Megaspore assemblages from the Åre Formation (Rhaetian–Pliensbachian) offshore mid-Norway, and their value as field and regional stratigraphical markers

A megaspore biozonation of the non-marine Åre Formation is proposed, based on a micropalaeontological analysis of key Haltenbanken area wells (Block 6608/11). The lower part of the Åre Formation is divisible into Banksisporites pinguis, Nathorstisporites hopliticus and Horstisporites areolatus zones, and subzones, occupying the Rhaetian–Hettangian interval. In the upper Åre Formation a marked turnover of megaspore assemblages is evident, with the appearance of several species of Trileites and the mesofossil Kuqaia quadrata. On this basis, the biozonation is extended into the Sinemurian–Pliensbachian, with the recognition of the Kuqaia quadrata Zone and subzones. Reference to selected wells in the Urd Field (Block 6608/10) and further south demonstrates that these biozones correlate across the northern Haltenbanken region. Biozonal boundaries are calibrated with miospore/microplankton markers where possible, to provide a robust bio-chronostratigraphical framework with which to evaluate the stratigraphy of the Åre Formation and its reservoir units. Comparison with published European biostratigraphical data shows that a similar megaspore succession exists through the Rhaeto-Liassic interval, with shifts in megaspore composition occurring within the same time intervals. On this evidence it is suggested that the megaspore biozones identified are regionally extensive and may reflect palaeoclimatic controls on the distribution of the megaspore-producing plants. It is concluded that megaspores are a stratigraphically important microfossil group, which should be utilized routinely in Upper Triassic–Jurassic oil field and regional biostratigraphical studies.


INTRODUCTION
Atlantic margin, Upper Triassic-Lower Jurassic sediments include reservoir units within the Åre Formation that form several major oil fields on the Heidrun-Norne trend (Fig. 1). Until recently, biostratigraphical evaluation of the Åre Formation in the Haltenbanken region was based primarily on palynological analysis, but the predominance of morphologically diverse, long-ranging miospores prevented reliable biozonation of reservoir units (see below). During such phases of evaluation, megaspores were generally excluded from palynological preparations because of their comparatively large size; they are usually removed along with plant debris through 'swirling' and filtering. Moreover, at least 100 g of sample is required to extract significant numbers of megaspores, compared with about 5 g or 10 g for miospores (e.g. see Batten & Koppelhus, 1993).
Although it has long been established that megaspores can occur abundantly in conventional micropalaeontological preparations of non-marine sediments (e.g. Keuper: Wicher, 1957), in which the minimum sieve size is c. 63 µm, the potential for megaspore extraction has generally been overlooked or ignored. Consequently, micropalaeontological analyses of non-marine sediments, including those of the Åre Formation, have not normally been undertaken during routine well analyses. Furthermore, since megaspore systematics has been perceived by many micropalaeontologists to fall within the domains of palaeobotany and palynology, encounters with megaspores, even in marginal marine and marine sections (e.g. the Båt and Fangst groups), has often led to assemblages being unspeciated. As a result, the stratigraphical value of offshore, Upper Triassic-Jurassic megaspores has remained largely unevaluated, especially in the North Sea and Atlantic margin regions.
During routine evaluation of Norwegian well 6608/11-4, (Figs 1a,b), micropalaeontological analysis of the marine Cretaceous section was extended downwards into the Lower Jurassic Åre Formation and Upper Triassic 'Grey' and 'Red' beds (Terminal Depth: Fig. 2). Analysis of cuttings and cores from this succession, which comprises 650 m of mainly nonmarine claystones, coals and sandstones, revealed a distinct succession of megaspore floras (Fig. 3). Following this first encounter, the biostratigraphical potential of the megaspores was further evaluated by StatoilHydro through the analysis of several exploration and production wells in the Urd Field area (Figs 1b,c) and further south on the Halten Terrace and Trondelag Platform (Fig. 1b).
The main objective of this paper is to present the biostratigraphical data from this initial phase of research with reference to selected wells from the Haltenbanken area. A megaspore/ mesofossil biozonal scheme is proposed for the latter region, which, by comparison with previously published data, is also shown to have regional application.
The Åre Formation comprises the lowest unit of the Båt Group, which succeeds the transitional facies of the continental 'Red Beds' and 'Grey Beds' (Fig. 2: see Dalland et al., 1988 for lithostratigraphical definitions). The lower Åre Formation was deposited during the Rhaetian-Hettangian at a time of relative tectonic quiescence, accumulating in established Permo-Triassic rifted basins (Ziegler, 1990;Blystad et al., 1995). Non-marine, fluvio-deltaic and possibly lacustrine facies were extensively developed during this period, with deposition of organic matter leading to the formation of coals at a number of horizons (Dalland et al., 1988;Pedersen et al., 1989). During the Sinemurian-Pliensbachian the development of NNE-SSWtrending growth faults led to higher clastic input, with the development of extensive sandstones, which form the main reservoirs in the upper Åre Formation (Dalland et al., 1988;Pedersen et al., 1989). Whereas deposition of the upper Åre Formation is considered to have been in marginal marine conditions, there is a general absence of marine microfaunas and palynofloras, compared to younger Jurassic units (e.g. the Not and Melke formations), but there is a significant shift in the composition of megaspore/mesofossil assemblages (see below).
The general lithostratigraphical nomenclature used in this study follows Dalland et al. (1988), and this is shown in relation to their chronostratigraphical assignments in Figure 2. As these authors pointed out, however, based on log character alone, problems arise in defining the upper and lower boundaries of the Åre Formation, especially the latter where the boundary with the underlying 'Grey Beds' is transitional. Subdivision of the Åre Formation has been undertaken informally by operators in a number of field areas, based primarily on log character. In the Norne Field area (Block 6608/10), StatoilHydro divides the unit into Åre-1 and Åre-2, with the latter further subdivided into eight reservoir zones (2.1-2.8), which are shown schematically in Figure 2.

