Taphonomic
controls on the preservation of vertebrate tracks in
ephemeral-braided river deposits of the Middle and Upper Members
of the Mississippian Mauch Formation, Eastern Pennsylvania
Smith, Casey
J., Moran, Kelli L., Fillmore, David L., Simpson,
Edward
L., and Lucas, Spencer G., 2009, Taphonomic
controls
on
the
preservation
of vertebrate tracks in ephemeral-braided river deposits of the
Middle and Upper Members of the Mississippian Mauch Formation,
Eastern Pennsylvania [abs.]: 2009 Portland GSA Annual Meeting
(18-21 October 2009), Geological Society of America Abstracts with
Programs, Vol. 41, No. 7, p. 119.
The
middle
member of the Mississippian/Visean Mauch Chunk Formation that crops
out in eastern Pennsylvania has produced a diverse assemblages of
both invertebrate and vertebrate trace fossils that span a critical
juncture in the evolution of terrestrial ecosystems. Analysis of the
facies succession from the middle to upper member indicates that
these deposits developed in an ephemeral-braided stream shifting to
a more perennial setting. Within this depositional system, the
presence or absence of the mudstone-draped ripple facies is the
factor that controls the taphonomic filtering of this important
paleoecosystem.
Fining-upwards, stacked channel sequences characterize the middle
member. Vertically, the channel fills are characterized by: 1)
scoured base, 2) either medium-grained, trough cross-bedded or
structureless sandstones with intense bioturbation, 3)
intermittently occurring parallel laminated sandstones, 4) ripple
deposits with various climb angles commonly preserving bedforms with
mudstone drapes increasing in abundance upwards and the preservation
of rain drop impressions, mud cracks, roots, as well as vertebrate
footprints including Palaeosauropus and Batrachichnus, and 5)
mudstones with pedogenic Bk horizons. This vertical facies
succession is best interpreted as the product of an
ephemeral-braided stream system. In the upper member, the coarsest
sediment size is gravel. Channel-fill bases are deeply cut and
filled with several upward-fining sequences that are characterized
by massive, clast-supported or trough cross-bedded conglomerate,
overlain by trough cross-bedded sandstone that fines upward. Tops of
smaller-scale fining-upward cycles may be overlain by mudstone. The
channel sequences are capped with massive mudstones that lack, or
have weakly developed, Bk horizons. Vertebrate tracks are not
present where the mudstone-draped ripple facies are not present or
well developed. This depositional system is best interpreted as a
higher-gradient braided stream system with mud sedimentation
restricted to overbank flooding events. Our study indicates that
terrestrial mudstone-draped sandstone should be explored extensively
to discover ichnofossils that fill gaps in our understanding of
vertebrate and invertebrate evolution.