The interpretation of cyclic successions of the Middle and Upper Triassic of the Northern and Southern Alps

Abstract

Theories regarding the formation of sedimentary cycles in the 3rd, 4th and 5th order bands are reviewed with reference to the Middle and Upper Triassic of the Northern Calcareous Alps (NCA) and Southern Alps. Milankovitch, autocyclic and tectonic theories are discussed, together with an evaluation of concepts of chaotic sedimentation and a case example from the NCA. Concerning eustasy, 3rd, 4th and 5th order sea-level fluctuations were probably a low-amplitude, low-rate phenomenon caused by fluctuations in the volume of mountain glaciers and ocean water during the Triassic. The Mid and Late Triassic was a non-glacial interval in which polar regions may have been ice-free, so glacio-eustasy can not be expected. Eustatic sea-level variations in the 3rd, 4th and 5th order bands seem to have left no useful imprint on cyclic successions 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 influenced by tectonic activity. This is as true for the Middle and Late Triassic of the NCA as it is for the Southern Alps. Tectonic activity may be responsible for large-scale cyclicity (4th to 3rd order scale). Although seismogenic structures have yet to be identified and described in carbonate successions of the Alps, candidates do exist. Slumped and microfaulted layers in laminated sediments of the Seefeld Basin (Upper Triassic, NCA) have been described as the products of fault movements. The sedimentary record from the NCA and Southern Alps also leaves little doubt that autocyclic processes were important in all environments except perhaps the deep, sediment-starved basins. Most small-scale platform cycles (5th order scale) in the region can be related to autocyclic processes and, in shallow basinal successions, to events such as storms. Previous workers have not been consistent in their interpretation of cyclic successions in the area, applying diverse theories to similar successions. So far, the Steinplatte-Hochkönig platform, with attached Kössen Basin, is the only example interpreted with reference to tectonics and autocyclicity; eustasy was probably not the most important factor in cycle generation in the Triassic of the NCA and Southern Alps. Such an approach could prove useful in future studies.