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Triassic-Jurassic Boundary Biotic Event in the Fatric Unit (West Carpathians, Slovakia)

A. Tomasovych
Wurzburg Universitat, Institut fur Palaontologie,Wurzburg, Germany

During the study of the uppermost Triassic (Rhaetian) sediments of the Fatra Formation from the Fatric Unit (Velka Fatra Mts., Central West Carpathians, Slovakia), new paleoecologic, taphonomic, and sedimentologic data related to the Triassic-Jurassic boundary event have been obtained.

The Fatra Formation is formed by predominantly carbonate peritidal-shallow subtidal lagoonal sediments with high facies vari ability, with abundant benthic macrofauna and very low terrigenous admixture. On the boundary with the Jurassic, it has been abruptly replaced by siltstones, claystones, and sandy limestones of the Kopienec Formation (Hettangian-lower Sinemurian), with impoverished macrofauna lacking stenohaline elements.

Six basic benthic assemblages have been recognized in the preliminary paleoecological analysis of the Fatra Formation in the Velka Fatra Mountains, differing in taxonomic composition, diversity and relative abundance of individual taxa. They are dominated by epifaunal suspension feeders, with typical terebratulid brachiopod Rhaetina gregaria, bivalves Rhaetavicula contorta, Atreta intustriata, and Placunopsis alpina, and corals of the genus Retiophyllia. Temporal pattern of the distribution of benthic assemblages is characterized by both reorganization and attrition types of the community replacement.

Benthic organisms are preserved in several types of taphofacies, often forming shell concentrations. The formation of shell concentrations resulted predominantly from the variation in three basic processes— the rate of sedimentation, the water energy related mainly to storm reworking of varying intensity and frequency, and the rate of change of environmental factors affecting shell-concentration processes. Relatively high shell productivity and their inherent preservational potential were prerequisite for the origin of shell-rich sediment.

AAPG Search and Discovery Article #90902©2001 AAPG Foundation Grants-in-Aid