LATE ORDOVICIAN CONODONTS FROM THE MARAVILLAS FORMATION NEAR PERSIMMON GAP, BIG BEND NATIONAL PARK, TEXAS

Paleozoic rocks near Persimmon Gap, Big Bend National Park, Texas, include the upper part of the late Middle to Upper Ordovician Maravillas Formation, the Lower Silurian to lowermost Mississippian Caballos Novaculite, and part of the Upper Mississippian-Lower Pennsylvanian Tesnus Formation. The Paleozoic strata are exposed in a thrust block juxtaposing a section of Maravillas, Caballos, and Tesnus rocks in the upper plate above lower plate Tesnus Formation. Thrust faults that deformed the block are northwest-vergent and formed during the Mississippian-Permian Ouachita-Marathon-Sonora orogen. The Paleozoic thrust block was subsequently deformed during the Cretaceous-Tertiary Laramide orogeny by southwest-vergent thrust faults, which juxtaposed the Paleozoic thrust block with Cretaceous sedimentary rocks above and below to form the northwest-trending Santiago Mountains at the southwest edge of the Marathon uplift.

We collected five limestone samples from the Maravillas Formation that yielded similar Late Ordovician conodont faunas, including representatives of Amorphognathus ordovicicus Branson & Mehl, Belodina spp. (of Late Ordovician morphotype), Oistodus venustus Stauffer, Panderodus gracilis Branson & Mehl, Panderodus unicostatus Branson & Mehl, 3 M elements of Periodon aculeatus Hadding (likely reworked), Phragmodus undatus Branson & Mehl, and Protopanderodus insculptus Branson & Mehl. These taxa are known from many collections of Late Ordovician age in North America from the eastern Appalachians westward through the Great Basin. With the exception of the panderodids, most of the conodonts are incomplete to fragmentary and include a mix of shallow- to deep-water taxa that indicate relatively high energy post-mortem transport basinward.