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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 957

Last Page: 957

Title: Resedimented Deposits and Evolution of Thornton Reef (Niagaran) Northeastern Illinois: ABSTRACT

Author(s): James E. McGovney

Article Type: Meeting abstract


The significance of the Thornton reef, long established as a major reef model by paleoecologic studies, is enhanced by the recent recognition that significant volumes of the reef were deposited by submarine sediment-gravity flows and that these resedimented deposits document a distinct stage of reef growth. The Thornton reef (2 km diameter) is an erosionally truncated carbonate buildup that consists mostly of flank beds that dip steeply (30 to 40°) and radially away from the reef's center. The flank beds are thin, even, continuous, and consist of syndepositionally cemented crinoid wackestone and packstone. They were deposited by in-place sediment accumulation. In contrast, the resedimented conglomerates, which comprise 10 to 20% of the reef, occur as wedges, tongues, and sheets 0.1 to 25 m thick of lithoclast/skeletal wackestone/packstone. Lithoclasts range from 1 mm to 20 m across, and are made up of wackestones and boundstones. Debris flows were probably responsible for the thick boulder-bearing tongues and sheets in the interreef facies. A more prevalent and enigmatic resedimented deposit consists of steeply dipping (to 15°) wackestone/packstone conglomerate wedges that mantle the reef's outer margin. These are interpreted as the product of grain cascades down the reef flank.

Clasts in the resedimented deposits, derived from the now-eroded part of the reef are more stromatoporoid-rich and faunally diverse than the contemporaneous flank. This difference is attributed to a more energetic environment in the eroded part of the reef. There is no evidence in these clasts for lagoonal restriction, emergence, or shallow-water surf action.

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