Publication Date


Document Type


First Advisor

McGinnis, Lyle D. (Lyle David), 1931-2017||Flemal, Ronald C.||Keighin, C. William

Degree Name

M.S. (Master of Science)

Legacy Department

Department of Geology


Gravity--United States--Illinois


A gravity survey of north-central Illinois was conducted as part of a state-wide gravity mapping program of the Illinois State Geological Survey. The three dimensional analysis technique of Talwani and Ewing (1960) was used to interpret the Bouguer gravity field. Two profiles across the study area were used to define several crustal models from which theoretical fields were then calculated. Theoretical fields that best fit the observed fields indicate the presence of a large, deep seated intrusive-like body that extends from the Precambrian basement surface to the base of the crust. A density contrast of +0 .04- gm/cm3 between the country rock and the "intrusive" and the presence of only acidic rocks in the basement in the midcontinent region suggests that the "intrusive" is a slightly higher density granitic phase than the older country rock. An inverse relation exists between small Paleozoic structures in this area, as has previously been observed in northern Illinois. It is concluded that variations in crustal densities are the chief cause of gravity anomalies in the area.


Includes bibliographical references.||Includes illustrations and maps.


31 pages




Northern Illinois University

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