Petroleum Geoscience - Volume 15, Issue 3, 2009

Seismic interpretation of well-calibrated, overlapping, high-fidelity, 3D seismic data volumes has shown that many of the features advanced in support of a meteorite impact origin for the enigmatic ‘Silverpit Crater’ (SC) are, in fact, not only widespread in the Southern North Sea (SNS) region but also are of the same age. The mapping leads to a comprehensive alternative explanation for the formation of the SC in which Zechstein Supergroup evaporite mobility (halokinesis) controlled the development and evolution of several WNW–ESE-striking Cenozoic folds, including a buried salt withdrawal syncline in the axis of which lies the SC. The results demonstrate that the removal of structural support resulting from halokinetic outflow at depth promoted syn-sedimentary growth of the synclines and brittle deformation and collapse in the overburden. Significantly, it can now be shown that the WNW–ESE-striking synclines are exactly coincident with Palaeogene igneous dyke intrusions. The nature and shape of fault patterns that affect the Chalk Group all appear to be dictated by the form of their transecting intrusions, the associated post-depositional diagenetic alteration and structural failure resulting from the geometry of their respective underlying salt withdrawal syncline. In the case of the circular SC, the spectacular brittle ring-faulting is interpreted to result from its location above a zone of cylindrical salt evacuation, the flow from which was probably induced by the intrusion of a pipe-like igneous body. As well as explaining the genesis of the SC, the results also aid our understanding of Triassic prospectivity in the Silverpit area. Intrusion-induced fracturing provides a mechanism by which gas migrated up section from sub-salt (Carboniferous) source rocks through the thick Rotliegend Group Silverpit Claystone Formation, Zechstein Supergroup evaporite and Lower Triassic, Bunter Shale Formation seals. The prospective, gas-prone Bunter Sandstone Formation reservoirs lie in anticlinal closures above Zechstein Supergroup salt-cored pillows formed during the same phase of Early Cenozoic halokinesis that caused salt withdrawal beneath the SC.

Across most rifted margins, the extension measured from fault geometries is far less than that required to explain whole crustal and lithospheric thinning. This ‘extension discrepancy’ is commonly explained through crustal depth-dependent stretching (DDS – perhaps better termed depth-dependent thinning, DDT). However, several independent lines of evidence (velocity structure, rheological modelling, reconstructions, ODP drilling results) show that the amount of DDT required to explain the extension discrepancy cannot have occurred. Instead, it is suggested that as extension increases, complex geometries arise which are not completely interpreted, leading to a massive underestimation of the amount of extension. The implications are that pre-rift and early syn-rift reservoir and source rocks are likely to be widely scattered across deep margins. In the absence of massive crustal DDT, the deficit in syn-rift subsidence observed at some margins can be explained by thermal and dynamic uplift, igneous addition or mantle serpentinization during rifting. But it is also possible that syn-rift subsidence has been systematically underestimated if local water level was substantially below global sea-level, as indicated at some margins by the formation of thick evaporites at the end of rifting.

There have been many studies into the post-Palaeozoic exhumation history of the Irish Sea basin system, which is thought by some to be the locus of Cenozoic exhumation in the British Isles. Few studies, however, have sought to constrain the history of Mesozoic–Cenozoic vertical motions in Northern Ireland, where the geological record of this time period is comparatively complete. Post-Triassic rocks are missing from large parts of the Irish Sea, but sediments of Lower Jurassic, Upper Cretaceous and Oligocene age are found in Northern Ireland, in addition to the Paleocene flood basalts of the Antrim Lava Group. Here we present apatite fission-track analysis (AFTA) and sedimentary rock compaction data from the Larne No. 2 borehole, NE Northern Ireland, which penetrated a c. 2.9 km thick Permian–Triassic succession intruded by Palaeogene dykes and sills. We show that the preserved section was more deeply buried by up to 2.45 km of Upper Triassic–Lower Jurassic sediments that were removed during exhumation episodes beginning during the mid-Jurassic and early Cretaceous. Our results suggest limited early Palaeogene exhumation, which is consistent with the preservation of Upper Cretaceous Chalks beneath the Antrim Lava Group. They also indicate deeper burial of the preserved section by up to 1.3 km prior to late Cenozoic exhumation. This additional section could include a substantial thickness of Paleocene basalt, which provides a likely explanation for the anomalously low porosities of the Chalk in Northern Ireland.

