Sedimentological and ichnological characterization of tide-dominated succession during transgression in the Lower Cretaceous Zubair Formation, central Mesopotamian Basin

Abstract

Depositional processes recorded by coastal deposits may vary rapidly during transgression due to the interactions of wave, river, and tidal processes, leading to the diversity of facies models. Reconstructing a reliable, detailed facies model associated with depositional processes remains challenging unless ichnological features are considered. Integrated sedimentological and ichnological analyses of high-resolution core samples, wireline log, and seismic data from the Lower Cretaceous Zubair Formation, central Mesopotamian Basin, are undertaken to propose a tide-dominated model within transgressive succession. The presence of sporadic river-influenced sedimentary structures, variability in ichnodiversity and bioturbation intensity, and abundant tidal physical indicators attest to the strong influence of tide-generated currents during deposition. In a transgressive upward-fining succession, seven facies associations are identified based on differences in sedimentological and ichnological indicators, and further grouped into two environments (tide-dominated estuary and open coast tidal flat), which are both considered as first-order tidal coastal environments. Deposits of estuaries are occupied by mixed Cruziana-Skolithos ichnofacies and dominance of channel-bar complexes, while local storm-influenced structures, higher ichnodiversity and bioturbation intensity, and well-expressed Cruziana and Skolithos ichnofacies clearly characterize the open coast. River-dominated paleovalley evolved into the tide-dominated estuary and then transformed into the open coast tidal flat as global sea-level rose, with increasing ichnodiversity, bioturbation intensity, and weakening fluvial influence. For the first time in the region, distinct deposits of estuary and open coast are recognized, indicating the development of facies diversity during long-term transgression. High accommodation, topographic features, limited sediment supply, and sea level rise are responsible for the spatiotemporal differential distribution of these two depositional systems. At the regional scale, tidal environments, river-influenced, and river-dominated deltas were distributed along the coastline from northwest to southeast due to the relatively increasing sediment supply from the Arabian Shield towards the southeast. The results of this study improve our understanding of regional paleogeography in the central Mesopotamian Basin, NE Arabian Plate, during the Barremian, and provide a case for understanding the diversity and sedimentary evolution of tidal facies models, which can be applied to other transgressive sequences in deep-time basins worldwide.