Early Cretaceous Vertebrates of Texas

  Although the lithology of the Glen Rose Formation is typically described as "limestone," the unit is variable ranging from quartz siltstones and sandstones to marls, micritic limestones, dolomites, and boundstones (Davis, 1974; Perkins, 1974). In the type area the Glen Rose is subdivided into three members. The lower member consists of limestone, sandy mudstone, and sandstone. The Thorp Spring (or "Middle") Member, the most extensive unit of the outcropping Glen Rose, consists of lime mudstones, wackestones, and calcarenites. The upper member consists of alternating limestones and marls (Boone, 1968; Perkins, 1987).  

  The Glen Rose Formation is approximately 75 m thick in the type area near Glen Rose, but 16 km to the west, at the Paluxy Townsite, it is 45 m thick (Rodgers, 1967; Perkins, 1987). Sediments at Cedar Brake Camp (SMU Loc. 223; Figs. 1, 3), in the lower member of the Glen Rose Formation (6.2 km east of Paluxy Church and stratigraphically above), represent primarily peritidal and marsh facies (Perkins, 1987). 

  West and north of the Glen Rose type section correlation becomes increasingly difficult because of the lateral replacement of limestones by sandy facies. For example, at the Twin Mountains type section (Fig. 3) the Thorp Spring Member is apparently absent and the Glen Rose Formation has been reduced to approximately 10 m of sandy limestones alternating with marly siltstones. Near the base of the Glen Rose Formation at Twin Mountains, vertebrate fossils (primarily isolated pycnodont teeth and scales; SMU Loc. 225; Fig. 3) were recovered from a micritic limestone and marly siltstone lens. These units are probably correlative with the middle to upper members of the Glen Rose because the upper portion of the Twin Mountains Formation increasingly thickens and replaces the lower member of the Glen Rose Formation in these western exposures. Eventually, the Glen Rose Formation feathers out and the Twin Mountains Formation merges with the Paluxy Formation as the Antlers Formation.