Geochemical analyses were performed on 11 Phosphoria Formation samples collected throughout the Wyoming-Idaho-Utah Overthrust belt. Maturation models incorporating the thermal effects of thrust faulting with Lopatin's techniques were then used to interpret these geochemical data. In particular, four of the sample sites were selected for detailed thermal modeling. These four sites include one sample site in an area unaffected by thrusting, two sites located in the hanging walls of major thrusts, and one site in the footwall of the Darby thrust.

The detailed thermal modeling shows overthrusting did affect the thermal history of the Phosphoria Formation. The effect of thrusting was to cool the overriding sheet and warm the sediments being overridden. Thus, the hanging wall samples had a different post-thrusting thermal history than the footwall sample. Cooling of the hanging wall slowed post-thrusting maturation in the two thrust-sheet samples. In contrast, the footwall sample was exposed to higher temperatures after thrusting, and the maturation process was accelerated.

Results of this study indicate that tectonism influences the thermal history of the rocks and should be included in any reliable source rock evaluation program involving overthrust strata. A simple evaluation based only on the thickness of the overburden, in general, will not be sufficient to explain fully the maturation data. In the absence of actual geochemical information such as R_o, TAI or CAI, the models described in this paper appear to be sufficiently accurate to give a reasonable estimate of thermal maturity of a potential source bed.