Journal of the Graduate Research Center


The Austin formation (Upper Cretaceous) in Dallas County is divisible into three members: a lower chalk, a middle marl, and an upper chalk. Because of the gentle dip of the Austin formation and the low surface relief of the area, stratigraphic sections of as much as 30 feet are rare, thus no complete surface section exists for any of the members. A composite stratigraphic section of the lower chalk member, 155 feet thick, was constructed from exposures in the upper reaches of Ten Mile Creek. Individual sections in excess of 10 feet were carefully measured and correlated by insoluble residues, in conjunction with bed thicknesses and weathering profiles, to obtain the composite section. The microfaunal content was examined at five foot intervals and points of significant lithologic change. Approximately 50 species of Foraminifera, representing 26 genera and 9 families, are present in the lower chalk member. The dominant species are Globigerina cretacea d'Orbigny, Globotruncana canaliculata (Reuss), G. marginata (Reuss), G. fornicata Plummer, Gumbelina striata (Ehrenberg), and Planulina sp.; cf. P. austinana Cushman. Globigerina cretacea d'Orbigny is the most common and persistent. All of the above are planktonic except Planulina. The only indications of limited vertical distribution within the lower chalk member were restricted occurrences of Rectogumbelina hispidula Cushman and Pleurostomella watersi Cushman. The former was observed only in a single sample taken approximately 8 feet above the Eagle Ford-Austin contact; the latter was found in samples of the uppermost 12 feet of the lower chalk section as one of the most common species; it probably, however, ranges upward into the middle and upper Austin.

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