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

Systematic variations in sequences, their component units and their stacking patterns exist within rift basins. These variations can be related to displacement gradients associated with the large-scale (12-50 km) segmentation of normal fault zones, which control accommodation, sediment 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 the centre of normal fault segments may cancel out the effects of glacio-eustatic sea-level fall, so that accommodation development is normally 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 relative 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 development will be more pronounced during greenhouse times than icehouse times.