Abstract

The Tomahawk volcanic area, located 11 km south of Lead, South Dakota, consists of an oval depression underlain by Tertiary lithic tuff and breccia. Xenolithic fragments consisting of Precambrian metamorphic rocks, Paleozoic and Mesozoic sedimentary rocks, and Tertiary rhyolite porphyry occur within the bedded tuffs. The youngest xenoliths are derived from lower Cretaceous shales, which occupied a position nearly one km above the present erosional level at the time of intrusion. Gravity data collected along closely spaced profiles reveals an elliptical gravity anomaly covering an area approximately 2.1 km (1.3 miles) long and 1.5 km (0.9 miles) wide with a gravity minimum of -7.5 mgal in the center. Three-dimensional gravity modeling of the ovate feature indicates that the observed gravity data is modeled best as a tapering feature filled with tuff and breccia extending 1.5 km below the present exposure surface. Modeling of the available regional aeromagnetic data suggests that the Tomahawk volcanic area is part of a much larger Tertiary intrusive system which was emplaced to the west of the Tomahawk diatreme. The Tertiary intrusive system appears to be dominated by a reversed remanent magnetization similar to other large intrusives in the northern Black Hills.