On the structural evolution of the Central Trough in the Norwegian and Danish sectors of the North Sea

On the structural evolution of the Central Trough in the Norwegian and Danish sectors of the North Sea Martin B. Gowers Geoform A/S, Oddenvn. 1 3a, 1 3 2 2 Hcvik, Norway

and Asbjcrn Saebce Norsk Hydro a.s., P.O. Box 4 9 0 , 1 301 Sandvika, Norway

Received 25 May 1985 The Central Trough of the North Sea is not a simple rift graben. It is an elongated area of regional subsidence which was initiated in mid Cretaceous times and continued to subside through to the late Tertiary. Its form is not representative of pre-mid Cretaceous tectonics. In Late Permian times the North Sea was divided into a northern and southern Zechstein basin bythe E-Wtrending Mid North Sea-Ringk~bing-Fyn High. The latter was dissected bya narrow graben trending NNW through the Tail End Graben and the Sogne Basin. The Feda Graben was a minor basin on the northern flank of the Mid North Sea High at this time. This structural configuration persisted until end Middle Jurassic times when a new WNW trend separated the Tail End Graben from the Sogne Basin. Right lateral wrench movement on this new trend caused excessive subsidence in the Tail End and Feda Grabens while the Segne Basin became inactive. Upper Jurassic subsidence trends continued during the Early Cretaceous causing the deposition of large thicknesses of sediments in local areas along the trend. From mid Cretaceous times the regional subsidence of the Central Trough was dominant but significant structural inversions occurred in those areas of maximum Early Cretaceous and Late Jurassic subsidence. Keywords: Central Trough; Structural Elements; Inversion; Tectonics

Introduction

Previous work

The area covered by this article has been extensively explored and is the site of several major hydrocarbon fields (see Figure 1). It was the focus of the first major phase of exploration in the Norwegian sector following the discovery of the Ekofisk field in 1969, but relatively little has been published concerning the regional structural setting. The area extends from the Norwegian sector into both the UK and the Danish sectors, a fact which has hampered the construction of a good regional understanding since most individual studies stop at the national borders. As a result of the early date of exploration the main body of data is of much poorer quality than that which we can now obtain. The initial exploration concentrated on Cretaceous and Paleocene targets and until recently little work was done on the deeper geology. Spurred by the Norwegian 6th licensing round in 1981 new studies with new data revealed that the pre-Cretaceous structural picture is complex, and to a large extent explains much of the post Jurassic geology. This article is based on work done from 1980 to 1982 and aims to define the major structural elements and their relation to the regional geology. The analysis is based on extensive seismic and well data mainly from the Norwegian and Danish sectors, but also in the UK sector. In this paper we follow the structural evolution from the Permian to the Tertiary. We believe that there is much to learn from the area and hope that other workers with a more detailed knowledge of local areas can be stimulated to publish refinements to the framework we suggest here.

Ronnevik et aL (1975) described the main structural elements of the area and established a solid basis for nomenclature. Hamar et aL (1980) contributed to the regional picture in the Norwegian Sector while Day et aL (1981) provided a useful overall view to link the structures in the various national sectors. Andersen et aL (1982) gave the first overall view of the structure in the Danish sector which provided an important perspective to the understanding of the Norwegian Sector. Skjerven et al. (1983) presented the most recent description of the structural development in the Norwegian sector and we extend their work by focussing in more detail on the Central Trough area. Nomenclature There appears to have been no formal proposal for structural nomenclature since 1975 (Rannevik et aL) even though several new names have been introduced. One of the difficulties is that structures are time dependent, and require an understanding of the structural history of the area in order that they may be properly defined. In this article we have used previously introduced nomenclature where appropriate and have proposed some new names for structural elements not previously recognized. One point deserves special mention. The proposal of Ronnevik et aL (1975) that the term Central Graben be replaced by the term Central Trough as the name of the broad graben system does not seem to have been followed by subsequent workers. This is unfortunate

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Central Trough of the North Sea: M.B. Gowers and A Soeb~e

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since, as we will show, the details of the main basin system are complex, and rather distinct from the broad area of post-Jurassic subsidence which is popularly loosely referred to as the Central Graben. In this paper we will follow the proposal of Ronnevik et aL (1975) and refer to the main subsidence system as the Central Trough and we will use names of the individual structural entities to define the constituent parts of the Central Trough, hence avoiding the term Central Graben. Confusion has arisen since extensive inversion tectonics have resulted in significant structural highs at top Cretaceous level in those areas where Jurassic subsidence has been greatest. Definitions of the new names used in this paper are given later. Lithostratigraphic nomenclature used follows the proposals of Vollset and Dor~ (1984) and

Deggan and Scull (1977) where applicable, although we have consciously kept the lithostratigraphic analysis and references to a minimum in order to concentrate on the major structural elements.

Data base The data used in this work comprises in the Norwegian sector both regional seismic data, with an average grid spacing of about 4 km and more detailed seismic data in some of the blocks. Most of the well data in the Norwegian sector was also available and used to calibrate the seismic interpretation. The regional seismic data used in the Danish sector has an average grid spacing of about 6 km, while the detailed seismic data varies in its grid spacing, being commonly better

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Central Trough of the North Sea: M.B. Gowers and A Soebee than 2.5 km. Danish well data used includes all wells released up to 1982 and some more recent wells. In the UK sector a limited number of regional seismic lines were examined and related to most of the relevant wells drilled before 1981. Seismic interpretation Much of the previously published work has relied largely on well control. In our study we have used the well control to calibrate the seismic at the wells, and then embarked on the main phase of the interpretation. Hence our analysis is largely based on the seismic interpretation. The supporting evidence for the interpretation, numerous seismic sections and discussions of their inter-relation does not lend itself to publication in an article of this nature and length. Because of this we have tried to restrict our analysis and conclusions to that which is drawn from relatively certain interpretations, and avoided conclusions drawn from speculations. Where less certain interpretations are involved then these uncertainties are stated. This is frustrating for those who are accustomed to document the majority of interpretations with well control. We ask for their tolerance and hope that they recognize that seismic control, when properly calibrated, can be just as valid as well control. Over such a large area the quality of individual reflections shows large variations and it is not feasible to detail these variations in the space of this paper. In general the sequence from top Zechstein to the Upper Jurassic is the most difficult to follow on the seismic, especially in the deeper areas where there is little or no well control. However it is mostly possible from those reflections which are apparent, to give maximum or minimum thicknesses for units within this age range. This enables us to use broad thickness ranges on the maps from which we may hopefully draw valid conclusions, even though the precise thicknesses are unknown. Basement structural zones The extensive and mobile Zechstein evaporite sequence in the area has resulted in a significant difference between the structure of the top of the preZechstein surface and that of the post-Zechstein strata. Structures on the latter are often completely absent on the former. It follows that the top pre-Zechstein surface is likely to give a better idea of the main basement related structural movements than shallower levels. Since in most of the study area we are unable to see below the base Zechstein on the seismic, we will loosely refer to this top pre-Zechstein as basement in this paper. Pre-Permian sediments have been tested in only a few wells. The response of the basement to the various tectonic episodes is complex and varied, and for the purposes of the study we have divided the area into 17 zones. Many of the boundaries between these zones coincide with major faults, but others are more subjective and the structural character changes more gradually over them. Figure 2 shows these basement structural zones superimposed on a map of the main faults in the base Zechstein surface. Most major differential tectonic movements within the Central Trough took place in

