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Documentation of Late Cretaceous Forebulge Migration in Southwestern Wyoming

Hongjun Luo, Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, phone: 307-766-2843, fax: 307-766-2737, [email protected] and Dag Nummedal, Colorado Energy Research Institute, Colorado School of Mines, Golden, CO 80401.

The recognition of a forebulge in the subsurface is difficult due to its low amplitude and wide extent. Three regional profiles are established to define the late Cretaceous forebulges in southwestern Wyoming based on detailed well log correlations and good outcrop control. The amplitude of forebulges is 40-80 m according to the stratal erosion and thinning. In response to eastward progressive movement of the Crawford, Early Absaroka, and Late Absaroka thrusts, the forebulges migrated eastward to the Moxa arch, the Rock Springs uplift, and the Washakie basin, respectively. Tectonic analysis show that the late Cretaceous forebulges resulted from the combined elastic response of the lithosphere by the Wyoming thrust belt and Wind River thrust. 3D flexural modeling results further supports this explanation. Following the formation of the forebulge, basement-involved uplifts formed at the Moxa arch and Rock Springs uplift. This probably implies that the forebulges may have weakened these zones of the lithosphere triggering basement-involved uplifting during the Laramide orogeny.