Abstract
The crustal and mantle dynamics of subduction-driven orogenesis and subsequent lithospheric thinning are preserved in Phanerozoic sedimentary deposits of the North American Cordillera. The hinterland of the North American Cordillera, the area between the magmatic arc and the frontal Sevier fold-thrust belt, records a protracted history of crustal thickening and surface uplift as well as subsequent orogenic collapse to form the modern Basin and Range Province. By the Paleogene, continued orogenesis led to the formation of a high-elevation plateau in what is now eastern Nevada and western Utah. The topography and hydrology of this plateau and its relation to tectonic processes is enigmatic, largely because this history is obscured by later Basin and Range extension. Terrestrial basins that were once situated across a large expanse of this plateau preserve a detailed record of this landscape and enable reconstruction of the paleogeographic and tectonic development of the hinterland.
Eocene fluvial and lacustrine deposits are spread across much of eastern Nevada and reflect the evolution of several long-lived terrestrial basins. These deposits provide a critical record of drainage patterns, paleolake evolution, and the nature of upper-crustal exhumation prior to orogenic collapse. Lacustrine strata of the Elko Formation and related fluvial and volcanic units were deposited across a large area of northeastern Nevada during the early–middle Eocene. The mechanisms that facilitated the accumulation of these Eocene sediments and the past controls on their observed heterogeneity remain uncertain (e.g., Satarugsa and Johnson, 2000; Haynes, 2003; Henry, 2008; Smith et al., 2017). To resolve this uncertainty, detailed study and correlation of time-equivalent hinterland strata is essential. The research presented in this dissertation includes a study of the sedimentology, stratigraphy, geochemistry, geochronology, and thermochronology of Elko Formation and equivalent strata in northeastern Nevada. This interdisciplinary approach permits the first detailed assessment of Eocene paleogeographic evolution in the central North American Cordilleran hinterland at high spatial and temporal resolution. The primary goal of this research is to gain a better understanding of the Paleogene-Neogene landscape evolution of the hinterland, and to determine tectonic and climatic drivers for changes to the surface record.
Chapter I presents a detrital geochronologic and thermochronologic evaluation of early–middle Eocene sedimentary rocks to determine sediment provenance and hinterland exhumation rates across an ~8 m.y. time span. By applying (U-Th)/(He-Pb) double dating of detrital zircon and (U-Th)/He thermochronology of detrital apatite from precisely dated Paleogene terrestrial strata (cf. Smith et al., 2017), we determine the timing and approximate magnitude of upper-crustal exhumation across this interval. This facilitates interpretation of possible links between tectonic unroofing, magmatism, and basin development. Nearly all Eocene fluvial strata sampled contain detrital zircon with long lag times (i.e., the time between cooling and deposition) of >100 m.y., indicating sediment recycling and prolonged retention at upper-crustal depths. However, 43–41 Ma strata contain nonvolcanic detrital zircon with <10 m.y. lag times, indicating rapid cooling of proximal source terranes. These data reflect accelerated exhumation (with rates of >1 km/m.y.) synchronous with the onset of magmatism in northeastern Nevada. This chapter is in press for publication in the Geological Society of America Bulletin in 2019.
