Geogr. Fis. Dinam. Quat. DOI 10.4461/GFDQ.2013.36.16 36 (2013), 189-198, 9 figg. MAURO SOLDATI (*), CHIARA TONELLI (*) & JORGE PEDRO GALVE (*) GEOMORPHOLOGICAL EVOLUTION OF PALAEOSINKHOLE FEATURES IN THE MALTESE ARCHIPELAGO (MEDITERRANEAN SEA) ABSTRACT: SOLDATI M., TONELLI C. & GALVE J.P., Geomorpho- INTRODUCTION logical evolution of palaeosinkhole features in the Maltese archipelago (Mediterranean Sea). (IT ISSN 0391-9838, 2013). The Maltese archipelago is characterised by the pres- Palaeosinkhole features characterise the Maltese islands, some of them ence of outstanding palaeosinkholes that may reach a few reaching relevant dimensions, especially in the Island of Gozo. They show varied morphostructures: sub-circular bays, large depressions and rounded hundreds of metres in diameter and stratigraphic throw, buttes or mesas (due to relief inversion) are the resulting geomorphic being comparable in size to the largest sinkholes of the expressions. A detailed geological and geomorphological study of the World. They display different landforms depending on the palaeosinkholes located in the NW sector of the Island of Gozo has been lithologies that crop out inside and outside the subsidence carried out with the aim of investigating the evolution of the associated structures. Sub-circular bays, large depressions and round- landforms. A field survey and a mapping campaign at a 1:5000-scale, cou- pled with aerial-photo interpretation, has been carried out and a spatial ed buttes are the resulting geomorphic expressions. De- database has been implemented within a GIS software. Existing evolution- spite their large size and impressive associated landforms, ary models have been critically analysed and new models proposed. Thanks these collapse palaeosinkholes have never been referred to to the detailed exploration of the collapse palaeosinkholes, it has been de- in the specialised karst literature and have received scarce ducted that at a certain stage, selective erosion has become the main factor influencing their geomorphological evolution. Positive reliefs (rounded but- attention from the geomorphological perspective. Previous tes or mesas) were formed where more resistant terrains were located inside authors provided very detailed sedimentological studies of the sinkholes; conversely, depressions and sub-circular bays developed the units filling the palaeosinkholes and proposed a genet- where the sinkholes infill was surrounded by more resistant rocks. Collapse ic model (Trechman, 1938; Pedley, 1974; Illies, 1980; Ped- structures do not seem to be active at present and their activity probably ley & alii, 2002), but they did not analyse in detail the evo- ceased during the Miocene, suggesting that karst subsidence processes are not the only responsible for the final shaping of the above-mentioned sub- lution of the current landforms controlled by these struc- circular depressions and bays. The Gozitan rounded bays related to the tures. Other papers have dealt with these morphostruc- palaeosinkholes have been compared with similar sub-circular coastal land- tures from the geomorphological point of view. Paskoff & forms located along the southern coast of the Island of Malta. The investi- Sanlaville (1978), based on a study on the coasts of the gation showed that the latter are not caused or influenced by karst process- es, but linked to the attitude of strata and to their different resistance to Maltese archipelago, described the rounded bays related erosion. The bays analysed in Gozo and Malta are morphologically similar to palaeosinkholes. Alexander (1988) focused on the tec- but genetically different, representing a relevant example of equifinality. tonic aspects of the Maltese palaeosinkholes and suggested The Gozitan sinkhole-related landforms have also been compared with to investigate the relations between surface hydrology and those of the Island of Malta, generally much smaller in size, which allowed vertical tectonics to better understand their mechanism of their different geomorphological evolution to be pointed out. formation. Coratza & alii (2012) have recently analysed KEY WORDS: Palaeosinkholes, Karst, Selective erosion, Malta, Medi- the Maltese sinkholes aiming at their recognition and as- terranean Sea. sessment as geosites. Within this frame, during recent surveys carried out in (*) Dipartimento di Scienze Chimiche e Geologiche, Università degli the Maltese archipelago, a detailed geological and geomor- Studi di Modena e Reggio Emilia, Largo S. Eufemia, 19, 41121 Modena, phological study of the palaeosinkholes has been carried Italy. E-mail: [email protected] out, with the aim of investigating the evolution of the asso- This paper is part of the Project «Coupling terrestrial and marine ciated