Counting marmots…
A Mid-Tertiary igneous intrusion into the Upper Cretaceous Mancos Shale provides an excellent natural laboratory to study the thermal effects of an intrusion on the organic geochemistry and water-rock system across a contact metamorphic profile. The intrusion, is believed to have established a high temperature gradient across a two mile long sampling profile. This study investigated the isotopic and chemical evolution of both kerogen and extractable organic material. Kerogen displayed a net weight loss of 11% due to methane generation. Kerogen showed essentially no carbon isotopic change. Bitumen contents decreased dramatically across the profile with a measurable 1.5 per mil depletion in 12C for the highest temperature samples. The relatively unique time-temperature history of the study area, and the difference in the kinetics of bitumen and kerogen degradation, is believed to be responsible for the difference in behavior of kerogen and bitumen. Oxygen isotope ratios of matrix calcite suggest that the calcite exchanged with connate water or a very small volume of meteoric water. The final oxygen isotopic ratio of calcite is a function of temperature and the water/calcite volume ratio. Finally, a thermal model involving conductive heating due to a subjacent extension of the White Rock pluton beneath the study area is shown to be capable of generating a temperature of 3°C.
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