IV. RESULTADOS Y DISCUSIÓN
4.4 PRUEBA T PARA DOS MUESTRAS INDEPENDIENTES RNA “NEH”
During the Triassic, the Northern North Sea was dominated by north-south faulting which formed deep and well defined grabens (Figure 3.2). Faulting commenced in the Permian, and rapid subsidence occurred throughout the Triassic (Ziegler, 1982; Doré and Gage,
1987; Badley et a l., 1988; Fisher and Mudge, 1990). It is assumed that in the basins
fault-bounded margins, with finer-grained fluvial or lacustrine sediments in the ‘lows’ (Fisher and Mudge, 1990).
/
A B S E N T \ f M ^ ; I S O P A C H M A P O N T H E T R I A S S I C IN T H E N O R T H S E A R E G I O NFigure 3.17: A map of Triassic isopachs and a possible compressional tectonic model. Contour interval = 250m. (After Olsen, 1987). Note the NNW-SSE orientation of maximum horizontal compression in the Southern North Sea Basin.
The Northern North Sea Basins are thought to have been separated from the Central North Sea Basins by a structural block in the vicinity of the triple junction between the Witch Ground, Viking and Central Grabens (Fisher and Mudge, 1990). The Triassic sequences in this area has been divided into the Cormorant Formation south of the latitude 60°N, and the Hegre Group to the north. Both groups are overlain by the Statfjord Formation (Fisher and Mudge, 1990).
The Cormorant Formation is mainly composed of argillaceous sandstones with some siltstones, shales, conglomerates, and coarser-grained sandstones deposited in a fluvio-
lacustrine environment (Fisher and Mudge, 1990). Correlation o f this sequence between separate fault blocks has proven to be very difficult due to the unfossiliferous nature o f the rocks and the use o f this term has been proposed to be restricted to attenuated sequences on structural highs (Fisher and Mudge, 1990).
The Hegre Group, comprising the Teist, Lomvi, and Lunde Formations, is used as a broad term to cover the Triassic sediments in this area. The rocks comprise interbedded sandstones, shales and claystones resulting from deposition in fluvial, aeolian and lacustrine environments (Fisher and Mudge, 1990). Detailed correlation between the Hegre Group and the Cormorant Formation is not feasible from the current published data (Fisher and Mudge, 1990).
The Statfjord Formation overlies both the Cormorant Formation and the Hegre Group. Its base is defined by an coarsening-upward succession of grey, green, and red shales interbedded with thin siltstones, sandstones and dolomites indicative o f a slow marine transgression (Fisher and Mudge, 1990). It is thought that the environment o f deposition o f the formation was from coastal alluvial fans or fan deltas which show evidence of repeated progradation onto a coastal plain (Roe and Steel, 1985).
3 .2 .6 Jurassic (208Ma to 145Ma)
From an economic standpoint, the Jurassic is the most important period in the history o f the North Sea. Jurassic sandstones provide reservoir rocks for many North Sea oil fields and, more importantly, rocks from this stratigraphie system provide the main source rocks in the central and northern North Sea (Brown, 1990). A generalised outline o f the Jurassic lithostratigraphy within the North Sea basin is illustrated in (Figure 3.18).
During the Jurassic the North Sea was flanked by the large, active rift systems within the Arctic-North Atlantic and Tethyan Oceans (Ziegler, 1981). Jurassic strata occur for the most part in fault-bounded basins related to the crustal extension initiated during the Permian (Brown, 1990). Fault-controlled differential subsidence, contemporaneous with sedimentation, had a marked influence on stratigraphie thickness and facies distribution throughout the area (Brown, 1990).
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East Shetland B#wn ^ ; : : : :3Figure 3.18: A generalised outline of the Jurassic lithostratigraphy within the North Sea Basin. (After Brown, 1990).
Many o f the recognised faults appear to follow older structural trends, including reactivated Precambrian, Caledonian and Variscan lineaments (Selley, 1975; Eynon, 1981; Johnson and Dingwall, 1981). However, Janet Watson (1985) pointed out the discordance o f the main graben system with the NE-SW grain o f the Caledonian orogenic belt and with major late Palaeozoic structures such as the Mid-North Sea-Ringkobing-Fyn High, and suggests that this raises questions about the importance o f inherited structural trends in determining the orientation o f major rift basins. Recent evidence from deep seismic reflection profiling suggests that many extensional structures within the basin must have been newly formed during the M esozoic, and evidence o f reactivation o f Palaeozoic compressional faults is limited (Klemperer and Hurich, 1990). However, as discussed in section 3 .2 .2 , the evidence for a Palaeozoic shear zone (Figure 3.6) which is thought to have predestined the development o f the Viking-Central Graben rift system suggests that at least some o f the major faults in the North Sea Basin have been active since the Palaeozoic and reactivation o f these lineaments has been important in the structural evolution o f the Basin.
The earliest interpretations o f the origin o f the main graben system envisaged plume generated crustal uplift centred on the mid-Jurassic volcanic pile at the intersection o f the Viking Graben, Central Graben and the Witch Ground Graben (Brown, 1990). Further interpretations emphasised the presence o f anomalously thin continental crust under much o f the main grabens (Christie and Sclater, 1980) which relied on a hypothesis o f regional lithospheric stretching (after McKenzie, 1978). Wood and Barton (1983) associated the extrusion o f basalts during the mid-Jurassic with the most significant and rapid phase of extension. This model envisaged stretching and fault-controlled subsidence throughout the mid- to late-Jurassic and into the Early Cretaceous, when, as extension in the North Sea area ceased, thermally induced subsidence became the dominant influence on basin formation (Brown, 1990).
In localised areas, detailed analyses o f seismic and borehole data (McQuillan et al.y 1982;
Badley et al.y 1984; 1988) have refined or suggested departures from this regional model
and incorporate initial Permo-Triassic rifting in the Viking Graben and Moray Firth followed by a second phase during the Jurassic.
After tectonism, the second major influence on Jurassic sedimentation was a eustatic change in sea-level (Brown, 1990). The Jurassic period is thought to have been characterised by a gradual rise in sea level until the Kimmeridgian (Brown, 1990) followed by a net fall into
the Lower Cretaceous (Rawson and Riley, 1982). However, the interplay between subsidence, sedimentation and eustatic sea-level change are difficult to disentangle.
The Jurassic sequences within the Central and Southern North Sea Basins have been analysed by Olsen (1987) who proposed that deposition in basins controlled by N-S and NW-SE trending faults are parallel to the strike-slip shear directions that a dominant compressional stress oriented NW-SE would give (Figure 3.19). This orientation is identical to that determined by Olsen (1987) for the Late Triassic successions in this area (Figure 3.17). a b s e n t ) A BS ENT A B SEN T I S O P A C H MAP O N T HE J U R A S S I C IN T HE N O R T H S E A R E G I O N
Figure 3.19: A map of Jurassic isopachs and a possible compressional tectonic model. Contour interval = 250m. (After Olsen, 1987). Note the NNW-SSE and NW-SE orientation of maximum horizontal compression in the Southern North Sea Basin and the Central Graben respectively.