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Incised drainage systems can provide a record of the timing, duration and magnitude of ancient vertical crustal motion. The NE Atlantic underwent rapid uplift at c. 56 Ma, resulting in widespread incision and the formation of Paleogene unconformities across the Faroe–Shetland Basin. We used 3D seismic data to map a newly identified incised drainage system in the northern Faroe–Shetland Basin that formed during this period of uplift and compared it with similar incision surfaces in the region. The c. 30 km × 50 km erosional surface is a composite feature, comprising the Upper Thanetian Unconformity, which was rejuvenated and eroded by the younger Flett Unconformity, and records prolonged regional uplift during the late Thanetian–earliest Ypresian punctuated by a phase of subsidence. The drainage system was influenced by igneous intrusions, resulting in the diversion of channels around forced fold structures. Several preserved palaeoshorelines occur close to the basinward limit of the incision surface, recording progressive back-stepping of the shoreline and marine transgression during early Ypresian subsidence. The drainage system may have supplied ‘clean’ sand, eroded from the North Shetland Platform, to the Flett Sub-basin, with the potential to form high-quality reservoir units. Such sands may have been concentrated along shorelines coeval with the drainage system, but potentially were also delivered further out into the basin.

Seismic reflection data and field observations have revealed the presence of voluminous igneous sill complexes emplaced into organic-rich shale formations in sedimentary basins worldwide. Damage and structures associated with sills have major implications for fluid flow through basins. Constraining the distribution of these structures requires a good understanding of the sill emplacement mechanism. However, most mechanical models of sill emplacement assume elastic host behaviour, whereas shale is expected to deform inelastically. This contradiction calls for new field observations to better constrain sill emplacement mechanisms. In this paper, we report on detailed field observations of spectacularly exposed fingers and a sill emplaced in shale at Cuesta del Chihuido, in the Neuquén Basin, Argentina. Exceptional outcrop conditions allow detailed descriptions of both (1) the entire cross-section of the intrusions, and (2) the deformation structures accommodating intrusion propagation in the host rock. All intrusions exhibit irregular, blunt or rectangular tips. The structures accommodating the tip propagation are systematically compressional, including reverse faults, folding and imbricate thrust system. Our observations suggest that the studied intrusions have propagated by pushing the host rock ahead, as a viscous indenter. Our observations suggest that the viscous indenter model is probably a dominant mechanism of sill emplacement in shale., Facultad de Ciencias Naturales y Museo

Voluminous volcanic intrusive activity took place in the Voring and More basins at the Paleocene–Eocene boundary at about 56 Ma. This event caused thermal maturation of Cretaceous sedimentary rocks in the basins. We have estimated the resulting thermogenic gas generation potential from contact metamorphism using numerical simulations calibrated using borehole data. The borehole 6607/5-2 from the Utgard sill complex in the Voring Basin contains two c . 100 m thick sills and is used as a case study. We present both new and compiled data showing that (1) the bulk organic content is reduced towards the sill intrusions, (2) a c . 1 km thick stratigraphic interval is thermally affected, based on vitrinite reflectance data, (3) relative emplacement timing can affect the gas yield by up to 25%, and (4) some of the thermogenic methane is still present in the aureoles. The numerical model is calibrated using data from 11 wells. We estimate that the total gas generation potential for the two Utgard sills equals that of the Troll field ( c . 10 Gt CH 4 ), the largest producing gas field offshore Norway. We show that in the Voring and More basins, the total gas generation potential is up to 1500 Gt CH 4 ( c . 1100 Gt C), even from the relatively organic-poor Cretaceous source rocks with c . 1 wt% organic carbon, with implications for the carbon cycle at the Paleocene–Eocene boundary.

The Karoo large igneous province, formed at c . 183 Ma, is characterized by the presence of voluminous basaltic intrusive complexes within the Karoo Basin, extrusive lava sequences and hydrothermal vent complexes. These last are pipe-like structures, up to several hundred metres in diameter, piercing the horizontally stratified sediments of the basin. Detailed mapping of two sediment-dominated hydrothermal vent complexes shows that they are composed of sediment breccias and sandstone. The breccias cut and intrude tilted host rocks, and are composed of mudstone and sandstone fragments with rare dolerite boulders. Sandstone clasts in the breccias are locally cemented by zeolite, which represents the only hydrothermal mineral in the vent complexes. Our data document that the hydrothermal vent complexes were formed by one or a few phreatic events, leading to the collapse of the surrounding sedimentary strata. We propose a model in which hydrothermal vent complexes originate in contact metamorphic aureoles around sill intrusions. Heating and expansion of host rock pore fluids resulted in rapid pore pressure build-up and phreatic eruptions. The hydrothermal vent complexes represent conduits for gases and fluids produced in contact metamorphic aureoles, slightly predating the onset of the main phase of flood volcanism.

