Journal of the Geological Society, London, Vol. 149, 1992, pp. 13-26, 12 figs, Printed in Northern Ireland

Tectonic and climatic control of sedimentation in the Beryl Basin, northern

L. E. FROSTICK’,T. K. LINSEY’ & I. REID2 ‘Postgraduate Research Institute for Sedimentology, The University, Reading RG6 2AB, UK 2Department of Geography, Birkbeck College, University of London, Malet Street, London WClE 7HX, UK,

Abstract: The Beryl Basin (Embayment) occupies a central position along the Viking Graben of the northern North Sea and has been a hydrocarbon play since the early 1970s. Detailed analysis reveals a complex half graben that was established during a -phase of development that occurred in the (Teist Formation), after which, a long period of thermal subsidence (Lomvi and Lunde Forma- tions) provided accommodation forin excess of 1OOOm of sediment. The Teist Formation sediments are complex, they include shales, sands and conglomerates, and their superimposition on Zechstein salts is indicative of both uplift and the development of a moderate relief. They give way to the comparatively clean and occasionally pebbly sands of the Lomvi Formation,for which an analysis of both bedding and texture suggests depositionby a westerly-directed river system draining the hanging-wall ramp. The Lunde Formation records the spreading influence of a bajaddplaya complex and its eventual conversion to a more persistent lake. The growing importance and eventual dominance of the succession by lacustrine sediments gives clear indication a of change in climate, and this has been attributed to the rapid northward drift of Pangea during the Triassic period.

Triassic sediments of the northern North Sea are geographi- The present study focuses attention on the Beryl Embay- cally extensive and attain considerablethickness (Lervik et al. ment, which sits on the eastern margin of the East Shetland 1989; Steel & Ryseth 1990). They occur across most of the Platform between the North and SouthViking Graben (Fig. 1). region, from the MureBasin in the north to theLing Graben in The area containsseveral productive oil fields, including Beryl the south and from the onshore and seabed outcrops of the and Bruce, whose reservoirs were discovered in theearly 1970s Inner Moray Firth in thewest to the deepsub-surface deposits and areMiddle (Brennandet al. 1990). As a result, the of the StordBasin off the Norwegian coast in the east. In some area has been drilled extensively and much of the data ob- areas, e.g. the Horda Platform, thicknesses of up to 1858m tained is now in the public domain. Seismic data arealso more have been proven(well N31/6-1). Yet the nature of theTriassic readily available than in areas of current prospect. basins and the factors that controlled sediment character are With this comparative wealth of information, it has been poorly and only partly understood. There are several reasons possible to explore the patterns of Triassic basin development for this. First, the main exploration targets of the northern and to infer the events and processes controlling the changing North Sea lie in younger rocks and, consequently, few wells nature of the sedimentary fill. penetrate more than a couple of hundred metres into Triassic sediment. Second, the Triassic sequence consists almostexclu- sively of continental redbeds in which biostratigraphical con- Structural setting trol is either poor, makinguse of sparse palynomorphs(Bertel- Post-Triassic structural evolution of the area is well known as sen 1974; Warrington et al. 1980; Lervik et al. 1989), or it is a result of oil exploration aimed at Jurassic, , and entirely absent. This presents major problems of correlation in Tertiaryplays (Heritier et al. 1979; Rochow 1981). But the any attempt at basin analysis. earlyrift-phase that belongs tothe Triassicperiod is only To date, most papers that dealwith the Triassic succession sketchily understood. Thekey to thisearly history ofthe basin of the region have consisted of broad surveys. These provide lies undoubtedly in careful analysis of the Triassic fill using useful overviews of Triassic sedimentation, but lack any de- all available well-log, core and seismic data. tailed analysis of sequences and lateral facies changes in in- The Beryl Embayment is situated at the southern endof the dividual basins. Recent examples include those of Lerviket al. North Viking Graben which hasan extensionalhistory (1989) and Fisher & Mudge (1990). The Inner Moray Firth extending backat least as far as the Trias (Whitemanet al. 1975; Basin is a notable exception. Here, access to bothonshore out- Zeigler 1982b; Glennie 1990). Beryl has a complex half graben cropsand offshoreexploration and production well data structure inwhich the north-south East Shetland Fault acts as gives a valuable advantage. Asa result, there havebeen several the major boundary on westernits margin (Fig.1). It occupies a detailed sedimentological studies, although not all have been 200 km by 180 km hanging-wall block which is fragmented by aimed at basin-wideanalysis (Peacock 1966; Clemmensen a number of antithetic andsynthetic faultsinto a series of tilted 1987; Frostick et al. 1988; Naylor et al. 1989).Besides the sub-blocks. Although the western margin of the basin is well- Moray Firth, the other area that hasreceived close attention is defined by the boundary fault, its eastern margin is more diffi- that of Tampen Spur in the Norwegian Sector, where R0e & cult to establish. The basin extends across the international Steel (1985) haveexamined theinterplay of rivers and Median Line into the Norwegian sector.But there, databecome shorelines at the edge of the / con- sparseand structural information isless available. In this tinent. study, thebasin is defined as extending toa set of NNE-SSW- 13

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160N East Shetland

c .-0, r

N25/8-1 \

\ -KEY \ ’r Major faults (after Jakobbsen et al.) f \ *%.Medianline 0 D ry well -$- Dry ‘\

P roduction well0 Production \ 0 0

G as and Gas oil well \\\.XN0 Fig. 1. Index maps of the Beryl Basin p,--- __ showing the pattern of faults according Oilwell + I\\ to Jakobbsen et al. (1980), the Gos and oil shows distribution of wells, and the lines of -0 Oil show seismic and stratigraphic sections. Note 0 km 100 that the most westerly fault depicted is 0 Other a southern extension of the East Shetland Fault.

