LAMINA PROPERTIES AND MICROSTRATIGRAPHY OF SHELF DEPOSITS OF THE UPPER CRETACEOUS TUSCALOOSA MARINE SHALE

Mudrock comprises two-thirds of all sedimentary rocks. Their fine-grained nature and multi-order heterogeneity in grain size and composition have hindered characterization of lamina thickness, abundance, and mineral composition. Laminae reflect the slightest changes in sedimentation conditions. Hence, accurately documenting their thickness and compositional variations would provide insights on the frequencies of these changes to understand the origin of mudrock. Manual counting of mm to sub-mm laminae on thin sections, hand specimens, cores, or outcrops only observes a small fraction of a stratigraphic interval and is ineffective. This study used grayscale data of mudrock cores to document lamina boundary, thickness, and color. This fast and precise method applies nested "IF" conditional statements in Python algorithm to detect significant multi-scale trends. It was applied to a 51-m-thick core of Upper Cretaceous Tuscaloosa Marine Shale (TMS) from the northern Gulf of Mexico (GOM). The results were cross-checked by direct observations of cores and thin sections. Of ~70,000 laminae, their thickness ranges from 0.13 to 69.05 mm with a mean of 0.67 mm, median of 0.41 mm, and mode of 0.27 mm. Light gray laminae commonly have abundant quartz, feldspar, and pyrite and are x-laminated and lenticular. In contrast, dark gray to blackish gray laminae are dominated by clay and muscovite, and parallel or rippled. Thin laminae (<3 mm) account for 96%, medium lamina (3-10 mm) 3%, and thick lamina (>10 mm) 1%. 80% of grains in thick laminae are equal to or finer than medium silt, while thin laminae show no correlation between thickness and grain size. Variations in lamina thickness are linked to dynamic sediment transport on the shelf. The dominance of thick laminae and thin beds of pelagic deposits in contrast to thin–medium laminae of tractional deposits suggests that sustained deposition of pelagic suspension load is the dominant mode of sediment transport and deposition on the shelf. Average lamina color shows an overall upward darkening trend, which correlates to the upward trend of decreasing energy and deepening of the TMS. Future work will utilize lamina properties to quantitatively understand the hierarchy of the TMS and their depositional mechanisms at variable scales, to accurately reconstruct the Late Cretaceous paleogeography of the GOM.