Sedimentology and paleontology of late
Cretaceous sag ponds, Upper Member, Wahweap Formation, Grand
Staircase-Escalante National Monument, Utah
Simpson, Edward L.,
Koch,
Robert, Heness, Elizabeth A., Wizevich,
Michael C., Tindall, Sarah E., Hilbert-Wolf, Hannah
L., Golder, Keenan B., and Steullet, Alex, 2013, Sedimentology and
paleontology of late Cretaceous sag ponds, Upper Member, Wahweap
Formation, Grand Staircase-Escalante National Monument, Utah: Geological Society of America National Meeting,
Denver, Colorado (27–30 October, 2013).
During development of the East Kaibab monocline, listric normal
faulting influenced sedimentation in the Late Cretaceous Wahweap
Formation recording the initial phases of the Laramide Orogeny.
Located along the East Kaibab monocline, are deposits of two sag
ponds that document seismogenic fault movement and disruption of
local drainage systems. Ancient sag pond deposits are likely
under-identified in the rock record; this study demonstrates their
significance and potential for unraveling fault histories.
At the northernmost syndepositional normal fault, the deposits of a
small sag pond are located at the boundary between the upper and
capping sandstone members of the Wahweap Formation. Pond infill
consists of laminated, gray mudstones and siltstones intruded by
sandstone dikes and sills. Adjacent to the southernmost
syndepositional normal fault, a larger and thicker sag pond deposit
is located near the base of the upper member. This southern older
sag pond preserves gray, seemingly structureless siltstones and
mudstones. The simple fine-grained fill is cut by brittle faulting
along the fault-side margin of the southern pond in contrast with
the pervasive clastic intrusions in the younger sag pond. The
geographic positions and thicknesses of the deposits suggest that
the older more southern sag pond has a protracted history of
extension, in comparison with the more northern sag pond locality.
The younger, northern sag pond deposit preserves small macerated
flora, but no discernable invertebrate fauna. Either the pond was
too small, was not active for long, or sedimentation rates were too
high (or a combination of factors) to sustain a well-developed
ecosystem. The older, larger sag pond fill comprises a series of
fossil horizons consisting of juvenile gastropods, Lioplacodes,
Planorbis, and Vivaparis and unionid bivalves, Unio, along with
unidentifiable macerated plant fragments. The immature fossil
assemblages suggest that the older, southern sag pond experienced
frequent significant sedimentation events that prevented the
invertebrate fauna from reaching maturity punctuated by periods of
low sedimentation rates. It is likely that fault movements triggered
these rapid catastrophic sedimentation events, but significant
precipitation events cannot be ruled out.
Sedimentology and paleontology of late
Cretaceous sag ponds, Upper Member, Wahweap Formation, Grand
Staircase-Escalante National Monument, Utah