PROPOSED: REVISE THE CONTACT BETWEEN ERNST AND SAN VICENTE MEMBERS, BOQUILLAS FORMATION, BIG BEND NATIONAL PARK, TRANS-PECOS TEXAS

Mapping in and near Big Bend National Park (BBNP) shows that the Allocrioceras hazzardi zone of the Boquillas Formation stratotype (southeastern BBNP) is very regular, is lithologically distinctive, and is widespread in the Big Bend region, Trans-Pecos Texas. The A. hazzardi zone is ≈1-1.3 m thick and consists of four patchily silicified (most localities) sparry micritic fine biopackstone intervals (5YR 6/1, fresh; to 10YR 6/2-5/4, weathered) separated by soft calcareous mud or clay shales (≈5YR 7/2). Colors stand out against more neutral light grey rocks above and below. Limestone intervals are laminated (1-3 cm), fine upward, and locally cross-bedded with basal shell hash. Bases are sharp and shallowly undulate, particularly for the three upper beds. Tops are narrowly gradational, and undulate. Three upper indurated intervals commonly have a prominent 3-7 mm layer of irregular 1-2 mm pyrite masses that form a red hematite/limonite stripe 2-4 cm below tops on a weathered surface. Intervening calcareous shales are thinly laminated (≈1-3 mm) with occasional basal fragmental shelly patches. These rocks constitute the taxon-range zone of the distinctive heteromorphic ammonite, A. hazzardi (found in nearly all outcrops), and record the last appearance of Inoceramus (Cremnoceramus) waltersdorfensis in the region. The most common fossil is a so-far unidentifiable baculitid; other taxa, not previously recognized in the A. hazzardi zone, include cf. Forresteria alluaudi, a new scaphitid species, other scaphitids, a squid, a gastropod (cf. Turritella sp.), and several taxa yet to be identified, including other inoceramids. The top of the taxon-range zone appears to coincide with the Turonian-Coniacian boundary. The original description of Boquillas members (Maxwell and others, 1967) placed the top of the Ernst Member at the top of the (almost entirely unfossiliferous) “Coilopoceras beds,” and noted a disconformity overlain by a minor conglomerate at the base of the San Vicente Member. Extensive fieldwork shows this contact to be gradational. We propose the A. hazzardi zone as the upper boundary-stratotype for the Ernst Member, Boquillas Formation, to replace the original less practical boundary between the Ernst and San Vicente members of the formation.