Influence of the Atlantic inflow and Mediterranean outflow currents on Late Quaternary sedimentary facies of the gulf of cadiz continental margin

摘要

The late Quaternary pattern of sedimentary facies onthe Spanish gulf of Cadiz continental shelf results from aninteraction between a number of controlling factors that aredominated by the Atlantic inflow currents flowing southeastwardacross the Cadiz shelf toward the Strait of Gibraltar. An inner shelfHolocene mud facies progrades offshore from river mouth sources,but Atlantic inflow currents caused extensive progradation alongshelf toward the southeast. Increased inflow current speeds nearthe Strait of Gibraltar and the strong Mediterranean outflowcurrents there result in lack of mud deposition and development ofa reworked transgressive sand dune facies across the entiresouthernmost shelf. At the outer shelf edge and underlying themid-shelf mud and inner shelf sand facies is a late Pleistocene toHolocene transgressive sand sheet formed by the eustatic shorelineadvance. The late Quaternary pattern of contourite deposits on theSpanish Gulf of Cadiz continental slope results from an interactionbetween linear diapiric ridges that are oblique to slope contoursand the Mediterranean outflow current flowing northwestwardparallel to the slope contours and down valleys between the ridges.Coincident with the northwestward decrease in outflow currentspeeds from the Strait there is the following northwestwardgradation of contourite sediment facies: (1)upper slope sand to siltbed facies, (2) sand dune facies on the upstream mid-slope terrace,(3) large mud wave facies on the lower slope, (4) sediment driftfacies banked against the diapiric ridges, and (5) valley faciesbetween the ridges. The southeastern sediment drift facies closestto Gibraltar contains medium-fine sand beds interbedded with mud.The adjacent valley floor facies is composed of gravelly, shellycoarse to medium sand lags and large sand dunes on the valleymargins. By comparison, the northwestern drift contains coarse siltinterbeds and the adjacent valley floors exhibit small to mediumsand dunes of fine sand. Because of the complex pattern ofcontour-parallel and valley-perpendicular flow paths of theMediterranean outflow current, the larger-scale bedforms andcoarser-grained sediment of valley facies trend perpendicular to thesmaller-scale bedforms and larger-scale bedforms and coarser-grained sediment of valley facies trend perpendicular to thesmaller-scale bedforms and finer-grained contourite deposits ofadjacent sediment drift facies. Radiocarbon ages verify that theinner shelf shoreface sand facies (sedimentation rate 7.1 cm/kyr),mid-shelf mud facies (maximum rate 234 cm/kyr) and surfacesandy contourite layer of 0.2-1.2m thickness on the Cadiz slope(1-12cm/kyr) have deposited during Holocene time when high sealevel results in maximum water depth over the Gibraltar sill andfull development of the Atlantic inflow and Mediterraneanoutflow currents. The transpressive sand sheet of the shelf, and themud layer underlying the surface contourite sand sheet of the slope,correlate, respectively, with the late Pleistocene sea level lowstandand apparent weak Mediterranean outflow current.