Tectnostratigraphic Interactions in Rift Basins - Constraints from Forward Stratigraphic Modelling

Rift basin stratigraphy reflects a complex interplay between first-order tectonic, climate and base-level controls. Forward stratigraphic modelling (FSM) provides a means of simulating and better understanding aspects of this interaction. The study reported in this thesis used Sedsim from Stratamod to investigate: (1) how drainage is steered into fault-controlled depocentres and the extent to which autogenic cycles can develop despite constant external forcing; (2) the impact of contrasting extensional fault growth mechanisms on how and where sediment is delivered from the footwall to hanging wall basins by antecedent drainage, and (3) drainage patterns and stratigraphy occurring during salt-influenced rifting when the geometry of fault-controlled accommodation can be extensively modified by subsurface salt migration. The workflow incorporated a first set of simulations replicating physical experiments that included a relay ramp and a fault-controlled depocentre. Sedsim was configured to match the inputs and it faithfully reproduced drainage and depositional patterns down to episodic sediment storage and release that drove autogenic stratigraphic cycles in the hanging wall basin. Additional models were able to show the autogenic cycles persisted when the lateral tank walls were removed and when the simulation continued beyond the duration of the original physical experiments. FSM allowed upscaling of the physical model to natural rift basin dimensions and the inclusion of more realistic surface deformation around the faults; this confirmed the physical model reasonably represents reality, although varying the inputs showed autogenic cycles developed for only a subset of the possible combinations of input variables. Sedsim models were then produced for antecedent drainage interacting with a fault array developing according to either the Isolated or Constant Length fault growth models during rift initiation. Differences in the footprint and rate of hanging wall accommodation creation, the pattern of footwall uplift, and the relay ramp evolution explain significant contrasts in tectonstratigraphy between the two growth mechanisms. The drainage was less likely to be captured by the smaller initial depocentres in the Isolated fault growth examples, and when it was, sediment supply was able to balance space creation more effectively, with sediment delivered mainly via the developing relay ramps right through to breaching, and beyond relay breaching in cases where the discharge was high enough. Relay ramps were only utilised in the earliest stages of rifting associated with faults growing by the Constant Length model before drainage was diverted around the fault array tips to fill the interconnected hanging wall depocentres axially. Focussing of drainage around relays in the isolated growth model drove extensive footwall erosion close to the fault tips, whereas in the Constant Length case, where flow was sufficiently erosive, drainage could excavate valleys through the footwall at the point of maximum surface displacement. FSM was also used to address rift basin filling where subsurface salt migration initially compensates for sub-salt fault displacements before coupling of a fault across the salt layer creates reverse drag at the surface, but with accommodation still modified by upwelling salt. A conceptual model for salt-influenced rifting, honouring the dimensions of the Irish offshore rifts and drawing on a range of basins and physical experiments, was implemented in Sedsim and various sensitivities explored. Rapid changes in accommodation creation reflecting linkage of subsurface fault segments, migration of the early depocentre towards the coupled fault, and salt migration impacted facies distribution and stratigraphic development and underpin a new model for salt-influenced tectonostratigraphy that compliments the well-established half-graben models for rifting in the absence of subsurface salt-layers