Slade, Laura¹, Rick Abegg¹, Chris Maples² 
(1) ChevronTexaco, New Orleans, LA
(2) Indiana University, Bloomington, IN

ABSTRACT: Widespread Anhydrites of the Lower St. Louis Limestone (Meramecian)

The lower St. Louis Limestone, deposited across the interior of North America during the middle Mississippian, represents a widespread and often underappreciated episode of evaporite deposition. Thick anhydrites interbedded with carbonates accumulated in the Illinois Basin in Indiana and in the Hugoton Embayment of the Anadarko Basin in southwestern Kansas. 
In the subsurface, the lower St. Louis Limestone in the Illinois Basin is composed of shallowing-upward cycles characterized by restricted subtidal algal and peloidal wackestones, intertidal laminated mudstones and dolostones, and supratidal nodular gypsum. The evaporite beds, ranging 0.15 to 5 m in thickness, are composed of bluish-white nodular anhydrite. The sedimentary structures and cyclicity within the lower St. Louis Limestone resemblance Recent deposits in the Persian Gulf coastal sabkhas. 
The lower St. Louis (Hugoton Member) in the Hugoton Embayment contains two evaporite units that are tens of meters thick, separated by up to 17 m of subtidal carbonates. Anhydrite beds are up to 8.2 m thick, typically contain less than 20% carbonate sediment and few supratidal indicators, and are interpreted to have been deposited in a shallow subaqueous saltern that was deposited throughout most of the Hugoton Embayment. 
The lower St. Louis is interpreted to be deposited during a third order transgressive systems tract. Restricted circulation allowed for extensive anhydrite deposits to accumulate, although enough connection to the open ocean was maintained to prevent halite precipitation. The widespread extent of these coeval evaporites indicates an arid climate and suggests that restriction was the result of a relative sea-level fall.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.