Early and Middle Callovian ammonites from the Pieniny Klippen Belt (Western Carpathians) in hiatal successions: unique biostratigraphic evidence from sediment-filled fissure deposits
In the Pieniny Klippen Belt (West Carpathians), the upper Middle Jurassic (Callovian) sediments are mostly represented by deep-water radiolarites that were deposited in slope and basinal environments of the Kysuca Basin in the northwestern Tethys. The contemporaneous depositional conditions from shallow environments of the Czorsztyn Ridge are poorly known owing to the Upper Bathonian – Lower Oxfordian hiatus that is extensively developed in the Pieniny Klippen Belt. However, submarine fissures (neptunian dykes) related to extensional fracturing were filled with unlithified carbonate during this period and thus provide unique information about sediments and fossil assemblages that are otherwise missing in normal stratigraphic position. In hiatal successions, such sediments were not deposited on exposed surfaces owing to sediment bypassing and winnowing. The absence of borings and encrusters on the walls of the dykes indicates that such fissures were filled relatively rapidly with sediment, thus also enhancing preservation of fossil assemblages. In addition to brachiopods, bivalves, and gastropods, some dykes contain ammonites that represent biostratigraphic zones that were not recorded in the West Carpathians until now. They include the Lower Callovian Gracilis Zone and the Middle Callovian Coronatum Zone. Ammonite assemblages in the Callovian dykes are dominated by phylloceratids and lytoceratids. Ammonitina are almost exclusively represented by microconchs or juveniles. This size-selective preservation can reflect sorting of shells within fissures that eliminated large-sized microconchs and macroconchs, and/or higher susceptibility to damage of large-sized ammonites on the exposed sea-floor. Relatively rare kosmoceratid ammonites that are preserved in the Lower Callovian assemblages indicate that the Czorsztyn Ridge was situated within the area of the Subboreal influence.
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Document Type: Research Article
Publication date: April 1, 2009
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