Operation Basement: Buried Precambrian Rocks of Missouri--their Petrography and Structure

Abstract

The buried Precambrian basement of Missouri is described on the basis of data from 550 drillholes, petrographic analyses of more than 1,000 samples, and geophysical and structural data. Prominent topographic and probable structural highs are identified as (1) the Central Missouri high, (2) the Southeast Missouri high, (3) the Southwest Missouri high, and (4) the Northeast Missouri high. The first two of these underlie the Ozark dome. Basement rocks associated with the Central Missouri high and locally found elsewhere are gneissic granite, gneiss, and schist. These rocks formed at deeper crustal levels, subsequently have been upwarped to the basement surface, and may be part of an earlier orogenic period. They include lenses of infolded metamorphosed sediments, as well as intrusive granite and basic plutonic rocks. It is assumed that the crustal complex is older than late Precambrian igneous rocks exposed in southeast Missouri, and that regionally persistent prominent northwest trends observed on geophysical maps and in Paleozoic structures are related to structural belts of the crustal complex. In western Missouri a northwest-trending basement fault is defined and interpreted as the northern fault of a graben wher supracrustal Precambrian and possibly younger rocks locally are preserved and brought into contact with metamorphic rocks. The association of late Precambrian rhyolitic volcanic and related shallow intrusive rocks is referred to as the St. Francois terrane. These rocks are exposed in southeast Missouri and extend in the subsurface northwestward. They are associated with the Southwest Missouri high and the Northeast Missouri high. Volcanic rocks identified from southwest Missouri may be part of a volcanic terrane extending into southeast Kansas and northeast Oklahoma; rhyolite in northeast Missouri may extend into Illinois. The granitic rocks of the St. Francois terrane are interpreted as postorogenic subvolcanic massifs that are exposed at the basement surface where pre-Paleozoic uplift and erosion removed their supracrustal cover. Northeast-southwest structural trends are believed to be related in part t the evolution of the St. Francois terrane and are superimposed on the older trends. The youngest igneous rocks of the basement complex are dikes and sills of tholeiitic diabase intruded along both sets of pre-existing structural zones. Local accumulations of unmetamorphosed clastic sediments on the basement surface may be Precambrian or younger.