This paper outlines the Precambrian geologic history of the continental interior of the United States by describing several selected areas from north to south. In this way the history is developed chronologically and the various areas illustrate the methods used in preparing a map of the buried basement. Lateral continental accretion is of lesser importance than is usually postulated; at least 50 percent of North America was in existence 2,500 m.y. ago. Crustal stabilization occurred at different times (about 2,500, 1,700, 1,350, and 1,000 m.y. ago) in the North American continental interior.

Buried basement rocks from more than 3,000 wells and scattered outcrops in the continental interior of the United States were studied petrographically. This information, in conjunction with geophysical data and isotopic ages, was used to outline geological units and to work out the geological history.

Except for the Cambrian igneous complex of the Wichita Mountains, southern Oklahoma, all basement rocks between the Rocky and Appalachian Mountains are Precambrian. Included here as basement are the widespread Precambrian sedimentary and volcanic rock sequences as well as the crystalline continental crust usually defined as basement.

Rocks 2,500 m.y. old or older exposed in northern Minnesota occur as belts of west-southwest-trending granite and gneiss alternating with greenstone and schist. They extend without interruption into eastern North Dakota and northeastern South Dakota. A large part of Wyoming also is underlain by rocks at least 2,500 m.y. old.

Well data for the Williston basin in eastern Montana and western North and South Dakota are too sparse to permit construction of a geologic map of the basement but isotopic ages and gravity anomalies suggest that this region is underlain by rocks deformed and consolidated about 1,700 m.y. ago during the Black Hills orogeny. The sedimentary parent rocks of the Precambrian rocks exposed in the Black Hills were deposited on an older basement that contains zircon dated at 2,550 m.y.

The Black Hills are the culmination of the long north-northwest-trending basement high that includes the Chadron and Cambridge arches of Nebraska and the Central Kansas uplift. Most wells to basement have penetrated metamorphic rocks along this uplift. A granite batholith underlies the

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area along the Nebraska-Kansas boundary. North of this batholith, along the uplift, isotopic ages of granite and gneiss are similar to those from the Black Hills (ca. 1,700 m.y.). The batholith intrudes mafic schist as well as the granite and gneiss; Rb-Sr ages are 1,490 m.y.; K-Ar ages of biotite from the granite batholith and schist as well as whole-rock ages of the schist are between 1,360 and 1,300 m.y. These 1,360-1,300-m.y. ages reflect a period of metamorphism or thermal activity subsequent to the original formation of the rocks but are not known to be associated with a granite-forming event here. K-Ar ages of biotite from cataclastic zones in southwestern Nebraska are 1,170 m.y. A body of anorthosite-gabbro in southwestern Nebraska has not been dated, but its cataclastic textu e and a zone of cataclasis that trends toward the body suggest that its intrusion is either contemporaneous with, or older than, the time of cataclasis.

Isotopic ages in central Kansas suggest two thermal events: 1,450-1,350 m.y. and 1,250-1,100 m.y. The latter is defined on the basis of Rb-Sr and K-Ar mica ages, the former on Rb-Sr feldspar or whole-rock analyses.

Granite in southeastern Nebraska, northeastern Kansas, and central Missouri is at least 1,450 m.y. old. Most of the Nemaha uplift as well as many granites at least as far west as south-central New Mexico give isotopic ages in the 1,450 to 1,350-m.y. range. During this period (the Nemaha igneous activity), rocks from New Mexico eastward to Ohio were consolidated to form the basement for younger extensive volcanic-intrusive complexes.

Keweenawan basalt and associated sedimentary rock units are nearly continuous from northeastern Kansas to the outcrop belt around Lake Superior. An offset in trend and a break in continuity near the Nebraska-Kansas border have been postulated to be the result of a post-Precambrian transcontinental wrench fault. Cataclasis, recognizable in some basement samples within this discontinuity, could be associated with either rifting of the Keweenawan trough or Paleozoic movement along the adjacent north-trending Nemaha uplift.

Extensive areas in southeastern Missouri, northeastern Oklahoma and vicinity, and in and near the Texas Panhandle are underlain by rhyolite and related granite. Rhyolite is found in isolated localities in Wisconsin and in some of the few basement tests in Illinois, Indiana, and Ohio. Three distinct volcanic-intrusive events can be defined: St. Francois igneous activity of southeastern Missouri (to Ohio?), 1,350-1,200 m.y. ago; Spavinaw igneous activity of northeastern Oklahoma, 1,300-1,150 m.y. ago (with a 1,200 ± 30 m.y. isochron); and Panhandle igneous activity of Texas and eastern New Mexico, 1,200-1,100 m.y. ago (1,140 ± 50 m.y. average).

Each igneous event produced micrographic granite as well as rhyolite (many samples show well-preserved welded tuff textures). Rhyolite from the Spavinaw and Panhandle igneous activities covers at least 24,000 and 21,500 sq mi, respectively; the original extent of the volcanic fields is unknown.

Exposed in the Llano uplift of central Texas is a thick sequence of sedimentary rocks that were deposited, folded, metamorphosed, and intruded 1,150 to 1,000 m.y. ago during the Llano orogeny. Rocks associated in time with this event are found in the subsurface of north-central Texas and Trans-Pecos Texas.

Extensive areas are covered by sedimentary rocks of probable or known Precambrian age. From north to south these include: Sioux Quartzite, minimum age 1,200 m.y.; upper Keweenawan sediments in the Keweenawan trough; sediments of southwestern Missouri-southeastern Kansas; Rice Formation of Scott, central Kansas; Tillman metasedimentary group of Ham et al., southern Oklahoma and adjacent parts of Texas; De Baca terrane of this report, south-central New Mexico (possibly correlative with the Swisher diabasic terrane = Swisher gabbroic terrane of Flawn, of the Texas Panhandle); and Fisher metasedimentary terrane of Flawn, north-central Texas (probably correlative with the Llano uplift metasedimentary rocks).

The last major igneous event of the central United States before the Paleozoic marine inundation was the intrusion and extrusion of the early to mid-Cambrian basalt-gabbro-rhyolite-granite igneous complex of the Wichita Mountains of southern Oklahoma.

Known Precambrian rocks have yet to be penetrated by wells in the Gulf coastal plain. Crystalline basement rocks in the inner coastal plain appear to be metamorphosed Paleozoic rocks of the Ouachita system. Southward the basement is covered by thick sedimentary rocks of the Gulf Coast geosyncline.