British Isles Middle Jurassic sequence stratigraphy ...

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Petroleum Geology Conference series

Potential reservoir and source rocks in relation to Upper Triassic to Middle Jurassic sequence stratigraphy, Atlantic margin basins of the British Isles N. MORTON Petroleum Geology Conference series 1993, v.4; p285-297. doi: 10.1144/0040285

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Potential reservoir and source rocks in relation to Upper Triassic to Middle Jurassic sequence stratigraphy, Atlantic margin basins of the British Isles N. M O R T O N Department of Geology, Birkbeck London WC1E 7HX, UK

College, University of London, Malet

Street,

Abstract: Atlantic margin basins from the Celtic Sea to the Hebrides and North Minch, west of the British Isles, and in the Northern North Sea, have the same major genetic sequences (sensu Morton 1989) in the Upper Triassic to Middle Jurassic. The basins were tectonically related, and six major sequences can be recognized throughout. The tectono-stratigraphic evolution of the basins was through two episodes of extension comprising: syn-rift phases (1) Late Triassic to Early Sinemurian and (4) Latest Toarcian to Late Bajocian; thermal and loading sag phases (2) Mid Sinemurian to earliest Toarcian and (5) Late Bajocian to Late Bathonian (except North Sea ?); and stabilization phases (3) Toarcian and (6) Late Bathonian and Callovian. Potential and proven reservoirs (sandstones) occur in the syn-rift sequences (e.g. Statfjord, Bearreraig, Brent) and in the upper parts of sag sequences (e.g. Cook, Scalpa, Elgol, Valtos). Several organic-rich shales (potential source rocks) occur: in the basal parts of sag sequences (e.g. Heather, Cullaidh, Pabba); in stabilization sequences after eustatic sea-level rises (e.g. Drake, Portree); and locally at the base of syn-rift sequences (e.g. Dun Caan).

The stratigraphical analysis of sedimentary basins has been revolutionized in recent years by the development of concepts of sequence stratigraphy, based on the recognition of packages of conformable strata separated by stratigraphical breaks. A major impetus has been the work of Peter Vail and the Exxon group, summarized in Van Wagoner et al. (1990). However, much of the associated interpretation and terminology has been based on two assumptions: 1. sedimentation occurred in a palaeogeographic setting of hinterland-coastal plain-coast-shelf-shelf edge-deeper basin, which may not always be appropriate for basins of different bathymetric profile; 2. the main mechanism controlling sequence development was eustatic sea level change, i.e. global as distinct from relative sea-level change within a basin. The assumption of a eustatic sea-level controlling mechanism has not been confined to the Exxon group but can also be found in, for example, analyses of outcrop successions such as the Carboniferous mesothems of Ramsbottom (1977). Sequence boundaries caused by eustatic sea-level events cannot be diachronous, in the sense of sediments above the boundary" being older than sediments below, but should be of chronostratigraphic significance as time-correlatable surfaces between tectonically unrelated sedimentary basins. Other authors have argued for a tectonic causal mechanism, from interpretation of seismic stratigraphy and depositional facies (e.g. Hubbard 1988; Morton 1989; Underhill 1991) or from theoretical modelling (e.g. Cloetingh et al. 1987; Watts and Thorne 1984). Sequence boundaries caused by tectonic events need not have chronostratigraphic significance but could be diachronous. One such diachronous sequence boundary is documented for the Lower Jurassic of the Hebrides (between Sequences A and B described later in this paper) where precise ammonite biostratigraphy is available (Morton 1989, 19906). Also there would be no reason for them to be recognizable, far less correlatable, between tectonically unrelated basins. In this paper, based originally on work on outcrops of the Jurassic in the Hebrides Basin (Morton 1989, 1990a, 19926), major basin-wide genetic stratigraphical sequences and sequence boundaries (cf. the megasequences of Steel and Ryseth

1990) have been recognized 'objectively' without preconceived ideas about causal mechanism, in terms of the following definition: Genetic stratigraphical sequences—packages of conformable strata within which facies changes are gradational in space and time (other than channel fills within a fluvial system, for example), and Walther's Law is applicable because of the gradual evolution through time of the depositional environments. The packages are separated by stratal surfaces across which there are major abrupt changes of facies, and these are frequently (but not always, at the very precise level of biostratigraphic resolution available in the Hebrides with ammonite subzones) associated with hiatuses or unconformities. Genetic sequences represent the naturally occurring episodes in the dynamic development of the sedimentary fill of a basin and are, therefore, the basic units for analyses of basin evolution. Note that this definition is not identical with those of either Galloway (1989) or Van Wagoner et al. (1990). The major genetic stratigraphical sequences described here are not synonymous with those of either the Exxon group or of Galloway (1989) (see Morton 1989, 19926 for more detailed discussion). In some basins, parts of the succession can be subdivided into lower-order sequences (e.g. see Mitchener et al. 1992), which are not discussed in this paper. The stratigraphical distributions of potential reservoirs (sandstones) and source rocks (organic-rich shales with > 2% TOC) were then found to have clear context relationships to the interpreted sequences, not just in the Hebrides but in other basins west of the British Isles and in part of the Northern North Sea. These are summarized in this paper, but discussion of other parameters is beyond its scope. Atlantic margin basins of the British Isles: distribution and palaeogeography The distribution of Upper Triassic and Jurassic rocks in western and northern parts of the British Isles and adjacent continental shelf is shown in Fig. 1. No information is shown for the Central and Southern North Sea or southern and eastern areas because basins in these areas have different

From Petroleum Geology of Northwest Europe: Proceedings of the 4th Conference (edited by J. R. Parker). © 1993 Petroleum Geology '86 Ltd. Published by The Geological Society, London, pp. 285-297.

