Calcareous Megaturbidites of Eocene South Pyrenean Foreland Basin

Pierre Labaume, Michel Seguret, Emiliano Mutti

In the Eocene foreland basin of the south-central Pyrenees--an elongated east-west-trending trough (180 × 30 km)--the siliciclastic deep-sea fan and basin-plain deposits of the Hecho Group contain nine spectacular intercalations of calcareous megaturbidites (MT). Siliciclastic turbidites were derived from fluviodeltaic complexes to the east. The MT were derived from northern and southern carbonate shelves flanking the turbidite trough, and form virtually basinwide (up to 130 × 30 km) and exceptionally thick (up to 200 m) units. Each MT consists of a basal clast-supported megabreccia division overlain by a well-graded sequence of microbreccia-calcarenite-calcareous mudstone. In its lower part the megabreccia contains slabs of shelfal limestones up to several tens of meters thick and several hundreds of meters across; toward its top, it includes metric to decametric clasts of slope mudstone. An MT represents the deposit of a mass flow (megabreccia) that generated an overlying turbidity current (graded sequence) through progressive mixing with surrounding water. A basal microbreccia of metric thickness occurring locally is injected upward into mushroomlike structures as much as 30 m high, which suggests that the mass flow moved at least partly over a sheared, overpressured basal carpet. The flows were probably triggered by major earthquakes (magnitude 7-7.5) along the fault-controlled basin borders. The MT internal facies distribution and paleocurrent pattern show that slump scars may have been as much as 100 km wide, the currents having flowed alo g the basin slope with a front of similar width.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.