INTERNAL DOCUMENT
IIL
THE DEVELOPMENT OF MESOZOIC SEDIMENTARY IBASINS AROUND THE MARGINS OF THE NORTH ATLANTIC
AND THEIR HYDROCARBON POTENTIAL D.G. Masson and P.R. Miles
Internal Document No. 195
December 1983
[This document should not be cited in a published bibliography, and is supplied for the use of the recipient only].
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THE DEVELOPMENT OF MESOZOIC SEDIMENTARY BASINS AROUND THE MARGINS OF THE NORTH ATLANTIC
AND THEIR HYDROCARBON POTENTIAL D.G. Masson and P.R. Miles
Internal Document No. 195
December 1983
Work c a r r i e d out under contract to the Department of Energy
This document should not be c i t e d i n a published bibliography except as 'personal communication', and is supplied f o r the use of the r e c i p i e n t only.
I n s t i t u t e of Oceanographic Sciences, Brook Road,
Wormley,
ABSTRACT
The d i s t r i b u t i o n of Mesozoic basins around the margins of the North A t l a n t i c has been summarised, and i s discussed in terms of a new e a r l i e s t Cretaceous palaeogeographic reconstruction f o r the North A t l a n t i c area. The Late Triassic-Early Jurassic r i f t basins of
I b e r i a , offshore eastern Canada and the continental s h e l f of western Europe are seen to be the fragments o f a formerly coherent NE trending r i f t system which probably formed as a r e s u l t of tensional stress between Europe, Africa and North America. The separation of Europe, North America and Iberia was preceded by a Late Jurassic-Early Cretaceous r i f t i n g phase which i s c l e a r l y d i s t i n c t from the e a r l i e r Mesozoic
r i f t i n g episode and was l i t t l e influenced by i t . The two periods of r i f t i n g are separated by a middle Jurassic t e c t o n i c a l l y quiet period. The d i s t r i b u t i o n of hydrocarbon finds w i t h i n the r i f t basin suggests that the best prospects are i n areas where the two r i f t i n g episodes are
superimposed. Lack of mature source rocks i n the l a t e r Mesozoic r i f t basins and an unfavourable temporal r e l a t i o n s h i p between hydrocarbon generation and tectonic a c t i v i t y i n the early Mesozoic basins are
INTRODUCTION
A number of authors have proposed that the evolution of the North A t l a n t i c Ocean north of the Newfoundland Ridge - Azores - Gibraltar boundary began with a Late Triassic r i f t i n g phase (e.g. Pegrum and Mounteney, 1978; Montadert et a l . , 1979; Boil l o t et a l . , 1979a, b). This is based p r i m a r i l y on the i d e n t i f i c a t i o n of red beds and evaporites of Late Triassic and Early Jurassic age in r i f t basins on the Grand Banks and along the western margins of Europe (Jansa and Wade, 1975; BRGM et a l . , 1974; Wilson, 1975; Jansa et a l . , 1980; Naylor and Shannon, 1982, Z i e g l e r , 1982). This r i f t i n g phase was followed by a period of r e l a t i v e quiescence before the main Late Jurassic and
Early Cretaceous r i f t i n g which preceded the i n i t i a t i o n o f sea-floor spreading between North America, Europe and Iberia (e.g. B o i l l o t et a l . , 1979a, b; Montadert et a l . , 1974, 1979; Roberts et a l . , 1981; Masson et a l . , i n press).
The purpose of t h i s paper i s to compare the d i s t r i b u t i o n of the Late Triassic r i f t basins with those formed during the Late Jurassic and Early Cretaceous, to assess the r e l a t i o n s h i p between these r i f t i n g
episodes and the Mesozoic sea f l o o r spreading h i s t o r y of the North A t l a n t i c , and to evaluate the c o r r e l a t i o n between basin age and hydrocarbon p o t e n t i a l . This has been achieved by p l o t t i n g the relevant geological data on a
revised North A t l a n t i c reconstruction. Note t h a t we are discussing r i f t basins only and t h a t we are not implying t h a t l a t e r a l connections between these basins, i n the form of epicontinental seas, did not e x i s t .
THE NORTH ATLANTIC RECONSTRUCTION
The reconstruction used in t h i s paper (Fig. 1) concentrates on the r e l a t i v e palaeopositions of the European, Iberian and North American plates. I t i s based on the magnetic anomaly i d e n t i f i c a t i o n of
Kristoffersen (1978), augmented by a considerable volume of both published and unpublished magnetic data held by the I n s t i t u t e of Oceanographic Sciences. P a r t i c u l a r a t t e n t i o n has been devoted to the i d e n t i f i c a t i o n of an ocean-current boundary (OCB) at the margin of each continental block, i n order to
constrain, as f a r as possible, the reconstruction to the 'time of i n i t i a l opening'. A f u l l description of t h i s reconstruction w i l l be published
-elsewhere (Masson and Miles, i n press; Miles and Masson, i n p r e p . ) .