Chapter II provides a reconstruction of the Paleogene exhumation history of the Copper Mountains and the development of the extensional Copper Basin during the middle Eocene through Oligocene. This area preserves one of the most complete records of Paleogene Cordilleran hinterland extension to date, which is preserved in both synextensional hanging wall strata and footwall rocks. We evaluate the precise timing and style of extensional deformation via 40Ar/39Ar and (U-Th)/He thermochronology of hanging wall and footwall rocks. This thermochronology demonstrates the Late Cretaceous Coffeepot Stock was uplifted in the footwall and cooled through the zircon (U-Th)/He closure temperature between ~90 and 70 Ma and the apatite (U-Th)/He closure temperature between 43 and 40 Ma. Plutonic grains of the Coffeepot Stock are identified in Copper Basin strata using (U-Th)/(He-Pb) double dating and REE chemistry. When combined with precise depositional ages for Copper Basin strata, detrital cooling ages reveal rapid exhumation of this source during 8–12 km of middle Eocene–early Oligocene slip along the Copper Creek normal fault. This history of rapid extension in the Copper Mountains is correlative to the pattern noted in the regional synthesis of exhumation rates in Chapter I. Taken together, these data indicate a substantial transition in the style of near-surface hinterland deformation and terrestrial ponding during the middle and late Eocene that was likely driven by dynamic and thermal effects of Farallon slab and/or lower lithosphere removal. Chapter II is accepted pending revision for publication in Geosphere.
Chapter III summarizes a geochemical study of Elko Formation strata that investigates Eocene lake water chemistry to constrain paleolake extent and connectivity, sediment provenance, and controls on lake evolution. Evaluation of the stable isotope geochemistry of lacustrine carbonates using multiple geochemical tracers (87Sr/86Sr, δ13C, and δ18O) permits assessment of intrinsic and external drivers for observed stratigraphic changes. Low 87Sr/86Sr ratios for most samples collected across the stratigraphic range of the Elko Formation (mean = 0.70857; σ = 0.0007) indicates that bedrock underlying the Elko Basin catchment consisted predominantly of late Paleozoic sedimentary strata and/or recent volcaniclastic sediments for the duration of lacustrine deposition. Covariance in 87Sr/86Sr, δ13C, and δ18O values across multiple field areas implies prolonged lacustrine connectivity across a >1000 km2 area between ca. 43 and 41 Ma. Heightened δ13C and δ18O values and a progressive 38–52% in δDglass values for tuff intercalated with Elko Formation lacustrine sediments deposited after ca. 42.1 Ma signifies enhanced evaporation prior to basin closure by ca. 40 Ma. Chemostratigraphic variations recorded in multiple proxies indicate substantial variation in the source, intensity, and chemistry of hydrologic inflow to and outflow from Elko Basin lakes. These data indicate lake expansion occurred coeval to the middle Eocene acceleration of source exhumation rates highlighted in Chapter I and that subsequent basin closure was closely related to the arrival of encroaching volcanism, likely triggered by Farallon slab removal. This chapter will be submitted to Geochimica et Cosmochimica Acta.
Chapter IV presents results of detailed sedimentology and stratigraphy of the Elko Formation, informed by the data presented in Chapters I–III and the age model of Smith et al. (2017). Several previous workers have described various aspects of the stratigraphy of the Elko Formation (e.g., Van Houten, 1956; Smith and Ketner, 1976; Solomon et al., 1979; Moore et al., 1983; Haynes, 2003; Horner, 2015; Johnson and Birdwell, 2016), but no study has integrated outcrop, core, and well data with detailed stratigraphic correlations. As a result, numerous hypotheses exist for how and why paleolake systems developed in the hinterland. Stratigraphic characterization indicates the Elko Basin was filled by >1.2 km of fluvial, lacustrine, and volcaniclastic sediment during two distinct phases of tectonic subsidence in the early and middle Eocene. Two lake-type transitions from fluvial-lacustrine to fluctuating profundal conditions (cf. Carroll and Bohacs, 1999) at 49.0 Ma and 42.4 Ma signify changes in lake water chemistry and depositional character driven by external controls. Lake expansion-contraction cycles can be correlated regionally in both fluctuating profundal and fluvial-lacustrine strata, potentially implying regional paleolake connectivity. Lake transgression occurred during intense regional magmatism and extension within the basin catchment, both of which are reflected in the lithology and structure of sedimentary facies. Reconstruction of the subsidence history of the Elko Basin and comparison with other hinterland basins reveals a similar tectonic subsidence history that is generally distinct from other basin types. This chapter will be submitted to Basin Research.