SEDIMENTOLOGICAL CHARACTERIZATION OF THE TUNUNK SHALE: PETROGRAPHIC AND CHEMICAL INTERPRETATION OF DEPOSITIONAL SYSTEMS FOR CRETACEOUS MUDSTONE PARASEQUENCES

Studies of mudstones, especially those at fine-scales, are still uncommon despite the prevalence of shales in the rock record. Detailed facies analyses of mudstones can reveal the mechanisms driving (4^thorder) parasequence (PS) development - changes that may not be as easily interpreted from region-scale sequence stratigraphy.

Three detailed sedimentologic sections were measured in the field to document mm- to cm-scale variations in sedimentary facies characteristics in three PS within the Turonian Tununk Member of the Mancos Shale Formation. Lithofacies were defined for all units based primarily on bed-by-bed analyses of sedimentary structures, degree of cementation, and bioturbation index (BI).

Coarsening upward sets of beds are usually increasing upward in both frequency and thickness through each PS. Lithofacies variations within the same PS are likely due to changes in localized energy (frequency and magnitude of storms) and/or the degree of BI. Lithofacies variations between different PS are probably influenced by changes in shoreline proximity, provenance, or both.

pXRF was used on slabbed samples to help differentiate lithofacies by tracking chemical changes. Chemical proxies appear to be particularly helpful in differentiating small scale sedimentological changes even in more homogenized sections.

Petrographic observations, under both light microscope and SEM, were used to characterize petrographic fabrics and identify clast composition. It is often found that the observed microfabrics are in turn responsible for many macro-features, including decimeter to meter-scale outcrop weathering patterns. Changes in the type and size of observed clasts between PS suggest shifts in shoreline proximity, bathymetry, and possibly provenance – interpretations consistent with those from outcrop observations.