Establishing detailed chemofacies of depositional environments in an epeiric seaway using high-resolution (500 μm) micro X-ray fluorescence core scanning data

Establishing depositional environments in ancient mudstone successions from epeiric seas is difficult due to the lack of obvious lithological changes, leaving long, largely undifferentiated mudstone intervals that complicate their correlation to near-shore environments. This problem is mainly the result of the limitations in analytical resolution using traditional methods, making it difficult, if not impossible, to accurately identify transitions between depositional environments. This study used elemental data collected from an Itrax micro X-ray fluorescence core scanner at 500-μm sampling interval to establish detailed chemofacies in a thick (17 m) distal mudstone deposit and compare them to the chemofacies of previously established near-shore (fluvial floodplain to prodelta) depositional environments. The chemofacies for the mudstone were created using a hierarchical clustering algorithm known as a self-organizing map, to develop detailed descriptions of elemental composition, which showed the variation both between environments and within them. The relationship between Fe (terrigenous proxy) and Ca (marine proxy) was effective at indicating proximity to shoreline while the relationships between Ti and K describe weathering and transport conditions at the sediment–water interface due to changes in bottom-water current energy. For the near-shore sediments, the average values in the elemental proxies were less effective at distinguishing the environments than the SDs of those proxies within each environment, which became more constrained (lower (Formula presented.) relative to (Formula presented.)) basinwards as fluvial input and water energy decrease. Basinwards of the prodelta, the values of terrigenous proxies decreased more rapidly and were associated with a rapid increase in mean Ca values and SDs, combined with an increase in V/Cr and Cu/Ti as a result of lower oxygen conditions and increased preservation of marine organic matter. These robust chemofacies can help to guide the lithological interpretation and allow for higher resolution mapping of ancient mudstone sequences from epeiric seas, which will improve their correlation to near-shore environments.