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Correlation of the early Paleogene in northwest Europe: an overview
R. W. O'B. KNOX British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
The last two decades have seen a major resurgence Rockall area (Legs 12, 48, 81), and the volcanic of interest, both commercial and scientific, in the successions of British Tertiary Igneous Province early Paleogene stratigraphy of northwest Europe. (BTIP) and the North Atlantic Igneous Province The commercial interest has arisen primarily as a (NAIP). Five, largely independent, areas of study result of major oil and gas finds in the central and can thus be identified: Northern North Sea, mostly in deep-water sand- (1) the onshore sections of the southern margin of stones of late Paleocene to mid Eocene age. the North Sea Basin, restricted to inner shelf, Increased interest in the onshore sections has been littoral and terrestrial facies; stimulated partly in response to the offshore hydro- (2) the offshore sections of the North Sea Basin carbon exploration, but largely through the and West of Shetlands area, dominated by activities of international (IUGS/IGCP) working outer shelf, slope and basinal facies, but groups, whose primary concern is the establishment including inner shelf to terrestrial facies around of a globally standardized system of series and the Scottish landmass; stages. The onshore sections of the southern North (3) the offshore sections of the Goban Spur area, Sea Basin area are of particular importance in restricted to bathyal facies (largely calcareous these investigations, because they include nannofossil oozes); the historical stratotypes for the Paleocene and (4) the offshore sections of the Rockall area, repre- Eocene series and for their constituent stages. senting inner shelf to bathyal facies; Unfortunately, these historical stratotypes are (5) the onshore and offshore stratified sections inappropriate as global stratotypes because of their of the BTIP and GFIP, dominated by lavas stratigraphic incompleteness, and their limited and tufts, but with intercalations of non- representation of the standard Paleogene biozones. volcanogenic sediments. Only through the fullest understanding of these historical stratotype sections, however, can we ensure that the standard stages are defined in a way that ensures the maximum compatibility with traditional assignments in NW Europe (Knox 1994; Schmitz 1994). For a long time the commercially driven and scientifically driven lines of investigation proceed- ed more or less independently, partly because of the _ !tI confidential nature of the offshore investigations and partly because of difficulties in correlating widely separated sections of strongly contrasting lithofacies and biofacies. For these reasons, the earlier stratigraphic compilations for the Paleogene of northwest Europe were concerned almost exclusively with the onshore areas (e.g. Curry et al. 1978; Cavelier & Pomerol 1986; Pomerol 1989). A notable exception is the compilation of data collected in relation to IGCP Project 124 (Vinken et al. 1988), which represents a remarkable achievement in the field of multidisciplinary and multinational stratigraphic collaboration. Fig.1. Distribution of early Paleogene sedimentary and Other areas that have been the subject of detailed igneous successions in NW Europe, with locations of the analysis are the oceanic successions of the eastern Bay of Biscay, Goban Spur and Rockall DSDP sites Atlantic (Fig.l), encountered during DSDP drilling (Legs 12, 48, 80, and 81) and central North Sea well in the Goban Spur area (Legs 48, 80) and in the 22/10a-4.
