The Thunderbird mass of Precambrian rhyolite porphyry in the Franklin Mountains of westernmost Texas is a low remnant mountain that protruded above the floor of the Early Ordovician seas. This extrusive rhyolite, dated at 950 m.y., is surrounded and overlapped by limestone of the Early Ordovician El Paso Group. Sparite-cemented masses of rhyolite talus border the mountain, grading into dolomitized biointrasparrudite containing many large angular clasts of rhyolite. This rhyolite mountain and other Precambrian-rock islands in the region provided detritus to the Early Ordovician seas of southern New Mexico, west Texas, and northern Mexico.
Preservation of such highlands for much of Early Ordovician time appears to be extraordinary. The El Paso limestone of the Franklin Mountains, however, reflects in entirety an environment ranging from supratidal to shallow-water marine, as evidenced by the occurrence of such features as polygonally cracked micrites, algal stromatolites, digitate algae, and archaeoscyphid mounds. Protection of the remnant mountains from high-energy waves can be attributed to the regional, virtually intertidal, mudflatlike environment; the occurrence of archaeoscyphid biohermal barriers; periods of submergence below wave base; or a combination of these factors. The sedimentary features imply that a vast region of remarkably uniform shallow-water to supratidal environmentals existed during Early Ordovician time.