Storm-graded layers on inner continental shelves: Examples from southern Brazil and the Atlantic coast of the Central United States

Abstract

Size-graded layers have been reported from the United States and Brazilian Atlantic continental shelves, amid a bedform association that includes ripples, megaripples, sand waves, and sand ridges. Graded layers are most apparent in the lag gravels and shell hashes that overlie an older substrate in the troughs between sand ridges, but also occur as thinner, isolated beds within the sand ridges. At least four different models seem to be applicable to graded bed formation. The stratigraphic graded model, grading occurs within and immediately above the basal lag gravel of the Holocene transgressive sand sheet. It is a consequence of the generation of the sand sheet by erosional shoreface retreat. Such graded sequences are formed by landward facies displacement over thousands of years, but the process is storm-mediated, and sequences may contain thinner storm-graded layers. The storm-graded model calls for entrainment of sediment by storm flow and subsequent graded deposition under waning energy. In the bedform migration model, the migration of megaripples and sand waves may produce thin graded beds at any level within the Holocene sand sheet, and sand ridge migration can cause graded sequences in the basal part of the Holocene sand sheet. The bottom liquefaction model relies on storm-wave pumping of the seafloor sands with their resultant liquefaction and settling of coarser clasts. Beds in which sorting is confined to coarse shell material may be of this origin. These models are believed to be complementary rather than mutually exclusive.