NEW 40AR/39AR AGES, GEOCHEMISTRY, AND STRATIGRAPHY FOR MAFIC AND RHYOLITIC VOLCANIC UNITS FROM BIG BEND NATIONAL PARK, TEXAS

Stratigraphic sections comprising volcanic rocks of the Chisos Group (CG) are exposed along the western part of Big Bend National Park. Based on detailed field relationships, geochemical analyses, and age determinations, we propose new units be added to the CG stratigraphy. These are: unnamed basalt flow(s) (Tul-1) above the Mule Ear Springs Tuff (MEST), Wasp Spring Member of the Burro Mesa Formation (WSM), Burro Mesa Rhyolite Member (BMRM), which together constitute the Burro Mesa Formation and unnamed lava flows capping the BMF (Tul-2). The WSM and BMRM were originally part of the South Rim Formation, which crops out in the High Chisos Mountains, but have been removed from that formation and elevated to formation status based on age and chronology (White and others, Lithos (2006) 91, 74-94).

New geochemical data (XRF and INAA) will aid in the correlation of numerous lava flows associated with the Alamo Creek lavas (ACL), the Bee Mountain lavas (BML), and the Tule Mountain trachyandesites (TMT). New single-crystal 40Ar/39Ar ages for numerous ash-flow tuffs and lava flows within the CG are helping to constrain the age range for the Tul-1 and Tul-2 lavas that have not been dated, as well as refining the age range for the ACL, BML, and TMT.

As reported by previous researchers, the ACL consist of a series of flows with variable geochemical signatures. This may also hold true for the Ash Spring lavas and TMT (30.3 Ma) based on recently collected geochemical data. The ACL are dated at 46.6 Ma, but we are currently analyzing a series of flows to determine if they represent multiple batches of 46.6 Ma lavas with wide geochemical variations or a distinct series of time-stratigraphic flows. The ages for two BML samples are 33.74 and 33.70 Ma. Another prominent series of basaltic lava flows (Tul-1) lie directly above the MEST. These flows are unique to all other lava flows as they contain numerous xenoliths of quartzite, which aid in mapping this horizon. The upper mafic lava flows (Tul-2) are directly above the BMR and have been identified in two areas of the park.