Maja Martinuš

Discontinuity surfaces recorded in shallow-marine platform carbonates: an example from the Early Jurassic of the Velebit Mt. (Croatia)

Discontinuity surfaces of different types and scales are common in successions of shallow-marine carbonate platforms because sediments there are deposited close to the sea level and therefore are sensitive to any significant physico-chemical changes of environmental factors. Discontinuity surfaces indicate breaks in sedimentation under subaqueous or subaerial conditions. Most discontinuities in shallow-marine carbonate successions are on a bed-scale, and can be determined only by analysis of sedimentologic, diagenetic, taphonomic, and ichnologic features of the rock. The study of small-scale discontinuities has been carried out on two Lower Jurassic successions of the Velebit Mt. Depending upon their common features and environment of formation, three groups of discontinuities are distinguished on simple bedding planes: subaerial exposure surfaces, erosion surfaces, and omission surfaces. The distribution of discontinuity types in both successions is evaluated. Exposure surfaces prevail in both sections, and four units (relatively thin intervals of the sedimentary record) with abundant subaerial exposures are recognized. Dated by biostratigraphy, these units are of earliest Sinemurian, middle Early Sinemurian, earliest Pliensbachian, and late Early Pliensbachian age. Omission surfaces are the least common type of discontinuity. Thickness variations of high-frequency peritidal and shallow subtidal shallowing-upward cycles, highlighted by the Fischer plots show a very similar long-term trend for the two sections. The units with common subaerial exposure surfaces coincide with the falling limb of the Fischer plots and the section with common omission surfaces coincides with the rising limb of the plots. The studied discontinuities are formed by autocyclic and/or allocyclic processes operating on the shallow platform, but the units with abundant subaerial exposures invoke allogenic forcing of the sedimentary record. The use of the units with abundant discontinuities instead of a single surface has proven useful for the correlation of the studied shallow-platform deposits because one type of discontinuity may change laterally into another type or features of different discontinuity types can be superimposed.

High-frequency sea-level changes recorded in deep-water carbonates of the Upper Cretaceous Dol Formation (island of Brač, Croatia)

High-frequency sea-level changes recorded in deep-water carbonates of the Upper Cretaceous Dol Formation (island of Brač, Croatia) The upper part of the Middle Coniacian/Santonian-Middle Campanian deep-water Dol Formation of the island of Brač is composed of countless fine-grained allodapic intercalations deposited in an intraplatform trough. Within the studied section 13 beds can be distinguished, each defined by its lower part built up of dark grey limestone with abundance of branched, horizontally to subhorizontally oriented burrows, and the upper part, in which the light grey to white limestone contains larger burrows, rarely branched, showing no preferential orientation. The lower, dark grey, intensively bioturbated levels are interpreted as intervals formed during high-frequency sea-level highstands, while the upper, light grey-to-white levels are interpreted as intervals formed during the high-frequency sea-level lowstands. Cyclic alternation of these two intervals within the fine-grained allodapic beds is interpreted as the interaction between the amount of carbonate production on the platform margin and the periodicity and intensity of shedding and deposition in the distal part of toe-of-slope environment, which is governed by Milankovitch-band high frequency sea-level changes.