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The AAPG/Datapages Combined Publications Database

Utah Geological Association


Geology of Northwest Utah, Southern Idaho and Northeast Nevada, 1984
Pages 35-44

Comparison of a Cambrian Medial Shelf Sequence With an Outer Shelf Margin Sequence, Northern Great Basin

Linda B. McCollum, Michael B. McCollum


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.

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