300

pre-Tertiary times and especially in pre-Cretaceous times for those related to rifting. Therefore we have established the basement structural zones on the basis of pre-Upper Cretaceous tectonic activity. The area of the study forms a broad NW-SE trend flanked to the southwest by Zone Q, the Mid North Sea High where only relatively thin sediments are encountered between the base Zechstein and the Upper Cretaceous. The northeastern flank is formed by the Ringkobing-Fyn High and the Southern Vestland Arch of zone A where thick Zechstein and Triassic deposits are overlain by relatively thin Jurassic and Lower Cretaceous strata. (We use the term 'Southern Vestland Arch' after Vollset and Dor~, 1984). Between the two flank zones are a series of highs, basins and intermediate areas with differing structural histories. There are three deep fault-bounded graben areas (zones F, L, and O), three minor basin areas (zones B, C, and N, and three faulted high areas (zones D, J, and P). The remaining zones m Figure 2 have an intermediate nature between the basins and highs. The Tail End Graben (zone L) is the area which experienced the most pre-Cretaceous subsidence. The Upper Jurassic sequence attains thicknesses of up to 3 seconds two way time (ca. 5 kin) and the base of the Zechstein cannot be seen on the seismic data in the deepest part. The graben has an asymmetric form, being deeper against the eastern bounding fault than the western one. The Feda Graben (zone F) is in many respects similar to the Tail End Graben, although the two are clearly not contiguous. The Upper Jurassic sequence is very thick, over 3 km in the deepest parts, and the base Zechstein is generally too deep for reliable interpretation on the seismic data. The Feda Graben is asymmetric with the maximum subsidence against the western bounding fault. Halokinetic activity increases northwards while in the Tail End Graben it increases to the south. Within the Feda Graben the faulting, folding and subsidence patterns are complex and not yet fully understood, in contrast to the relatively undisturbed nature of the Tail End Gaben. Although not conclusive, the indications from the seismic suggest that sequences of Middle Jurassic and Triassic age are not thick in either of these two graben areas. Zone O has not been studied in detail. It is less asymmetric than Zone F and appears to be seperated from it. The exact nature of the boundary between the two is uncertain, the base Zechstein reflector being often below the seismic record length, but we suspect faulting. In contrast to the similarity of the graben zones, the minor basins are of differing structural character. The Sogne Basin (zone C) is a clearly defined faulted basin in which the base Zechstein consistently dips towards the major eastern boundary fault similar to the asymmetry of the Tail End Graben to which it is closely related. Apart from halokinetic movements there has been relatively little tectonic movement within the basin, except perhaps in the south where the data quality becomes poor. In contrast to the fault controlled basins described above the Outer Rough Basin (zone N) appears to have subsided by a process of gentle flexure over a limited time interval. Zone B is an area with complex faulting of the base Zechstein surface, and a thick Triassic sequence. It can be regarded as a sub-basin although it probably owes its

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Figure 1.) Zones D, J, and P are largely responsible for the much used expression Mid Graben Highs which we believe to be misleading. The Mandal High (zone D) is a flat fault bounded high where Upper Cretaceous chalk rests directly on Precambrian gneiss in the well 3/7-1 (Ofstad, 1983). Zone J (the Dogger High) has a similar fiat fault-bounded form at base Cretaceous level, although the faulting is more complex than that of the Mandal High. The chalk is underlain by a

volcanic Rotliegendes sequence which rests unconformably on eroded Carboniferous sediments and Caledonian basement in the well P-1 (Michelsen, 1982). We use the term Dogger High to refer only to zone J of which the P-1 well is representative and not to any supposed extensions to the northwest or southeast of which a variety can be found in the literature. Zone P, which we call the Josephine High after the discovery of that name in the UK block 30/13 is very different from the highs discussed above. At the base Zechstein level it appears to be a tilted horst block where sediments of Lower Cretaceous, Jurassic and Triassic age rest on Zechstein evaporites with structure controlled by halokinetic and other tectonic movements.

M a r i n e and P e t r o l e u m G e o l o g y , 1 9 8 5 , V o l 2, N o v e m b e r

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Central Trough of the North Sea: M.B. Gowers and A Soebce Apart from the basins and highs mentioned above the remaining structural zones in Figure 2 have an intermediate, or less well defined nature. In zone I (the Hidra High) the basement was structurally high in Jurassic times due to movements on the N W trending faults bounding zones B and H. With the exception of these faults the boundaries to zone I are transitional and arbitrary. Zone G (the Grensen Nose) may be considered as an extension of the Mid North Sea High. Zone H is a complex faulted block in which the basement dips towards its northern bounding fault, It appears to have affinities to both the Feda Graben and zone B. Zones E and I~ which together with the Mandal High constitute the Vigeland Ridge, are faulted shelf areas bordering the two major grabens, and separating them from one another. The boundary between zones E and K is arbitrary and could also be drawn further north nearer the national sector boundary. Zone M is an intermediate zone between the deep Tail End Graben and the Ringkobing-Fyn High. It has accumulated sediments of Triassic and Jurassic age which rest on a mobile Zechstein evaporite sequence. The division of the area into the 17 zones enables us to discuss and better understand the structural history and we will refer to the zone letters, or to the names given in Figure2 throughout this paper. We believe that many refinements and clarifications of these zones are required but a more detailed description of them is beyond the scope of this paper. Structural evolution

Pre- Triassic Evidence for the timing of the initial formation of the rifts of the Central Trough is difficult to find, although many have assumed an early Permian age. Poor seismic data and little well control prevent any detailed analysis of the pre-Triassic section, and hence we will merely review the main data available. Metamorphic rocks of Caledonian age were found in wells P-I, B/4-1, 3/7-1, and 30/16-5 (FrostetaL 1981) and in Per-1 (Zeigler, 1982) shown in Figure 3 and indicate that the crystalline basement is of this age in the study area. The seismic data indicates that the Devonian to loE