MATERIAL AND METHODS
During initial routine well evaluation of 6608/11-4, the extraction of megaspores involved conventional micropalaeontological processing methods, including soaking of disaggregated core chips and washed cuttings samples (c. 120 g.) in 200 ml of 10% w/v hydrogen peroxide. Owing to the differences in density between plant material (including megaspores) and mineral grains, a technique was subsequently developed to 'pan-out' the former during the wet-sieving phase in order to concentrate megaspores in residues with high sand content. This technique was particularly suitable for the reservoir sections of the Åre Formation in the Urd Field where the thermal maturity is generally low. Minor corrosion and fragmentation of megaspores occurred frequently using these extraction methods owing to the oxidizing effects of hydrogen peroxide and mechanical abrasion, but this normally did not significantly impair species identification.
Since the initial recovery of megaspores in 6608/11-4, more than 20 wells have been evaluated for megaspores in the Haltenbanken region. In these studies, conventional core was sampled at intervals of less than 1 m, and cuttings at less than 6 m, where possible. Sampling points in individual wells were closely tied to wireline logs to focus on low density, carbonaceous claystones.
Analysis of picked samples involved quantitative counts of megaspores and other mesofossils, and semi-quantitative counts of plant material, which was classified informally as: (a) miscellaneous cuticles and stem and leaf fragments that are noncarbonized and generally translucent; (b) indeterminate filaments, which are often associated with megaspores; (c) sporangial fragments; (d) fusain, used informally to include all carbonized plant material. These components are not, however, considered in this paper because little biostratigraphical significance could be attached to their distribution, although a positive correlation exists between the occurrence of megaspores and cuticle, filament and sporangial frequency.
Not all of the megaspores were identifiable because of poor preservation. These are plotted on Figures 3-5 as 'Indet. megaspore-like bodies (casts)' and 'Indet. megaspore spp.', the former comprising diagenetic (commonly sideritic) infills of megaspores, their exines having been lost as a result of abrasion through reworking and, in some cases, possibly during laboratory processing. Also recorded were indeterminate miospore species which were observed within the matrix of sediment pieces in coarser sieved residues, but these are not discussed because they are beyond the scope of this paper. Koppelhus & Batten (1992) noted that the formal system of megaspore taxonomy proposed by Potonié (1956Potonié ( , 1958Potonié ( , 1960 is difficult to apply and, owing to its hierarchical nature, may separate genera of similar appearance and group together others that are clearly distinct. Hence, the megaspores recorded herein are grouped generically according to overall morphology and degree of exine sculpture and complexity. Since the main aim of this paper is biostratigraphical rather than taxonomic, only brief descriptions are provided, with just the original species citation presented. Bibliographical details for all records of species described before 1989, and the ages reported for these occurrences, are given in Batten & Kovach (1990); their ranges, plotted in Kovach & Batten (1989), are based on these data. References to more recent records are cited below where required.

Classification and description
The descriptive terminology used is standard. The dimensions of specimens are given as the maximum diameter of the megaspore body, excluding sculptural elements. The spore wall (exine) is a non-rigid substance that is prone to deformation, shrinkage and fracturing; hence, the measurements made are only approximate. Colour is described for dry specimens only. This varies according to the thickness, structure and degree of thermal alteration of the exine; as a result, although worth recording, it has little taxonomic significance.  Dalland et al., 1988), and reservoir zonation used in this study (StatoilHydro unpublished). All of the specimens illustrated on Plates 1-3 were deposited in the collections of the British Geological Survey, Keyworth, Nottingham (catalogue numbers prefixed by MPK). They were photographed in reflected light using a Bresser, 1.3 megapixel 'Microcular' digital camera, in combination with a stereomicroscope. In conventional photomicroscopy of uncompressed megaspores and other microfossils the amount of detail in focus is limited by the depth of the focal plane: this reduces with increasing magnification so that much of the image in any single photograph is blurred. This optical limitation was overcome here through the use of 'Helicon Focus Tm' imaging software, which allows several focal-plane images (taken in stages through the field of focus) to be combined digitally into one. All of the photomicrographs in this paper here were handled in this way.

Systematic descriptions
The descriptions below are based on observations of specimens examined only under a binocular microscope: no scanning electron microscopy was undertaken. Mineral grains are attached to some specimens because the assemblages were not 'cleaned' by immersion in hydrofluoric acid. Several taxa were identified in open nomenclature including those attributed to Trileites: formalization of these species is intended in a later publication.

