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Palaeokarst surfaces recognized in Lower Jurassic carbonates on Velebit Mt., Croatia

Numerous surfaces showing evidence of ancient exposure to subaerial conditions and dissolution from meteoric water are recognized in shallow-marine carbonates recorded on Velebit Mt.. Each of these surfaces point to subaerial exposure of various duration, but few of them show macroscopic and microscopic diagenetic features produced during prolonged chemical dissolution and associated modification of a carbonate rock, i.e. show signs of karstification. Such surfaces are therefore called palaeokarst surfaces (Choquette & James, 1988). Two potential palaeokarst surfaces are recognized in the studied succession and will be here described. Two sections of Lower Jurassic carbonate succession that belonged to the shallow inner ramp environment of the Adriatic-Dinaridic carbonate platform (ADCP) were studied in detail. Both sections are situated in the Velebit Mt. area (Kubus and Mali Alan passes). Today, these localities are 50 km apart. According to first appearance of foraminifer Mesoendothyra sp. and other characteristic fossil assemblage, studied succession is determined to be of Early Jurassic, or more precisely of Early Sinemurian age (Velić, 2007). Palaeokarst surfaces are recognized due to the data collected in the field (sedimentary structures) and during laboratory investigations (microfacies analysis from thin sections). In the field, these surfaces are of irregular relief, often show brecciated appearance due to the dissolution processes. Upper bedding planes show centimetre to decimetre sized irregular and circumgranular cracks filled with lighter coloured, often early diagenetic dolomitized limestone rock. Rarely these cracks contain yellow-brownish marly sediment. Cracks observed on upper bedding planes often penetrate as irregular fissures up to 15 cm downwards into the bed. The angle of dip of studied beds is 65°, but observed fissures pervade perpendicularly into the bed in relation to the upper bedding plane, pointing to their palaeokarst character and excluding the possibility that they are result of a recent karstification. Fissure walls are sharp and rough indicating that dissolution occurred in completely lithified rock. Fissure infill contains many angular millimetre to centimetre sized fragments of host rock. Dolomitized limestone found as fissure infill probably formed during marine transgression that followed exposure period, while yellow-brownish marly sediment, found on rare occasions, is probably washed away due to the same event. Limestones showing palaeokarstic features mostly have micritic texture (mudstones and wackestone) and under the microscope show small cracks and voids sometimes with preserved brownish coating on its wall indicating corrosion in subaerial conditions. According to the observed features, two surfaces studied in Lower Sinemurian carbonates are recognized as possible palaeokarst surfaces. Whereas these surfaces display micro-relief (penetrating few cm up to 15 cm into the rock) and no caves or larger dissolution features were observed, they can be attributed to a micro-karstification of lithified carbonates or formation of epikarst underneath a soil cover (Wright, 1992 ; Hillgärtner, 1998). These two palaeokarst surfaces are recorded at both studied sections and correlate very well, indicating contemporaneous subaerial exposure event that took place on the studied part of the ADCP and lasted long enough for palaeokarst structures to develop.

palaeokarst surfaces; carbonates; Early Jurassic; Velebit Mt.

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