trending faults that lie immediately east of the Median Line as feature through much of the Mesozoic. In addition, basement shown in Fig. 1. rocks are closer to the present-day surface here than elsewhere. In all published structural analyses of the northern North It may be possible, therefore, to infer a transfer fault as the Sea, the Beryl Basin (or ‘Embayment’ as it has been loosely reason for the southern ‘closure’ of the ‘embayment’. called) is depicted as a westward kink in the East Shetland Fault. The precise nature and cause of this deviation from a Triassic sediments of the Beryl Basin generalized north-south alignmentis the subject of speculation (Donato & Tully 1982; Swallow1986). The presence of a The Triassic sediments of the Beryl Basin have been largely Caledonian intrusion to the northwest (Threlfall 1981) may ignored in previous studies. Regional analyses of the Triassic have caused a curvature of the fault around its margin, so give only a general treatment of the sequence across an area producing the westward embayment. However, the reason for covering the whole of the Viking Graben (e.g. Brennand 1975; ‘closure’ ofthe basin in the southis less obvious. The boundary Lervik et al. 1989) and none describes the sediments in detail. fault is undoubtedly offset eastward in this area,while regional There are 93 published well-logs for the Beryl region of seismic sections reveal that there wasa positive topographical which 54 contain information about Triassic rocks. However,

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of these, only 10 penetrate the whole of the succession into either very thin or absent in manyof the wells in theUK sector olderrocks. This means that there are 14305m of well in- of the basin especiallytowards the south, althoughit does form formation in total, but within this framework there is only part of the oil reservoirin the Beryl field and reaches a respect- 148m of publicly available core. The distribution of wells is able thickness in the Norwegian Sectorwells (e.g. 251 m in the inevitably patchy, tending to concentrate close to structural type-sequence well N33112-2). For this reason, and because of ‘highs’(Fig. 1). Thismakzs basinwide interpretation of its equivocal status on the Triassic-Jurassic boundary, it has palaeoenvironments rather speculative. In addition, the Lower not been included in this study. Triassic strata arevery poorly represented in the record. These The contrasting characterof the three Triassicformations is shortcomings of the available database are not unique to the evident froman analysis of percent shale volume ineach bed as Triassic Beryl Basin. However, they serve to highlight the im- defined by the Automatic Bedding Disciminator (Fig. 3). From portance of maximizing the extraction of useful information. this it can beseen that beds of the LomviFm. are dominatedby Well-log interpretation is traditionallylaborious and highlyindividualistic. In this study, data-handling has been facilitated by the use of an Automatic Bedding Descriminator (Reid et al. 1989). In addition, regional seismic sections were deployed not only in the analysis of basin structure but also as a way of augmenting the patchywell data through an interpre- H Lunde Fm. tation of broad features of the sedimentary sequence such as Lomvi Fm. thicknessvariations. Only sections with good well control Teist Fm. were used and, in these, reflectorscorresponding to stronghori- * zons within the Trias were identified and followed laterally. 0 In the few wells that penetrate toolder rocks, the Trias was 0” 30 C 0) found to overlie thin Zechstein salts in the south. However, in 1 the east, sediments of age are absent and the Triassic g sequence sits unconformably on or older rocks. At ct: the top of the sequence, thereis an unconformity in 85% of the F9 wells in the UK sector of the basin,and in some, the Jurassicis represented by only a few metres, before giving way to a thick Cretaceous blanket. This contrastswith the few wells that are available in the Norwegian sector, where the Triassic sequence 0 passes conformablyupward into LowerJurassic strata in 0 50 100 almost all cases. Shale volume 5% For thisstudy, the Beryl Triassicsediments have been Fig. 3. Histograms of shale volume percentage for beds of the three divided broadlyinto three formations: Teist,Lomvi and Triassic formations that are recognized in the Beryl Basin. Lunde, by comparison with the type-sequences of Vollset & Individual beds are defined using the Automatic Bedding Dore (1984) (Fig. 2). The overlyingStatfjord Formation is Discrimator (Reid et al. 1989).

Northern North North Sea, NorthSea. N.of I BerylBasin. I LOCAT l ON N. of 60QN 60°N (revised by (this study). Lervik et 01 1990

Statfjord R- Formatio:\> F- v, -- Lunde Lunde Lunde Formation Fm. Lower/ Middle W Memb. a -7

C - Formation Lomvi W 2 LADlNlAN --- Fm . c l LL ‘ ANlSlAN W

C SPATHIAN 0 Teist I I b 0 0 a Formation FormationFormation Fig. 2. Triassic stratigraphy in various E W b parts of the northern North Sea 0 W according to (left to right) Deegan & 0 Scull (1977), Vollset & Dore (1984), --lLower I GRIESBACHIAN I TeistFm. Lervik et al. (1990), and this study.

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9/13-3a 9/8-5 9/13a-22 9/13-12 9/13-9a 9/10-1 25/4-5 25/4-1 25/4-5 9/10-1 9/13-9a 9/13-12 9/13a-22 9/8-5 9/13-3a

A’

Fig. 4. Stratigraphic sections, based on gamma-ray log profiles, constructed by drawing nearby wells towards lines that lie roughly orthogonal to the local rift axis. The section lines are located in Fig. 1. N.B. Section A-A’ has been ‘hung’ from the top of the Lunde Fm., while Section B-B’ is ‘hung’ from the base of the Lomvi Fm.

