DEPTH AND INTENSITY OF BIOTURBATION IN CHANNEL DEPOSITS OF THE MISSISSIPPIAN-AGE MAUCH CHUNK FORMATION
The middle member of the Mauch Chunk Formation consists of a braided-ephemeral river deposit developed in a semiarid setting. The bioturbation in the channel was evaluated by the application of Bedding Plane Bioturbation Index (BPBI of Miller and Smail, 1997), a semiquantitative scale from 1 (no bioturbation) to 5 (60% to 100% bioturbation). The channel samples collected have a BPBI of 4 to 5. The intense bioturbation mixes the channel sand body up to 1.6 meters in depth. Other facies in the middle member of the Mauch Chunk Formation have BPBIs of 2 to 3. The channel may have developed either as a single or a multiple stage fill, and bioturbation, consisting of the trace fossil Planolites, has homogenized any evidence of the complex fill. The channel may have acted as a conduit, a high permeability zone that acted as a refuge for invertebrates to survive during periods of drought.
The Mauch Chunk channel thus indicates intensive subsurficial bioturbation in a nonmarine setting during the Mississippian Visean time, long before such bioturbation was thought to have occurred. Previous failure to recognize such bioturbation in upper Paleozoic deposits maybe due to a limited sampling of what was then a rare (but present) phenomenon that did not become widespread until the Mesozoic.