MONOLITHOLOGIC, POLYMICTIC BRECCIAS REFLECT LATE CRETACEOUS TECTONIC DEVELOPMENT OF THE NEVADAPLANO CREST AND WESTWARD ENCROACHMENT OF THE SHEEP PASS BASIN, SEVIER OROGENIC HINTERLAND, CENTRAL NEVADA

‘Crypto-bedded’ to massive, unsorted, ungraded, unfossiliferous, monolithologic, polymictic, breccias, derived exclusively from Devonian carbonate rocks, discontinuously crop out in an east-west band from northern Railroad Valley to the southern Fish Creek Range, covering ~500 km². This informally named “Pancake Range breccia” unit (PRB) reflects evolution of the Sevier hinterland between piggyback and graben deposition of the late Early Cretaceous Newark Canyon FM (KNC) and the high elevation, latest Cretaceous Sheep Pass (TKSP) basin. Thus PRB age is likely early Late Cretaceous.

The most common lithology is a tight, fragmentary, light grey brown to brown dolostone with a few, small and discontinuous areas of laminae and a web-work of post-depositional silica between and within fragments. A second, limestone breccia is also present but displays crypto-bedding. Both depofacies have historically been mapped as their own source rocks, usually as Late Devonian Bay State and Simonson DS and Devils Gate LS, respectively.

Regionally, the PRB has been recognized in the Fish Creek Range while a stack of unnamed LK, monolithologic breccias crop out north of Eureka, NV. Fractured and disrupted dolostone-similar to the PRB are scattered to the NW, mapped as part of the Roberts Mountains Thrust system. Similar rocks crop out in Little Smokey Valley and are overlain by scraps of KTSP fossiliferous lacustrine limestones.

PRB was deposited as large, permeable rock falls, some likely long-traveled, but only after KNC was eroded from most areas. Source areas, likely along the Nevadaplano crest to the West, were distal from PRB outcrops as no proximal, bouldery facies has been recognized.

Highest elevation of the Nevadaplano was probably reached in PRB time as contraction began to pass East, towards the Late Cretaceous Sevier front in Utah. As the crest subsided, the KTSP basin edge moved west, first eroding the PRB (Member A), then onlapping its scattered remnants (Member B).