High Resolution Biostratigraphy of the Juana Lopez Member of the Mancos Shale Formation and the Gallup Sandstone Formation, New Mexico

Biostratigraphy and chronostratigraphy is becoming increasingly useful in calibration to astrochronological charts as well as in the development of allostratigraphy. Biozones allow subdivisions of units that are constrained by absolute radiometric dates using bentonite beds as datums. As a result, previous studies on sequence stratigraphy within the Juana Lopez can be more accurately correlated regionally, with the integration of biostratigraphy and chronostratigraphy. This study tests whether the astrochronology described by Joo & Sageman (2014) is consistent with the Milankovitch cyclicity found within the Turonian Juana Lopez Member of the Mancos Shale in the San Juan Basin. The Juana Lopez was found to contain four Upper Turonian ammonite biozones; Prionocyclus macombi, Prinonocyclus wyomingensis, Scaphites whitfieldi, Scaphites warreni and two co-occurring Upper Turonian inoceramid biozones; Inoceramus dimidius and Inoceramus perplexus. The Prionocyclus macombi, Inoceramus dimidius and Inoceramus perplexus biozones are constrained by the upper and lower Juana Lopez bentonites from 91.07+/-0.10Ma to 90.62+/-0.25Ma. The middle bentonite correlated to both measured sections. These bentonites are identified as being the upper and lower boundaries of the Juana Lopez and it can be therefore stated that the unit was deposited in approximately 0.92+/-0.36Ma. Correlations between the 2 measured sections show there are 2 coarsening-upward parasequences within the time period between the middle and upper bentonite (0.45+/-0.35Ma), indicating cyclicity of approximately 225kyr. This is in the realm of Milankovitch cyclicity, and therefore is evidence for a glacio-eustatic origin and aquifer-eustatic influence of the sedimentary cycles and supports the presence of ice in the Cretaceous Period.