Remarks.
Our material compares closely with the holotype of Marcinkiewicz (1960) from the upper Lias (Toarcian) in Poland. It is also similar in size and gross morphology to Margaritatisporites regalis Marcinkiewicz, 1962, but lacks granules between the triradiate ridge, characters that were later considered by Marcinkiewicz (1981) to be included within the circumscription of Trileites candoris. Material. Twenty-one specimens from several Urd Field wells.

Description.
A small to medium-sized trilete megaspore, subtriangular-triangular in equatorial outline, 200-400 µm in equatorial diameter; pronounced triradiate ridges 15-20 µm broad extend to, or almost to, equator producing a highly angular, trigonal-shaped spore. Linear compaction folds or undulations often present between, and extend away from, ridges. Exine 10-15 µm thick, commonly modified by impressions of minerals from the associated sediment; in well-preserved specimens it appears smooth, semi-lustrous and light-medium orange-brown.
Stratigraphic occurrence. Recorded from the upper Åre-2 Formation in several wells where it is the index taxon for Subzone B of the Kuqaia quadrata Zone.

Remarks.
The trigonal profile of this taxon distinguishes it from other published species of Trileites. It is similar in gross morphology to Trileites sp. 2 (described below), but lacks curvaturae. It has been assigned previously to Trileites sp. 5 in unpublished StatoilHydro reports.
Stratigraphic occurrence. Recorded from the lower Åre-2 Formation in several wells where it is restricted to the Kuqaia quadrata Zone, Subzone A.
Remarks. This species is similar in gross morphology to Trileites sp. 1, but differs in possessing distinct curvaturae. It has been assigned previously to Trileites sp. 6 in unpublished Statoil-Hydro reports. Material. Two hundred and thirty-four specimens from several Urd and Haltenbanken exploration wells.
Remarks. The generally poor preservation of this spore makes comparison with published species difficult: certain forms of Trileites from the upper Lias (Toarcian) may, however, be conspecific (e.g. Trileites sp. in Marcinkiewicz, 1960, pl. 2, figs 3, 4). It has been assigned previously to Trileites sp. 7 in unpublished StatoilHydro reports.

Remarks.
Our spores are similar in size to the two specimens from East Greenland on which Harris (1935) based his species Triletes pedinacron, now Tasmanitriletes pedinacron, as indicated above, but the species he described from the Yorkshire Jurassic (Harris, 1961) as Triletes murrayi, now Trileites murrayi (Harris, 1961) Marcinkiewicz, 1971, appears to be more closely comparable to the morphology of our material. On the other hand, almost all of the records of this species are from younger strata than we have investigated in this paper (for an exception, see Wierer, 1997). Hence, for the time being, we prefer to compare our species to Tasmanitriletes pedinacron rather than to Triletes murrayi.
Stratigraphic occurrence. Abundant in the 'Grey Beds' transition and lower Åre-1 Formation, where it is the index taxon for the Banksisporites pinguis Zone (Middle-Upper Rhaetian).
Stratigraphic occurrence. This is generally a rare species, ranging through the Åre 2.4-2.2 units, Kuqaia quadrata Zone, Subzone A.

Remarks.
Our material is similar to that described by Jung (1960) and Bertelsen & Michelson (1970), although the verrucae are more subtly developed: as a result it is difficult to identify this species, especially when preservation is poor.
Stratigraphic occurrence. Recorded commonly from the lower Åre-1 Formation where it is restricted to the Banksisporites pinguis Zone (Middle-Upper Rhaetian).

Remarks.
Our material differs from that of Bertelsen & Michelsen (1970) in having narrower triradiate ridges and curvaturae, and in lacking striations on the contact surface. A wide range of variation is seen in the size and density of verrucae, with many specimens appearing almost smooth: this may be partly attributable to preservational differences.

Description.
A large, trilete megaspore, circular in equatorial outline, 700-800 µm in equatorial diameter (mature specimens). Laesurae straight, extending to 0.9 radius of spore on prominent ridges c. 40 µm high except near equator where they become lower. Curvaturae evident either as a faint line of discontinuity between coarse and fine verrucae or as a ridge some 20 µm high. Exine 10-15 µm thick, dark orange-brown, sculptured with coarse, rather irregularly shaped verrucae up to 40 µm in diameter, sometimes coalescing to form wedge-shaped elements (Pl. 2, fig. 7); size of verrucae on contact face generally somewhat reduced, being mainly 10-20 µm in diameter.
pinguis, and appears to be restricted to the Banksisporites pinguis Zone, Subzone A (Middle-Upper Rhaetian).
Remarks. This species is similar to Verrutriletes utilis but differs in being both more robust and more coarsely sculptured.

Description.
A small trilete megaspore, subtriangular-circular in equatorial outline, 150-180 µm in equatorial diameter. Laesurae straight, extending to 0.6-0.8 radius of spore on ridges that are commonly obscured by dense, small bacula <5 µm in diameter and c. 10 µm high, which cover whole surface of spore. Contact area between triradiate ridges tends to be concave as a result of compaction, which produces a trigonal-shaped apex (Pl. 2, fig.  10). Exine thick (unable to determine precise thickness from any specimens), light-dark orange-brown.

Remarks.
Our specimens differ from those of Bertelsen & Michelsen (1970) in having a smaller average equatorial diameter. The presence of a fine, imperfect reticulate sculpture, described by Bertelsen & Michelsen (1970), is not apparent under reflected light; examination under a scanning electron microscope is necessary to see it.