From Knox, R. W. O'B., Corfield, R. M. & Dunay, R. E. (eds), 1996, Correlation of the Early Paleogene in Northwest Europe, Geological Society Special Publication No. 101, pp. 1-11. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
2 R.W. O'B. KNOX
A broad stratigraphic framework has now been SE England provide a more or less continuous tran- established for each of these areas. For example, sect from sublittoral facies in the historical extensive studies have now been carried out on Thanetian stratotype sections of Kent to outer shelf all aspects of biostratigraphy, such that regional 'North Sea'facies in northern parts of East Anglia. zonation schemes are firmly established for the The major challenges for the future are (i) inter- more important fossil groups. Similarly, comp- regional correlation within northwest Europe, and rehensive magnetic polarity zonations have been (ii) calibration of the northwest European established for the onshore areas of the BTIP and sections in terms of the standard biozones and the the southern North Sea Basin, providing not only a geological timescale. Because it is clear that no one means of correlation, but also a direct link to the discipline can solve all these problems, a multi- geological timescale. It is only now that these disciplinary approach is paramount. The purpose of comprehensive stratigraphic frameworks have been this overview is to provide a brief assessment of established that correlation betweeen the individual how the different disciplines have contributed areas (referred to as 'interregional correlation' in towards interregional correlations, and how the this account) can be realistically attempted. ultimate aim of a chronologically calibrated, Problems in interregional correlation arise integrated stratigraphy for the entire northwest primarily from differences in the type and nature European region might be achieved. of the available stratigraphic data. Examples of limitation due to data type are seismic data, which Essential elements of an interregional are available only for offshore sections, and magnetic polarity data, which can be obtained only correlation from cored sections or outcrops. Even within a The geological timescale single discipline, interpretation can be hampered by different methods of data acquisition. Thus bio- The publication of a new Paleogene timescale stratigraphic data for the offshore hydrocarbons (Cande & Kent 1995) is a welcome development basins are largely based on cuttings, and thus in view of the significant discrepancies between dependent on first downhole occurrences (FDOs), earlier timescales, especially over the late whereas for the onshore sections they are based on Paleocene to early Eocene interval. The incor- the standard criteria of first and last appearance poration of a radiometric date obtained from a datums (FADs, LADs). A more serious limitation tephra layer of early NP10 age puts the new on interregional correlation is the effect of facies on timescale on a much firmer footing. This is the nature and diversity of fossil assemblages. Thus especially important for assessing the influence of while the Goban Spur sections in the eastern 'Atlantic' tectonism on stratigraphic events, as it Atlantic possess rich calcareous microfaunas and allows an improved correlation between the bio- nannofaunas, allowing assignment to the standard stratigraphically dated sedimentary successions and Paleogene biozones, equivalent strata in central the radiometrically dated lava successions of the parts of the North Sea Basin are commonly North Atlantic borderlands. However, as cautioned devoid of calcareous fossils. Conversely, whereas by Berggren & Aubry (1996), timescales are con- palynomorphs are ubiquitous in the North Sea structed on a series of assumptions that will be Basin, they are reported to be absent from the subject to continuous reassessment, and the new Goban Spur sections. Under such circumstances, Paleogene timescale is no exception. The Cande & correlation between the two successions must rely Kent (1995) timescale is nevertheless considered to on a combination of techniques, with particular provide a much improved overall chronological emphasis on those that are less facies dependent calibration of the standard biozones and magnetic (e.g. magnetostratigraphy and tephrostratigraphy). polarity zones (see Berggren et al. in press). Its Fortunately, not all of the correlation problems adoption by the lUGS Subcommission on are so severe. For example, the early Paleogene Paleogene Stratigraphy should ensure that it succession of Denmark, accessible through outcrop becomes the common standard until such time as sections and cored boreholes, is of deep-water sufficient data are accrued to warrant further refine- facies, and provides a valuable insight into the ment. The use of such a common standard will succession of the central North Sea. It is there- greatly facilitate comparison of stratigraphic data of fore possible to apply techniques such as all types and from all parts of the world. magnetostratigraphy to a basinal North Sea succes- sion, and thereby assess the relationship between Biostratigraphy the zonally based biostratigraphic schemes established for onshore sections and the FDO-based Calcareous nannofossils. A more or less com- schemes established for the offshore hydrocarbons plete record of the early Paleogene standard cal- boreholes. Additionally, the Paleocene sections of careous nannofossil zones is present in the Goban Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
