Mapping the chemostratigraphy of the Upper Cretaceous Mancos Shale, New Mexico, USA, using high-resolution XRF core scanning techniques.

Abstract

High-resolution (500 um) elemental data were collected from marine sedimentary rocks of the Upper Cretaceous Western Interior Seaway of North America using an Itrax uXRF core scanner in order to create detailed chemofacies and chemostratigraphic profiles to aid in the construction of sequence-stratigraphic frameworks in mudstone successions. Chemofacies were created for several depositional environments associated with siliciclastic shorelines such as fluvial channels, delta front, prodelta, mudbelt, and sediment-starved shelf using an unsupervised, hierarchical clustering algorithm known as a Self-Organising Map, which is topologically invariant so the clusters could be further grouped into superclusters to investigate variability within the depositional environments at a number of scales. Chemostratigraphic mapping was done on a 94-metre long section of Mancos Shale from a single well. The high-resolution data showed marine transgressive-regressive cycles at several orders that could be correlated to shoreline trajectories established in near-shore facies. Both the point of maximum regression, which indicates the initial flooding during a relative sea-level rise, and maximum transgression (i.e., the maximum flooding surface), could be accurately identified indicating that a complete cycle was recorded at each order. The data between the two represents a correlative conformity, which can provide palaeoenvironmental information that is not retained in near-shore facies. This research proves that marine mudstones retain a more complete record than near-shore sandstones and that they contain a wealth of palaeoenvironmental information. The high-resolution sampling of a uXRF core scanner is the only method that can fully map the chemostratigraphy of these mudstones, which are often visually homogeneous. By applying these techniques to other mudstone cores from the Western Interior Seaway or another Upper Cretaceous epeiric sea, high-resolution sequence-stratigraphic frameworks can be developed that can be used to reconstruct palaeoclimates at local to global scales.