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