SEQUENCE DEVELOPMENT IN TECTONICALLY ACTIVE BASINS: IMPLICATIONS FOR THE PALEOZOIC OF EASTERN LAURENTIA

It is widely assumed that sequence boundaries develop more or less instantaneously as a result of "relative" falls in sea level that are almost universally attributed to eustasy. The role of tectonic phenomena is usually relegated to longterm basin development or to localized enhancement of stratal discordance. Neither view can be rigorously defended in tectonically active basins, particularly during warm intervals such as much of the pre-Carboniferous Paleozoic, for which there is only limited evidence for continental glaciation. Sequence boundaries in most cases develop over a finite interval of time during progradation, and their existence alone implies nothing about either amplitudes or rates of eustatic change, or even that eustasy was involved (in the absence of other data). Recent work in Cretaceous and Cenozoic foreland basins suggests a role in sequence development for a range of tectonic mechanisms, at both regional and local scales, and at timescales as short as 10-100 k.y. Regional mechanisms include the three-dimensional flexural response of the lithosphere to vertical loads, and potentially to variations in in-plane force. Local mechanisms relate to the growth of faults and folds (in piggyback basins), to the inversion or reactivation of existing structures beneath the foreland, and to the movement of buried salt. Fault and fold growth are particularly interesting phenomena for sequence analysis because they can lead to the development of diachronous unconformities. The key to all tectonic mechanisms is that they involve changes in patterns of subsidence and uplift that cannot be related sensibly to eustasy or even to relative sea-level change. These emerging ideas are pertinent to the interpretation of the Paleozoic foreland basins of eastern Laurentia. Tests require confidence in the interpretation and lateral tracing of surfaces at high resolution, in three dimensions, and at length scales of up to hundreds of kilometers. Whether or not that is possible, it is probably not sensible to assume that eustasy was the only player.