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

The late Quaternary pattern of sedimentary facies on the Spanish Gulf of Ca diz continental shelf results from an interaction between a number of contr olling factors that are dominated by the Atlantic inflow currents flowing s outheastward across the Cadiz shelf toward the Strait of Gibraltar. An inne r shelf shoreface sand facies formed by shoaling waves is modified by the i nflow currents to form a belt of sand dunes at 10-20 m that extends deeper and obliquely down paleo-valleys as a result of southward down-valley how. A mid-shelf Holocene mud facies progrades offshore from river mouth sources , but Atlantic inflow currents cause extensive progradation along shelf tow ard the southeast. Increased inflow current speeds near the Strait of Gibra ltar and the strong Mediterranean outflow currents there result in lack of mud deposition and development of a reworked transgressive sand dune facies across the entire southernmost shelf. At the outer shelf edge and underlyi ng the mid-shelf mud and inner shelf sand facies is a late Pleistocene to H olocene transgressive sand sheet formed by the eustatic shoreline advance. The late Quaternary pattern of contourite deposits on the Spanish Gulf of C adiz continental slope results from an interaction between linear diapiric ridges that are oblique to slope contours and the Mediterranean outflow cur rent flowing northwestward parallel to the slope contours and down valleys between the ridges. Coincident with the northwestward decrease in outflow c urrent speeds from the Strait there is the following northwestward gradatio n of contourite sediment facies: (1) upper slope sand to silt bed facies, ( 2) sand dune facies on the upstream mid-slope terrace, (3) large mud wave f acies on the lower slope, (4) sediment drift facies banked against the diap iric ridges, and (5) valley facies between the ridges. The southeastern sed iment drift facies closest to Gibraltar contains medium-fine sand beds inte rbedded with mud. The adjacent valley floor facies is composed of gravelly shelly coarse to medium sand lags and large sand dunes on the valley margin s. By comparison, the northwestern drift contains coarse silt interbeds and the adjacent valley floors exhibit small to medium sand dunes of fine sand . Because of the complex pattern of contour-parallel and valley-perpendicul ar flow paths of the Mediterranean outflow current, the larger-scale bedfor ms and coarser-grained sediment of valley facies trend perpendicular to the smaller-scale bedforms and finer-grained contourite deposits of adjacent s ediment drift facies. Radiocarbon ages verify that the inner shelf shorefac e sand facies (sedimentation rate 7.1 cm/kyr), mid-shelf mud facies (maximu m rate 234 cm/kyr) and surface sandy contourite layer of 0.2-1.2 m thicknes s on the Cadiz slope (1-12 cm/kyr) have deposited during Holocene time when high sea level results in maximum water depth over the Gibraltar sill and full development of the Atlantic inflow and Mediterranean outflow currents. The transgressive sand sheet of the shelf, and the mud layer underlying th e surface contourite sand sheet of the slope, correlate, respectively, with the late Pleistocene sea level lowstand and apparent weak Mediterranean ou tflow current. (C) 1999 Published by Elsevier Science B.V. All rights reser ved.