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Abstract

Flat-lying Lower Miocene deposits in the northwestern part of the Ebro Basin, Spain, are considered to be the deposits of a single, radial distributary system, the Luna System, which formed in this subsiding foreland basin. Facies distribution patterns and vertical sequences were controlled by the structure of the adjacent south Pyrenean thrust belt, the rate of basin subsidence and the rates of evaporation and infiltration. The apex of the Luna System is well defined and is adjacent to a structural low in the southern Pyrenean front. Proximal pebbly braided stream deposits show two major pulses of sedimentation which are related to two tectonic episodes and rejuvenation of erosion in the drainage basin within the thrust belt. The medial part of the system is dominated by sandy channel-fill and overbank facies. The channel bodies show a low divergence of paleocurrent directions. They contain little evidence for lateral migration and have a low interconnection. This suggests that the channels were relatively straight and stable and that the rates of subsidence and sediment accumulation were high compared to the frequency of avulsion. Evaporation and infiltration in the basin may have resulted in a radial decrease in discharge. Poorly channelized sheets are more abundant in the distal parts of the system (40 km from the apex) where fluvial sediments interdigitate with lacustrine muds and carbonates. It is suggested that the vertical sequences and lateral facies patterns in this exceptionally well preserved system may be characteristic of subsiding foreland basins with internal drainage.

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