137-7 Detrital Signals of Quaternary Tectonics, Climate, and Sea Level from the Sierra to the Sea, California, USA

Session: Joint SGD-SEPM-IAS Focus on the Sedimentary Record of Climate Change

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One of the fundamental challenges to understanding short time-scale (10² – 10⁴ years) landscape change, and humanity’s role in it, is our limited understanding of the full range of “natural” variability in Earth surface processes. For this, we can look to Earth’s recent past to reconstruct millennial-scale patterns of erosion, transport, and deposition. Of the many drivers known to trigger changes in sediment routing (climate, tectonics, sea level, etc.), all have been at play during the Quaternary in California’s largest drainage system. The coastal outlet for the Sacramento-San Joaquin valley established its current marine outlet, Carquinez Strait, in San Franciso (SF) Bay just 600,000 years ago when a tectonically impounded lake (Lake Clyde) broke a new path through the San Andreas Fault system. At the supply end of this system, Quaternary glacial and interglacial cycles can shift the dominant locus of sediment generation between southern and northern Sierran sources, respectively. During interglacial periods such as now, the Sacramento River provides seven times more sediment than the San Joaquin River. However, detrital provenance comparisons between Pleistocene relic sediment in SF Bay, offshore sediment, and the modern drainage configuration suggest that southern Sierran sources were more dominant during the Last Glacial Maximum. Finally, within SF Bay, this difference in northern and southern Sierran provenance sets the stage for identifying the role of Holocene sea level rise in modifying coastal sediment routing. Sand provenance results from optical petrography, bulk geochemistry, and detrital zircon mixture modeling of Holocene and relic Pleistocene sediment indicate that sand in the bayhead region has a distinct source from sand in the baymouth region. Sand residence times dated via optically stimulated luminescence from bayfloor dredge samples indicate that sand in the Suisun and Central embayments is relic (ca. 4 k.a.); whereas sand in the baymouth region (Central Bay) was more recently bleached (ca. 1 k.a.), likely by littoral transport. We attribute these differences to the bayhead region sequestering terrestrially-sourced sediment and the baymouth region being sourced by coastal erosion and marine sediment supply. New results build on previous efforts to interpret a new regional sand transport history that describes the influence of post-LGM sea level rise and regional onshore climate in shifting the sources of sand between structural embayments in SF Bay.

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

doi: 10.1130/abs/2025AM-11170

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