Coarse-grained sediment waves in hyperpycnal clinoform systems, Miocene of the Austral foreland basin, Argentina

Abstract

In this paper we present an outcrop example of coarse-grained sediment waves generated by hyperpycnal discharges at the toe of Miocene clinoform systems exposed in the northeast Atlantic coast of Tierra del Fuego, Argentina. An individual set of these sediment waves is formed during the waxing and waning phases of the hyperpycnal discharge. During the waxing phase, accumulation of coarse-grained sediments with generation and migration of dune bedforms occurs at the base of the clinoforms. These deposits show transitions of tractive sedimentary structures with evidence of bedload transport, and a continuous reworking by the overpassing hyperpycnal flow. The continued erosion and reworking during the waxing phase affect mainly the top and the stoss side of the dunes, resulting in isolated lenses having upcurrent aggradation structures. During the waning phase, these isolated lenses are covered by fine-grained heterolithic strata that are thicker on the stoss side than on the lee side, constituting a differential draping. The pulsating and sustained character of the hyperpycnal flows and the steep depositional slope clinoforms controlled the stacking pattern of the coarse-grained sediment waves during successive hyperpycnal events. In this way, small-scale, coarse-grained sediment waves, with wavelengths of 10–40 m and up to 5 m amplitude, and a stacking pattern showing an up-current growth, are generated by high-density hyperpycnal flows on slopes having steep gradients.