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

The sedimentary record tends to be cyclic in nature, and high-frequency sedimentary cycles (sub-million-year parasequences) are observed throughout the Cretaceous section of the Western Interior Seaway. There has been much debate whether the observed high-frequency sedimentary cycles are climate-driven Milankovitch-scale cycles, suggesting glaciers during the Cretaceous Period.

The Juana Lopez Member of the Mancos Shale is a marine Turonian deposit found above the Dakota Sandstone and below the Gallup sandstone within the San Juan Basin in New Mexico. There have been no previous high-resolution facies and shoreline trajectory analysis on the Juana Lopez. Preliminary work shows that the Juana Lopez displays high-frequency stratigraphic cycles. Nine measured sections have been collected. The sections are between 30 to 40m in height, with 42km between the southernmost and northernmost section. The sections consist of 20 to 30 upward coarsening sequences, and 12 to 16 bentonite beds/lamina.

There are three main objectives for this project: (1) to further analyze the nature and frequency of cyclicity and sea level fluctuations in the Juana Lopez through extensive regional correlation of upward coarsening sequences; (2) to date the bentonite layers to determine the time range within the Juana Lopez; (3) to compare the frequency of cyclicity in the Juana Lopez with previously analyzed cycles of other Turonian units in the Cretaceous Western Interior Seaway. Stratigraphic outcrop logs are regionally correlated with the aid of bentonite datums. Through the regional correlation, the extent of lateral heterogeneity of this heterolithic Mancos Shale Member is determined.

There is a lack of studies on thin-bedded heterolithic units, such as the Juana Lopez. This study provides high-resolution descriptions of the lateral and vertical variability of heterolithic marine units that are of great interest to the oil and gas industry due to recent discoveries of fracking to obtain hydrocarbons. This detailed analysis provides the energy industry a high resolution analog of a potential unconventional heterolithic reservoir. Also, this study contibutes to the future detailed analysis of overlooked shale units worldwide.