PINNACLE REEFS Source: Unknown Source: Unknown Some of these reefs can be seen today on the south side of Chicago, and I-94 crosses over a giant quarry into one of them. Assuming that the Silurian reefs formed in an environment similar to modern reefs, we can infer something about the Great Lakes area during this time. All modern reefs grow on a shallow bottom, no deeper than the depth of light penetration (generally about 60 m). Furthermore, coral reefs are restricted to tropical regions where water temperatures are consistently above 77oF. Thus, the presence of extensive reefs around the Michigan Basin during the Silurian suggests a shallow tropical or sub-tropical environment. Source: Unknown Well-preserved fossils in most of these ancient reefs substantiate this assumption. Outward from the reefs, in the deeper water, muds rich in CaCO3 accumulated on the floor of the vast shallow seas, generally producing thinly bedded dolomite that contains few fossils. Thus, visualize a subsiding, circular basin in lower Michigan, surrounded by shallow waters and coral reefs.
Source: Unknown Meanwhile, in shallower waters, carbonate bank (shelf) deposits were being formed (landward of the pinnacle reefs) (see below). Source: Unknown Source: Unknown
Source: Unknown Source: Unknown
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