Large floating clasts in turbidites: a mechanism for their emplacement

Postma, G., Nemec, W. and Kleinspehn, K.L., 1988. Large floating clasts in turbidites: a mechanism for their emplacement. Sediment. Geol., 58 :47 -61 The transportation mode for large, isolated clasts "f loating" in turbidites is a problem that has been difficult to resolve. New observations from experimental, high-density turbidity currents indicate that large, outsized clasts can be transported along a rheological interface which develops within the flow. The clasts "glide" along the top of an underlying, pseudolaminar inertia-flow layer, partly submerged in it, and are driven by the downflow component of turbulent shear-stresses transmitted from the overlying, faster-moving turbulent layer. As the inertia-flow layer freezes and a new one forms, or as the layer thickens, the gliding clast may be forced to a progressively higher level within the flow. With deceleration, the inertia-flow phase of the flow freezes entirely and the large clast is then trapped "suspended" above the base of the resulting turbidite. Both a (p)a (i) and a (t)b(i) orientations of the floating clasts are observed. It is suggested that, hypothetically, a similar mechanism for megaclast emplacement may also operate in other types of viscous sediment flows, subaqueous or subaerial, wherever there is a distinct rheological interface developed within the flow.