HIGH-FREQUENCY STRATIGRAPHIC CYCLICITY ANALYSIS OF THE UPPER CRETACEOUS JUANA LOPEZ MEMBER OF THE MANCOS SHALE, NEW MEXICO

The Earth is considered to have been in a “greenhouse state” during the Cretaceous Period. High-frequency sedimentary cycles are observed throughout the Cretaceous section of the Western Interior Seaway. Even though this warm Cretaceous climate suggests an ice-free planet Earth, there has been much debate as to whether the observed high-frequency sedimentary cycles are climate-driven Milankovitch-scale cycles that would suggest glaciers during the Cretaceous Period. This study tests the hypothesis of a glacio-eustatic origin of high-frequency cyclicity in the Turonian Juana Lopez Member of the Mancos Shale in the San Juan Basin, New Mexico. Data for this study was obtained from two stratigraphic measured sections which are 3.2 km apart, and located southwest of Shiprock. The two sections are found approximately 60 km away from the Turonian shoreline in an offshore marine environment. A high-resolution thin bed facies analysis on both sections reveals the Juana Lopez to be deposited in a fluvial-dominated, mixed wave- and fluvial-influenced environment. The Juana Lopez is shown to be an overall coarsening-upward sequence, displaying a shallowing regressive environment. Correlations between the two sections reveal 13 correlated parasequences identified through the violation of Walther’s Law. An average cyclicity frequency of ca 90 kyr was determined for the sequences by bracketing the Inocermus dimidus and Scaphites whitfieldi biostratigraphic zones within the Juana Lopez sections. This cyclicity represents short eccentricity Milankovitch cycles. Many studies have confirmed that the observed Milankovitch-scaled cyclicity in the Cretaceous must be controlled by glacio-eustasy. With similar Milankovitch cyclicity found in the Juana Lopez, the short eccentricity Milankovitch cycles are interpreted as being glacio-eustatic in origin, which supports the presence of ice in the Cretaceous Period.