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.