Description.
A medium-sized trilete megaspore, circular in equatorial outline, 350-500 µm in equatorial diameter. Laesurae straight, extending to 0.6-0.8 radius of spore on ridges that are often obscured by sculpture of closely spaced bacula 5-15 µm in diameter and height, which cover whole surface of spore, sometimes coalescing to form truncated cones with comparatively wide spaces (20-30 µm) in between (Pl. 2, fig. 13). Curvaturae present as depressions or raised ridges 10-15 µm broad, although in most specimens they are obscured by bacula. Exine 10-15 µm thick, light-dark orange-brown.
Stratigraphic occurrence. Ranges from the lower Åre-1 Formation into the upper Åre-2 Formation, occurring most consistently in the Åre-1, Banksisporites pinguis Zone (Middle-Upper Rhaetian). Specimens from the Åre-1 Formation tend to be more heavily ornamented and robust than those from the younger Åre-2 Formation.

Remarks.
Our material displays a wide range of variation in the size and density of the bacula, in accordance with the description of Bertelsen & Michelsen (1970).
Stratigraphic occurrence. This is a rare species which occurs sporadically through the Åre 2.2-2.5 units, Kuqaia quadrata Zone, subzones A-B.

Remarks.
Our material is similar to Horstisporites planatus but appears to possess coarser verrucae on the proximal face.
Stratigraphic occurrence. This species occurs commonly in the Banksisporites pinguis Zone, Subzone A, ranging to the upper Åre-1 Formation (Middle-Upper Rhaetian).

Description.
A medium-sized trilete, zonate megaspore, circular to subtriangular in equatorial outline, 400-450 µm in equatorial diameter. Laesurae straight, reaching equator, bordered by highly elevated membranous lips that are deeply incised and partly comprise bifurcating capilli that extend up to 170 µm at proximal pole, decreasing in height towards equator (Pl. 3, figs 10, 11). Contact face delimited by a low exinous elevation that is parallel to equator. Exine 15-20 µm thick, light-medium orange-brown, covered with coni or spines c. 25-30 µm in diameter at base and 20-25 µm in height; in many cases elements probably originally higher than this, extending into long capilli on proximal face in between triradiate flange but now largely missing as a result of exine degradation; bases of elements generally appearing somewhat broader and evenly distributed on distal surface, intervening spaces being c. 20-50 µm wide.

Stratigraphic occurrence.
A common species in the Åre-1 Formation where its consistent occurrence defines the Nathorstisporites hopliticus Zone (Lower-Middle Hettangian).

Remarks.
Our material compares closely with previously published descriptions and illustrations of specimens of this species, including those previously assigned to Lycostrobus scotti Nathorst (e.g. by Lundblad, 1956).
Stratigraphic occurrence. A very common species, ranging from the uppermost Åre-1 Formation to the uppermost Åre-2 Formation where it characterizes the Kuqaia quadrata Zone (Sinemurian-Lower Pliensbachian).

