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Abstract Modern and ancient lacustrine carbonate build‐ups provide uniquely sensitive sedimentary and geochemical records for understanding the interaction between tectonics, past climates, and local and regional scale basin hydrology. Large (metre to decametre), well‐developed carbonate mounds in the Green River Formation have long been recognized along the margins of an Eocene lake, known as Lake Gosiute. However, their mode of origin and significance with respect to palaeohydrology remain controversial. Here, new sedimentological, Sr isotope data and structural evidence show that significant spring discharge led to the formation of a decametre size complex of shoreline carbonate mounds in the upper Wilkins Peak Member of the Green River Formation at Little Mesa and adjacent areas in the Bridger Basin, Wyoming, USA. Sedimentological evidence indicates that spring discharge was predominantly subaqueous but was, at times, also subaerial, which produced tufa cascades and micro‐rimstone dam structures. The87Sr/86Sr ratios measured from these subaerial spring deposits are less radiogenic (87Sr/86Sr = 0.71040 to 0.71101) than contemporaneous palaeolake carbonates (87Sr/86Sr = 0.71195 to 0.71561) because their parent groundwaters likely interacted with less‐radiogenic Palaeozoic carbonate. Calcite‐cemented sandstone cones and spires (87Sr/86Sr = 0.71037 to 0.71057) in the Wasatch Formation directly below the spring deposits suggest that groundwaters derived from Palaeozoic carbonates preferentially flowed along thrust faults. These results imply that high spring discharge coincided with lake high stands of the upper Wilkins Peak Member, suggesting that recharge at the north‐west margin of the Bridger Basin contributed to Lake Gosiute’s water budget and lowered the salinity of an underfilled, evaporative lake basin. The findings of this study provide criteria for the recognition of groundwater discharge in palaeolake systems which may lead to the discovery of palaeospring systems in other ancient lake deposits.
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Geochemical Evolution of Eocene Lakes in the Nevada Hinterland of the North American Cordillera
Abstract Eocene strata of the Elko Formation record lacustrine deposition within the Nevada hinterland of the North American Cordillera. We present a detailed geochemical stratigraphy enabled by high‐sampling‐resolution geochronology from lacus trine limestone and interbedded volcanic rocks of the Elko Formation. Two intervals of lacustrine deposition, an early Eocene “Lake Adobe” of limited aerial extent and a laterally extensive middle Eocene “Lake Elko,” are separated by ∼5 m.y. of apparent unconformity. Sediments deposited in the apparently short‐lived (49.5–48.5 Ma) early Eocene Lake Adobe exhibit high‐amplitude covariation of δ18O, δ13C and87Sr/86Sr, which suggests a dynamically changing catchment and precipitation regime. Lake Elko formed during the middle Eocene, and its strata record three geochemically distinct phases, indicating it was a single interconnected water body that became increasingly evaporative over time. The lower Elko Formation (44.0–42.5 Ma) was deposited in a freshwater lake. Middle Elko Formation (42.5–41.2 Ma) lithofacies and geochemistry suggest that an increasingly saline and alkaline Lake Elko experienced salinity stratification‐induced hypolimnion disoxia and burial of12C‐rich organic matter. The upper Elko Formation (41.2–40.5 Ma) records a shallow final phase of Lake Elko that experienced short residence times and a breakdown in stratification. A sharp decline of87Sr/86Sr in the upper Elko Formation reflects an increasing aerial extent of low‐87Sr/86Sr volcanic deposits from nearby calderas. Middle Eocene strata record ponding of paleodrainage, increasing hydrologic isolation and volcanism, consistent with progressive north to south removal of the Farallon flat slab and/or delamination of the lower lithospheric mantle of the North American plate.
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- Award ID(s):
- 1848563
- PAR ID:
- 10360044
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geochemistry, Geophysics, Geosystems
- Volume:
- 22
- Issue:
- 10
- ISSN:
- 1525-2027
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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