Field and Laboratory


Louis C. Read

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The Midlothian quadrangle in northwest Ellis County, Texas, is underlain by the Eagle Ford Shale and Austin Chalk, both of Upper Cretaceous age. The upper 75 to 100 feet of Eagle Ford shale crops out within the quadrangle. It is predominantly a dark gray clay containing flaggy detrital limestone beds. Separating the shale from the overlying chalk is a six-inch conglomerate bed containing black phosphate pebbles, phosphatized fish teeth and molluscan shells, glaueonite, quartz, gypsum, and calcareous foraminifera tests. The lower Austin Chalk consists of massive chalk beds interbedded with thin beds of marl or bentonitic shale. In the upper 30 or 40 feet of this unit the beds are less massive. Above the lower chalk there is a 30-foot transition zone in which the rock is intermediate in lithology between the chalk and the middle marl unit above. The middle marl unit contains thick beds of marly chalk and chalk. Terrace deposits consisting largely of chalk gravel are probably correlative with the Marsalis Terrace of Dallas County. A northeast-striking fault with a throw of at least 90 feet is exposed along Long Branch in the east-central part of the quadrangle near Sardis. There are three other faults of similar trend with throws of at least 25 feet, in the area. These faults are probably related to the Balcones Fault system. Northeast-and northwest-trending joints are numerous.

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This work is licensed under a Creative Commons Attribution-Noncommercial 4.0 License

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