THE INTERPRETATION OF CYCLIC SUCCESSIONS OF THE MIDDLE AND UPPER TRIASSIC OF THE NORTHERN AND SOUTHERN ALPS

Theories regarding the formation of sedimentary cycles in the 3rd, 4th and 5th order bands are reviewed with reference to the Middle and Upp er Triassic of the Northern Calcareous Alps (NCA) and Southern Alps. M ilankovitch, autocyclic and tectonic theories are discussed, together with an evaluation of concepts of chaotic sedimentation and a case exa mple from the NCA. Concerning eustasy, 3rd, 4th and 5th order sea-leve l fluctuations were probably a low-amplitude, low-rate phenomenon caus ed by fluctuations in the volume of mountain glaciers and ocean water during the Triassic. The Mid and Late Triassic was a non-glacial inter val in which polar regions may have been ice-free, so glacio-eustasy c an not be expected. Eustatic sea-level variations in the 3rd, 4th and 5th order bands seem to have left no useful imprint on cyclic successi ons in the region; whatever record there may be is inextricably mixed with two other signals (tectonic activity and autocycles). The review shows how sedimentation in the Triassic of the area was strongly influ enced by tectonic activity. This is as true for the Middle and Late Tr iassic of the NCA as it is for the Southern Alps. Tectonic activity ma y be responsible for large-scale cyclicity (4th to 3rd order scale). A lthough seismogenic structures have yet to be identified and described in carbonate successions of the Alps, candidates do exist. Slumped an d microfaulted layers in laminated sediments of the Seefeld Basin (Upp er Triassic, NCA) have been described as the products of fault movemen ts. The sedimentary record from the NCA and Southern Alps also leaves little doubt that autocyclic processes were important in all environme nts except perhaps the deep, sediment-starved basins. Most small-scale platform cycles (5th order scale) in the region can be related to aut ocyclic processes and, in shallow basinal successions, to events such as storms. Previous workers have not been consistent in their interpre tation of cyclic successions in the area, applying diverse theories to similar successions. So far, the Steinplatte-Hochkonig platform, with attached Kossen Basin, is the only example interpreted with reference to tectonics and autocyclicity; eustasy was probably not the most imp ortant factor in cycle generation in the Triassic of the NCA and South ern Alps. Such an approach could prove useful in future studies.