LATE ORDOVICIAN COOL WATER LIMESTONES IN THE CARNIC ALPS (AUSTRIA-ITALY)

Late Ordovician limestones in the Carnic Alps (Austria-Italy) include two facies. The massively-bedded Wolayer is composed primarily of medium to coarse sand-size grains that are crinozan skeletal elements. Bryozoan fragments comprise the second most abundant component. Brachiopods, trilobites and conodonts occur in the facies as well. This facies appears to have formed from crinozoan-bryozoan thickets that grew on a shallow marine shelf. Its coeval facies, the Uggwa, is composed of grains derived from the Wolayer facies that were swept downslope episodically as debris flows to accumulate in a moderate depth shelf environment. The Uggwa limestones are graded and include orthoceroid nautiloids as well as shell fragments of organisms that compose the Wolayer facies. Each flow is separated by thin, commonly tan to orange-colored remains of microbial mats. The basal portions of each flow locally disturbed the underlying mat such that small parts of the mats incorporate the coarsest skeletal grains. The stratigraphically higher flows are significantly thinner than the lower flows, are less well graded, and tops of certain of them display ripple marks. These Late Ordovician carbonates bear textural and other similarities to the cool water carbonates forming today at 47 to 49 degrees south latitude on the Snares Platform off the southern coast of the south island of New Zealand. Paleomagnetism studies of Carnic Alps Late Ordovician rocks indicate that they formed at about 45 – 50 degrees south latitude. Faunas and carbon isotope analyses of these Carnic Alps limestones indicate that they accumulated prior to onset of glaciation. The stratigraphically highest flows in the Uggwa facies appear to reflect increasing storm activity preceding glaciation. Glacio-eustatic sea level fall led to termination of Wolayer-Uggwa limestone deposition.