Control of Alluvial Sedimentation at Foreland-Basin Active Margins: A Case Study from the Northeastern Ebro Basin (Southeastern Pyrenees, Spain)

Abstract

Abstract Alluvial fans are the main geomorphological and depositional features at active margins of nonmarine foreland basins. The characteristics of these fans have already been extensively studied in relation to mountain-emergent thrust fronts. However, how an entire mountain front (from emergent to blind thrusts) controls the depositional environment, organization, and evolution of a foreland fan system is not yet well explored. These controls are discussed here through a case study of the Berga Formation, deposited in the late Eocene–early Oligocene in the eastern Ebro foreland basin, south of the Pyrenean chain. Near the village of Sant Llorenc de Morunys, an extensive facies analysis of the Berga deposits suggests the occurrence of a wide range of depositional processes such as subaerial stream flows and sheet flows, subaerial to subaqueous gravity flows, and subaqueous suspension fallout. This indicates two distinct sedimentary environments. The first is a large, coarse-grained, gently sloping, braided-stream-flow-dominated alluvial fan that periodically prograded into intermittent shallow lakes. It developed with sediments provided by a regional drainage basin extending from the outer Pyrenean thrust sheets to the axial Pyrenean zone. In the study area, the flow directions on this large fan had a significant component subparallel to the orientation of the mountain-front structures. In the fan, the deposits were organized in large lithosomes (tens of meters thick and tens of kilometers long), interpreted as single fan lobes. These lithosomes are arranged into packages (tens to hundreds of meters thick) of proximal to distal sediments. These packages are interpreted as short-term and long-term progradation–retrogradation cycles of the fan, driven by low-frequency climatic changes and/or tectonic events. The second sedimentary environment is a small, coarser-grained, stream-flow- and gravity-flow-dominated alluvial fan. It developed later, with sediments coming from a local drainage basin extending only on the outermost thrust sheets of the chain. The flow directions on this fan were roughly perpendicular to the orientation of the local mountain-front structures. Therefore, during the deposition of the Berga Formation, the southeastern Pyrenean foreland presented at least one large, long-lived alluvial fan within the basin and probably several small, short-lived alluvial fans at its edge. The large fan was a permanent element of the foreland geomorphological and depositional system. The geometry and evolution of the southeastern Pyrenean thrust belt controlled its location, size, drainage pattern, depositional environment, and long-term stratigraphic sequences at a regional scale. Additionally, the growth of foreland folds modified the local bed geometries, transport pathways, environmental features, and long-term stratigraphic trends of its deposits. In contrast, the small fans were temporary, jumping from the footwalls of one active emergent thrust front to another, as a result of local reorganizations into river networks and sediment sources. The adjoining emergent thrust front controlled the locations, sizes, drainage patterns, depositional environments, and long-term stratigraphic sequences of these small fans at a local scale. Finally, it is possible to speculate whether or not feedback interactions existed between the foreland deposition and the deformation of the mountain front. We suggest that the overall organization and evolution of the Berga fans, as well as their interactions with the deformation, can be extrapolated to many other active margins of foreland basins.