Abstract
Compaction was measured in six drill-core samples of marls from the Wilkins Peak and Tipton Shale Members of the Eocene Green River Formation, Sweetwater County, Wyoming, by the degree of preferred orientation of clay mineral grains. The preferred orientation was measured with an x-ray goniometer, and the compaction was calculated by application of the theory of March (1932), in which one assumes that the clay grains were originally oriented at random. Associated organic materials were exhaustively extracted by solvents and were identified by gas and thin-layer chromatography. The amount of solvent-soluble organic compounds increases with depth. The proportional share of alkanes is approximately constant. Odd-carbon dominance of the nC 17 alkane decreases rapidly and monotonically with depth, while the ratio of nC 16 plus nC 18 alkanes to alkenes increases in a reciprocal fashion. The carbon preference index of alkanes of higher molecular weight is fairly constant at a value of 2 except in one sample (CPI = 3.2), which is much less compacted than all the others. There appears to be a correlation between compaction and the content of pristane relative to the Nc 17 alkane, superposed on a general decrease of pristane concentration with depth. Phytane, on the other hand, is concentrated in shallow and deep samples, with a minimum in between, and it shows no correlation with compaction. The phytane content reflects either the nature of contributing biotas or possible early alteration of organic detritus, whereas pristane seems to respond to later, gradual alteration, possibly governed by catalysis on sediment grain surfaces. This catalysis could be enhanced by mechanical scraping and breaking of grains during compaction. Attenuation of the odd-carbon preference of the freshly deposited organic compounds may be similarly affected by catalytic processes.
Published Version
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