Depositional model, pebble provenance and possible reservoir potential of cretaceous conglomerates: Example from the southern slope of Medvednica Mt. (northern Croatia)

Abstract

Upper Cretaceous deposits in Medvednica Mt. are composed of coarse-grained conglomerates, sandstones, shales and the pelagic Scaglia Limestones. Such deposits in the wider region possess reservoir potential, not previously studied in Northern Croatia. Modal composition of conglomerates, size and distribution of clasts, porosity and permeability were studied from one new exposure in Medvednica Mt., and the results were compared with previously published data from neighboring successions. Conglomerates are polymictic, clast-to matrix-supported, with clasts and matrix entirely composed of local bedrocks. Porosity varies between 4.98 to 10.89% and permeability from 1.13 to 43.3 mD. Overlying pelagic Scaglia Limestones contain pelagic foraminifera of the latest Santonian to Early Campanian age (83 to 85 Ma). Clasts were eroded from the local hinterland, probably transported to the beach by short-term torrents and deposited along the shelves of the proto-Medvednica Island. Previously presumed alluvial transport is not likely. Deposition took place in a Gosau-type basin during the subsidence phase, additionally controlled by a third-order sea-level change at the Santonian-Campanian boundary. According to this study, Upper Cretaceous clastites possess possible reservoir potential, and deserve more attention in future hydrocarbon research in Croatia.