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.