Determining Sedimentary Processes and Environments of Argillaceous Rocks by Fabric Analysis Techniques

Neal R. O'Brien, Roger M. Slatt

A detailed study of shale and mudstone sedimentary fabrics within cores through the Ferron Sandstone Member of the Mancos Shale (Lower Cretaceous, central Utah) was conducted to test the utility of micro-fabric and macro-fabric analyses for interpreting the depositional environments of argillaceous rocks. Methods of fabric analysis included x-radiography, scanning electron microscopy (SEM), and thin-section petrography; total organic carbon (TOC) and x-ray diffraction mineralogy were also analyzed. The environments of deposition of Ferron sandstones had been determined previously from an outcrop and core study so environments of associated argillaceous rocks could be inferred.

Characteristic primary sedimentary fabrics can be documented for argillaceous rocks deposited in delta plain, interdistributary bay, delta front, and marine shelf environments. These primary fabrics can be used to trace depositional environments vertically through progradational sequences or laterally across facies tracts. Bioturbation of argillaceous sediments has its own characteristic fabric and acts primarily to destroy variation in primary fabric among different environments of deposition.

Although mineralogy does not vary significantly with depositional environment, TOC varies significantly. TOC ranges from 0.2 to 0.8 wt. % in the upper delta plain, 2.4 to 4.8 wt. % in the lower delta plain and interdistributary bay, 1.3 to 2.8 wt. % in the delta front, and 0.4 to 1.5 wt. % in marine shelf rocks.

This study indicates that certain sedimentary processes leave clues to their origin in argillaceous rocks which, if not destroyed by bioturbation, may be revealed by using micro-fabric and macro-fabric and certain compositional analysis techniques.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.