The oldest magnetic l i n e a t i o n which can be c l e a r l y mapped i n the North A t l a n t i c i s anomaly 34 ( K r i s t o f f e r s e n , 1978) although some
doubt exists as to i t s i d e n t i f i c a t i o n i n the Bay of Biscay. Reconstruction to anomaly 34 time (84 Ma) i s r e l a t i v e l y s t r a i g h t f o r w a r d ; beyond
84 Ma to the 'time of i n i t i a l opening' the reconstruction depends on OCB i d e n t i f i c a t i o n , with l i m i t e d supplementary geological and geophysical constraints. For Europe, the QCB i s well-defined i n the Bay of Biscay and west of Goban Spur (Montadert et a l . , 1979; Masson et a l . , i n press). For North America, i t i s also reasonably constrained from Flemish Cap northwards (IDS unpublished data). This allows an accurate North
America - Europe reconstruction which i s also constrained by the proposed spreading d i r e c t i o n in the southern Rockall Trough (Roberts et a l . ,
1981).
To f i n d the i n i t i a l position of I b e r i a , i t i s f i r s t necessary to compensate f o r the T e r t i a r y convergence between Iberia and Europe which gives r i s e to the overlap between Iberia and North America on the
anomaly 33 reconstruction of K r i s t o f f e r s o n (1978) (Grimaud et a l . , 1982). This was achieved by a 6.4° clockwise r o t a t i o n of I b e r i a around a pole at 34J°N, 19|°W (Masson and Miles, i n press; Grimaud et a l . , 1983). From i t s p o s i t i o n at anomaly 34 time, defined p r i m a r i l y on the basis of North A t l a n t i c anomaly 34 coincidence, Iberia was then r o t a t e d 32° clockwise to close the Bay of Biscay and the Iberian-Newfoundland Basin (Masson and Miles, i n press; Miles and Masson, i n p r e p . ) . This r o t a t i o n was c o n t r o l l e d by the well-defined OCB i n the northern Bay of Biscay
(Montadert et a l . , 1979), the OCB i n the Newfoundland Basin proposed by Sullivan (1983) and a magnetic anomaly lineament west of I b e r i a which i s also l i k e l y to mark the p o s i t i o n of the OCB (Masson and Miles, i n press). Note t h a t a 32° clockwise r o t a t i o n also s a t i s f i e s the palaeomagnetic evidence (Pitman and Talwani, 1972).
The new reconstruction places Galicia Bank east of Flemish Cap, rather than to the south of i t as suggested by, f o r example,
of the North A t l a n t i c region (e.g. Le Pi'chon et a l . , 1977; Lefort et a l . , 1980). Le Pichon et a l . (1977) have suggested that t h i s area might
be composed of e i t h e r older (?Hercynian) ocean f l o o r or thinned and subsided continental crust. The width of the 'gap' might also be lessened i f allowance were made f o r a spreading centre propagating from the south during the l a t e Lower Cretaceous, as suggested by Sullivan (1983) and Masson and Miles ( i n press).
LATE TRIASSIC-EARLY JURASSIC RIFT BASINS
R i f t basins containing a c h a r a c t e r i s t i c redbed and evaporite sequence of Triassic and Lower Jurassic age (Jansa et a l . , 1980) have been i d e n t i f i e d on the Grand Banks (Jansa and Wade, 1975), west and south-west of the B r i t i s h Isles (Pegrum and Mounteney,
1978; Reeves et a l . , 1978; Kammerling, 1979; Naylor and Shannon, 1982; Z i e g l e r , 1982), i n western Portugal (Wilson, 1975) and i n south-west France (Winnock, 1971; BRGM et a l . , 1974) (Fig. 2). Some authors have tended to label the redbed sequence "Permo-Triassic" following c l a s s i c a l B r i t i s h s t r a t i g r a p h i c nomenclature (e.g. Evans et a l . , 1981; Naylor and Shannon, 1982) but there are, as y e t , no published
examples of unequivocal Permian dates, and the formation of the r i f t basins probably began in Triassic times as suggested by Pegrum and Mounteney (1978) and Jansa et al. (1980). Redbed and evaporite deposition was followed, i n the Early and Middle Jurassic, by normal marine sedimentation with deposition of shales, carbonates and sand-stones (Pegrum and Mounteney, 1978).
Superposition of the d i s t r i b u t i o n of these 'Lower Mesozoic' basins on our revised reconstruction c l e a r l y demonstrates t h a t the scattered basins are the components of a formerly coherent r i f t system. The Lusitanian Basin (Fig. 2 ) , traced offshore by Rehault and Mauffret (1979), is continuous to the north-east with the Western Approaches Basin (Figs. 2, 3). This c o r r e l a t i o n i s enhanced by the occurrence of a t h i c k p r e - l a t e s t Jurassic sediment sequence i n the v i c i n i t y of DSDP s i t e s 400 and 401 on the North Biscay margin (Montadert et a l . , 1979; Fig. 2). S i m i l a r l y , the Porcupine Basin (Fig. 2) is a northeastward continuation of the basin complex on the Grand Banks. In t h i s case,
the c o r r e l a t i o n is made through one or possibly two previously undescribed
Lower Mesozoic basins i n the region between Flemish Cap and Orphan Knoll (Fig. 2).
