Models of miocene marginal lacustrine sedimentation in response to varied depositional regimes and source areas in the Madrid Basin (Central Spain)

Abstract

A succession of lake complexes formed through Miocene times in the Madrid Basin (Central Spain). The complex formed during the Aragonian stage provides an outstanding opportunity to evaluate the role of source areas and depositional regimes in the construction of coeval marginal lacustrine deposits. Sedimentary facies of the Aragonian lake indicate a shallow lake that was fringed to the NW and S by arkosic alluvium, to the N by litharenitic alluvial fans, and to the NE and E by alluvial fans distally dominated by large floodplains. Marginal lacustrine deposits in the arkosic realm mostly consist of Mg-smectites and dolomite sequences that laterally interfinger with sepiolite beds which were deposited in small discontinuous ponds in the distal alluvial fan areas. Occasionally, micaceous-rich sandy streams entered the lake forming sheets at their entry points. In contrast, the northern margin of the lake was formed by muddy flats which were influenced by fluctuation in the water table. In this setting, illitic mudstones are commonly interbedded with biotite-rich flood sands, showing a sheet-like geometry. Meandering channels occasionally cut into the mud flats. Facies assemblages on this side of the lake were mainly derived from low-grade metamorphic areas. To the northeast, the marginal lacustrine facies consist of silty clays, carbonate paleosols and paludal to lacustrine limestones. These facies reflect an interfingering between the lake and a wide stream-floodplain system derived from a carbonate source area. The mosaic of marginal lacustrine facies in the Madrid Basin during the Aragonian is discussed in the light of lithological and geochemical factors related to variations in source areas.