Abstract

Dating of orogenic rock units in the central part of the Madison Range shows that Laramide deformation was virtually completed by the end of the Cretaceous. Early Campanian K-Ar dates of about 79 m.y. were obtained from welded tuffs in the basal part of the Livingston Formation, a volcanic and volcaniclastic assemblage that is conformable with underlying Cretaceous clastic rocks and with the overlying Sphinx Conglomerate. No datable materials were obtained from the Sphinx, but both it and the Livingston were deformed by the Hilgard fault system, a series of thrust faults and associated folds which extend along the western side of the southern two-thirds of the range. This north-trending fault system represents the culmination of Laramide shortening within the range. K-Ar and 40Ar/39Ar dating of hornblende from dacitic laccolithic rocks that intruded the Hilgard fault system in the central part of the range indicates an approximate date of 68–69 m.y. B.P. for emplacement of the igneous suite. The dacite postdates movement along faults of the Hilgard fault system, and postdates the synorogenic Sphinx Conglomerate.

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