relatively pure sands (<25% shale) and the Lunde Fm. has sandstones are generally white to grey,fine to coarse grained, beds with higher shale volumes (generally >40%). The Teist well to poorly sorted, and subangular, with a sparse calcar- Fm. exhibits an apparent mixture of sands andshales, but this eous, dolomitic or siliceous cement. The grains are dominantly is largely a product of gamma-ray emissions fromthe feldspar- of quartz, someof them pink to red in colour.In some richconglomerates and its sediments are, in fact,generally localities, the sands are conglomeratic with quartzite pebbles much coarser than those of the Lunde Fm. up to 45mm in diameter, and there is evidence of repeated small-scale fining upwards. Traces of carbonaceous material Teist Formation and mica are common. The sands of the interbedded sandstone/siltstone sequence The Teist Formation is the least well-known sequence in the are very similar incharacter tothose in the basal deposits, except basin since only nine of the 54wells penetrate to sufficient that they tendto be finer-grained, more micaceousand patchily depth. Where it is present, it varies in thickness between97 m feldspathic. The associated green to red-brown siltstones are and 310m. One of the best examples of the Teist sequence is also micaceous, and contain a lot of calcareous material and recordedin well 9/13a-22(Fig. whereit comprises con- 4) clays as well as dolomite, anhydrite and carbonaceous frag- glomeratesand sandstones with subordinate siltstones. The ments. In some well-logs,and in the small amountof available sandstones range in grain-size from fine to very coarse and in core, some of the sediments show evidence ofoil staining. places areconglomeratic. They are buff to redin colour, The gamma-log signature of the sand unit is characteris- poorly sorted and withsub-angular to sub-rounded grains. tically blocky. The kickat the baseofthe unit towardslower and Cements are patchy and can be calcareous, dolomitic or silic- more constant API units(typically around 30 to 40) is often the eous.The siltstones are generally red and micaceous, with first thing to be identified on awell log and is verysimilar to the traces of calcite, dolomite and anhydrite. The conglomerates type-log signature identified further NE byVollset & DorC are of considerable interest in that they consistof fragments of (1984). The upward transition into interbedded siltstones and quartzite, granite, gneiss and shale in a coarse sandy matrix. sandstones is marked by a gradualshift back towards higher and The feldspars in the granitic and gneissic cobbles producevery more variable API values (Fig. 4), but without the extreme distinctive kicks in the gamma-log, in some instances to> 200 spikiness of the typical Teist Formation log. API units and this gives the characteristically spikey gamma- log signature which is easily recognized. Lunde Formation Lomvi Formation The Lunde Formation is the thickest of the three units in this The Lomvi Formation sediments vary in thickness from50 m to basin, as in others. It reaches 550m inwell 9/17-1A, which 205 m, reaching a maximum inwell 9/13a-22. They comprise a is within 5 km of the main border fault (a fact that will be basal sequence that is dominated by relatively pure sands, but significant in the discussionof the changing roles of tectonism this gives way to interbedded sandstones and siltstones. The and climatic change in basin evolution). However,its thickness

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U Lu Upper Lunde M & L Lu Middle & Lower Lunde L0 Lomvi Te Teist

B'

is very variable, ranging from zero over much of the southwes- Member. The Upper Lunde Member has a log signaturewhich tern portion of the basin to 713 m in the north (9113A-22). is much less variable and averages 90 API. Much of the variabilityis due to post-Triassic erosion,which is evident onlyin the UK sector of the basinand which sometimes removes all of the Lunde and part of the Lomvi sediments. This Evolution of the Beryl Basin can be seen on regional seismic sections as well as in the well- The deposits of the Beryl Basin show many features that are logsof this area (Fig. 4, B-B'). The fact that the Statfjord consistent with sedimentation in a half graben where fault ac- Formation and much of the Lower Jurassic sequence are also tivity varied in intensity and character through the Triassic absent from some of the UK sector of the basin, especially in period. the south,is again evidence ofstrong post-Triassic movements in this area. These may have resulted from the up-doming and Gross geometry the sediments associated volcanism knownto have affected the centralNorth of Sea during the Jurassic period (Larsen& Jaarvik 1981; Brown Isopachs of the Triassic interval in Berylthe Basin show a very 1990). complex pattern of variations in sediment thickness (Fig. 5). In contrast with Vollset & Dore (1984), who made three Thiscomplexity arises, at least in part,from post-Triassic subdivisions, the formation hasbeen divided into two members erosion which was probably a function of the Jurassic doming in the Beryl Basin. These have been designated Lower/Middle of the central North Sea and which affected not only the area Lunde and Upper Lunde. Thelower of these two membersis a south of Beryl but also the East Shetland Basin to the north series ofinterbedded fine sandstones,siltstones and shales, (Hallet 1981; Larsen & Jaarvik 1981; Brown 1990). However, with subordinate fine, mottled limestones. The sands are gen- there are several features of the isopach pattern which suggest erallyangular to subangular,poorly sorted and calcite- fault-controlled sedimentation. First, sediment thickness de- cemented. The silts and claystones are both red-brownto grey- creasesrapidly to zero in thevicinity of the East Shetland green, micaceous and slightly calcareous, with some evidence Fault, at least in the northern two thirdsof the basin. Toward of oil staining. The upper memberis dominated by red-brown the south, thick sediments appear to cross thefault as defined to green-mottled claystones which grade into marls. Towards by Jakobbsen et a/. (1980). However, thismerely suggests that the top, there are a few thin beds of siltstone and very fine theirregional structural analysis requires a slight westward sandstone which show some evidence of glauconite. adjustment of the main boundary fault for the Triassic period, At the base of the Lunde Formation, the gamma-log kicks at least here in Beryl. Second, within the basin, sedimentsgen- towards relatively high API values (60-loo), symptomatic of a erally thicken westward towards the fault-zone. Thisis evident generally finer-grained unit.The strongvariability of the log is on a cross-section drawn fromwell log data (Fig. 4, A-A') and afunction of interbedding in the Lower to MiddleLunde on seismic sections (Fig. 6, C-C').

