Abstract

The Moenave Formation (Dinosaur Canyon Member) and the closely associated Wingate Sandstone in southern Utah represent the southwestern erg margin of the Lower Jurassic Wingate erg. The erg-margin deposits, which extend across a 50- to 100-km-wide region, are composed of dune, eolian sand-sheet, mud-flanked eolian, sabkha, ephemeral-stream and lacustrine facies. The succession is divided into a basal erg-margin association (8-27 m) dominated by eolian facies and an overlying erg-margin association (as much as 90 m thick) composed mainly of fluvial and lacustrine facies.

The sediments of the basal association are composed of three drying-upward or locally drying-wetting-upward sequences that can be traced from the erg margin 100 km slightly upwind (that is, toward the west-northwest) into the outer erg-margin zone. Sequences of the inner erg-margin zone contain dune facies in their upper parts, sequences in the intermediate erg-margin zone contain sand-sheet facies, and those in the outer erg-margin zone contain sabkha facies. The basal association thins markedly toward the west-northwest.

Sediments of the overlying association contain up to six drying-upward or drying-wetting-upward sequences but are apparently only fully preserved in the outer erg-margin zone. Individual sequences show much lateral variation but characteristically contain lower, muddy, ephemeral-stream deposits and upper, sandy, ephemeral-stream deposits; sand-sheet deposits; or incipientdune deposits.

The general time trend is interpreted as a gradual change toward a more humid climate. The stacked drying-upward or drying-wetting-upward sequences suggest that this long-term climatic change was characterized by a number of fluctuations between relatively arid and humid conditions. These climatic fluctuations apparently controlled both sediment yield to the basin and the depositional conditions in the erg-margin region. During arid intervals, ephemeral braided streams transported sandy bed-load into the erg-margin area, where eolian reworking took place. During humid intervals, sheet-floods and high-sinuosity rivers transported a more mud-rich sediment load into the basin, where eolian activity was minimal.

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