Morphologic and facies trends through the fluvial–marine transition in tide-dominated depositional systems: A schematic framework for environmental and sequence-stratigraphic interpretation

Abstract

Most tide-dominated estuarine and deltaic deposits accumulate in the fluvial-to-marine transition zone, which is one of the most complicated areas on earth, because of the large number of terrestrial and marine processes that interact there. An understanding of how the facies change through this transition is necessary if we are to make correct paleo-environmental and sequence-stratigraphic interpretations of sedimentary successions. The most important process variations in this zone are: a seaward decrease in the intensity of river flow and a seaward increase in the intensity of tidal currents. Together these trends cause a dominance of river currents and a net seaward transport of sediment in the inner part of the transition zone, and a dominance of tidal currents in the seaward part of the transition, with the tendency for the development of a net landward transport of sediment. These transport patterns in turn develop a bedload convergence within the middle portion of all estuaries and in the distributary-mouth-bar area of deltas. The transport pathways also generate grain-size trends in the sand fraction: a seaward decrease in sand size through the entire fluvial–marine transition in deltas, and through the river-dominated, inner part of estuaries, but a landward decrease in sand size in the outer part of estuaries. A turbidity maximum (i.e., a zone of significantly elevated suspended-sediment concentrations) is developed within estuaries and the delta-plain region of deltas as a result of flocculation and density-driven water-circulation patterns. This leads to an area within the estuary or delta plain where the abundance and thickness of the mud drapes are greatest, including the potential for the development of fluid-mud deposits (i.e., structureless mud layers more than 0.5–1 cm thick that were deposited in a single slack-water period). A monotonic seaward increase in salinity characterizes both estuaries and deltas. The brackish-water conditions in the transition zone, accompanied by the high turbidity and physically harsh conditions, produce a biologically stressed environment, in which bioturbation is generally not pervasive. The ichnofossil assemblage in this zone is characterized by the low diversity of ichnogenera, small size of the individual burrows (typically smaller than their open-marine counterparts), and highly variable population densities, ranging from unbioturbated to very high-density mono-specific assemblages in local areas. This review begins with a survey of how and why each depositional process varies through the fluvial-to-marine transition and then examines the sedimentological responses to these processes, focussing on the observable, longitudinal variations in the development and/or abundance of each deposit characteristic (e.g., sand grain size, paleocurrent patterns, mud drapes, and biological attributes). The review ends with a summary of the characteristics of each major facies zone through the transition, with separate discussions for both estuaries and deltas. It must be noted that any attempt to generalize, as is done here, will undoubtedly contain statements that are inappropriate for individual deposits or systems. Exceptions must be expected and the concepts must be applied with due consideration for the local context.