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Only fragmentary direct information is available on the basement complex of the Southern Peninsula of Michigan because of limited and poorly distributed basement drill holes. This has encouraged the use of geophysical methods, primarily gravity and magnetic, to study the Precambrian formations. A basement configuration map prepared from magnetic depth estimates and basement drill tests confirms that the basement surface under the Southern Peninsula has the form of an oval depression reaching a maximum depth of more than 15,000 ft (4.5 km) below sea level on the western shore of Saginaw Bay. A basement topographic high is associated with the Howell anticline and a roughly north-south-striking basement trough plunges into the basin from the common boundary point of Indiana, Ohio, and Michigan. Aeromagnetic and Bouguer gravity anomaly maps, together with isotopic dates of samples obtained from basement drill holes and extrapolation of known Precambrian geologic trends, indicate that four basement provinces underlie the Southern Peninsula. The Penokean province can be traced geophysically from Lake Michigan into the Southern Peninsula, where it is characterized by east-southeast-striking geophysical anomalies. Central and southwestern Michigan is underlain primarily by felsic rocks correlating with the Central province. Basement rocks in southeastern Michigan strike north-northeast and are interpreted to be metamorphosed intrusive and extrusive rocks and mafic and felsic gneisses of the Grenville province. The Grenville front strikes south-southwest from Sagi aw Bay to west of the Howell anticline and from there due south to the Michigan Ohio boundary. A Keweenawan rift zone characterized by mafic intrusive and extrusive rocks and by uplifted gneisses transects all of the provinces and extends from Grand Traverse Bay to southeastern Michigan. Another subparallel ancient rift zone may be present in southwestern Michigan. These zones were formed during an episode of crustal extension in Keweenawan time. Subsequent deformation of the sedimentary rocks within the basin generally has been associated with movement along lines of basement weakness apparently related to the rift zone and Penokean structural trends.
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