-5-These have been i d e n t i f i e d from seismic r e f l e c t i o n data by comparison with s i m i l a r data across known Lower Mesozoic basins on the Grand Banks
(L.M. Parson, pers. comm).
However, there is l i t t l e evidence f o r a south-westerly continuation
of Triassic or Lower Jurassic age to the B r i s t o l Channel - C e l t i c Sea - Fastnet basin complex (Fig. 2). Published maps of t h i s basin complex (e.g.
Pegrum and Mounteney, 1978; Robinson e t . a l . , 1981; Naylor and Shannon, 1982) show t h a t i t does not extend to the s h e l f edge south-west of Ireland.
B o i l l o t et a l . (1979a) have speculatively interpreted a "lower Mesozoic" basin west and south-west of Galicia Bank, d i r e c t l y to the south-west of the Fastnet Basin. However, t h i s speculation is not confirmed by dredge data which seems to indicate that Late Jurassic rocks d i r e c t l y o v e r l i e Palaeozoic basement ( B o i l l o t et a l . , 1979a).
The south-west truncation of the C e l t i c Sea-Fastnet Basin complex appears to be controlled by north-westerly trending s t r i k e - s l i p f a u l t s (Robinson et a l . , 1981). This s t r i k e - s l i p d i r e c t i o n i s c l e a r l y important i n the Early Mesozoic evolution of the North A t l a n t i c area ( L e f o r t , 1980 and i n press) and has been detected in the Porcunine Basin (Lefort and Max, 1983), B r i s t o l Channel Basin (Kammerling, 1979), Kish Bank Basin
(Jenner, 1981) and Western Approaches Basin ( Z i e g l e r , 1982), as well as in the C e l t i c Sea-Fastnet Basin complex. I t might also be invoked to explain the configuration of the offshore extension o f the Lusitanian Basin as proposed by Rehault and Mauffret (1979), although B o i l l o t et a l .
(1979) preferred a NE-SW o f f s e t d i r e c t i o n i n t h i s area. I t i s not clear to what extent such an o f f s e t d i r e c t i o n occurs i n the Grand Banks area.
Lefort (1980 and i n press) has noted the possible association between the NW-SE s t r i k e - s l i p d i r e c t i o n and the postulated Biscay-Labrador transform f a u l t which Laughtcn (1972) considered to be an important element in the early h i s t o r y of the North A t l a n t i c . However, there is no d i r e c t evidence f o r a Biscay-Labrador f a u l t and i t s occurrence need only be postulated i f the r i f t i n g and possible s e a - f l o o r spreading i n the Rockall Trough are older than the r i f t i n g between Goban Spur and Flemish Cap (Fig. 1), which is now known to be l a t e Lower Cretaceous i n age
Late Cretaceous (Roberts et a1., 1981) although no conclusive evidence has yet been produced. We note that the Rockall Trough is p a r a l l e l to other Lower Mesozoic r i f t basins in the North A t l a n t i c area, and that the NW trending transform d i r e c t i o n proposed by Roberts et a l . , (1981) i n the southern Rockall Trough is p a r a l l e l to the NW s t r i k e - s l i p d i r e c t i o n observed in other basins on the western European s h e l f .
However, Ridd (1981) has shown that an important r i f t i n g event west of the Shetland Isles ended around the end of the Lower Cretaceous. This i s compatible with Lower Cretaceous r i f t i n g and possible Albian to e a r l y Late Cretaceous sea-floor spreading in Rockall Trough, and suggests that t h i s area had a s i m i l a r h i s t o r y to that o f Goban Spur
(Masson et a l . , in press). Accordingly, we cannot support the
proposed Biscay-Labrador transform f a u l t although i t i s c l e a r , at least
on the European continental s h e l f , that a pervasive, NW trending, predominantly s i n i s t r a l s t r i k e - s l i p f a u l t system was operating during the formation
of the Lower Mesozoic basins. The magnitude of these s t r i k e - s l i p motions appears to have been small, with the main e f f e c t being the t r a n s f e r
of extensional movement between basins. For example, the south-west truncation of the C e l t i c Sea and Fastnet Basins might be compensated f o r by the apparent southward increase in extension i n the Porcupine Basin (Buckley and Bailey, 1975). North-west trending s t r i k e - s l i p f a u l t s are c l e a r l y documented i n t h i s area (Fig. 2; Robinson et a l . , 1981; Lefort and Max, 1983).
