HIGH RESOLUTION BIOSTRATIGRAPHY AND CHRONOSTRATIGRAPHY OF THE JUANA LOPEZ MEMBER OF THE MANCOS SHALE AND THE GALLUP SANDSTONE FORMATION, NORTHWESTERN AND NORTHCENTRAL NEW MEXICO

Biostratigraphy, the morphological evolution of fossils throughout the rock record, is an important component to large scale regional chronostratigraphic correlations. Due to their rapidly evolving anatomy, ammonites and inoceramids are extremely useful tools in biostratigraphy. The Juana Lopez Member of the Mancos Shale Formation and the Gallup Sandstone Formation were deposited in the Middle Turonian to Lower Coniacian Western Interior Seaway. A total of 209 ammonite, and 164 inoceramid specimens were collected along vertical measured sections for this study. Samples were collected from 7 Juana Lopez and 3 Gallup Sandstone outcrops, spanning a total distance of 300km across Northwestern and Northcentral New Mexico. Four ammonite species were identified in the Juana Lopez Member as Prionocyclus hyatti, Prionocyclus macombi, Prionocyclus wyomingensis, and Scaphites whitfieldi. Two ammonite species were identified in the Gallup Sandstone as Scaphites whitfieldi and Scaphites preventricosus. Two inoceramid species were identified in the Juana Lopez as Inoceramus dimidius and Inoceramus perplexus. Two inoceramid species were identified in the Gallup Sandstone as Inoceramus dakotensis, and Cremnoceramus deformis erectus. Layers of bentonite (volcanic ash) were also collected and used for regional correlations and for constraining biozones. Sanidine crystals were extracted from two samples within the Juana Lopez and dated using radiometric argon-argon dating methods. Two bentonite dates from the Upper and Lower Juana Lopez bentonites constrain the P.macombi, I. dimidius and I. perplexus biozones to 91.07Ma +/-10Ma to 90.62Ma +/-0.25Ma. Both the Upper and Lower Juana Lopez bentonites correlate across 6 of the 7 sections, spanning a total distance of 260km. Spatial changes in lithology are observed across the study area, however the fossil assemblage of both units remains relatively unchanged. This study provides high-resolution biostratigraphy of the Juana Lopez Member and the Gallup Sandstone Formation and regionally correlates 5 ammonite biozones, and 4 inoceramid biozones as well as constrains 3 biozones using absolute age dating of sanidine crystals. Finally, the Juana Lopez Member and the Gallup Sandstone Formation are correlated across Northwestern and Northcentral New Mexico.