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Carboniferous sequence is over 2500 m thick northwest and southeast of the 2/10-1 well but that the basement dips to the southwest relative to the Carboniferous in this area. The Carboniferous is absent to the west but oversteps the Devonian in the east to lie directly on the basement in P-1. Since there is no well or seismic control of the pre-Permian in the basinal areas we conclude that it is at present not possible to show whether the Central Trough system was active at this time or not. In the Auk field the well 30/16-1 drilled 485 m of Rotliegendes sands above the Devonian (Brennand and van Veen, 1975) while the maximum thickness reported from the Argyll Field well 30/24-2 was 454 m. (Pennington, 1975). In the well 2/10-1, 311 m of Permian Rotliegendes are reported with traces of volcanic activity in the lower part (Ofstad 1983). The well 2/7-2 (see Figure 3 for locations) bottomed in Lower Permian basalt below a series of sands thought to belong to the Rotliegendes Group (Ofstad, 1981), indicating at least 90 m of Permian. The Danish well P-1 drilled through 58 cm claystones followed by 152 m ofbasalts and associated sediments before entering the Carboniferous, while the wells Q-1 and W-1 and 3/7-2 drilled 71 m, 148 m, and 165 m respectively of Rotliegendes volcanics without penetrating older rocks. Thus it seems that the volcanic deposits, which are found where the Norwegian and Danish parts of the Central Trough meet, die out westward as the preserved Permian becomes thicker. Permian rocks are absent in 3/7-1 probably due to Jurassic erosion of the Mandal High. They are absent in the well Per-l, also probably due to erosion, although in this case we cannot rule out non-deposition. There is thus insufficient data to determine whether the volcanics are related to the Central Trough area, or to the Horn Graben area (Olsen, 1982) where Lower Permian volcanics of similar petrographic and geochemical character are found (Dixon et aL 1981). We must therefore agree with Larsen and Michelsen (1982) that it is not known whether the initial formation of the grabens of the Central Trough was associated with this volcanism (and hence of early Permian age), although this may be a resonable assumption bearing in mind the distribution and nature of the Zechstein interval. Zechstein deposits in the study area can be divided into 2 types; thin carbonate dominated sequences (marginal facies) on the major highs, and thick mobile halite dominated (basinal facies) in the major basins. Both types are often difficult to recognize on the seismic. The marginal facies usually occurs with other thin sequences overlying it such that the seismic response is complex and good well control is required for reliable interpretation. The basinal facies is easy to identify due to the evidence of halokinetic activity. However, the acoustic impedance contrast between the salt and the Triassic sediments is often very low and hence the details of the top Zechstein event can be difficult to follow. This is especially true where diapiric activity ceased before deposition of Jurassic and younger sediments such that the record of movement is entirely within the Triassic sediments, which commonly have poorly defined bedding reflections. In such cases early diapirism has sometimes resulted in a complete withdrawal of the mobile Zechstein salt such that the Triassic appears to lie conformably on the

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Figure 4 Seismic expression of the Zechstein sequence. The Sogne Basin - Tail End Graben trend dissects the major high

Rotliegendes. Poor data in the deep areas of the Feda Graben and the Tail End Graben compound the problems of mapping present or original Zechstein thickness. We have chosen to map the seismic expression of the Zechstein instead of its original or present thickness and have recognized 3 types. Type 1 is the diapiric province where the seismic shows clear evidence of diapirism. Type 2 is an intermediate province where there is evidence of mobile evaporites (mostly disharmonic relations between the Triassic and Rofliegendes reflectors) but that there is no evidence of diapirism. Type 3 includes all those areas where there is no evidence of mobile Zechstein at all. Figure4 shows a sketch of these three provinces.

The geological interpretation of these seismic indications is subject to several uncertainties. We can assume that in the type 1 diapiric area the Zechstein was deposited as thick evaporites of the basin facies. Type 2 areas can be interpreted as intermediate type with a thin (relative to type 1) evaporite sequence which did not produce diapirism but enabled a certain amount of detachment between the structure of preand post- Zechstein strata. However in much of the type 2 area shown in Figure 4 the Triassic is thin and eroded such that evidence of pre - mid Jurassic diapirism may have been removed and thus the seisinic expression that of a remnant evaporite layer. Thus we cannot automatically assume that type 2 areas in the Feda and Tail End grabens were not sites of depostion

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Central Trough of the North Sea: M.B. Gowers and A Soeb~e of thick basin evaporites in Zechstein times. The interpretation of type 3 areas is easy where we have well control. Marginal Zechstein carbonate facies are reported from Auk (Brennand and van Veen, 1975), Argyll (Pennington, 1975), 2/10-1 (Ofstad, 1983), B-1 (Jacobsen and Larsen, 1982), and the Mid North Sea High. Hence areas surrounding these wells can be assumed to have been marginal to the main basin in Zechstein times. We note that the presumed Zechstein carbonate interval in 2/10-1 is of uncertain age (Ofstad, 1983). Similarly marginal Zechstein facies are reported from the Ringkobing-Fyn High further east in the well L-1 (Jacobsen and Larsen, 1982). However, between these two major highs there are no certain datings of Zechstein in type 3 area, and significant erosion in Late Triassic times may have removed any Zechstein deposits - marginal carbonates or evaportites. In the four most relevant wells: 2/7-2, P-l, Q-l, and W-1 (see Figure 1 for locations) this erosion has not removed the Rotliegendes volcanic section and the sediments directly overlying the volcanics are of uncertain age (from Rotliegendes to Triassic). The post-Triassic erosion has exposed deposits of similar stratigraphic age in the Q- 1 and W- 1 wells of the basin area and the 2/ 7-2 and P- 1 wells on the high s. Thus we consider it most likely that the four locations were at similar elevations in Late Permian times and hence that the Zechstein evaporite basin did not originally extend southeast from the Feda Graben in any significant thickness. New data may prove this conclusion to be in error. There is one isolated type 2 area just south of the Dogger High. This area is difficult to interpret due to the local structural intensity. Compressive movements have considerably deformed a thick pile of sediments, probably of Upper Jurassic age in a manner which suggests the presence at depth of soft sediments. Some evaporties, or plastic shales may be presentThe observations of the Zechstein shown in Figure 4 clearly suggest at least a partial connection of the northern and southern Zechstein basins. This connection occurred through the Tail End Graben and the Sogne Basin. Note that on Figure 4 this connection is separated from the Feda Graben by a ridge, (The Vigeland Ridge), which stretches from the Mandal High southwards to the German sector. This ridge, which was certainly active in Late Triassic times, may thus already have been active in the Zechstein. The Zechstein distribution may indicate a gentle doming of the Mid North Sea High - Ringkobing-Fyn High trend and the initiation of the Tail End Graben - Sogne Basin rift-

Triassic The reconstruction of original Triassic depostion suffers from difficulties similar to those for the Zechstein. In all borehole occurrences the Triassic is incomplete due to erosion, and marked angular unconformities seen on the seismic data indicate that large thicknesses of Triassic have been removed in many places. In the study area the observed Triassic sequence attains thicknesses of over 3500 m in places, even after erosion, while in other places it is completely removed by erosion. The onset of Triassic sedimentation seems to have occurred at the same time in both high and basinal areas since Scythian 304

