Sedimentology and depositional history of the late Cenozoic Gila Group in the central Duncan Basin, Greenlee County, southeast Arizona




Reid, Bradley Hamilton

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This study documents the sedimentation and depositional history of the Upper Pliocene to Lower Pleistocene Upper Gila Group basin fill in the central Duncan basin, Greenlee County, southeast Arizona. The north-northwest trending intermontane Duncan basin is uniquely situated near the junction of the southern Basin and Range, Transition Zone, Colorado Plateau, and Rio Grande rift physiographic provinces, all of which had some influence on the structural evolution of the slightly asymmetric basin. Forty-one stratigraphic sections, mostly the Upper Gila Group, located between the towns of Duncan and Three Way, Arizona, were measured and described to determine a facies classification of the basin fill deposits. Seven depositional facies were recognized. Their lateral and vertical distribution within measured sections and mapped areas were the basis for defining six significant depositional elements used to reconstruct the Plio-Pleistocene depositional history and paleogeography of the central Duncan basin. The six depositional elements include: streamflood-dominated alluvial fans, distal sheetflood alluvial fan, axial braided fluvial, floodbasin, lacustrine, and high basin stand deposits. The streamflood-dominated alluvial fans are prominent along the southwest margin of the central Duncan basin, and are characterized by four distinct fan associations that were all locally sourced from the Peloncillo Mountains to the west. These fans generally prograded east and onlapped and interfingered with an areally extensive floodbasin that occupied much of the central Duncan basin. This floodbasin acted as a catchment area for the fine-grained sediments sourced from the alluvial fans systems along the basin margins. Parts of the floodbasin near Duncan were occupied intermittently by extensive freshwater lakes with benthic conditions that supported diatom growth. A prominent distal sheetflood alluvial fan occupied the northern part of the field area. It represents the distal portion of a prominent southeast prograding axial alluvial fan system sourced from the elevated Morenci block at the head of the basin. An axial braided fluvial system also was sourced from the Morenci block and flowed southeast along the southwest margin of the basin and dispersed into the floodbasin. Deposition of the Gila Group basin fill terminated with the deposition of dominantly alluvial fan deposits locally sourced from the northeastern basin margin. These gravel deposits cap high mountain-front mesas in the Duncan basin and are similar to terminal basin fill deposits in other basins that are known regionally as the high basin stand (HBS). They were deposited just prior to and/or during entrenchment by the modem Gila River between 2.0 - 1.0 Ma, and are a coarse-grained response to a climatic and/or a tectonic shift during the latest Pliocene and early Pleistocene. A Blancan-aged horse molar, Equus simplicidens, found in the Upper Gila Group, permits stratigraphic correlation to other nearby studies, and suggests that the Gila Group deposits in the central Duncan basin are primarily late Pliocene to early Pleistocene (3.7 - 1.0 Ma)