Abstract Three-dimensional seismic reflection data provide a means to assess the impact of injection on parent sands, and to quantify the character of the resulting injectite networks. The morphology of a series of large injectite structures hosted in the Paleocene Lower Lista Formation were mapped using broadband 3D seismic data from the North Sea to investigate their relationship with parent sands. Fourteen bowl-shaped structures were identified within the Lista Formation in the study area (60–85 m in height, and 200–900 m in width). Sand is absent (below resolution) below these large-scale bowls, suggesting that the parent sand is the underlying Maureen Formation and that sand ‘welds’ formed, rather than sand-prone channelized deposits within the Lista Formation. Identification of injectite networks can be ambiguous, which impacts geological model development. Observations from exhumed systems and core offer high-resolution insights into the complexity of injectite networks. To advance our understanding of this scale gap, we argue for injectites being scale invariant in their shape and grain size. This permits the application of outcrop-scale knowledge to seismic-scale interpretation. The demonstrable depletion of parent sands, and their scale invariance, can be applied to basin-fills worldwide to reduce uncertainties of the impact of sand injectites on hydrocarbon reservoirs.

Abstract Many mass transport complexes (MTCs) contain up to kilometre-scale (mega)clasts encased in a debritic matrix. Although many megaclasts are sourced from the headwall areas, the irregular basal shear surfaces of many MTCs indicate that megaclast entrainment during the passage of flows into the deeper basin is also common. However, the mechanisms responsible for the entrainment of large blocks of substrate, and their influence on the longitudinal behaviour of the associated flows, have not been widely considered. We present examples of megaclasts from exhumed MTCs (the Neuquén Basin, Argentina and the Karoo Basin, South Africa) and MTCs imaged in three-dimensional seismic reflection data (Magdalena Fan, offshore Colombia and Santos Basin, offshore Brazil) to investigate these process–product interactions. We show that highly sheared basal surfaces are well developed in distal locations, sometimes extending beyond their associated deposit. This points to deformation and weakening of the substrate ahead of the flow, suggesting that preconditioning of the substrate by distributed shear ahead of, and to the side of, a mass flow could result in the entrainment of large fragments. An improved understanding of the interactions between flow evolution, seabed topography, and the entrainment and abrasion of megaclasts will help to refine estimates of run-out distances, and therefore the geohazard potential of submarine landslides.

Abstract Late Cretaceous-Eocene deep-water deposits of the Guipúzcoa basin are well exposed along the Basque coastline. Early Eocene deposits provide an opportunity to analyze the facies distribution and architecture of a thick-bedded, sand-rich, turbidite system, a commonly identified seismic facies. The Guipúzcoa basin is a narrow east-southeast-to-west-northwest-trending foreland basin that formed during the Cretaceous and Eocene. Sediment was supplied to deep-water settings via the Aquitanian basin in the northeast and the South Pyrenean foreland basin in the southeast. In the late Cretaceous and Paleocene, the shelf area was largely covered with carbonate platforms, and turbidity currents transported carbonate-rich sediment into the deeper parts of the basin. The deep-water sediment supply changed through early Eocene times to a siliciclastic-rich system, and the stratigraphy comprises a series of prograding submarine-fan systems. The deposits change upward from thin beds to sheets to channels. Three sedimentary cycles have been identified that successively increase in their proportion of siliciclastic material, average grain size, and bed thickness. The uppermost cycle, the only cycle dealt with in this paper, is the Higuer-Getaria Formation, which is exposed between Hondarribia and San Sebastian ( Figure 1 ). Seven higher order cycles have been identified and correlated more than 15 km (9 mi) downdip ( Figure 5 ). In the Higuer-Getaria Formation, 20–80-m (66–262-ft)-thick intervals of thick-bedded, immature, coarse-grained sandstones with widespread amalgamated and dewatered contacts, are separated by 15–40-m (49–131-ft) - thick intervals of thin-bedded turbidites that preserve a wide diversity of ichnofauna. Updip, bottom currents are interpreted to have reworked the thin-bedded turbidite bed-tops