Geometry, Paleontology, and Petrography of Thornton Reef Complex, Silurian of Northeastern Illinois

Abstract

A detailed study of the geometry, paleontology, and petrography of a Silurian Reef Complex, which lies about 10 miles southwest of the southern tip of Lake Michigan, demonstrates that the complex was a true reef 575 feet thick at its maximum development. The reef was characterized by an organic framework or core and a surrounding, preponderantly bioclastic, flank deposit on which many satellite reefs developed. Initial reef development was probably from coalescing coral growth centers on the floor of a Silurian epicontinental sea. Upon attainment of wave base by the beginning of the Niagaran Epoch, a prevailing west-by-southwest wind, in conjunction with a surrounding water depth of about 200 feet, controlled the development of a definite southwest-to-northeast ecologic zoning in the form of distinct biotopes and lithotopes. The reef core, represented by a preponderance of skeletal material in growth position, is in successive order the site of a windward wave-resistant stromatoporoid ridge, a coral rampart, a dead-reef flat, a lagoon beach, a lagoonal crinoid meadow, and a leeward wave-resistant stromatoporoid ridge. The core is surrounded by predominantly bioclastic flank deposits, consist ng of rhythmic alternations of fossil fragments, calcareous muds, and argillaceous sediments. The flank was the site of organic communities dominated by brachiopods, gastropods, or sponges. Other areas of the flank were characterized by a lack of a distinctive fauna. Continued rise of sea-level allowed further upward and lateral reef growth with the result of an atoll stage of reef development by early Cayugan time.