SEQUENCE STRATIGRAPHY IN ACTIVE EXTENSIONAL BASINS - IMPLICATIONS FORTHE INTERPRETATION OF ANCIENT BASIN-FILLS

Systematic variations in sequences, their component units and their st acking patterns exist within rift basins. These variations can be rela ted to displacement gradients associated with the large-scale (12-50 k m) segmentation of normal fault zones, which control accommodation, se diment supply and basin physiography. Sediment supply is also strongly influenced by the nature of pre-rift drainage networks and variations in bedrock lithology. High rates of hangingwall subsidence close to t he centre of normal fault segments may cancel out the effects of glaci o-eustatic sea-level fall, so that accommodation development is normal ly characterized by the continual addition of new space. The resulting sequences lack type 1 sequence boundaries and lowstand systems tracts , and stack into aggradational sequence sets. The adjacent footwall is subject to uplift, which may lead to subaerial exposure and incision, generating a composite type 1 sequence boundary. Away from the fault zone and near segment boundaries, slip rates are much lower and hence relative sea-level change is dominated by eustasy. Here, falls in rela tive sea level are important, resulting in type 1 sequences. Sequence stacking patterns reflect not only eustasy and local fault-controlled subsidence, but also the interaction of these with adjacent uplifting footwalls and/or regional uplift. Tectonic influence on sequence devel opment will be more pronounced during greenhouse times than icehouse t imes.