M. J. P. Welland, J. H. Makurath, T. M. Sarniak, T. D. Lee, R. J. Marchel
Detailed analysis of selected sections of the Vinini and Valmy Formations (Ordovician) in the Toquima and Toiyabe Ranges in north-central Nevada indicates that the depositional characteristics of components of these formations are consistent with the model developed by Horne and Ferm for sedimentation in deltaic environments.
The Vinini Formation in the Pete's Summit area (Toquima Range) is dominated by a coarsening-upward sequence of interbedded shales, siltstones, and minor amounts of chert and limestone near the base, overlain by thick-bedded to massive orthoquartzites. The lower part of the section consists of very thin to thin-bedded, dark-gray to black shales and siltstones. Bedding is laterally continuous and has sharp planar to wavy contacts with underlying and overlying strata. Primary structures in siltstones, including small-scale trough cross-bedding, climbing ripples, and planar laminations, indicate episodes of rapid sedimentation in the generally low-energy regime in which shale accumulated. The lower section also contains sandstones (typically 1.5 m thick) that become finer upward and pinch out laterally over several meters; the bases of the sandstones are conglomeratic and sharply truncate the underlying beds.
The capping rock is a thick-bedded to massive, well-sorted orthoquartzite with a minimum thickness of 12 m, displaying only poorly preserved primary structures; bedding thickness decreases toward the top. The unit is a linear, elongate body, 150 to 200 m long, cut in many places by trough-shaped surfaces.
The lower part of the sequence is interpreted as representing bay-fill deposits, with minor channel-filling sandstones, of a lower deltaic system. It is suggested that the upper orthoquartzite unit represents a period of transgression of barrier-island environments following delta-lobe abandonment.
The Valmy Formation in the Big Creek area (40 km west of Pete's Summit) is composed of interbedded shales, siltstones, and cherts. The units are thin to thick bedded and are laterally continuous. Ripple lamination, climbing ripples, current laminations, and cross-bedding are dominant primary structures in both siltstones and cherts. Small stringers of sand within the siltstones indicate sporadic influx of coarse detritus into a relatively quiescent environment.
The presence of marginal to shallow-marine components of the Roberts Mountains allochthon imposes significant restrictions on possible modes of origin and emplacement of this tectonic unit.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma