Morphology and hydrogeology of a complex sinkhole system in a remarkable archaeological site along the Adriatic coastline (Apulia, S Italy)

The Grotte della Poesia karst system is a complex of caves, sinkholes and submerged galleries, located along the Adriatic side of southern Apulia (Italy). In detail, the system consists of two main collapse sinkholes (Grotta della Poesia Grande and Piccola), connected through sumps with an intervening cave, and linked to the sea on two sides. Sinkhole development was strongly favoured by hyperkarst processes due to intermixing between fresh and salt water, and by the resulting increased aggressivity on carbonate rocks. The overall system is within the remarkable archaeological site of Roca, which incorporates remains from late Bronze to Medieval age (Scarano 2010). In particular, Grotta della Poesia Piccola hosts along its walls thousands of Messapian inscriptions dating back to IV-II centuries B.C., which are still the object of study by archaeologists.

Local stratigraphy in the area consists of weak, laminated calcilutites and fine calcarenites alternated to coarser macro-fossiliferous and bioturbated calcarenites (Middle-Upp. Pliocene). Differences in permeability among the layers originate a multi-layered water table. To this, name of the caves has probably to be related, since the word poesiacomes from the local dialect (in turn, from ancient Greek), to indicate a spring or water emergence (Parise et al. 2003). A spring would therefore have been present within Grotta della Poesia Piccola, but at present is not visible anymore, probably due to lowering of the water table.

Tectonically, wide folds with N 150 E axes (about parallel to the coast) characterize the area. They determine the presence inland of a wide marshland (Tamari), that has been interpreted as the inner and protected harbour for the ancient town of Roca.

The Adriatic coastal landscape is characterized by a number of marine terraces resulting from the combined action of regional uplift and glacio-eustatic sea level changes. The coastline is very articulated, with 10-15m high cliffs, intensely affected by slope instabilities (Delle Rose and Parise 2004; Lollino et al. 2021). Within this geological setting, we are carrying out detailed speleological and diving explorations aimed at fully surveying the intricate system of caves (in both aerated and flooded conditions), as a mandatory step in order to identify the sites most susceptible to rock instabilities. Since the site is highly touristic, it is crucial to recognize the likely hazards, and to properly delimit the most dangerous areas. The surveys are also going to be used to better understand the hydrogeological situation, and to verify the possibility of presence of submarine springs in the coastal area and its surroundings.

 

References

Delle Rose M. & Parise M., 2004, Slope instability along the Adriatic coast of Salento, southern Italy. Proc. IX Int. Symp. Landslides, 1, 399-404.

Lollino et al., 2021, Multi-scale approach to analyse the evolution of soft rock coastal cliffs and role of controlling factors: a case study in South-Eastern Italy. Geomatics 12 (1), 1058-1081,

Parise et al., 2003, Karst terminology in Apulia (southern Italy). Acta Carsologica 32, 65-82.

Scarano T., 2010, Roca. Le fortificazioni della media età del Bronzo. Ann. Scuola Normale Sup. Pisa, s. 5, 2 (2), 151-204.