Abstract

Gravitational mapping by the University of Wisconsin has delineated what appears to be the largest positive-anomaly feature on the North American continent, extending from the Lake Superior region southwest into Kansas. For the greater part of its length this midcontinent gravity high is flanked on both sides by gravity lows. Because the southern part of the anomalous area is blanketed by Paleozoic sediments, the cause of the anomaly was sought first at its northern end, around Lake Superior, where Precambrian rocks crop out to facilitate a correlation of gravity and geology.

Around western Lake Superior the positive anomalies correlate with Keweenawan lava and gabbro. The negative anomaly on the Bayfield Peninsula reflects a thick accumulation of sandstone and shale which was deposited in the subsiding Lake Superior syncline during Upper Keweenawan time. A second thick accumulation of sedimentary rocks may underlie the gravity low at Cumberland. Steep gravity gradients indicate the Douglas fault. A second major fault symmetric to the Douglas fault is mapped in northwestern Wisconsin on the opposite side of the Lake Superior syncline. The center of the syncline has been thrust upward between the two faults as a horst. A traverse along the spit at Duluth fails to detect the North Shore fault as it is usually mapped; if the fault exists, no great amount of sandstone is placed in juxtaposition to basaltic lava. Detailed correlation of gravity and geology is presented on maps and structure sections along lines of gravity traverse.

The usual isostatic correction cannot reduce the gravity differentials around western Lake Superior. A geological correction which allows for the effects of near-surface geology does account for most of the anomalies. Any attempt to compute the extent of crustal warping at depth without allowing for the near-surface geology would have led to considerable error.

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