Deep marine sediment in the Alboran Basin, as represented by samples collected from Sites 976, 977, 978, and 979, is remarkably homogeneous throughout the upper Pliocene to Holocene section. Most is structureless to burrowed clay and silty clay in which calcareous nannofossils are the principal component, with subordinate sand-sized foraminifers.
Throughout the sequence quartz and feldspar are minor but persistent components, generally making up less than 10%. Grains range in size from sand to clay, and at least in the case of quartz, they are detritally derived as evidenced by the coarser grain sizes and rounding seen on SEM images. The detrital input to the basin, represented by the quartz and feldspar fractions in the holes, appears to have been controlled by local sedimentation events such as immediate source and short-lived restricted bottom-flowing currents, rather than by regional ones. The main evidence for this is the lack of correlation between cycles in the quartz distributions. Given the proximity and elevation of the adjacent tectonic hinterland, it was hypothesized that regional tectonic events may have been reflected in detrital input that could be regionally correlated,; however there is no discernible evidence of such events. Variation is either not detectable at the sampling scale used, or the events did not occur.
There is no evidence to support the contention of shipboard scientists (Shipboard Scientific Party, 1996b) that the detrital input represented by quartz + feldspar + rock fragments + mica (QFRM) in Hole 976B was arranged into upward-increasing cycles, each ~150 m thick (similar to the thickness of the turbidite channel fill represented by lithostratigraphic Unit II at Site 976). Overall, detrital sediments compose less than 10% of the sediment and are interpreted to be biologically mixed, relatively thin, distal turbidite deposits. This is not to say that submarine fans or base-of-slope aprons do not exist in the Alboran Basin, only that none were intersected by holes drilled during Leg 161, with the exception of Unit II at Site 976. On the basis of seismic evidence and the presence of an erosional base, this sequence appears to be a localized channel-fill deposit located on the northern slope of the Western Alboran sub-basin. The lack of coarse-grained deposits at other sites in the upper Pliocene to Holocene is not surprising given the physiographic locations of the drilling sites.
A regional correlation exists in cycles based on the proportion of carbonate, and these can be separated into <20-µm (mainly calcareous nannofossils) and >63-µm (mainly foraminifer) fractions. In the former, cycles have a frequency of occurrence of ~500 k.y., whereas in the latter, which are less well defined, the repetition rate is ~750 k.y. Both can be equated with fourth- to fifth-order sea-level cycles and are attributed to climatic control, most likely caused by orbital eccentricity variability.