Depositional Facies of Mississippian Clastics, Antler Foreland Basin, Central Diamond Mountains, Nevada

Abstract

ABSTRACT In the central Diamond Mountains of eastern Nevada, about 2000 m of Mississippian clastic strata in the Chainman Shale and Diamond Peak Formation were deposited in the Antler foreland basin. Paleocurrents and petrographic data confirm that the provenance to the west was the Antler orogenic belt formed by emplacement of the Roberts Mountains allochthon. The foreland clastic sequence rests disconformably upon and is overlain gradationally by shelf carbonates that were deposited along the Cordilleran continental margin both before and after the Antler event. The basin fill thus contains a full record of the foreland subsidence that accompanied and followed the Antler orogeny. Detailed facies mapping indicates that the clastic succession includes two main divisions: a) a lower retrogradational sequence (750-1000 m) of basin-slope and submarine-fan sediment deposited as active tectonic subsidence increased water depths from neritic to bathyal, and b) an upper progradational sequence (1000-1250 m) of delta-slope and delta-platform sediment deposited as passive isostatic subsistence accompanied progressive shoaling of the sediment surface. The retrogradational facies contain prominent turbidites and include, in ascending order: 1) upper-slope facies of shale with resedimented carbonates, 2) lower-slope facies of shale with olistostromal beds and lenticular sandstones, 3) inner-fan facies of shale with amalgamated conglomerate and sandstone packets, and 4) midd e-fan facies of interbedded shale and sandstone in thinning-upward packets. The progradational facies constitute a fan-delta assemblage that includes 1) delta-slope facies of shale and siltstone with ravine-fill lenses of conglomerate and sandstone that become more prevalent upward, 2) delta-front facies of interbedded distributary coarse clastics and interdistributary fine clastics and bioclastic limestones, and 3) delta-plain facies of conglomeratic sandstones and red hutites in fining-upward channel-floodplain cycles. The Antler foreland basin thus contains both flyschlike and molasselike sequences in a characteristic succession that probably reflects collision orogeny followed by erosional denudation of orogenic highlands. Initial tectonic downflexure to form a deep-water trough was later counteracted by sedimentation even though isostatic subsidence under sediment loading continued.