The Eocene Elko Basin and Elko Formation, NE Nevada: Paleotopographic Controls on Area, Thickness, Facies Distribution, and Petroleum Potential

Abstract

Based on published and my ages, the Elko Basin developed at ~46 Ma. Sediments that make up the Elko Fm accumulated in the basin until ~38 Ma; deposition ended diachronously, oldest to the southwest where input from the Robinson Volcanic Field (Piñon Range) overwhelmed other sources. Published depictions of the Elko Basin have ranged from large rectangular areas (~17,000 km2, present day) to a contiguous area (~8000 km2) with a partly dividing intrabasin high in the Adobe Range. The basin probably resulted from early extension of the Ruby Mts metamorphic core complex, although the amount of early extension is debated. All depictions effectively assume that the basin was continuous and developed on flat topography.

In contrast, the distribution of 45-40 Ma ash-flow tuffs show that they were channelized into major east-trending paleovalleys. E.g., the 40.2 Ma tuff of Big Cottonwood Canyon is preserved in three curvilinear belts between its source caldera in the northern Tuscarora Mts eastward 180 km to the Pequop Mts. I interpret this distribution to reflect flow and deposition in three east-draining paleovalleys. The caldera is within the NW part of the previously interpreted Elko Basin, and the Pequop Mts in or east of the basin. If the tuff had erupted onto the interpreted large, flat, continuous basin, it would have spread out as an areally restricted, proximal sheet. The tuff’s actual distribution precludes such a large, continuous basin. I interpret the Elko Basin to have developed initially as separate depocenters where northwest-dipping normal faults cut paleovalleys. Individual depocenters were generally separated by topographically high interfluves (as much as 1.5 km relief) but may have been hydrologically connected intermittently during basin evolution. The thickest, especially lacustrine, potentially hydrocarbon-generating sediments accumulated and are preserved in these depocenters. My interpretation predicts considerably thinner deposits over interfluves, consistent with observation. Sedimentary structures in Elko Fm lacustrine deposits indicate generally shallow water deposition, so sedimentation kept up with basin subsidence. This interpretation reduces the potential total volume of source rocks in the Elko Basin but suggests the possibility of additional prospective basins along known paleovalleys. This interpretation also has implications for the distribution of sediments and source rocks in other intermontane basins of the western US.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018