Uplifted flank margin caves in telogenetic limestones in the Gulf of Orosei (Central-East Sardinia—Italy) and their palaeogeographic significance

Abstract

This work reports the results of geomorphological observations carried out in the coastal Fico Cave and surrounding areas (Baunei, Central East Sardinia) in the Gulf of Orosei. A tidal notch, generally believed to be of Eemian (MIS 5e) age, is barely visible at 8.5 above present sea level (asl), some metres below the main entrance of the cave. Old cave passages, now partially opened by cliff retreat and parallel to the coastline, are clearly visible at around 14m asl and correspond to the main level of Fico Cave. Two more notches are located higher, at 22 and 50m asl. Fico Cave itself is composed of at least 6 clearly distinguished more or less horizontal levels (−10m below present sea level (bsl), and +14, +22, +40, +50, and +63m asl), independent of the stratal dip, arguing for a sea-level, and hence, fresh-water lens control. Cave passages develop along main fractures more or less parallel to the coastline and never extend landward for more than 150m, mostly ending blindly, or diminishing in their dimensions progressively landward. Most passages only contain clay deposits, lacking fluvial or marine sediments or typical fluvial erosion morphologies (i.e. scallops).It is suggested from this body of evidence that Fico Cave was formed in the coastal mixing zone along major discontinuities during several Quaternary interglacial periods, when sea level was high and relatively stable for enough time to develop large dissolutional voids. The geomorphological observations indicate the main +14m asl level of the cave to have formed during MIS 9, and was heavily reworked during MIS 5, while the higher levels are relative to older interglacial highstands that occurred between 1Ma and 325ka. The small active branch developed below present sea level has formed during MIS 7 (225ka). These observations shed new light on the position of the MIS 5e highstand markers in this area of the coast, much higher than previously thought.