Stratigraphy and Facies Across Lower Devonian-Middle Devonian Boundary, Central Nevada

Abstract

A regressive episode near the end of the Early Devonian caused a nearshore quartzose carbonate facies to shift to the edge of the carbonate shelf in central Nevada. Transgression followed in the latest Early Devonian and continued in the early Middle Devonian. Seaward of the shelf-wide unconformity produced by regression, a dolomitized, dark-gray lime wackestone-packstone with abundant crinoidal debris formed on the inner slope of a silled outer shelf basin. This dolomite is here named the Sadler Ranch Formation. The dolomite front forms the facies boundary between the Sadler Ranch and the upper Coils Creek limestone, which was deposited in the shallow outer shelf basin. Dark lime muds that characterize Sadler Ranch and upper Coils Creek sediments probably collected only uring low stands of sea level when the outer sill of the basin was emergent. Crinoid meadows flourished on the ramp when sea level rose. Conodonts in the Sadler Ranch Formation date the regression as serotinus Zone and the subsequent transgression as spanning the upper part of the serotinus Zone, the patulus Zone, and the lower part of the costatus costatus Zone. The transgression correlates closely with the Onondaga Limestone of New York and the Couvinian-Eifelian of western Europe and is of probable eustatic origin.