RAMIREZ, PEDRO C.
Department of Geological Sciences, Cal State Los Angeles, 90032 ([email protected])

Abstract: Climate and Post-depositional Alteration of Paleogene Sandstones of the Silverado Formation, San Joaquin Hills, California.

Field and petrographic analysis of coarse grained sandstones of the Paleogene Silverado Formation exposed in San Joaquin Hills reveal that significant post-depositional alteration and dissolution of framework grains occurred and that these postdepositional changes were a consequence of existing climatic conditions. Compositionally, the sandstones range from lithic arenites to arkoses. Sedimentary structures in the sandstones such as trough and planar crossbeds, channels, and lenticular beds indicate deposition in a probable high energy braided stream environment. In some beds following deposition, feldspars were altered to kaolinite and/or partially dissolved, biotite grains were kaolinized, and quartz grains were etched and partially dissolved. Additionally, polycrystalline metamorphic and plutonic quartz grains were disaggregated. In places, complete alteration of unstable framework grains led to an approximately ten meter thick kaolinite layer containing scattered and etched quartz grains. These layers are mottled purple and brown, locally contain iron pisolites, and are interpreted as paleosols developed on emergent areas in the fluvial system. The paleosols present in the Silverado are important climatic indicators because they commonly form in tropical climates. Decaying vegetation produced corrosive pore fluids that modified the framework grains. Silicified wood fragments and locally silica-cemented sandstone suggest the release of significant amounts of silica during the alteration and dissolution of the framework grains. Generally, the paleosols studied record a period of relative stability and non-deposition in the fluvial environment. However, paleosol rip-up clasts in the sandstones indicate the erosion of previously formed soil horizons as periodic shifting of the high energy Paleogene river occurred.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California