The remaining areas of doubt concerning the d i s t r i b u t i o n of Lower Mesozoic basins in the North A t l a n t i c area a l l occur along the margins of I b e r i a . To the north, i t has been proposed by Winnock (1971)
and Montadert et a l . (1974, 1979) t h a t the proto-Bay of Biscay area was the s i t e of a Triassic r i f t system, l i n k i n g the Aquitaine Basin with the Trevelyan Escarpment region where a t h i c k p r e - l a t e Jurassic sediment sequence is known (Fig. 2). However, our Lower Mesozoic reconstruction (Fig. 3) shows the Trevelyan Escarpment area to be part of a NE-SW trending r i f t system at t h i s time. Furthermore, a NE-SW r i f t axis has also been proposed i n the Aquitaine Basin during the Late T r i a s s i c (Biju-Duval et a l . , 1977). We suggest, therefore, that two d i s t i n c t NE-SW trending basins crossed the proto Bay of Biscay area during the Lower Mesozoic and that a deep E-W trending r i f t basin was not formed u n t i l the Late Jurassic-Early Cretaceous, as is documented
-7-onshore i n the Parentis Basin (Dardel and Rosset, 1971) which we propose is the true onshore extension of the E-W trending Bay of Biscay
r i f t - s y s t e m (Fig. 4), Further evidence f o r a Late Jurassic-Early Cretaceous age f o r the r i f t i n g in Biscay comes from the Armorican marginal basin,
the deeper sediment f i l l of which is "probably mainly o f Lower Cretaceous age" (Montadert et a l . , 1974). I t i s not c l e a r , however, whether
a shallow NE-SW seaway connected the Aquitaine Basin with the Western Approaches/Lusitanian Basin complex during the Lower Mesozoic or whether t h i s area was the s i t e of a NE trending basement ridge l i n k i n g the
Hercynian massifs of Iberia and B r i t t a n y (Fig. 3).
The largest area of uncertainty in the Lower Mesozoic reconstruction is the apparent 'gap' between Iberia and Newfoundland. Various authors have interpreted t h i s region, which includes the Tagus Abyssal Plain, as being underlain by thinned and subsided continental crust (e.g. Le Pichon et a l . , 1977; S u l l i v a n , 1983), but there i s no data r e l a t i n g to the d i s t r i b u t i o n of possible Lower Mesozoic sediments. As discussed previously, the present authors are sceptical regarding evidence f o r possible Lower Mesozoic sediments west and south-west o f Galicia Bank ( B o i l l o t et a l . , 1979a). A f u l l description of the plate tectonic
h i s t o r y of t h i s region w i l l be presented elsewhere (Masson and Miles, in press).
MIDDLE JURASSIC QUITE PERIOD
Several authors who have worked i n the Aquitaine Basin and around the margins of Iberia note t h a t the Late T r i a s s i c - E a r l y Jurassic and Late Jurassic-Early Cretaceous periods of active r i f t i n g are separated by a
'middle Jurassic' t e c t o n i c a l l y inactive period, although t h i s term is not well-defined (Winnock, 1971; BRGM, 1974; B o i l l o t et a l . , 1979a; Montadert et al . , 1979; Roberts et a l . , 1981).
The middle Jurassic tectonic quiet period i s best seen i n the
Aquitaine Basin where a t h i n , uniform succession of shallow water carbonates was l a i d down between lower Bathonian and lower Kimmeridgian times
Evidence f o r a middle Jurassic quiet period on the northern margin of the Bay of Biscay (Montadert et a l . , 1979; Roberts et a l . , 1981) appears to be based e n t i r e l y on a westward extrapolation of the Aquitaine Basin data. However, since we have shown that the Aquitaine Basin and the Bay of Biscay may not have formed a continuous r i f t basin p r i o r to the Late Jurassic (Figs. 2, 3) the v a l i d i t y of t h i s e x t r a p o l a t i o n is . obviously suspect.
A large volume of geological data i s available concerning the basins on the continental shelf south-west of the B r i t i s h I s l e s where i t is clear that the Middle Jurassic saw a slowing down of basin-forming tectonic a c t i v i t y over most of the area. In the B r i s t o l Channel and Fastnet Basins, t h i c k Triassic and Lower to lower Middle Jurassic sediments are followed by the patchy development of a regressive Upper Jurassic facies (Kammerling, 1979; Robinson et a l . , 1981). S i m i l a r l y , in the C e l t i c Sea Basin, an Upper Jurassic section including "appreciable q u a n t i t i e s " of non-marine sediments unconformably o v e r l i e s a middle
Jurassic sequence (Naylor and Shannon, 1982). In the Porcupine Bank, and perhaps f u r t h e r a f i e l d at DSDP Site 111 on Orphan K n o l l , Middle Jurassic sediments o f continental facies may also represent a regressive facies which corresponds to a cessation of f a u l t - c o n t r o l l e d basin
subsidence ( Z i e g l e r , 1982; Laughton, Berggren et a l . , 1972).
In contrast to the above, the l a t t e r part of the Middle Jurassic and the early Late Jurassic was a period of major transgression i n the Scotian Basin, to the south-west of the Grand Banks (Jansa and Wade, 1975; Fig. 3). These authors also suggested that transgression affected the eastern
Grand Banks, even though an i n f l u x of c l a s t i c material was observed at t h i s time; they explained the c l a s t i c input by proposing u p l i f t of the source area. However, we would propose that the c l a s t i c i n f l u x i s yet f u r t h e r evidence f o r a Middle Jurassic regression which apparently
affected the marine basins i n the North A t l a n t i c north of the Newfoundland,
Azores-Gibraltar boundary, although t h i s regression was apparently contemporaneous with a transgression i n the Scotian Basin.