It is common practice for geological studies based on interpretation of 3D seismic data to utilize two-way time datasets for convenience. Even depth-imaged or depth-converted 3D datasets have limitations if palaeogeometries are important in understanding the geological process under investigation. In this study I introduce the production of a back-stripped 3D seismic cube to allow examination of restored geometries and depths, which requires time-to-depth conversion and 3D back-stripping. For both processes, either standard functions or more complex models can be utilized as merited. To illustrate the method, I have produced a back-stripped 3D seismic cube across a saucer-shaped igneous intrusion. The sill's restored original geometry fits recent observational and experimental intrusion models, with some modification. Magma supply appears to have been to a deep off-centre point within the inner sill, 3000 m below palaeosurface, with a possible contribution to a more peripheral feeder system below the inclined sheet, and magma propagation to the outer sill being outwards and upwards. The value of a back-stripped 3D seismic cube is not limited to studies of igneous intrusions, but may have application in many areas of subsurface sedimentary basin studies, such as fluid migration path modelling and evolution of hydrocarbon trap geometries.

Basic, doleritic sills form a key component of subvolcanic complexes in the prospective parts of volcanic margins where they add considerably to the structural and thermal complexity of the subsurface geology. Whilst recent 3D seismic studies of sill complexes in volcanic margins have added considerable new insights into their development and significance, field studies at the sub- and trans-seismic scale are still required. The Theron Mountains in Antarctica offer perhaps the best exposures of a dolerite sill complex to be seen anywhere in the world and they create an unprecedented opportunity to understand the processes, on a variety of scales, that are important in individual sill emplacement. Large- and small-scale evidence indicates that forceful roof uplift is the fundamental emplacement mechanism. The sills contain an abundance of ‘bridge structures’, which are the remains of country rock that once lay between thin and closely separated precursor sills. Viewed on a variety of scales and inferred to be preserved at different stages in their development, these indicate that small proto-sills propagate ahead of the main sill body, exploiting thin weak horizons, such as coal and shale. The proto-sills expand laterally as they move downstream and come to overlap vertically along their lateral margins. Country rock bridges form between the sill overlaps and are rotated, bent and eventually broken as magma flow creates vertical inflation. Thus, a network of proto-sills exploiting a variety of weak horizontal horizons ahead of the main sill body unite, inflate and progressively allow the main sill body to move downstream. Although most thick sills are probably created by many thin proto-sills merging, some sills and some parts of sills are produced by the merging and inflation of single pairs of proto-sills and, in these examples, vertical inflation may run to more than 1000% of their original vertical separations.

Recently acquired aeromagnetic data for the Falkland Islands have shown that previous interpretations of the dolerite dyke swarms are inadequate. In particular, most of the dykes previously described from West Falkland as forming a ‘north–south’ swarm of Jurassic age are associated with a set of NE–SW linear magnetic anomalies that are entirely separate from another set of truly N–S anomalies. Very few dykes had been previously reported from East Falkland, but the aeromagnetic survey demonstrates clearly that dykes of both the NE–SW and the N–S swarms are present. Ar–Ar age dating of East Falkland dykes has confirmed the Jurassic age of the NE–SW dykes but has established an early Cretaceous age for the N–S dyke swarm. The Jurassic dykes are generally considered a part of the regional Karoo–Ferrar magmatism linked to the initial break-up of Gondwana. We consider the Cretaceous dykes to be associated with the later opening of the North Falklands Basin during the early development of the South Atlantic Ocean. The Jurassic and Cretaceous dykes must respectively pre-date and post-date the microplate rotation envisaged in most models for the Falklands break-out from Gondwana. The shapes of the aeromagnetic anomalies associated with dykes from each of the swarms support the hypothesis that the early Jurassic dykes have experienced a pre-Cretaceous, clockwise microplate rotation of about 120°.