Llandoverian to Ludlovian Barrier Reef Complex in Southeast Wisconsin

Charles W. Rovey

Subsurface exploration in the Michigan basin established that a carbonate bank and barrier reef complex prograded basinward during the late Wenlockian to early Ludlovian, but the corresponding Niagaran Series is generally undifferentiated. In southeast Wisconsin the series is well exposed; thus, a better record of depositional history is available.

Until now, reefs in the Racine formation of southeast Wisconsin (upper Wenlockian through lower Ludlovian) were interpreted as patch reefs built landward of the barrier complex. However, the following criteria are consistent with an extension of Michigan's northern barrier complex beneath Lake Michigan to southeast Wisconsin: (1) Ubiquitous presence of reef facies along a southwest to northeast trend. This trend is coincident with thickening and a facies change indicative of a deep to shallow water transition. (2) similarity in depositional sequence of the overlying Salina Group in Wisconsin and Michigan. The Salina sediments surround, but are absent over, structures interpreted as pinnacle reefs and form a feather edge against the thicker belt interpreted as a barrier complex. Hence, the Racine reefs are reinterpreted as a barrier and pinnacle reef complex.

Similar facies changes are also present in older formations. Intraformational truncation surfaces in the underlying Waukesha Dolomite (upper Llandoverian to lower Wenlockian) clearly indicate the presence of a nearby carbonate slope. Therefore, the carbonate buildup originated prior to the Wenlockian and migrated further basinward than previously believed.

AAPG Search and Discovery Article #91023©1989 AAPG Eastern Section, Sept. 10-13, 1989, Bloomington, Indiana.