Remarks. Li Wen-ben (1993) distinguished three species of
Kuqaia from the non-marine Yangxia Formation (Lower Jurassic) in Xinjiang, China, based on the relative prominence of the concentric versus the radial ridges that comprise the ornament. In a later paper, a further two species were erected and one new combination of a species previously assigned to the megaspore genus Aneuletes was established by Cui et al. (2004) based on other Early Jurassic material from Xinjiang. No attempt has been made to distinguish different species in our study because the relative robustness of the concentric and radial ridges was found to vary with the degree and angle of compaction, and state of preservation. The morphology of uncompacted, well-preserved specimens is closely comparable to that of Kuqaia quadrata, as described by Li (1993); hence, this species name is applied here in a broad sense to all of the specimens recovered.
The interval immediately above the 'Red Beds' in both wells has not yielded any significant in situ microfauna or palynoflora: consequently, it is unzoned (Figs 3, 4). In 6608/11-4, megaspores first appear within the 'Grey Beds' with the identification of Banksisporites pinguis at 2178 m, although, in 6608/11-5, the first definite record of this species is higher, coinciding with the base of the Åre-1 Formation (Fig. 4). This 'event', which can be widely correlated (see below), defines the base of the Banksisporites pinguis Zone.
In 6608/11-4, a modest increase in megaspore abundance and diversity is evident through beds of the lower Åre-1 Formation compared with 6608/11-5 (Fig. 4); however, in the former well, the samples were more widely spaced (12 m) and processed for megaspores at an early stage in the investigation when laboratory procedures were less refined: consequently, the megaspore biostratigraphy of 6608/11-5 is considered to be more representative of this part of the formation. Relatively diverse megaspore floras are established within the lower part of the lower Åre-1 Formation, represented by Banksisporites pinguis, Bacutriletes tylotus, B. reticuliferus, Verrutriletes utilis, Verrutriletes sp. 1 and Horstisporites sp. cf. H. rexargenteus. Towards the top of the unit, a changeover in this flora is evident, with extinctions of a number of taxa at or near 1568.50 m, within a thick carbonaceous claystone sequence (Fig. 4). Last occurrences of Verrutriletes sp. 1, Horstisporites sp. cf. H. rexargenteus and Bacutriletes reticuliferus at this level mark the top of Subzone A within the Banksisporites pinguis Zone. Above this level, which is cored in 6608/11-5, increased species dominance of B. pinguis is evident, with the appearance of Tasmanitriletes sp. cf. T. pedinacron, this marking Subzone B of the B. pinguis Zone (Fig. 4). For the reasons already stated and the lack of core, this subzone is not recognized in 6608/11-4; however, the B. pinguis Zone can be widely correlated and, on regional evidence, extends beyond the Haltenbanken area with the last occurrence of B. pinguis marking top Rhaetian (see below).
In well 6608/11-5, megaspores above the B. pinguis Zone are missing owing to erosion beneath the base Cretaceous Unconformity. In 6608/11-4, however, which is downflank of the Rødøy High (Fig. 1b), a more complete Åre Formation succession is present and, in this, the highest occurrence of B. pinguis occurs in carbonaceous claystones some 60 m below 'Coal A' (Fig. 3). This species is succeeded by Nathorstisporites hopliticus, which ranges up to the middle part of 'Coal B', defining the N. hopliticus Zone. Based on the 6608/11-4 well data, N. hopliticus reaches its acme within 'Coal A', above which Horstisporites areolatus becomes dominant. These changes provide a provisional basis for placing the boundary between Subzone A and Subzone B of the N. hopliticus Zone within 'Coal A' (Fig. 3).
In 6608/11-4, megaspore assemblages dominated by Horstisporites areolatus extend from the middle part of 'Coal B' up into the overlying claystones of the upper Åre-1 Formation, defining the Horstisporites areolatus Zone (Fig. 3). Towards the top of this zone there is overlap between H. areolatus and the succeeding assemblage, with the first common occurrence of Kuqaia quadrata, a microfossil of hitherto undetermined affinity, as noted above, coinciding with the last occurrence of H. areolatus at 1878 m, near the top of the Åre-1 Formation. This overlap provides a basis for defining an upper Subzone B within the H. areolatus Zone (Fig. 3). Above this, at 1854 m, a rare occurrence of Trileites candoris was recorded near the base of the Kuqaia quadrata Zone, and this can be correlated with other wells (e.g. 6608/10-6). Low diversity megaspore assemblages dominated by Kuqaia quadrata extend across the Åre-1/Åre-2 boundary.

Åre-2 Formation
Megaspore/mesofossil distribution through the Åre-2 Formation in 6608/11-4 highlights the predominance of Kuqaia quad-rata, with few associated megaspore species present to indicate floral changes within this unit (Fig. 3). This is attributed to the quality of cuttings analysed and the processing technique used in the initial phase of our study. Infill core analysis of upper subunits, however, revealed the presence of the large, but comparatively fragile megaspore Trileites sp. 3.
More detailed megaspore analysis of the Åre-2 Formation was undertaken in the Urd Field area, which is sited some 17 km south of 6608/11-4 (Block 6608/10; Fig. 1b, c). Evaluation of the megaspore/mesofossil biostratigraphy in this area was based on the analysis of eight exploration and production wells, representing the stratigraphy in all field sectors. Data from well 6608/10-J-1H is presented here to illustrate the biostratigraphy of, and key 'bioevents' in, the Åre-2 Formation (Fig. 5).
The appearance of Trileites spp. in the lowermost part of Åre-2 (units 2.1-2.3) is characteristic, with Trileites sp. 3 being most notable. In other wells (not illustrated) the reappearance of Bacutriletes tylotus is also seen within claystones of these subunits.
Further floral changes are associated with claystones of the Åre 2.4-2.5 units, namely the inception of Trileites sp. 1, which, despite the difficulties caused by caving, can be correlated in most Urd Field wells, the 'event' defining the base of Subzone B of the Kuqaia quadrata Zone (Fig. 5). Associated with this appearance is an increase in numbers of Trileites sp. 3, this species tending to be consistently common-abundant through Subzone B. In other field wells Bacutriletes tylotus occurs sporadically within the upper part of this subzone, eventually petering out within Åre unit 2.7.
Moderately diverse megaspore assemblages characterized by Trileites sp. 1, common-abundant Trileites sp. 3, sporadic Bacutriletes tylotus, along with common-abundant Kuqaia quadrata, continue to the top of Åre 2.7, this marking the limit of Subzone B. Above this level (lower Åre 2.8), there is a reduction in megaspore/mesofossil abundance and diversity, with Kuqaia quadrata and Trileites sp. 3 only occurring in the uppermost unit where this is present (e.g. in 6608/10-J-1H; Fig.  5) and not eroded away.
The termination of megaspore floras at the base of the Tilje Formation is abrupt, reflecting a shift towards coastal plain deposition in the Upper Pliensbachian (Pedersen et al., 1989).