-9-LATE JURASSIC-EARLY CRETACEOUS RIFT BASINS
The main phase of r i f t i n g which preceded the i n i t i a t i o n of sea-floor spreading i n the North A t l a n t i c north of the Newfoundland-Azores
Fracture Zone i s widely believed to have occurred during the Late Jurassic and Early Cretaceous (Montadert et a l . , 1974, 1979; B o i l l o t et a l . ,
1979a, b; Group Gal i c e , 1979; Roberts et a l . , 1981; Masson et a l . , i n press). Examination of the d i s t r i b u t i o n of the Late Jurassic-Early Cretaceous
r i f t basins, hereafter referred to as the Late Mesozoic basins, indicates that they occur almost exclusively on the present North A t l a n t i c continental margins, and that in general they have margin p a r a l l e l trends. This
is i n marked contrast to the consistent n o r t h - e a s t e r l y trend of the
Lower Mesozoic basins, which are thus often cross-cut by the l a t e r continental margins.
The l a t e r Mesozoic evolution of the margins of I b e r i a and the Bay of Biscay, including the Lusitanian and Aquitaine Basins, has been studied by Dardel and Rosset (1971), Montadert et a l . (1974, 1979); Wilson (1975); B o i l l o t et a l . (1979a, b) and Group Gal ice (1979). An important r i f t i n g phase appears to have begun i n the Oxfordian or Kimmeridgian and to have terminated i n the Aptian. Thick Lower Cretaceous s y n - r i f t sections may be present (e.g. Dardel and Rosset, 1979; Montadert, 1974; Group Galice, 1979). On the opposing Grand Banks margin, i t i s clear that an important tectonic rejuvenation occurred during the Late Jurassic-Early Cretaceous
(Jansa and Wade, 1975) although l i t t l e detailed"geological data i s available because a major erosional episode (Avalon Unconformity, Jansa and Wade, 1975) occurred on the Grand Banks shelf during the Early Cretaceous, and the deeper water areas of the margin are deeply buried under l a t e r
sediments. Fault controlled sedimentation during the Early Cretaceous i s however known from Flemish Pass (Fig. 4, McKenzie, 1981).
Further north, the main r i f t i n g phase between Goban Spur and Flemish Cap began somewhat l a t e r in Hauterivian or e a r l i e s t Barremian time (Leg 80 S c i e n t i f i c Party, 1982; Masson et a l . , i n press). I t i s not known to what degree t h i s r i f t i n g episode affected the Porcupine Seabight or the Rockall Trough. A published cross-section through the Porcupine Seabight ( Z i e g l e r , 1982) suggests that the most intense period of f a u l t i n g occurred w i t h i n the "Permo-Triassic" and Jurassic, but t h a t some f a u l t s remained active into the Early Cretaceous. In the Labrador Sea, e a r l i e s t
deposited i n f a u l t bounded basins (Umpleby, 1979), i n d i c a t i n g a l a t e Early Cretaceous r i f t i n g phase.
In d e t a i l , therefore, two d i s t i n c t Late Mesozoic r i f t systems can be recognised; one between Iberia and Europe/North America which began to form i n the Late Jurassic, and a second, between Europe and North America which was not i n i t i a t e d u n t i l the Early Cretaceous. This is l i k e l y to be related to the episodic northwards propagation of North A t l a n t i c sea-floor spreading, which may have begun as e a r l y as the Hauterivan immediately north of the Newfoundland-Azores Fracture Zone and which had propagated into the Bay of Biscay by the Upper Aptian (Masson and Miles, in press. Fig. 5). A new spreading axis developed between Goban Spur and Flemish Cap during the Albian (Leg 80
S c i e n t i f i c Party, 1982; Masson et a l , i n press) - t h i s may have penetrated northwards i n t o Rockall Trough (Roberts et a l . , 1981). Yet another change i n the spreading configuration occurred towards the end of the Cretaceous, when a spreading axis was i n i t i a t e d i n the Labrador Sea (Srivastava, 1978).
DISCUSSION
The evidence c i t e d in the previous sections c l e a r l y shows that the pre-seafloor spreading h i s t o r y of the proto-North A t l a n t i c area during the Mesozoic can be divided into three main phases, namely a Late
Triassic-Early Jurassic r i f t i n g phase, a 'Middle' Jurassic quite period and a Late Jurassic-Early Cretaceous r i f t i n g phase. The second r i f t i n g phase may i t s e l f be s u b - d i v i s i b l e i n t o a Late Jurassic-Early Cretaceous phase which r i f t e d Iberia from Europe/North America and an Early
Cretaceous phase which r i f t e d Europe from North America.
