Erosional vs. accretionary shelf margins: the influence of margin type on deepwater sedimentation: an example from the Porcupine Basin, offshore western Ireland

Abstract

ABSTRACTA 1000 km2 three‐dimensional (3D) seismic data survey that extends out from the western margin of the Porcupine Basin, offshore western Ireland reveals the internal geometry and depositional history of a large Palaeogene (Palaeocene–Early Eocene) shelf‐margin. Two wells intersect the margin thereby constraining the depositional environments. The 34/19‐1 well (landward end) intersects slope, shelf, marginal marine to coastal plain facies. The 35/21‐1 well (basinward end) intersects seismically imaged shelf‐margin clinoforms where base of slope back up to coastal plain deposits (source‐to‐sink) are represented. The basin‐fill stratal architecture of the Palaeogene succession reveals sediment deposition under two end member, basin physiographic styles: (1) an erosional margin style and (2) an accretionary or progradational margin style. Uplift of the western margin of the basin is suggested as the major cause of the initially oversteepened shelf‐slope erosional profile. Key characteristics of an erosional margin include sediment bypass of the shelf, canyon formation, and the development of significant onlapping submarine fan deposits on the lower slope. Failure on the slope is also revealed by several mass–transport complexes (MTCs) that carve out major erosive features across the slope. Three‐dimensional seismic analysis illustrates variations in size, geometry and depositional trend and transport mechanisms of the MTCs. Confined, thick chaotic seismic facies, erosional basal scours and syn‐depositional thrusting (pressure ridges) at terminus as opposed to thin, high‐amplitude discontinuous facies with an unconfined lobate terminus are interpreted to indicate slump‐ and slide‐dominated vs. debris flow‐dominated MTCs, respectively. The erosional margin was transformed into an accretionary margin when the gradient of the shelf‐slope to basin‐floor profile was sufficiently lowered through the infilling and healing of the topographic lows by the onlapping submarine‐fan deposits. This shallowing of the basin allowed nearshore systems to prograde across the deepwater systems. The accretionary margin was characterised by a thick sediment prism composed of clinoforms both at the shoreface/delta (tens of metres) and shelf‐margin (hundreds of metres) scales. Shelf‐margin clinoforms, the focus of this study, are the fundamental regressive to transgressive building blocks (duration 10–100 kyr) of the stratigraphic succession and can be observed on a larger scale (∼1 Myr) through the migration and trajectory patterns of the shelf‐edge. Trajectory pathways in the accretionary margin are accretionary in a descending or ascending manner. The descending style was characterised by a shelf‐slope break that migrated seawards and obliquely downwards as a result of a relative sea‐level fall. The descending trajectory geometry is lobate along strike suggestive of a point source progradation. Internally, the descending trajectory consists of downward stepping, steeply dipping shelf‐margin clinoforms that display extensive slumping and deposition of sediment on the lower slope indicative of rapid deposition. Furthermore, basin‐floor fans and associated ‘feeder’ channels extend basinwards beyond toe of slope. The ascending trajectory reflects a shelf‐slope break that is interpreted to have migrated seawards during steady or rising relative sea level. The ascending trajectory geometry is associated with significant lateral sediment dispersal along the shelf‐edge, reflecting distributary systems that were less ‘fixed’ or a greater reworking and longshore drift of sediment. Accretion involving the ascending shelf‐edge trajectory characteristically lacked significant basin‐floor deposits. Variable ascending trajectories are recognised in this study, as read from the angle at which the shelf‐slope break migrates. Horizontal to high angle ascending trajectories correspond to dominantly progradational and dominantly aggradational shelf‐edge trajectories, respectively. The sequence stratigraphic analysis of the Porcupine deltaic complex reveals a long‐term relative sea‐level rise.