MEGASPORE BIOZONATION AND RELATIONSHIP TO MIOSPORE/MICROPLANKTON BIOEVENTS
Composite ranges for megaspore species and Kuqaia quadrata in the Åre-1 and Åre-2 formations based on our well studies are shown in Figure 6. It is notable that whilst the number of megaspore species present at any one stratigraphical level is low, with a total of 17 species recorded from the Åre Formation, many individual ranges appear to be stratigraphically restricted, as do certain acme 'events' (e.g. Bankisporites pinguis, Nathorstisporites hopliticus, Horstisporites areolatus). This provides the basis for a high resolution zonation of the Åre Formation, which is relatively easy to apply, given the processing and logging techniques described.
As previously stated, palynological assemblages in the Åre Formation are dominated by morphologically diverse, longranging miospores, which have proved biostratigraphically unreliable in the northern Haltenbanken region. Nevertheless, a number of miospore bioevents were used for correlation within the Åre Formation at field level further south (Heidrun: Pedersen et al., 1989), and some of these also have regional chronostratigraphical value (Lund, 1977;Batten & Koppelhus, 1996). An attempt was also made to calibrate the occurrences of Nathorstisporites hopliticus and Banksisporites pinguis with miospore events in the Statfjord Formation, North Viking Graben (Charnock et al., 2001;see below).
At the time of writing only a limited number of Haltenbanken exploration wells have been analysed for both megaspores and miospores, including 6608/11-5, 6608/10-7 and recently completed 6507/6-3. A summary of miospore/microplankton events based on data from these wells is shown against the megaspore biozonation in Figure 6. The miospore bioevents indicated (H) are also defined in the Heidrun Field (Pedersen et al., 1989) and overall these can be used to provide additional chronostratigraphical calibration of megaspore bioevents beyond the known occurrence of megaspore species (see below). On this basis the Banksisporites pinguis Zone can be confidently placed in the Middle-Upper Rhaetian, with its top corresponding to that of the uppermost Triassic, Ricciisporites tuberculatus/Limbosporites lundbladii event, which is also recognized in the North Viking Graben (Charnock et al., 2001).
The succeeding Nathorstisporites hopliticus Zone and Horstisporites areolatus Subzone A coincide with the last occurrences of common Ricciisporites tuberculatus and Limbosporites lundbladii at its base and the first occurrence of Trachysporites fuscus at its top, giving a Hettangian assignment, with the top of this stage occurring close to the upper Horstisporites areolatus Zone boundary (Fig. 6). In the North Viking Graben the extinction of Nathorstisporites hopliticus coincides with the appearances of Cerebropollenites mesozoicus and C. macroverrucosus at the base of the Upper Hettangian (Charnock et al., 2001) in support of this assignment, and tentatively indicates a Middle Hettangian age for the top of the Nathorstisporites hopliticus Zone (Fig. 6). The base of the Kuqaia quadrata Zone, which occurs near the top of the Åre-1 Formation, coincides with the first occurrence of common/abundant Cerebropollenites thiergartii at the base of the Sinemurian.
Bioevents within the Kuqaia quadrata Zone of the Åre-2 Formation are poorly constrained by miospores; however, the top of Subzone A, which is marked by a number of megaspore inception and extinction events within the 2.4/2.5 mudstone subunit (Fig. 6), coincides with the last occurrences of common Eucommiidites troedssonii and abundant Quadraeculina Fig. 6. Megaspore/mesofossil biozonation scheme for the Åre Formation, Haltenbanken region, showing composite ranges of key taxa plotted against palynology and interpreted chronostratigraphy (this study). Palynological events, mainly miospores, are based on occurrences in wells 6608/11-5, 6608/10-7 and 6507/6-3. H indicates palynogical event identified in the Heidrun Field (Pedersen et al., 1989).  anellaeformis. This event marks top Sinemurian (StatoilHydro unpublished scheme) and is used here tentatively to date the Kuqaia quadrata Subzone A/B boundary (Fig. 6). The first occurrence of marine microplankton, represented by Nannoceratopsis gracilis and N. senex, is defined at the base of the Tilje Formation and is used regionally to indicate basal Upper Pliensbachian (see Pedersen et al., 1989). This provides an upper age constraint on the Kuqaia quadrata Subzones B-C, which can be placed within the Lower Pliensbachian (Fig. 6).
The chronostratigraphical calibration of megaspore biozones proposed here is compared with the published occurrences of megaspore species in the regional biostratigraphy section below.

CORRELATION OF BIOZONES, HALTENBANKEN AREA
The initial phase of our research involved wells from Blocks 6608/11 and 6608/10, the results of which are presented above. Subsequently, megaspores and other mesofossils from the Åre Formation were evaluated in several exploration wells including 6508/5-1 (Trondelag Platform) and 6507/12-3 (Halten Terrace; see Fig. 1b). In these and other study, wells the same succession was identified, enabling correlation of the biozones recognized with stratigraphical sections within these areas, as shown in Figure 7. This correlation encompasses sequences extending over a distance of 120 km and three structural units, providing insight into (a) the age and stratigraphical relationship of the transition between the 'Grey Beds' and the Åre Formation; (b) the age and lateral extent of coals and carbonaceous-rich deposits, and reservoir horizons within the Åre Formation; and (c) the degree of erosion of units of this formation. Hence, the potential exists to evaluate the stratigraphy of the Åre Formation across large areas of the Haltenbanken region using megaspores, and to extend the biozonal scheme into other basins where extensive Lower Jurassic, non-marine deposition has occurred (see below).
In addition, Figure 7 highlights the areal extent of the Kuqaia quadrata Zone, which is also identified in 6507/12-3. Correlation of subzones within the K. quadrata Zone across the Haltenbanken area has not been possible using the current database. However, our detailed analysis of the Åre-2 Formation in the Urd Field demonstrates that the changes in megaspore composition described above may be correlated across the field. Correlation of these subzones is shown in Figure 8 using selected study wells (see Fig. 1c for well locations). Key 'marker' shales within the reservoir interval are indicated, and changes in the megaspore assemblages broadly correspond to these shale breaks (e.g. the transition from Subzone A to Subzone B coincides with the Åre 2.4/2.5 boundary). Although the Åre-2 reservoir sequence can only be subdivided into three subzones, these can be used to reduce uncertainties in log-based correlation, especially where there is faulting-out or erosion of reservoir units.

