Provenance and Sedimentary Context of Clay Mineralogy in an Evolving Forearc Basin, Upper Cretaceous-Paleogene and Eocene Mudstones, San Joaquin Valley, California

Mudstone samples from the Moreno (Upper Cretaceous-Paleocene) and Kreyenhagen (Eocene) formations are analysed using X-ray diffraction (XRD) fluorescence (XRF) to determine their mineralogy. Smectite (Reichweite R0) is predominant phyllosilicate present, 48% 71.7% bulk rock mineralogy (excluding carbonate cemented highly bio siliceous samples) 70% 98% of <2 μm clay fraction. Opal CT less so cristobalite concentrations cause main deviations smectite dominance. A common only in Upper Kreyenhagen. In fraction, Fm significantly more smectite-rich than Fm. was derived alteration volcaniclastic debris contemporaneous rhyolitic-dacitic magmatic arc volcanism. No tuff preserved. intense sub-tropical weathering granitoid-dioritic terrane during hypothermal period early mid-Eocene; derivation local volcanism unlikely. All had chemical indices (CIA) indicative source terrane. Ferriferous enrichment occurrence locally kaolinite contributory evidence for intensity weathering. Low concentration (max. 7.5%) clinoptilolite Lower possibly open marine conditions There no volumetrically significant silicate diagenesis. The diagenetic mineralisation restricted low-temperature silica phase transitions.