Large floating clasts in turbidites: a mechanism for their emplacement

The transportation mode for large, isolated clasts "floating" 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.