LATE ORDOVICIAN CRINOZOAN-BRYOZOAN REEFS IN THE CARNIC ALPS

The Late Ordovician stratal succession in the Carnic Alps, an area that lay at about 50 degrees south latitude in the Ordovician, includes bioclastic carbonates derived from reefs. Two distinct facies have been distinguished in the Carnic Alps Late Ordovician succession: an inner shelf, Wolayer, facies and an outer shelf-upper slope, Uggwa, facies. Wolayer facies carbonates are massive, structureless, and accumulated as reefs which developed as thickets of crinozoans, primarily cystoids, and trepostome bryozoans. Cystoid calices are common at some sites. Conodonts, chitinozoans and trilobites are present in the reef rocks. These crinozoan-bryozoan reefs developed in relatively shallow-shelf, subtidal environments. Reef-derived detritus was swept downslope from time to time to accumulate in a sequence of graded debris flows in deep shelf-upper slope settings. These carbonate clast debris flows are a characteristic component of the Uggwa facies. Siliclastics subjacent to the Uggwa carbonates bear Rawtheyian shelly faunas. Blocks within cross-bedded sandstones and siltstones superjacent to Uggwa carbonates bear Hirnantia faunas. Persculptograptus persculptus and Mucronaspis occur in interbedded silty shales. Correlations based on faunas in strata subjacent and superjacent to the carbonates indicate that the reefs developed during the Normalograptus extraordinarius graptolite zone. Late Ordovician glaciation on Gondwana occurred during that zone. Siliclastics are absent from the reefs and reef-derived carbonates. If the source for the siliclastics was a portion of Gondwana, then reef development during glaciation suggests that the reef-building organisms were permitted to grow because the reef sites were not impacted by fluxes of siliclastics. The abundance of crinozoans and bryozoans suggests that small plankton were plentiful and nutrient availability high.