THE CARBONATE-CLASTIC CYCLES OF THE EAST-ALPINE RAIBL GROUP - RESULT OF 3RD-ORDER SEA-LEVEL FLUCTUATIONS IN THE CARNIAN

The Carnian Raibl group of the Eastern Alps consists of three 50-100 m thick, alternating carbonate and clastic third-order cycles, each of which can be traced for hundreds of kilometers. Tectono-eustatic sea-level fluctuations of a few tens of meters, spanning a few millions of years, are the driving mechanism of this cyclicity. The carbonate intervals represent restricted marginal marine, tidal and evaporitic environments. The clastic intervals represent inner and outer shelf facies, and are related to the fluviatile "Schilfsandstein" of the Germanic facies belt. In the Raibl group, contrary to other carbonate/clastic depositional settings, relative sea-level lowstands are dominated by carbonate production, and highstands are dominated by clastic deposition. Each of the three Raibl cycles corresponds to a type-2 sequence, containing shelf margin, transgressive and highstand systems tracts. During sea-level lowstands, deltaic point sources were near the shelf margin, allowing clastics to bypass the carbonate platform. This setting corresponds to a shelf margin system tract. Transgressive and highstand systems tracts developed during the subsequent sea-level rise, as deltaic clastics were reworked and redistributed over the carbonate platform, and the deltas retrograded to the inner shelf. The highstand systems tracts are capped by a type 2 sequence boundary, which is conformable in the study area. The systems tracts can be further subdivided into shallowing upward subcycles, caused by fourth-order sea-level fluctuations, believed to represent Milankovitch rhythms. The middle Raibl cycle is consistently thinner, and may represent a shorter termed, third-order sea-level fluctuation. Our data also corroborate a second-order transgressive trend for the Carnian.