Sands in the Alboran Sea: A Model of Input in a Deep Marine Basin

Abstract

The Alboran Sea, an almost totally land-enclosed, mountain-bounded (Rif, Betic ranges) basin, lies east of Gilbraltar in the westernmost Mediterranean. A petrologic study of the sand fraction in river, river mouth, and beach samples collected on the coast of the Alboran Sea defines the composition and distribution of the principal light and heavy mineral groups along its margins. The investigation details 20 mineralogical provinces on the southern Iberian and northern Moroccan margins and the Strait of Gibraltar sector and identifies the major source terrains and fluvial and marine point sources of terrigenous sediment entering the basin. Significant sample-to-sample changes in the proportion of mineralogical components are attributed to marine processes, particularly nearshore currents, which move sands laterally along the coast and, while so doing, modify the proportions of light and heavy mineral components. Lateral trends observed within Moroccan and Spanish mineralogical provinces provide evidence on the actual sense of nearshore sediment dispersal. Marine transport agents have a more pronounced effect on the light mineral fraction, while even unstable heavy mineral species appear to suffer less modification as a result of the transport in the marine environment. The paths followed by the sands between source terrain and final depositional site in deepwater environments are complex ones. A comparison of mineral assemblages in coastal sands and in sands in deep-sea cores shows a provenance from the Serran?a de Ronda complex in the Betic range west of M?laga. After initial deposition on the coast, these river-borne sediments are transported in a southwestward direction toward Gibraltar and then eventually are funneled downslope in a southeastward direction toward the Western Alboran Basin through the Gibraltar Canyon and submarine valley. In geological terms, the Alboran. Sea study can serve as a model for sedimentation in one type of elongate enclosed basin bounded by regions of high relief. Although the geographic and geologic configuration of the Alboran Sea and contiguous land conforms to a multisource basin model, the transport paths of sediment since the late Quaternary have been essentially longitudinal. This longitudinal input, with filling as a result of currents primarily from the Strait of Gibraltar sector, is independent of a major delta source and is thus unlike many elongate, deep-sea basins examined in present oceans and troughs (including flysch) mapped in the ancient rock record.