Abstract

The Cambrian system, as exposed along the Utah–Nevada border in the Great Basin, exhibits two strikingly different sequences above the Prospect Mountain Quartzite. A fairly typical, shallow medial-shelf sequence in the Deep Creek Mountains, south of Gold Hill, is contrasted with a previously undescribed outer shelf and slope sequence in the mountains near Wendover, Nevada. The approximately 2,100-m-thick Cambrian carbonate platform in the Deep Creek section is dominated by shoaling shelf, oolitic carbonates; and intertidal to subtidal silty wackestones and fenestral lime mudstones, often dolomitic. These massive cliff-forming carbonates are punctuated by cratonally derived siliciclastic shales, carrying an endemic benthic fauna; as well as volcanic ash and open shelf elastics, containing a mixed benthic and cosmopolitan pelagic fauna.

In contrast, a Lower to Middle Cambrian outer shelf sequence, overlain by a westward progradation of the Upper Cambrian carbonate platform, occurs near Wendover. The 2,300-m-thick Cambrian sequence above the Prospect Mountain Quartzite begins with approximately 300 m of dark, graphitic phyllites with thin sandstone and laminated limey mudstone interbeds. This is overlain by a 900-m-thick Middle Cambrian sequence of rhythmically bedded and laminated limey mudstones, carrying a sparse, pelagic, cosmopolitan trilobite fauna. This hemipelagic facies shows soft sediment deformation and slumping which increases from east to west. The increased slumping denotes the shelfbreak region on a distally steepened ramp, which migrated westward as a prograding rimmed outer carbonate margin reached this area by the uppermost Middle Cambrian. Upper Cambrian sedimentation shallowed, as evidenced by occasional oolitic facies and bioturbated silty carbonates.