Abstract
ABSTRACT In the inner Coastal Plain of Alabama, chalky marl is a distinctive sedimentary facies which contains more original biogenic calcareous-nannofossil component than the adjacent marls. The chalky marl is one facies in the cyclically arranged genetic packages which 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 the outcrop belts of the thick, fracture-prone chalky marls. Joint systems in the marls and chalky marls have two main strike trends (N40E to N70E and N30W to N50W) and two secondary strike trends (North to N30E and east-west), which are nearly parallel and perpendicular, respectively, to crystalline bedrock structural trends and fracture zones. Thus, it appears that bedrock structural trends and fracture patterns have propagated through the overlying Mesozoic rocks, dictating the orientation of joints in the chalky marls.
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