The Alboran Sea basin contains a Neogene sequence, up to 7 km thick, formed by marine facies sediments that fill basement grabens or half grabens (Fig. 2). The architecture of the sedimentary cover beneath the Alboran Sea basin is well documented from seismic profile data (Maldonado et al., 1992; Comas et al., 1992; Watts et al., 1993; and references therein).
Commercial exploration wells provide information about lithologies and ages of the sedimentary sequences, up to 3 km thick, that occur in the basin beneath the Spanish continental shelf (Comas et al., 1992; Jurado and Comas, 1992). Analysis and correlation through a dense grid of multichannel seismic reflection lines and wireline log interpretations allowed Jurado and Comas (1992) to identify six lithoseismic units within the sedimentary cover of the Alboran Sea basin (Fig. 4).
The lithology and ages of sediments forming these units are known from cutting samples from the commercial wells (Jurado and Comas, 1992). The upper Aquitanian-Burdigalian (seismic Unit VI) deposits lie directly over the basement and contain olistostromes and overpressured shales. The middle to upper Miocene (Langhian to lower Tortonian) deposits (Units V and IV) consist of overpressured shales at the base (Subunit Vb; Fig. 4), and change upward into graded sand-silt-clay turbidites (Unit IV). Unit III (late Tortonian and earliest Messinian in age) is formed of thick sandstone intervals that alternate with claystone and silty clay beds, which correspond to turbidite layers. The Messinian deposits (Unit II) consist of distal marine or shallow carbonate facies, as well as and gypsum and anhydride intervals, although thick evaporite sequences are missing.
Seismic profile images show Burdiglian-Holocene (seismic Units VI to I) sequences filling the main depocenters of the Western Alboran Basin. In the central Alboran and Eastern Alboran Basin, however, most of the sedimentary depocenters seem to be filled mainly by deposits that are probably younger than late Tortonian (seismic Units III to I), although the easternmost major depocenters probably contain lower or middle Miocene deposits (Fig. 2).
In seismic images, all the lithoseismic units appear to be bounded by unconformities of probable tectonic significance, although major regional unconformities occur at the top of the Burdigalian deposits (Unit VI), within upper Tortonian sediments (base of seismic Unit III), and at the base of Pliocene-Pleistocene seismic Unit I. The reflector marking the base of Pliocene-Pleistocene Unit I (Fig. 4) reveals a highly erosional and locally angular unconformity, which can be correlated with the M-reflector (Ryan, Hsü, et al., 1973) and corresponds to the top of the Messinian evaporites recognized elsewhere in the Mediterranean (Comas, Zahn, Klaus, et al., 1996).
Volcanic and volcanoclastic levels, intercalated throughout the middle and upper Miocene sequences, and large mud diapirs in the Western Alboran Basin, formed of undercompacted shales that are rooted in seismic Units VI and V, are particularly characteristic of the entire Alboran Sea basin (Comas et al., 1992).
Sample and stratigraphic data for the deep-sea deposits in the basin are provided solely from drilling at Deep Sea Drilling Project (DSDP) Site 121 (Ryan, Hsü, et al., 1973) and Leg 161 Sites 976- 979 (Comas, Zahn, Klaus, et al., 1996). Sediments recovered at these sites consist mainly of a variety of fine-grained sediments, ranging from nannofossil clay to clay and silty clay, with minor sand levels. Sedimentary facies are dominated by calcareous to terrigenous hemipelagites and terrigenous muddy turbidites (Comas, Zahn, Klaus, et al., 1996).
Hole 976B, in the Western Alboran Basin, a region from which the Messinian deposits were eroded, cored the entire 650-m-thick sedimentary cover and penetrated 267 m into metamorphic basement rocks. The stratigraphic sequence, ranging from uppermost middle Miocene to the Pleistocene-Holocene (Fig. 4), records three major hiatuses located between the upper and lower Pliocene (Zanclean and Piacenzian), the lower Pliocene and uppermost Miocene (Zanclean and Messinian), and within the upper Miocene (Tortonian; Shipboard Scientific Party, 1996b).
Sites 977 and 978 in the Eastern Alboran Basin sampled 598.5 m and 485 m (from 213 to 698.0 mbsf), respectively, of upper Miocene (from Hole 978A only) and Pliocene-Holocene sediments (Fig. 4). A hiatus was recognized in the lower Pliocene between 490.59 and 490.63 mbsf at Site 977. A conglomerate interval containing volcanic rock pebbles, recovered at the base of the Pliocene sequence, has been correlated with the M-reflector and interpreted as corresponding to the M-unconformity (Shipboard Scientific Party, 1996a).
The stratigraphic section recovered at Site 979, which penetrates into the South Alboran Basin, ranges from upper Pliocene to uppermost Pleistocene/Holocene (Fig. 4).
An attempt to correlate the seismic units and isochrons from the sediments drilled by commercial wells on the Iberia shelf with those from Leg 161 sites in the deeper Alboran Sea basin is shown in Figure 4.