Tectonic
control on the alternating fluvial style in the Late Cretaceous
Upper Member of the Wahweap Formation, Southern Utah
Wizevich, Michael C., Simpson,
Edward
L., Tindall,
Sarah E., Bernard, Jonathan, Wolf, Hannah L., Simpson, Wendy S.,
Storm, Loran
P., and Paese, Steven, 2008, Tectonic
control
on
the
alternating
fluvial style in the Late Cretaceous Upper Member of the Wahweap
Formation, Southern Utah [abs]: Cordilleran Section (104th
Annual) and Rocky Mountain Section (60th Annual) Joint Meeting
(19–21 March 2008), Geological Society of America Abstracts with
Programs, Vol. 40, No. 1, p. 42.
The
Wahweap
Formation in Grand Staircase–Escalante National Monument, Utah
consists of lower and middle members that contain isolated fluvial
sandstone units encased in floodplain mudstone, an upper member of
amalgamated sandstone units with little mudstone, and the capping
sandstone member, which contains sandstone and conglomerates with
very little mudstone. The sequence reflects a transition from
deposition by mixed-load meandering streams to that of
bedload-dominated braided streams; however the transition is not
simple. Paleocurrents and lithology of the capping sandstone member
indicate a change in source area from the south (upper member) to
west. Furthermore, detailed analysis of the 115 m thick upper member
near the Gut, revealed multistoried, inclined heterolithic units
alternating with three thick sandstone units, which are interpreted
as meandering and braided stream deposits, respectively.
Changes in fluvial style, and thus the form and architecture of
channel deposits, are either a response to external controls,
tectonic, base level and climate changes, or they can be related to
the inherent geomorphic variability of the river system (e.g.,
avulsions). In the upper member, deep (several meters) incision of
thick sandstone units into heterolithic units suggests external
controls are responsible for changes in fluvial style. In addition,
the study area is located close to a series of normal faults that
were apparently active during deposition of the upper member. Highly
distorted sediments (seismites) and preserved sand volcanoes and
dikes indicate intense high-magnitude seismic activity, and suggest
that the primary control on deposition was related to fault
activity.
Cyclic alternations of thick sandstone units and multistoried
heterolithic bodies represent pulses of sedimentation, a result of
tectonic movements of nearby faults. Individual cycle reflect the
development of a high-gradient braided alluvial-fan system, which
ultimately changed to a low-gradient alluvial plain before fault
reactivation led to the next cycle.