On the western side of the A t l a n t i c , geological s i m i l a r i t i e s between the Scotian Basin and the basin complex on the Grand Banks c l e a r l y indicate that the Lower Mesozoic tensional episode is r e l a t e d to the i n i t i a l r i f t i n g between A f r i c a and North America (Jansa and Wade, 1975). When plotted on our reconstruction, the obviously coherent nature of the e n t i r e north-easterly trending Lower Mesozoic r i f t system strongly implies that the e f f e c t s of the tensional episode which eventually resulted in the separation of A f r i c a and North America extended at least as f a r north as the l a t i t u d e of northern B r i t a i n . We note t h a t
-the north-westerly 'transform' d i r e c t i o n , p a r t i c u l a r l y well-displayed o f f south-west B r i t i a n , ( F i g . 3) is p a r a l l e l to the i n i t i a l d i r e c t i o n of opening between A f r i c a and North America as defined by
Le Pichon et a l . (1977) (Fig. 4). I t is not e n t i r e l y clear how the Aquitaine Basin f i t s i n t o t h i s overall p i c t u r e , since i t s formation i s l i k e l y to have been affected by tectonic events in the Tethys as well as i n the proto-North A t l a n t i c r i f t system (Biju-Duval et a l . , 1977; Z i e g l e r , 1982).
The change to a less active tectonic regime which occurred j u s t p r i o r to or during the e a r l y part of the Middle Jurassic apparently correlates with the separation of A f r i c a and North America, which can be dated between Sinemurian and Bajocian time (e.g. Pitman and Talwani, 1972; Jansa and Wade, 1975; Vogt and Einwich. 1979; Hinz et a l . , 1982). Lateral movement between Europe/North America and North A f r i c a was accommodated by a transform f a u l t along the southern margin of the Grand Banks (Auzende et a l . , 1970; Le Pichon et a l . , 1977) and tensional movements north of t h i s transform apparently ceased.
The regressive sediments deposited in many of the Early Mesozoic basins during the Middle and early Late Jurassic r e f l e c t the cessation of r i f t i n g , probably because decreasing subsidence rates permitted f i l l i n g of the r i f t basins giving r i s e to shallow marine or even non-marine sedimentation.
During the Late Jurassic-Early Cretaceous, a new phase of r i f t i n g was i n i t i a t e d , f i r s t l y between Iberia and Europe/North America and
l a t e r between Europe and North America. Only i n the south, between Iberia and the Grand Banks, did these r i f t s f o l l o w the Early Mesozoic northeasterly trend. Further north, WNW-ESE trends predominated i n the Bay of Biscay region, with NW-SE trends in the European-North American r i f t zone. Indeed, the Early Mesozoic r i f t basins had remarkably l i t t l e influence on the l a t e r continental break-up.
HYDROCARBON POTENTIAL
Much of the area under discussion can be regarded as ' f r o n t i e r
explored, although scattered hydrocarbon occurrences are known from the Labrador Sea i n the north-west to the Parentis Basin i n the south-east (Fig. 6 ) .
The Parentis Basin, where exploration commenced as e a r l y as 1951, i s as y e t the only o i l producing basin i n the study area (Dardel and Rosset, 1971). However, commercial q u a n t i t i e s of o i l have
r e c e n t l y been discovered i n the Hibernia F i e l d t o the east of Newfoundland (McKenzie, 1981) and i n the Porcupine'Basin west of I r e l a n d (Naylor
and Shannon, 1982), and as y e t unevaluated o i l f i n d s have been made i n the Asturian Basin o f f north-west Spain and i n the C e l t i c Sea basin (Soler et a l . , 1981; Naylor et a l . , 1982. N o r o i l , 1983). Commercial gas f i e l d s have also been discovered i n the C e l t i c Sea and Northern I r i s h Sea Basins (Colley et a l . , 1981; Ebbern, 1981) w h i l e the Labrador Shelf Basin may also be a p o t e n t i a l gas producer (Umpleby, 1979).
Despite the successes l i s t e d above, a very large number of dry
holes have also been d r i l l e d , p a r t i c u l a r l y i n the Lower Mesozoic basins south-west of the B r i t i s h I s l e s and, with the exception o f the
Hibernian area (Fig. 6) i n the Lower Mesozoic basins on Grand Banks. A comparison between the d i s t r i b u t i o n of the various r i f t basins, t h e i r h i s t o r y of development, and the success or f a i l u r e o f hydrocarbon
e x p l o r a t i o n i s discussed below (Fig. 6 ) .
