Abstract

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 and bedrock lithology controlled the location of broad, terraced paleovalley segments, which formed in the central and eastern metamorphic bedrock belts and subsequently became areas of significant fluvial deposition. High-gradient, high-energy deposits are present near the edges of these areas, suggesting that tributaries and outlets resided in steeper, narrow paleovalleys. Multiple strath terraces are common in broad paleovalleys, indicating that incision events occurred both before deposition and during paleovalley widening, followed by periods of aggradation. Broad paleovalley deposits contain predominantly braided stream facies deposited in an energetically variable system. Multiple upward-fining cycles, bounded by erosional surfaces, are composed of alternating coarse-grained channel-belt and fine-grained floodplain packages. An overall upward-fining succession suggests that the locus of fluvial deposition shifted and depositional energy waned over time as the system backfilled successive broad paleovalleys.

Paleochannel slope estimates of 0.004–0.055 were obtained based on average grain sizes of coarse channel fills. Anomalously thick (up to 11 m), laterally continuous (1–3 km) floodplain units, containing kaolinite-rich clay and Oxisol intervals, record high fine-grained sediment supply, driven by high rates of chemical weathering in the warm Eocene climate. Based on sedimentary characteristics, combined with floral and detrital zircon data, braided stream deposition within the basin occurred from the late early Eocene through late Eocene, and aggradation primarily occurred in broad, lower-gradient reaches behind a major bedrock knickpoint, created in the late Mesozoic–early Cenozoic, possibly due to asymmetric batholith unroofing. The diachronous, localized nature of paleovalley incision and braided stream aggradation within the northern Sierra Nevada invalidates previous estimates of the timing and amount of range uplift based on paleochannel reconstructions and gradients. Both internal and external drivers influenced the evolution of the fluvial system through (1) the pattern of basin filling, as the locus of deposition migrated east to west, (2) the warm Eocene climate, which increased chemical weathering and contributed a high sediment supply, and (3) changes in base level due to eustasy and/or Laramide tectonism. This record of Laramide tectonics in the Sierra Nevada and Basin and Range is important for the timing of Sierra Nevada uplift, for tectonic reconstructions of western North America, and for studies of landscape evolution in tectonically active orogens.

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