AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
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AN INTEGRATED PETROLEUM EVALUATION OF NORTHEASTERN NEVADA
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STRATIGRAPHY The stratigraphic section of this report is a description of the stratigraphic units, formations, groups, and assemblages which have been described within the study area. A great deal of effort has been made to systematize the description of units with formational or greater rank within a rigid format which includes the type section, geologic age, general lithology, average thickness, areal distribution, depositional setting, and where appropriate, a discussion of exploration significance. This format does not allow for the incomplete handling of stratigraphy common to most published reports. It does allow a great deal of information to be summarized in a useable and understandable form. The published literature has been relied on heavily in this synthesis and has been augmented by field observations and discussions with several other workers. Formations that are not in common use, are poorly or inadequately defined, or are facies of other units have been eliminated in some cases. Available data on sedimentation has been compiled for the stratigraphic units and is supplemented by this writer’s interpretations and observations. Unfortunately, little is known about the specifics of depositional environment for many of these units. The stratigraphic units are described within several sequences or assemblages that combine units of similar age, tectonic setting and/or lithology. These designations include Cenozoic Sediments, Cenozoic-Mesozoic Volcanic and Plutonic Rocks, Mesozoic Sediments, the Upper Paleozoic Western Siliceous and Volcanic Assemblage, Upper Paleozoic Transitional Detrital-Carbonate Assemblage, Upper Paleozoic Eastern Carbonate-Detrital Assemblage, Lower Paleozoic Western Siliceous and Volcanic Assemblage, Lower Paleozoic Transitional Assemblage, and Lower Paleozoic Eastern Carbonate Assemblage. For convenience, assemblages and sequences are described from youngest to oldest. Individual stratigraphic units within each designation are also described from generally youngest to oldest. The second portion of the stratigraphic volume is a brief and concise paleogeography from Precambrian through Quaternary time, for the evaluation area. This discussion relies heavily on a selective compilation and summary of published information. The depositional setting and paleogeography are poorly understood, particularly in light of structural complexity that has strongly influenced present facies and thickness distributions. Environmental reconstruction is exceedingly difficult both along and within the Roberts Mountains allochthon and in the eastern low-angle normal fault terrane. The abundance of high-angle normal faults and poor level of exposure across much of the area makes attempts at reconstruction fanciful at best. The author cautions the reader to be aware of the problems with arches, highs and positive elements introduced by so many Nevada stratigraphers. Most of these are representations of structural complexities, not original depositional elements. Sorting this out in the literature is a difficult and frustrating, albeit a necessary experience. The author encourages the reader to develop more than a passing acquaintance with the stratigraphic units described in the following sections. An understanding of the structure and tectonics, thermal history and geochemical trends, and ultimate economic potential of northeastern Nevada is impossible without a concise understanding of the stratigraphic setting. |
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