49-4 Paleolake response to shift from contraction to extension across the hinterland, wedge-top, and broken foreland of the North American Cordillera

Session: Exploring Feedbacks Between Tectonics and Climate on Lithospheric Evolution Using Multidisciplinary Approaches

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Abstract:

We compare three Paleogene paleolake systems which formed synchronously across the North American Cordillera at a time of significant change in the kinematics of deformation and thermal boundary conditions resulting from removal of the shallow Farallon slab. In the hinterland, the oldest (~49 Ma) deposits of the lacustrine Elko Formation of NE Nevada fill conglomerate-floored paleovalleys incised into deformed Paleozoic bedrock. The Elko Formation consist predominantly of laminated marlstone with thin interbeds of microbialitic carbonate and tuff. While there are no clear fault scarps or coarse clastic lithofacies associated with these deposits, accommodation was likely the consequence of deep flow of overthickened crust that acted to pond surface drainage across the central hinterland. These lakes closely preceded voluminuous local magmatism and the formation of a regional unconformity that persisted for 25 m.y. prior to the beginning of Basin and Range faulting. In the Sevier thrust belt and wedge-top at the boundary between the Cordillera and its foreland, laminated Green River Formation accumulated in piggy-back basins formed when thrusts advanced into the adjacent foredeep. Following a short period of fluvial infill and unconformity from 51-49 Ma, narrow grabens formed aligned with the strike of the Sevier belt and record a local shift from contraction and extension. Freshwater-lacustrine and volcaniclastic strata of the Fowkes Formation filled these grabens over a 2-3 m.y. interval. In the foreland region of the central Rocky Mountains, paleolakes and paleorivers were confined to structural downwarps from the late Paleogene to early Eocene. New geochronology and stratigraphy of the Wagon Bed Formation of central Wyoming shows that following 49 Ma, paleolakes and rivers migrated towards structural highs and away from structural basins, lapping onto beveled Precambrian basement. Lithofacies and stable isotope values from the associated deposits indicates evaporative concentration of solutes in these lakes, which were coeval with the Rattlesnake Hills and Absaroka volcanic fields. Regional unconformity affected the region after ca. 45 Ma, and was periodically interrupted by accumulation of fluvial and aeolian strata assigned to the White River Formation. These data indicate that the downward adjustment of isostatically unsupported Laramide arches began 25 m.y. earlier than previously documented, consistent with removal of nearly half of the aerial extent of the Farallon slab during a short interval of the late early Eocene.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9847

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