Abstract

Passively rising diapirs generate seabed relief, controlling deep-water sediment dispersal and the resultant stratigraphic architecture of synkinematic (growth) strata. Near-diapir deformation of these growth strata result in the deposition of unconformity-bounded packages known as halokinetic sequences (HS), which stack to form composite halokinetic sequences (CHS). Although the general geometry and halokinetic significance of CHS are well known, the way in which deep-water sediments are distributed and deformed in these packages is more poorly constrained.

We integrate 3-D seismic reflection, core, and petrophysical data from the Pierce Field, East Central Graben, North Sea, offshore UK, to examine the stratigraphic and structural development of deep-water CHS developed adjacent to passively rising diapirs. Seismic reflection data imaged four tapered CHS in Unit 6 (Ekofisk Formation), Unit 5 (Sele, Lista, and Maureen formations), Unit 4 (Horda and Balder formations), and Unit 3 (lower Lark Formation), which record protracted passive diapirism and, more specifically, periods when sediment-accumulation rate was rapid relative to diapir-rise rate.

At the sub-seismic scale, core data reveal fractures, veins, and stylolites in the Ekofisk Formation (i.e., Unit 6), reflecting drape-fold-induced brittle deformation of carbonate that was lithified during the very earliest stages of burial. In contrast, in the younger, turbidite-rich Forties Sandstone Member (Sele Formation; i.e., lower Unit 5), fractures are absent, whereas zones of bedding-parallel slip and debrite-dominated intervals are relatively common, the latter inferred to record reworking of sediment from the crest and most proximal flanks of the rising diapir. The absence of fractures within these clastic-dominated units, unlike their carbonate counterparts, when constrained by core and petrophysical data, reveals stratigraphic thinning and onlap of deep-water deposits towards the diapirs. This depositional geometry is consistent with observations from seismic reflection data, and indicates that salt diapirism generated syndepositional relief. More specifically, turbidite-rich deep-water lobes pinch out in bounding mudstones towards, and are inferred to be absent immediately adjacent to, the diapir flanks. Our study demonstrates the value of using an integrated, multi-scale dataset to understand near-diapir stratigraphy and deformation in deep-water sequences.