REGIONAL UPPER TRIASSIC JURASSIC MEGASPORE BIOSTRATIGRAPHY
In reviewing the stratigraphical occurrences of megaspore species world-wide, Kovach & Batten (1989) highlighted the inherent difficulties in accepting authors' identifications at face value. Consequently, they indicated that the total ranges given for some species are over-extended, with Banksisporites pinguis cited as a case in point. This species, which is an index taxon in the current study, is shown to range from Griesbachian (Lower Triassic) to Aptian/?Santonian, with a large gap through the Sinemurian-Berriasian. This suggests that different taxa are included or that morphologically similar forms evolved independently through the Mesozoic.
Other zonal taxa, including Nathorstisporites hopliticus and Horstisporites areolatus, are also shown to have broad stratigraphical ranges (Carnian-Toarcian and Carnian-Bathonian, respectively), which would appear to downgrade their regional biostratigraphical value. Indeed, the restricted stratigraphical ranges of key taxa evident in the Haltenbanken study wells are difficult to reconcile with their distribution based on the compilation of world-wide occurrences (Kovach & Batten, 1989). On this basis, therefore, the ranges of key megaspore species in the Haltenbanken area (Fig. 6) appear to have been influenced by relatively local palaeoenvironmental controls, albeit over a wide offshore area. In considering the succession of Boreal megaspore floras in general, however, assemblage zonation rather than total range zonation provides greater resolution regionally.
During early palaeobotanical studies on outcrops, megaspore occurrences were tied to the established plant biozonation for the Rhaeto-Liassic section of East Greenland (Harris, 1935). In this and subsequent work (e.g. by Jung, 1960), distinct megaspore/plant associations were identified within the Rhaetian Lepidopteris ottonis and the Hettangian Thaumatopteris schenki zones. The stratigraphic ranges of megaspore species common to our study and previously published European investigations are given in Table 1.
It can be seen from this table that a distinct succession of megaspore floras is evident through the Rhaeto-Liassic interval. Initially Banksisporites pinguis assemblages become established within the Rhaetian and are replaced by Nathorstisporites hopliticus in the Hettangian. This flora is in turn succeeded by Horstisporites areolatus, which ranges with other species (e.g. Trileites candoris) into the Sinemurian. The similarity with the distribution of megaspores in the Åre-1 Formation is striking, indicating that this was controlled by regional factors rather than by more localized palaeoenvironmental changes.
Where more continuous stratigraphical sections have been analysed in borehole studies (e.g. Marcinkiewicz, 1962;Bertelsen & Michelsen, 1970) further similarities are evident between the 'Red Beds'-Åre-1 Formation and the Upper Triassic-Lower Jurassic (Keuper-Lias) megaspore successions. In the Rødby-1 Borehole, offshore Denmark (Bertelsen & Michelsen, 1970), unspeciated ostracod microfaunas are succeeded by Banksisporites pinguis megaspore assemblages in the Rhaetian, as in the 'Red Beds'-Åre Formation sections of 6608/11-4/5. It is noteworthy that the ostracod faunas, which have also been reported from North German Upper Triassic sections, consist of the same species of Rhombocythere (see Simon & Bartenstein, 1962;Christensen, 1962). Following the initial appearance of Banksisporites pinguis in this well, the inceptions of Bacutriletes reticuliferus, Verrutriletes utilis and Tasmanitriletes pedinacron compare sequentially with those of the Åre-1 Formation; similarly the extinction of these species along with Banksisporites pinguis occurs at the same level (top Rhaetian).
The overlying Lias equivalent in the Rødby-1 and Mechowie-1 boreholes yielded the same megaspore succession as the Åre-1 Formation, with low diversity floras dominated by Nathorstisporites hopliticus succeeding Banksisporites pinguis assemblages in the basal Hettangian (probable pre-planorbis beds; see Bertelsen & Michelsen, 1970). The overlap in stratigraphical range between Nathorstisporites hopliticus and Horstisporites areolatus, evident in the Åre-1 Formation (Fig. 6), is also apparent in both Rødby-1 and Mechowie-1 boreholes, despite increasing marine influence evident in the former from the association of the index ostracod Ogmoconchella aspinata through the N. hopliticus-H. areolatus zones. In both boreholes the upper Lias sequence is either absent or unsampled, so further comparisons are not possible.
The above similarities in the megaspore succession strongly suggest that the megaspore biostratigraphy of the Åre-1 Formation was influenced by regional factors that affected a large geographical area, encompassing a significant part of the Central European Basin. One possibility is that the evolution and distribution of the plants involved were responding to widespread climatic change. A progressive increase in seasonal humidity through the Rhaetian (e.g. Ahlberg et al., 2002) would have favoured heterosporous lycopsids, aiding their rapid colonization of wet, poorly drained lowland tracts, conditions conducive to megaspore dispersal and germination.
Owing to the widespread development of deepening marine facies through the Sinemurian-Pliensbachian of the Central European Basin (Ziegler, 1990), regional comparison of megaspore assemblages from the Åre-2 Formation is more difficult, with the most characteristic species, the non-megaspore Kuqaia quadrata, not reported in previous European studies. The original recovery of this species from the Lower Jurassic in Xinjiang, China (Li, 1993) suggests, however, that it is widely distributed. This is further supported by its occurrence in the Rønne Formation of Bornholm (Batten, pers. obs.) and the Statfjord-lower Amundsen formations of the North Viking Graben (see below).
Most of the megaspores recorded from the Åre-2 Formation are species of Trileites identified in open nomenclature, so it is not possible at present to evaluate their regional distribution on the basis of comparisons with the published literature. However, Bacutriletes tylotus becomes extinct and Verrutriletes franconicus and Horstisporites sp. cf. H. planatus have their inceptions and extinctions within this unit (Fig. 6). From Table 1 it can be seen that the stratigraphic occurrence of these species is broadly consistent, although Bacutriletes tylotus ranges above the Hettangian in the Haltenbanken area. These limited comparative data suggest that the Åre-2 Formation is partly equivalent to Lias (Sinemurian): it also indicates that regional lycopsid evolution was maintained, although at this time it may have become more restricted geographically to the northern Boreal realm.

