Cenozoic paleosols San Diego area,California

Ancient weathering profiles provide mute evidence of the major climatic regimes that operated in the southern California area during the Cenozoic Era. A tropical climate during part of the Paleocene and Early Eocene caused severe weathering of Jurassic andesitic rocks, Cretaceous granitic rocks and Late Cretaceous and Early Eocene terrigenous rocks. Erosional remnants of this paleosol exist as buried soils up to 30 meters thick and as exhumed soils on some modern hills. The main weathering horizon is overwhelmingly composed of kaolinite with residual quartz grains and minor amounts of iron oxides. This tropical climate was in effect at 35° to 40° north latitude.Late Eocene sedimentary rocks were deposited under a semi-arid climate with 50 to 60 cm of annual rainfall that was concentrated during one season. This interpretation is based on: 1) the dominance of conglomerate and immature sandstone in the Upper Eocene section compared to the relative scarcity of claystone, 2) the immaturity of the clay mineral suite which is dominated by smectite and vermiculite, 3) the presence of fractured clasts presumably split by growth and differential expansion of salts, 4) the multiple caliche beds that individually are up to one meter thick, and 5) the paleohydrology of the fluvial deposits which transported rhyolite gravels over 300 km distance from an eastern bedrock source about 4,000 m high in a river of 1.3 sinuosity with 100 year flood discharges up to 30,000 m³/sec.The extensive marine terraces in San Diego are mantled by relict soils that record composite weathering characteristics from glacial and interglacial climates from Late Pliocene to present. Maximal development of the paleosol includes an ironstone concretion layer (B_ir horizon) above an illuvial clay layer (B_t horizon) which rests on an iron- and silicacemented hardpan (C_m horizon).