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Illinois Basin--Tectonically Influenced Ramp During Chesterian

Janis D. Treworgy

Vertical and lateral distributions of lithofacies of the siliciclastic Fraileys-Big Clifty and carbonate Haney Formations in the Illinois basin, as determined from subsurface mapping and outcrop studies, provided the basis for determining depositional settings and paleogeography. The Illinois basin appears to have been a shallow epeiric sea of a low-angle paleoslope and low-relief sea floor, and is interpreted further to have been a tide-influenced ramp with no marked shelf edge, rather than a rimmed platform. Sedimentation was influenced by limited synsedimentary tectonism along the Du Quoin monocline, the La Salle anticlinal belt, and the Rough Creek lineament, which are Precambrian-rooted structural hinge lines that separated what was generally the deeper part of the b sin--the Fairfield basin--from shallower shelf areas, including the western, eastern, and southern shelves. Throughout Fraileys, Big Clifty, and Haney deposition, the western shelf and the eastern part of the eastern shelf were generally shallow, nearshore, upper ramp settings. The southern shelf was a lower upper ramp setting that functioned as a shallow sill separating the Illinois basin from the epicontinental sea to the south. The vertical sequence of lithofacies from the base of the Fraileys-Big Clifty, which overlies the Beech Creek Limestone, to the top of the Haney indicates three changes in relative sea level: (1) regression I, which occurred between the end of Beech Creek deposition (lower ramp) and emergence of shelf areas during late Fraileys-Big Clifty deposition; (2) transg ession, which occurred after Fraileys-Big Clifty emergence and prior to deposition of basal Haney limestone; and (3) regression II, which continued throughout Haney deposition.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.