Abstract
The late Miocene pre-evaporitic succession of the Lorca basin is characterized by alternations of marls, limestones and diatomites of the Varied Mb interbedded between the basinal marls of the Hondo Fm and the evaporitic unit of la Serrata consisting of gypsum and halite (Geel 1976, Montenat et al., 1990; García-Veigas
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et al., 1994; Rouchy et al., 1998). This succession records the progressive isolation of the basin during the late Tortonian (Krijgsman et al., 2000). Seven layers of sulphur-bearing limestones are described within the Varied Mb in the central part of the basin and correlated to eight gypsum beds cropping out at the NE margin of the basin (Rouchy et al., 1998). These gypsum beds are preserved as secondary gypsum (after anhydrite) and present nodular lithofacies with chicken-wire textures. The sulphur limestones were considered as the product of diagenetic transformations, driven by bacterial sulphate reduction, of precursor gypsum layers formed in a shallow water basin in response of precessional-driven climate changes and were used to tie the succession to the astronomical polarity time scale (Krijgsman et al., 2000). In particular, the sulphur-bearing limestones were correlated to arid phases at times of precession maxima (insolation minima), when evaporation exceeded precipitations. However, this interpretation contrast with the sedimentological features and the paleontological content of the hosting sediments (marls and diatomites), which provide evidence for a relatively deep marine environment characterized, at least temporarily, by anoxic to dysoxic bottom conditions. To elucidate the origin of these peculiar layers, we carried out new sedimentological and petrological studies in two sections exposed along the La Serrata ridge. Field observations show that these limestones layers, together with the sediments in which are intercalated, exhibit evidence of sinsedimentary deformation (slumps) and contain intraclasts of various size of diatomites and marls and locally, abundant sponge spicula. In addition, the sulphur bearing limestones occupy different positions in the Varied Mb cycles: some of them are found at the base of the diatomite packets, others at their top or even within the marly intervals. A prominent feature in these diagenetic limestones is the presence of millimetric to centimetric gypsum crystal pseudomorphs included in a carbonate matrix. The origin of the gypsum in the depocenter of the basin can be related to saturate interstitial waters related to periods of water stratification that favors the presence of brines in the deep part of the Lorca basin or to the emplacement of detrital gypsum from older evaporitic units that occurs at the East margin of the basin (Playa et al 2000). In both cases, the mixing of organic-rich sediments and sulphate provided the ingredients necessary for bacterial sulphate reduction reactions, responsible for intense carbonate precipitation. In conclusion, we suggest that the sulphur limestones from Lorca are the product of diagenetic transformation of layers formed in a relatively deep basin and not to gypsum facies deposited in a shallow environment.
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