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Discontinuity surfaces related to subaerial exposure of shallow-water platform carbonates (Early Jurassic of Gorski kotar and Velebit Mt., Croatia)

Area of Gorski kotar and Velebit Mt. are mainly built of Mesozoic carbonate deposits, which were formed on a vast Adriatic Carbonate Platform (AdCP). From the late Early Jurassic until the end of the Cretaceous, the AdCP was an area of carbonate production that resulted in an up to 5 km thick succession of shallow marine carbonates and was punctuated by periods of subaerial exposure and several pelagic drowning episodes. The subject of this research is Lower Jurassic carbonate succession with common discontinuity surfaces, which is nowadays well exposed in the area of Gorski kotar (locality Gornje Jelenje) and Velebit Mt. (localities Kubus and Mali Alan). The biostratigraphic age of the studied succession ranges from the latest Hettangian to the Late Pliensbachian. The aim of our work is to describe and interpret macro- and microscopic features observed on discontinuity surfaces (DS), which can be related to ancient subaerial exposure of this shallow-water platform carbonates. In the studied section, emersion surfaces are characterized by subaerial exposure ranging from (1) occasional influence of meteoric waters (development of diagenetic discontinuities), (2) intermittent subaerial exposure (inter- to supratidal hardgrounds) to (3) prolonged exposure leading to pedogenesis and/or karstic dissolution [palaeosols (3a) and paleokarst/microkarst (3b)]. (1) Diagenetic discontinuities show meteoric influence visible only in thin-sections as dissolution and recrystallization of skeletal and nonskeletal particles, and vadose cements (pendant and meniscus cements). These features indicate periods of occasional influence of meteoric waters without subaerial exposure. Diagenetic discontinuities are overlain by rocks showing features of marine diagenesis. (2) Inter- to supratidal hardgrounds (HG) show circumgranular and desiccation cracks (macro- and microscopic scale), sheet cracks, brecciation, birdseyes, black pebbles, and pisoids. These diagenetic features imply formation by repeated sediment drying and wetting in subaerial conditions. Fillings of the polygonal desiccation cracks are preserved on many bedding surfaces, but tepee structures have been seen only rarely. Many microbial and fenestral limestone laminites are capped by dolomitic crusts with dolomitization being most intense in the upper part of the bed and penetrating downwards for about 10 cm. Some bedding surfaces covered by such stromatolites show strong reddish-brown to dark reddish staining (possibly by iron oxides) indicating oxidation during subaerial exposure. Stained surfaces often show laminated microcrystalline carbonate in thin-sections, i.e., incipient calcretization. However, the possibility that this alteration is due to outcrop weathering should also be taken into account. Possible vertebrate footprints have been found on the top of one inter- to supratidal HG. (3a) Palaeosols show features of incipient soil development, plant activity, and vadose conditions indicating prolonged subaerial exposure. In the field, they show irregular relief and a brecciated appearance due to dissolution processes and rhizoturbation. The alteration of limestones (dissolution, recrystallization, and calichification) reaches 10–20 cm into the strata below the DS and is characterized by reworked lithoclasts within a brownish to yellow clayey matrix. Preliminary XRD analyses of clayey matrix show the presence of illite. Contact with the overlying bed is mostly sharp and a thin clayey covering is rarely preserved on the upper bedding plane, probably because of the erosion during the following marine flooding. Blackened clasts comparable to the black pebbles typical of pedogenic settings are observed. Microfacies analysis revealed characteristic circumgranular cracking, solution vugs, and root-related structures (tubular branched voids of decayed roots filled with sparite, alveolar-septal and nodular fabrics, i.e. glaebules). (3b) Paleokarst (microkarst) surfaces show dissolution vugs and fissures penetrating not more than 15–20 cm into the bed and never reaching into the underlying bed. Irregular cracks and fissures pervade the bed perpendicularly in relation to the upper bedding plane, indicating their paleokarst character and excluding the possibility that they result of Recent karstification. Fissure walls are sharp, indicating that dissolution occurred in completely lithified rock. The fissure infill contains many angular millimetre to centimetre sized fragments of host rock and, on rare occasions, yellow to brown clayey to marly sediment. Small dissolution cracks and voids seen in thin-sections sometimes have a brownish coating preserved on their walls, indicating ancient corrosion in subaerial conditions. Discontinuity surfaces related to subaerial exposure, recognized in the studied succession, represent breaks in sedimentation occurring periodically on the shallow marine carbonate platform. Their formation is dependent upon many interrelated factors, such as 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. It can be assumed that the duration of emergence events that produced palaeosols, paleokarst or inter- to supratidal HG-s was not long, due to the fact that features of mature palaeosols or paleokarst were not found. Nevertheless, common subaerial exposure - related discontinuities bring into question the stratigraphic completeness of the studied succession and give valuable information about carbonate platform evolution. Also, parts of the succession with abundant discontinuity surfaces (e.g. the one of earliest Pliensbachian age) can be used as useful field markers for stratigraphic correlation.

Discontinuity surfaces; subaerial exposure; shallow-water carbonates; Early Jurassic

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