The Messinian Guadalhorce corridor: the last northern, Atlantic±Mediterranean gateway Jose  M. Martõ Ân, 1 * Juan C. Braga 1 and Christian Betzler 2 1 Departamento de EstratigrafõÂa y Paleontolog õa, Universidad de Granada, Campus de Fuentenueva, 18002 Granada, Spain; 2 Geologisch- Pala Èontologisches Institut, Bundesstr. 55, D-20146 Hamburg, Germany/Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK Introduction Gateways are narrow passages that connect oceans and serve as pathways for water-mass exchanges (e.g. Hay, 1996). Plate Tectonics controls the position of gateways, although minor tectonic events may open or close them. The present-day Atlantic±Mediter- ranean connection is through the Straits of Gibraltar. This passage is the only natural link of the Mediterra- nean Sea with the world's oceans. The Straits of Gibraltar is a Pliocene struc- ture (Comas et al., 1999), which formed after the `Messinian Salinity Crisis'. This episode of evaporitic drawdown of the Mediterranean Sea (HsuÈ et al., 1977), which started at around 6 Ma (Riding et al., 1998), is believed to have been triggered by the isolation of this sea from the world's oceans, by a combination of global sea-level lowering and tectonic uplift closing the Mediterranean-Atlantic connection. Prior to the Messinian (7.2±5.3 Ma), the Mediterranean was connected to the Atlantic Ocean via several Iberian and north African gateways (Esteban et al., 1996). The Iberian seaways, which existed in the area of the Betic Cordillera (southern Spain), became progressively reduced in number and extent by middle-to-late Miocene tectonics, with the last survi- ving gateway occurring during the early Messinian north-west of MaÂlaga in the region of the Guadalhorce River valley. The Guadalhorce gateway pro- vided a signi®cant control on Messin- ian pre-evaporitic oceanic circulation in the Mediterranean Sea: it acted as a major out¯ow channel of Mediterra- nean water into the Atlantic and its closure induced water-mass restriction and strati®cation in the western Medi- terranean, immediately prior to the `Messinian Salinity Crisis'. Geological setting Horizontal to gently dipping layers of Upper Miocene sediments in the Gua- dalhorce River valley occur as discon- tinuous but closely spaced exposures within a N±S belt some 30 km in length (Figs 1 and 2). This succession rests unconformably on a deeply eroded Palaeozoic-to-Lower Miocene basement palaeorelief (Martõ Ân- Algarra, 1987; LoÂpez-Garrido and Sanz de Galdeano, 1999). Orueta (1917) ®rst proposed that these sedi- ments represent the remnants of a Miocene strait connecting the Medi- terranean MaÂlaga Basin to the south with the Atlantic Guadalquivir Basin to the north. Subsequent researchers have agreed with Orueta's interpret- ation and attributed an upper Torto- nian age to these deposits on the basis of regional lithostratigraphic correla- ABSTRACT Messinian marine deposits of the Guadalhorce River valley in southern Spain record evidence of the last northern gateway that existed between the Mediterranean and the Atlantic. They comprise sandstones and conglomerates with unidirectional cross-bed sets up to nearly 1 km long in their down-sedimen- tary-dip direction. These cross-bed sets relate to extremely fast (1.0±1.5 m s )1 ) bottom currents ¯owing from the Mediterra- nean into the Atlantic. The Guadalhorce gateway (which had a maximum width of 5 km and a maximum water depth of 120 m) was an important element controlling the Messinian pre- evaporitic oceanic circulation in the Mediterranean Sea, as it acted as a major out¯ow channel. Its closure limited the exchange of water between the Atlantic and the Mediterranean to the Ri®an corridors of Morocco, inducing water-mass restriction and strati®cation in the western Mediterranean immediately prior to the `Messinian Salinity Crisis'. Terra Nova, 13, 418±424, 2001 Fig. 1 Geographical and geological setting of the Guadalhorce corridor. Box marks its precise location. *Correspondence: Dr Jose M. Martõ Ân, Departamento de Estratigrafõ Âa y Pal- eontologõ Âa, Universidad de Granada, Campus de Fuentenueva, 18002 Granada, Spain. Tel.: +34 958-243335; fax: +34 958-248528; e-mail: jmmartin@ugr.es 418 Ó 2001 Blackwell Science Ltd