P. C. Richards

The geological evolution of the Falkland Islands continental shelf

Abstract The Falkland Islands are surrounded by four major sedimentary basins: the Falkland Plateau Basin to the east, the South Falkland Basin to the south, the Malvinas Basin to the west, and the North Falkland Basin to the north. The four main basins appear to have formed initially as Triassic through earliest Cretaceous extensional rifts associated with the break-up of Gondwana. A ?Valanginian end to rifting was followed by thermal sag. There is evidence of Cenozoic uplift in at least the North Falkland Basin, possibly coincident with Andean compression and the development of overthrusting along the plate boundary to the south of the islands resulting from opening of the Scotia Sea. There is no evidence from offshore seismic and gravity-magnetic data to support interpretations that the Falkland Islands have rotated clockwise through up to 180° during Gondwana separation. With the exception of the South Falkland Basin all the major basins probably underwent initially, more or less east-west extension, and had a similar orientation to adjacent South American and western southern African basins. The Falkland basins probably shared a similar geological history with the offshore southern African and South American basins.

Cretaceous dykes discovered in the Falkland Islands: implications for regional tectonics in the South Atlantic

New aeromagnetic data resolve the dykes of the Falkland Islands into three swarms. A hitherto unrecognized suite of north–south dykes is established as early Cretaceous by an Ar–Ar date of about 121 Ma. Swarms of NE–SW and east–west dykes are both early Jurassic: the former gives an Ar–Ar age of about 178 Ma, whereas the latter has been previously dated to about 190 Ma. The intrusion of the Cretaceous dykes marks the onset of oceanic crust generation in the South Atlantic and so restricts to mid-Jurassic the microplate rotation envisaged in most models for the Falklands break-out from Gondwana.

Evolution of deep-water climbing dunes in the Rockall Trough — Implications for overflow currents across the Wyville-Thomson Ridge in the (?)late Miocene

Abstract A (?)late Miocene dune field covering an area of 350 km2 has been identified at and below the sea bed in the northern part of the Rockall Trough, in 1080–1180 m of water. Seismic reflection records across the area reveal details of the internal bedform arrangement of the dunes. A three-fold subdivision of the dunes can be made, into basal climbing dunes, an intermediate transitional phase and an upper sinusoidal package. These three subdivisions are similar to the phases observed elsewhere in small-scale, sand-ripple drift systems evolving under conditions of decreasing energy. The basal climbing dunes show evidence of southwards migration, along the axis of the Rockall Trough. They were probably deposited primarily from traction currents formed in dense, cold, bottom water overflow moving from the Norwegian Sea, over the Wyville-Thomson Ridge and into the North Atlantic during the late Miocene. As this current energy decreased due to the increasing influence of the Wyville-Thomson Ridge as a barrier to overflow, the depositional style of the dune system changed. The decrease in current strength was accompanied by a decrease in migration rate and an increase in suspension load deposition, with a corresponding change in dune geometry. When the current strength decreased further as the ridge became a more permanent barrier to overflow, the dune system evolved into a sinusoidal phase, with deposition dominated by the fallout of suspension load material. The dune system is erosionally overlain in places by a younger, undated deposit that may represent a slump sheet derived from the eastern side of the Rockall Trough.

Rotation of the Falklands microplate reassessed after recognition of discrete Jurassic and Cretaceous dyke swarms

ABSTRACT 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°.

Upper Jurassic stratigraphy and sedimentary facies in the Central Outer Moray Firth Basin, North Sea

Abstract The Upper Jurassic of the Outer Moray Firth Basin can be divided into two main stratigraphic units — the Piper and Kimmeridge Clay Formations. In each of these formations five major sedimentary facies can be recognized. The Piper Formation, of late Oxfordian to early Kimmeridgian age, comprises very fine to coarse-grained sandstones and minor mudstone of clastic shelf to shoreline origin. Large scale upward-coarsening sequences are well developed in some areas, particularly in the reservoir sands of the Tartan oilfield, and are interpreted as regressive, possibly deltaic deposits. The unconformably overlying Kimmeridge Clay Formation ranges in age from late Oxfordian through Volgian to Ryazanian. The formation is predominantly argillaceous, but also contains locally thick accumulations of sandstone deposited by gravity flow processes. The Claymore Sandstone Member is proposed as a new name for these sandstones in the region of the Claymore oilfield, where they form the major reservoir. Sands of the Piper Formation were derived mainly from the south-west, although some input from the north may also have occurred. Deposition may have extended further eastwards than the present erosional limit of the sands. Thick sand sequences in the Kimmeridge Clay Formation are probably restricted to the margins of the Witch Ground Graben, where contemporaneous faulting occurred.