(lowermost Triassic) deposits are reported from the Mid North Sea High in wells 30/16-2 of the Auk Field (Brennand, 1975) and B-1 (Bertelsen, 1980). The seismic data indicates significant intra-Triassic tectonic activity, in some areas, partly related to movement of Zechstein sail, notably in zone B of Figure 2. Other areas show thick sequences of parallel bedded Triassic, for example in the south of the Sogne Basin. Since dating of these sequences, and determination of the thickness removed by erosion is at present not possible, we are unable to describe from the data the structural evolution within the Triassic and must therefore rely on the broad regional picture. The reader is referred to Zeigler (1982) for an overview of the latter. On the basis of the data we can only attempt to map the present Triassic distribution. Figure 5 is a sketch map of the distribution of Triassic sediments in the study area. It is mostly based on seismic data since well control is not abundant- In detail an isopach of Trias sediments shows large local variations due to movement of Zechstein salt and to erosion and hence Figure 5 indicates only the approximate maximum Triassic thickness. Over most of the area there is some uncertainty as to the pick of the base Triassic/top Zechstein which is generally a poor reflector. There is considerable uncertainty as to the pick of the top of the Triassic in the Feda Graben and areas just to the north of it- In much of the Tail End Graben the data is poor and/or the base of the Triassic is below the bottom of the seismic record. The map shown in Figure 5 should therefore be used with care. We have divided the Triassic sequence into 4 areas. On the major highs the Triassic is thin (< 200 m) or absent due to subsequent erosion. A second area is of intermediate thickness (200 m to 1000 m approx.) and in the third area the Triassic is judged to be over 1000 m thicE In the fourth area there is evidence of significant intra Triassic movements, both tectonic and halokinetic and an average thickness is not meaningful due to the very large variation in local thickness. The Triassic is generally very thick in this area, especially in the north. The boundaries between these 4 areas are somewhat subjective in many cases but we believe that the broad picture is representative. The map shows the Mid North Sea High, the Ringk~bing-Fyn High, and the Vigeland Ridge as high areas, and the connection from south to north of the Tail End Graben with the Sogne Basin. This is similar to the Zechstein map (Figure4). Thick parallel bedded Triassic sequences indicate that the tilting, especially in the Sogne Basin and Tail End Graben must have post dated the deposition of the observed strata. If this is the case in all areas then the relative subsidence illustrated byFigure5, which is clearly controlled by the major faults between the structural zones, occurred in conjunction with the erosional episodes following the main deposition, rather than during deposition of the Triassic. There is no indication that the Feda Graben or the northern Tail End Graben have retained Triassic sediments in thicknesses of over 1000 m although we stress that these are areas of uncertainty. Thus the broad picture is consistent with continued subsidence of the southern and northern Permian basins during the Triassic with post-depositional establishment of the Tail End Graben, S~gne Basin,

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Central Trough of the North Sea: M.B. Gowers and A Soebee

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and Vigeland Ridge. The relative subsidence illustrated in Figure 5 may have occurred at any time from Middle Triassic to Middle Jurassic times. Skjerven et aL (1983) discuss two possibilities for formation of the Central Trough: development in Permo-Triassic times along NW trending faults, or Triassic N-S trending faults dissecting the Mid North Sea - Ringkobing-Fyn Highs followed by NW-SE block faulting of Kimmerian age. The data we have studied indicates that the NW-SE trend was not dominant in pre-Middle Jurassic times but that the structural grain was NNW-SSE parallel to the Sogne Basin - Tail End Graben and the Vigeland Ridge. Faulting may be attributed to the Early Cimmerian movements of Late Triassic age, the Mid-Cimmerian movements of early Middle Jurassic age, or as part of

an ongoing subsidence initiated in Early Triassic times. A combination of all three is likely although a comparison with the regional geology suggests that the main movements were of Middle Jurassic age. Early Jurassic

Lower Jurassic sediments are reported from wells in the south in zone M (Koch et aL, 1982). In the Norwegian sector Lower Jurassic sediments are reported from 7/81 (Strass, 1978), 7/8-2, 7/9-1 (Olsen and Strass, 1982), and the Ula field (Bailey et al. 1981) although there is some doubt as to the dating. Lower Jurassic is also reported, with some considerable doubts as to the dating from the Danish Q-1 well (Michelsen and Anderson, 1983). Apart from the above there is little further information available. Many wells have not

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Central Trough of the North Sea: M.B. Gowers and A Soeboe drilled through the Middle Jurassic, some have drilled into diapiric Zechstein salt directly under formations younger than Lower Jurassic, and some have proved the Lower Jurassic to be absent. There is a general consensus that the uplift and erosion during the midCimmerian tectonic phase in the early Middle Jurassic, has removed all but a few isolated remnants of a Lower Jurassic sequence which may well have been initially deposited over the whole area, including the major highs. The impression is, therefore, that even though there may have been differential subsidence during the Triassic, the depositional surface showed little relief from the Triassic through to the major uplift of the midCimmerian phase. Thus the map in Figure 5 and the discussions of it in the preceeding section may be mostly relevant to early Middle Jurassic uplift. Outside the study area Lower Jurassic deposits are retained to the northeast and to the south in a pattern which is interpreted to represent the mid-Cimmerian uplift as a major crustal doming centred on the Mid North Sea High - Ringkobing-Fyn High trend. The regional context of this dome is illustrated by Ziegler (1982). The major dissection of this rift dome was the Tail End Graben - Sogne Basin trend which is so clearly apparent in Figures 4 and 5. Note that the Feda Graben represents a less important feature at this time and the NW-SE Central Trough is not a subsidence trend, which emphasises the misleading nature of the popular use of the term Central Graben. Middle Jurassic" Sediments of Middle Jurassic age are found in the north and south of the area studied. In the north numerous wells on the Southern Vestland Arch (zone A) prove a thin sequence of sands, silts, and coals variously described as fluviatile, deltaic, swamp or even shallow marine deposits. These belong to the Middle Jurassic Bryne Formation (previously called the Haldager Fro) and are remarkably consistent in thickness: 100 - 200 m (see Hamar et aL 1983). In the south Koch et aL (1982) describe an equivalent sequence, which they call the J-2 Unit from four wells in zone M where the thickness varies from 286 m to 39 m. Seismic data indicate that this unit is widespread and does not exhibit sudden thickness changes besides those directly attributable to salt movements. The J-2 Unit in zone M and the Tail End Graben appears to rest unconformably, though with little angular discordance on the underlying Lower Jurassic and shows only a gradual thinning northwards. Little significant tilting to the east is indicated within zone M but in the Tail End Graben there is some evidence of a thinning to the west. In the Sogne Basin the Bryne Fm has been drilled, for example in 2/6-1 (Frodesen, 1979) and seismic data indicate a significant increase in thickness towards the main fault in the east, with maximum thicknesses of over 500 m. Therefore we assume that the Sogne Basin and the north of the Tail End Graben were actively subsiding due to normal movements of the major fault bounding the Ringkobing-Fyn High during Bajocian and Bathonian times. Koch etal. (1982) also report 55 m of J-2 Unit in the Q-1 well though there is some doubt as to the age of the sediments. Dating the Middle Jurassic deposits is often problematic, presumably since both it 306

