MUDSTONE GEOCHEMISTRY AS AN INDICATOR OF PROVENANCE IN AN ARC-UNROOFING SEQUENCE IN NORTHERN CALIFORNIA, USA

Due to their formation at subduction zones, forearc basins preserve integrated records of magmatic arc unroofing in their stratigraphy. These records are commonly studied using the coarse-grained fraction of forearc basin fills, often neglecting the more abundant finer-grained mudstone fraction. Geochemical analysis of mudstone can be used to identify the presence of unstable mafic minerals and volcanics and provide a more homogenized provenance signature. The Great Valley forearc basin in central California is an excellent example of remnant forearc basin stratigraphy and has been well studied utilizing U-Pb detrital zircon analysis, sandstone composition, and mudstone proxies. Since the Great Valley Group contains sediment from the dissection of the Sierra Nevada arc, we hypothesize that geochemical signatures of unstable mafic minerals and volcanics should decrease over time as the volcanic arc is eroded and the intermediate to felsic Sierra Nevada batholith is exhumed.

To test for a geochemical arc dissection trend, we analyzed major and trace element geochemistry of the Cortina, Boxer, Lodoga, and Stony Creek formations of the Great Valley Group, which all consist predominantly of mudstone. We measured and sampled sections from each of the four formations at 1-m intervals to provide a high-resolution stratigraphic unit-by-unit picture of provenance trends in the mudstone geochemistry. This allows us to compare trends between and within the units itself. Preliminary results indicate concentrations of volcanically derived elements V and Sc decrease up-section while more felsic indicators such as Th gradually increase. Moreover, the increasing up-section fractionation of rare earth elements (La/Yb) reflects a transition from volcanic to plutonic provenance during arc dissection. Elements characteristic of felsic plutonic sources such as potassium and zirconium diverge from arc-dissection trends, with volcanic and plutonic derived sediment having equal concentrations, which may correspond to variations in weathering and zircon fertility in the arc. Future work aims to collect additional analyses to capture stratigraphic variability within each of the units and examine geochemical weathering proxies.