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Authigenic illite is a prominent pore-fill in the Nugget Sandstone, the main reservoir rock of most fields in the southwest Wyoming Overthrust belt. Illite, a good K/Ar clock, was dated from several well samples, all from the Absaroka thrust sheet. This includes a producing well in the Clear Creek field where seven samples traverse the gas, oil, and water zones. The ages of the Clear Creek suite are virtually concordant at 110 ± 2 m.y. Assuming hydrocarbon emplacement would have arrested authigenesis in the oil and gas zones, the similarity of ages from the hydrocarbon zones with the water zone indicates hydrocarbon emplacement was post 110 m.y. ago (middle Cretaceous). Ages obtained from the other Absaroka sheet Nugget samples fall in the narrow range of 102 to 120 .y.b.p. This indicates illite authigenesis was a relatively short-lived "event" for the Nugget in the Absaroka sheet.
The Wyoming-Idaho-Utah overthrust belt involves several thrust sheets each of which was emplaced over its foreland sequentially from west to east over a time spanning tens of millions of years. We attribute the mid-Cretaceous illite growth in the Absaroka sheet to burial conditions established when that part of the Nugget was thrust upon by the Crawford sheet. The burial was accomplished both tectonically and by synorogenically derived sediment. If true, our illite dates imply a somewhat older age for the Crawford sheet than previously interpreted. We attribute the post-illite hydrocarbon emplacement in the Absaroka Nugget as a result of thrusting of the Absaroka sheet on top of its foreland containing middle Cretaceous petroleum source-bed shales. These beds were thermally matured wh n buried by the emplacement of the Absaroka sheet and its derived sediment in the Late Cretaceous.
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