GEOLOGY OF THE UNIVERSITY OF TEXAS MCDONALD OBSERVATORY, DAVIS MOUNTAINS, TEXAS

A new geologic map of the University of Texas McDonald Observatory in Jeff Davis County, Texas, is presented, along with thirty new whole rock analyses of volcanic rocks from the Barrel Springs Group (Fort Davis Tuff, Mount Locke Formation, Wild Cherry Tuff, and Casket Mountain Formation) and Eppenauer Ranch Formation. Mount Locke is the type locality of the ~36 Ma Mount Locke Formation and consists of a relatively widespread, highly porphyritic metaluminous trachyte. A newly recognized rhyolite occurs within beds mapped by Anderson (1968) as Mount Locke Formation and appears to correlate with the Wild Cherry Tuff. The overlying quartz trachyte lava, originally included in the Mount Locke Formation, correlates to a Casket Mountain Formation flow reported by Henry et al. (1994). The Mount Locke Formation is thus restricted at its type locality to the lower, highly porphyritic metaluminous trachyte. The Mount Locke summit is capped by rhyolite originally mapped as Wild Cherry Formation by Anderson (1968) but is likely part of the Casket Mountain Formation. Lower breccia is rare in the rhyolite-trachyte-rhyolite sequence on Mount Locke but a distinctive upper breccia, vitrophyre and tuff above each unit is interpreted to represent block and ash flow tuffs.

Mount Fowlkes, 1.2 km northeast of Mount Locke, hosts a unique sequence of lavas and tuffs consisting of younger Casket Mountain flows. The oldest common Casket Mountain flow is succeeded on Mount Fowlkes by a highly porphyritic, low-silica quartz trachyte, a partially reworked quartz trachyte tuff, trachyandesite of the Eppenauer Formation, an aphanitic trachyte, a variably welded trachyte and tuff breccia and, a vesicular trachyandesite porphyry. The first two lithologic units and the last three are informally assigned to the Casket Mountain Formation. Anderson Jr. (1968) originally mapped the last three lithologies as Wild Cherry Formation. The Fort Davis Tuff and Wild Cherry Formation are ignimbrites that erupted from the Paradise Mountain caldera at ~36.2 and 35.9 Ma (Parker & Henderson, 2021, Lithos, 106453), and the Mount Locke trachyte lavas are also thought to have originated from this caldera, but the presence of Block and Ash flow tuffs may indicate a local origin for some of these rocks.