with thin layers of shale on the order of one centimeter thick. The sandstone must be pried up to reveal the prints as casts on the underside of the slabs. The sandstone slabs also display mudcrack casts, raindrop impressions, and have numerous fern frond impressions. The overall succession of thinly interbedded sandstone and shale is as much as a meter thick and covers an area of 30 meters along the outcrop and back 3-4 meters along an excavated bench, before extending beneath overlying strata, a fortunate result of recent road improvements. This succession is the top-most meter of a large, complex ten meter thick sandstone channel fill that makes up the Upper Marietta Sandstone. Overlying strata consist of a thick succession of shales interpreted as floodplain deposits that are primarily paleosols and red beds with thin, discontinuous greywackes. The trackways are interpreted to have been preserved in successive fining upward flood or splay deposits. The sandstone casts are the initial sands at the base of a splay deposit that filled trackways impressed into fine muds capping the previous splay. This apparently happened during the final filling of the large erosional channel scour making up the Marietta Sandstone that formed during a significant downcutting event, likely cuased by a drop in base level. The channel subsequently filled with sands. Trackways are preserved in the transition from the coarse grained channel fill to the finer grained deposits of floodplain deposition as erosional topography was filled, perhaps because of a subsequent rise in base level.