Paleovalley morphology and fluvial system evolution of Eocene-Oligocene sediments ("auriferous gravels"), northern Sierra Nevada, California: Implications for climate, tectonics, and topography

摘要: In the northern Sierra Nevada, California, Eocene–Oligocene fluvial sediments (the “auriferous gravels”) rest unconformably in well-defined basement-incised paleovalleys, providing a record of paleolandscape geomorphology and drainage system evolution. Paleovalley surface mapping shows that local topography bedrock lithology controlled location broad, terraced paleovalley segments, which formed central eastern metamorphic belts subsequently became areas significant deposition. High-gradient, high-energy deposits are present near edges these areas, suggesting tributaries outlets resided steeper, narrow paleovalleys. Multiple strath terraces common broad indicating incision events occurred both before deposition during widening, followed by periods aggradation. Broad contain predominantly braided stream facies deposited an energetically variable system. upward-fining cycles, bounded erosional surfaces, composed alternating coarse-grained channel-belt fine-grained floodplain packages. An overall succession suggests locus shifted depositional energy waned over time as backfilled successive Paleochannel slope estimates 0.004–0.055 were obtained based on average grain sizes coarse channel fills. Anomalously thick (up to 11 m), laterally continuous (1–3 km) units, containing kaolinite-rich clay Oxisol intervals, high sediment supply, driven rates chemical weathering warm Eocene climate. Based sedimentary characteristics, combined with floral detrital zircon data, within basin from late early through Eocene, aggradation primarily lower-gradient reaches behind major knickpoint, created Mesozoic–early Cenozoic, possibly due asymmetric batholith unroofing. The diachronous, localized nature Nevada invalidates previous timing amount range uplift paleochannel reconstructions gradients. Both internal external drivers influenced evolution (1) pattern filling, migrated east west, (2) climate, increased contributed (3) changes base level eustasy and/or Laramide tectonism. This tectonics Basin Range is important for uplift, tectonic western North America, studies landscape tectonically active orogens.