RELATIVE AGES OF DETACHMENT FOLDING AND BASIN INVERSION, FORELAND BASIN SYSTEM OF SIERRA MADRE ORIENTAL, NORTHEASTERN MEXICO

New data from growth strata and cross-cutting relations in the Parras and La Popa basins indicate two fold trends were active during latest Cretaceous-Paleogene time in the foreland of the Sierra Madre Oriental (SMO). E-W trending detachment folds dominate the proximal Parras basin and the SMO to the south. In the SMO, these folds are >2km in amplitude and detach in Jurassic evaporites. Northward, the folds decrease in amplitude to >1km as the detachment ramps upward into Cretaceous shale where the evaporites pinch out. Growth strata indicate these E-W folds were forming during the late Maastrichtian and early Paleocene. Recent mapping indicates E-W trending, shale-detached folds continue northward into the La Popa basin, which is dominated by NW-SE trending large-amplitude folds that formed during rift-basin inversion and are detached on isolated Jurassic evaporites. Growth-stratal analysis from northern La Popa basin indicates these NW-SE folds began to form in mid-Maastrichtian time. Where the fold domains overlap, the small-amplitude E-W folds are refolded by the NW trending La Gavia and El Gordo anticlines, indicating initiation of the E-W fold trend prior to the mid-Maastrichtian. Shortening continued into the Paleocene on both the E-W trend south of the La Gavia anticline, which separates the Parras and La Popa basins, and on the NW trend in La Popa basin, with the result that both fold trends were active simultaneously above different detachment levels in different parts of the foreland.