Page 284 - Petroleum Geology

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SILURIAN REEFS OF THE GREAT LAKES AREA

Lowenstam (1950) demonstrated that the Silurian (Niagaran) reefs of the
Great Lakes area could properly be called reefs because the stromatoporoids
not only had the potential to build a wave-resistant structure, but clearly did
so. Silurian reefs are exposed at the surface around Lake Michigan and around
the north shore of Lake Huron to the Bruce Peninsula, from which the out-
crop extends south-east (see Fig. 12-3). Chamberlin (1877) provided the first
description of fossil coral reefs in North America from the Silurian reefs of
Wisconsin. The subsurface reef trends are to the east of Lake Michigan, around
the eastern and southern shores of Lake Huron, in Michigan and Ontario.
In this broad area during Niagaran and earliest Salinian times, there was a
general carbonate accumulation with associated evaporites. Along the margin
of the Michigan basin, the Silurian reefs seem to have developed into effective
barrier reefs, leading to the accumulation of evaporites in the back-reef areas.

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0 100miles
100 km
Fig. 12-3. Palaeogeographic map of north-western Michigan basin. Horizontal ornament:
platform-shelf carbonates. Diagonal : platform-margin barrier reef. Blank: platform slope
pinnacle reefs. Vertical: basinal carbonates. (After Gill, 1979, p. 609, fig. 1.)