A new palaeogeographic reconstruction for the Middle Jurassic of the northern North Sea

An estuarine sequence equivalent in age to the Brent Group deltaic deposits further north is recognized in the central Viking Graben. It indicates a persistent marine connection southwards from the Boreal Ocean during deltaic accumulation, and suggests that the Brent delta did not prograde from the south, along the graben as previously suggested. Evidence from the UK north Viking Graben indicates the Brent delta was probably derived transversely rather than axially, as were the other Jurassic clastic systems infilling the graben.

The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity modelling

The lithospheric structure of the Falkland Plateau region has been modelled by making an initial estimate on the basis of local isostasy and then refining the geometries by inversion of gravity anomalies. The model predicts a crustal thickness beneath the Falkland Plateau Basin that is more than twice that previously inferred from seismic evidence. The preferred explanation is that the seismic survey detected high-velocity (?underplated) lower crust rather than upper mantle. This is compatible with the results of deep seismic experiments over the conjugate Filchner Block of Antarctica and also appears likely on the basis of the position of the Falkland Plateau Basin in relation to the Karoo–Ferrar magmatic province at the time of extension. Continental crust is inferred to be continuous beneath the northern part of the Falkland Plateau, as there are consistent magnetic and flexural anomalies associated with the continent–ocean boundary. Flexural modelling of the southern margin of the plateau indicates lateral variations in strength, with the strongest lithosphere beneath the Falkland Plateau Basin. This may indicate thick oceanic crust beneath the southern part of the basin or the effect of thinning relatively weak continental crust and replacing it with relatively strong underplating.

Evolution of Lower Jurassic coastal plain and fan delta sediments in the Beryl Embayment, North Sea

Coastal plain and fan delta sediments accumulated throughout the Sinemurian to late Pliensbachian in the Beryl Embayment area, North Sea. These sediments represent a marginal, clastic fringe along the southern extension of the Boreal Ocean into the northern North Sea. Two informal formations are recognized in these predominantly sandy deposits. The lower formation comprises interbedded sandstones, siltstones and coals of minor fluvial channel and associated coastal floodbasin marsh, lake and sheetflood origin. The overlying formation represents a fan delta system, possibly generated as a result of footwall uplift and associated erosion. This formation is dominated by probable channel sandstones, with marine-influenced debris flow deposits developed at a number of levels indicating an active, coastal fan setting. Fan sediments are capped by a marine sandstone, beds of which interfinger with the overlying offshore siltstones. Thickness variations are related to major normal faults, suggesting at least a limited fault control on the deposition and distribution of facies during a time conventionally regarded as tectonically quiescent in the northern North Sea. There is no clear evidence for a eustatic control on sedimentation, although deposition of post-fan delta shallow marine siltstones may have been principally controlled by eustatic sea level rise recording the acme of the early Jurassic expansion of Boreal waters into the Viking Graben.

Discussion on a new palaeogeographic reconstruction for the Middle Jurassic of the northern North Sea. Reply

L. M. Fält & R. J. Steel write: Richards et al. 1988 have offered a new palaeogeographic reconstruction for the Middle Jurassic of the northern North Sea. We consider their suggestions as an interesting contribution to a continuing debate about Jurassic sand-body development and evolution in the northern North Sea basin. Nevertheless, we suggest from the perspective of the Norwegian sector that they are mistaken in some of their main points and, at best, they have not documented any convincing case. Richards et al. 1988 reject the ‘traditional’ model for the Middle Jurassic deltaic system of the Brent Group, one of general northwards progradation with a broadly convex deltaic-front morphology. They suggest instead a more complex model with an axial estuary (without any trunk river system!) and deltaic flanks with sediment supplied primarily from the East Shetland Platform and from the Horda Platform (Richards et al. 1988, fig. 1). Our main problems with the suggestions of Richards et al. 1988 are as follows. (1) They have attributed, to the ‘traditional’ model, a greater degree of precision than it has ever had. The nation of a general northwards progradation of Brent delta system, as developed through the works of Eynon (1981), Johnson and Stewart (1985), Graue et al. 1987, Brown et al. 1987, Helland-Hansen et al. 1989 and Fält et al. 1990 has never been dependent on the presence of a southerly dome as principal source area. There has been an increasing awareness of the likelihood of sediment contribution from the