Abstract

This work presents results of geological mapping of the easternmost part of the Logudoro basin: a roughly NNW trending half-graben of Burdigalian age infilled by continental and marine volcanic and sedimentary successions (Funedda et alii, 2000). The study focuses on the volcanic succession made up of pyroclastic flows which lap on to a metamorphic basement of Variscan age. The succession, roughly spanning from Aquitanian up to middle Burdigalian, consists of, from the bottom: a) continental conglomerates (Badiamenta Conglomerate) made up of metamorphic rounded pebbles with sandy matrix; b) andesites showing porphyritic texture (Monte Umulu Andesite); c) well bedded conglomerate cemented by calcite and analcime, containing clasts deriving from the reworking of andesite and Paleozoic basement (Monte Umulu Conglomerate); d) an ignimbrite more than 150 m thick (Lower Ignimbrite), which in turn has been split into three different cooling sub-units; e) an epiclastic stratigraphic unit (Badu e Giaga Epiclastics); f) a second less welded and less thick ignimbrite (Upper Ignimbrite). Resting on this essentially volcanic succession is the well known transgressive upper Burdigalian-Langhian sequence. In some places Plio-Pleistocene within-plate basaltic flows seal all the previous volcano-sedimentary and tectonic features. The main goal of the field mapping was to evaluate, within the thick volcanic succession, the sedimentary nature and lateral continuity of Badu e Giaga Epiclastics. This deposit of lacustrine environment has been recognised for the first time as an important stratigraphic marker within a mainly volcanic area where the lithologic units show limited lateral continuity as well as highly variable thickness. For these reasons a sedimentary unit having regional continuity facilitates the reconstruction of chronological and spatial relationships among the different volcanic successions and also between volcanic and sedimentary successions. A synthesis of such reconstruction results from the attached map; it could represent a useful tool for reconstructing the structural, magmatic and sedimentary evolution of the region. Moreover, as the Badu e Giaga Epiclastics occurs between two main ignimbrites, it also testifies to a significant time span during which the volcanic activity in the area was almost quiescent. The present day structural framework of the area is mainly controlled by N60 degrees -N70 degrees and NS trending normal faults. The latter, of Pliocene age, are responsible for the general uplift of the region; the former, even though they experienced probable Serravallian and Pliocene reactivations, are consistent with the Oligocene-Aquitanian trend which characterizes the well known sinistral strike-slip tectonics of north Sardinia and Corsica (Carmignani et alii, 1994; Oggiano et alii, 1995). Those faults controlled circulation of fluids which caused hydrothermal alteration of the glassy fraction in both pyroclastic and epiclastic rocks (Cerri et alii, 2001). This hydrothermal alteration resulted in a strong zeolitization that appears more severe and uniform in the Badu e Giaga Epiclastics than the underlying and overlying thick pyroclastic flows (CERRI, 2000; Cappelletti et alii, 2000). As a consequence, this study could represent a basic contribution to further prospecting and evaluating zeolite deposits. Finally the research highlights the differences between the lacustrine epiclastics of the studied area and the more common lacustrine epiclastics which are characterised by alternances of tuffites and basin-wide flint strata.

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