Silurian sequence stratigraphy of the Carnic Alps, Austria
Abstract Sequence stratigraphy provides an alternative approach to correlation that may also serve as a predictive framework for the interpretation of sea-level and sedimentological change. The main aim of this study is to apply sequence stratigraphic concepts to the biostratigraphically well constrained shallow to moderately deep shelf carbonates and basinal graptolitic shale facies of the Silurian successions in the Carnic Alps of Austria in order to correlate the sequence packages and sea-level changes established there with those identified in other areas of North America and Europe. Documenting local sea-level curves is essential for determining global eustasy. The sea-level curve for the Silurian of the Carnic Alps has been elaborated within a refined stratigraphic framework for the Silurian based on conodont and graptolite biozonation [Melchin, M.J., Cooper, R.A., Sadler, P.M., 2004. The Silurian Period. In: Gradstein, F.M., Ogg, J.G., Smith, A.G. (Eds), A Geologic Time Scale. Cambridge University Press, Cambridge, pp. 188–201.]. In particular, the minor and frequent sea-level changes within 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 interpretation of the field and microfacies analysis indicates major sequence boundaries in the Llandovery (3), Wenlock (3), Ludlow (3) and Přidoli–Lochkovian (2) which may be correlated with coeval disconformities in the Appalachian Foreland Basin of eastern North America and/or in the Welsh Basin of the British Isles. The following times appear to represent relative sea-level highstand maxima in the Silurian of the Carnic Alps, as indicated by dark, graptolitic shales in deep shelf to basinal carbonate-dominated sections: a) early Aeronian, approximately the Coronograptus cyphus graptolite Zone/ Demirastrites triangulatus graptolite Zone), b) the early Telychian ( Oktavites spiralis graptolite Zone; Pterospathodus celloni conodont Superzone), c) late Telychian (lower Pterospathodus a. amorphognathoides conodont Zone); d) early to middle Sheinwoodian ( Kockelella ranuliformis to Ozarkodina sagitta rhenana conodont Zones; Monograptus riccartonensis graptolite Zones); e) mid-Wenlock ((?upper Kockelella walliseri conodont Zone; Cyrtograptus rigidus graptolite Zone); f) mid Homerian ( Ozarkodina bohemica conodont Zone; Gothograptus nassa graptolite Zone); g) near the Wenlock–Ludlow boundary ( Neodiversograptus nilssoni graptolite Zone); h) Polygnathoides siluricus conodont Zone; i) near the Ludlow–Přidoli boundary (upper Ozarkodina snajdri Interval Zone); j) lower Přidoli ( Monograptus parultimus graptolite Zone) and k) at the Silurian–Devonian boundary (earliest Lochkovian: Icriodus woschmidti woschmidti conodont Zone. Of these, the earlier (at least b–e) are well represented in the Appalachian Basin, as in Avalonian sections in Great Britain and in Baltica. Johnson [Johnson, M.E., 2006. Relationship of Silurian sea-level fluctuations to oceanic episodes and events. GFF 128, 115–121.] documented eight major highstands in global sea-level during the Silurian. The temporal resolution obtained in the Carnic Alps for local sea-level changes allows for refinement of the Telychian to Přidoli sea-level curve, as the stratigraphic successions are chronologically well-defined using both conodont and graptolite biostratigraphy and K-bentonite levels. Nearly all inferred deepenings in the Carnic Alps section, with the exception of that in the Polygnathoides siluricus conodont Zone, approximately match highstands recorded on the Silurian sea-level curves of Johnson [Johnson, M.E., 1996. Stable cratonic sequences and a standard for Silurian eustasy. In: Witzke, B.J., Ludvigson, G.A., Day, J. (Eds.), Paleozoic sequence stratigraphy – Views from the North American Craton. Geol. Soc. Am. Spec. Pap. vol. 306, Boulder, pp. 203–212., Johnson, M.E., 2006. Relationship of Silurian sea-level fluctuations to oceanic episodes and events. GFF 128, 115–121.]; however, the two Telychian highstands are not distinguished but rather are combined as highstand 4 on the Johnson curve, although they are recognized by Loydell [Loydell, D.K., 1998. Early Silurian sea-level changes. Geol. Mag. 135 (4), 447–471.], and the upper Kockelella walliseri conodont Zone deepening is not explicitly numbered by Johnson. These similarities suggest pervasive and probably eustatic events that are manifested in the Apulia Terrane [Cocks, L.R.M., Torsvik, T.H., 2002. Earth geography from 500 to 400 million years ago: a faunal and palaeomagnetic review. J. Geol. Soc. Lond. 159 (6), 631–644.], Laurentia and Avalonia during these time intervals.