--> The tectonic-erratic hypothesis presents a new strategy for petroleum exploration in north-central Nevada

AAPG Pacific Section and Rocky Mountain Section Joint Meeting

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The tectonic-erratic hypothesis presents a new strategy for petroleum exploration in north-central Nevada

Abstract

During the Antler orogeny, a thick, Cambrian-Devonian succession of largely siliciclastic strata, which accumulated in a continental rise and adjacent ocean floor setting, was thrust ~200 kilometers eastwards along the Roberts Mountains thrust over a thick, largely carbonate succession, which had accumulated in an extensive epicontinental sea and composes much of the Cordilleran miogeocline. Thus, throughout north-central Nevada, the intensely deformed siliciclastic strata of the Roberts Mountains allochthon (RMA) structurally overlie the coeval, relatively undeformed, autochthonous, carbonate succession. During the Permian-Triassic Sonoma orogeny, the emplacement of the Golconda allochthon outboard of the RMA generated further deformation in north-central Nevada with the initiation of the Henderson thrust at the level of the Roberts Mountains thrust. As its upper plate was displaced eastwards, the Henderson thrust ramped upwards and thus duplicated the stratigraphic and structural succession within the RMA. In the process, carbonate blocks were plucked from the lower plate of the Roberts Mountains thrust and incorporated and transported in the upper plate of the Henderson thrust. Over large areas, the siliciclastic strata of the RMA in the upper plate of the Henderson thrust were removed by erosion, but the carbonate blocks carried in the lower part of the upper plate remain as topographic highs structurally overlying the siliciclastic strata of the RMA in the lower plate of the Henderson thrust. The carbonate blocks bear an analogy to glacial erratics and are termed tectonic erratics. Along a north-south trending belt in Eureka County, both upper and lower plates of the Henderson thrust dip to the east, and the tectonic erratics occur in the base of the upper plate. There these masses of fractured carbonate directly ovelie great thicknesses of Ordovician strata in the RMA that include four stratigraphic intervals of oil shale (>20% TOC). Thus, tectonic erratics in the subsurface are potential reservoir rocks directly overlying potential source rocks. The Devonian reservoir rock for the Blackburn oil field in Eureka County, previously interpreted as a horst of the autochthonous succession, may instead be a large tectonic erratic. Given the size of erratics identified at Lone Mountain and Devils Gate (several kilometers long and up to 300 m thick), erratics in the subsurface are potentially large reservoirs and likely common in Eureka County. Their recognition provides a new strategy for exploration. Instead of targeting horsts of the allochthon, exploration should be focused on erratics at the base of the upper plate of the Henderson thrust.