Abstract
Diamictite-bearing strata present in Neoproterozoic successions in the Death Valley area of southeastern California, northern Utah, and southeastern Idaho, USA record glacialmarine sedimentation. Though no radiometric dates have been obtained from these rocks, lithostratigraphic correlations suggest that they were deposited during the Sturtian (Rapitan) glacial epoch. These strata were deposited within and along the margins of a number of differentially subsiding basins that are inferred to have developed between 780 and 730 Ma, during an episode of rifting that preceded the formation of a passive continental margin in latest Neoproterozoic or early Cambrian time.
Three diamictite-bearing associations of sedimentary facies are recognized. The massive diamictite association is interpreted to represent rain-out and/or redeposition by sediment gravity flow, of debris derived from a partially floating or disintegrating ice sheet, perhaps near its grounding line, in the absence of bottom currents. Two bedded diamictite-bearing facies associations are recognized. The stratified diamictite and graded sandstone association includes heterogeneous diamictite, graded sandstone, bedded conglomerate and grit, as well as fine-grained strata including rhythmite and dropstone-bearing laminite. It is interpreted to include subwave base sediment gravity flow deposits, in part redistributed by thermo-haline currents, with a variable component of ice-rafting. Generally this facies represents more ice-distal or basinal paleogeographic settings than the massive diamictite association. The diamictite and laminated sandstone association contains heterogeneous diamictite and distinctive, texturally mature, massive to parallellaminated sandstone beds. It is interpreted to represent ice-proximal deposits which form in the presence of subglacial or glacialfluvial meltwater. Two non-glacial facies associations are recognized: the carbonate, shale and sandstone association (mainly marine), and the cross-bedded sandstone association (fluvial).