J. H. Makurath, M. J. P. Welland, T. D. Lee, R. J. Marchel, T. Sarniak
The Elder Sandstone (Silurian?) in the Shoshone Range of north-central Nevada is a complex unit of interbedded shales, siltstones, cherts, and feldspathic and calcareous sandstones. Internally the unit is characterized by coarsening-upward cycles 5 to 30 m thick. Lower parts of each cycle typically are composed of interbedded shales and siltstones which are rippled, laminated, and burrowed and become coarser upward. Upper parts of cycles contain coarsening-upward beds and medium to thick-bedded units of feldspathic and calcareous sandstone. Large- and small-scale trough cross-lamination, climbing ripples, local erosion surfaces, and lenses of conglomerate and coarse sandstone are common in the upper parts of Elder cycles.
Sedimentary textures, structures, and vertical organization of lithologies are consistent with the model developed by Horne and Ferm for sedimentation in deltaic environments. Application of this model suggests that the Elder sequence was deposited in a lower delta-plain setting. Lower parts of Elder cycles (shales and siltstones) were deposited in prodelta and/or interdistributary bays, whereas upper parts of cycles (silts and sands) represent deposition associated with crevasse splays and/or distributary-mouth bars.
Parts of the (Silurian?) Fourmile Canyon formation in the neighboring Cortez Mountains show similar trends in textures, structures, and vertical sequence. This unit consists of interbedded cherts, argillites, siltstones and sandstones, in a coarsening-upward sequence also interpreted as representing deltaic deposition.
This interpretation dictates a marginal to shallow-marine origin for parts of the Roberts Mountain allochthon and, in combination with evidence of complex tectonic upper and lower contacts of these units, sets serious constraints on possible pre-Antler paleogeographies in north-central Nevada.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma