THE ORIGIN AND DEVELOPMENT OF NEOGENE BASINS IN THE SE BETIC CORDILLERA (SE SPAIN): A CASE STUDY OF THE TABERNAS-SORBAS AND HUERCAL OVERA BASINS

Neogene- Quaternary sedimentary basins in SE Spain contain a record of the geodynamic evolution of the Internal zone of the Betic Cordillera. The basement of the Internal Zone is composed of Palaeozoic and Mesozoic metasediments which have undergone variable degrees of metamorphism. The External zones consist of largely unmetamorphosed sedimentary rocks which were deposited on the SE margin of the Iberian Plate during the Mesozoic and Early Cenozoic. Westward tectonic emplacement of these terranes onto the Iberian Plate took place between the end of the Palaeogene and the middle Miocene. In this paper, we investigate the late Miocene (Tortonian-Messinian) stratigraphy of two basins in the Internal Zone - the Tabernas-Sorbas and Huercal Overa Basins. We also consider some recently-acquired structural data. The Tabernas-Sorbas and Huercal Overa Basins are east-west trending depressions bounded to north and south by sierras in which basement rocks are exposed. The basins contain very similar sedimentary successions in which planktonic foraminifera have been preserved. However, the faunal composition is very variable, and the observed sporadic and abrupt changes in foraminiferal populations imply palaeo-ecologic and palaeo-oceanographic instabilities which may be associated with local tectonism. Stratigraphic markers were affected by these changes, making precise dating difficult near the Tortonian-Messinian boundary. Our data indicate that Messinian rocks are more widely distributed than has hitherto been suspected. A Messinian age for the prominent coral limestones in the Tabernas-Sorbas Basin has long been accepted; similar coral limestones in the Huercal Overa Basin have previously been dated at Tortonian. However, our data show that these carbonates are of Messinian age in both basins. The origin and development of the Tabernas-Sorbas and Huercal Overa Basins have previously been interpreted in a number of ways. Many (but not all) models favour strike-slip movement on NE-SW or east-west trending basin-bounding faults. The formation and deformation of the basins occurred during the Tortonian and Messinian, at the same time as the uplift of the sierras. The sierras are here interpreted to represent structural culminations above westward-verging, deep-seated thrust faults, and the basins to be lateral folds (or lateral ramps, i.e. oriented parallel to the thrust transport direction). The east-west trending strike-slip faults at the margin of the Sorbas-Tabernas Basin may be compatible with such a model. Major NE-SW trending, left-lateral wrench faults have been described in this area. These faults cut through late Miocene and Pliocene deposits, and are still active at the present day. Although these faults were initiated during the late Miocene, they do not appear to have influenced the development of the Tabernas-Sorbas and Huercal Overa Basins during the early Tortonian.