Downloaded from http://pgc.lyellcollection.org/ by Nicol Morton on February 7, 2014 N. MORTON

Fig. 1. Map showing distribution of Upper Triassic to Middle Jurassic rocks in the British Isles and adjacent continental shelf, excluding Central and Southern North Sea, southeast England and Channel. Jurassic positive areas, subject to net erosion and acting as hinterland source areas, are also shown. No attempt has been made to extrapolate to the deeper-water areas of the Porcupine, Rockall and Faeroe troughs beyond the line marked by U. Names of basins or outliers—A: Faeroe; B: West Shetland; C: North Lewis; D: North Minch; E: Flannan; F: Barra; G: Hebrides; H: Donegal; I: Erris; J: Slyne; K: Porcupine; L: Loch Indaal; M: Rathlin; N: Antrim (North Channel); O: Keys (East Irish Sea); P: Kish Bank; Q: North Celtic Sea (incl. Cardigan Bay); R: South Celtic Sea (and Bristol Channel); S: Fastnet; T: Goban Spur; U: East Shetland; V: North Viking Graben; W: Horda Platform; X: Beryl Embayment; Y: South Viking Graben; IMF: Inner Moray Firth; OMF: Outer Moray Firth; Ar: Arran; Ca: Carlisle; Pr: Prees. In all the basins shown, similar major Upper Triassic to Middle Jurassic genetic stratigraphical sequences can be recognized, therefore the basins are technically related. Exceptions are: (1) the basins north of the Wyville-Thomson Ridge, and south of the Clare Lineament west of the British Isles; (2) basins in the Moray Firth (and further south in the North Sea); (3) basins in the Irish Sea and Northern Ireland-southwest Scotland. These show different patterns of tectono-stratigraphic evolution and are not discussed in this paper.

Downloaded from http://pgc.lyellcollection.org/ by Nicol Morton on February 7, 2014 U. TRIASSIC-M. JURASSIC SEQUENCE STRATIGRAPHY

Potential reservoirs

287

Potential source rocks

Fig. 2. Upper Triassic to Middle Jurassic stratigraphy of the Hebrides Basin, showing: chronostratigraphy to stage level; diagrammatic representation of facies and its lateral variation from Mull (S) to northern Skye (N) (not to horizontal scale); lithostratigraphical nomenclature; the major genetic stratigraphical sequences (SEQ); mean thicknesses (M) and coefficient of variation (V) for sequences or parts of sequences; distribution of potential reservoirs (sandstones) and source rocks (organic-rich shales). Standard lithofacies symbols are used, with gradation from sandstones (stipple) through siltstones (stipple + dash) to shales (dashes); S S: palaeosols (mainly calcretes); modifications of standard limestone symbol are stipple for sandy limestones, dashes for argillaceous limestones, circles for oolitic ironstones. SBF: Staffin Bay Formation; RIF: Raasay Ironstone Formation; PG: Penarth Group.

tectonic histories (see Ziegler 1990) and are not being discussed in this paper. The deeper-water areas of the Porcupine, Rockall and Faeroe troughs are insufficiently known because of a thick cover of Cretaceous and younger rocks, so that no attempts can be made at present to extrapolate conclusions to these areas. The tilted fault block structures containing Upper Triassic and Lower to Middle Jurassic rocks (Fig. 1) partly reflect the subsiding basins of the Late Triassic to Middle Jurassic. However, subsequent structural evolution has included inversion and erosion in some areas, notably north of Ireland and especially during the Early Cretaceous. This has left isolated

remnants of an originally continuous system of basins (see Morton 1992a for discussion of basins west of the British Isles, Ziegler 1990 and Cope et al. 1992). Evidence for the locations of basin margins and of the positive hinterland 'land' areas is based mainly on detrital mineralogy and facies distributions in the basins. This is clearest for the Hebrides Basin, where sediment derivation from an Outer Hebrides landmass to the west (faulted margin along the Minch Fault Zone) and a Scottish Highlands landmass to the east (unfaulted margin near the present coastline of the mainland) can be demonstrated (Hudson 1964; Harris 1989, 1992). Other 'positive' areas shown on Fig. 1 in relation


288

N. MORTON

to the basins are in part presumed by analogy with the Hebrides (Morton 1992a) and partly from palaeogeographic maps by Millson (1987) and Petrie et al. (1989) (Celtic Sea basins), and Morton et al. (1992), Richards (1991) Steel and Ryseth (1990), Ziegler (1990) and others (North Sea basins). There is general agreement with other published palaeogeographical maps (Ziegler 1990; Cope et al. 1992, etc.) with two exceptions: 1. basins west of the Shetlands (with the possible exception of the Faeroe Basin) consistently lack Lower Jurassic sediments and are interpreted (Morton et al. 1987; Morton 1992a) as not having undergone significant Early Jurassic subsidence. 2. the Irish Sea area north of the Celtic Sea basins contains isolated remnants of a thick uniform shale succession of Lower Jurassic rocks but no evidence, even indirect, of Middle Jurassic. For reasons discussed in Morton (1992a) the area is interpreted as not having undergone subsidence during the Middle Jurassic, rather than the Middle Jurassic having been subsequently removed by erosion.

3. A final phase of stabilization with greatly reduced rates of subsidence (or possibly limited uplift, see Morton 1987) in basins resulted in stabilization sequences which are thin and condensed, and associated with hiatuses. Sequence A (Upper Triassic to lower [First syn-rift sequence]

Sinemurian)

This sequence consists of continental red beds, resting unconformably on pre-Mesozoic basement, and passing up diachronously into marine sandstones and limestones (nearshore) or limestones and shales (offshore). Facies are extremely variable laterally, sometimes over very short distances (e.g. sandstones to limestones in the lower Sinemurian over 5 km in Raasay). Thickness variation decreases upwards through the sequence, which is interpreted as having formed during a latest Triassic to early Sinemurian episode of lithospheric extension. Potential reservoirs occur in the sandstones of the Broadford Beds Formation and, more locally, in channel sandstones of the alluvial plain sediments of the Stornoway Formation. Laminated dark shales occur, especially in the Rhaetian and lower Hettangian, but are infrequent and no TOC measurements are available.

Sequence stratigraphy in the Hebrides Basin Of the basins shown in Fig. 1 and being discussed in this paper, only the Hebrides Basin has significant onshore outcrops. These enable more detailed sedimentological and stratigraphical analyses to be made and related to the precise level of biostratigraphy available in the Jurassic at ammonite zonal and subzonal level. This provides a tectono-stratigraphical model against which the other basins can be compared. However, it is not yet possible in the Hebrides to relate observations made at outcrops to seismic or well data, so that the broader three-dimensional architecture of stratigraphical units is less well known. Detailed description and discussion of the genetic sequences recognized in the Hebrides are given elsewhere (Morton 1989, \992b) and in the summary given here emphasis is on description of the occurrence of sandstones (potential reservoirs) and dark-grey or black laminated organic-rich shales lacking benthic fauna (potential source rocks). The chronostratigraphy, lithostratigraphy and genetic stratigraphical sequences recognized in the Hebrides are summarized in Fig. 2, together with diagrammatic representation of the fades and its lateral variation from Mull (S) to Skye (N). Present thicknesses of sequences, or parts of sequences, are summarized as the mean and coefficient of variation (which gives a measure of variability independent of the value of the mean) of complete successions. Decompacted thicknesses have not been used for this purpose (see Morton 1987). Interpretation of the sequences, based on analysis of the stratigraphical architecture integrated with subsidence history, is explained in detail in Morton (1989, 1990a). Three phases of basin evolution and sedimentary fill can be identified. 1. A phase of lithospheric extension resulted in differential subsidence in basins associated with rejuvenation of hinterland topography, giving influx of coarse siliciclastic sediments and diverse depositional environments. These syn-rift sequences are highly variable in thickness and facies. 2. Extension is followed by a phase of more uniform and broader basinal sag caused by thermal contraction of crust and sediment loading, giving onlap at basin margins and abrupt facies change to fine-grained shales, possibly associated with a submarine hiatus. Laterally more uniform subsidence and depositional environments result in post-rift sag sequences which are much less variable in thickness and facies.