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6OoN

J I

Fig. 5. Isopachs of the Triassic interval in the Beryl Basin. N.B. the fault pattern is that of Jakobbsen et al. (1980); lines other than faults or isopachs are those of seismic sections analysed to supplement well information and so reduce extrapolation in areas where wells are sparce.

c 7100 7000 6900 6800 6700 c'

Fig. 6. Part of regional seismic line CNST-82-10 (Geophysical Company of ) that crosses the rift axis and is located in Fig. 1. Note that the westernmost fault is synthetic to the nearby East Shetland Fault and is, therefore, part of the main boundary structure of the Beryl Basin. The foreset structures are thought to be indicative of delta construction by hanging-wall rivers that debouched into a permanent lake of restricted extent during Lomvi times. Much of the Lunde Formation has been eroded in this part of the Basin as a result of Jurassic uplift.

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TRIASSICSEDIMENTATION, NORTH SEA 19

Notwithstanding these generalized trends, the isopach pat- ? 'I 0 km 50 tern (Fig. 5) is dominated by the exceptionally thick deposits I ? found in the southern central part of the basin. These could I >/- - I indicatethe location of thebasin depocentre, although 4- / thicknesses of over 1600m (double the average found close to I / the border fault both north and south of this area) suggest a I I- / degree of differential subsidence thatis difficult to explain. One cause may be a transfer fault,and this has been postulated here as cutting the basin just to the south of the area of thickest Triassic sediments (Fig. 7). Such a fault would have acted as a focus of greater subsidence in the adjacent sub-basin,so giving an opportunity for localized thickening of the sedimentary fill. The Triassic sequence thins rapidly at the northern margin of the basin. However, towards the south, the pattern is less simple with variations in thicknessthat maybe related eitherto theoblique closure of theembayment by another transfer fault, or to small-scale salt tectonics (since this particular area is underlain by Zechstein halite).Towards the east, up ontothe ramping margin, the sediments consistently become thinner. Any deviation from this general trend can be explained easily by synthetic faults.

Triassic fault patterns Faults thought to have been active during the Triassic period are shown in Fig. 7. The evidence for this pattern lies in: (1) thinning of the sequence over structural highs as seen in the Fig. 7. Reconstruction of faults that were active in the Beryl Basin isopachs and in cross-sections; (2) thickening of the sequence during the Triassic interval using seismic evidence and sedimentary into faults and theaccompanying 'fanning' of the beds as seen inference. on seismic sections(Figs 6 and 8); and (3) theerosion of

Fig. 8. Part of regional seismic line CNST-82-09 (Geophysical Company of Norway), located in Fig. 1. The East Shetland Fault is westernmost and is complicated by a flower structure which has been taken, with other evidence, to indicate the presence of a transfer fault that was active during the Trias. Note the onlap of the Lunde Formation, indicative of the spreading influence of a low-angle bajadalplaya complex and, later, of a permanent lake that resulted from decreasing aridity as the supercontinent drifted northward.

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sediment from footwallblocks as seen,again, in seismic Jurassic sediments are more orless undisturbed (Fig. 6). Fault sections and inferred from therecord of coarse detritus inwell activity has also been inferred where well-logs and cores in- logs and observed in cores. dicatesudden and localizedintroductions of coarse con- Movement on the East Shetland Faultis evident on both the glomeratic debris inknown fault zones (generally during Teist isopach map (Fig. 5) and on seismic section C-C’ shown in times, e.g. well 9/13a-22; Fig. 9). Fig. 6. The footwallblock remained astructural ‘high’ The natureof faultingon the southernmargin of the basin is throughout deposition of the Triassic sequence. The transfer obscure due to a local paucity of data. What is evident is that faults cutting this main boundary fault are inferred either from there was a topographic ‘high’ just to the west of the inter- the recognition of patterns on seismic sections that areusually national Median Line throughout the Triassic period. This is indicative of strike-slip movement(e.g. in section=D’, Fig. 8, interpreted as part of an intra-basinal tilt-block on Fig.7, but the western fault has a positive flower structure), or from the it is possible that the areawas maintained asa basement ‘high’ presence of basement ‘highs’ inN-S seismic sections (not illus- by the basin-oblique transfer faultthat hasbeen inferredin this trated), orfrom the sympatheticdisposition of thicker study. Afurther complicationin thissouthern areacomes from sequences on either side of these ‘highs’ (Fig.5), a pattern that the presenceof the Zechstein salts. Although theseare notvery suggests a number of sub-basins and one which is consistent thick in published wells (<84 m), there is a suggestion in re- with evidencefrom modernrift settings (Rosendahl et al. 1986; gional seismic sections of small-scale salt tectonics which may Frostick & Reid 1987~). have complicated patterns of deposition during the Triassic. Synthetic faults that were active during Triassic times have been drawn onseismic sections where Triassic sequencescan be seen to vary abruptly in thickness, but where the overlying Discussion Models of sedimentation Several models of sedimentation have been developedfor half graben in the early stagesof their development. Theseare based largely on observationsin modemand youngrift systems (Frostick & Reid 1987a, b, 1988; Leeder & Gawthorpe 1987) and differ from earliermodels that are basedlargely on a hypothetical considerationof the changing disposition of axial drainage(Allen 1978; Bridge & Leeder 1979; Alexander & Leeder 1987) which are likely to be more appropriate to later stages of rift development (Frostick & Reid 1990). In general, the early rift modelsgive greater emphasis to the role of ramp drainage thatis aligned orthogonal tothe rift axis.In addition, Leeder & Gawthorpe (1987) have suggested that obsequent streams draining the boundary fault scarp playa significant, if variable,role in controlling thepattern ofsedimentation, although Frostick & Reid (1987~)relegate the contribution made by these foot-wall fans relative to that of lake deposits. Despite any differences between these models,each portrays a systematic variationin sediment characteracross the axisof an active rift.

