Field and Laboratory

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A partial composite section totalling 300 feet was constructed for the Upper Cretaceous Austin Chalk through correlation of overlapping portions of some of the larger incomplete sections. The correlations were effected primarily by insoluble residues in conjunction with thicknesses and weathering profiles. Bar graphs based on insoluble percentages indicate that the formation is composed of the following principal lithologic types: marly limestone, marlstone-limestone, limy marlstone, and marlstone. These are variably distributed vertically and to a lesser degree laterally. The subdivisions of lower chalk, middle marl, and upper chalk are a function of the proportions in which the basic lithologic types are present in these portions of the column. Based on insoluble residue correlations, composite sections of approximately 100 feet each were constructed for the basal lower chalk, the lower chalk-middle marl, and the middle marl-upper chalk contact. The intraformational contacts are both conformable and transitional. The use of insoluble residue determinations to correlate incomplete sections is an effective, but exceedingly time-consuming procedure. Correlations are most readily made with proximate sections 30-40 feet or more, but the number of such sections in any given area is limited. The method is not recommended as a readily applicable technique for construction of a detailed stratigraphic section of the Austin Chalk in Dallas County.

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