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.