Sequence B (middle Sinemurian [First sag sequence]

to lowermost

Toarcian)

The base of this sequence is marked everywhere in the Hebrides by an abrupt facies change to shales. This change is associated with a hiatus of approximately one ammonite zone, but both are diachronous with strata above the hiatus in the south (Mull) belonging to the same ammonite subzone as strata below the hiatus in the north (Raasay) (Oates 1978; Morton 19906). Laminated dark shales lacking significant benthic fauna always occur in the basal part of the sequence (base of Pabba Shale Formation), but sedimentation rates remained high (58 m Ma" 1 ) and available TOC measurements only reach 1.5% (Thrasher, pers. comm.). Local sandstones occur in the lower part, but the sequence is characterized by an overall coarsening-upwards to fine-grained sandstones with porosities at outcrop of 10-15%. Potential reservoirs occur in the upper parts of the Scalpa Sandstone Formation.

Sequence C (lower to upper Toarcian) sequence]

[First stabilization

The top of the Scalpa Sandstone Formation is characterized by widespread development of calcareous sandstones, and local ironstone, in the basal Toarcian. This is overlain abruptly by dark laminated shales containing ammonites but with restricted or no benthic fauna. The shales are synchronous throughout the basin, belonging to the Exaratum Subzone and therefore correlative with a widespread deepening event recognized in many parts of the world and interpreted as a eustatic sealevel rise (Hallam 1989). The anoxic bottom conditions associated with this sea-level rise (Hallam 1967) resulted in deposition of the widespread hydrocarbon source rock, the Posidonienschiefer in Germany and Schistes cartons elsewhere in Europe. The correlative Portree Shale Formation in the Hebrides has TOC measurements up to 4% (Thrasher 1992). In all areas except parts of north Skye the shales pass up into an oolitic ironstone, followed by a major hiatus with most of the middle and upper Toarcian proved to be missing. However, there is no evidence of significant erosion so that this sequence is interpreted as representing an episode of basin (and hinterland) stabilization, during which eustatic sea-level events had a marked influence.


Sequence D (uppermost [Second rift sequence]

Toarcian to upper

Bajocian)

Renewed sedimentation began just before the end of the Toarcian as a result of lithospheric extension causing strongly differential subsidence in the basin. The Bearreraig Sandstone Formation is extremely variable in thickness and facies, sometimes over very short distances (see Morton 1965). In some areas the basal Dun Caan Shale Member is sufficiently organic rich (up to 4.7% TOC, Thrasher 1992) to be a potential source rock, but tectonism also rejuvenated hinterland topography, resulting in influx of large quantities of medium to coarse sand. Rates of subsidence kept pace with sedimentation rates so that the depositional environment remained marine, with sand mostly redistributed by tidal currents parallel to the basin axis and apparently influenced by intra-basinal faults (Morton 1983). These tidal sandstones frequently contain significant percentages of shell debris and have low porosities because of calcite cementation and diagenetic silicification, but elsewhere porosities at outcrop range up to 15-20%, and the Bearreraig Sandstone Formation is a major potential reservoir. Sequence E (upper Bajocian to Bathonian) sequence]

[Second sag

Above the various sandstones there is a basin-wide abrupt change to dark shale or clay. The lower part (Garantiana Clay Member) is marine with ammonites, a restricted benthic fauna and TOC measurements of up to 2% (Thrasher 1992). This is overlain by the brackish-lagoonal Cullaidh Shale Formation (formerly called the Oil Shale, Great Estuarine Group) which is even more organic rich with TOC values of up to 15% and tested samples yielding 12-12.8 gallons of crude oil per ton (Lee 1920, p. 53). Higher parts of Sequence E include laminated black shales (Leak and Duntulm formations) with TOC values up to 2% and prograding deltaic sandstones (Elgol and Valtos formations) which are thick enough and have high enough porosities (est. 15%) to be potential reservoirs. The sequence is characterized by remarkable lateral continuity of facies, including some thin algal limestones (Hudson 1980), and the lower coefficients of thickness variation typical of a sag sequence. Sequence F (and part of G) (upper Bathonian Callovian) [Second stabilization sequence]

to

A marine transgression, possibly diachronous from the north, during the latest Bathonian and early Callovian resulted in deposition of the thin Staffin Bay Formation, associated with hiatuses and resting with abrupt facies change on the nonmarine Great Estuarine Group. The sandstones are too thin to have significant reservoir potential and no organic-rich source rocks occur in the Staffin Bay Formation. However, the overlying Callovian part of the Staffin Shale Formation (Dunans Shale Member) is also very thin and has an abrupt facies change at the base, interpreted (Morton 1989) as a deepening event probably of eustatic origin and identified as the base of Sequence G. With TOC measurements ranging up to 10.4% (Fisher and Hudson 1987), this part of the succession includes laminated bituminous shales of hydrocarbon source potential. In the Hebrides Basin, therefore, a tectono-stratigraphic model for the occurrence of potential reservoir and source rocks in relation to the main genetic stratigraphical sequences can be summarized as follows (see Fig. 2). 1. Potential reservoirs are sandstones occurring in two main situations: (a) throughout the syn-rift sequences (Stornoway and

