The main sediment type comprising the upper Pliocene-Holocene stratigraphy of the deep marine Alboran Sea is nannofossil clay to silty clay that contains a wide variety of subsidiary minerals and a range of particle sizes. On the basis of shipboard visual estimates of the component proportions from smear-slide analyses, a typical texture is 88% clay-sized, 10% silt-sized, and 2% sand-sized, and a typical mineralogy is 51% clay minerals, 30% nannofossils, 6% quartz, 5% other carbonate (bioclasts, dolomite, foraminifers, micrite), 4% siliceous organic material (diatoms and sponge spicules), and 2%-3% other detrital silicate minerals (feldspar, mica, opaque minerals) and rock fragments (Sample 161-979A-20X-4, 54-55 cm).
The nannofossil clay and silty clay from all sites is poorly to moderately sorted, with particles ranging from clay (<4 µm) to sand size (>63 µm). The clay and fine silt sizes dominate, but coarse silt and sand are present everywhere. In some samples, sand-sized material was estimated to make up more than 20% of the clay/silty clay. In many cores, identifiable trace fossils are present, and poor sorting can most probably be attributed to biological mixing. However, a significant part of the sediment is either structureless (Fig. 4) or displays only an indistinct wispy or mottled structure.
From the typical composition cited above, it can be seen that the main sediment type comprises (1) a carbonate component, principally calcareous nannofossils, but also with foraminifers, shell fragments and other bioclasts, micrite, and some inorganic calcite and dolomite, and (2) a siliceous component, which the shipboard scientists determined is mainly clay minerals, but commonly includes quartz, feldspar, glauconite, muscovite, opaque minerals and rock fragments, and relatively rare diatom tests and siliceous sponge spicules.
In terms of a relationship between grain size and composition, only a qualitative estimate was made on board ship. In the carbonate fraction, grain-size distribution is a function of taxonomy and preservation state. Nannofossils, the main carbonate component, are confined to the fine silt and clay fraction (>20 µm). Whole foraminifers constitute the majority of the carbonate fraction of sand. Fragments of shells and foraminifers are present in both sand and silt fractions, and may also be present in the clay fraction.
The main siliciclastic components, quartz and feldspar, are commonly present in both sand and silt fractions and may be present in the clay fraction. No estimate of the proportional distribution was made by shipboard scientists, and one of the aims of the present study is to quantify the grain-size distribution of the siliciclastic components.