SEQUENCE STRATIGRAPHY OF THE CAMBRIAN BONNETERRE FORMATION IN THE ILLINOIS BASIN: IMPLICATIONS FOR CARBONATE PLATFORM DEVELOPMENT AND ITS EVOLUTION IN SOUTHERN ILLINOIS AND THE ADJACENT REELFOOT-ROUGH CREEK RIFT

The Bonneterre Formation in the Illinois Basin is a dominantly carbonate succession and constitutes the upper part of the Sauk II supersequence. It was deposited along a southward-thickening carbonate platform, thickening from less than 150 ft (46 m) in the northwest of southern Illinois to over 3000 ft (914 m) in the Rough Creek Graben of western Kentucky. The Bonneterre conformably underlies the Davis member of the Franconia Formation and overlies the Mt. Simon Sandstone or Precambrian basement where the Mt. Simon is absent. To evaluate carbonate platform development, platform to off-platform transition, and its evolution, facies and vertical stacking of the Bonneterre Formation has been studied using the available subsurface data.

The Bonneterre Formation is characterized by distinct platform and basinal facies. Shaly fossiliferous lime mudstone-wackestone and packstone-grainstone facies containing bioclasts, ooids, peloids, and intraclasts representing inner platform, platform margin, and basinal environments are recognized. The Bonneterre platform carbonates consist of two transgressive-regressive sequences that can be correlated across the platform to basin transition. Within the rift, however, the Bonneterre succession includes three sequences composed of shale and shaly or sandy limestone that grade to clean oolitic limestone or dolomite upward. Deposition of Bonneterre carbonates occurred when sea level rise during late Middle Cambrian resulted in the development of a vast carbonate platform and terrigenous sedimentation was confined to the northern part of the basin. The platform was developed along the northwest shoulder of the Reelfoot rift and adjacent craton, with a platform margin that was facing the deep and rapidly subsiding Reelfoot-Rough Creek rift basin. High carbonate productivity during highstand along with cessation of tectonic movement along the rift border faults during middle to upper Bonneterre sedimentation, resulted in overproduction of shallow marine carbonates, leading to vertical and lateral expansion of the platform. Thus, progradation across the rift and gradual infilling of the basin led to gradual establishment of a homoclinal ramp platform by the time of deposition of the Upper Cambrian Derby-Doerun/Franconia Formation.