Sag
pond development at the Upper and Capping Sandstone Member
contact, Upper Cretaceous Wahweap Formation, Grand Staircase –
Escalante National Monument, Utah
Wolf, Hannah
L., Simpson,
Edward L., Bernard, Jonathan J., Tindall,
Sarah E., Simpson, Wendy S., and Jenesky, Timothy A., 2007, Sag
pond
development
at
the
Upper and Capping Sandstone Member contact, Upper Cretaceous
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. 12.
The
transition from the upper to the capping sandstone member of the
Upper Cretaceous Wahweap Formation is marked by an abrupt change in
petrology from lithic to quartz sandstones and a shift in
paleocurrents from northeast to southeast modes. At Bull Flat in
Grand Staircase -Escalante National Monument, examination of this
contact near the tip line of a probable normal fault revealed the
presence of a preserved sag pond filled with seismically disturbed
strata. Sag ponds develop along the surface expression of active
faults and are characterized by a limited areal extent, fine-grained
fill and localized internal drainage. At the Bull Flat locality, sag
pond deposits are characterized by carbonaceous mudstone and
siltstone with interbedded lenses of fine- to medium-grained
sandstone. The deposit can be traced for approximately 100 meters
and reaches a maximum thickness of 5.0 meters. The carbonaceous
mudstones and siltstones appear massive with rare, faint, diffuse,
folded laminations. The sandstone lenses pinch and swell in
thickness with irregular bases and tops and are best interpreted as
sedimentary sills. Vertical dikes, containing inclusions of the
mudstone, connect sills to the top of the sandstone underlying the
sag pond deposit. The contact between the mudstone fill within the
sag pond and the underlying sandstone is convolute. At least two
seismic events occurred to generate the observed sequence. The
initial event tilted underlying strata and produced the sag pond.
The second event generated the dikes and sills, probably sourced
from the underlying sandstone. The overlying capping sandstone
member contains ripped up clasts of the sag pond and is
characterized internally by convolute bedding. The generation of the
convolute bedding may be related to the second seismic event that
generated the dikes and sills or may be a discrete younger event.