Sedimentology of a Middle Cambrian Outer Shelf Margin with Evidence for Syndepositional Faulting, Eastern California and Western Nevada

Abstract

ABSTRACT The Middle to Upper Cambrian Emigrant Formation at Horse Thief Canyon, California, is a transitional section between the outer shelf Bonanza King Formation and the basin facies represented by the Emigrant Formation in Nevada. At Horse Thief Canyon the Emigrant is subdivided into four members: the Limestone-Siltstone Member, the Papoose Lake Tongue, the Megabreccia Member, and the Limestone-Chert Member. For a portion of Middle Cambrian time the shelf structure was that of a ramp. Bedded and burrowed pelletal dolostones were deposited in the deeper waters of the outer shelf. Sediment for the outer shelf may have been derived from a mid-shelf shallow subtidal to peritidal zone. Cycle patterns of burrowed and bedded dolostones are probably related to periods of little sediment input accompanied by intense burrowing of the bottom followed by periods of heightened current activity and bedded dolostone deposition. Later in the Middle Cambrian the shelf structure changed to a platform. Megabreccia and breccia beds representing debris flow deposits and interbedded allodapic calcarenite and calcisiltite are evidence for the existence of a significant slope between shelf and basin. This slope may have been caused by syndepositional faulting and collapse of a portion of the outer shelf. The fault hypothesis is based on a rust-colored marker unit recognized in the Emigrant at Horse Thief Canyon and in the outer shelf Bonanze King section 13 km to the south along with the presence of distinctive algal boundstone clasts in the Megabreccia Member. The megabreccia beds lie above the marker unit but all known source rocks for the algal boundstone occur stratigraphically well below the marker. Uncertainty egarding the syndepositional fault hypothesis centers on whether the Horse Thief and Last Chance Range sections are in the same or different allocthons.