DETAILED GRAVITY PROFILE ACROSS THE WAUKESHA FAULT, SE WISCONSIN

A detailed E-W gravity survey was conducted in Ozaukee County, Wisconsin, along a ~7.2 mile segment of Highland Road ending at Port Washington Road. The purpose of the survey was to further delineate the location and geometry of the Waukesha fault in this area. Previous surveys identified a large, complex gravity anomaly associated with the fault. Comparison between parallel E-W gravity surveys along Pioneer Road (2 miles north of Highland Road) and County Line Road (5 miles south of Highland Road) indicated a significant change in fault geometry over a relatively short distance.

Gravity anomalies for the Highland Road traverse were computed relative to Station 28 (Waukesha, Wisconsin) of the Wisconsin Gravity Base Station Network, with an absolute gravity of 980341.98 mGal. Gravity data were collected at ~1000 foot intervals and reduced to Bouguer anomalies using a Bouguer density of 2.67 g/cm³.

In general, gravity anomalies along the Highland Road traverse are high west of the fault and low east of the fault, ranging from ~14 mGal on the western end of the survey to ~4 mGal to the east. This change in gravity occurs over a horizontal distance of ~4 miles in the center of the survey, with a gradient of ~2.5 mGal/mile. A total range of ~13 mGal with a gentler gradient was previously recorded along the Pioneer Road traverse, while the survey along County Line Road yielded a total range of ~8 mGal with a much steeper gradient.

Our model, combined with previous models, indicates that the Waukesha fault is a northeast-striking, southeast-dipping normal fault that displaces Paleozoic strata but not Quaternary till. The projected surface trace of the fault along Highland Road is ~7 miles west of the Lake Michigan coast. Modeled estimates of vertical displacement of Precambrian basement range from 860 feet to several thousand feet depending on the density chosen for the basement. Even though the three gravity traverses are within 7 miles of one another, the observed differences indicate that the fault geometry changes significantly along strike.