IMPROVING THE ACCURACY OF GEOLOGIC MAPS THROUGH THE USE OF AMMONITE BIOSTRATIGRAPHY

The Galisteo lowlands, located approximately 33.2 km southeast of Santa Fe, New Mexico, contain a large exposure of west-dipping Cretaceous strata. Lisenbee (1999; 2000) prepared 1:24,000-scale geologic maps of the Galisteo and Ojo Hedionda quadrangles, but lithologic character alone did not adequately differentiate Cretaceous units. Lisenbee’s division of Cretaceous stratigraphy into Dakota Sandstone, Graneros Shale, Greenhorn Limestone, Carlisle Shale (with members), and Niobrara Formation was useful for initial map preparation but several stratigraphic and structural relationships remained unresolved.

Varriale (2003) collected and catalogued the molluscan (especially ammonite) and selachian fauna of problematic units. The collected material yielded 225 specimens assignable to 47 vertebrate and invertebrate species. To improve map accuracy the biostratigraphy of faunal constituents was used to correlate units with local stratigraphy and surrounding San Juan Basin and Great Plains units.

Map accuracy was improved by identifying a previously undivided Graneros Shale as intertonguing with members of the Dakota Sandstone. Formerly unrecognized intertonguing Dakota-Graneros members include the Cubero Member of the Dakota Sandstone and the Clay Mesa and Whitewater Arroyo Shale Members of the Graneros Shale. Members of the Dakota Sandstone and Carlile Shale once thought to be the Twowells and Codell Sandstone are now identified as the Paguate and Semilla Sandstone Members respectively.

Sandstone once thought to be a homogeneous Niobrara unit is now recognized as the Mulatto, Satan, and Hosta-Dalton Sandstones of the Mancos Shale and Crevasse Canyon Formation. Furthermore, the structural complexity of the study area has increased due to the recognition of what were once two Niobrara sandstones, as the same sandstone repeated by normal faulting.