CrosJ-basin trends in sediment character Structural evidence suggests that the TriassicBeryl Basin was partitioned into threesub-basins by east-west basement ‘highs’ that were associated witha number of transfer faults (Fig.7). If this were the case, thesetopographic barrierswould have exer- cised considerable controlboth over the drainage networkand over the size and disposition of perennial lakes or playas, at least in the early phase of basin development. There are, however, insufficient data to establish any sys- tematic cross-basin variation in the styleof sedimentation dur- ing deposition of the Teist Formation, since only a small num- ber of wells penetrate deep enough, and those that do are awkwardly located. What is evident isthat thislowest Triassic Fig. 9. Part of the core removed from well 9/9-1 at a depth of sequence is very variable in character, both in time and space. 3629m. This lies on the dipslope of one of the intrabasinal The Teist Fm. includes the coarsest sediments of the basin tilt-blocks and is of Teist Formation age. The deposit was laid (well 9/13A-22). However, interestingly, these are not associ- down by streams that were backshed from a fault scarp and is ated with foot-wall alluvial fans. They lie rather on the dip among the coarsest in the Beryl Basin. Pebbles of granitic gneiss slope of a tiltblock andare, therefore, back-shed from a and sedimentary lithics reach 30 mm in diameter, are angular to synthetic fault scarp. In fact, sediments that lie immediately in sub-rounded, and are probably rederived fromthe underlying front of the main boundary fault(well 9/17-1A) are reported to Rotliegende. N.B. The core has a width of c. 1OOmm. be only of coarse sand and granule calibre. As a result, the

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Lomvi Lu nde

ym= 3.20 - 0.01~m, r = -0.88 3.2 - 3.4 1 m

m - 2.8 r = -0.28

DD m 2.4 -

D m

m 2.2 ! I I I I 2.0 l 1 0 20 40 60 20 60 100

60 -0.29 y.= 91.20~~, r = -0.92 60 -

r = 0.09 B 50 m D 40 m 40 m m

m

=m 30 20 -l- 1 20 40 60 20 60 100

40 - ym= 68.97~.~'~~,r = -0.76 80 m D D

'm 20 m

m m 40 B r = 0.28

0 0 I l I 20 40 60 l 20 60 100 1 Distance from the EastShetland Fault, km Fig. 10. Cross-basin patterns of mean bed thickness, percentage of beds that coarsen upward, and shale volume percentage for the Lomvi and Lunde Formations of the Beryl Basin. Distance is calculated up the hanging-wall and away from the East Shetland Fault along the line of Section A-A' (Fig. 1) in the case of the Lunde Fm. and along an extended Section B-B' for the Lomvi Fm.

basinward size-gradation of traditional rift-fill models cannot Information for both the Lomvi and Lunde Formations is be found here in Beryl. Indeed, the diminutive significance of more prolific and shows some interesting cross-basin trends. footwall drainage and the increased importance attributed to Thevariations of threeenvironmentally significant sedi- hanging wall drainage that are embodied in morerecent early- mentary parameters aregiven for both formations aas function phase rift models, appears to make them very appropriate in of cross-basin distance from the East Shetland Faultin Fig. 10. explaining the patterns of sedimentation.However, it has to be These parameters are: (1) average bed shale volume percent- stressed that data for theTeist Fm. are sparse andpalaeogeo- age; (2) percentage of beds which have a coarsening-upward graphical inferences are inevitably extremely tentative. signature; and (3) mean bed thickness. All have been derived

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with the Automatic Bedding Discriminator (Reid et al. be expected to rise abruptly at the distal end of the section 1989). (Fig. 10). In the same part of the drainage system, the inter- Immediately striking is the contrast between the Lomvi and digitation of lake muds and channel sands, common in such the Lunde Formations. InLomvi times, there is evidence of an rift-lake deltas (Frostick & Reid 1986), would ensure higher increase in the amount of shale, in bed thickness and in the shale volume percentages. Since drainage on the footwall dip- percentage of beds coarsening-up when moving down the rampslope is generally backshed away from a basin because of the towards the boundary fault. These patterns are consistent with uplift and doming that accompanies rifting, the fault scarp is those that might be expected of an integratedriver system that rarely a source of large spreads of coarse detritus (Frostick & drains to an enclosed lake from the hanging wall. In its upper Reid1989a, b). Footwall fans tend to be small and do not reaches,channel slopes may have been maintained by penetrate far into a basin. Indeed, the modal radius of 588 spasmodic movements eitheron themain boundary fault or on modernalluvial fans from a variety of physiographic and synthetic faults. As a transport conduit in these upper reaches, structural settings inthe western USA is only 2.2 km, whilethe there would be only limited evidence ofaggradation, and beds maximum is 8 km (Anstey 1965). As a result, in Beryl, it is not would be comparativelythin (Fig. 10). Onthe other hand, surprising that the down-ramp trends in each sedimentary para- becausethis is proximal to theheadwaters, any alluvium meter are not reversed as the boundary fault is approached would be relatively coarse and so the shale volume percentage (Fig. 10). should be low. In moving down the trunk stream,bed thickness Corroborating these patterns is the wedge-like geometry of might be expected to increase and sediment should getfiner (i.e. the Lomvisequence (Fig. 4, B-B'), whichthickens from shale volume percentage should increase). At the bottom end < lOOm to > 200m in an east-west direction down-ramp to- of the system, it would be reasonable to assume that the river wards the fault scarp. In addition, east-westthe seismic section debouched into a lake, bothby analogy with modernrift basins of Fig. 6 shows some evidence of large-scale foresetsat a point insemi-arid settings and because the influenceof thelake wheredelta growth might be expected to havetaken place becomes more and more obvious towards the topof the Lomvi spasmodically during Lomvi times. Fm. Short- and medium-term lake levels in the Triassic will In contrast, the Lunde sediments show no evidence of sys- have been as unstable as they are in present-day environments, tematic changes acrossthe basin. This, combined with the fact where evaporationdominates the hydrologicalregime. In that these are thick fine-grained sequences in which there are thesecircumstances, deltaic progradation over a lake bed thin beds of evaporite, suggests deposition in conditions similar temporarily exposed by falling water-stage might be expected to those inferred for the Upper Triassic sediments of south to have occurred reasonably frequently, and because of this, Wales by Tucker (1977), i.e. broad, low-gradient bajadas with the percentage of beds with a coarsening-up signature might shallow but widespread playa lakes all sitting in a subsiding basin in which there is almost no fault activity.

