Late Silurian Depositional Patterns in the Proto-Illinois Basin

Abstract

Late Silurian Depositional Patterns in the Proto-Illinois Basin

MIKULIC, Donald G., Illinois State Geological Survey (ISGS), University of Illinois, [email protected], Barrick, James E., Texas Tech University; [email protected], Butcher, Anthony, University of Portsmouth, UK, [email protected], Kluessendorf, Joanne, Weis Earth Science Museum, [email protected], Loydell, David K., University of Portsmouth, UK, [email protected], Merrell A. Miller: irf group, [email protected], Norby, Rodney D., (ISGS), University of Illinois, [email protected]

Silurian depositional trends of the proto-Illinois Basin have been of interest since the discovery of reef related hydrocarbon reservoirs in the 1940s. Several models of late Silurian shelf/basin development were proposed to account for the distribution of these reefs; however, precise data needed to demonstrate the validity of specific models have been lacking. New chemo-, bio-, and sequence stratigraphic data derived from a reevaluation of a frequently referenced core from White County, Illinois, provide clarification.

These new data demonstrate that a continuous late Silurian (Ludlow and Pridoli) section is represented in the basin, which helps to redefine the Silurian/Devonian boundary. Also, the 100-meter-thick Bailey Limestone, formerly considered to be wholly or partly Silurian, is now assigned to the Devonian. This dramatically reduces the thickness of the late Silurian basinal rocks to just 69 meters. In contrast, contemporaneous individual shelf-slope reefs, which are truncated, and the surrounding eroded shelf area have a thickness of 200–300 meters. The age of the preserved portions of these structures is not younger than Ludlow.

Some previous studies favored ramp models in which a limited reef/shelf depositional topography resulted from greater rates and volumes of basin sedimentation, which included Bailey sediments. These new data favor a depositional model with rapid rates of carbonate deposition in the reef and shelf areas that contrast with much slower rates of deposition in the deeper basin. The result is a distinct reef/shelf topography that ranged 100–200 meters in elevation above the basin floor.

AAPG Datapages/Search and Discovery Article #90218 © 2015 Eastern Section Meeting, Indianapolis, Indiana, September 20-22, 2015