CORRELATION OF THE EARLY PALEOGENE IN NORTHWEST EUROPE" AN OVERVIEW 3
Spur DSDP Sites 549 and 550, which together may the onshore and offshore basins of NW Europe (see be regarded as providing a composite oceanic Powell 1992; Heilmann-Clausen 1994). reference section for NW Europe. This composite Terrestrial palynomorphs have an even greater section, which has recently been the subject of a potential for interregional correlation, as they occur detailed reassessment (Aubry et al. 1996), provides in both marine and continental facies, including a unique opportunity for linking the epicontinental sedimentary intercalations within the lava succes- successions with the global oceanic record. Direct sions of the Hebridean Province and the Faeroes. correlation is admittedly hampered by the patchy Regional stratigraphic analysis of the terrestrial distribution and in part endemic nature of palynomorph assemblages is, however, less calcareous nannofossil assemblages in NW Europe, straightforward than for marine palynomorphs, but, as early studies on the onshore sections of the because of greater variability in assemblages North Sea Basin (e.g. Aubry 1986; Siesser et al. arising from terrestrial climatic and physio- 1987) are augmented by new data (e.g. Steurbaut graphical controls, and because of the strong 1990; Hine 1994; Ellison et al. 1996), a significant geographical control exerted by fluvial catchment, part of the NW European succession can now be transport and deposition. Nevertheless, the poten- at least broadly linked to the oceanic calcareous tial for local and regional high-resolution stratigra- nannofossil record. phy has been demonstrated in studies on the Thanet Formation of southern England (Jolley 1992) and Microfossils. The Goban Spur sites also provide on the Harwich Formation and its Central North an important record of the standard planktonic Sea equivalents (Jolley 1996). foraminiferal zonation for the eastern Atlantic, Calibration of the palynomorph zonations which has recently been the subject of a compre- against the standard zonal schemes is possible in hensive review (Berggren & Aubry 1996). those parts of the succession where the appropriate However, planktonic foraminifera are of limited calcareous nannoplankton are present, including value in the epicontinental successions, where the early Paleocene, part of the late Paleocene (late faunas are characteristically dominated by endemic NP6 to early NP9), and much of the early Eocene. assemblages. Only in the early Paleocene and part Calibration over the interval spanning late NP9 to of the early Eocene is there potential for direct NP10 cannot, however, be achieved within the assignment of the standard planktonic zonations. North Sea Basin, and application of calibrations For the remainder of the succession, microfossil developed for other areas (e.g. Rockall area: zonal schemes have been established largely for the Morton et al. 1983) may be unsafe because of pos- deep-water, basinal areas of NW Europe, either on sible diachroneity of dinoflagellate cyst influxes whole faunas (e.g. King 1989; Mudge & Copestake resulting from isolation of the North Sea Basin at 1992) or selected elements (e.g. Mitlehner 1996). this time. While these zonal schemes have potential for Many of the limitations apparent from existing detailed correlation between the deep-water facies palynological studies will be overcome from the of the North Sea Basin, West of Shetlands basins combined use of terrestrial and marine groups in and the offshore Norwegian basins, they cannot be establishing correlations between terrestrial, shal- applied to the shallow-water onshore successions of low marine and deep marine facies. Published the southern North Sea Basin margins because of a accounts in which such an approach have been used fundamental change in biofacies. (e.g. Jolley 1996) are relatively few, but the Zonal schemes that apply to the entire region method is being increasingly used in the study of (e.g. Gradstein et al. 1994) are therefore neces- the offshore hydrocarbons basins. sarily less detailed than the correlations developed specifically for the basinal areas. However, they Mammals. Though the mammal faunas of NW have the advantage that they can be directly linked Europe are of very restricted occurrence compared with the calcareous nannoplankton zonation and with other zonally significant fossil groups, they magnetostratigraphy established for the onshore provide important information on the evolution of sections. the regional palaeogeography Hooker 1996). The succession of NW European mammal faunas indi- Palynomorphs. Following their early use in the cates that interchange with North American faunas correlation of the marginal successions of the took place twice during the Paleocene and Eocene, southern North Sea Basin, dinoflagellate cysts have firstly in the early late Paleocene and secondly in proved to be particularly valuable in basinal areas the Paleocene/Eocene 'boundary interval' (see (e.g. Heilmann-Clausen 1985; Powell 1988; Mudge below). Both events correspond to periods of & Bujak 1996). They are now established as maximum lowstand (middle Maureen Formation perhaps the most effective means of correlating and basal Sele Formation lowstands of the central across the broad spectrum of facies encountered in North Sea), which presumably resulted in exposure Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
