Early and Middle Devonian Shelf-Slope Transition, Southern Mahogany Hills, Eureka County, Nevada

Robert A. Schalla, Kathleen M. Benedetto

In the southern Mahogany Hills of Nevada, the abrupt lateral transition between the lower McColley Canyon Formation (Kobeh Member) and Beacon Peak (Sevy) dolomite (Lower Devonian) and the pinch-out of the Oxyoke Canyon sandstone into the upper Sadler Ranch formation (Middle Devonian) document lithofacies relationships between carbonate shelf and slope deposits. The McColley Canyon-Beacon Peak transition is marked by a change from thin-bedded dolomite and limestone with occasional chert nodules and scattered fossil material to poorly bedded, largely unfossiliferous, sandy dolomite. This change suggests shallowing and concomitant circulation restriction at the shelf edge during the early stages of platform (ramp) development. Measured sections in the vicinity of Combs Peak uggest that the location of the shelf edge is the result of both transgression and paleotopographic relief on the subjacent platform. A thin oxidized zone at the top of the Beacon Peak in section CP II, interpreted as a paleosol, indicates subaerial exposure of the outer shelf prior to renewed transgression and deposition of the Bartine Member.

The location of the carbonate shelf edge is further documented by the westward pinch-out of unfossiliferous, arenaceous dolomites of the Oxyoke Canyon sandstone into fossiliferous foreslope dolomites of the Sadler Ranch formation. This facies change is the result of regression, which caused shoaling along the shelf edge.

Lithofacies relationships exposed in the southern Mahogany Hills document two transgressive-regressive cycles and help to establish the location of an Early and Middle Devonian carbonate shelf-slope break in southern Eureka County.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.