Late Oligocene to Miocene sedimentary strata of the Lake Mead, Nevada, region hold evidence that can test models of central Basin and Range extension and tectonic evolution and illuminate paleodrainage systems related to the carving of the Grand Canyon. In this region, Sevier contraction and a period of relative tectonic quiescence predate Miocene extension and magmatism. The herein revised Rainbow Gardens Formation, formerly the oldest member of the Horse Spring Formation, is found throughout the Lake Mead area and spans the transition from quiescence to the start of extension. New detailed stratigraphic work within the Rainbow Gardens Formation, including paleocurrent analyses, stable isotope geochemical data, new 40Ar/39Ar dates, tephrochronologic data, and ongoing structural studies, was used to build a detailed chronostratigraphic framework and reconstruct the Rainbow Gardens basin through time. Our results show that the basin formed by a combination of inherited topography and tectonic events outside the Lake Mead region prior to ca. 25 Ma. Sedimentation rates, sedimentary facies patterns, and the lack of evidence for syndepositional faulting indicate that the basin did not form during extensional faulting, although recent work suggests that this basin was modified by uplift to the south between 18 and 20 Ma. Initially, the basin recorded fluvial sedimentation within the center of an E-NE-trending inherited valley, with two facing gravel-dominated bajadas on either side. As the valley filled, rivers from the Caliente volcanic field to the north delivered an influx of volcaniclastic material into a broad area of marshes and ephemeral lakes. In the final stages of sedimentation, just prior to the onset of extension at ca. 17 Ma, the basin filled with limestone in a series of shallow lakes. Understanding the development and evolution of this basin is crucial for testing models of extension as recorded by the younger Horse Spring Formation. In addition, the detailed sedimentology and stratigraphy place constraints on models of the formation of the Grand Canyon and suggest that a paleo-Colorado River did not deposit sediment in the Lake Mead area between 25 and 17 Ma. This work highlights the importance of detailed basin analyses, which are crucial companions to structural, tectonic, and geomorphologic studies aimed at understanding the complex evolution of lithospheric and landscape changes across a large region.
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