Beach-rock's contribution in the study of the modalities of the olocenic sea rising. Post-glacial beach-rocks of the North-East Sardinia

Abstract

As a part of multidisciplinary research studies aimed at the reconstruction of the modalities and mechanisms of Mediterranean sea level rise in the past 20,000 years, we believe it is useful to concentrate our attention on the contribution that may derive from the study of beach-rocks. This work represents a critical re-examination of the existing information framework, both published and unpublished, having the objective of contributing to integrate the curve of post-glacial sea level rise, as regard the centre-western Mediterranean Sea. The complicated issue of the use of beack-rocks as marine palaeo-levels has reduced their role, in the past years, in reconstructing the curves of eustatic rise; despite the wide scientific production on submerged beack-rocks in the Mediterranean. Numerous studies and surveys on beack-rock in the Sardinian continental shelf have found several evidences of stationary levels of the Holocene sea, both in the deposition facies, beach rocks and relict littoral sediments, and in the erosion fades, abrasion surfaces and ledges cutting the substrate. From the recent bibliography it can be deduced that during the Holocene the whole Island reached remarkable tectonic stability; the marine deposits related to the Middle and Upper Pleistocene lie at consistent and correlate levels except for a few rare sites where mild vertical blocks are observed. On one hand, this may justify the particularly conservative characters of the Sardinian continental shelf compared to the shorelines, whilst on the other one it qualifies Sardinia as a key area in redrawing the post-glacial sea rise in the western Mediterranean. In the Italian seas, the first shorelines related to the Versilian transgression and, among these, the relict submerged sandbanks at - 40 m in the Golfo di Palmas (south-western Sardinia) were found by Segre in 1969. Subsequently, after surveying the beach-rock at - 70 m in the Straits of Bonifacio a complex system of perfectly preserved submerged shorelines along the Sardinian north-eastern continental shelf (between - 55 and -145 m) was observed. This evidence of the variation of the deglacial sea level therefore enables the bypass between Sardinia and Corsica up to the present level (-75m) to be recognised. The economic significance of the relict littoral sediments as heavy minerals-bearing sediments has increased the interest of researchers (involved in the "Oceanografia e Fondi Marini - Risorse Minerarie - Placers" Project of C.N.R.) on submerged shorelines- The geophysical prospecting techniques were refined, the first cartographic experiences were accomplished for the southwestern Sardinia and the first underwater samplings of a beach-rock was collected at -33 m offshore Cala Liberotto. The wide geophysical prospecting and sampling on the entire continental shelf have completed the present knowledge on the geographical and bathy-metric distribution of the beack-rocks, and have suggested the eastern part of the shelf as more conserving than the western one. The set of data was represented in the first geomorphological-cartographic synthesis of the entire Sardinian continental shelf. The early results obtained with in the mining Project stimulated a specific research activity along the submerged shorelines. The detailed cartography methods, the underwater prospecting techniques and the procedures of sedimentoiogical and mineralogical analysis were developed on the perfectly preserved beach-rock in the sample area of Fontanamare in south-western coast. The contribution of the underwater survey in supplementing the geophysical data was of a great impulse to geomorphologic cartography of the sea bottoms. Consequently, a detailed mapping of beack-rocks in the south-eastern continental shelf at -40 and -55 m, in the Ria of Arzachena (north-eastern sector) at -2 and -10, in the Gulf of Asinara at -45 m, and in the Straits of Bonifacio at -30m was performed. Littoral-transitional depositional complexes characterised by beack-rocks in different arrangements have been observed: the Gulf of Cagliari at -25, -40, -55 m; the northern sector of the Gulf of Orosei at - 35, -40 e -55 m; and the Canale di San Pietro at -5 m. At the same time, on the western Sardinian continental platform, relict sedimentary bodies related to the Versilian transgression have been studied and their geometries and sedimentoiogical aspects analysed. The lack of information on the occurrence of beack-rocks on the -10 to 30 m bathy-metric range in the Sardinian seas seems to depend on the masking of the outcrops by Posidonia oceanica. A part of the surveys aimed at establishing Marine Reserves; detailed mapping of the marine phanerogam meadows shows beack-rocks discontinuous outcropping out of vegetable bioconstructions in the Bay of Tavolara at -5, -25 28 m, in the area of Capo Carbonara and in Sinis - Gulf of Oristano. This paper shows the methodological evolution of the geophysical surveys by Side Scan Sonar and Sub-bottom profiler; the scuba diving and remote operate vehicle survey; the analysis of sediments and cements: mineralogical and geochemical - S.E.M. Both classic and accelerator mass spectrometry A.M.S. isotopie C14 methodology was particular emphasised. The research proceeded uninterruptedly increasing the detail on both geomorphological s.s and geochemical-sedimentological aspects. Further geochronologi-cal studies have shown the comparison between radiometric data globally obtained on the beack-rocks. From a palaeodimatic and palaeoenvironmental point of view, it has been possible to read the isotopie ratios of both Oxygen and Carbon in the beach-rock cements of north-eastern Sardinia and southern Corsica. By these data results, it has been possible to draw the first curve of the Holocenic rise based on the beach-rock data only. From the analysis of the cements, supported by the new elements from the A.M.S. radiometric age of the beack-rock of Cala Liberotto (- 33 m), it has been possible to justify the curve of rise of the post Late Glacial Maximum sea. Moreover, the depositional characters and the facies of cement precipitation were better define. In particular, the impor-tance of the bacterial activity in the initial percipitation of Mg-calcite was recognised. The notable scientific interest of these outcrops in reconstructing the Holocene eustatic curve is related to the possibility of correlating the data obtained in these studies with those from the south-eastern basin of the Mediterranean. The regional scale view of the problem would thus lead to a reliable reconstruction of the palaeogeography of the Holocene sea of the central-western area of the Mediterranean Sea. The most significant limiting factors in using the beach rock, data in the sea-level rise curve have proved to be the discontinuity of the outcrops, the often bad condition of conservation, genetic characteristics (that are not always clear), the distribution in continental shelves of different origin and often characterised by significant depositional processes or their location in neotectonically active areas, and the difficulty in dating palaeo-beach sediments (apart from a few particular cases). These factors require: - A detailed analysis of the coastal and shelf geomorphological context; - Great care in dating both the cements, which can bear different mineralogical, genetical, chronological and diagenetic features, and the bioclasts, which could be subjected to a policyclical elaboration. However, to date, the data belonging to beack-rocks do not allow us to determine the paleo sea level with a detail grater than 5 m.