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Post-Cretaceous uplift of the Sioux Quartzite ridge in southeastern South Dakota

Evidence of a post-Cretaceous uplift of the Sioux Quartzite ridge in southeastern South Dakota consists of deformation of the Dakota Formation, Graneros Shale, Greenhorn Limestone, Carlile Shale, and Niobrara Formation of Cretaceous age. The Greenhorn is warped upward about 400 feet on the Sioux Quartzite with a formation dip ranging from 30 to 50 feet per mile. Elsewhere in eastern South Dakota the dip of the Greenhorn ranges from 3 to 8 feet per mile. INTRODUCTION During the Late Creataceous-early Tertiary Rocky Mountain orogeny, several isolated areas in the plains to the east were subjected to uplift. The most notable uplift is the Black Hills which included the intrusion of Tertiary igneous rocks. Deformation also resulted in the Central Kansas uplift and the Nemaha uplift in southeastern Nebraska and western Iowa (Bunker, 1981). Although there is no evidence of Tertiary intrusives in southeastern South Dakota, there is stratigraphic evidence to support the presence of a post-Cretaceous up-lif t. The purpose of this report is to present evidence that a post-Cretaceous uplift occurred in southeastern South Dakota. TECTONIC SETTING Although the interior of North America is commonly referred to as technically stable, the area has actually been subjected to episodic tectonism from Precambrian to present. The most recent major tectonism occurred about 60 million years ago during early Tertiary time when the Black Hills in western South Dakota and Wyoming began rising (fig. 1). The cause of the Black Hills uplift is poorly understood; however, the contributing forces operated within or below the Precambrian crystalline rocks. The only igneous intrusives of Tertiary age in South Dakota are, in the northern Black Hills, trending in a N. 74 W. direction. Redden (U.S. Geological Survey and U.S. Bureau of Reclamation, 1975) pointed out that the Tertiary igneous rocks in the northern Black Hills are in line with the Nye-Bowler fault zone to the northwest in south-central Montana (fig. 1).