Abstract

The Permian Rancho Rojo Member of the Schnebly Hill Formation in central Arizona displays distinctive large-scale cross stratification in compound sets that are several meters thick and tens of meters long. Based on geometry, stratigraphic sequences, sedimentary structures, and the processes which formed these features, the Rancho Rojo is interpreted as a submarine or estuarine sand wave complex. Modern sand wave processes produce large-scale intraset cross-stratification and superimposed dunes which form complex internal sedimentary structures like those of the Rancho Rojo. The Rancho Rojo is subdivided into five facies. The planar-tabular foreset facies is composed of giant westward-dipping foresets that probably formed in large, steep, strongly asymmetrical sand waves. The intraset cross-stratified facies comprises complex compound cross stratification bounded by westward-dipping planes of erosion. It was deposited by large, less asymmetrical sand waves with superimposed dunes on the surface. The trough cross-stratified facies consists of complex trough-shaped intrasets. The internal geometry of this facies probably resulted from deposition in complex, nearly symmetrical sand waves that responded to oscillatory tidal currents. The ripple-laminated sand facies and homogeneous sand facies comprise ripple-laminated and bioturbated sandstone that formed during waning sand-wave deposition. Although the facies are complexly interbedded, their overall stratigraphic succession from oldest to youngest typically is similar to the order just presented. This suggests that through time large, asymmetrical, rather simple sand waves were succeeded by more complex, lower, less asymmetrical sand waves due to change of current regime from unidirectional to bidirectional. This interpretation of a sand body that was once thought to be part of a continental redbed sequence, requires revision of paleogeographic reconstruction for part of the Permian of central Arizona.

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