Precambrian Rocks at North Franklin Mountain, Texas

Abstract

At North Franklin Mountain in the western tip of Texas, more than 5,000 feet of sedimentary and volcanic rocks lie unconformably beneath the Bliss sandstone (Upper Cambrian). Except for algal structures, these older rocks are non-fossiliferous, and are thought to be Precambrian in age. Granite, probably Precambrian, is intruded into the sequence. In ascending order the Precambrian rocks consist of the Castner limestone (new name), the Mundy breccia (new name), the Lanoria quartzite, and an unnamed rhyolite unit. The Castner is 1,100 feet thick and includes limestone, hornfels, and chert, as well as diabase sills. It contains mound-like algal structures near the base and rests on intrusive granite. The Mundy, ranging from a trace to 250 feet thick, consists of basalt rubble of unknown source resting unconformably on the Castner. The Lanoria quartzite, 2,600 feet thick, is composed of fine-grained sandstone, siltstone, and shale, all metamorphosed. It is divided into three members by a middle unit of massive quartzite. The rhyolite unit contains rounded pebbles of Lanoria quartzite at its base and attains its maximum known thick ess, 1,400 feet, at North Franklin Mountain. The Castner limestone may be equivalent to the Allamoore limestone of the Van Horn area, 100 miles to the east. However, other Precambrian formations in the two areas are so unlike that positive correlation can not be made.