Abstract

The footwall geometry of the Marathon fold-thrust belt is similar to the structural contour map on top of the cratonic “Siluro-Devonian”; both are characterized by a northwest-trending, structural low. Post-thrust deformation did not effect either the footwall geometry or the “Siluro-Devonian” structure. The geometry of the cratonic basin in the Early and Middle Paleozoic was north-trending, narrow, and sinuous. Sedimentation was controlled by three structural highs in the cratonic rocks: the North Brewster, Del Norte, and Allison Ranch highs. The basin deepened to the north toward the Tobosa Basin and to the south toward the passive continental margin. Slopes of the isochore contours suggest that the Early and Middle Paleozoic basin was shallow and deepened rapidly to the north in Wolfcampian time. In Desmoinesian time limestone facies and isochore thicknesses suggest that an east-trending high developed across the southern end of the older Paleozoic basin. After Desmoinesian time and prior to middle Wolfcampian time, the east-trending high subsided as thrusts overrode it. Although the exact geometry of the east-trending high is uncertain, the high should have influenced the early footwall geometry of the Marathon fold-thrust belt. Structural and stratigraphic interpretations suggest that structural highs in the Early and Middle Paleozoic cratonic rocks were reactivated and influenced the footwall geometry at the end of thrusting in middle Wolfcampian time.