TRANSPORT DYNAMICS OF MASS FAILURES ALONG WEAKLY COHESIVE CLINOFORM FORESETS

The initiation mechanisms of sediment gravity flows are poorly understood. Previous studies have modeled sediment gravity flows by releasing dense water-sediment mixtures into ambient water. A problem with these studies is that the flows are artificially generated rather than forming on their own. Factors such as the density, composition and turbulence of the flows are predetermined by the experiment, precluding observation of the driving factors that initiate the flows. This study uses a new experimental method that allows these flows to self-generate. By building a clinoform using a cohesive mixture of crushed walnut shells and kaolinte, the foreset builds up and fails periodically, generating spontaneous sediment gravity flows. Slopes undergo a series of morphological changes prior to failure. The slope develops a concave shape which becomes exaggerated as deposition continues. This morphology leaves the slope in a metastable state. There are two mechanisms that trigger the destabilization of the slope, slumping and bed load transport. Once the slope is destabilized, failure is initiated. Also, in this study we investigated the role of clinoform progradation rates and their influence on failure size and frequency. We conducted experiments over a range of water and sediment discharge rates (7.09 to 36.6 cubic centimeters of water per second, 0.50 to 1.28 grams of sediment per second). Neither failure size nor failure frequency changes with discharge rate; instead, increases in sediment supply appear to be taken up by changes in the partitioning of sediment between the steep upper foreset and the more gradual delta-front apron below. Increased sediment supply rates dampen the influence of failure events by increasing background sedimentation, which takes the form of semi-continuous slow creep along the foreset; a failure mode that we think has been under-appreciated in the submarine mass flow literature. This finding suggests that the presence of mass failure deposits does not provide insight on the rate of which the sediment was transported. If these relationships hold at field scales, this would imply that sediment gravity flows are relatively insensitive to changes in water and sediment supply.