Flow behaviour of ponded turbidity currents

Sea floor topography can constrict, deflect or reflect turbidity currents resulting in a range of distinctive deposits. Where flows rebound off slopes and a suspension cloud collects in an enclosed basin, ponded or contained turbidites are deposited. Ponded turbidites have been widely recognised in slope mini-basins and on small, structurally-confined basin floors in strike-slip and foreland basin settings. They can have a variable internal structure the significance of which remains poorly understood in terms of flow behaviour. New experiments demonstrate that the ponding process can comprise up to four phases: 1) cloud establishment, 2) inflation, 3) steady-state maintenance, and 4) collapse. The experiments explored the behaviour of sustained turbidity currents draining into small basins and show that the ponded suspensions that form are characterised by an important internal interface; this divides a lower outbound-moving layer from an upper return layer. The basal layer evolves to constant concentration and grain size, whereas the upper layer is graded (concentration and grain size decrease upward). During the cloud inflation stage, the concentration and velocity profiles within the ponded suspension evolve and this phase can dominate the resulting deposit. Outbound internal waves can travel along the interface between the outbound and return layers and impinge against the confining slope and their amplitude is highest when the inter-layer density contrast is greatest, e.g., when the input flows are thin and dense. The experiments show that flow reversals can arise in several ways (initial rebound, episodic collapse of the wedge of fluid above the counter slope, ‘grounding’ of the internal velocity interface) and that despite steady input, velocities decay and the deposit grades upwards. Internal waves emanate from the input point, i.e., do not form as reflections off the counter slope. The internal grain size interface within the suspension may dictate textural trends in sands onlapping the confining slopes. Where flows are partially ponded, internal waves can generate pulsing overspill to basins down dip.