and possible overlying Upper Jurassic sands are derived from similar source areas, assumed to be the eroding Triassic of the Mid North Sea and Ringkobing-Fyn Highs. Elsewhere in the study area the Middle Jurassic is found in zones B, I and E but is generally thin and probably only present locally. Many wells are drilled on salt influenced structures and in these wells it is common for Upper Jurassic sediments to lie directly on salt. Movements of salt in Triassic times, especially Late Triassic in the Norwegian sector, allowed salt to be extruded at the surface at numerous locations both in the Central Trough area, and on the Southern Vestland Arch. In the Middle Jurassic at many of these locations the salt remained at the surface while deposition continued between the salt areas. Thus at the end of Middle Jurassic times the Upper Jurassic sediments were deposited on a low relief depositional plain with many areas of exposed salt. Some post Jurassic movement of the salt in these areas created the structures of which so many have been drilled, with Upper Jurassic resting directly on the salt. Due to this lack of adequate well control we are unable to demonstrate the presence or absence of Middle Jurassic in the Feda Graben. Seismic interpretation leads us to expect that it is in fact thin and we believe that zones O, F, and K were relatively higher than the zones to the northeast in Middle Jurassic times. Figure 6 shows the situation envisaged during Bajocian and Bathonian times. It leads to the important conclusion that the Sogne Basin - Tail End Graben trend was the only area of active subsidence relative to the stable plain to the north. Also, as envisaged for the Late Permain and Triassic, the NWSE trend of the Central Trough was not active at this time. It is possible that the thinning southwards from zone A towards the Feda Graben is indicative of the active doming of the latter in the Middle Jurassic. Note also that the area of maximum subsidence occurs where the Tail End Graben - Sogne Basin trend traverses the assumed doming of the Mid North Sea H i g h - Ringkobing-Fyn High trend. Late Jurassic Relative to older formations there is good data control for the Upper Jurassic deposits. This is due to the relative clarity of the seismic response of the sequence, particularly the top, and the significant number of wells which test the Upper Jurassic. The rising Upper Jurassic seas eventually transgressed most of the study area. Sediments of this age are proved to be absent on the Dogger High and the Mandal High but, at least in the case of the latter, the lack of erosion products around the high indicates that thin Upper Jurassic sediments may have been deposited only to be removed in early Cretaceous times. On the other hand immature erosion products in the W-1 well (Koch et al., 1982) indicate emergent local highs (Dogger High and the southwest of zone K) in early Upper Jurassic times. Erosion products in the Late Jurassic of the well 2/7-3 (Ofstad, 1981) and those of the Fulmar and Clyde Fields (Gibbs, 1984) indicate that the Mid North Sea High was at least partially emergent during Late Jurassic times. The Mid North Sea High and the RingkabingFyn High may have been covered by end Jurassic

Marine and Petroleum Geology, 1985, Mol 2, November

Central Trough of the North Sea: M.B. Gowers and A Soebee

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times. Evidence for this has been removed by erosion. Seismic definition of the Upper Jurassic interval is the most difficult in areas where the sequence is very thin because of resolution limitations, and also in areas where it is very thick, and hence the base is very deep, for example in the Feda Graben. Figure 7 shows a map of the average thickness of Upper Jurassic deposits. The interval mapped probably includes some Ryazanian sediments. It is evident from Figure 7 that the tectonic pattern in the Upper Jurassic is significantly different from that which had dominated since Permian times. The Segne Basin became relatively inactive and ceased to act in unison with the Tail End Graben, from which it is separated by a W N W trending fault. The fault forms

part of a major W N W trend through the northern part of the Tail End Graben, the Feda Graben, zone O in the UK sector, and the central part of the Vigeland Ridge. The Dogger High became an independent feature as the Outer Rough Basin subsided relative to the Mid North Sea High for the first time. The fault west of the Sogne Basin became gradually inactive in early Upper Jurassic times and was dormant thereafter, The subsidence in the Feda Graben and the northern end of the Tail End Graben shows many similarities. Both appear to be asymmetric. The subsidence is greatest in the southwest of the Feda Graben and in the north east of the Tail End Graben along the N N W trending bounding faults and in both cases we estimate that the Upper Jurassic shale

M a r i n e and P e t r o l e u m Geology, 1 9 8 5 , Vol 2, N o v e m b e r

307

Central Trough of the North Sea: M.B. Gowers and A Soebee

2° E

30E

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Figure 7 Approximate average thickness of Upper Jurassic sediments. Note the subsidence in the Tail End Graben is not continued into the Sogne Basin, and a subsidence trend runs WNW from the Feda Graben sequence attains thicknesses of well over 3000 m, or over twice the maximum thickness seen in the rest of the area, with the exception of zone M which is intimately linked to the Tail End Graben. Most of the salt movement in the Tail End Graben (south) and zone M occurred during Upper Jurassic times while in contrast the salt in the Norwegian sector, which was mostly active in Late Triassic times, was relatively dormant during the Late Jurassic. Neither the Feda Graben nor the northern Tail End Graben coincide with the areas of maximum salt activity. An explanation of the subsidence patterns in the Late Jurassic can be found in the interaction of the old NNW faulting trends with the new WNW trend first seen in the Late Jurassic. Right lateral wrench movement along the new WNW

308

trend creates tension along the NNW trending faults, especially next to the more stable blocks as shown in Figure8. The Feda Graben and Tail End Graben might then be thought of as pull apart basins in the sense of Crowell (1974). It is interesting to note the parallel with basins flanking the major highs further south in the North Sea. These basins subsided in the Late Jurassic as tension gashes thought to be due to right lateral wrench movements between the Ringkobing-Fyn-Pompecki Block to the north and the London-BrabantBohemian Massif to the south (Ziegler, 1982). Not shown on Figure 7 is a subtle Upper Jurassic subsidence trend between zones A and B. Along this trend synsedimentary faulting resulted in thicker

Marine and Petroleum Geology, 1985, Vot 2, November

Central Trough of the North Sea: M.B. Gowers and A Soeb~e

~,

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Figure 8 Possible relation of Upper Jurassic subsidence in Feda and Tail End Graben to interaction between old NNW fault trends and a right lateral wrench movement along the new WNW trend. Compare with Figure 7

Upper Jurassic deposits on the downthrown southwesterly side, and the trend can be followed south through the Hidra High. Although small in scale the faulting along this trend formed an important control over the depostion of Upper Jurassic sands in the relatively low relief area north of 56°30' in the Norwegian sector, and the Ula Field reservoir (Bailey et aL 1981) is one of the fortunate results.

Early Cretaceous The boundary between the anaerobic Mandal Formation (previously called the Kimmeridge Clay Fm) and the aerobic Valhall Formation is a lithological change which generally gives rise to a very large acoustic impedance contrast, and hence a strong reflection on the seismic data. A strong reflection is also common at the base of the Late Cretaceous sequence such that, compared with older intervals the Early Cretaceous interval is relatively easy to map. In addition well control is abundant. However, there was much tectonic activity in the Early Cretaceous and its distribution is not easy to understand, In addition, as is often the case, most wells are drilled on local structural highs and often give a misleading picture of the overall trend. Widespread faulting activity resulted in many small local basins where no well control is available to confirm seismic reflection identifications and hence, despite the generally good data quality, we can expect surprises. Figure 9 shows a sketch map of the distribution of the Lower Cretaceous interval as mapped from the seismic data. The base of the interval is probably Late Ryazanian in age and the top probably Mid Albian (see Rawson and Riley, 1982). We have defined in Figure 9 those areas of anomalous thickness of about 700 rn, or more, and those areas where the Lower Cretaceous interval is absent or very thin. The details of the thickness within the intermediate area are too complex for a map of this scale since numerous faults active at this time have resulted in significant but local thickness changes. In common with the Upper Jurassic, the Lower Cretaceous interval shows a subtle thickness increase along the boundary between zones A and B and across the Hidra High, and rollover-

