CYCLIC CARBONATE SEDIMENTATION IN THE UPPER TRIASSIC DACHSTEIN LIMESTONE, AUSTRIA - THE ROLE OF PATTERNS OF SEDIMENT SUPPLY AND TECTONICS IN A PLATFORM-REEF-BASIN SYSTEM

Factors that controlled the deposition of (i) peritidal Lofer cycles o n a carbonate platform, and (ii) platform margin (reef-slope) deposits have been deduced at localities in the Austrian Alps (the Steinernes Meer and Hochkonig Massif). These locations are part of an entire Late Triassic platform-reef-basin system that is preserved with most origi nal stratigraphic relationships intact. Platform Lofer cycles shallow upward from a subtidal grainstone through a variety of intertidal dolo mitic mudstones to a supratidal weathering horizon (soil), Lofer cycle s show random, non-hierarchical stacking patterns, limited lateral con tinuity, varied progradation directions, complete shoaling (98% of cyc les), and very low stratigraphic completeness (only 1-20%). Exponentia l frequency distributions of cycle thickness suggest random, aperiodic cycle deposition, rather than regular deposition in response to regul ar eustatic sea-level oscillations. Sediments in the adjacent reef com plex: record storms and the lateral migration of sand shoals and strom atolite-capped banks, not sea-level fluctuations and intermittent suba erial exposure. On the basis of these observations, in contrast to man y previous interpretations, Lofer cycles are interpreted to be most ly autocycles formed within a tidal-Rat island system by lateral migrati on of wide, low, emergent banks separated by shallow subtidal areas. P reservation potential of individual cycles is thought to have been low ; reworking was almost certainly very important in this system. Extens ional regional tectonics in the Late Triassic exerted a long-term cont rol over the development of the tidal flat island system on the platfo rm top, and appears to have left a tectonic overprint in Lofer cycle s uccessions, Differential subsidence of individual platforms across the region is suggested by substantial regional thickness variations (120 0-3000 m) in the Norian/Rhaetian platform carbonates of the Northern C alcareous Alps. Two important tectonic deepening events in the Steiner nes Meer section almost caused platform drowning, and correlate with a lowermost Rhaetian transgression in the Western Tethys. Many other te ctonic events may have gone unrecorded on the platform, Within a 716 m thick measured succession of Lofer cycles, intervals of enhanced pale okarst development and stacked intertidal to supratidal beds are prese nt with 20-75 m vertical spacing. These platform units are interpreted to represent prolonged periods in which greater areas of the platform were occupied by intertidal to supratidal sediments. These units corr elate with thick units of debris flows on the reef slope. What may be ''tectonic highstands'' (the result of a vertical tectonic movement) a re recorded as packstone to grainstone deposition on the reef slope. A holistic model driven by aperiodic fault-controlled downdropping, res ulting in switching loci of sediment export patterns from a continuous ly operative subtidal carbonate factory (the reef complex) best explai ns vertical facies patterns in the platform and reef-slope successions . It is not a static sea-level model, although third-, fourth-, and fi fth-order eustasy is not required. The model could explain the sporadi c occurrence of shallowing-up cycles in the adjacent Kossen Basin. Lat e Triassic eustatic sea level fluctuations were ineffective in control ling sedimentation as a result of the processes described above.