4 R.W. O'B. KNOX of the Greenland-Scotland landbridge. The initial early diagenetic (pedogenic) carbonate from the highly cosmopolitan nature of the North American Argile Plastique Bariolre (equivalent to the and European faunas within the Paleocene-Eocene Reading Formation) in the Paris Basin. This may boundary interval suggests that migration was well correlate with the major negative short-term exceptionally rapid, with favourable palaeo- excursion recorded from oceanic sections in late geographical conditions probably being enhanced NP9, in which case it will corroborate the earlier by short-term climatic amelioration (Hooker 1996, findings of Koch et al. (1992) and constitute an and see below). important breakthrough in linking the Paleocene/Eocene succession of NW Europe with Integrated biostratigraphic framework. As the oceanic record. discussed above, there is a distinct dichotomy between the marine biostratigraphic record in Radiometric dating the oceanic Atlantic succession and that in the epicontinental successions of the North Sea Basin Radiometric dating has been carried out on both and West of Shetlands area. Even within the high temperature minerals and glauconites. With epicontinental basins, biofacies changes from the two exceptions, the high temperature dates have centre to the margins of the basins puts severe been obtained from volcanic rocks in the BTIP and constraints on long-distance correlations. No one GFIE These dates have contributed to knowledge fossil group can be relied on to correlate between of the relative timing of volcanic events within and all sections, and the key to developing a biostrati- between individual igneous centres, and also to the graphic correlation framework for the entire NW gross timing of volcanism in the region (e.g. European region thus lies in determining the inter- Mussett et al. 1988; Noble et al. 1988; Ritchie & relationships between bioevents and biozonal Hitchen 1996). They are also of major significance schemes established for all fossil groups. in assessing the relationship between phases of volcanism and the tectonic and sea-level history of the region as inferred from coeval sedimentary Isotope stratigraphy successions. The standard techniques of carbon and oxygen Dating of the sediments themselves has been isotope analysis are carried out on whole-rock based on K-At analysis of glauconite, and has carbonate or, for more informative results, on played a significant part in the construction of some selected species of calcareous benthic and plank- timescales (e.g. Harland et al. 1989). Single-crystal tonic foraminifera. Analysis is therefore usually (sanidine) Ar/Ar dates from early Eocene tephras in restricted to carbonate-rich facies. Knowledge of Denmark have played a more specific role in the the Paleogene isotopic record has therefore been construction of the Cande & Kent (1995) timescale, built up primarily from oceanic sections encoun- with the age of ash-layer-17 being used as a tie- tered in DSDP and ODP drilling (e.g. Charisi & point for the lower part of calcareous nannofossil Schmitz 1996; Corfield & Norris 1996; Stott et al. zone NP10. 1996). The long-term trends within the early Paleogene are now well established, and attention Magnetostratigraphy is currently focusing on specific aspects, such as the occurrence and age of one or more short-term Magnetostratigraphy has also played an important negative carbon isotope excursions in the late role in the establishment of the igneous history of Paleocene (e.g. Corfield & Norris 1996; Stott the region, supplementing the data obtained from et al. 1996) and their relationship to other major superpositional/crosscutting relationships and events, such as the widespread oceanic 'benthic radiometric dating (Mussett et al. 1988; Ritchie extinction' event (Thomas & Shackleton 1996). & Hitcben this volume). In the sedimentary Appropriate facies for carbon and oxygen successions of the onshore southern North Sea area, isotope analysis within the lower Paleogene are a detailed knowledge of the reversal history has largely restricted in NW Europe to the lower now been established for many areas (see Ali et al. Eocene. A detailed study on the Danish lower 1993; Ali & Jolley 1996), though reassessment and Eocene has revealed both long-term trends and refinement are still possible through the application short-term events, as well as demonstrating the of improved techniques or through the study of new influence of fresh water input on isotopic values sections (e.g. Ali et al. 1996). As a means of (Schmitz et al. 1996). No such comprehensive providing precise chronological correlations, study has been carried out on the NW European magnetostratigraphic data have unique potential in Paleocene, because the facies are largely unsuit- correlating between igneous and sedimentary able. However, Stott et al. (1996) have identified a successions and in assessing synchroneity of sea- distinct negative carbon isotope excursion within level change both within NW Europe and beyond. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