Regional analysis of new 2D and 3D multichannel seismic data has improved interpretation of the crustal configuration and structural style along the Norwegian margin. Five domains with different structural styles and evolutions are defined along the outer Voring Basin: (1) the Nyk High–Naglfar Dome; (2) the north Gjallar Ridge; (3) the Gleipne saddle; (4) the south Gjallar Ridge; (5) the Ran ridge. Early Campanian–Early Paleocene and Early to mid-Cretaceous extensional events are evidenced. Timing of deformation and structural styles observed along each segment reflect a lateral variation of the rifted system, probably affected by magma-tectonic processes. Correlation with the deep structures of the outer Voring Basin shows that the shallow structure in that basin is directly controlled by a deep-seated, strong, high-amplitude reflection (the T reflection), marking the top of a high-velocity body ( V p >7 km s −1 ). The relation between the lower-crustal architecture and the subsurface basin structures has implications for the margin evolution and for the nature of the high-velocity body.

Voluminous volcanism characterized Early Tertiary continental break-up on the mid-Norwegian continental margin. The distribution of the associated extrusive rocks derived from seismic volcanostratigraphy and potential field data interpretation allows us to divide the Møre, Vøring and Lofoten–VesterÅlen margins into five segments. The central Møre Margin and the northern Vøring Margin show combinations of volcanic seismic facies units that are characteristic for typical rifted volcanic margins. The Lofoten–VesterÅlen Margin, the southern Vøring Margin and the area near the Jan Mayen Fracture Zone show volcanic seismic facies units that are related to small-volume, submarine volcanism. The distribution of subaerial and submarine deposits indicates variations of subsidence along the margin. Vertical movements on the mid-Norwegian margin were primarily controlled by the amount of magmatic crustal thickening, because both the amount of dynamic uplift by the Icelandic mantle plume and the amount of subsidence due to crustal stretching were fairly constant along the margin. Thus, subaerial deposits indicate a large amount of magmatic crustal thickening and an associated reduction in isostatic subsidence, whereas submarine deposits indicate little magmatic thickening and earlier subsidence. From the distribution of volcanic seismic facies units we infer two main reasons for the different amounts of crustal thickening: (1) a general northward decrease of magmatism due to increasing distance from the hot spot and (2) subdued volcanism near the Jan Mayen Fracture Zone as a result of lateral lithospheric heat transport and cooling of the magmatic source region. Furthermore, we interpret small lateral variations in the distribution of volcanic seismic facies units, such as two sets of Inner Seaward Dipping Reflectors on the central Vøring Margin, as indications of crustal fragmentation.

B. E. Lock & J. A. Robey write: We were interested to read the paper by Svensen et al . (2006) on South African hydrothermal vents within the Karoo Basin, particularly concerning two vent complexes known as Witkop II and Witkop III. One of us (B.E.L.) had barely initiated a study of the Stormberg diatremes when he left South Africa to accept a faculty position in Louisiana in 1977, and the project was not continued. However, the Svensen paper reminded us of features we had seen in the field at the Mackay's Kop diatreme, near Jamestown, in the mid-1970s. This diatreme is in the same general area as those described by Svensen et al . but had not been visited by the latter because of their difficulty in locating the absentee landowner (G. Marsh, pers. comm.). In particular, we note the description of ‘a partly undulating border zone. . . typical for the pipes at Witkop III’ complex, which the authors interpreted as a ‘deformation zone’. Based on figures in the original paper, particularly their figure 7c, a similar zone surrounds the Mackay's Kop diatreme, but our interpretation differs significantly from that of Svensen et al ., and bears on the mechanisms involved in diatreme formation and evolution. The pipe at Mackay's Kop has a central fill consisting of a variety of lithofacies: massive sandstone, heterolithic breccias with sandstone and mudstone clasts, and small pebble conglomerates consisting of well-rounded sandstone clasts (Fig. 1). A narrow marginal zone (15–40 cm wide) of sandstone is the equivalent, we believe, of the ‘deformation zone’ of Svensen et al . …