Abstract

Modern interpretations of discrete midwestern Silurian reefs have become so diverse that attempts to construct reef models have resulted in a veritable geological collage. We believe, in contrast, that a truly representative Silurian reef model should be constructed more along the lines of a mural—as an artists' mural fashioned by a continuum of Old Masters and modern artists, thus giving the model order, purpose, and continuity throughout.

Therefore we propose a comprehensive shallow-water model for Silurian reefs in the southern Great Lakes region, a truly muralistic model that contains the following major elements: (1) the great majority of the reefs originated in water no more than a few tens of meters deep and were able to maintain their reef-top surfaces near or within the zone of wave activity during much of the life of the reefs; (2) reef-framework-builders had prominent roles in the construction of virtually all Silurian carbonate buildups of this area that are frequently called "reefs"; (3) virtually all of these buildups called "reefs" contain recognizable structural cores in which the framework-builders are (or were before diagenesis) more prominent than in other parts of the reef; (4) most of the reefs grew both upward and outward through time, some coalescing to produce large complexes; (5) all of the larger reefs developed steeply dipping flank beds around the margins of the cores; (6) typical flank beds consist of a combination of transported skeletal debris and in situ reef-dweller and reef-builder fossils along with some detrital siliciclastic muds and silts; (7) compaction, fracturing and solution processes acting differentially on reef cores, flank beds, and interreef beds have caused most flank beds to be rotated into steeper-than-original inclinations; (8) differential compaction and other processes also have resulted in subreef sag and suprareef drape in the nonreef beds associated with most reefs; and (9) subreef sag has counteracted some of the upper flank-bed rotation and has resulted in negative rotation angles for some lower flank beds, causing them to dip toward the reef centers.

Evidence presented both by us and by many other workers over the past several decades has been carefully used to establish these elements of the shallow-water Silurian reef model. Additional support for the model comes from modern stratigraphic determinations that permit distinctions among several different reef generations and that place restrictions on how the model may be refined. In addition, new geophysical evidence presented by other workers now strongly supports the concept of diagenesic steepening of flank beds.

Some modern-day proposals for modifications of the model, however, including suggestions of deep-water environments and coreless so-called "clinothem reefs", appear to have little applicability to the model presented here. They are contradicted by compelling petrologic and paleontologic indications of shallow water, observance of reef cores in most well-exposed reefs, and stratigraphic evidence of former reef growth above now-eroded benches, that is, so-called "wave-swept benches". The modifications have failed, in fact, just as the artists' mural fails if one or a few parts are rendered in style and concept so disharmonic as to negate the beauty achieved through harmony of the whole. We conclude, therefore, that the modern and comprehensive shallow-water Silurian reef model outlined above is a truly representative model and that it applies to virtually all aspects of midwestern Silurian reefs.

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