Abstracts

Paleosol Development and Quartz Dissolution in the Paleogene Silverado Formation, Northern Peninsular Ranges, California

S. M. LEYVA; P. C. RAMIREZ; R. LIMA; Department of Geology, California State University, Los Angeles, 5151 State University Drive, Los Angeles, California 90032

Exposed in the middle part of the Paleogene Silverado Formation in the San Joaquin Hills are several well to poorly defined paleosols. These paleosols, which occur in texturally immature arkoses and lithic arenites, have not been well documented until recently. The lowermost part of one of these paleosols is a 1.75 meter thick unit, consisting of a pallid zone, grading up into a red and white mottle zone and capped by an hematite-rich zone. A buff-colored, one meter-thick, indurated, iron-rich quartz and kaolinite sandstone caps this lower unit. A two meter-thick mottled red and white zone, similar to the mottled zone in the lower unit, overlies this iron-rich sandstone. The mottled zone is capped by 5 meters of white sandy claystone composed entirely of quartz and kaolinite. Feldspars, biotite and polycrystalline quartz grains, present in the arkoses, are mostly absent in all zones of the paleosols. Partially dissolved, etched, angular, and predominantly monocrystalline coarse quartz grains float in a kaolinite matrix throughout the paleosol. Hematite-staining of the matrix and poorly defined iron nodules occur within the mottled and iron crust zones. An anomalous quartz-rich (90% quartz) sandstone overlies the paleosols with a sharp contact.

Deposited arkoses were subjected to intense weathering and alteration resulting from humid tropical climates. Iron nodules, mottled red and white kaolinite 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.

Poster presented at: 1999 GSA Cordillerian Section Annual Meeting, Berkeley, California

Created by Sonjia Leyva (copyright 2001)

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