FLOOD-GENERATED TURBIDITY CURRENTS AND TURBIDITES IN ANCIENT AND RECENT ENVIRONMENTS

River flood generated turbidity currents (hyperpycnal flows) form at river mouths with discharge to the coastal ocean at a frequency variyng from one event per year, to one event every millennium or so. Most of the suspended sediment supplied to the river mouth is transported during flood-generated flows, because: (1) hyperpycnal flows form during major flood events, and (2) the rating curve is represented by a power law relationship between discharge and load. Flood-generated flows transport mainly fine grained suspended particles, but also a significant amount of sand as bed-material load. Thus a distinguishing feature of hyperpycnal floods, is long distance that sand can be transported despite the smaller proportion of sand relative to many classical slide-induced turbidity currents. Classical hyperpycnal-flow deposits (hyperpycnites) are located seaward of 1) dirty small-sized rivers (e.g. Var), and 2) cleaner rivers that are occasionally subjected to landslides or bank-failure (e.g. Saguenay), or jökulhlaups, or lahar events. Unusual flood-generated deposits can form by reconcentration of hypopycnal deposits forming off of the mouths of clean rivers through density cascading and reconcentration processes (Nile type). These examples show that (1) hyperpycnal flows can generate deposits in many deep-sea environments; (2) As a consequence, these long duration, quasi-steady, flows are good candidates to explain canyon and channel sinuosity. (3) Because they are related to floods, frequency and thickness of flood deposits is related to frequency, intensity and duration of the hyperpycnal flows. In that sense, flood-deposits represent a good deep-sea marker on climatic change across continents.