The regional maturation history of the Phosphoria Formation in the Idaho-Wyoming Overthrust belt was determined from a suite of Phosphoria samples collected throughout the region. Samples were collected from both the footwall and hanging wall of as many thrust sheets as possible. The results of this study indicate that inclusion of the thermal effects of thrust faulting on the temperature history of the region is critical to a thorough explanation of the variations in maturity levels observed in the Phosphoria. A simple evaluation based only on the thickness of the overburden will not be sufficient to accurately interpret the maturation data. Instead, maturation models incorporating the thermal effects of thrust faulting with techniques developed by Lopatin are used to ex lain the geochemical maturation data.

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Modified Lopatin diagrams were used for detailed maturation modeling of four of the Phosphoria sampling sites. This detailed modeling clearly demonstrates that the thermal effects of overthrusting do significantly influence the maturation history of organic material incorporated in overthrust sediments. Briefly, the result of thrusting is to cool the overriding sheet and warm the sediments being overridden. A unit in the hanging wall of a thrust will not be subjected to high temperatures for nearly as long a time as the equivalent unit in the footwall of the thrust. Comparison between observed reflectance values for the four sampling sites with vitrinite reflectance values calculated from Lopatin diagrams indicates that better agreement between observed and calculated values is obtain d when thermal modifications due to thrusting are incorporated within the Lopatin diagrams.

Finally, in the absence of actual geochemical information such as R_o or T.A.I., the models here described appear to be accurate enough to give a reasonable estimate of the thermal maturity of a potential source bed. Such a predictive technique can be of great value in planning an exploration program in previously untested overthrust terrains.

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