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.
