Windblown desert sands in coeval shallow marine deposits: a key for the recognition of coastal ergs in the mid-Cretaceous Iberian Basin, Spain

IntroductionThe action of the wind in ancientsedimentary records is commonlybased on the direct observations ofdune and interdune deposits within ahierarchy of aeolian bounding surfa-ces. A direct observation of ancientaeolian systems is not always possibleas they are prone to erosion and to theremoval of sand into other, fluvial andmarine systems. Although the aeoliandeposits may no longer be present,their former existence can be inferredon the basis of grain-size characteris-tics and aeolian features on grainsurfaces. Aeolian sediments indeedconstitute an important source formarine deposits (Kukal and Saadal-lah, 1973; Fryberger et al., 1983).Widespread, fine-grained, very well-sorted quartz sandstones characterizeshallow-marine facies in the mid-Cre-taceous of the eastern Spain (IberianBasin). The good sorting and round-ness of the grains hint at the presenceof a nearby erg system, suggestive ofan arid climatic zone along the west-ern Tethyan Margin during the mid-Cretaceous.Geological settingThe Mesozoic Iberian Basin along thewestern Tethyan Margin has one ofthe more complete lower-middle Cre-taceous sedimentary records in Eur-ope. The Iberian Range is a NW–SEstriking intraplate fold belt generatedby the tectonic inversion of theIberian Basin during the Palaeogene(Fig. 1A,B) (Salas and Casas, 1993;Capote et al., 2002). This basin wasformed by crustal thinning duringthe Late Permian–Triassic and LateJurassic–Early Cretaceous rift stages(Salas and Casas, 1993; Arche andLo´pez-Go´mez, 1996). During thesecond rift stage, extensional sub-basins were formed as part of thelarger Maestrazgo Basin (i.e. the east-ern sector of the Iberian Basin;Fig. 1B) (Soria, 1997). Sands of boththe last synrift unit of these sub-basins(the Escucha Formation) and of thefirst post-rift unit of the Iberian Basin(the Utrillas Formation) were depos-ited across the estuarine, deltaic andfluvial environments (Fig. 1C) (Quer-ol et al., 1992). The sands are domi-nantly fine-grained, show excellentsorting, and as they consist almostpurely of quartz, they are mineralog-ically very mature.The Escucha and the UtrillasformationsThe upper Aptian-middle Albian Es-cucha Formation (Aguilar et al., 1971;Sole´ de Porta et al., 1994) (Fig. 1C)was formed during the last phases ofthe second rift stage that affected theIberian Basin (Salas and Casas, 1993).It is a tidally influenced heterolithicunit dominated by very well-sortedfine sand and clay, with coal-bearingdepositsinitslowerpart,andlocallyinthe basal part coarse-grained siliciclas-tic levels. It lies upon the limestonesand marls of the Urgonian Platform(Aptian), and is covered by sandstonesof the Utrillas Formation (Fig. 1C)(Pardo, 1979; Querol, 1990). The Es-cucha Formation was interpreted as adeltaic–estuarine succession (Querolet al., 1992), with a barrier-island sys-tem in the northern part of the OlieteSub-basin (Rodri´guez-Lo´pez et al.,2005). It was deposited in extensionalsub-basins bounded by palaeogeo-graphic thresholds produced by listricand block-rotation faults. The UtrillasFormation overlies the Escucha For-mation and is late Albian–early Cen-omanian in age and represents fluvialdeposits (Pardo, 1979). However, theABSTRACT