Eolian
strata of the Upper Cretaceous Capping Sandstone Member, Wahweap
Formation, Cockscomb Area, Grand Staircase-Escalante National
Monument, Utah
Simpson,
Edward L., Wolf, Hannah, Simpson, Wendy S., Tindall, Sarah,
and Jenesky,
Timothy, 2006, Eolian
strata
of
the
Upper
Cretaceous Capping Sandstone Member, Wahweap Formation, Cockscomb
Area, Grand Staircase-Escalante National Monument, Utah [abs]:
Rocky Mountain Section–58th Annual Meeting (17–19 May 2006),
Geological Society of America Abstracts with Programs, Vol. 38,
No.6, p. 6.
Eolian
processes
can be distinguished from subaqueous deposition by the presence of
centimeter-scale, inversely graded wind-ripple stratification.
Detailed examination of the capping sandstone member of the Wahweap
Formation in the Cockscomb area in Grand Staircase and Escalante
National Monument revealed the presence of eolian stratification.
This eolian stratification is the first occurrence reported from the
Upper Cretaceous age strata of Utah.
In measured sections, wind-ripple stratification, composed of
inversely graded fine- to medium-grained sandstone, is present as
(1) filling within lenticular scours that cap ephemeral fluvial
bars, and (2) trough cross bed sets that record the presence of
small dune fields between the braided river channels. Lenticular
wind-ripple stratification sets, capping bars are at most 20 cm
thick, indicating reworking by highly variable winds during
prolonged exposure of the fluvial bar tops.
Stratification from small dunes occurs at two stratigraphic heights
in the section north of the Gut Road. Both occurrences record the
migration of the dune field across the braided stream deposits. In
the uppermost occurrence of the section, the thickness of
wind-ripple trough sets changes vertically from 15, 32, 25 to 20 cm,
recording the encroachment of the dune field margin. Laterally,
these thin trough sets are truncated by a 2 m thick set of
wind-ripple strata containing cyclic wind-ripple stratification.
During this time, wind was trending to the southeast. The thick
trough set indicates the maximum extent of the dune field and
records the migration of barchanoid dune types. Cyclic wind-ripple
stratification in the main dune field may record annual
fluctuations. Braided stream deposits truncate the eolian strata.