COMPARATIVE SEQUENCE STRATIGRAPHY AND EUSTASY IN THE SILURIAN OF THE CARNIC ALPS, AUSTRIA, AND EASTERN NORTH AMERICA

The Carnic Alps of Austria expose thin, but relatively complete sections of Mid to Upper Silurian strata developed in mixed carbonate-siliciclastic facies that represent outer shelf and slope environments. Application of sequence stratigraphic concepts to the well-dated Silurian successions from various sections permits comparison of stratal patterns and inferred sea-level changes with those previously established in North America and Britain. Conodont and graptolite biostratigraphy provide a framework for making detailed temporal comparisons. The relative sea-level curve for the upper Llandovery-lower Ludlow interval of the Carnic Alps sections and the magnitude of variations in sea-level also compare favorably with those inferred by M. Johnson and colleagues for global sea-level changes during the Silurian. Detailed comparisons between the sequence boundaries and facies changes determined for the Carnic Alps and those of North America suggest similar and synchronous patterns of sedimentation in the Carnic Alps terrane on one side and Laurentia and Avalonia on the other during this time interval. Widespread thin carbonate intervals in all areas mark lowstand to early transgressive deposits and associated siliciclastic starvation and carbonate production. Iron and manganese enriched mineralized deposits appear to mark condensed intervals coincident with maximum flooding/starvation intervals. Dark shale intervals are noted at several levels throughout the Silurian and into the Early Devonian and appear to correspond to highstands represented in other parts of the world that were associated with widespread hypoxia. These similarities of pattern in disparate paleogeographic areas support a strong eustatic control on sedimentation patterns.