The Relationship Between Silurian Reefs and Structural Features in the Michigan Basin

By

MIKULIC, DONALD G.

Illinois State Geological Survey, 615 E. Peabody Dr., Champaign, IL 61820

Joanne Kluessendorf

Weis Earth Science Museum, University of Wisconsin-Fox Valley, 1478 Midway Rd., Menasha, WI 54952

     Early Silurian strata in eastern Wisconsin exhibit previously unrecognized depositional patterns and structural features that played a significant role in the later development of reef trends in the northern portion of the Michigan Basin.

      Slow subsidence and shallow, marginal-marine deposition characterized northeastern Wisconsin and adjacent portions of the Upper Peninsula of Michigan beginning in the late Ordovician and extending into the early Silurian (early Llandovery). In the Illinois Basin to the south, normal-marine shelf deposition predominated. During the mid-Llandovery, a prominent uplift occurred in east-central Wisconsin which resulted in a broad topographic high displaying a southwest-northeast trend. This high area became emergent and was karstified. Subsequent onlap during the late Llandovery and early Wenlock produced a shallow water carbonate bank, which overlies and runs parallel to this uplifted area. This bank separated areas of continued shallow water deposition to the north that was commonly marginal marine in nature from basinal environments to the south. Numerous individual reefs developed down slope and basinward of the bank deposits. 

     This trend appears to extend across Lake Michigan to the northeast where considerable amounts of oil and gas have been recovered from deeply-buried Silurian reefs in the Lower Peninsula of Michigan. Only limited data are available for early Silurian and nonreef rocks in this region, but those that are available fit the depositional trends seen in eastern Wisconsin. Currently there is no information suggesting similar tectonic controls on the location of reef trends in the Silurian rocks of southern Michigan or the Illinois Basin.

     Previous studies of the Silurian paleogeography of the Michigan Basin suggested that general basin subsidence was the primary control in the development and distribution of the Silurian reefs. It is now clear that a more localized short-term tectonic event was probably the cause for both the development and location of the northern reef trend.