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Modern and Ancient Alluvial Systems
Sedimentology and Depositional Setting of the Upper Proterozoic Scanlan Conglomerate, Central Arizona
The Upper Proterozoic Apache Group crops out throughout central Arizona. Radiometric dating of underlying granites and intrusive diabase sills bracket deposition between 1.45 and 1.15 b.y. The basal unit of the Apache Group, the Scanlan Conglomerate, unconformably overlies older Precambrian igneous and metamorphic rocks. Relief on the basement is as great as 30 m and defines paleovalleys into which Scanlan sediments were shed.
Facies analysis of the Scanlan indicate deposition in an alluvial fan/proximal braided stream complex. Two generalized facies sequences are common. Facies sequence 1 comprises interbedded matrix-supported, disorganized conglomerate and crudely to horizontally bedded clast-supported conglomerate which represent, respectively, debris flows on alluvial fans and in-channel sheet deposits and longitudinal bars. Facies sequence 2 consists of clast-supported, imbricate conglomerate and large sets of planar-tabular cross-bedded conglomerate. Associated facies include trough cross-bedded conglomerate and cross-and horizontally stratified sandstone. The second facies sequence formed in a proximal braided fluvial system and reflects in-channel deposition and both longitudinal and transverse bar development. Paleocurrent and mineralogic data indicate a south-sloping paleoslope away from a large Precambrian highland. Local variations in flow are related to structurally controlled paleovalleys.
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