The most obvious point to emerge from f i g u r e ' 6 i s t h a t most of the s i g n i f i c a n t o i l discoveries to date occur i n areas of Lower Mesozoic r i f t basins which have been involved i n the l a t e r Mesozoic r i f t i n g event. A l l these areas have a h i s t o r y of sedimentation beginning
i n the T r i a s s i c and extending through to the T e r t i a r y , and are the
s i t e s of some of the t h i c k e s t sedimentary accumulations on North A t l a n t i c margins. For example, over 15 km of sediment may occur i n the area
of the Hibernia f i e l d (McKenzie, 1981) over 5 km i n the Porcupine Seabight (Roberts et a l . , 1981; Naylor and Shannon, 1982) and over 8 km i n
the Parentis Basin (Dardel and Rosset, 1971). In c o n t r a s t , the m a j o r i t y o f the areas involved only i n the l a t e r Mesozoic r i f t i n g phase, i . e . those areas underlying the present day continental slopes, are generally
sediment starved (e.g. Roberts et a l . , 1981), although exceptions
such as the Armorican marginal basin, wich contains t h i c k lower Cretaceous s y n - r i f t sediments (Montadert et a l . , 1974) and the Orphan and Labrador Sea
-13-Basins, which contain exceptional thicknesses of T e r t i a r y sediments (A.C, Grant, personal communication) are known to occur. Lack of
deep b u r i a l and r e s u l t i n g thermal immaturity are thus l i k e l y to prevent generation of hydrocarbons i n the majority of the l a t e r Mesozoic r i f t basins, although t h i s has not yet been proven by d r i l l i n g in many areas, p r i n c i p a l l y on account of the great water depths in which a large
proportion of these basins occur.
An exception is the Labrador Shelf Basin ( d i s t i n c t from and to
the west o f , t h e Labrador Sea Basin) which has water depths of less than 400m (Umpleby, 1979). A number of wells have been d r i l l e d in t h i s
basin, three of which have detected s i g n i f i c a n t amounts of gas. However, Umpleby (1979) points out that well temperature data suggests t h a t
even the most deeply buried Cretaceous sediments i n the Labrador Basin should only be capable of. generating o i l . He suggests t h a t the gas accumulations indicate higher heat flow i n the geological past.
This would presumably be related to Late Cretaceous-Early T e r t i a r y sea-floor spreading i n the Labrador Sea (Srivastava, 1978). However, he does not consider gas derivation from w i t h i n the "economic basement", here
consisting mainly of Pre-Cambrian metamorphics but w i t h s i g n i f i c a n t amounts of Ordovician and Carboniferous sediments. A Carboniferous gas source has been considered a p o s s i b i l i t y in the C e l t i c Sea (Naylor and Shannon, 1982) and is highly l i k e l y f o r the Morecambe Gas F i e l d i n the North I r i s h Sea Basin, where the Permo-Triassic basin f i l l has v i r t u a l l y no source rock p o t e n t i a l , i n d i c a t i n g .that the gas source must be located i n the underlying Carboniferous.
Lack of deep burial cannot be c i t e d as a reason why no s i g n i f i c a n t hydrocarbon finds have been made in the purely Lower Mesozoic basins on the Grand Banks, and, with the exception of the North C e l t i c Sea
graben (Naylor and Shannon, 1982), i n the Lower Mesozoic Basins south-west of the B r i t i s h I s l e s . Sediment thicknesses reach 6 km on the Grand Banks
(Jansa and Wade, 1975) and between 3 and 9 km i n the basins south-west of the B r i t i s h Isles (Kammerling, 1979; Naylor and Shannon, 1982) although i t should be noted that the lower part of these sequences consists largely of Triassic and lower Jurassic c l a s t i c s and evaporites
The major unknown concerning the Lower Mesozoic basins relates to the timing of hydrocarbon generation compared to the timing of tectonic events which could form potential hydrocarbon traps but could also allow escape of e a r l i e r formed hydrocarbons. I t might be suspected t h a t increased heat flow leading to hydrocarbon
maturation could have occurred during the Late Jurassic-Early Cretaceous r i f t i n g episode which preceded the formation of the North A t l a n t i c . I f , f o r example, hydrocarbon generation occurred p r i o r to the
Early Cretaceous u p l i f t and erosion on the Grand Banks (Jansa and Wade, 1975) or p r i o r to the Upper Cretaceous and Palaeocene inversion of the C e l t i c Sea Basin (Naylor and Shannon, 1982), then p o t e n t i a l hydrocarbon accumulations might have been l o s t . On the other hand, the Kinsale Gas Field occurs i n an Early T e r t i a r y a n t i c l i n e
(Colley et a l . , 1981), although, of course, t h i s may be a feature of secondary migration i n i t i a t e d by the Early T e r t i a r y earth movements.
In conclusion, the most a t t r a c t i v e areas f o r f u t u r e hydrocarbon exploration would appear to occur where Lower Mesozoic r i f t basins Intersect continental margins formed during the Late Mesozoic. In p a r t i c u l a r , the l i t t l e explored Flemish Pass - Porcupine Basin trend and the southern part of the South-West Approaches - Lusitanian Basin trend seem to be a t t r a c t i v e , although deep water over much of these areas makes exploration uneconomic at the present time. The presence of mature source rocks coupled with the low success rate-of-hydrocarbon exploration wells d r i l l e d i n the purely Lower Mesozoic Basins of the Grand Banks and the continental shelf south-west of B r i t a i n and Ireland suggests t h a t r e l a t i v e timing of hydrocarbon generation and t e c t o n i c a c t i v i t y may have been largely unfavourable to hydrocarbon accumulation and preservation. However, isolated successes, p a r t i c u l a r l y i n the C e l t i c Sea, are l i k e l y to continue to stimulate i n t e r e s t i n these areas.
