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J.L. Clayton

Publications and source records attributed to J.L. Clayton.

22 records · Page 2Linked to original sources
USGS1987

Effects of weathering on biological marker and aromatic hydrocarbon composition of organic matter in Phosphoria shale outcrop

GC-MS analyses were performed on core samples collected from a shale outcrop of the Permian Phosphoria Formation in Utah, U.S.A., to study effects of weathering on selected biological marker and aromatic (phenanthrene) hydrocarbon compounds. Among the biological markers, the most important weathering effects are a decrease in the 20S 20R "> 20S20R diastereomer ratio of the C 29 steranes and loss of low molecular weight triaromatic steroids. A decrease in the C 19 through C 22 tricylcic terpanes occurs relative to the total C 19 -C 26 tricyclic fraction. Pronounced loss of methyl-substituted phenanthrenes occurs relative to phenanthrene. No major effect on the overall distribution of pentacyclic terpanes is evident.

Geochimica et Cosmochimica Acta↗
USGS1986

Temperature effects on kerogen and on molecular and isotopic composition of organic matter in Pierre Shale near an igneous dike

A suite of siltstone samples from the Upper Cretaceous Pierre Shale from the contact zone of a 130-cm thick igneous dike near Wolcott, Colorado, U.S.A., was taken from the contact to 170 cm from the dike to study the effects of temperature on the organic matter. The sampled bedding interval was about 10 cm thick, so variation in lithology and type of organic matter is minimal. Vitrinite reflectance values ( R 0 ) increase from 0.4 far from the dike, to 3.3% near the dike contact. Geochemical measurements show systematic thermal effects analogous to those often observed for catagenesis and metagenesis in the depth range of 1–4 km within a sedimentary basin. The H/C ratio of kerogen and the hydrogen index (Rock-Eval) decrease most rapidly in the 0.6–1.7% R 0 range, in which the transformation ratio (Rock-Eval) increases from 0.1 to 0.3. Based on extraction of C 15+ compounds, the main increase of hydrocarbons and total extractable organic matter occurs between 0.6 and 1.0% reflectance. The saturated/aromatic hydrocarbon ratio increases almost twofold in this range of maturity. However, the pristane/phytane ratio is essentially constant through the hydrocarbon generation zone but decreases slightly at high levels of thermal alteration ( R 0 > 1.2%). The δ 13 C values for aromatic and saturated hydrocarbons are about −27 and −29‰, respectively, and are constant to about 1.0% R 0 , then both become heavier by about 2‰ at higher R 0 values.

Colorado↗
USGS1978

Subaerial weathering of sedimentary organic matter

Small diameter core samples were taken from outcrops of the Permian Phosphoria Formation and the Cretaceous Pierre Shale of the Western United States to determine the effects of weathering on organic matter in shale outcrops. While the Pierre Shale core showed no evidence of weathering, the Phosphoria Formation showed significant reduction of overall organic content and pronounced changes in organic composition over the near-surface interval of the core. Total organic carbon is lower by as much as 60% over the upper 2 ft of the core. Chloroform-soluble organic matter and total hydrocarbon (C 15+ ) concentrations are 50% lower over this same interval. The ratio of saturated to aromatic hydrocarbons decreases steadily with core depth over the upper 2.6 ft of the core. Aromatic hydrocarbons are enriched in the stable carbon-13 isotope by an average of 1.7%. over this same interval. Shallow core samples also show a loss of n -paraffins relative to branched/cyclic compounds in the saturated C 15+ fraction. Although the extent of weathering is variable, certain characteristic effects are recognizable and can be applied to the interpretation of outcrop data in organic geochemical studies.

Geochimica et Cosmochimica Acta↗