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

Abstract

ABSTRACT Factors that controlled the deposition of (i) peritidal Lofer cycles on 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 original stratigraphic relationships intact. Platform Lofer cycles shallow upward from a subtidal grainstone through a variety of intertidal dolomitic mudstones to a supratidal weathering horizon (soil). Lofer cycles show random, non-hierarchical stacking patterns, limited lateral continuity, varied progradation directions, complete shoaling (98% of cycles), and very low stratigraphic completeness (only 1-20%). Exponential frequency distributions of cycle thickness suggest random, aperiodic cycle deposition, rather than regular deposition in response to regular eustatic sea-level oscillations. Sediments in the adjacent reef complex record storms and the lateral migration of sand shoals and stromatolite-capped banks, not sea-level fluctuations and intermittent subaerial exposure. On the basis of these observations, in contrast to many previous interpretations, Lofer cycles are interpreted to be mostly autocycles formed within a tidal-flat island system by lateral migration of wide, low, emergent banks separated by shallow subtidal areas. Preservation potential of individual cycles is thought to have been low; reworking was almost certainly very important in this system. Extensional regional tectonics in the Late Triassic exerted a long-term control over the development of the tidal-flat island system on the platform top, and appears to have left a overprint in Lofer cycle successions. Differential subsidence of individual platforms across the region is suggested by substantial regional thickness variations (1200-3000 m) in the Norian/Rhaetian platform carbonates of the Northern Calcareous Alps. Two important deepening events in the Steinernes Meer section almost caused platform drowning, and correlate with a lowermost Rhaetian transgression in the Western Tethys. Many other events may have gone unrecorded on the platform. Within a 716 m thick measured succession of Lofer cycles, intervals of enhanced paleokarst development nd stacked intertidal to supratidal beds are present 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 correlate with thick units of debris flows on the reef slope. What may be tectonic highstands (the result of a vertical movement) are recorded as packstone to grainstone deposition on the reef slope. A holistic model driven by aperiodic fault-controlled downdropping, resulting in switching loci of sediment export patterns from a continuously operative subtidal carbonate factory (the reef complex) best explains vertical facies patterns in the platform and reef-slope successions. It is not a static sea-level model, although thi d-, fourth-, and fifth-order eustasy is not required. The model could explain the sporadic occurrence of shallowing-up cycles in the adjacent Kossen Basin. Late Triassic eustatic sea-level fluctuations were ineffective in controlling sedimentation as a result of the processes described above.