Flood-Generated Turbidity Currents and Related Deposits in Ancient and Recent Environments

Thierry Mulder¹, James P.M. Syvitski², Guillaume St Onge³, Emmanuelle Ducassou¹, Sébastien Migeon⁴, Julien Bourget¹, John D. Milliman⁵, Bruno Savoye⁶, Sébastien Zaragosi¹, Y. Callec⁷, and O. Parize^8
1Université Bordeaux 1, Talence, France, [email protected]
²INSTAAR, Univ. Colorado, Boulder CO
³Université du Québec à Rimouski, Québec, Canada
⁴Géosciences Azur, Villefranche-sur-Mer, France
⁵Virginia Institute of Marine Science, William & Mary, VA
⁶Ifremer, GM, Plouzané, France
⁷BRGM, Orléans, France
⁸Areva, France

River flood generated turbidity currents (hyperpycnal flows) form at mouths of flooding rivers with discharge to the coastal ocean at a frequency varying from one event per year, to one event every millennium or so. Particular geological environment (presence of easily erodable sediment) and climate (long wet season or monsoon) are the main causes for exceptional high-frequency of hyperpycnal flows (Chinese and Taiwanese rivers). Excess density when compared to density of the ambient water in the receiving basin allows the transport along the basin floor. 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. Consequently, flood-generated deposits can represent a significant part of basin-fill in basins supplied by a siliciclastic source. Flood-generated flows transport mainly fine grained suspended particles, but also a significant amount of sand as bed-material load. Because of the duration of flood-generated turbidity currents, a distinguishing feature of hyperpycnal flows 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 mountainous rivers (e.g. Var, Arabian sea, and the Eocene-Oligocene Annot Sandstone Basin in southeast France), and 2) cleaner rivers that are occasionally subjected to landslides or bank-failure in areas of earthquake activity (e.g. Saguenay, Taiwan), 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). Taking into account reconcentration, as much as 84% of the world river can generate hyperpycnal flows with initial concentration at the river mouth as low as 5 kg m^-3. Classical hyperpycnal deposits have two superposed units: a basal inversely-graded unit deposited during the increasing discharge period at the river mouth (waxing flow) and a top unit deposited during the decreasing discharge period at the river mouth (waning flow). During major floods the basal part may be eroded. During short-duration high magnitude events (jökulhlaups, natural or artificial dam breaks, such as the Malpasset dam in southeast of France) the peak in discharge occurs instantaneously and the basal part is not deposited. Most of the transport occurs as bedload and may prevent deposition seaward of the shelfbreak. This analysis shows 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. This is particularly the case of major flood deposits that formed during the dismantlement of the North-American ice sheet at the end of tle last glacial period (Lake Bonneville or Lake Missoula floods).

AAPG Search and Discovery Article #90079©2008 AAPG Hedberg Conference, Ushuaia-Patagonia, Argentina