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

Hybrid Event Beds Generated By Local Substrate Delamination On A Confined-Basin Floor

The outer parts of deep-water fans, and the basin plains into which they pass, are often described as areas where erosion is negligible and turbidite systems have net aggradation. Nevertheless sedimentological and stratigraphic analysis of outer fan lobe and confined basin plain deposits in Cretaceous-Paleocene Gottero Sandstone (NW of Italy) has revealed extensive but cryptic bedding-parallel substrate-delamination features at the base of many sheet-like event beds. These comprise a variety of shallow but wide scour structures showing evidence of lateral expansion by sand-injection. The scours commonly occur at the base of beds made up of a basal clean sandstone overlain by argillaceous sandstone containing abundant mudstone clasts and locally large substrate rafts (up to 20 meters long). These strata are interpreted as a type of hybrid event bed. Field observations suggest that mud-clast entrainment occurred by delamination at the base of dense sandy flows. The large rafts, in some cases only partly detached, were incorporated in the flows locally and then carried for short distances (100s m to a few km) before partly disaggregating and undergoing deformation due to internal shearing. The development of such features may be common in flat and/or confined basin settings where high-volume flows interact with a cohesive and well layered substrate (e.g. muddy outer fans or confined or ponded basins with thick mudstone caps). Delamination is therefore suggested as an alternative mechanism leading to the formation of hybrid event beds following local substrate entrainment on the basin floor as opposed to on more remote slopes and at channel-lobe transition zones

Rheological complexity in sediment gravity flows forced to decelerate against a confining slope, Braux, SE France

Hybrid event beds are now recognized as an important component of many deep-sea fan and sheet systems. They are interpreted to record the passage of rheologically complex sediment gravity currents (hybrid flows) that comprise turbulent, transitional, and/or laminar zones. Hitherto, the development of hybrid flow character has mainly been recognized in system fringes and attributed to distal and lateral flow transformations and/or declining turbulence energy expressed over lateral scales of several kilometers or more. However, new field data show that deposition from hybrid flows can occur relatively proximally, where flows meet confining topography. Turbidity currents primed to transform to hybrid flows by up-dip erosion and incorporation of clay may be forced to do so by rapid, slope-induced decelerations within 1 km of the slope. Local flow transformation and deposition of hybrid event-beds offer an alternative explanation for unusual facies developed at the foot of flow-confining seafloor slopes