The OIS 5c along Licosa cape promontory (Campania region, southern Italy): morphostratigraphy and U/Th dating

A geological, geomorphological and topographical survey has been carried out, together with the air photo interpretation and the analysis of topographic maps (1:5000 scale) along the Licosa cape promontory. Four different orders of well-preserved wave-cut terraces are identified at 20-25, 6.5 -10, 5 and 1,5 m. asl. respectively. The relative age of these four orders of marine terraces can be inferred on the basis of their correlation with the morphostratigraphical data coming from the surrounding area of S.Maria bay (northen boundary) and Ogliastro bay (southern boundary) that has been studied in a previous paper (CINQUE ET AL. 1994). Particularly the highest order can be ascribed to the end of the middle Pleistocene, oxygen isotope stage (OIS) 7, while those located at lower altitude seem to correlate quite well with the main sea level high stands of the OIS 5 (5e, 5c and 5a respectively). To test these hypotheses we try to obtain data from U-series dating of the few marine deposits we found associated with the +5 wave-cut terrace and for this aim an accurate sampling of the biogenic calcilutitic deposits has been carried out. These deposits consist of two basic components: a first detrital phase, more or less bioclastic, and another biogenic mainly formed by crusts of coralline algae with minor bryozoans. This represents a first attempt to apply U-series disequilibrium method to algae samples. The present authors believe that a rigorous protocol must definitely be followed in sample selection and preparation and independent evidences must confirm the reliability of U-series ages. By means of cursory petrographic examination, we have selected the sample for radiometric analysis that contained the thicker red algae crusts. Very thin slices (around 0.5 cm) have been cut with an ordinary saw; these have been then broken to isolate fragments consisting almost uniquely of algal crust. In this way we have tried to keep to the minimum the contamination by the calcite cements. The fibrous carbonate in the conceptacula is apparently very early in origin and should not introduce a significant error in the dating. The red algae sample was then cut into thin sections with a small diamond. Any sections was broken into several-millimetre-diameter pieces with a stainless-steel chisel. The fragments were accurately examined under a binocular microscope and any pieces that showed signs of secondary alteration were discarded. Sampling was rigorously restricted to areas dominated by talli of encrustant red algae. Areas with trace of detritus (rich in quartz, mica and feldspar grains), bioclatic fragments (bivalvia, anellida, foraminifera, echinodermata and bryozoa) and post-depositional carbonate cement were discarded. The remaining pieces were ultrasonically cleaned in distilled water twice. The U-series dating of the red algae has given an age of 102 +/- 4 ky which is in agreement with the morphostratigraphical data and can be accepted as real Some considerations have been added related to the altitude of the various orders of wave-cut terraces and the paleo-sea level eustatic curve reconstructed in other area of the Mediterranean basin. For what concern the sea level fluctuations of the last interglacial (substage 5e) recordered in Licosa cape, we underline the occurrence of two paleo sea level, respectively testified by the terraces of +9 and +6.6 m, that resembles the behaviour of the sea level during the substage 5e reconstructed in several mediterranean and extra-mediterranean coast. Moreover we established that the Licosa headland can be considered tectonically stable since the substage 5a, i.e. during the last 70 ka.