EVIDENCE FOR CHANGES IN DRAINAGE PATTERNS AND TECTONIC ACTIVITY AT ~19 MA IN THE OLIGOCENE-MIOCENE RAINBOW GARDENS FORMATION IMMEDIATELY NORTH OF THE GOLD BUTTE FAULT, SOUTHERN NEVADA

The Lake Mead domain of the central Basin and Range province provides an excellent example of large-scale continental extension. There are many proposed models that attempt to explain the magnitude and timing of extension but with no consensus. The Rainbow Gardens Formation of the Lake Mead domain is understood to largely represent deposition during a period of tectonic quiescence, prior to the beginning of widespread extension at ~17 Ma. Recent work focused in the southern portion of the Rainbow Gardens basin has documented a distinct crystalline pebble-conglomerate that interrupts the overall fining upward trend of the Rainbow Gardens Formation, and contains Proterozoic clasts. The crystalline pebble-conglomerate is first observed in the southwestern portion of the basin at ~25 Ma, when deposition began, and progrades north and east into the basin at ~19 Ma. Above the ~19 Ma crystalline pebble-conglomerate, initial sedimentation rates increase and are suggestive of syn-depositional faulting. Presence of this crystalline pebble-conglomerate suggests changes in drainage patterns that may be related to initial extensional foundering of the Kingman Uplift and/or uplift of the Proterozoic Gold Butte-White Hills fault block south of the basin.

Initial detrital zircon data and sandstone petrography confirm ongoing studies that sediment for the majority of the Rainbow Gardens Formation were recycled from Paleozoic and Mesozoic strata in the surrounding Sevier and Laramide-age uplifts and directly from on-going volcanism to the north; detrital zircon ages cluster at 1000-1200 Ma (Grenville) and 27-17 Ma (Great Basin ignimbrite province). Detrital zircon ages for the crystalline pebble-conglomerate have age peaks at 90-110 Ma (Cordilleran Magmatic Arc) and 1650-1750 Ma (Yavapai-Mazatzal), with a minor population of Great Basin ignimbrite province ages (36-17 Ma), confirming a different source terrain during deposition. Evaluation of growth structures in the Rainbow Gardens Formation and Ar/Ar dating of tuffs are in progress to evaluate faulting as part of the explanation for the changes at ~19 Ma and to confirm increases in sedimentation rates after ~19 Ma. These results will be used to help redefine the timing of deformation, paleogeography, and drainage patterns of the Rainbow Gardens basin.