EOLIAN STRATA OF THE UPPER CRETACEOUS CAPPING SANDSTONE MEMBER, WAHWEAP FORMATION, COCKSCOMB AREA, GRAND STAIRCASE-ESCALANTE NATIONAL MONUMENT, UTAH

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.