Megaturbidites from Eocene of Southern Pyrenees: Alternative Interpretations

Cai Puigdefabregas

Several megaturbidite beds occur in the turbidite-filled South Pyrenean foreland basin (Hecho Group, lower to Middle Eocene). They have been recognized and described by several authors since 1970. The given interpretations agree that the mega-beds were the result of seismicity-triggered large-scale collapse of the adjacent carbonate platform. The main differences concern the paleogeographic location of the related platforms, transport directions, and transport mechanisms. We think there are arguments and field evidences enough to make the following statements.

The Hecho Group corresponds to the turbidite fill of the South Pyrenean foreland basin. It comprises (1) a northern active margin characterized by south-vergent thrust sheet emplacement, (2) a deeper trough filled by siliciclastic turbidites, and (3) a southern passive margin developing extensive carbonate platforms. This threefold scheme is displaced southward with time in response to thrust-sheet emplacement. No evidence is found of a northern carbonate platform from where megaturbidites could derive.

Lithology, reworked fauna, field geometrical relations, and current indications suggest a southern provenance. We believe most of the arguments indicating northern provenance come from misinterpretation of field data.

Once the previous points are accepted, there is no need to look for special mechanisms to explain the transport of huge olistolites from the northern active slope, across the basin as far as the base of the southern slope. We do not believe the Hecho Group megaturbidites result from one simple large-scale turbidite. Because of the complexity and propositions of the collapse, a succession of transport mechanisms were involved, ranging from block failure, debris flow, amalgamated turbidites, and suspension fall-out, resulting all of them from one single event.

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