Depth and Intensity of Bioturbation in
Channel Deposits of the Mississippian-Age Mauch Chunk Formation
Smith, Casey
J., Fillmore, David L., Simpson, Edward, and Lucas,
Spencer G., 2010, Depth and Intensity of Bioturbation in Channel
Deposits of the Mississippian-Age Mauch Chunk Formation [abs]:
Geological Society of America Northeastern Section (45th Annual)
and Southeastern Section (59th Annual) Joint Meeting (13-16 March
2010).
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.