Magnetic and gravity anomaly maps of West Virginia

Abstract

The magnetic and gravity maps of West Virginia show anomalies or disturbances in the Earth’s magnetic and gravity fields. These maps are part of a series of geologic, geochemical, structural, and geophysical maps compiled at the scale of 1:1,000,000 for mutual comparison and are produced in cooperation with the West Virginia Geological and Economic Survey. All maps of the series present the mineral resources and pertinent background earth science information of West Virginia. Because the magnetic and gravity anomaly maps are produced at a small scale, they show only anomalous features that are larger than 20 square miles. Smaller anomalous features appear on anomaly maps previously published at the scale of 1:250,000 (U.S. Geological Survey, 1974a,b, 1976, 1978; Kulander and Dean, 1987). West Virginia lies within the Appalachian Plateaus and the Valley and Ridge Province (see fig. 1) and northwest of the exposed crystalline rocks of the Blue Ridge and Piedmont Province (Bayer, 1983). The Appalachian Plateaus and Valley and Ridge Province comprise the Appalachian basin, which contains a thick sequence of Paleozoic sedimentary rocks that are folded and faulted; deformation is stronger to the east in the Valley and Ridge Province than in the Appalachian Plateaus. Most of West Virginia occupies a broad recess of subdued folding between two arcuate salients of the Appalachian fold belt, the South Mountain salient to the north and the Mount Rogers salient to the south (Rankin, 1976; see fig. 1). The basement of Precambrian igneous and metamorphic rocks that underlies the Appalachian basin is poorly known; in West Virginia the basement is penetrated by only ten deep drill holes (fig. 2 and table 1), all in the western part of the state (Schwietering and Roberts, 1988; Schwietering, unpub. data, 1991). On the basis of drilling and seismic reflection data (Cardwell and others, 1968; Bayley and Muehlberger, 1968; Cardwell, 1977; Pohn, 1994), the basement rocks underlying West Virginia are buried to depths of about 7,000 to 36,000 ft below sea level. These rocks are metamorphosed intrusive and sedimentary rocks of Middle Proterozoic (Grenville) age, which has been determined by K-Ar dating on biotite at 950±150 Ma from similar rocks exposed in Canada (Moore and others, 1986). Basement rocks in West Virginia that have been intersected by deep wells include gneisses and intrusive rocks classified as syenite, granite, granodiorite, and tonalite, most of which are highly weathered near the top of the basement surface (table 1). Ammerman and Keller (1979) have postulated that basalt flows may occur in the basement in the deepest parts of the Rome trough (fig. 1); however, no such flows have been intersected in deep wells. Ages of 870 Ma (Bass, 1960, map number 9 in fig. 2 and table 1) and 939 Ma (Heald, 1981, map number 7 in fig. 2 and table 1) were obtained by rubidium-strontium and potassium-argon dating of biotite from basement rocks. Similar ages have been determined from cores of basement rocks in adjacent parts of Ohio and Kentucky (Summerson, 1962; Ammerman and Keller, 1979). All ages fall between 1100 Ma and 800 Ma, the period of the Grenville orogeny, when a broad swath of the eastern part of ancestral North America was extensively intruded and metamorphosed during the collision of North America and a continental mass to the southeast (Moore and others, 1986). Following a half billion years of erosion of the orogenic region, the Rome trough (Woodward, 1961; McGuire and Howell, 1963) developed as part of an intra-continental rift system extending from western Tennessee to Pennsylvania (Webb, 1980).