engleski

Discontinuity surfaces on a bed and intra-bed scale recorded in shallow-water platform carbonates, and their relationship to shallowing-upward cycles (Early Jurassic of Gorski Kotar and Velebit Mt., Croatia)

Discontinuity surfaces (DS) recognized in the Gorski Kotar and Velebit successions represent breaks in sedimentation occurring periodically on the shallow marine carbonate platform. Their formation is particularly dependent on platform morphology, subsidence rate, high-frequency and low-amplitude sea-level changes, action of waves and currents, climate and other locally active processes such as wind patterns and nutrient levels. The study of DS-s on bed and intra-bed scale has been carried out on three Lower Jurassic successions exposed in the areas of Gorski Kotar (Gornje Jelenje) and Velebit Mt. (Kubus and Mali Alan). The under- and overlying rocks of each bedding plane were studied and detailed bed-by-bed logs were made, recording petrographic data, sedimentary structures and textures. Depositional facies and early diagenetic alteration were analysed from thin-sections and XRD analyses were performed on clayey-marly material collected from DS-s. Studied carbonates belong to the inner carbonate platform area and have a stratigraphic range from the latest Hettangian to the Late Pliensbachian. They are built of subtidal and mostly peritidal facies showing organization into shallowing-upward (coarsening- and fining-upward) cycles of variable thickness. Predominance of peritidal cycles implies a flat-topped platform. Fischer plots of the three sections are constructed to show cycle thickness variations and depict changes in accommodation. The plots indicate very good correlation and similar long-term trend. Depending upon their common features and environment of formation, three groups of DS-s are distinguished on simple bedding planes: subaerial exposure surfaces, erosion and omission surfaces. Among the identified DS-s, subaerial exposure surfaces prevail and the least represented type of DS-s are omission surfaces, which are related to periods of non-deposition or low-sedimentation rates. Erosion surfaces are mostly associated with both exposure and omission surfaces. The Lower Sinemurian succession is characterized by the falling limb of the Fischer plots and cycles thinner than average, which were formed during period of reduced accommodation space. During the Late Sinemurian an increase in accommodation space is reflected in the rising limb of the plots with maximum positive departure from mean cycle thickness and generally thick shallow subtidal bioclastic and dominantly oolitic platform sediments. The Pliensbachian is represented by a falling limb of the plots, which show a rising trend toward the end of the Pliensbachian. Generally, during periods of relative sea-level fall (falling limbs on the plots) decreased accommodation space caused tidal flat progradation and formation of thin, peritidal shallowing-upward cycles commonly terminating with subaerial exposure surfaces. According to facies evolution, which implies changes in water depth and the trend of the plots, studied successions can be designated as highstand deposits with the Upper Sinemurian and uppermost Pliensbachian succession parts showing the characteristics of transgressive deposits. Five units with very common exposure surfaces (of earliest Sinemurian, middle Early Sinemurian, earliest Pliensbachian, late Early Pliensbachian, and Late Pliensbachian age) are recognized and correlated between the sections. They all coincide with the falling limb of the Fischer plots and the thinnest cycles, whereas only the Upper Sinemurian part of the successions contains almost exclusively omission surfaces and coincides with the rising limb of the plots.

discontinuity surfaces; shallowing-upward cycles; Early Jurassic

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