ABSTRACT

The Upper Cretaceous Vallcarga Formation in the south-central Pyrenees, Spain, consists of a thick succession of gravity-flow deposits, forming the aggradational, fining-upward and thinning-upward fill of an extensional basin. The stratigraphically lower part of this succession contains a number of two-layer beds consisting of a basal sandy carbonate (micro)breccia and an overlying turbidite. The breccia shows blunt frontal terminations (snouts) and synsedimentary thrusting. We interpret the breccia as the result of density-modified grain flow, a process intermediate between true debris flow and grain flow. Clast interlocking is a major effect in the strength of the debris and the behavior of the flow. Generation of the modified grain flows was apparently restricted to slopes at sites where the basin deepened as a result of local structural control; filling of the basin gradually reduced the slope angles and precluded generation of modified grain flows later in basin history.