Petrographic characterisation of the Algeciras Unit, Campo de Gibraltar Complex, S Spain

The Cenozoic-age Algeciras Unit is a deep-marine unit cropping out between Algeciras and Tarifa and forms part of the Campo de Gibraltar Complex (Flysch Trough Units) of southern Spain. The Campo de Gibraltar Complex is situated along the contact between the internal and external zones of the Betic Cordillera. The various units range from Cretaceous to Miocene in age, but mainly comprise Paleogene and Lower Miocene sediments. The Paleocene- to Miocene-age Algeciras Unit comprises mudstones, siltstones, and silty marls with intercalated (calcareous) turbiditic sandstones and conglomerates. Five major outcrops (Punto de Getares, Cala Arenas, Punta de Carnero I, Punta de Carnero II, and Tarifa) were sampled for detailed petrographic analysis (n=18). The clastic sediments comprise medium-grained siltstones to coarse-grained sandstones, which contain varying amounts of feldspar and can be compositionally classified as feldspathic litharenites, litharenitic feldsarenites and subfeldsarenites, or more rarely, litharenites. Carbonate components, both extrabasinal and intrabasinal are negligible, indicating that the clastic sediments are non-carbonate extra-arenites. In contrast, interstitial carbonate cement is always present, though quantities vary from 0.5 to ~40%. Other cement types (iron oxide/hydroxide cement) and matrix are present, but rare. Lithic fragments are largely metamorphic in nature, although sedimentary, plutonic and volcanic fragments are all also found in the clastic sediments sampled. Of particular interest is the fact that all of the samples from the Punto de Getares outcrop are carbonates and could thus not be evaluated by the Gazzi-Dickinson point-counting method. The bioclasts present, include, benthic and planktonic foraminifera, as well as echinoderm, red algae and shell fragments. Glauconite grains were also noted. The carbonate- and bioclast-rich nature of this succession contrasts with previous studies which ascribed the Punto de Getares outcrop a more clastic character. These new petrographic results provide new insights into the depositional history of the various sedimentary units within the Algeciras Unit cropping out in the Tarifa-Algeciras area. Clearly, the source areas of the Getares sediments is very different to that of the other sediments, with a carbonate-rich provenance suggested. In addition, this particular unit has yielded a rich ichnofaunal assemblage, characteristic of a deep-marine setting. Although there are no recorded coeval shallow-marine deposits in the area, this work would suggest the possible presence of a carbonate-dominated shelf landwards of the deep-marine sediment lobe from the Getares area.