OCEANOGRAPHIC CONDITIONS AS A TRIGGER FOR THE FORMATION OF AN EARLY MIOCENE (AQUITANIAN) KONSERVAT-LAGERSTAETTE IN THE CENTRAL PARATETHYS SEA

An exceptional Early Aquitanian (Miocene) Konservat-Lagerstätte with well-preserved vertebrate and invertebrate fossil assemblages has been studied in the North Alpine Foreland Basin of Upper Austria. The finely laminated sediments containing the abundant and well-preserved megafossils were deposited along the northern shelf of the Central Paratethys Sea. The most spectacular fossil is the giant sunfish Austromola (>3 m), the largest teleost fossil of the Cenozoic Era.

Micropaleontological (foraminifers, dinoflagellates) and geochemical (organic carbon, sulphur, carbonate content, biomarker, stable isotopes) proxies indicate intense upwelling around the area of deposition. In addition, episodically increased coastal runoff provided large amounts of nutrients stimulating primary productivity. All evidence suggests deposition within dysoxic–anoxic bottom waters of an oxygen minimum zone along the outer shelf and upper slope.

Fossil assemblages show specific planktic and nektic associations in distinct intervals of the section. Various mechanisms are discussed to explain their origin: (1) blooms of pteropods and calcareous nannoplankton reflect short-term peaks in primary productivity. Increased coastal runoff and/or intensified upwelling activity are considered as trigger mechanisms for providing the nutrients. (2) Allochthonous associations of the cephalopod Aturia with brown algae suggest a two-fold transport mechanism: shells of the offshoreliving cephalopods were transported post mortem to the coast by surface currents and/or wind currents. Episodic flooding events and storms mixed the accumulated shells with the algae and moved them offshore. The latter process also seems to apply to several pipefish accumulations observed in the section. (3) Multispecies vertebrate accumulations of fish and dolphins are considered parautochthonous as their habitat is in good agreement with the reconstructed paleoenvironment. (4) Benthic macrofauna is scarce and of low diversity. It mainly consists of bivalves adapted to dysoxic environments and is thus interpreted to be autochthonous.