Abstract

Hyperpycnal flows originate when sediment-laden fluvial discharges enter standing, lower-density water. Because of their excess density, the flows plunge near the river mouth and continue to travel basinward as a quasi-steady and fully turbulent underflow. The related deposits are hyperpycnites, and constitute a particular type of turbidite with poorly known facies and facies tracts. Although hyperpycnal flows seem to be quite common in present times, their occurrence in fossil strata is poorly documented. This paper addresses the characteristics and depositional processes of shallow lacustrine sandy hyperpycnites, on the basis of the field analysis of well-exposed Lower Cretaceous strata (Rayoso Formation) in the Neuquen Basin of west-central Argentina. The Rayoso Formation is composed of clastic (and minor evaporitic) red beds up to 1200 m thick, deposited in a shallow perennial lake of variable salinity affected by long-lived hyperpycnal flows. Main clastic facies are composed of fine-grained sandstones with climbing ripples and plane beds. Other common sandstone facies include massive beds and low-angle cross stratification. Most sandstone facies are related to traction plus fallout processes, and often show a vertical fluctuation between sedimentary facies originated under different traction-plus-fallout conditions within single beds. These fluctuations are interpreted to be evidence of deposition from flow fluctuations in sustained hyperpycnal flows. Most beds internally show the existence of three depositional phases, acceleration (AP), erosion plus bypass (EP), and deceleration (DP), which record the complete evolution of a single long-lived hyperpycnal flow at a fixed point. Additionally, the depositional evolution of a single long-lived hyperpycnal flow with distance records initially the progressive basinward migration of the AP and EP phases, and finally an overall deposition under the DP phase both in proximal and distal areas. This evolution provides an adequate explanation for the basinward extension of channelized features, and for the occurrence of fine-grained sandstones with climbing ripples both in proximal and distal positions within the same hyperpycnal system. Consequently, facies analysis derived from application of the Bouma sequence is not valid for deposits of quasi-steady hyperpycnal flows.