Gordo megabed: a possible seismite in a tortonian submarine fan, tabernas basin, province almeria, southeast spain

Abstract

The mainly Tortonian sediments of the Tabernas Basin contain the elements of a well-developed, small submarine fan. Synsedimentary unconformities and randomly intercalated megabeds in both basin-plain and fan-lobe deposits indicate tectonic activity during fan growth. One of the megabeds, the Gordo megabed, overlies dislocated and deformed fan-lobe and basin-plain deposits. The megabed, with a thickness of up to 40 m, has a sheet-like geometry and consists of four superimposed sedimentary units. Unit I is a matrix-supported conglomerate and contains reefal limestone debris, metamorphic rocks and, mainly at its top, large slabs of ripped-up fan sediments up to 60 m across. The irregular surface of the Unit is filled in by Unit II: a cobble-pebble conglomerate. This conglomerate is covered by Unit III, a sandstone layer of 6–8 m thickness which grades into Unit IV, a normal graded to massive mudstone. The extra basinal content of the megabed is conservatively estimated at 0.9 km 3, its total volume at 6 km 3. From sedimentary structures and textures it is inferred that Unit I represents a deposit from a debris flow, while Unit II consists of a complex mixture of deposits of debris-flow and turbulent-flow and Units III and IV are deposits from a turbulent flow. The megabed is interpreted to originate from a single catastrophic collapse along the basin margin. Because of its exceptional volume, the sheet-like geometry and the random position within the fan sequence the megabed is referred to as a seismite. The depositional history succeeding a tectonic triggering is envisioned as follows: the collapse of the basin border evolved through stages of sliding and slumping into a debris flow; body transformation of the debris flow produced turbulent flow. It is tentatively assumed that the debris flow outraced its turbidite.