MARINE HYPERPYCNAL FLOWS: INITIATION, BEHAVIOR AND RELATED DEPOSITS. A REVIEW

Abstract

Hyperpycnal flows form in the marine environment when river discharge enters the ocean with suspended concentrations in excess of 36 kg m-3 due to buoyancy considerations, or as little as 1-5 kg m-3 when convective instability is considered. They form at a river mouth during floods of small to medium size rivers including extreme events such as jokulhaups, dam breaking and draining, and lahars. Associated with high-suspended concentration, they can transport considerable volume of sediment to ocean basins. The typical deposit or hyperpycnite sequence is a compound of a basal coarsening-up unit, deposited during the waxing period of discharge, and a top fining-up unit deposited during the waning period of discharge. Hyperpycnites differ from other turbidites because of their well-developed inversely graded facies and intrasequence erosional contacts. These observations lead to a complete redefinition and interpretation of fine-grained turbidites. Hyperpycnite stacking can locally generate high-sedimentation rates, in the range of 1-2 m per 100 year. Because hyperpycnites are related to climate through flood frequency and magnitude, their record should vary with sea level and climate change. They can also be associated with proximal ice-melting settings. Hyperpycnal flows could also be involved in the formation of meandering canyons and channels.