MAPPING THE UPPER CRETACEOUS ALLOCRIOCERAS HAZZARDI ZONE, BOQUILLAS FORMATION, BIG BEND NATIONAL PARK, TEXAS

In and near Big Bend National Park, a thin regionally significant set of isochronous and lithologically distinct (iron-bearing) marker beds contains a unique fossil assemblage that we designated the Allocrioceras hazzardi Zone (AHZ). Based on index fossil identification and lithostratigraphy, we determined that the base of the AHZ correlates with the Turonian-Coniacian Stage Boundary. The top of the AHZ is the top of the lower or Ernst Member of the Boquillas Formation. It is overlain by the San Vicente Member of the Boquillas Formation. We used the AHZ as the basis of a long-term, classical mapping project of the Boquillas Formation that began with reconnaissance and identification in 1994; developed with XRD, SEM, thin-section analysis and specimen identification; and (by 2012) resulted in the publication of five new 7.5’ geological quadrangles (Texas Bureau of Economic Geology, Misc. Map No. 50), inclusion in the USGS remap of Big Bend National Park. (USGS SI Map 3142), and publication of a Field Guide to the Late Cretaceous geology of the Big Bend Area (Houston Geological Society, in press). The primary techniques used were “old-school” - pencil, field notebooks, mapping on a topographic base, and the use of a consumer grade GPS device. We chose Adobe Illustrator for digitization since Adobe products are proven “industry standard” applications, which allowed a seamless transition into the publications. We tried various computer technologies in the field as they became available and feel that they substantially increased the time spent at individual localities, decreased the actual areas mapped, were more trouble then they were worth, and did little to enhance our mapping effort. We strongly feel that the use of modern technology in the field must be tempered with the challenges of the environment and the limitations of the technology.