wafer balance envlronmenal imwpretatm Tectonism or climatic change? 1 In closed rift basins, the effects of fault movement and climatic events of low frequency but high magnitude are often indis- tinguishable and easily confused (Frostick & Reid 1989a, b). 1 Basin wide lake Bothinduce river incision and downstream aggradation of 4 coarse sediment. Both can bring about fluctuations in lake- 3'L stage and thereby influence the positionof the shore-line. Dis- tinguishingbetween climatically and tectonicallydriven i Ephemeral lake (phya) impulses requires integration of sedimentological and struc- tural data and often turns on details of sediment geometry or character. The Triassic sediments of the Beryl Basin show evidence of both tectonism and climatic change. A synopsis of events as they have been interpreted here is given in Fig. 11. The nature of the topography at the close of the Permian is Tectonic activky? uncertain. The Utsira HighiHorda Platform,east of what was (well 9/13-12) to become Beryl, appearsto havebeen subaerial since the Zechstein salts are absent (e.g. in well N2518-1, the Triassic sits unconformably on Devonian sediments). The East Shetland Fault does not appear have to had any topographic expression. Prograding drainage, So, for example, in well 9116-1, 20 km west of the fault align- ephemeral lake ment, the Zechstein evaporites are 60m thick, while at well

- ~~ -~ I 9/17-1A, a couple of kilometres east of the fault, they have a Integrated hanging wall similar thickness i.e. 84m. However, at well 9/13a-22, 30 km drainage east of the fault and movingup onto the Utsira High,they have thinned to 19m, while in the northern part of Beryl, they are absent. The picture that emerges fromthis pattern is of a low- lyingcoastal plain to the east and north, that partially Fig. 11. The natural gamma-ray log of well 9/13a-22 and an encloses a shallow embayment of the Zechstein Sea. interpretation of tectonism and climatic change in the Beryl Basin It is inferred, therefore, that significant movement on the during the Triassic. East Shetland Fault took place at the inception of the Triassic.

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Upper Lunde

Lomvi

Teist

Fig. 12. Cartoons that depict progressive alteration of the Triassic landscape as a function of rifting (Teist) and climatic change during the ensuing phase of thermal subsidence (Lornvi and Lunde).

This is consistent with the pattern ofevents outlined byBadley While the inception of the basin appears to be elusive, the et al. (1 988) in their assessment ofthe Viking Graben asa whole existence of local conglomerateson the dipslopes of intra-basin and concurs with long-held views concerning the inception of tilt-blockshigh in the Teist Fm. (well9113a-22; Fig. 9) has rifting in the NorthSea (Zeigler 1982~;Wood & Barton 1983). been takenas symptomatic of small-scale, syntheticfault However, there is no obvious sedimentary evidencefor tecton- movements. These conglomerates are locally derived, mainly ism, except for the fact that continental conditions prevailed from underlying Rotliegendes deposits which containthe same everywhere in Beryl at this time. The lackof strong sedi- distinctive suite of granitic and gneissic pebbles. The effects of mentary signatures, perhapsin the formof very coarse deposits, these fault movements are also seen in seismic sections, where may be merely because there is only one well sufficiently close small-scale erosional features can be detected on the ramping to the fault scarp to register any influx of sediment that might margins of the tilt-blocks and where sedimentary fills can be have been encouraged by a rapid increase in local relief. A seen to thicken into the faults (Fig. 6). It would seem, there- journey along the scarps of many modern serves as a re- fore, that a topography generating rivers capable of carrying minder of the sporadic nature of obsequent scarp streams and,coarse detritus did develop during the earlyTriassic, but that therefore, of thefact that the probabilityof penetrating a foot- this may have takenplace in several spasms. No major climatic wall fan with a single well may be fairly low. shifts can be inferredfor thisperiod. Throughout thesequence,

