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abstract

Fission-track dating is a valuable method for studying the thermal history of sedimentary basins because apatite and zircon fission-track annealing temperatures span the main temperature range for oil generation, and both minerals are present in the heavy- mineral suites of many sedimentary rocks. Fission tracks in apatite are totally annealed at temperatures of about 105°C (221°F) to 150°C (302°F) over periods of heating of 108 to 105 yr duration, respectively. The temperatures at which fission tracks in zircon anneal are not as well known but are probably in the range of 200°C ± 30°C (392°F ± 54°F).

Fission-track ages of detrital apatite separated from Upper Cretaceous and lower Tertiary sedimentary rocks in the El Paso Natural Gas Wagon Wheel no. 1 well on the Pinedale anticline, northern Green River basin, Wyoming, indicate that there has been significant cooling in the rocks in this part of the basin. The data suggest that at least the latest phase of cooling began only 2 to 4 m.y. ago and involved a relatively rapid temperature decrease of 20°C (36°F) or more. The magnitude of cooling is supported by several other paleotemperature indicators in the drill hole.

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