Possible
debris flows in the Upper Cretaceous Capping Sandstone, Wahweap
Formation, Grand Staircase – Escalante National Monument, Utah
Bernard,
Jonathan J., Simpson,
Edward
L., Wolf,
Hannah L., Simpson, Wendy S., Tindall, Sarah E.,
and Jenesky,
Timothy J., 2007, Possible
debris
flows
in
the
Upper Cretaceous Capping Sandstone, Wahweap Formation, Grand
Staircase – Escalante National Monument, Utah [abs]: Rocky
Mountain Section - 59th Annual Meeting (7–9 May 2007), Geological
Society of America Abstracts with Programs, Vol. 39, No. 5, p. 42..
The capping sandstone member of the
Late Cretaceous Wahweap Formation is interpreted as a braided stream
system deposit with limited preservation of overbank fines. At
Henrieville Creek and Bull Flat localities, pebbly conglomerate and
pebbly sandstone deposited in various types of braid bars
characterize the middle part of the section. However, within the
upper portion of the conglomerate zone in both sections, graded and
sandy siltstone to pebbly siltstone horizons occur, and these are
best interpreted as debris flow deposits.
The debris flow deposits extend laterally for up to 100 meters
before being truncated by the overlying channel complexes. The
preserved thickness is up to 70 cm with a basal relief of up to 30
cm. Internally the bed fines upward from a pebbly sandy siltstone
with ripped up clasts of conglomerate, pebbly sandstone and
sandstone from the underlying bed, to a pebbly siltstone. Dewatering
structures are present at the base and top of some beds. Some tops
of beds are rooted with possible burrows. Within one bed at the Bull
Flat locality two depositional units are recognizable.
A debris flow interpretation is supported by: 1) the lack of
internal primary structures, 2) floating pebbles in a sandy
siltstone matrix, 3) ripped up clasts of hydrodynamically heavier
particles, 4) dewatering structures at the base and tops on some
beds and 5) weak normal grading. The scoured bases of some debris
flows indicate that some flows were initially turbulent followed by
matrix-supported flow. Dewatering structures indicate rapid
sedimentation and fluid expulsion. These debris flows most likely
followed topographic lows, such as channels between braid bars.
Superposition of two flows and rooting and burrowing at the tops of
flows indicate that a significant time span occurred between
truncation by younger fluvial processes.