Revision of Precambrian stratigraphy in the Prescott-Jerome area, Yavapai County, Arizona

Abstract

The metamorphosed volcanic, volcaniclastic, and sedimentary rocks that predate the oldest plutonic rocks in the Prescott-Jerome area (1,770±10 m.y.) are placed in the Yavapai Series, a provincial time-stratigraphic term denned as the 'time interval from 1,770±10 to 1,820+m.y. Two rock-stratigraphic groups are recognized; the Ash Creek Group is1 separated from the Big Bug Group by a major fault and a large pluton of quartz diorite. Isotopic dating now indicates that the Ash Creek Group is older. The name Big Bug Group replaces the term Alder Group, used in earlier publications on the Prescott-Jerome area. Within the Big Bug Group the Indian Hills' Volcanics and Green Gulch Volcanites have been shown to be the same formation, and the latter term is retained. Where first described the Chaparral Volcanics was bounded by faults, but subsequent mapping has demonstrated that it occupies the upper part of the Spud Mountain Volcanics; so the term Chaparral Volcanics is abandoned. The Texas Gulch Formation is removed from the Yavapai Series because it is separated from the rocks of the Big Bug Group by a period of plutonism, represented by the Brady Butte Granodiorite, and subsequent erosion.