engleski

End Permian Regression-Evidence from Meiobenthic Fossil Communities from the Velebit Mt. (SW Croatia)

In the area of the Middle Velebit Mt. (SW Croatia) a thick sequence of carbonate sediments were deposited during the Middle and Upper Permian, with continuous transition to the Lower Triassic. These, predominantly dolomitic sediments were deposited in shallow marine, supratidal to shallow subtidal environments. Changes in microfossil communities and petrographic features provide the evidence for several regressive episodes during the Upper Permian. These episodes become more common in the uppermost part of the sequence, towards the Triassic boundary. Fossil associations from these sediments were partly transported by turbide currents, waves or tempests, but many of them were deposited "in situ", or not far from their life positions. These "autochtonous" communities are of different biotic diversity, and reflect the environmental changes during the sea level oscillations. High biotic diversity communities occur within packstones-grainstones. Dominant fossils - gymnocodiacean and dasycladacean algae, together with fusulinids, small foraminifers and encrusting taxa, represent shallow subtidal environment with favourable life conditions. Moderately diverse communities in wackestones are typical for the beginning of the shallowing upward episodes. Number of specimens of calcareous algae decreases, and presence of large fusulinid foraminifera becomes an exception. On the contrary, globivalvulines, lagenids, miliolids and paleotextulariaceans take the advantage of environmental changes, and become dominant constituents of the communities. Cyanobacterial oncoids are also common in these sediments. Mudstones with earlandiaceans and scarce lagenids and/or other small foraminifera represent the extremely stressed phase of the regressive cycles. They appear at several horizons during the Upper Permian, but become dominant during the Upper Dorashamian. Fenestral fabrics, vadose cements, dessication craks and pyrite grains are also common in these sediments, reflecting the temporary emersions during the uppermost Permian. Early diagenetic stromatolitic dolomites have been found near the Midian-Dzhulfian boundary, also reflecting the regressive cycle, which is typical for this period in Paleotethys. Although the uppermost Permian deposits in the Velebit Mt. undoubtfully show the regressive tendencies, which can be well correlated with the regional drop of the sea level, regression in this area does not seem to be as prominent as some of the authors propose for the end of the Permian. Estimated maximum depth of the basin was probably not more than 20 meters during the phase of the optimal life conditions, and the possible periods of emersion were not long enough to enable the formation of some typical continental markers. End Permian anoxic event, which was reported from numerous localities in the area of former Paleotethys did not affect the extremely shallow environments, and thus could not much affect the investigated sediments. Intercalations of red shales, which succesively become more common in the uppermost part of the sequence, support the hypothesis of the continuous transition into the Lower Triassic.

Upper Permian; Regression; Platform Sediments; Fossil Communities; Western Part of the Central Paleotethys; Croatia

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