Abstract

Lithic arenites and lithic wackes of the National Petroleum Reserve in Alaska were never buried deeply. Maximum depth was generally less than 2,000 m, and the paleotemperature, less than 60 degrees C. Nevertheless, a wide array of diagenetic processes markedly affected these rocks. In the eogenetic zone, authigenic clays (chlorite, smectite, and kaolinite) developed as coatings around detrital grains, as matrix, and as aggregate structures. Also occurring in the near-surface environment was the precipitation of siderite cement. With deeper burial, beginning at a subsurface depth of perhaps 500 m, ductile rock fragments and micaceous minerals suffered physical compaction. Syntaxial quartz overgrowths precipitated in this zone. The last stage of diagenesis, at depths in excess of 1,100 m, included continued compaction, which significantly reduced primary porosity, and the formation of calcite cement. Continuing throughout all stages was the alteration of unstable rock fragments. Chemical breakdown of these grains provided ions necessary for authigenic minerals. Because of the rocks' shallow burial, the effects of near-surface diagenesis are preserved; they have not been overprinted or masked by deep, late-stage processes.

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