Variable style of transition between Palaeogene fluvial fan and lacustrine systems, southern Pyrenean foreland, NE Spain

Abstract

Two Palaeogene fluvial fan systems linked to the south-Pyrenean margin are recognized in the eastern Ebro Basin: the Cardona–Su´ ria and Solsona–Sanau¨ ja fans. These had radii of 40 and 35 km and were 800 and 600 km2 in area respectively. During the Priabonian to the Middle Rupelian, the fluvial fans built into a hydrologically closed foreland basin, and shallow lacustrine systems persisted in the basin centre. In the studied area, both fans are part of the same upward-coarsening megasequence (up to 800 m thick), driven by hinterland drainage expansion and foreland propagation of Pyrenean thrusts. Fourteen sedimentary facies have been grouped into seven facies associations corresponding to medial fluvial fan, channelized terminal lobe, nonchannelized terminal lobe, mudflat, deltaic, evaporitic playa-lake and carbonate-rich, shallow lacustrine environments. Lateral correlations define two styles of alluvial-lacustrine transition. During low lake-level stages, terminal lobes developed, whereas during lake highstands, fluvial-dominated deltas and interdistributary bays were formed. Terminal lobe deposits are characterized by extensive (100–600 m wide) sheet-like fine sandstone beds formed by sub-aqueous, quasi-steady, hyperpycnal turbidity currents. Sedimentary structures and trace fossils indicate rapid desiccation and subaerial exposure of the lobe deposits. These deposits are arranged in coarsening–fining sequences (metres to tens of metres in thickness) controlled by a combination of tectonics, climatic oscillations and autocyclic sedimentary processes. The presence of anomalously deeply incised distributary channels associated with distal terminal lobe or mudflat deposits indicates rapid lake-level falls. Deltaic deposits form progradational coarsening-upward sequences (several metres thick) characterized by channel and friction-dominated mouth-bar facies overlying white-grey offshore lacustrine facies. Deltaic bar deposits are less extensive (50–300 m wide) than the terminal lobes and were also deposited by hyperpycnal currents, although they lack evidence of emergence. Sandy deltaic deposits accumulated locally at the mouths of main feeder distal fan streams and were separated by muddy interdistributary bays; whereas the terminal lobe sheets expand from a series of mid-fan intersection points and coalesced to form a more continuous sandy fan fringe.