slump type faults characterize this trend. In zone A the interval is commonly 50-100 m thick along the southwestern boundary but thickens gradually to the north-east towards the Egersund Basin. In the Sogne Basin some thickness changes directly related to salt movement are found but otherwise there is no evidence of subsidence relative to zone A, In the Tail End Graben, where Late Jurassic subsidence was greatest, the Lower Cretaceous deposits attained thicknesses of up to 500 m in the centre but the areas of greatest thickness are concentrated along the faults at each side, giving the appearance of basin inversion. Further south in the Tail End Graben thickness variations become dominated by salt movements and the clear pattern seen in the north is not found. The WNW trend connecting the south of the Feda Graben with the north of the Tail End Graben, which was first established in Late Jurassic times was clearly active in the Early Cretaceous and exhibits a pattern of subsidence with maximum thickness developed against the boundary faults and a thinner sequence in the centre. This gives a pronounced WNW anticlinal trend shown inFigure 9. This anticlinal trend is also evident in Late Jurassic times, in sharp contrast to the Tail End Graben which shows the least Early Cretaceous subsidence in the centre where the greatest Upper Jurassic thicknesses are found. In the Feda Graben the Early Cretaceous subsidence pattern is more complex than in any other area. Well control along the axis of the Feda Graben shows variation from 100 to over 500 m in Lower Cretaceous thickness (Ofstad, 1983) and seismic data indicates a variation from zero to well over 1000 m within the area of maximum Late Jurassic subsidence. The variation in the thickness of the Lower Cretaceous in the Feda Graben is due to faulting and folding of the Jurassic sequence along with WNW trend. There is no evidence of Early Cretaceous subsidence along the fault separating the Feda Graben and the Grensen Nose similar to subsidence found in the Tail End Graben and just north of the Dogger High. However it is possible that such subsidence has indeed occurred. The internal seismic reflections from the Lower Cretaceous sequence often appear to be more or less parallel with the base of the sequence in those areas which can be satisfactorily resolved on the seismic data. This would indicate that the relief on the base Cretaceous surface was not formed in earliest Cretaceous times, but only after deposition of a considerable thickness of the Early Cretaceous Valhall formation. If this is the case then the thinning of the Lower Cretaceous towards the main western bounding fault of the Feda Graben is probably due to erosion after faulting and folding occurring during the Early Aptian as part of the Austrian Tectonic Pulse (see Ziegler, 1982). Much of the detail of the Lower Cretaceous sequence is obscure on the seismic data over the main part of the Feda Graben due to the gas charged Tertiary sediments which degrade the data quality. There are also some indications of some inversion movements in the south east of the Feda Graben but we do not at present consider these to be conclusive. Thick Lower Cretaceous deposits are also interpreted in the northwest Feda Graben and the adjacent zone O in the UK sector. These areas are less

Marine and Petroleum Geology, 1985, Vol 2, November 309

Central Trough of the North Sea: M.B. Gowers and A Soebge

2OE

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4°E

5°E

57°N

56°N

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50KM.

Figure 9 Approximate distribution of Lower Cretaceous sediments. Note subsidence occurs along WNW trend well defined and are complex to map due to extensive halokinetic movements. There appears to be less nonsaltrelated structure in these areas than in the main part of the Feda Graben. The last significant area of anomalous Lower Cretaceous thickness is found in zone H where subsidence which is directly related to movements on the northern boundary fault of the zone has allowed the accumulation of over 1000 m of Early Cretaceous sediments. This area differs from the Feda Graben in that it did not, according to our interpretation subside very much during the Late Jurassic. (Late Jurassic subsidence cannot be completely ruled out since there is at present no well control on the relevant sequence of reflections but for the present we interpret a thin Late Jurassic sequence under the thick Early Cretaceous deposits in zone H).

310

Erosion of Early Cretaceous sediments can be recognized in the west and in the south of the Sogne Basin, presumably including the Mandal High in a manner which indicates that the Sogne Basin block behaved as one with the Ringkobing-Fyn High. Similarly erosion of the Early Cretaceous is recognized in the northeast of the Outer Rough Basin, presumably including the Dogger High, and the southern part of zone K. These two erosional areas represent uplift along the flanking blocks of the now pronounced WNW tectonic trend joining the south of the Feda Graben with the north of the Tail End Graben. This trend has an anticline in its centre such that together with the uplifted flanks it has the structural style of a symmetrical elongated down-faulted dome feature. Although the WNW trend first established in the Late Jurassic it is also clearly active in the Early

Marine and Petroleum Geology, 1985, Vol 2, November

Central Trough of the North Sea: M.B. Gowers and A Soebee Cretaceous and we cannot merely invoke a continuation of the same right lateral wrench mechanism along this trend to explain the distribution of Early Cretaceous sediments. The foci of Late Jurassic subsidence do not coincide with those seen in Figure 9, although they are clearly related. Nor is there significant evidence of the major tectonic or rift movements popularly ascribed to the Late Cimmerian Tectonic Phase. The so called Late Cimmerian Unconformity would seem to be of minor importance in this area when compared with the major tectonic changes between the Mid and Late Jurassic, and with the Mid Cimmerian movements. We suspect that the Austrian Tectonic Pulse may also be more significant than the Late Cimmerian Movements. (The over emphasis on the structural significance of the Jurassic/

2° E

Cretaceous boundary is well exposed by Rawson and Riley, (1982)).

Late Cretaceous Since the main hydrocarbon finds in the area have been made in the Late Cretaceous Chalk, this sequence is relatively well documented in the literature (Brewster and Dangerfield, 1984; D'Heur, 1984; Van den Bark and Thomas, 1980; Skovbro, 1983; AndersenetaL 1982; among others). Thus we do not intend to describe the Late Cretaceous distribution in detail and will focus mainly on the tectonics which are relevant to the structural evolution thus far described. The thickness of the Chalk Group and its individual members varies considerably in the study area and

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Figure 10 Late Cretaceous inversion and subsidence. The inversion area west of 2°E is deduced from data presented by Ham (1983). Subsidence follows similar pattern to Tertiary subsidence. Major inversions correlate with major late Jurassic subsidence M a r i n e and Petroleum Geology, 1 9 8 5 , Vol 2, N o v e m b e r

