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Central Mediterranean and Carpathians |
1 Department of Earth Sciences, Oxford University, Oxford OX1 3PR, UK
2 Department of Earth Sciences, Leeds University, Leeds LS2 9JT, UK
The Messinian ‘salinity crisis’ marked a dramatic climatic change in the Mediterranean region. The timing and extent of this event remain controversial, with conflicting models ranging from predictions of rapid and catastrophic dessication of all of the Mediterranean basin at once, to progressive draw-down of sea-level causing gradual onset of evaporite formation starting at the margins of the Mediterranean Sea. In this paper the preliminary magnetostratigraphy is described of two sections in the Neogene Caltanissetta basin, Sicily, containing the early Messinian Tripoli Formation, which pre-dates evaporite formation, and the overlying Calcare di Base shallow water carbonates that mark the first evaporite phase. The magnetisation of the sediments has been extensively overprinted by recent weathering, but primary or early diagenetic remanences which pass reversal and fold tests have been isolated. This has been possible because the sections have been tilted by Pliocene tectonic movements, thus rotating the pre-tilt remanence direction away from the later overprint direction, allowing resolution of the components by great circle analysis. The sections have been correlated with Krijsman et al.’s revision of the geomagnetic polarity time scale (GPTS) of Cande & Kent. This was achieved by comparing variations in the sediment rate derived from the alternative matches with the GPTS with the observed proportions of diatomite (relatively fast deposition) and clay (slow deposition). The proposed correlation puts the base of both sections at or close to the start of the Messinian, but predicts diachroneity in the onset of Calcare di Base deposition between the two sections, as the base of the Calcare di Base falls in opposite polarity chrons in the two sections. The sections lie in two separate sub-basins controlled by thrust anticlines of the Maghrebian thrust belt. Structural and sedimentological evidence suggests that the onset of evaporite formation in these perched basins was strongly controlled by the extent and development of the emerging anticlines. The magnetostratigraphy provides a first positive test of this prediction of tectonically controlled diachroneity of facies. Further palaeomagnetic work is in progress to assess the relative timing of evaporite formation on Sicily in more detail.