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traces of calcite, dolomite and anhydrite, aswell as the general Fm.,eventually blanketing the topography. The same geo- red colour of the sediments combine to suggest continuous morphologicalchange is demonstratedfor basins on the aridity. Horda Platform by Steel & Ryseth (1990), but their explana- The Lomvi Formation shows noevidence of active faulting. tion differs, as we shall see. In Beryl duringUpper Lunde In fact, the likelihoodof an integrated drainagesystem crossing times,the sequence becomes dominated by claystone and the ramping marginof the basin suggests that either the small- marl, and this suggests a further change in the hydrological scale synthetic faults of Teist times had ceased movingor that balance of the basin, shifting towards even greater humidity the rivers had had time to establish courses which could be (Fig. 11). maintained irrespective of minor tectonicdisruption. However, The general fining upwardsof the Triassic sequencemay be there is some difficulty in explaining the sudden change insedi- linked to the exceptionally rapid northward drift of Pangea ment character tl~attook place at the onset of Lomvi times. during this period of Earth history (Zeigler 1982~).The Beryl Almost everywhere throughout Beryl and similarly elsewhere region moved through c. 10" of latitude, taking it from a pos- in the northern North Sea (e.g. in the East Shetland Basin; ition equivalent to, say, present-day Syria to the steppes of the Jones 1991), the confused gamma-ray signature of the Teist northern Caspian Sea region. Besides this drift from latitudes Fm., with its feldspar spikes, gives way abruptly to that of a that weredominated by north-northeasterlytrade winds monotonously clean sand with quartz-like signature. Further blowing from Fenno-Scandinavia (Clemmensen1987) towards north, Steel & Ryseth (1990) suggest that the sands represent those that were influenced by the seasonal southward penetra- thecoalescence ofprograding clastic wedges andthe tionof rain-bearing Westerlies, the increasing humidity of blanketing ofall but the centresof each basin,but this does not Upper Lunde times may alsoreflect a growing maritime influ- explain the sharp change in petrophysical character in Beryl, ence as the sea level began to rise, a rise which anticipated the where streams are not progradingover muds but over coarse- Jurassic transgression (Vail & Todd 1981) as the Boreal Sea grained deposits of Teist age, presumably laid down by the penetrated southward and exploited the topographic oppor- same drainage system. The ubiquity of the change seems to ruletunities offered by the early stagesof continental fragmentation out erosional incision and a sudden erosional exploitation of (R0e & Steel 1985). country rocks of different character since this is unlikely to While we haveinvoked climatic change asmajor a have occurred everywhere and certainly not at approximately determinant of the temporal and spatial changes in sedimen- the same time throughoutBeryl and beyond. Instead, the wide- tary style that occurred through the Triassic period, Steel & spreadnature of thesudden change in sediment character Ryseth (1990) have proposed an alternative mechanism. They suggests a climatic driving force, working perhaps, through a have suggestedthat the controlling factorwas the rate of basin fairly sudden alteration of the vegetation and a shift in the subsidence, and donot mention climate. They draw what they rainfallkunoff ratio. consider to be a set of three megasequences in the Triassic- Leaving this unsolved problem asidefor the time being, the earlyJurassic post-rift succession, largely, butnot exclu- influence of climatic change becomes more obvious towardsthe sively, in basinsnorth of 60"N. It is their contention that each of top of the formation. The hanging-wall river that we have these megasequences starts with fines that were deposited in reconstructed,however tenuously, debouched into alake rapidlysubsiding basins in whichriver sediments remained which periodically spread and retreated in a manner similar to marginal; the fines were superseded by alluvial sands towards thatrecorded in modernlakes that liein semi-aridclosed the top of each sequence as subsidence rate decreasedand the basins under a seasonal rainfall regime (Butzer 1971; Frostick one-time marginal rivers were able to prograde out into the & Reid 1987~).These lake-level fluctuations are recorded in basins. We agree with thebroad pattern of tectonism, thatis a the interbedding of river sands with micaceoussilts and clays, comparatively short rift-phase followed by a long period of deposits which arelikely to have been laid down in the quieter thermalsubsidence. However, the succession inBeryl lacks waters of thelake margin. Towards the top of the Lomvi some ofthe features which have enabled Steel& Ryseth (1990) sequence, these interbeds become more frequent and thicker, to establish their megasequences elsewhere. For instance, the suggesting notonly an exaggeration ofthe transgressive- lowerTeist Fm. is not fine-grained, and its conglomeratic regressivecycle, but also that the lake had becomea more upper part is often much coarser than the succeeding Lomvi persistent feature of the landscape. This is taken as positive Fm. whichis thought by Steel & Ryseth to represent the evidence of ashift in the hydrological balance ofthe basin and culmination of the first coarsening-upward megasequence. In is attributed to a changeof climate. The precipitation of anhyd- addition, there does not appearbe toany gross coarsening-upin rite, dolomite and calcite all suggesta continued degree of the Lunde Fm. that would signify the second megasequence. aridity. However, the presence of carbonaceous debris in some The trend in Beryl is exactly the opposite. wells couldbe interpreted as a symptom of an increase in Subsidence undoubtedly accommodated sedimentation in vegetation and therefore of shift a in the climate towards higher Beryl throughout the Middle and Late Triassic, otherwise the rainfall. great thickness of sediment would not have been accommo- The influence of the lake reaches a maximum in the Lunde dated inwhat appears to havebeen a topography of only Formation and this has implications for bothclimate and tec- moderate relief. But it does not in itself explain the progressive tonism. In the Lower Lunde,the basinwide spread of interbed- expansion of the Lunde lake. This post-Lomvi increasein lake ded fine sands, silts and clays and theabsence of coarsedetritus water volume (whether seasonal or perennial) must have re- both combineto suggest a landscape of subdued topography in sulted from either a change in catchment area through river which fault activity had become negligible or completely ab- capture (unlikely given that neighbouringbasins show the sent. The intrabasinal fault blocks of the Teist had been par- same trend towards widespread sedimentation of fine-grained tially eroded during Lomvi times and then swamped by the material and such drainage diversion would have caused lake expanding lake or playa of the Lunde Fm. This picture of shrinkage rather than growth in those basins that had been landscapeevolution is corroborated by the seismic sections robbed), or aregional change in water-balancei.e. a change in (e.g. D-D', Fig. 8) where the Lunde Fm. onlaps the Lomvi climate leading to higher runoff.

