Sedimentary models for extensional tilt-block/half-graben basins

Summary Extensional tectonism produces characteristic half-graben/tilt-block systems whose facies mosaics are influenced by tectonically induced slopes resulting from hanging wall downtilting and footwall uplift. The characteristic asymmetrical subsidence vectors that therefore develop across the graben also exert a fundamental control upon facies distributions. A number of predictive tectono-sedimentary facies models are presented in which these various influences are explored. Alluvial fans and cones react to tilting by becoming segmented, those in the hanging wall showing down-dip hanging wall off-lap and those sourced in the footwall showing progradation from the apex. Lake and coastal waters react instantly to tilting, causing transgression and seiche-induced erosion. Axial through-flowing river channels and delta lobes tend to migrate or avulse towards the axis of maximum subsidence but may be constrained by the toes of footwall-sourced fans. Peat accumulation or soil development are accentuated up the hanging wall dip slope away from the locus of maximum deposition. In coastal areas, fan deltas sourced in the footwall pass offshore into small submarine fans whilst axial fans issue from delta fronts where individual fan lobes may migrate under fault control. In carbonate provinces the footwall scarp may become a bypass margin whilst the hanging wall dip slope may undergo a ramp-to-rimmed shelf evolution with time.