Surface facies and sediment dispersal patterns: southeastern Gulf of Cadiz, Spanish continental margin

Abstract

Grain size of 294 samples and mineralogical analysis of 364 surface samples from the Spanish continental shelf and upper slope of the Gulf of Cadiz delineate four sediment facies that derive mainly from siliciclastic sources. The northern area is formed by the mud-rich prodelta of the Guadalquivir River, parallel to the coast and progradational towards the southeast. It exhibits a typical bulk mineral association of illite ≫ smectite > kaolinite + chlorite. In the north-central area, there is a smaller, coarser-grained prodelta in the Bay of Cadiz, linked to the Guadalete River, which offshore becomes connected to the Guadalquivir prodelta. Its mineral association is quartz ≫ illite > kaolinite + chlorite. In the southern area, where fluvial input is negligible, the characteristic coarse-grained facies generally corresponds to relict, Late Pleistocene reworked deposits. The typical mineral association is quartz > calcite > aragonite ≫ smectite–mixed layer clay minerals. The fourth area corresponds to the shelf break and upper slope offshore Cadiz Bay, where a westward grain-size coarsening differentiates the deposits of the last low sea-level stand and no typical mineral association exists. The mineralogy of the terrigenous deposits indicates a source area from Paleozoic sequences of the Sierra Morena to the north and the Subbetic Units and Gibraltar flysch to the east. The Neogene deposits of the Guadalquivir depression are an additional source of shelf sediments. The surface sediment distribution on the inner continental shelf is largely determined by the southeastern Atlantic water flow, the terrigenous input from rivers, and the shelf physiography. Sediment dispersal on the outer continental shelf and upper slope, in contrast, largely reflects the location of low sea-level stand, the development of palimpset deposits, and the location of the Mediterranean undercurrent over the margin.