2000 GSA Annual Meeting -- Reno, Nevada

Abstract 52976

FROM ARKOSES TO LATERITES: COMPOSITIONAL ANALYSIS OF PALEOSOLS IN THE PALEOGENE SILVARADO FORMATION, NORTHERN PENINSULAR RANGES, CALIFORNIA

Presented by Leyva, S. M.
Authors:      Ramirez, P. C..

Key words: paleosols, laterite, Paleocene, Silverado

In Session 41     Paleoclimatology and Paleoceanography (Posters) Monday, November 13, 2000 PM in Room: Hall C at 01:30 PM for .

Abstract: A white kaolinitic claystone bed in the San Joaquin Hills area is a sequence of paleosol horizons recording a probable subtropical to tropical Paleocene climate. This paleosol occurs primarily in texturally immature arkoses and lithic arenites in the middle part of the Paleogene Silvarado Formation.

Paleosol 1 is a nine-meter thick sequence of four distinct paleosol horizons. These beds are, in stratigraphic order: Horizon A, comprising of a kaolinitic pallid zone, grading into a red and white mottled zone, and capped by a hematite-rich crust; a hematite-stained clayey sandstone; Horizon B, a red and white mottled zone with a hematite-rich crust; Horizon C, a white kaolinitic claystone; and Horizon D, a kaolinitic pallid zone. A quartz-rich sandstone, containing kaolinite rip-ups, partially altered feldspars and subrounded etched quartz clasts, caps the paleosol. In all horizons, monocrystalline quartz grains are dissolved, etched and pitted, and disaggregation of polycrystalline quartz along crystal boundaries is evident. The red and white mottles are produced by hematite staining of the kaolinite.

Paleosol 1 grades laterally into a highly weathered coarse-grained sandstone/saprolite that is poorly exposed. The saprolite consists of partially dissolved, etched, angular, and predominantly monocrystalline quartz, highly altered mica, and partially dissolved feldspars incorporated in a kaolinitic matrix. Kaolinite rip-up clasts and weak relict bedding are also present. Distinct horizons in the saprolite are not evident.

High precipitation rates associated with subtropical/tropical climates resulted in intense weathering and alteration of the arkose. Iron nodules, mottled red and white zones, kaolinite-rich intervals and partially dissolved quartz grains are consistent with laterite development. Feldspars and other labile constituents were altered to kaolinite, while quartz grains were partially etched and dissolved as intense leaching occurred. The degree of feldspar alteration into kaolinite and of feldspar and quartz dissolution indicates that a humid and wet climate must have occurred during the Paleocene.

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