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ABSTRACT: Grain-Cement Relationships, Sespe Sandstone, Ventura Basin

L. A. Hathon, D. W. Houseknecht

Diagenesis within sandstones of limited compositional variability has been documented over a depth range of 17,000 ft in the Oligocene Sespe Formation of the Ventura basin. Nonmarine, lithic arkoses from the Oxnard shelf, a subprovince of the basin, display evidence that grain-pore fluid interactions controlled the sequence, volumes, and types of authigenic products formed during burial diagenesis.

During shallow burial, mechanical compaction and/or tectonism induced fractures in brittle grains, which were subsequently enhanced by dissolution. Clay grain coats are the earliest authigenic phase recognized; predominantly smectite at shallow depths, clays grade to corrensite and then chlorite with depth. The concomitant decrease in Si and Ca and increase in Fe and Mg with depth may reflect ion sourcing from dissolution of volcanic lithic grains during shallow burial, and biotite and heavy minerals during deeper burial.

Although dissolution of K-feldspar and plagioclase grains is observed, K-feldspars more commonly appear unaltered whereas plagioclase may be dissolved, altered to quartz plus clay, or partially albitized. K-feldspar overgrowths occur on both K-feldspar and plagioclase grains. Mn-carbonate cement, with Ca and Mn perhaps sourced from dissolution of plagioclase grains, postdates K-feldspar cement. Laumontite cement is observed in two depth zones, 7600-8000 ft and >17,000 ft. Sources of Ca, Al, and Si probably included dissolution of plagioclase in the shallower zone, and albitization and intergranular pressure solution in the deeper zone. Although quartz overgrowths occur throughout the sample set, they are present in greater volumes in deeper samples, suggesting Si was sourced from i tergranular pressure solution.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990