CORRELATION OF THE EARLY PALEOGENE IN NORTHWEST EUROPE: AN OVERVIEW 5
Tephrostratigraphy surprising, considering the complex crustal history that is recorded in the British and North Atlantic Though a detailed tephrostratigraphy had been igneous provinces, culminating in the opening of established in the early Eocene ash-series of the North Atlantic in early Eocene times (Ritchie & Denmark around the turn of the century, the Hitchen 1996). Under such circumstances, the erec- significance of these tephras as regional or inter- tion of an interregional sequence statigraphic regional correlation tools was not appreciated until scheme will inevitably be hampered by geographi- their wide geographical extent was revealed by off- cal variation in the amount of uplift, and by the shore drilling in the North Sea. Equivalents of the interplay between uplift and eustatic sea-level Danish ash-series are now known to extend beyond change. the North Sea Basin, into the West of Shetlands area and into the Goban Spur area, providing a valuable interregional marker for the lower NP10 Paleocene/Eoeene boundary events interval. Several phases of ash deposition have now The Paleocene/Eocene boundary has yet to be been recognized, with compositional changes formally defined, but historical considerations reflecting progressive stages in the volcanic history require that it will eventually be placed within the of the northeast Atlantic rift zone (Morton & Knox NP9 to early NP10 interval, which, in the mean- 1990). time, is often referred to as the Paleocene/Eocene boundary 'interval' or 'transition'. This interval Sequence stratigraphy and crustal history includes several events of global significance, of which the following may be considered the most As with the other methods of stratigraphic analysis, important: sequence stratigraphy has to a large extent devel- oped separately in the offshore areas (e.g. (1) a pronounced short-term negative shift in Armentrout et al. 1993; Den Hartog Jager et al. carbon isotope values (Corfield & Norris 1996; 1993; Galloway et al. 1993; Mitchell et al. 1993; Stott et al. 1996); Jones & Milton 1994) and the onshore areas (e.g. (2) an oceanic 'benthic extinction' event, involv- Plint 1988; Grly & Lorenz 1991; Jolley 1992; ing a dramatic reduction in both numbers and Hardenbol 1994; Knox et al. 1994; Powell, et al. diversity of benthic foraminifera in the oceans 1996; Vandenberghe in press). It is only recently (Thomas & Shackleton 1996); that studies have been published that combine the (3) an influx of kaolinite into both the oceans and two. Studies of this kind range from broad shelf seas (Thomas & Shackleton 1996); overviews (e.g. Neal 1996), in which depositional (4) interchange of mammals between North systems are linked to both long-term and short-term America and Europe, leading to cosmopolitan sea-level change, to detailed, often biostratigraphi- faunas with an exceptional level of species in cally driven, analysis of restricted stratigraphic common (Hooker 1996); intervals (e.g. Jolley 1996; Powell et al. 1996). (5) extensive uplift and volcanism associated with Sequence stratigraphy provides a useful vehicle the lead up to opening of the North Atlantic for the compilation of diverse, multidisciplinary between Greenland and Rockall (Ritchie & stratigraphic data. However, it is clear from the NW Hitchen 1996). European record that the sequence stratigraphy of The relative timing of these events has yet to be the region cannot be assessed independently of its fully established, and is the focus of IGCP Project crustal history, which during the early Paleogene as 308. Until this timing is properly established, the a whole was strongly influenced by both Atlantic ultimate cause of all these changes will not be and Alpine processes. While the overall aim must known. Explanations for the individual features be to develop a sequence stratigraphic scheme for include: (1) a change in the pattern of oceanic the whole of NW Europe, it is clear that proper circulation, with warming and/or increased salinity appreciation must be given to the effect of local tec- of bottom waters causing mass mortality among tonics on the relative sea-level curve for different the benthic communities; (2) rapid changes in parts of the region. The influence of tectonism productivity in the oceans; (3) climatic warming, during the Paleocene has been amply demonstrated with enhanced chemical weathering leading to an by studies on the Paleogene uplift history of the increased production of kaolinite; (4) the develop- central and northern North Sea, with an uplift of ment of a land-bridge between North America and over 400 m proposed for the Outer Moray Firth Europe as a result of regional, probably plume- area (Nadin & Kusznir 1996). Tectonic control is related, uplift, perhaps enhanced by eustatic sea- also proposed as the underlying mechanism for the level fall. generation of large-scale sequences in the Eocene The NW European succession, with its detailed, (Joy 1996). Such a strong tectonic signal is hardly multi-component stratigraphic, tectonic and Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