289

Broadford Beds formations in Sequence A; Bearreraig Sandstone Formation in Sequence D); (b) during the later stages of development of post-rift sag sequences (Scalpa Sandstone Formation in Sequence B; Elgol and Valtos Sandstone formations in Sequence E). 2. Potential source rocks occur as organic-rich shales in three situations: (a) during early stages of post-rift sag sequences (base of Pabba Shale Formation in Sequence B; Cullaidh Shale Formation in Sequence E); (b) during stabilization phases due to eustatic sea-level rise (Portree Shale formation in Sequence C; part of Staffin Shale Formation in Sequence G); (c) locally during the early stages of rift sequences (Dun Caan Shale Member in Sequence D). Comparison with basins west of the British Isles West of the British Isles a complex series of tilted fault blocks containing rocks of Upper Triassic to Middle Jurassic age extends from west of the Shetland Isles to south-west of Ireland (Fig. 1). Several of these are relatively unexplored by drilling, and reconstruction of regional stratigraphy and palaeogeography is also hampered by post-Jurassic erosion. This is especially the case north and west of Ireland where only lowermost Jurassic appears to be preserved in the Erris (and Donegal ?) troughs. Original continuity of basins from the Hebrides (and North Minch) through to the Slyne and Porcupine troughs was indicated by Trueblood and Morton (1991) and Morton (1992a) on the basis of identity of sedimentary facies and successions, and palaeobiogeography. This is further supported by the occurrence in the Porcupine Trough of conchostracan faunas typical of the Great Estuarine Group in the Hebrides (Chen and Hudson 1991). For a more detailed description and discussion of basins in this area see papers in Parnell (1992). In analysing the Upper Triassic to Middle Jurassic stratigraphy of most of the basins in the area (see Fig. 3) the major genetic stratigraphical sequences recognized in the Hebrides were found to be applicable, first in the Slyne Trough (Trueblood and Morton 1991), then in other basins (Morton 1992a). Exceptions include: 1. the Irish Sea, where relatively uniform Lower Jurassic contrasts with the more varied successions elsewhere, and it is postulated (Morton 1992a) that little or no Middle Jurassic was ever deposited; 2. basins north of the Wyville-Thomson Ridge west of Shetland (except Faeroe), where little or no Early Jurassic subsidence occurred (cf. basins further south) and Middle or Upper Jurassic rests unconformably on basement or Trias; 3. basins south of the Fastnet Basin (e.g. Goban Spur) where Middle Jurassic subsidence is generally described (Hiscott et al. 1990) as insignificant ('Middle Jurassic quiet period' of Masson and Miles 1986) by comparison with basins further north. The boundary between these differently evolving basin systems appears to coincide with an extension of the Clare Lineament. Sequence

A

In comparison with the Hebrides this is generally more finegrained in the Triassic with mudstones and anhydrite common. The late Triassic (Rhaetian) marine transgression appears to have been earlier and in the Hettangian there is a greater development of carbonate sediments, on a carbonate platform (e.g. Fastnet, Petrie et al. 1989) or alternating with shales (cf. Blue Lias). These pass laterally and upwards into shales


290

N. MORTON CELTIC PORCUPINE

SLYNE

ERRIS

HEBRIDES

W.SHETL

CALLOVIAN

BATHONIAN

BAJOCIAN

AALENIAN

TOARCIAN

PLIENSBACHIAN

SINEMURIAN

HETTANGIAN

! Potential reservoirs Fig. 3. Synthesis of the Upper Triassic to Middle Jurassic stratigraphy of Atlantic margin basins west of the British Isles, with diagrammatic representation of fades variation (not to horizontal scale); major genetic stratigraphical sequences (A to G) recognized in the Hebrides (see Fig. 2) applied to the other basins; distribution of potential reservoirs (sandstones) and source rocks (organic-rich shales). See Fig. 1 for location of basins and Morton (1992a) for sources of data. Lithostratigraphical nomenclature (where established): (a) Celtic Sea (after Millson 1987)—LF: Lilstock Formation; CF: Croyde Formation (BM, Brayford Member); KF: Kilkhampton Formation (SM, Saunton Member); SF: Stratton Formation; GHF: Galley Head Formation; MZF: Mizen Head Formation; BG: Ballycotton Group; DF: Dungarvon Formation; (b) Porcupine, Slyne, Erris, Hebrides (see Fig. 2 and after Trueblood and Morton 1991; Morton 1992a; Trueblood 1992)—SF: Stornoway Formation; BLF: Blue Lias Formation; BBF: Broadford Beds Formation; PaSF: Pabba Shale Formation; ScSF: Scalpa Sandstone Formation; PoSF: Portree Shale Formation (plus Raasay Ironstone Formation); BSF: Bearreraig Sandstone Formation; GEG: Great Estuarine Group; SBF: Staffin Bay Formation; StSF: Staffin Shale Formation; (c) West Shetland—no established lithostratigraphical terminology. For explanation of lithofacies symbols see Fig. 2.

(Murphy and Ainsworth 1991). The Sinemurian upper part of the sequence becomes more similar to the Hebrides Broadford Beds Formation, with sandstones in the Slyne (Trueblood and Morton 1991; Trueblood 1992), Fastnet (Petrie et al. 1989, unit J10 of Murphy and Ainsworth 1991) and Celtic Sea basins (Brayford Member of Millson 1987). Highly variable facies development is again characteristic of this sequence. In the Slyne Trough reservoir quality sandstones with porosities of 712% were identified by Trueblood (1992) in the Broadford Beds Formation.

(Murphy and Ainsworth 1991) and Pabba Shale Formation in Slyne (Trueblood and Morton 1991). The basal part is marked by the widespread occurrence of organic-rich shales with high gamma ray log signatures, especially in the Celtic Sea and in the Slyne where Trueblood (1992) reports up to 4% TOC measurements. There is, as in the Hebrides, general upward shallowing, with periodic influx of sandstones in Fastnet (Petrie et al. 1989) and the Celtic Sea (Saunton Member of Millson 1987), though in Slyne (at least) these did not reach reservoir quality (Trueblood 1992). Eastwards, carbonates are more common.

Sequence B As in the Hebrides there is a widespread facies change to shales at the base of this sequence, Kilkhampton Formation in the Celtic Sea (Millson 1987), Liassic Shale unit (Jll) in Fastnet

Sequence C There is throughout the region a major marine transgression identified in the Lower Toarcian (comparable to that dated as


