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.
