Variations in fluvial style on the Eocene–Oligocene alluvial fan of the Scala Dei Group, SE Ebro Basin, Spain

Abstract

The Scala Dei Group represents one of several coarse clastic wedges along the eastern margin of the Tertiary Ebro Basin. This group, of Eocene–Oligocene age, is partly concealed by faulted and locally overthrust older sediments, but a segment of Oligocene age is spectacularly exposed for about 15 km along the newly constructed A2 motorway. Near the edge of the basin multistorey conglomerate channel-bodies are composed of large scale foresets up to 3 m high, representing side bars or possibly point bars. Smaller (up to 40 cm) normally graded beds with imbricated clasts represent diagonal or transverse bars, and horizontally-stratified sheets may be longitudinal bars. Flow was highly variable. The multistorey conglomerate bodies are restricted to several crudely radiating axes representing major feeder systems and give rise to a ‘goose-foot’ depositional palaeomorphology. Towards the basin centre and in quiet zones between the main feeder axes single-storey channel-bodies of conglomerate and sandstone are common. Some plug-like conglomerate and sandstone channel-bodies are ribbons which are essentially straight in plan, and other, generally asymmetrical, sandstone channel-bodies preserve the point bars of channel bends where limited lateral migration took place. Many channel-bodies possess extremely well-developed wings representing levees and locally amalgamated crevasse splay sequences. Conspicuous crevasse channels cut through the levees. The wings and crevasse channel-bodies contain carbonate-rich palaeosols. This sequence is interpreted as an essentially distributive but ephemeral fluvial system.