Initial stages of foreland basin formation along the Adriatic carbonate platform margin of the Dinarides (Gacko, Bosnia and Herzegovina)

During much of the Mesozoic, deposition on the eastern part of the Adria microplate contrasted between two major paleogeographic domains: the deep-water realm of its subsided margin flanking the Neotethys Ocean and the shallow-water carbonate platform environments of its interior. Since the late Jurassic tectonic processes related to Europe-Adria convergence progressively affected both of these regions. Ophiolite obduction and continued nappe propagation involving Adria basement caused local uplift and subsidence related to lithosphere flexure and migration of foreland basin depocenters. The area of Gacko in Bosnia and Herzegovina offers a well exposed sedimentary record of initial foreland basin encroachment onto the margin of the long standing Adriatic Carbonate Platform.
Along a transect north of the town of Gacko, Lower Cretaceous shallow marine deposits are overlain by massive bioclastic floatstone - rudstone interpreted as deposited in a ramp environment, marking an early stage of foreland basin formation. Upwards the succession changes into a well-bedded Santonian-Campanian sequence of alternating pelagic wackestones, carbonate turbidites, hyperconcentrated density flows, and slumps deposited in a slope environment. Clast composition includes various bioclasts (benthic foraminifera, rudists), intraclasts of pelagic wackestone-packstones hosting Late Cretaceous foraminifera, and extraclasts of Cretaceous limestones. A dominance of pelagic sedimentation suggests continued relative deepening during the Campanian. Further in the succession sedimentation shifts to coarse grained carbonate breccias and calcarenites possibly indicating coeval tectonic influence along the basin flank. In the Maastrichtian and Paleocene, deposits are characterized by an alternation of marls, sandstones, calcarenites, and carbonate breccias. Thick (>10 m) megabeds consist of poorly sorted, clast-supported, angular carbonate clasts (up to 40 cm in size) in their lower segments, topped by several meter thick massive calcarenite upper segments. Their clast composition includes various carbonate lithoclasts, which together with E-NE-directed paleocurrent orientations suggest a dominant derivation of material from the carbonate platform realm to the SW. The siliciclastic component in the sandstones is largely composed of quartz and foliated quartz-mica lithoclasts, while the heavy mineral fraction includes Cr-spinel, zircon, garnet, rutile, and tourmaline, indicating contribution of material from units exposed in the advancing orogenic wedge.