The Relationship Between Silurian Reefs and
Structural Features in the Michigan Basin
MIKULIC, DONALD G.
Illinois State Geological Survey, 615 E. Peabody
Dr., Champaign, IL 61820
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.
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
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.