A NEW GRAVITY MAP OF WISCONSIN: PROGRESS IN GRAVITY SURVEYING

A new preliminary Bouguer anomaly gravity map of Wisconsin has been created from more than 38,700 gravity measurements that have been collected between 1948 and 2000. Prior to the present project, more than 28,000 stations north of 44° N. were compiled by Ervin and Thompson. The current USGS effort is directed at completing the gravity coverage of the state south of 44° N. with a comparable station density. To date we have added measurements at more than 6800 new stations to achieve a nominal spacing of one to two miles. Older data from the National Geophysical Data Center (NGDC) have been replaced by this new data. The new data are tied to the Wisconsin First-order Gravity Base Station Network, which is in turn tied to the International Gravity Standardization Network-1971 (IGSN-71). The data were then gridded at an interval of 500 m. The map will be presented at a scale of 1:500,000.

Some notable features shown on this map include 1) the Mid-continent gravity high, 2) the flanking gravity lows corresponding to the Keweenawan sedimentary basins, 3) gravity highs over inferred mafic plutons in southeastern Wisconsin, 4) a low centered on the Precambrian Wolf River batholith, and 5) several northwest trending linear features of unknown origin.