USING DELTA-FRONT TURBIDITES TO ASSESS THE OCCURRENCE AND PROPERTIES OF SAND-RICH HYPERPYCNAL FLOWS EMANATING FROM RIVERS

River flows with high suspended-sediment concentrations can be sufficiently dense to plunge underneath seawater and continue down-slope hyperpycnal flows. The potential importance of direct river underflows to construction of the seascape has only recently come under systematic study and the connection of these currents to deposition of sandy turbidites is not yet clear. The small number of documented modern flows all are affiliated muddy deposits. Fortunately, sandy turbidites that are unambiguously linked to the deltaic channels from which underflows would have emanated, can be used to study potentially sand-rich hyperpycnal flows. Sedimentary structures and textures within these deposits will be used here to assess the occurrence and properties of sandy turbidity currents directly derived from river underflows.

Many delta-front turbidites possess an internal stratification produced by trains of climbing ripples. This stratification, together with grain-size data, is used to estimate properties of the depositing currents that include average velocity, suspended-sediment concentration, flow thickness, as well as duration. These estimates can be checked against a record of the tidal cycle that is often embedded within the stratification. This record occurs in the form of a repetitive change in the angle of ripple climb that is interpreted as the consequence of superimposing flood and ebb currents on a sustained unidirectional underflow. Where tidal modulation is present it constrains both current velocity and duration. Agreement between the independently derived estimates of current properties reduces the uncertainty associated with the flow reconstructions thereby enhancing their utility for addressing questions related to sand-rich hyperpycnal flows. Exceptionally well-exposed examples of tidally modulated climbing ripples from the Cretaceous Ferron Sandstone Member of the Manchos Shale, central Utah, will be used here to examine what fraction of the sediment constituting these delta-front turbidites could have been derived from direct underflow of suspended-sediment laden river discharge versus from other sources.