Geochronology of the midcontinent region, United States: 3. Southern area

Remnants of the vast 1600- to 1800-m.y.-old terrane of the northern United States and Canada occur in Colorado, northern New Mexico, and northeastern Kansas. The Precambrian basement rocks of the area record essentially continuous igneous activity during Late Precambrian time. Granitic rocks dated at 1450 to 1600 m.y. are found in Kansas and Missouri. Widespread intrusion of granite occurred from 1350 to 1450 m.y. ago. This igneous activity ranged from New Mexico to Colorado and Missouri. It is well developed in the Nemaha uplift of Kansas, from which the name Nemaha igneous activity is derived. Around the northern and western margins of the area the gneisses into which the granites were intruded retain ages near 1700 m.y. Granite and rhyolite 1100 to 1350 m.y. old are abundant. The St. Francois igneous activity of Missouri ranged from 1200 to 1350 m.y.; the Spavinaw igneous activity of northeastern Oklahoma and vicinity, from 1150 to 1300 m.y.; and the Panhandle igneous activity of Texas, from 1100 to 1200 m.y. ago. The cycle of sedimentation, deformation, metamorphism, and intrusion, 1000 to 1150 m.y. ago, in central Texas is here named the Llano orogeny. Igneous and metamorphic rocks in Trans-Pecos Texas are closely related in time. The Wichita Mountains igneous activity occurred in Early to Middle Cambrian time (500 m.y. ago).