Fracturing in Upper Cretaceous Selma Group Chalky Marls, Inner Coastal Plain of Alabama: Stratigraphic (Facies) Control of Joint Development and Regional Joint-Strike Orientations

Enid Bittner, David T. King, Jr., Ira Holston

In the inner coastal plain of Alabama, chalky marl is a distinctive sedimentary facies that contains more original biogenic calcareous-nannofossil components than the adjacent marls. The chalky marl is one facies in the cyclically arranged genetic packages that comprise the Mooreville and Demopolis Chalks of the lower part of the Upper Cretaceous Selma Group. Analysis of genetic-package stratigraphy shows that the chalky marls were deposited during relative sea level high-stands. The chalky marls are selectively cemented by calcium carbonate, thus making a relatively well-indurated brittle rock. The chalky marls, in three sections over 15 m thick, host most of the systematic joints in the Mooreville and Demopolis. As a result, fracture-density mapping clearly delineates t e outcrop belts of the thick fracture-prone chalky marls. Joint systems in the marls and chalky marls have two main strike trends (N30°E to N70°E and N20°W to N50°W) and two secondary strike trends (north to N10°E and east-west), which are nearly parallel and perpendicular, respectively, to crystalline bedrock structural trends and fracture zones. Thus, bedrock structural trends and fracture patterns appear to have propagated through the overlying Mesozoic rocks, dictating the orientation of joints in the chalky marls.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.