Proposal to Establish a Type-Section for the Allocrioceras hazzardi Zone Preliminary to Establishment of a Formal Taxon-Range Zone Defining the Uppermost Ernst Member, Boquillas Formation, Big Bend National Park, Trans-Pe

The Allocrioceras hazzardi zone (AHZ) is a taxon-range biozone infomally named for the heteromorph ammonite that is the most readily identifiable and diagnostic taxon within a regionally distinctive 1.13–1.74 meter thick stratigraphic interval. Based on correlation with established ammonite taxon-range zones and inoceramid interval zones, the lowermost indurated layer of the AHZ is less than a meter above the Coniacian/Turonian Stage Boundary (89.3 Ma). The uppermost of the four indurated ferruginous layers that comprise this interval has been previously proposed (Cooper et al, 2005) as defining the contact between the Ernst and San Vicente Members of the Boquillas Formation. Nineteen measured sections of the AHZ have been completed extending from Tornillo Creek in the Hot Springs area to the Nine Point Draw area near Persimmon Gap in Big Bend National Park and two fossils census surveys have been done in widely separate geographic localities. Seventeen invertebrate species (including a belemnite species with preserved soft body parts) and one vertebrate species have been identified from the AHZ fossil assemblage. Site 19, the proposed type section for the AHZ, is located within Big Bend National Park ≈205 meters east of the turnoff to the Hot Springs along the paved road. The lowermost layer is ≈2 meters above roadbed on the east side of a low ridge (N29°11.777', W102°59.805'). The 1.29 meter average thickness of the AHZ is characteristically developed at Site 19 and easily recognizable due to the distinctive brownish coloration of the interval. Site 19 has also been chosen because of its close proximity to the type section for the Ernst Member, Hot Springs Trail Reference Section (Cooper et al, 2007).