THE STONY GAP SANDSTONE EXISTENTIAL CRISIS: PROBLEMS WITH USING CHANNEL SANDSTONES TO DEFINE FORMATION CONTACTS

Facies of the Middle to Upper Mississippian Mauch Chunk Group represent the evolution of a meandering fluvial system, deposited along a passive margin in southern West Virginia. In this region the Mauch Chunk is divided into four distinct formations: the Bluestone, Princeton Sandstone, Hinton, and Bluefield Formations, in vertical succession. Formation contacts are defined by prominent channel sandstone members. The Stony Gap Sandstone, a quartz arenite at the base of the Hinton, forms the contact with the underlying Bluefield. Recent 24k scale bedrock mapping in southeastern West Virginia suggests delineation of the formations should not be contingent on the presence of a channel sandstone due to its laterally discontinuous nature. Field mapping, cross-sections, and measured stratigraphic sections provide a means to: 1) highlight difficulties in using channel sandstones to define formation contacts, and 2) reassess the stratigraphic boundary between the Hinton and underlying Bluefield based on facies analysis. In areas where the Stony Gap is absent, red shale/mudstone or a different sandstone lithology is present instead, and the Hinton and Bluefield cannot be differentiated. Another problem is older channel sandstones, like the Droop Sandstone (Bluefield Formation), may represent the same channel as the Stony Gap, just an earlier channel migration path. Channel sandstone facies consist of pure-white, cross-bedded, lenticular-bodied quartz arenites. Floodplain facies consist of finer-grained, red micaceous mudstones, shales, and siltstones, often with fossil root casts. Marine flooding surfaces are represented by fissile noncalcareous to calcareous shales with some fossiliferous layers. Facies analysis shows the Stony Gap may be absent in some sections simply because there was no active channel in the area. Red mudstone and shale represent floodplain sediments, deposited adjacent to the active channel. Previous field geologists held the notion that rock units are laterally continuous, however, recent work shows that members of the Mauch Chunk adhere to Walther’s Law, representing a series of laterally and vertically adjacent depositional environments in a meandering fluvial system. A formal revision of the Mauch Chunk Group stratigraphy and formation boundaries is recommended for future work.