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.