311

Central Trough of the North Sea: M.B. Gowers and A Soeb~e Skovbro (1983) and Lieberkind et aL (1982) describe some of these variations in the Norwegian and Danish areas respectively. Thickness variations can occur due to major subsidence patterns, salt diapir growth, facies changes, intra chalk erosion, structural inversions, and variation in original depositional thickness and examples of all are found within the study area. Considerable evidence of reworking, slumps and turbidite activity is found in the various chalk sequences and for these and other reasons we are unable to assume that the depositional surface was fiat during the Late Cretaceous, thus limiting the precise reconstruction of subsidence patterns. Hence we use only 3 broad divisions on the basis of average thickness in our attempt to sketch Late Cretaceous subsidence patterns in Figure 10. On the established major highs on the southwest and northeast flanks of the Central Trough the chalk is relatively thin and does not show much variation in average thickness except on a local scale. The average chalk thickness increases from these stable platform areas towards a well defined depocentre centred in zone H and the southern part of zone B where the Chalk Group attains thicknesses of over 1500 m. In the southern end of the Tail End Graben and zone M the chalk becomes thinner and more affected by large scale halokinetic movements. Thus we infer from Figure 10 that subsidence during the Late Cretaceous followed a NW-SE trend with a clearly defined depocentre. This is a radical departure from earlier times when many depocentres reflected complex interactions between the various basement zones of Figure 2. The broad subsidence picture in Figure 10 is in fact rather similar to the Tertiary subsidence patterns shown in Figure 1. Superimposed on the general subsidence pattern are local areas of Late Cretaceous uplift shown in Figure 10. These uplifted areas generally correspond to areas of local subsidence in the Early Cretaceous and are therefore inversions of former basins. Reverse rejuvenation of normal faults occurred in most of those areas which experienced an anomalous fault related subsidence during the Early Cretaceous and those faults with clear reverse movement are indicated in Figure 10. Inversion of the Feda Graben was so strong that a pronounced asymmetric ridge was formed, named the Lindesnes Ridge (Skjerven et aL 1983). Basin inversion in the northern Tail End Graben was not as pronounced though reverse movements on the western boundary faults were similar to those in the Feda Graben. The close correlation of Late Cretaceous inversion and reverse rejuvenation to the patterns of Jurassic and Early Cretaceous subsidence may indicate that there was at this time a simple reversal of the wrench movements shown in Figure 8 such that the former extensional basins in a right lateral regime became compressional under a left lateral regime. Similar reversals were described by Zeigler (1975). Some conjugate wrench movement along the old NNW-SSE trend is also evident from the en-echelon tendency of some of the associated faulting. These movements were superimposed on the broad subsidence pattern described above, which is thought to represent the post rift thermal subsidence phase of the North Sea Rift system (Sclater and Christie, 1980). Our studies suggest that the subsidence does not

312

coincide with the areas of main graben development and hence that its cause may be rather more complex. The precise dating of inversion movements requires detailed correlation within the Chalk Group which we do not intend to discuss. It seems that movements on the Lindesnes Ridge had occurred before the deposition of the Tor Formation (Skovbro, 1983), i.e. pre-Late Campanian and according to Munns (1985) initial growth of the Valhall structure can be dated as Cenomanian. The most significant phase of inversion seems to have occurred around Santonian times and resulted in extensive erosion of the Hod Formation in the Eldfisk Field (Brewster and Dangerfield, 1984). Hardman and Kennedy (1980) infer significant movements at end Maastrictian, and end-Danian times in the Hod Field and although they link these movements to halokinetics, we believe them instead to be related mostly to the inversion movements under discussion. In the Tail End Graben there is less well control and hence it is more difficult to date the inversion movements. The data presented by Lieberkind et aL, 1982 are compatible with inversion movements of smilar age to those suggested for the Feda Graben. Andersen et aL, 1982 suggest basin inversion during the deposition of their Chalk-5 unit which is of Maastrictian age.

Tertiary During the Tertiary the Central Trough continued to subside as a broad elongate downwarp. The centre of subsidence in zone B, and the form of the subsidence were the same as during the late Cretaceous. Apart from this regional subsidence significant halokinetic activity persisted throughout the Tertiary and some continuation of the Cretaceous inversion trends occurred in the Early Tertiary. In particular the Lindesnes Ridge experienced continued inversion. The increase in halokinetic activity began to complicate the structures as the inversion movements weakened and often resulted in a shift of the axis of doming northeastwards as described by D'Heur (1984). In the Feda Graben Early Tertiary inversion movements were most intense in the south while halokinetic activity dominated in the north. In contrast the northern part of the Tail End Graben shows little Early Tertiary inversion but significant salt movement along its western boundary. Further south the Tail End Graben and zone M experienced a broad asymmetric inversion in the Early Tertiary with most of the differential movement concentrated on the western side. This can be seen in the contours of Figure 1 and we might conclude that zone M was not included in the general subsidence of the Central Trough in the Tertiary. The form of the Tertiary subsidence shown in Figure 1 shows little relation to the pre-Cretaceous subsidence sketched in Figure 11. Discussion The Central Trough has long been depicted in the literature as a structural entity and since 1975 both the Mandal High and the Dogger High have been recognized as elements within i t - although not always in consistent locations! The Dogger High in particular

Marine and Petroleum Geology, 1 985, Vol 2, November

Central Trough of the North Sea: M.B. Gowers and A Soebee has taken many different forms, often extending into the Norwegian sector as a major element as in Hamaret aL (1983). The introduction of the term Lindesnes Ridge by SkjervenetaL (1983) provided a much needed clarification of the main structure in this area of the Norwegian sector and showed for the first time that this high area was in fact a rapidly subsiding basin in preCretaceous times. We believe that the Central Trough should be treated as a broad NW trending zone of subsidence with some local highs only when discussing post mid-Cretaceous geology. When considering the geology of the area in pre mid-Cretaceous times we recommend that the individual structural elements within the area of the Central Trough should be recognized.

2° E

The best basis for a map of structural elements relevant to pre mid-Cretaceous times is the structure of the base Zechstein as it appeared at end Jurassic times. An isopach of the Zechstein, Triassic and Jurassic intervals is a good approximation of the desired structural picture since there has been relatively little erosion of uppermost Jurassic sediments. It is also the best map to use since the top Jurassic and base Zechstein reflectors are among the strongest reflections of the pre-Tertiary, and hence less liable to interpretation errors. In Figure 11 we have chosen to present the isopach of this interval divided into five thickness ranges rather than as a detailed contour map. This avoids the problem that the map can be drawn in great detail in some areas, but only very generally in

3° E

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Marine and Petroleum Geology, 1 985, Vol 2, November

313

Central Trough of the North Sea: M.B. Gowers and A Soebge

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others. It also forces us to concentrate on the main features, of which it should give a reliable presentation. Using the information displayed in Figure 11 the main pre mid-Cretaceous structural elements can be outlined as in Figure 12. Figure 13 shows the corresponding map of the main post mid-Cretaceous structural elements, and is considerably simpler. Several comments should be made regarding the individual structural elements. The M a n d a l High was defined by Ronnevik et aL (1975) as a N N W - S S E trending eastward rotated high with shallow basement. Although it is associated with the eastward rotated Sogne Basin, the major fault block on which the 3/7-1 well drilled is not rotated. We 314

suggest that the name Mandal High be restricted so as to refer only to the central horst as shown in Figure 14ct The Mandal High can then be described as a N N W SSE trending flat fault bounded horst on which the 3/71 well encountered Late Cretaceous Chalk resting on crystalline basement. The areas around the central horst block, especially to the north should be considered as part of the Vigeland Ridge and not as some authors show part of the Mandal High. The Hidra High was also defined by RonneviketaL (1975). Hamar et aL (1980) considered the Hidra High to be too small to be called a high and proposed to change the name to Hidra Fault Zone. The name Hidra Fault Zone has subsequently been applied to the fault bounding the northeast side of the Sogne Basin by

