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Deep to shallow lacustrine evaporites in the Libros Gypsum (southern Teruel Basin, Miocene, NE Spain): an occurrence of pelletal gypsum rhythmites



ABSTRACT A number of non-marine evaporite units composed of primary gypsum were deposited in saline lakes that developed in the southern Teruel Basin (NE Spain) during the Miocene. In the basin depocentre, a continuum of lacustrine evaporite lithofacies influenced by the activity of organisms is displayed. The Libros Gypsum was deposited in a deep lake, in which water stratification became unstable with progressive shoaling. Rhythmites, composed of laminae of pelletal gypsum and laminae of very fine lenticular gypsum crystals mixed with siliceous microorganisms, formed in addition to gypsum turbidites, intraformational gypsum breccias and slump structures. The pelletal laminae originated from the faecal activity of animals (crustaceans?) ingesting gypsum crystallites in the lake water during episodes of maximum evaporation, whereas the laminae of very fine lenticular gypsum mixed with microorganisms accumulated during episodes of relative dilution. In the wide marginal zones of the basin, the Libros Gypsum unit consists of massive to thin-bedded bioturbated gypsum and thin-bedded clotted gypsum, which formed in intermediate to very shallow (palustrine) water depths. The bioturbated gypsum lithofacies were produced by the action of diverse organisms, presumably worms and coleopterans, and chironomid larvae to a lesser extent; the massive lithofacies precipitated in very shallow water; and the thin-bedded lithofacies formed in shallow to deeper settings. The thin-bedded clotted gypsum is a relatively deep facies that may have diverse origins (e.g. bioturbation, compaction, disruption of soft sediments and early diagenesis). There is a well-developed metre-scale cyclicity in the marginal lake sequences, which is not observed in the inner lake deposits. This suggests a depth control in the various lacustrine subenvironments to record cyclic evaporitic processes. The isotopic composition of the gypsum indicates early sulphate-reducing bacterial activity in the bottom of the lake and suggests that the sulphate was derived from the chemical recycling of Triassic evaporites of the country rocks.