Platy limestone – geologic definition and its use as a mineral commodity

Detailed geological investigation of platy limestone has revealed a widespread occurrence of platy limestone in the RoofOfRock project area along the karstified Adriatic coast. At many locations, limestone plates are of good quality and are assessed as appropriate for use as building material. Furthermore, platy limestone was traditionally extensively exploited for local use, and there are many examples of best practice of its usage. Despite the fact that almost all platy limestone types were deposited on the same former (mostly late Cretaceous and late Paleogene) shallow-marine carbonate platform; the local geological and geomorphological conditions have resulted in some important differences between the various types of platy limestone. The platy limestone geological units in the project area are irregularly distributed within the carbonate succession and are of different ages. Most horizons are up to a few tens of metres thick, while in many places their thickness is less than one metre. In addition, the spatial distribution of platy limestone depends to a large extent on the relation of geological (bed dip) and morphological (slope dip) features. In the areas where the dip of the limestone beds is gentle and the surface is relatively flat, platy limestone can be exposed over wide areas. Spatially, most extensive types of platy limestone are known in central and south-western parts of the project area in Herzegovina, on the central Dalmatian islands (Šolta, Brač, Hvar and Korčula), the Pelješac peninsula, Trogir and Benkovac areas, as well as the western part of Istria in Croatia and in the central part of Kras in Slovenia, respectively. Platy limestone outcrops were also evidenced at numerous other locations where it is exposed in thin, dispersed and spatially limited horizons (e.g. the Kras/Carso in Italy). Regarding the platy limestone quality it should be noted that most of the platy limestone outcrops and even single beds are not lithologically homogenous, as a result of its genesis in its natural environment. Geomechanical analyses and expert field geological assessment of the limestone quality has revealed that the homogenous very fine-grained limestone (micritic lithotypes) and horizontally laminated (microbial) muddy limestone is of the best quality.

Generally, it can be concluded for the project area in Croatia that some of the most widely used materials (e.g. “Benkovac stone”, Rogač PL from Šolta, Gračišće PL from Brač, Vrboska PL from Hvar and some localities of Milna PL in general) are characterized by the best quality. The highest potential for further exploitation of platy limestone in Herzegovina is in the Podveležje, Raška gora, Zvirovići, Vrdi, Drinovci and in Borićevac sites. It is worth mentioning that in the southern and central part of Kras in Slovenia a special type of limestone plates of relatively good quality was widely used for roofing. It was named fractured limestone, as its origin is tectonically related. In the project region in Italy, appropriate reserves of this precious material that could represent a typical mineral commodity are lacking. No typical platy limestone quarries have been identified. On the other hand, massive limestone is much appreciated and has been exploited since the past thanks to their beautiful appearance and their excellent geomechanical properties. In the past, in the Kras/Carso in Italy platy limestone slabs were mainly excavated from local small outcrops, mainly from fractured limestone of tectonic origin.

Possible future reconstructions of historical objects should be based on the geological evaluation of a similar material from the active quarries

Limestone building materials used in more complex show-case urban objects in Dalmatia, representing major historical heritage, were mostly derived from a few major quarries in the Adriatic coastal region of Dalmatia, capable of supplying huge quantities of the building material, and situated as close as possible to the coast suitable for access by boats. Possible future reconstructions of historical objects should be based on the geological evaluation of a similar material from the active quarries. Furthermore, some of the major quarries and related stone factories were traditionally also able to produce standardized limestone building elements necessary for the construction of some crucial architectural elements, even in traditional rural architecture (e.g. standardized window frames and door posts). These elements were used all over Dalmatia. There are many stonemasonries in the region today which are able to produce such elements. On the other hand, the use of platy limestone as a building material has been following a rule of being used as a local source where it was possible. Plates used in objects mostly originate from local sources, situated in the vicinity of the objects.

However, today, especially in Croatia, there is a general trend of using the yellowish Benkovac platy limestone in more recent replacements on some objects, since there are no other legal quarries of the local greyish PL in the project region. Thus, we generally recommend the legalization of simple traditional "stone-gathering“, but only for local reconstructions and which should be controlled by the local government, at the very least.

It is important to note that even today in the Federation of Bosnia and Herzegovina, the laws and regulations related to the commercial exploitation of mineral resources are not well defined. This is especially the case at the county level and the consequences are clearly visible in the field, particularly in small local not quarries without mining right. However, the possibility for further exploitation of platy limestone in the project area in Herzegovina is generally estimated as very high. Preferential use of local stone is recommended from a geological point of view for future conservation and building purposes. These will considerably contribute to the preservation and protection of the natural and cultural heritage of Herzegovina. Selected show-case examples of best practice in the usage of PL as a building material should serve as milestones for implementations in the future. Gathering and picking up of PL on the surface with hand tools, which was also best practice in the past, is also highly recommended for the future, in order to preserve and protect the non-material heritage of Herzegovina.

Significant elements of natural heritage that should be properly presented to the public and locally also protected may sometimes be in conflict with the exploitation of platy limestone as a building material. Besides, some of the PL sites and areas are much more interesting for landscape protection and for the development of geo-tourism than for the commercial exploitation of mineral resources. However, according to historical data, it is clear that the majority of fossils found during the excavation of platy limestone were in small quarries which means that without excavation fossils cannot be found. Therefore excavation of small amounts of platy limestone for the reconstruction of typical karst architecture is not estimated to have too drastic an effect on a sensitive environment but digging should be under organized paleontological supervision of the competent institutions.