DEPTH AND INTENSITY OF BIOTURBATION IN CHANNEL DEPOSITS OF THE MISSISSIPPIAN-AGE MAUCH CHUNK FORMATION

Complex terrestrial communities, represented by significant subsurficial bioturbation, are reported to have evolved in the early Mesozoic Era. This is thought to reflect the fact that the substrate ecospace was the last to be exploited, including depth of burrowing. However, recent studies of the Mississippian-age middle member of the Mauch Chunk Formation have revealed an intensely bioturbated fine-grained sandstone preserved in an ancient channel fill. This new information opens up for discussion the intensity and depth of bioturbation in early (late Paleozoic) terrestrial ecosystems.

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.