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The action of wind and water in a mid-Cretaceous subtropical erg-margin system close to the Variscan Iberian Massif, Spain

Authors

  • JUAN PEDRO RODRÍGUEZ-LÓPEZ,

    1. Grupo de Análisis de Cuencas Sedimentarias (UCM-CAM), Departamento de Estratigrafía-Instituto de Geología Económica (UCM-CSIC), Facultad de Ciencias Geológicas, Ciudad Universitaria, 28040 Madrid, Spain (E-mail: jprodrig@geo.ucm.es)
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  • NIEVES MELÉNDEZ,

    1. Grupo de Análisis de Cuencas Sedimentarias (UCM-CAM), Departamento de Estratigrafía-Instituto de Geología Económica (UCM-CSIC), Facultad de Ciencias Geológicas, Ciudad Universitaria, 28040 Madrid, Spain (E-mail: jprodrig@geo.ucm.es)
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  • POPPE L. De BOER,

    1. Sedimentology Group, Department of Earth Sciences, Utrecht University, Budapestlaan 4, 3584 CD Utrecht, The Netherlands
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  • ANA ROSA SORIA

    1. Grupo de Análisis de Cuencas Sedimentarias Continentales (DGA), Departamento de Ciencias de la Tierra, Facultad de Ciencias, Universidad de Zaragoza, C/ Pedro Cerbuna 12, 50009 Zaragoza, Spain
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Abstract

Aeolian processes and ephemeral water influx from the Variscan Iberian Massif to the mid-Cretaceous outer back-erg margin system in eastern Iberia led to deposition and erosion of aeolian dunes and the formation of desert pavements. Remains of aeolian dunes encased in ephemeral fluvial deposits (aeolian pods) demonstrate intense erosion of windblown deposits by sudden water fluxes. The alternating activity of wind and water led to a variety of facies associations such as deflation lags, desert pavements, aeolian dunes, pebbles scattered throughout dune strata, aeolian sandsheets, aeolian deposits with bimodal grain-size distributions, mud playa, ephemeral floodplain, pebble-sand and cobble-sand bedload stream, pebble–cobble-sand sheet flood, sand bedload stream, debris flow and hyperconcentrated flow deposits. Sediment in this desert system underwent transport by wind and water and reworking in a variety of sub-environments. The nearby Variscan Iberian Massif supplied quartzite pebbles as part of mass flows. Pebbles and cobbles were concentrated in deflation lags, eroded and polished by wind-driven sands (facets and ventifacts) and incorporated by rolling into the toesets of aeolian dunes. The back-erg depositional system comprises an outer back-erg close to the Variscan highlands, and an inner back-erg close to the central-erg area. The inner back-erg developed on a structural high and is characterized by mud playa deposits interbedded with aeolian and ephemeral channel deposits. In the inner back-erg area ephemeral wadis, desiccated after occasional floods, were mud cracked and overrun episodically by aeolian dunes. Subsequent floods eroded the aeolian dunes and mud-cracked surfaces, resulting in largely structureless sandstones with boulder-size mudstone intraclasts. Floods spread over the margins of ephemeral channels and eroded surrounding aeolian dunes. The remaining dunes were colonized occasionally by plants and their roots penetrated into the flooded aeolian sands. Upon desiccation, deflation resulted in lags of coarser-grained sediments. A renewed windblown supply led to aeolian sandsheet accumulation in topographic wadi depressions. Synsedimentary tectonics caused the outer back-erg system to experience enhanced generation of accommodation space allowing the accumulation of aeolian dune sands. Ephemeral water flow to the outer back-erg area supplied pebbles, eroded aeolian dunes, and produced hyperconcentrated flow deposits. Fluidization and liquefaction generated gravel pockets and recumbent folds. Dune damming after sporadic rains (the case of the Namib Desert), monsoonal water discharge (Thar Desert) and meltwater fluxes from glaciated mountains (Taklamakan Desert) are three potential, non-exclusive analogues for the ephemeral water influx and the generation of hyperconcentrated flows in the Cretaceous desert margin system. An increase in relief driven by the Aptian anti-clockwise rotation of Iberia, led to an altitude sufficient for the development of orographic rains and snowfall which fed (melt)water fluxes to the desert margin system. Quartzite conglomerates and sands, dominantly consisting of quartz and well-preserved feldspar grains which are also observed in older Cretaceous strata, indicate an arid climate and the mechanical weathering of Precambrian and Palaeozoic metamorphic sediments and felsic igneous rocks. Unroofing of much of the cover of sedimentary rocks in the Variscan Iberian Massif must therefore have taken place in pre-Cretaceous times.

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