Abstract

The Permian Rancho Rojo Sandstone (0 to 18 m thick), which crops out along 40 km of the southern edge of the Colorado Plateau, central Arizona, was first recognized as a marine sand wave deposit by Blakey (1979). It conformably overlies the fluvial Hermit Formation and grades upward into the marine Bell Rock Member of the Schnebly Hill Formation. We have defined three facies which are interpreted as having been formed in a transgressive estuarine to open marine succession. Facies 1: the basal Rancho Rojo Sandstone comprises giant-scale (3 to 12 m) cross-bed sets interpreted as being estuarine tidal sand wave deposits. Cross-bed foresets dip westward, reflecting sand wave migration related to the dominant tidal current that was probably reinforced by saline density currents. The hypersaline Holbrook Basin (Blakey, 1980) to the east was the likely source of the density currents. Where the giant foresets dip gently (< 17°), they contain east-dipping, tabular- to wedge-shaped sets of large-scale (5 to 80 cm) cross-beds formed by megaripples that climbed the sand wave lee-slope under the influence of the subordinate tidal current. Where giant foresets dip more than 17°, megaripples were inhibited and sand wave foresets consist of simple, tabular beds, 1 to 25 cm thick. The influence of tidal currents is recorded in these beds by pause planes lined with mud drapes, small mud intraclasts and mud intraclast/mud drape couplets. Facies 2: the overlying facies consists of large-scale (0.1 to 1 m) trough and tabular cross-beds that formed in a shallow marine environment established by continued transgression. Paleocurrents of this facies are strongly biomodal north and south, 90° rotated from the dominant east-west paleocurrents of the underlying facies. A similar relatonship exists today between tidal current directions from onshore (estuarine) and offshore areas in the North Sea. Facies 3: a ripple cross-laminated facies, which indicates further reduction of tidal energy in a deeper marine setting occurs at the top of the succession. This facies association may be generally representative of tide-dominated, transgressive, shallow marine sequences.