SEDIMENTATION, VOLCANISM AND INTRUSION IN THE LOWER CHISOS GROUP, BIG BEND NATIONAL PARK, TEXAS

Mapping at 1:12,000 in the NW Emory Peak quadrangle and SW corner of The Basin quadrangle shows the following sequence in the lower Chisos Group, the older Cenozoic volcanic rocks in the Park. Deposition of floodplain siltstones and channel sandstones continued from the latest Cretaceous into the early Cenozoic. These clastic deposits contain quartz, plagioclase, sanidine, clinopyroxene and biotite, but no pumice or shards. The lowest lava, 5 to 50 m thick, is mugearite with stellate clusters of plagioclase phenocrysts 0.5 to 1 cm long. The base is locally hyaloclastic, and prismatic jointing indicates only one cooling unit. Above the lowest lava are nonwelded crystal-vitric tuff and minor breccia and sandstone; this interval totals 100 m. The second lava unit is 10-20 m of benmoreite with strong flow texture emphasized by parallel phenocrysts of plagioclase the size and shape of coins. This is followed by >200 m of nonwelded lithic-crystal-vitric tuffs, welded crystal-vitric tuffs, and sandstones. NE-striking hawaiite to benmoreite dikes cut this sequence; some dikes have peperite margins and contain clots of unlithified wall rock. Down-to-the-SW displacement on a NW-striking fault obscures an interval of unknown thickness which is followed by more sandstone and conglomerate, and then by three packages of hawaiite lava showing pillows and hyaloclastite. The three lava packages, 15 m, 50 m, and 10 m thick, are separated by two 3-m-thick welded crystal-vitric tuffs and by local conglomerates. Topographically prominent WNW- to NNW-striking rhyolite dikes, some traceable for 6 km, cut all of the preceding. Above the section described, densely welded tuffs, proximal surge and airfall deposits, and trachyandesite to rhyolite lava flows dominate the upper Chisos Group, when caldera-related volcanism overwhelms the area.