M a r i n e and Petroleum Geology, 1 9 8 5 , Vol 2, N o v e m b e r

Central Trough of the North Sea: M.B. Gowers and A Soebee

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Figure 13 Main structural elements in the Central Trough area in post mid-Cretaceous times

among others Hamar et al. (1983). At the end of the Jurassic the Hidra High as defined by Ronnevik et aZ had a structural closure at base Zechstein level of at least 1750 m and was bounded to the southwest by faults with over 3000 m of throw at the base Zechstein level. We cannot agree that this feature is too small to be called a high and suggest that the original name be used for the high as illustrated in Figure 12. The name Hidra Fault Zone should therefore be avoided. Instead the name Coffee Soil Fault can be applied to the major fault separating the Ringkobing-Fyn High from both the Tail End Graben and the Segne Basin. (Coffee Soil is the name of an area of the seabed at about 56°05'N, 5°E. The name is used by Ziegler, (1982) for the western edge of the Ringkobing-Fyn High). The Coffee Soil Fault runs from the Dutch sector northwards

through the Danish sector and can be followed north as far as 57°N, 3°35'E in the Norwegian Sector at base Zechstein level. The confusion over its relation to the Hidra High stems from the WNW-ESE trend of the developing boundary between the Central Trough and the Southern Vestland Arch which became evident first during the Cretaceous. The Dogger High is probably the most used name but the least defined high in the Central Trough area. It was tested by the P-1 well which found Rotliegendes and Carboniferous sediments overlying crystalline basement. At the base Zechstein horizon the high has a fiat horst like appearance as illustrated in Figure 14b. To the northeast the high is separated from the Feda Graben by several faults which trend WNW. To the southwest the high is separated from the Outer Rough

Marine and Petroleum Geology, 1985, Vol 2, November

315

Central Trough of the North Sea: M.B. Gowers and A Soebee

_ MANDAL _ _ HIGH v -

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Basin by a N W - S E trending fault which has probably undergone some compressive strike slip movement. To the northwest the high is linked to the main Mid North Sea High just north of the Outer Rough Basin and the exact nature of this boundary is not yet clear. To the east a series of normal faults separate the Dogger High from the Vigeland Ridge. To the southeast of the Dogger High there is a complex area which includes a small but very deep downfaulted block where Jurassic or Triassic sediments are thick. Strong inversion and compression, probably due to strike slip movemenL are also evident on the seismic data. This area clearly separates the Dogger High from the high to the southeast at the southern end of the Vigeland Ridge (the well B/4-1 drilled this high). Although most authors connect the two areas together, we strongly believe that they are two distinct entities. Evidence for this is also found in the bouguer gravity map presented by Andersen et aL (1982) where the Dogger High is plainly linked to the WNW, the B/4-1 high is linked to the Vigeland Ridge, and the two are separated by a marked gravity low. We therefore suggest that the term Dogger High be confined to the area of which the well P-1 is representative, as illustrated in Figure 12. The Outer R o u g h Basin is a new term derived from the name of the seabed in that area in accordance with 316

the practice in the Danish Sector. The basin extends from the ridge between the Dogger High and Mid North Sea High towards the southeast as shown in Figure 12. Up to 1000 m of sediments are found in the basin in the interval between the Upper Cretaceous Chalk and the pre-Zechstein sequence, which probably consists of a thin Rotliegendes volcanic sequence resting on eroded Carboniferous strata. No direct dating of the interval is available but correlation with the well B-1 suggests thick Upper Jurassic shales may rest on some Triassic sediments, with a thin Lower Cretaceous sequence which has been removed by erosion in the eastern part of the basin. The basin experienced gentle inversion in Late Cretaceous times but has subsequently been tectonically passive. (Michelsen and Andersen (1983) show this basin to contain Lower Cretaceous sediments overlying a questionable Jurassic sequence). Figure 14c shows a cross section of the Outer Rough Basin. The S~agne Basin is a new term derived from the name of a town on the Norwegian coast just east of Mandal. It is bounded to the east by the related major Coffee Soil Fault and to the west by the Mandal High. The Sogne Basin is bounded in the south by a large fault of probable Callovian age which throws down to the Tail End Graben. To the northwest a series of smaller faults downthrown to the west mark the boundary. The Sogne Basin has the general form of a tilted fault block active from Permian to late Jurassic times. The major dip within the basin is towards the Coffee Soil Fault The Feda G r a b e n is a new term derived from a place on the Norwegian coast west of Mandal. It is a major faultbounded asymmetric graben with major subsidence during the Late Jurassic along its southwestern boundary. It is tectonically closely related to the Tail End Graben although it is physically separated from it by the Vigeland Ridge. It extends northwest from the Dogger High, under the Lindesnes Ridge and takes on an easterly trend into the UK sector south of the Josephine High. Details of its deep structure, especially in the north are not yet established due to the poor quality of the seismic response. The Vigeland Ridge is a new term derived from a town on the Norwegian coast between Mandal and Feda. It refers to a pre-Cretaceous structural high trend forming the western flank of the Tail End Graben and Sogne Basin as shown in Figure 12. It includes the Mandal High in the north, and the B/4-1 high in the south. The Vigeland Ridge was a major feature in Late Triassic and in Jurassic times but ceased to be active when the younger W N W trend became dominant during the Early Cretaceous. Figure 14d shows a cross section of the Vigeland Ridge. The G r e n s e n Nose is a term we have used for the structural nose on the Norwegian side of the southern UK/Norway border. Grensen is a Norwegian word which means the border. We have not fully defined the nature of the boundary of this feature in the UK sector. The Grensen Nose was a part of the Mid North Sea high until the Central Trough subsidence was established in Late Cretaceous times. Conclusions The Central Trough itself was formed in mid Cretaceous times and has been a dominant area of

Marine and Petroleum Geology, 1985, Vol 2, November

Central Trough of the North Sea." M.B. Gowers and A Soebge subsidence thereafter. Before this time the dominant regional subsidence ocurred in the Norwegian-Danish Basin northeast of the Central Trough trend. The latter formed a transition zone between the subsiding area and the Mid North Sea High and Ringkobing-Fyn High trend to the south. This major high trend was transected in Zechstein to Middle Jurassic times by the Tail End Graben and the Sogne Basin. The Central Trough area in pre mid-Cretaceous times should not be depicted as a simple rift basin. The interaction of a Permian or older NNW-SSE trend with a Late Jurassic WNW-ESE trend has resulted in a complex series of basins and highs which show the effects of both tension and compression. We believe that wrench movements were of major importance in the structural evolution of the area and have resulted in a number of pre Cretaceous en-enchelon grabens along the Central Trough trend. Hence we recommend that the simple structural element picture at present in use be replaced by a more realistic presentation of the structural elements in pre Cretaceous times, and that the use of the term Central Graben be avoided.

Acknowledgements We would like to thank Norsk Hydro A/S for providing the data and for permission to publish, Kari Nilsen for typing the manuscript and Michael Raberg for drafting the diagrams. Thanks are also due to Roald Faerseth who critically read the manuscript

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