Exaratum Subzone in the Hebrides) with abrupt facies change to dark organic-rich mudstones with high gamma ray log signatures, especially at the base (Stratton Formation of Millson 1987 in the Celtic Sea; in unit Jl 1-12 of Murphy and Ainsworth 1991 in Fastnet; Portree Shale Formation of Trueblood and Morton 1991 in the Slyne). Trueblood (1992) identified this stratigraphical level as an excellent source rock, with TOC measurements up to > 7 % , similar to the correlative Posidonienschiefer/Schistes cartons (see Fleet et al. 1987). Sequence D Direct comparison of this sequence with the Bearreraig Sandstone Formation in the Hebrides is possible only for the Slyne (Trueblood and Morton 1991) and the Celtic Sea (Galley Head Formation, already compared with Bearreraig Sandstone Formation by Millson 1987). In the only Slyne well, near the southern margin, the Bearreraig Sandstone Formation is reported by Trueblood (1992) not to be developed in reservoir quality sandstones. Elsewhere (Fastnet, north Porcupine), correlations are hampered by the widespread development of non-marine sediments, at least in the marginal areas explored. However, circumstantial palaeoclimatic evidence was used by Morton (1992a) to suggest correlation of the lowest Middle Jurassic Unit 1 of MacDonald et al. (1987) in the northern margin of Porcupine with the Bearreraig Sandstone Formation. Sequence E In the Hebrides, this sequence is interpreted as having been deposited mainly in brackish-lagoonal environments which remained throughout very close to sea-level (Hudson 1980). This poses problems of correlation, especially when slight variations in relative rates of subsidence and sedimentation could result in very different successions. However, the succession in the Slyne is identical wih the Hebrides, enabling detailed lithostratigraphical correlations (Trueblood 1992). Reservoir quality sandstones are identified in the Elgol Sandstone Formation with porosities of 17-24%, but there is no clear evidence of an organic-rich shale equivalent to the Cullaidh Shale Formation of the Hebrides. Correlations with Porcupine are less clear but Chen and Hudson (1991) described conchostracan fossils from Porcupine and commented that the fossils and lithology (of the core samples) were indistinguishable from typical Hebridean Kilmaluag Formation. In the Celtic Sea the equivalent strata are the Mizen Head Group (Millson 1987) which, though marine and passing northeastwards into the carbonate rocks of the Ballycotton Group, show evidence of near-shore influence. Sequences F (and G) Available evidence from the other basins west of the British Isles allows identification of these Hebrides sequences only in very general terms (Morton 1992a). There are also some differences suggesting, inter alia, that the Boreal marine transgressions from the north which characterize the sequence boundaries in the Hebrides cannot be recognized in the basins further south.

Comparison with basins in the Northern North Sea In the North Sea there is much greater regional variation in Upper Triassic to Middle Jurassic stratigraphy, including strong north to south differentiation (summarized by Brown 1990), than in the basins to the west of the British Isles. Within the part of the North Sea shown on Fig. 1, there are three

291

Upper Triassic to Middle Jurassic tectono-stratigraphic provinces: Inner Moray Firth; Outer Moray Firth-Forties-northern part of Central Graben; and Viking Graben-Beryl Embayment-East Shetland Basin to the north of the 'triple junction'. In this last province similarities with the Hebrides are very strong (Morton et al. 1987), and only the basins in this area are discussed in this paper. The Upper Triassic to Middle Jurassic stratigraphy of the basins in the part of the Northern North Sea north of the Moray Firth is summarized diagrammatically in Fig. 4. The area is extensively explored and there are numerous published papers describing individual fields or discussing particular aspects of the geology, especially in Abbotts (1991) and proceedings of previous 'North Sea' conferences (Woodland 1975; Illing and Hobson 1981; Brooks and Glennie 1987). The main lithostratigraphical framework and nomenclature, originally established by Deegan and Scull (1977), has been revised by Vollset and Dore (1984) (see also Brown 1991). The stratigraphy of the Triassic rocks has been summarized by Fisher and Mudge (1990) and of the Jurassic by Brown (1990), while Ziegler (1990) discussed the tectonic and palaeogeographical evolution. A major difficulty in developing a stratigraphical synthesis is the comparative lack of published biostratigraphical data enabling independent verification of correlations, especially with other basins such as the Hebrides. Most biostratigraphy is based on palynological studies which are mainly confidential and not always consistent or comparable between companies (e.g. for the Middle Jurassic compare data presented by Mitchener et al. 1992 with Whitaker et al. 1992). In addition there are unresolved problems of palaeobiogeographic provincialism in parts of the succession. The rare ammonites found in cores are therefore particularly valuable, for example in the reinterpretation of the age of the Brent-Heather boundary suggested here. The major genetic stratigraphical sequences recognized in the Hebrides can also be identified in the Northern North Sea, and provide a framework for comparison. Precise chronostratigraphical correlation is not implied, and differences in age of sequence boundaries are possible because the main controlling mechanism is tectonic. Sequence

A

The base of this sequence is taken at approximately the base of the uppermost Triassic to lowermost Jurassic Statfjord Formation. In many parts of the area (e.g. Viking Graben, eastern part of East Shetland Basin, Horda Platform; Steel and Ryseth 1990; Inglis and Gerard 1991; Struijk and Green 1991) the Statfjord Formation rests with apparent conformity on older Triassic Cormorant or Lunde formations. The lower boundary is marked by an increase in input of coarser sandstones to the basins. Elsewhere (e.g. western part of East Shetland Basin; Taylor and Dietvorst 1991; Wensrich et al. 1991), a thinner incomplete upper part of the Statfjord Formation rests unconformably on older Triassic. The rocks are red beds, predominantly fluvial sandstones, in the lower part but pass up into marine sandstone-dominated sequences in the upper part (cf. the Stornoway and Broadford Beds formations of the Hebrides). The transition appears to be diachronous, and in marginal areas such as the Beryl Embayment marine transgression may not have occurred until at least late Hettangian (Richards 1991). Thicknesses vary greatly laterally, and thickening across faults is demonstrated by Steel and Ryseth (1990). This sequence is described in detail by Steel and Ryseth (1990), and constitutes their Megasequence PR3. It is interpreted by them, and other authors, as a sag sequence following Triassic rifting. However, the sequence is characterized by coarser grain-size compared with older Triassic, and by greater


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BERYL EMB.

E.SHETLAND

N. VIKING

HORDA P.

