Abstract

The transport and deposition of sand in deepwater submarine environments has traditionally been linked mainly to dilute sediment gravity flows of turbulent character, generally referred to as turbidity currents. Based on laboratory investigations of sand-rich experimental flume flows, we analyze an alternative mechanism for transport and deposition of deepwater sand. We argue that originally sand-rich debris flows with moderate clay content may evolve into fluidized flows able to transport and deposit clean sand at distances far from the site of initial mass failure. We show that the key process in this mechanism is a gradual separation of mud (clay + silt) and sand by the development of a fluidized region, resulting in a gradual sprinkling of sand onto the channel or basin floor as the flow proceeds downslope, due to sand grains settling through the fluidized region. This process results in a deposit consisting of a lower part of structureless and massive sand that becomes laminated in its upper part and grades further upwards into partly laminated mud that is deposited by material settling from the turbulent suspension cloud. The deposit corresponds to a bipartite “Bouma sequence” of turbidite beds with the Ta (structureless) to Tb (laminated) sandstone intervals overlain by the Td (laminated) to Te (structureless) intervals of mudstone.

The protected rear part of the flow may, due to its distance from the highly turbulent and disintegrating front, maintain its original composition for longer and thus develop fluidization gradually. This may increase the lifespan and runout of the fluidized flow, facilitating transport and deposition of rather clean, structureless or massive sand far from the source. The rear part may in some cases override distal sand settled from the fluidized flow and result in a muddy debrite being deposited en masse on top of rather well-sorted sand. This model thus also contributes to the discussion of linked debrites and transitional flows. We suggest that the fluidized-flow mechanism is one of a number of ways in which sand is transported and deposited by subaqueous sediment gravity flows, but may be widely applicable where such flows originate through the failure of sediments of varying clay content. As field analogues of sandstone beds that may have formed according to this fluidized-flow mechanism, we present examples of outcrop data from the Eocene Ainsa–Jaca basin, southern Pyrenees.