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Conclusions extensional modes of basin formation. Journal of the Geological Society, London, 145,455472. The sedimentary fill of the Beryl basin is consistent with that BERTELSEN,F. 1974. Triassic palynology and stratigraphy of some Danish North expected of continental rifts developing in areas undergoing Sea boreholes. Danish Geological Survey Report 17-32. BRENNAND, T.1975. P. The Triassic ofthe NorthSea. In: WOODLAND,A. W. (ed.) activelithospheric extension, and McKenzie’s (1978) struc- Petroleum and the Continental Shelfof North- West Europe.The Institute of tural modelprovides an apt basis for understanding the Petroleum, London, 295-312. sequence of events. The main boundary fault lay on the west -, VAN HOORN,B. & JAMES,K. H.1990. Historical review of North Sea side of the basin throughout the Triassic and was probably a exploration. In: GLENNIE,K. W. (ed.) Introduction to thepetroleumgeologyof southward continuationof the East Shetland Fault. A phaseof the North Sea. Blackwell Scientific Publications, Oxford, 3rd Edition, 1-33. BRIDGE,J. S. & LEEDER,M. R. 1979. A simulation model of alluvial stratigra- fault activity during the early Triassic (Teist) produced a half phy. Sedimentology, 26, 617444. graben in which local drainage systems developed. Some of BROWN,S. 1990. Jurassic. In: GLENNIE,K. W. (ed.). Introduction to thepetroleum these were subsequently rejuvenated by synthetic fault move- geology of the NorthSea. Blackwell Scientific Publications, Oxford, 3rd ment and the consequent rotation of intra-basinal tilt-blocks Edition, 219-254. BUTZER,K. W. 1971. Recent history of an Ethiopian delta. University of Chicago (Fig. 12, bottom cartoon). Thebasin as awhole was segmented Research Papers in Geography, 136. by a series of east-west transfer faults which preventedthe inte- CLEMMENSEN,L. B. 1987. Complex star dunes and associated aeolian bedforms, gration of axial drainage. This earlyrift phase was followed by Hopeman Sandstone (Permo-Triassic), Moray Firth Basin, . In: a much longer post-rift phase (Lomvi-Lunde) in which fault FROSTICK,L. E. & REID, I. (eds.). Desertsediments: ancient and modern. Geological Society, London, Special Publication, 35, 213-231. activity was absent and thermal subsidence controlled struc- DEEGAN, C.& E. SCULL, B. J. 1977. A standardlithostratigraphicnomenclature for tural development. At first, during Lomvi times, the drainage the Central andNorthern North Sea. Report No. 77125 Institute of Geo- systemwas able to integrateon the hanging wall (Fig. 12, logical Survey HMSO, London. middle cartoon). However, any intra-rift topography was des- DONATO,l. A. & TULLY,M. C. 1982. A proposed granite batholith along the western flank of the North Sea Viking Graben. Geophysical Journal ofthe tined to be swamped during Lunde times, first, by the fine- Royal Astronomical Society, 69, 187-96. grainedsediments of a growing bajada/playa-lake complex, FISHER, M.J. & MUDCE, D. C.1990. Triassic. In: GLENNIE,K. W. (ed.). Introduc- and subsequently by the even finer sediments of a perennial tion to the petroleum . Blackwell Scientific Publica- lake (Fig. 12, top cartoon). This overall pattern of upwards- tions, Oxford, 3rd Edition, 191-218. fining is attributed to both progressive a reduction inrelief and FROSTICK,L. & REID,I. 1986. Evolution and sedimentary character of lake deltas fed by ephemeral rivers in the Lake Turkana Basin. In: FROSTICK,L. E., a shift towardsmore humid climate as the area drifted RENAUT, R., REID,& I. TIERCELIN,l. J. (eds) Sedimentation in the African northward. Rifts. Geological Society, London, Special Publication, 25, 113-25. Roberts et al. (1989) have suggested that phases of fault -& -1987a. Tectonic controlof desert sediments in rift basins ancient and activity followed by thermal subsidence are characteristic of modern. In: FROSTICK,L. E. & REID,I. (eds) Desert sediments: ancient and modern. Geological Society, London, Special Publication, 35, 53-68. the northern NorthSea rift basins and that thesame pattern is - & - 19876. A new look at rifts. Geology Today, 3, 122-126. repeated as a result of successive bouts of extension in both -&- 1989~.Climaticversus tectoniccontrolsoffansequences: lessonsfrom Triassic and post-Triassic times. the Dead Sea, Israel. Journalof the GeologicalSociety, London, 146,527-538. In fact, Triassic basinsin adjacent provinces ofNW Europe -& -19896. Is structure the main control of river drainage andsedimenta- show temporal patterns of development that are remarkably tion in rifts? Journal ofAfrican Earth Sciences, 8, 165-182. -&- 1990. Structural control of sedimentation patterns andimplications similar. For example, the Somerset and Moray Firth basins, for the economic potential of the East African rift basins. Journal African among others, show evidence of fault activity early in their Earth Sciences, 10, 307-318. development, followed by a long phase of subsidence. How- ~~ . , JARVIS,J. & EARDLEY, H.1988, Triassic sediments of the Inner Moray Firth, Scotland:Early rift deposits. Journalof the Geological Society, ever, as in Beryl, the environment in these neighbouring parts London, 145, 235-248. of the supercontinent also appears to have become wetter in GLENNIE,K. W. (ed.) 1990. Introduction to thepetroleumgeology of the North Sea. Middle and Upper Triassic times (Jones 1991; Frostick et a1 Blackwell Scientific Publications, Oxford, 3rd Edition. 1988). HALLETT,D. 1981. Refinement of the geological model of the Thistle Field. In: ILLING,L. V. & HOBSON,G. D. (eds) Petroleum Geology of the Continental She!fofNorth west Europe. The Institute of Petroleum, London, 315-325. 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Received 30 November 1990; revised typescript accepted 8 August 1991

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