WAVES AND SAND AND . . . MUD? SEDIMENTARY ENVIRONMENT OF AN EXPOSURE OF THE PLACID SHALE AND RANGER LIMESTONE, NORTH CENTRAL TEXAS

The Upper Pennsylvanian is characterized by an interval of clastic and carbonate deposits in north Central Texas. The recently uplifted Ouachita Mountains were shedding sediments to the northwest, and these deposits are well represented in the exposed Canyon Group formations exposed in and around Ranger, Texas. The Canyon Group at the study location consists of the Placid Shale and is capped by the overlying Ranger Limestone formations. Here, the Placid Shale consists of grey, silty shales and smaller units of calcareous, brown-to-grey cross-bedded sandstones and siltstones. Distinct within the sandstone units is the presence of petrified wood, fossil leaves, and ferruginous concretions, as well as occasional herringbone crossbedding. Past descriptions of the Placid Shale interpret the formation as deltaic, with channel-fill deposits. Alternatively, the shale as observed at this locality may represent a ‘clastic swamp’ marginal facies between terrestrial and swamp depositional environments. This is evidenced by the high percentage of plant material within this rock unit, as well as the lack of classical delta bedding. The overlying Ranger Limestone is the cap rock in this area, and consists of sandy, fossiliferous limestone. This unit has traditionally been interpreted as indicative of a cyclic rise in sea level as part of the Absaroka transgression. Supporting the sedimentary evidence is the presence of numerous marine fossils in the uppermost Ranger Limestone, including brachiopods, rugose corals, crinoids, bryozoans, and other marine invertebrates. Despite the marine nature of this unit, the influence of the nearby coastline is evident, as shown by the small, rounded grains of carbonized plant material observed under magnification. In addition, the oxidation of the Ranger Limestone is typical of well-oxygenated shallow marine units, further supporting the nearshore nature of this marine unit. As such, the outcrops in Ranger and the surrounding area contain abundant evidence of the interplay between channel filled sandy coastlines and shallow marine sequences.