Vladimir Jelaska

Cenomanian carbonate facies and rudists along shallow intraplatform basin margin-the island of Cres (Adriatic Sea, Croatia)

SummaryThe island of Cres is located in the northern part of the Adriatic Sea. The island is built up of predominantly Cretaceous carbonates deposited in north-western part of extensive and long-lasting Adriatic Carbonate Platform. Owing to the influence of synsedimentary tectonics supported by eustatic changes during the latest Albian/Early Cenomanian, different sedimentary environments were established: from shallow intraplatform basin and related slope, across basin margin to protected shallow-platform. During the Early to Middle Cenomanian rudist communities (ichthyosarcolitid/caprinid/radiolitid)flourished along a relatively high-energy intraplatform basin margin. Fair amounts of coarse-grained bioclasts, derived almost exclusively from broken rudist shells, were deposited over a marginal depocenter. Contemporaneously, pithonellid wackestone-packstones containing microbioclasts and planktonic foraminifera were deposited basinward while marginal bioclastic sediments and limestone blocks of the basin margin origin were sporadically deposited within the basin. The opening of the Cres intraplatform basin was aborted and the basin was finally filled up during the Late Cenomanian. Since the Cres intraplatform basin was established at the beginning of the Cenomanian it probably represented the initiation phase in the north-western extension of the later Adriatic Trough development.

Upper Cretaceous Organic-Rich Laminated Limestones of the Adriatic Carbonate Platform, Island of Hvar, Croatia

Several organic-rich samples taken from upper Santonian--late Campanian(?) laminated limestones of Hvar Island (Croatia) were investigated to characterize palynofacies, macerals, type of organic matter, and organic geochemistry. These characterizations, as well as the reconstruction of the depositional environment, are interpreted in terms of a source rock perspective. The results suggest that the laminated limestones were formed within shallow depressions of the carbonate platform in an anoxic, restricted-marine environment with increased salinity. The Hvar Island laminated limestones contain type II-S kerogen, and they are found to be marginally mature and oil prone.

Upper Cretaceous Peritidal Deposits of Olib and Ist Islands (Adriatic Sea, Croatia)

Upper Cretaceous carbonate deposits of Olib and Ist islands are characteristic of peritidal sediments. They consist of shallow subtidal deposits alternating with intertidal laminites (shallowing-upward cycles). Subtidal beds with micritic matrix predominate over peritidal sediments. Determination of the micro- and macrofauna revealed two distinctive assemblages: one of Middle to Upper Cenomanian and the other of Upper Turonian to Lower Santonian age. The relatively high proportion of subtidal over intertidal sediments (subtidal/intertidal ratio 2.72) indicates that the Middle to Upper Cenomanian beds were deposited during a fall in the third order relative sea-level curve (late HST to LST). Sediments of the next carbonate sequence (Upper Turonian to Lower Santonian) with a higher proportion of the subtidal over intertidal sediments (subtidal/intertidal ratio 4.57) indicate deposition during relative sea-level rise (TST) and highstand (HST). Senonian limestones are overlain by sediments of Lower Lutetian age.

Facies Variability in Lower Liassic Carbonate Successions of the Western Dinarides (Croatia)

SummaryIn the Western Dinarides the Lower Liassic carbonates are underlain by Upper Triassic “Hauptdolomit”, whereas the first appearance of the foraminiferOrbitopsella praecursor (Gümbel) marks the beginning of the Middle Liassic. Their composition, observed at several localities in Western Croatia, shows a correlation of sedimentation events, which took place during Early Liassic on the Adriatic-Dinaridic carbonate platform. Facies variability is interpreted as result of autocyclic sedimentary processes on which the carbonate platform reacted by periodical oscillations of sea-bottom near the fair-weather wavebase. As a consequence, the Lower Liassic carbonate successions in the Dinarides is characterized by stacking of two main types of coarsening-upward parasequences: (1) the basal part of the Lower Liassic succession is represented by parasequences composed of mudstones or pelletal-bioclastic wackestones as their lower members, and peloidal-bioclastic wackestone/packstones to grain-stones as their upper members; and (2) the upper part of the Lower Liassic succession with parasequences consisting of mudstones or pelletal-bioclastic wackestones overlain by ooid grainstones. Judging from the composition of parasequences and thickness relations of their members, the first type is interpreted to comprise late transgressive system tract (ITST) and/or early highstand system tract (eHST), while the second type corresponds to a late highstand system tract (1HST) and/or early lowstand system tract (eLST) of a third-order sequence.