During the Miocene, the Alboran basin spread out north-south beyond the present limits of the Mediterranean Sea. A comparison of onshore (Betic Neogene Basins) and offshore (Alboran Sea basin) deposits, both from the ancestral Alboran Basin, reveals similarities in ages, but some lithological differences in the Neogene sedimentary sequence. Major lithological differences probably resulted from the development of different types of depositional environments on the northern margin of the Miocene Alboran Basin throughout its paleogeographic evolution.
Several extensional episodes could be responsible for the main unconformities present in the onshore and offshore Neogene sedimentary record. The extensional episodes described onshore have been dated as early Burdigalian, middle Langhian, and late Serravallian-early Tortonian (García-Dueñas el al., 1992; Crespo-Blanc et al., 1994). The base of the lower Burdigalian sequence (Sequence 1 in Fig. 3 and seismic Unit VI in Fig. 4) corresponds to the transgression after the Burdigalian extensional episode. This transgression signals the beginning of sedimentation in the entire Alboran Basin (Comas et al., 1992; Rodríguez-Fernández and Sanz de Galdeano, 1992).
In the Betic Neogene Basins, the lower middle Miocene transition is characterized by a scarcity of sediment, probably as a consequence of significant coeval extensional tectonics (the Langhian extension of García-Dueñas et al., 1992, and Crespo et al., 1993). In the Western Alboran basin, large olistostromes (seismic Unit VI) were probably deposited as a result of backthrusting in the Gibraltar Arc (Balanyá and García-Dueñas, 1987, 1988; García-Dueñas et al., 1992). Two important periods of rifting and subsidence occurred in the basin during the Langhian and Serravallian (Figs. 5B, 5C), followed by significant shallowing in the environments at the end of the middle Miocene. The marine or continental character of the Serravallian sediments probably depended upon the location of the sedimentary realm in relation to the extensional detachment on which the basin depocenters were seated. Offshore seismic data indicate that the extensional evolution of the Alboran Basin was complete by the late Tortonian (Comas et al., 1992), which agrees with dates reported for the Betic Neogene Basins.
The red continental conglomerates forming the lower Tortonian deposits in the continental environments of the Betic Neogene Basins can be correlated with an important hiatus in marine sediments, recognized in both the Betic Neogene Basins and the Alboran Sea basin. The hiatus comprises the NN9 calcareous nannofossil zone (Martini, 1971) and the Neogloboquadrina acostaensis planktonic foraminifer zone (Cande and Kent, 1995).
A significant upper Tortonian transgressive episode is marked by the existence of upper Tortonian deposits lying unconformably over metamorphic basement or ancient Neogene sediments throughout the entire Alboran Basin. In the Alboran Sea basin, this transgression corresponds to Reflector III (Fig. 4), at the base of Unit III (Comas et al., 1992). In the Betic Neogene Basins the transgressive episode is marked by the top of red continental conglomerates from stratigraphic sequence 4 (Fig. 3).
At the end of the Tortonian, a contractive reorganization of the Alboran Sea basin produced folding and strike-slip faulting with tectonic inversion of many previous structures (Comas et al., 1992; Rodríguez-Fernández and Martín-Penela, 1993; Comas, Zahn, Klaus, et al., 1996, and references therein). In the western and central part of the Betic Cordillera, the Alboran Domain emerged during the early Messinian (Rodríguez-Fernández and Sanz de Galdeano, 1992; Sanz de Galdeano and Vera, 1992).
A fall in sea level and tectonic reorganization during the late Messinian resulted in a general desiccation of the Betic Neogene Basins. The flooding in the early Pliocene affected the Betic Neogene Basins close to the Alboran Sea basin (Rodríguez-Fernández and Sanz de Galdeano, 1992; Sanz de Galdeano and Vera, 1992). During the Pliocene, a hiatus is recorded in the NN13 and parts of the NN12 and NN14 nannofossil zones (Martini, 1971) in the Betic Neogene Basins and in the Alboran Sea basin.
The latest tectonic reorganization, during the late Pliocene and Pleistocene, resulted in substantial paleogeographic changes in the marine realm and the present seafloor morphology of the Alboran Sea basin (Comas et al., 1992; Watts et al., 1993).