Sedimentary and diagenetic features in saline lake deposits of the Monegros region, northern Spain

The Monegros region in northern Spain is marked by the occurrence of a large number of ephemeral to dry lake basins, occupying small karstic depressions. The lacustrine sediment fill of these basins contain various carbonate and silicate minerals whose origin and palaeoenvironmental significance is poorly understood. For the present study, 14 lake basins were sampled in order to establish vertical, lateral and regional variations in mineralogical and textural characteristics, aimed at determining the mode of formation of the various mineral phases present. In nearly all basins, the same sequence of three lithological units is recognized, including a basal clayey unit, a middle magnesite-bearing and gypsum-rich unit, and a calcite- and dolomite-dominated surface unit. Distribution patterns of carbonate minerals indicate that magnesite is a synsedimentary precipitate, dolomite formed as a diagenetic authigenic phase, and calcite is partly authigenic and partly allogenic. All clay minerals, including sepiolite and smectite, appear to be allogenic. Regional variations are marked by similarities between groups of neighbouring basins, but no overall trend related to regional drainage patterns is recognized. The middle lithological unit records a lake stage with predominantly chemical sedimentation (Unit II), overlying a less well documented interval corresponding to a perennial lake stage with lower salinity (Unit III), whereas the surface unit formed during a period with predominantly clastic sedimentation. Based on a comparison with other regional records, the middle unit is attributed to an Early Holocene humid stage, separated from the overlying Late Holocene deposits by a hiatus that corresponds to a Mid Holocene arid stage.

Research Highlights
► Vertical, lateral and variations in composition of salt lake deposits.
► Similar sequence of lithological units in all 14 studied lake basins.
► Correlation of typical sequence with dated regional records.
► Synsedimentary magnesite, diagenetic dolomite, allogenic sepiolite.