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

Carbonate rocks exposed along the eastern Adriatic coastal area from Istria to Dalmatia and Herzegovina were deposited within a broad marine shallow-water area called the Adriatic/Dinaric carbonate platform (Vlahović et al., 2005). Carbonate depositional environments on the platform were very similar to the conditions on the present day Bahamas. Due to the diverse topography on the former Cretaceous carbonate platform surface, a variety of depositional environments existed on it and various limestone types were deposited.

At the bottom of the, up to a few metres deep, shallow and warm tropical sea, carbonate mud, peloids (micritic grains) and carbonate skeletal sands (grainy sediment) were deposited. Skeletal grains are most commonly represented by micro and macroscopic benthic organisms and their debris (e.g. mollusc shells, foraminifers, calcispheres). Beside the uniform shallow-marine areas, lowlands (islands) and intraplatform depressions/deeper lagoons on the platform also occurred. Spatially limited deeper lagoons were particularly suitable for the deposition of platy limestone (Fig. 2.14). This lagoonal limestone is commonly rich in macrofossils, especially in fossil land plants, fish and vertebrates (Fig. 2.15). Due to the occasional anoxic and dysoxic conditions on the bottom, the limestone is locally rich with organic matter.

Later, from the beginning of the Cenozoic period (66 My ago) when Adria collided with Europe, carbonate and siliciclastic rocks were deposited within the so-called foreland basins (Korbar, 2009 and references therein). Material from the uplifted and eroded rocks began to accumulate over the carbonate platform deposits. Later, during the Cenozoic, carbonates were deposited during short time intervals only. Platy limestone from this period can be found in smaller areas in the project area in Herzegovina. Cretaceous and Paleogene platy limestone types are characteristic for shallow-marine environments, while the youngest Miocene deposits were formed in continental freshwater environments.

The types of platy limestone studied have been deposited on the vast platform area (covering over 550km in the recent tectonic position) and over a relatively long period, from the Cretaceous to the Neogene. Therefore, various sedimentary environments on the carbonate platform conditioned significant differences among platy limestone types along the project area. Each studied platy limestone type has different lithological characteristics.

According to some of the most cited geological classifications (Trefethen, 1950), limestone is a carbonate sedimentary rock that is characterized by more than 90% calcium carbonate (CaCO₃). Various depositional processes, as well as later processes of diagenesis (maturation and lithification of previous soft sediments), enabled the formation of bedded/layered rock textures. According to classifications (see Appendices 2.2 and 2.3 and references therein) carbonate sedimentary rocks can be generally divided into laminated rocks (<1cm in thickness), thin-bedded (10-30cm), bedded (30-60cm), thick-bedded (60-120cm), very thick-bedded (>120cm), and massive rocks. Platy rock is considered to be very thin-bedded (up to 10cm) rock. Therefore, platy limestone is a very thin-bedded limestone that is characterized by a bed thickness from 1 to 10cm.

The sequences of PL belong to various geological (lithostratigraphical) units (Fig. 2.16), or represent a separate unit themselves. In the project area, platy limestone (PL) usually appears within sequences which could be from less than a metre to up to a few tens of metres thick.

It must be noted that most of the PL sequences are only a few metres thick, and in places less than a metre. According to a general approach agreed at the coordination meetings, and during the more detailed fieldwork on PL (lithostratigraphic mapping in scale 1:50,000), we recognized three categories of PL potentiality, primarily defined according to the main criterion – a percentage of PL within the lithostratigraphic unit thickness. Namely, each stratigraphic unit defined spatially during lithostratigraphic mapping is characterized by a substantial thickness of the sedimentary succession (on average up to a few tens of metres). Every sedimentary succession is a stack of sedimentary strata (beds) of various thicknesses (from a centimetre up to a few metres). In most cases, just a certain portion of the succession is characterized by a platy appearance (beds 1-10cm in thickness). This means that PL appears only in a restricted part of the unit at the surface (outcrops), although some units can be completely characterized by a PL appearance (100%). However, the superficial spatial appearance of PL highly depends on the relation of the geological (bed dip angles) and morphological (flat or steep relief) features: as concordant these two geometrical values are, as broad is the occurrence of a PL unit on the surface. 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. This criterion was also use for the characterization of the PL in the three categories:

• LOW POTENTIAL (<10% of PL, grey colour on the maps, Maps 2.3 and 2.4),
• POTENTIAL (10-30% of PL, yellow colour on the maps),
• HIGH POTENTIAL (>30% of PL, orange colour on the maps).

For the purpose of the project, the potential and high potential units are spatially defined.