THE HOT SPRINGS TRAIL REFERENCE SECTION (CENOMANIAN-TURONIAN-CONIACIAN), ERNST MEMBER, BOQUILLAS FORMATION, BIG BEND NATIONAL PARK, TEXAS

The Hot Springs Trail (HST) measured section is to be established as the formal stratigraphic reference for the Ernst Mbr, Boquillas Fm, Big Bend National Park, TX. The name 'Ernst' derives from an atypical section near Ernst Tinaja, which is not the type locality or area. Basal HST rocks lie disconformably on the Buda Fm with the top defined as the top of the Allocrioceras hazzardi Zone, conforming to the Ernst Mbr as redefined by Cooper and others (2004). Detailed mapping, stratigraphy, and paleontology of the rocks over a strike-distance of 80 km indicate that the HST is the most representative in terms of biostratigraphy, lithostratigraphy and thickness. Accessible and protected by the National Park Service, the HST is gently tilted and unfaulted. Differences in lithologic appearance arise from variations in proportions of micrite, microfossil tests, shell material, and terrigenous clays and sand, and sedimentary structures. Coarse-fine mm-scale couplets are near-ubiquitous in the lower half of the section and in the top 10 m, though some resistant ledges are cored with massive micrite or foraminiferal limestone. Ledges in the intervening section are thin-bedded, and terrigenous sand is rarer. Coarse material may be shell fragments or terrigenous sand. Recessive layers are dominated by clay or have many clay partings separating sub-mm–mm limestone laminae, or carbonate laminae sets. Decimeter to cm scale cross-lamination is common at tops of resistant beds. Limonitic zones follow or approximate bedding, and result from oxidation/hydration of sulfide accumulations. Identified fossils (1995-present: W.A. Cobban, USGS) correlate the HST with established Western Interior Seaway, northern European, Tethyan and Pacific Rim inoceramid interval and ammonite taxon range zones. The HST spans the Cenomanian-Turonian and Turonian-Coniacian Stage Boundaries, and extends the North American geographic range of some species up to 1100 km further south than previously known.