ACKNOWLEDGEMENTS
The work described here is part of a programme of continental margin studies carried out f o r and funded by the UK Department of Energy. We thank T.J.G. Francis of lOS and A.C. Grant of Bedford I n s t i t u t e of Oceanography, H a l i f a x , Nova Scotia, Canada f o r c r i t i c a l l y reviewing an e a r l i e r version of t h i s manuscript.
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FIGURE CAPTIONS
Figure 1. Lower Early Cretaceous North A t l a n t i c reconstruction i l l u s t r a t i n g the r e l a t i v e positions of the various continents immediately p r i o r to the onset of sea-floor spreading (from Miles and Masson , i n p r e p . ) . Also shown are the locations of the main geographical features discussed in the t e x t .
*Figure 2. Compilation of geological data used to define the d i s t r i b u t i o n of the Lower Mesozoic r i f t basins p l o t t e d on the North A t l a n t i c reconstruction, (Bathymetric contour i n t e r v a l 1000m but
with 200 and 500m contours added). Main data sources: Biju-Duval et a l . , 1977; B o i l l o t et a l . , 1979a, b; Dardel and Rosset, 1971; Jansa and Wade, 1975; Jansa et a l . , 1980; Jenner, 1981;
Laughton, Berggren et a l . , 1972; Masson et a l . , i n press; Montadert, Roberts et a l . , 1979; Rehault and Mauffret, 1979; Soler et a l . , 1981; Stevaux and Winnock, 1974; Wilson, 1975; Z i e g l e r , 1982. The northern edge of the Aquitaine basin has been defined as the northern l i m i t of the evaporite basin (Stevaux and Winnock, 1974).
Figure 3. Probable configuration of Lower Mesozoic r i f t system based on the data summarised i n Figure 2 and p l o t t e d on the North A t l a n t i c reconstruction. Note that the d i r e c t i o n of early opening between A f r i c a and North America (heavy dotted l i n e , Le Pichon et a l . , 1977) i s approximately p a r a l l e l to the s t r i k e - s l i p d i r e c t i o n w i t h i n the r i f t system.
Figure 4. Probable configuration of Late Mesozoic r i f t system super-imposed on the North A t l a n t i c reconstruction. Main data sources: Boil l o t et a l . (1979a, b ) ; Dardel and Rosset (1971); Group Gal i c e , 1979; Masson et a l . ( i n press); McKenzie (1981); Montadert, Roberts et a l . (1981a, b); Umpleby (1979) and
Wilson (1975).
* f o l d - o u t , see map pocket.
23-Figure 5. Summary of the general northward propagation of sea-floor spreading in the North A t l a n t i c during the Late Mesozoic. The ages given are those of the r i f t i n g to s e a - f l o o r spreading t r a n s i t i o n .
Figure 6. Compilation of both Early and Late Mesozoic r i f t basins p l o t t e d on the North A t l a n t i c reconstruction with known hydrocarbon occurrences superimposed. 'Not y e t evaluated' refers to p o t e n t i a l l y commerical hydrocarbon discoveries which have not yet been f u l l y evaluated. Note that the m a j o r i t y of hydrocarbon discoveries ( p a r t i c u l a r l y o i l ) occur in basins where a Lower Mesozoic r i f t has been 'overprinted' by a Late Mesozoic r i f t . Data concerning hydrocarbon discoveries drawn from Col ley e t a l . , 1981; Dardel and Rosset, 1971; Ebbern, 1981; McKenzie, 1981;
Naylor and Shannon, 1982; Soler et a l . , 1981; Umpleby, 1979),
24J
-Porcupine Bank
Celtic Sea Orphan
Knoll
o
Orphan Basin
Goban
FRANCE NEWFOUNDLAND
Flemish Grand Banks
IBERIA
45-40"
am J
JIM
Scotian
\ Faults
I I I Lower Mesozoic Basins % Lower Mesozoic Basins ? ®
35:
Strike-slip Faults
Hercynian Massifs of Iberia and Brittany
Cornish Granite Trend *•«
25
4
-# -# ) r 5 4 9
% q p - 4 0 2 ^ Flemish Pass/•
Parentis Basin Late Jurassic
Early Cretaceous Rift Basins:
^ Minor Late Jurassic-Early Cretaceous Rifts and/or Reactivation of Early Mesozoi Rift Basins
83Ma
83Ma
100Ma?
O lOOMd
lOSMa
i
L'Morecamb
Porcupine ^Kinsalq Basirm^Head
Hibernia
Parent! Basin sturian Basi
Gas Field
Not yet evaluated
Areas where two Rift Phases
overlap *
Late Jurassic-Early Cretaceous Rifts ®