FUTURE POTENTIAL OF UPPER TRIASSIC-JURASSIC MEGASPORE BIOSTRATIGRAPHY
The regional value of megaspore biostratigraphy, which was clearly evident from the earlier studies on northern Haltenbanken wells, is currently being rigorously tested in several field areas by StatoilHydro. The possibility that non-marine facies of the Statfjord Formation, further south in the Viking Graben, could be zoned using megaspores, was evaluated in the Gullfaks Field area (Block 34/10) in 2007. The initial pilot study, involving six well sections, confirmed the presence of the Banksisporites pinguis, Nathorstisporites hopliticus, Horstisporites areolatus and Kuqaia quadrata zones, and their subzones. Remarkably, the megaspore biozonation proposed could be applied directly to the Statfjord reservoir succession with little modification, providing additional evidence for regional (palaeoclimatic?) controls on the distribution of heterosporous plants and, hence, on the dispersal of their spores. Since this pilot study, a field-wide evaluation has been established to provide a comprehensive biozonation of the Statfjord Formation reservoir, and is currently in progress. Presentation of these oil-field data is beyond the scope of the present paper; it is mentioned simply to reinforce our argument that the potential  (1962), Reinhardt (1963), ; F, South Germany: Jung (1960); P, Poland: Marcinkiewicz (1960Marcinkiewicz ( , 1962; S, Scandinavia: Lundblad (1956), Gry (1969); R, Rødby-1, Denmark: Bertelsen & Michelsen (1970).
biostratigraphical value of megaspores at both field and regional levels should not be overlooked. In addition to the application of megaspore biostratigraphy to Lower Jurassic successions, there is growing evidence in the Haltenbanken area that megaspore assemblages also occur commonly in the Middle Jurassic Not and Melke formations, and can be correlated over a wide area (Morris, pers. obs.). The species encountered (e.g. Paxillitriletes phyllicus, Hughesisporites galericulatus) display a wide regional distribution, as for the Lower Jurassic. This indicates the possibility of extending the regional and field application of megaspores into younger Jurassic sections, especially where there is high reservoir potential (e.g. Brent Group, North Viking Graben).

CONCLUSIONS
The present study demonstrates that by using standard micropalaeontological techniques, megaspore assemblages can be effectively extracted from non-marine sediments of the Åre Formation. The results obtained show that a distinct megaspore succession is present within the unit, which can be divided into several assemblage zones. Associated miospore events and the known stratigraphic occurrence of megaspore taxa are integrated to provide a chronostratigraphical framework to constrain biozonal boundaries. A number of biozonation and age assignments are proposed. defined on the presence of the mesofossil Kuqaia quadrata without Horstisporites areolatus. The lower zonal boundary coincides with the first common/abundant occurrence of the miospore Cerebropollenites thiergartii and the upper with the first common occurrence of the dinoflagellate cyst Nannoceratopsis gracilis/senex. The zone can be divided into three subzones in the Urd Field using the ranges of several informal species of Trileites, Verrutriletes franconicus and Bacutriletes tylotus.
Using selected wells the megaspore biozones are shown to correlate across the northern Haltenbanken region, providing a robust bio-chronostratigraphical framework with which to evaluate the stratigraphy of the Åre Formation, which has previously relied heavily on log-based correlation.
Evaluation of published megaspore occurrences through European Rhaeto-Liassic sections, together with continuing research in the Viking Graben, strongly suggest that the Haltenbanken megaspore biozones extend across the Central European Basin, reflecting regional, palaeoclimatic controls on the distribution of the megaspore-producing plants.
Finally, our research shows that megaspores are a stratigraphically important microfossil group, which deserves to be used more widely in Triassic-Jurassic oil field and regional biostratigraphical studies.