Geology of the Lavon Area, Collin County, Texas

Abstract

The Lavon area, in southeastern Collin County, Texas, includes approximately 35 square miles. The rocks are of Cretaceous and Cenozoic age. Cretaceous rocks are represented by the marine Taylor Formation, here divided on the basis of chemical and physical characteristics into four members: siltstone, lower marl, glauconitic limestone, and upper marl, named in ascending order. The siltswne consists mostly of dark gray silt, and averages approximately 20 per cent solubles. The lower marl, the base of which is marked by a thin zone of irregular phosphatic nodules, is a pale brown, calcareous clay averaging about 55 per cent solubles. A disconformity separating the siltstone and lower marl is interpreted as a minor stratigraphic break of submarine origin. The glauconitic limest0ne is a very pale brown, sandy, and glauconitic fragmental rock that averages approximately 65 per cent solubles. The upper marl is similar in lithology to the lower marl but averages 47 per cent solubles. A hard, light gray layer of limest0ne occurs in the upper portion. Tentatively, the siltstone is correlated with the Wolfe City member of the Taylor Marl. The overlying lower marl and glauconitic limestone members presumably are the equivalent of the Pecan Gap Chalk though radically different in lithology. The Cenozoic strata are fluvial deposits in terraces and floodplains. The alluvium is composed mostly of clay and silt with subordinate amounts of pebble gravel. In the Cretaceous sequence the attitude of bedding can be determined at only a few localities. The siltstone and glauconitic limestone members strike NNE, and even though slight steepenings occur locally, the average dip probably is less than one degree toward the east. Joints are abundant in the Cretaceous rocks, especially in the lower marl. Several sandstone dikes, similar to those near Rockwall, Texas, cut the upper marl.