THE LION MOUNTAIN SANDSTONE AS AN EXEMPLAR OF COMPLEXITY IN TRANSITIONAL ENVIRONMENTS

During the Upper Cambrian, the North American craton was inundated with the shallow seas of the Sauk transgression. Evidence of this fluctuating shoreline can be seen in the tidal, near shore deposits of the Lion Mountain Sandstone member in Burnet, Texas. This glauconitic sandstone is the uppermost exposed member of the Riley Formation, and are part of the oldest exposed sedimentary rocks in Texas. The clastic constituents of the outcrop consist primarily of rounded, frosted quartz grains and glauconite pellets. These clastic deposits are interpreted as having eroded from the unconformable basement granites and gneisses of the craton. Interbedded sequences of silts, shales, and dolomitic to partially-dolomitized lenses permeate the green sandstone. The Lion Mountain sand has been interpreted as tidal flat and shallow marine shelf depositional environments as evidenced by preserved sedimentary structures and fossils. Herringbone cross-bedded structures in particular are numerous at the study site, and reinforce the high energy, tidal depositional model. The study site differs in several ways from established literature, however, with a complete absence of trilobite fossils and burrows.