The South Pyrenean Eocene carbonate megabreccias revisited: new interpretation based on evidence from the Pamplona Basin

The South Pyrenean Foreland Basin contains numerous units of Eocene carbona te megabreccias intercalated with siliciclastic turbidites and derived by r esedimentation of shallow-marine carbonate platforms. Previous studies were limited mainly to the foreland eastern part, known as the Jaca Basin. The present study from the Pamplona Basin, a western part of the foreland troug h, sheds new light on the origin and regional significance of these South P yrenean Eocene carbonate megrabreccias (SPECMs). The number of the SPECM un its in the foreland basin is higher than previously recognized and their ag e is somewhat older than originally assumed. The SPECM units appear to occu r as time-stratigraphic clusters, which can be correlated with the relative sea-level lowstands and linked with phases of tectonic activity. The megab reccias were derived from a carbonate-platform system hosted by the forelan d basin's southern (passive) margin. The episodic instability and mass wast ing were triggered by phases of structural steepening (forebulge uplift) ac companied by high-magnitude earthquakes, with the former causing platform e mergence, increased load stresses and excess pore-water pressure in the car bonate ramp. The SPECM deposits were emplaced by cohesive debris flows evol ving into high-density turbidite currents. An ideal SPECM unit consists of (1) an immature, homogeneous debrite in the proximal part; (2) a differenti ated, bipartite debrite and turbidite in the medial part; and (3) an incomp lete, base-missing debrite overlain by turbidite, or a turbidite alone, in the distal part. The debrite component volumetrically predominates in the S PECM units, and the original terms 'megaturbidite' and 'seismoturbidite' th us seem to be inappropriate for these deposits. (C) 1999 Elsevier Science B .V. All lights reserved.