CALLOVIAN

BATHONIAN

BAJOCIAN

AALENIAN

TOARCIAN

PLIENSBACHIAN

SINEMURIAN

HETTANGIAN RHAETIAN

NORIAN

Potential source rocks

Potential reservoirs

Fig. 4. Synthesis of Upper Triassic to Middle Jurassic stratigraphy of Viking Graben system of basins in the Northern North Sea, with diagrammatic representation of facies variation (not to horizontal scale); interpretation of succession in terms of major genetic sequences recognized in the Hebrides Basin (see Fig. 2); distribution of potential reservoirs (sandstones) and source rocks (organic-rich shales). Based on or reinterpreted from data in Brown (1990), Fisher and Mudge (1990), Graue et al. (1987), Richards (1990, 1991), Steel and Ryseth (1990) and other unpublished sources (released well logs). Lithostratigraphical terminology: BRB: Beryl Red Beds; StfG: Statfjord Formation; BLF: Beryl Lower Formation; BMF: Beryl Middle Formation; BUF: Beryl Upper Formation; DunG: Dunlin Group; AmF: Amundsen Formation; JoF: Johansen Formation; BuF: Burton Formation; CoF: Cook Formation; DrF: Drake Formation; BrtG: Brent Group; BrF: Broom Formation; OsF: Oseberg Formation; RaF: Rannoch Formation; EtF: Etive Formation; NeF: Ness Formation; TaF: Tarbert Formation; SlpF: Sleipner Formation; HeaF: Heather Formation; HugF: Hugin Formation; KrF: Krossfjord Formation; FeF: Fensfjord Formation. For explanation of lithofacies symbols see Fig. 2.

lateral variability of facies and thickness. Fault activity controlled sedimentation, especially across the Alwyn-NinianHutton and Tampen Spur alignments (Steel and Ryseth 1990; Inglis and Gerard 1991). Renewed rapid basin subsidence (Roe and Steel 1985) and associated hinterland rejuvenation occurred, suggesting analogy with the 'first syn-rift' Sequence A of the Hebrides. Development of reservoir sandstones in this sequence can be confirmed for the Northern North Sea basins. Commercial hydrocarbons occur at this stratigraphical level in several fields, most notably Statfjord (Kirk 1980), Brent (Bowen 1975; Struijk and Green 1991) and Alwyn (Johnson and Eyssautier 1987; Inglis and Gerrard 1991).

Sequence

B

Over most of the Northern North Sea there is an abrupt facies change from the sandstones of the Statfjord formation to shales (Amundsen Formation) at the base of the Dunlin Group (Brown 1990) and this marks the base of Sequence B. Dating varies between extremes of Hettangian and Pliensbachian, but most authors favour a mid-Sinemurian age and this seems more likely. The Dunlin Group is characterized by remarkable homogeneity of facies (Inglis and Gerrard 1991) compared with the Statfjord Formation (cf. Broadford Beds Formation to Pabba Shale Formation in the Hebrides). Marine shales and siltstones are dominant but upwards coarsening to sandstones


(Cook Sandstone Formation, cf. Scalpa Sandstone Formation in the Hebrides) is not so widely developed in the Northern North Sea basins. Marginal sandstones may persist throughout much of this sequence (e.g. in the Beryl Embayment, Richards 1991). Thicknesses are also more uniform with more gradual thinning towards the basin margins (e.g. from Brent to Hutton to Cormorant) in the 'Transitional Shelf (Wheatley et al. 1987) and north of the Horda Platform (Steel and Ryseth 1990). Marginal onlap onto older pre-Statfjord rocks occurs, for example in Emerald (Stewart and Faulkner 1991) (cf. minor unconformity at the base of the Pabba Shale Formation in Morvern; Oates 1978; Morton 1989). The changes in stratigraphical architecture and marginal onlaps seen in this sequence in the Northern North Sea basins are comparable with those in Hebrides Sequence B, and are consistent with interpretation of change to thermal and loading sag (first sag sequence). Development of organic-rich shales with higher gamma ray log responses appears to be restricted to only a few horizons at the base of this sequence (e.g. Johnson and Eyssautier 1987). However, the Dunlin Group is identified as an oil and gas source in Frigg (Brewster 1991). Reservoir sandstones are proven in both the marginal sandy facies (Beryl; Richards 1991) and in the Cook Formation (Gullfaks; Eriksen et al. 1987).

Sequence C Above the siltstones or sandstones of the Cook Formation there is a second abrupt facies change to shales (Drake Formation) which can be recognized throughout the Northern North Sea, even in marginal areas like the Beryl Embayment (Richards 1991). Dating of this event is imprecise at about the Pliensbachian-Toarcian boundary. The close similarity of the event and its age to the base of the Portree Shale Formation in the Hebrides, and to the widespread eustatic sea-level event discussed above, suggest that correlation and an early Toarcian age is likely for the base of the Drake Formation. This is consistent with the available palynomorph biostratigraphy. Within the Drake Formation a widespread chamosite marker can be recognized in the East Shetland Basin and Viking Graben. No precise dating for this is available, but similarity with the Raasay Ironstone Formation of the Hebrides is striking. However, this raises problems with interpretation of the upper boundary of the sequence and of the upper part of the Drake Formation, which is dated as ranging into the upper Toarcian. An alternative interpretation is that the upper part of the Drake Formation, especially in areas where it is very thick, may be stratigraphically equivalent to the uppermost Toarcian to lower Aalenian Dun Caan Shale Member in the Hebrides. The major sequence boundary would be within the Toarcian Drake Shale Formation, as in the Hebrides Basin (cf. Fig. 2) and Slyne Trough (cf. Fig. 3). This is supported by evidence of a transitional boundary in some areas (Cannon et al. 1992; Richards 1991) between the Drake Formation and the Broom Formation at the base of the Brent Group, which would indicate that they are part of the same major genetic sequence. The relatively thin Drake Formation, with possibly a hiatus of part of the Toarcian above the chamosite marker, may represent a 'first stabilization sequence' equivalent to the Hebrides Sequence C. The Drake Formation, especially in the lower part, is characterized by high gamma ray log signatures in some areas, suggesting high organic content and hydrocarbon source-rock potential. According to Thomas et al. (1985) this applies only to the Oseberg area, with elsewhere the Drake Formation being organically lean. Field (1985) reported TOC measurements of up to 3% in the North Viking Graben.

