Sequence Stratigraphic Controls on Stratal Architecture Within Active Rift Basins

John A. Howell, Stephen S. Flint

In conventional sequence stratigraphic analysis, relative sea-level changes are considered to occur across a topographically simple basin margin with constant subsidence in a strike direction.

Within rift basins significant variations exist in the rate of subsidence and accommodation creation in a depositional strike direction. These are comparable to those typically modelled in a depositional dip direction. In areas where slow subsidence is exceeded by the rate of eustatic sea level fall, (towards fault tips and relay zones), sub-aerial exposure occurs. In areas such as the centre of fault ellipses, where the rate of sea level fall is exceeded by the rate of subsidence, no sub-aerial exposure occurs and a type-2 sequence boundary is formed. The architecture of the type-2 sequence is strongly controlled by the mechanism of rapid accommodation creation. Fault derived accommodation sequences exhibit a significant asymmetry, occurring as large coarsening upward cycles whilst s quences where the rapid subsidence was generated by sediment loading of a ductile subcrop, (typical of halokinetic rim synclines) greater symmetry is exhibited.

The lowering of regional base level across a highly differentiated topography results in localized forced regressions on shallowly dipping fault blocks (local ramp profile) while deep water hangingwall troughs receive time-equivalent turbidite deposits (local shelf-slope break topography) fed by fluvial systems which are channelled through and incise into, transfer zones. Significant changes will also occur in shoreline orientations.

This model is illustrate with an example from the syn-rift, shallow marine deposits of the Upper Jurassic Humber Group from the United Kingdom South Central Graben.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995