TURBIDITE FACIES, FLUID-ESCAPE STRUCTURES AND MECHANISMS OF EMPLACEMENT OF THE OLIGOMIOCENE ALJIBE FLYSCH, GIBRALTAR ARC, BETICS, SOUTHERN SPAIN

The Aljibe Flysch, the largest tectono-sedimentary unit in the Gibralt ar Are, provides a record of Oligo-Miocene sedimentation in the Betic Orogen, southern Spain. The Aljibe Flysch can be subdivided into two s ub-units, the Beneiza Flysch (Late Oligocene) and the Aljibe Arenites (Early Miocene), which have very different depositional histories and mechanisms of emplacement. The Beneiza Flysch is characterised by late rally continuous and thin-bedded lithic arenites, siltstones and mudst ones which contain complete and partial Bouma sequences. These are int erpreted to have been deposited in a sheet-turbidite system which may have formed part of the North African Flysch Trough. By contrast, the Aljibe Arenites comprise of thick-bedded super-mature quartz arenites which are dominated by fluid-escape structures. These sandstones are s imilar to Facies Group B1-B2 disorganised sands of Pickering et al. (1 989), which are interpreted to be the product of deposition from high- density turbidity currents. Fluid-escape structures have been classifi ed according to the scheme of Lowe (1975) and their vertical arrangeme nt indicates that a large volume of sediment was deposited extremely r apidly during frequent remobilisation events. Consideration of the ori gin of the fluid-escape structures and their vertical arrangement has identified problems in their interpretation, and identified key areas for future research. The boundary between the Beneiza Flysch and Aljib e Arenites is interpreted to be a significant stratigraphic boundary a cross which there is a major change in sediment source and mechanism o f sediment emplacement. Several models can be put forward for the gene ration of this stratigraphic break, and these serve to set the agenda for future research into the emplacement of other similar flysch seque nces and the origin of the Gibraltar Are Flysch. (C) 1998 Elsevier Sci ence B.V. All rights reserved.