landforms. A preliminary analysis of the palaeosink- datasets for coastal hazard assessment and risk reduction in changing envi- holes depicted in the geological map of the Maltese islands ronments» funded by the EUR-OPA Major Hazards Agreement of the Council of Europe. The Authors are grateful to Francisco Gutiérrez and (Oil Exploration Directorate, 1993) has been carried out to an anonymous reviewer for their precious suggestions. and then the western sector of the Island of Gozo has been 189 selected for a detailed geological and geomorphological Oligocene (Chattian) to Late Miocene (Messinian) succes- study. The Gozitan palaeosinkholes have also been com- sion of marine sedimentary rocks, mainly limestone and pared with other sinkhole features and similar landforms marls, which compose the Maltese archipelago (Oil Explo- in the Maltese archipelago. ration Directorate, 1993; Pedley & alii, 2002). The rock se- A field survey and a mapping campaign at a 1:5000- quence is divided into five main lithostratigraphic units scale has been developed, an aerial-photo interpretation that lie almost horizontally across the islands (fig. 2). From has been performed and a spatial database has been gener- the bottom to the top the units are as follows: (i) «Lower ated using a GIS software (ArcGIS 8.3®). Existing evolu- Coralline Limestone Formation», a hard pale grey lime- tionary models have been critically analysed and updated, stone with abundant fossil corals and marine calcareous al- and new models of the Gozitan morphostructures and gae forming sheer cliffs up to 140 m; (ii) «Globigerina other similar coastal landforms have been developed. Limestone Formation», a softer yellowish fine-grained limestone unit forming irregular slopes with terrace-like steps, 20 to over 200 m-thick; (iii) «Blue Clay Formation», GEOLOGICAL AND GEOMORPHOLOGICAL a unit composed of silty sands, marls or clays, 20 to 70 m- SETTING thick; (iv) «Greensand Formation», a greenish glauconite- rich sand, rarely more than one m-thick; (v) «Upper The Maltese archipelago is located in the Mediter- Coralline Limestone Formation», a hard, pale grey lime- ranean Sea, about 90 km south of Sicily and 290 km north- stone unit, very similar to the Lower Coralline Limestone. east of Tunisia (fig. 1). The Archipelago is composed of The Island of Gozo constitutes a horst within the Mal- three main islands: the larger one is Malta, followed by tese graben system. Several ENE-WSW faults in its south- Gozo located to the North, and Comino, the smallest, lo- eastern coast (Illies, 1981) and other main faults located in cated in between, with a total area of 316 km2 (Magri, the western part of Gozo, generally associated with the 2006). From a geodynamical viewpoint, it is located in the collapse palaeosinkholes, have controlled the development Sicily Channel, which has been affected by continental of most of the river valleys. The Gozitan landscape is char- rifting during Neogene-Quaternary period (Finetti, 1984; acterised by the presence of many gorges, named wied by Dart & alii, 1993; Civile & alii, 2010). The tectonic setting locals, formed when the rivers downcutted the Lower is characterised by two intersecting fault patterns: the Coralline Limestone bedrock because of the uplift of the NW-SE-trending Pantelleria Rift and the ENE-WSW land or the fall of the sea level (Pedley & alii, 2002). Past graben system (Illies, 1981). These faults displace the Late sea-level changes have indeed significantly modified the FIG. 1 - Geological sketch map of the Maltese archipelago and loca- tion of study areas. 1: NW coast of Gozo; 2: SE coast of Malta. 190 palaeogeography of Gozo. The Mediterranean coastlines sediments deposited in river valleys, caves and lakes (Hunt, have changed through time, the last large exposure being 1997; Pedley & alii, 2002). during the Last Glacial Maximum, when the coastline Especially significant are karst landforms, in particular around Malta was some 130 m below the present sea level the palaeosinkholes of the western coast of Gozo investigat- (Caruso & alii, 2011; Lambeck & alii, 2011; Furlani & alii, ed in the present study. Recently a few investigations have 2013; Micallef & alii, 2013). been carried out on the Maltese karst features (Marmarà, Apart from the structural features, the different erodi- 2004; Saliba, 2008; Calleja, 2010; Tonelli & Galve, 2011; bility of the five rock units is the main factor controlling Coratza & alii, 2012; Tonelli & alii, 2012). Pedley & alii the Gozitan landscape of plateaus, mesas, buttes and (2002) have recognised two cavern systems, one developed gorges (fig. 2). The coralline limestone formations usually in Miocene times and the other in recent times. The former form sheer cliffs when located along the