293

Sequence D The base of this sequence approximates to the base of the Brent Group. The lower boundary of the sequence is locally an unconformity, but erosion appears to be limited to the crests of some tilted fault blocks and elsewhere there is a transitional boundary with the Drake Formation. Some refinement of the stratigraphy of the Dunlin-Brent boundary is required (see discussion of Sequence C). The Brent Group in the Northern North Sea was originally interpreted (e.g. Leeder 1983; Wood and Barton 1983) as a syn-rift sequence related to lithospheric extension, but more recently thermal subsidence following Triassic rifting has been emphasized (see Yielding et al. 1992). Variations in thickness of the Brent Group were ascribed to differential compaction and to passive infill of older remnant Triassic rift topography (Mitchener et al. 1992). However, Yielding et al. (1992) have shown that differential compaction cannot account for all observed thickness variations. Further, any remnant Triassic rift topography must have been effectively blanketed during deposition of the laterally homogeneous Dunlin Group. Fault activity is required (Yielding et al. 1992) and this has been demonstrated by Cannon et al. (1992), Graue et al. (1987) and Helland-Hansen et al. (1992). At the same time there was rejuvenation of hinterland topography resulting in influx of coarse terrigenous material into the basins. This cannot be explained by eustatic changes of sea-level (cf. Mitchener et al. 1992), and a tectonic event is required to cause differential subsidence in the basins together with hinterland uplift. The Brent Group in the Northern North Sea is, therefore, precisely analogous with the Bearreraig Sandstone Formation in the Hebrides and must be interpreted as a second rift sequence (though the rifting is less significant than later Jurassic rifting). The main difference is that the balance of subsidence and sedimentation rates (controlling accommodation space) meant that the sequence remained marine in the Hebrides while in the Northern North Sea delta progradation and delta-top nonmarine environments were established during parts of the succession. More detailed sequence stratigraphy, at a lower order, is described by Mitchener et al. (1992) and the numerous detailed descriptions (with references) are summarized by Brown (1990) and in Morton et al. (1992). The age of the Brent Group, and biostratigraphical correlations within the group, are less well established, and have varied between Aalenian and Bathonian (Brown 1991). However, dating of the underlying strata to upper Toarcian and of the overlying Heather Group to near the base of the Bathonian (see discussion of Sequence E) constrain the Brent Group to mainly Aalenian and Bajocian in age. There is no published record of potential source-rock shales (cf. parts of Dun Caan Shale Member in the Hebrides) occurring in the basal part of Sequence D in the Northern North Sea, though higher parts contain sufficient coals and land-plant material to be gas-prone source rocks. However, the various sandstone formations of the Brent Group are the most important reservoirs in the Northern North Sea (Brent, Cormorant, Statfjord, etc., Abbotts 1991; Morton et al. 1992). Sequence

E

In many areas the topmost part of the Brent Group (Tarbert Formation) was deposited in a marine environment as a result of transgression of the Boreal seas from the north (Graue et al. 1987). The sandstones are abruptly overlain by the shales and siltstones of the Heather Formation, and this is where the base of Sequence E is recognized. Age assignment of the base of the shales has varied widely from Bathonian to Callovian or even Oxfordian (Brown 1990). Graue et al. (1987) and other authors have suggested partial lateral equivalence, and a diachronous


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boundary between the Tarbert Formation and the Heather Formation which is approximately Middle Bathonian (in Tethyan scale) in the Brent Province (East Shetland Basin) but Callovian in the Beryl Embayment (Mitchener et al. 1992). However, ammonites have been found in cores of the basal part of the Heather Group from several wells ranging geographically from the East Shetland Basin (211/21, Callomon 1975) to the South Viking Graben (9/10, Callomon 1979) and Beryl Embayment (9/13, unpublished). These consistently give precise correlations with the Boreal Bathonian ammonite zones established originally in East Greenland, especially with the Arcticus Zone. Strong biogeographic provincialism has resulted in there being no reliable correlation of the Boreal Bathonian ammonite zones with the standard Bajocian/Bathonian (e.g. of Europe). However, it is generally accepted that the lowest Boreal Bathonian zones (at least the Borealis Zone and Indistinctus Zone) probably equate with the standard Upper Bajocian. The Arcticus Zone and, therefore, the base of the Heather Group must be Lower Bathonian in at least the blocks cited. Further, the consistency in age over such a wide area suggests that this may be generally correct and that limited diachronism of the boundary is more likely. This conclusion has important consequences for interpretation of the stratigraphical evolution of the area, and requires further investigation. In contrast, there are no reliable identifications of Callovian ammonites (Callomon, pers. comm.). There must be a suspicion that the widespread assignment of the Heather Group to the Callovian rather than Bathonian on the basis of micropalaeontology may be the result of problems of biogeographic provincialism. Only limited migration of marine faunas/floras between the Boreal (including Northern North Sea) and Tethyan seas appears to have been possible during the Bathonian, but open connections were established at the beginning of the Callovian. The abrupt change to shales and to laterally more uniform facies in the Heather Group compared with the Brent Group suggest comparison with the change from Sequence D (Bearreraig Sandstone Formation) to Sequence E (mainly Great Estuarine Group) in the Hebrides. The main difference between the Northern North Sea and the Hebrides (and other basins west of British Isles) is that the marine/non-marine roles were reversed compared with the preceding Sequence D. Doubts (expressed above) about correlations make interpretation of the tectonic evolution of the Northern North Sea uncertain. Mitchener et al. (1992) suggest that a major rifting episode began at about this time. In the basal part of this sequence in the Northern North Sea there are organic-rich shales in the lower part of the Heather Group with high gamma ray log signatures (the Heather 'hot shale') and TOC measurements of up to 11% (Thomas et al. 1985).

Sequence F There is limited evidence for recognition of a separate Sequence F distinct from Sequence E, partly because of lack of information about detailed correlations. Mitchener et al. (1992) identify a major sequence boundary between the lower and upper parts of the Heather Formation associated with basin-wide Middle (and Late in places) Callovian condensation. This is supported by the circumstantial evidence cited above about lack of reliable ammonite biostratigraphical proof of the Callovian. Mitchener et al. (1992) refer to a phase of tectonic quiescence in basin development (their J40 tectonostratigraphic unit). This would equate with Sequence F (plus basal part of G) (second stabilization sequence) recognized in the Hebrides, suggesting that there was a widespread Callovian interval of basin stabilization. In some areas (see Mitchener et

