A lithofacies-coupled palynofacies model for meandering river floodplains in the Late Cretaceous: Insights from the Dinosaur Park Formation, Alberta, Canada

Abstract

Alluvial floodplains accommodate various sub-environments that preserve evidence of fluvial dynamics, aiding in the reconstruction of terrestrial paleoenvironments. While floodplains serve as major sources and sinks for organic matter, characterizing sub-environments within ancient floodplains using fossil organic matter (kerogen) is sparsely applied due to its unclear relationship with organic matter dynamics in modern fluvial settings. This study develops a lithofacies-coupled palynofacies model for fluvial environments to address this gap and reveal organic matter dynamics in paleo-floodplains. The sedimentary successions of the Upper Cretaceous Dinosaur Park Formation in Dinosaur Provincial Park, Alberta, Canada, offer an ideal fluvial setting to develop this model, compare it across geological time intervals, and refine paleoenvironmental reconstructions of the Dinosaur Park Formation.

A floodplain-dominated stratigraphic section of the Dinosaur Park Formation comprises six recurring lithofacies and two facies associations representing channel-belt and floodplain deposits. All observed kerogen assemblages originated from terrestrial environments. Channel-belt deposits are characterized by sandstones containing opaque and amorphous non-biostructured phytoclasts, as well as amorphous organic matter. Levee deposits are characterized by sandy mudstones, while proximal floodplains are characterized by gray mudstones. Despite these distinctions, kerogen assemblages are relatively similar and heterogeneous, with opaque phytoclasts remaining prevalent. Backswamps (carbonaceous mudstones and shales) and distal floodplain environments (brown mudstones) exhibit a high abundance of cuticles, membranes, sporomorphs and amorphous non-biostructured phytoclasts. Total organic carbon content discriminates between different sub-environments, with higher organic carbon content found in backswamp and distal floodplain environments. The results show that lithofacies and kerogen assemblages (i.e. palynofacies) in the Dinosaur Park Formation's floodplains were mainly influenced by channel proximity and floodplain topography. When compared with modern and deep-time fluvial examples, the model supports palynofacies analysis as a robust proxy for distinguishing floodplain sub-environments and demonstrating its applicability in understanding fluvial dynamics over geological timescales.