REVISED VOLCANIC STRATIGRAPHY IN THE SOUTHEASERN HALF OF THE TULE MOUNTAIN 7.5' QUADRANGLE, BIG BEND NATIONAL PARK, TEXAS

Detailed mapping of volcanic units of the Oligocene Chisos Formation (Tch) in northern Burro Mesa and at Tule Mountain provides new stratigraphic information to that of Maxwell (1966). From oldest to youngest, above the Alamo Creek Basalt Member (Tacb) and overlying undivided tuffaceous Tch at Tule Mountain, previously mapped Ash Springs Basalt Member (Tasb) does not contain distinctive "turkey track" plagioclase crystals of the Tasb type locality, and therefore, probably is a different, unnamed basalt, more similar to Tacb. Above this new basalt flow, is a newly recognized, crystal-poor, siliceous tuff on the W side of Tule Mountain. A distinctive crystal-rich, brown, ash-flow tuff, either previously unmapped or mapped as the Bee Mountain Basalt Member (Tbmb), forms cliff tops on the NE and W sides of Burro Mesa and is exposed below Tbmb. Above more undivided Chisos and the Mule Ear Springs Tuff, an interval consisting largely of ash-flow tuffs and minor lava flows was mistakenly called undivided "lavas" (Tul) by Maxwell. On the E side of Apache Canyon, a new, thin, red to brown, ash-flow tuff upholds the rim of the cliff. W of Burro Mesa, a second trachyandesite within Tul overlies the Tule Mountain Trachyandesite (Ttmt) and underlies the Wasp Spring Flow Breccia Member of South Rim Formation (Srm), but is difficult to distinguish from Ttmt. This unit extends in the SW into the cliff of Burro Mesa. The Burro Mesa Riebeckite Rhyolite Member of the Srm tops the sequence. In addition to new stratigraphic information, in several cases the localities and areal extent of units differ from previous mapping; for example, Ttmt is far less extensive E of Burro Mesa, and the dike NE of Tule Mountain extends much farther toward the SE. Large landslides are now recognized on the S spur of Tule Mountain, on the W side of Burro Mesa, and within Apache Canyon.