Multi-Isotope Geochemistry of the Eocene Elko Formation, Northeastern Nevada

Abstract

Hinterland basins accumulate archives of orogenic landscapes that are otherwise largely erosional and thus rarely preserved in the geologic record. In northeastern Nevada, Eocene terrestrial strata of the Elko Formation represent ~10 Myr of high-elevation (2.5-3 km) lacustrine deposition in the hinterland of the Sevier retroarc. Elko Basin strata contain kerogen-rich strata that are an economically feasible target for petroleum exploration, but structural deformation associated with Neogene extension limits lithostratigraphic reconstructions of the spatial and temporal distribution of sedimentary facies. To correlate sedimentary lithofacies and petrofacies across the Elko Basin and test if hinterland paleolakes were connected through time, we couple decimeter-scale stratigraphy of the Elko Formation with gamma ray spectroscopy and isotope geochemistry (⁸⁷Sr/⁸⁶Sr, δ¹³C, δ¹⁸O, and δD). ⁸⁷Sr/⁸⁶Sr ratios of 32 rock samples collected from lacustrine carbonates and marls and 12 stream water samples collected from watersheds draining a variety of potential rock sources to the Elko Basin also tests if metamorphic rocks of the adjacent Ruby Mountains-East Humboldt Range were exposed during the Eocene. When combined with lithostratigraphic correlations, published single-crystal sanidine ⁴⁰Ar/³⁹Ar geochronology, and δD paleohydrology, these datasets highlight subtle changes in paleolake hydrology that are linked to lacustrine deposition, organic preservation, and stratigraphic architecture.

Lacustrine lithofacies show two lake-type progressions from overfilled to balanced-fill conditions between 49 and 41 Ma that are correlative across the basin and evident across multiple isotopic proxies. Carbonate mineralogy shows a dominant shift from calcitic to dolomitic composition at the interpreted boundary between fluvial-lacustrine and fluctuating-profundal lake types that likely signifies increased Mg/Ca concentration within hypoxic lake waters. δ¹⁸O values commonly range from 1‰ to -7‰ near the basin depocenter, but samples spanning ~8 Myr of the basin record have values between -15‰ and -21‰ that reflect regional paleoprecipitation values. Both δ¹⁸O and δ¹³C ratios are typically highest in fluctuating profundal zones, which suggests closed lake conditions with higher rates of primary productivity. This chemostratigraphic record indicates variations in the source, intensity, and chemistry of hydrologic inflow to and outflow from Eocene lakes in the Sevier hinterland.

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