al. 1992) high gamma ray readings occur in the upper part of the Heather Formation. Summary and conclusions 1. The distinctive pattern of major genetic stratigraphical sequences established for the Upper Triassic to Middle Jurassic of the Hebrides Basin can also be recognized in Atlantic margin basins west of the British Isles from the Fastnet and Celtic Sea basins south of Ireland to the North Minch Basin north of the Hebrides. South of the projection of the Clare Lineament, the Atlantic margin basins have a different tectono-stratigraphical history, especially in the Middle Jurassic. North of the Wyville-Thomson Ridge the basins west of the Shetland Isles generally have a different history in both the Early and Mid-Jurassic. However, the Hebridean pattern of tectono-stratigraphic evolution also occurs further east in the basins of the Northern North Sea, north of the Moray Firth. 2. In the basins discussed, six major genetic sequences can be recognized. These are interpreted as mainly caused by tectonic mechanisms but with eustatic sea-level influences becoming significant during episodes of basin stabilization. The tectonostratigraphic evolution of the basins can be summarized in terms of six phases as the basins twice evolved from extensiondriven syn-rift phases through thermal and loading sag phases to stabilization phases: Al. Late Triassic (Rhaetian, ?Norian) to Early Sinemurian: first syn-rift phase with strongly differential subsidence resulting in a sequence which is laterally highly variable in thickness and facies; hinterland rejuvenation resulted in influx of coarse siliciclastic sediments; red beds diachronously passing up into shallow marine sandstones and limestones. A2. Mid-Sinemurian to earliest Toarcian: first thermal and loading sag phase with broader area of more uniform subsidence giving laterally more uniform thicknesses and facies, and marginal onlap; abrupt change to dark possibly organic-rich shales at base then gradually coarseningup to siltstones or sandstones. A3. Early to late Toarcian: first stabilization phase with reduced rates of subsidence in basins and reduced hinterland topography; thin sequences with proven or probable hiatuses and eustatic deepening events resulting in organic-rich shales. Bl. Latest Toarcian to late Bajocian: lithospheric extension and second syn-rift phase with differential subsidence in basins and rejuvenation of hinterland topography; sequence highly variable in thickness and facies, but characterized by rapid influx of coarse terrigenous sediment. B2. Late Bajocian or early Bathonian to late Bathonian: second phase of thermal and loading sag (? or further rifting in the Northern North Sea) giving abruptly finergrained sediments, laterally more uniform thicknesses and facies, and marginal onlap. B3. (Late Bathonian to) Callovian: second stabilization phase with reduced subsidence in basins and lower topography in hinterland; thin sequences, possibly with hiatuses. Variations in space and time between marine and non-marine depositional environments, and transgression into the basin system from either the Tethyan seas to the south or Boreal seas to the north, were determined by the changing balance between rates of subsidence and sedimentation. 3.

Potential (proven) reservoir rocks (mainly sandstones) occur mainly in (i) syn-rift sequences:

Downloaded from http://pgc.lyellcollection.org/ by Nicol Morton on February 7, 2014 U. TRIASSIC-M. JURASSIC SEQUENCE STRATIGRAPHY Sequence A. Statfjord F o r m a t i o n in the N o r t h e r n N o r t h Sea (Alwyn, Brent, Statfjord, Gullfaks); Broadford Beds F o r m a t i o n in the Hebrides a n d the Slyne; Sequence D . Brent G r o u p in t h e N o r t h e r n N o r t h Sea (Alwyn, Brent, C o r m o r a n t , Ninian, etc.); Bearreraig Sandstone F o r m a t i o n in the Hebrides; Galley H e a d Formation in the Celtic Sea. (ii) the upper parts of sag sequences: Sequence B. C o o k Sandstone F o r m a t i o n in the N o r t h ern N o r t h Sea (Gullfaks); Scalpa Sandstone F o r m a t i o n in the Hebrides; Sequence E. Elgol a n d Valtos Sandstone formations in the Hebrides a n d the Slyne. (iii) basin margin settings: Sequence A / B . Sandstones in part of t h e N o r t h e r n N o r t h Sea (Beryl); Sequence E. Emerald Sandstone F o r m a t i o n in the N o r t h e r n N o r t h Sea (Emerald). 4.

Organic-rich shales with > 2 % T O C measurements occur in the following tectono-stratigraphic settings: (i) near the base of sag sequences: Sequence B. Basal part of A m u n d s e n F o r m a t i o n in the N o r t h e r n N o r t h Sea; base of P a b b a Shale F o r m a t i o n in the Hebrides a n d t h e Slyne; K i l k h a m p t o n F o r m a t i o n in the Celtic Sea; Sequence E. Basal part of Heather F o r m a t i o n in t h e N o r t h e r n N o r t h Sea; Cullaidh Shale F o r m a t i o n in t h e Hebrides, (ii) in stabilization phases following eustatic sea-level rise: Sequence C. Lower part of D r a k e F o r m a t i o n in the N o r t h e r n N o r t h Sea; Portree Shale F o r m a t i o n in t h e Hebrides a n d the Slyne; Stratton F o r m a t i o n in the Celtic Sea and equivalent lower Toarcian shales in other areas; Sequence F/G. U p p e r part of H e a t h e r F o r m a t i o n in the N o r t h e r n N o r t h Sea; part of Staffin Shale F o r m a t i o n in the Hebrides, (iii) very locally at the base of syn-rift sequences: Sequence D . Part of D u n C a a n Shale M e m b e r in t h e Hebrides. Research School of Geological and Geophysical Sciences, Birbeck College and University College, London, Contribution no. 2.

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Discussion Question (J. R. Underhill, University of

Edinburgh):

Would you agree that the Dun Caan Shale/Ollach Sandstone contact in Skye may represent the western correlative conformity to the intra-Aalenian 'mid-Cimmerian' event described by Mark Partington and me? Answer (N.

Morton):

The Dun Caan Shale-Ollach Sandstone boundary at Bearreraig is defined by a rapidly transitional facies change from silty shales to silty limestones which are very fossiliferous through much of the Murchisonae Zone (possibly due to relatively slower rates of sedimentation compared with higher and lower beds at Bearreraig). The boundary appears to coincide with the Scissum-Murchisonae zonal boundary, although Scissum Zone faunas are not well preserved at Bearreraig. However, on

Answer (N.

Morton)

The major sequence boundaries discussed are at present dated with sufficient precision (ammonite zones and subzones) only in the Hebrides. Correlation with other areas is interpretative but (with the one exception of the base of the Heather Formation, discussed in the paper) generally consistent with available biostratigraphy. Detailed comparisons of the Hebrides sequence boundaries and relative sea-level events with those shown on the Exxon curve (version in Haq et al. (1987) Science, 235, 1156-1167) are given elsewhere (Morton 1989, 1990a). Briefly, only the base of Sequence F (Upper Bathonian) and a local minor sequence boundary at the base of the Rhaetian correlate, the other nine major or minor Hebrides boundaries/events in the Upper Triassic-Middle Jurassic do not; conversely only two of twenty of the Exxon boundaries can be recognized in the Hebrides. Interpretation of events in the Hebrides (and related basins) based on integration of sequence stratigraphy with subsidence history and stratigraphical architecture, identified tectonic mechanisms for four of the sequence boundaries discussed here (bases of Sequences A, B, D and E). The interpreted eustatic sea-level events at the bases of Sequences C (base of Falciferum Zone, Lower Toarcian), F (?Discus Zone, Upper Bathonian) and G {Jason Zone, Middle Callovian) all correlate precisely with eustatic deepening events identified by Hallam (1989). Using the latest published revised version of the Exxon curve (Haq et al. 1987) there is indeed agreement with events in southern England and South Wales. This is hardly surprising considering that calibration of the curve against ammonite biostratigraphy at outcrop was established in southern England.