Silurian Sequence Stratigraphy in the Carnic Alps

Stratal sequences and their internal cycles which can be traced across sedimentary basins provide a powerful tool for stratigraphic correlation and paleogeographic reconstruction. In recent times the concept of stratigraphical sequences has gained wide acceptance in sedimentology and has been used to interpret the evolution of Phanerozoic sedimentary basins. Based mainly on seismic stratigraphy VAIL et al. (1977) identified depositional sequences bounded by unconformities or their correlative conformities on the order from 1-2 million years in duration. They attributed the origin of these sequences to global or eustatic changes in sea level. Sequence stratigraphy distinguishes first, second, third and fourth or higher order sequences. Higher order cycles which lasted less than 0.5 My are called parasequence cycles. The importance of sequence stratigraphy studies in the Silurian was emphasised by BRETT et al (1990) as they highlight the utility of detailed correlation of thin stratal packages and emphasise that it is only with the increased stratigraphic resolution provided by new physical correlation of sequences and subsequences that deductive models of eustacy and basin dynamics may be tested. Continued stratigraphic investigations are needed in order to produce a consistent global sequence-stratigraphic framework. JOHNSON (1996) states that eight highstands in sea-level occurred on a global basis during the Silurian, some of which may be linked to interglacial episodes, the cause of others however, remains unknown. It is stressed that only by interregional and intercontinental comparisons of numerous local relative sea-level curves placed within a refined chronostratigraphic framework can it be possible to gain an accurate picture of eustacy. Recognition of the paleohydrologic consequences of sea-level changes studied through sequence stratigraphy analysis has opened new avenues for research on many facets of sedimentary geology. The Carnic Alps represent one of the very few places in the world in which an almost continuous fossiliferous sequence of Palaeozoic age has been preserved. An almost complete Silurian succession of shallow to deep water fossiliferous marine sediments is preserved whose constituent units are biostratigraphically well constrained and hence, can be easily correlated. It is proposed to apply sequence stratigraphy concepts to the well-dated slope to basinal graptolitic shale and carbonate sequences of the Silurian from various sections in the Carnic Alps in order to - obtain a detailed sea-level curve for the Silurian of the Carnic Alps. - elaborate sequence stratigraphy packages across the Silurian depositional basin. - determine if sea-level change is controlled by local tectonism or eustatic changes. - correlate the sea-level changes and stratigraphic sequences determined on a global scale. A preliminary study by BRETT & SCHONLAUB of the Llandovery - lower Ludlow of the Cellon and Oberbuchach sections demonstrated that a sequence stratigraphy study is feasible in the Carnic Alps as they showed that a correlation may be possible with the sequences in NE America in the Appalachian Foreland Basin and those being studied for the Bristish Isles.

The main aim of this project was to apply sequence stratigraphy concepts to the well-dated slope carbonate sequences and basinal graptolitic shales of the Silurian from various sections in the Carnic Alps in order to correlate the sequence packages and sea-level changes determined with those established in other areas of Europe and North America. Unravelling local sea-level curves is essential for determining global eustasy. The sea-level curve for the Silurian of the Carnic Alps has been elaborated at a centimetre scale within a refined stratigraphical framework based on conodont, graptolite and chitinozoa biozonation. In particular, the minor and frequent sea level changes within the upper Llanodvery (11 transgressive episodes) and Pridoli (8 transgressive episodes) of the Silurian of the Carnic Alps are of special interest as these stratigraphic intervals are poorly preserved and not well studied in other Silurian localities. The preliminary interpretation of the field and microfacial analysis indicates sequence boundaries in the upper Llandovery (1), Wenlock (3), Ludlow (2) and Pridoli (2) which may be correlated with the sequences in NE America in the Appalachian Foreland Basin and those being studied for the British Isles JOHNSON (1996) states that eight highstands in sea-level occurred on a global basis during the Silurian. In some cases the temporal resolution obtained in the Carnic Alps for local sea level changes may be much more refined than those from other areas as the stratigraphic successions are chronologically well defined using biostratigraphy and K-bentonites.