6 R.W. O'B. KNOX volcanic record, provides a unique opportunity to sections in onshore Denmark) similarly have their unravel many of these temporal relationships. limitations. In particular, the absence of calcareous However, the coverage of this critical interval is facies precludes both the identification of the somewhat fragmented, and little attempt has been standard planktonic biozones and the application of made to piece it together other than in the general standard techniques of carbon and oxygen isotope context of regional stratigraphic compilations. analysis. However, in their favour, these sections do Attempts at identifying the influence of global provide a more or less continuous record across the events on the NW European stratigraphic record Paleocene/Eocene boundary interval and must have been largely concerned with the role of surely contain some reflection of any major change eustasy in the history of relative sea-level change. in the global environment. To assess the influence of specific global events, attention has focused largely on the early part of the Paleocene-Eocene boundary interval, during which The basinal record of the central North Sea major changes are known to have taken place in the oceanic environment and in the distribution of In the context of a relatively deep-water, non- terrestrial faunas. Such studies include assessment calcareous, clastic facies, and bearing in mind the of the history and mechanisms of mammal events recorded from oceanic sections, reflections migration (Hooker 1996) and the recent search for of any change in oceanic circulation or climate isotopic signatures in the Paleocene of the Paris might be expected in (i) the benthic community Basin (Stott et al. 1996). Unfortunately, both (dominantly agglutinated foraminifera), (ii) the studies have of necessity been concerned primarily nature and relative dominance of terrestrial with the terrestrial sections of the southern North palynomorph assemblages,~ and (iii) the compo- Sea margin, which suffer from the limitations sition of the clay mineral assemblages. Evidence imposed by incompleteness of the stratigraphic from published and unpublished sources indicates record. There is a real possiblity that some of the that significant environmental changes are recorded events identified in the oceans will be represented by all three of these components. in these sections by hiatuses. The offshore sections In terms of central North Sea lithostratigraphy, of the central North Sea (and equivalent deep-water the 'Paleocene/Eocene boundary interval' com-
Fig.2. Stratigraphic events in the early part of the Paleocene-Eocene boundary interval in well 22/10a-4, and possible correlation with the onshore succession of southern England. Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
CORRELATION OF THE EARLY PALEOGENE IN NORTHWEST EUROPE: AN OVERVIEW 7
prises the uppermost Lista Formation, the Sele characterized by an increase in the proportion of Formation, and the Balder Formation. Tephro- marine forms and by the incoming of the genus stratigraphic correlation indicates that much of Apectodinium. A peak abundance of Apectodinium the Balder Formation and the uppermost Sele occurs in the upper part of the turbidite sandstone Formation (unit $3) is of NP10 age, with the unit, immediate below the high-gamma spike. NP9/10 boundary probably occurring in the upper Clay mineral assemblages also display marked part of unit $2. The remainder of the Sele changes over this interval. The Lista Formation Formation is probably of NP9 age. is characterized by kaolinite-free assemblages, A pronounced upward reduction in the abun- dominated by smectite or chlorite (both probably dance and diversity of benthic foraminiferal assem- derived from alteration of fine-grained volcanic blages has long been recognized to occur near the material). Similar assemblages initially persist into base of the Paleocene/Eocene boundary intervals the lower part of unit S l a, but kaolinite appears offshore, corresponding to Sele/Lista formation about half way up the unit, reaching a low peak boundary as defined by Knox & Holloway (1992). within the turbidite sandstone unit. After a slight The existence of cores through this interval in a rel- fall-off in the lower part of unit Slb, kaolinite atively expanded Central North Sea section (well increases rapidly, paralleling an overall increase in 22/10a-4) has allowed other features associated grain size. with this 'benthic extinction' event to be recorded It is clear that important environmental changes in detail for the first time (O'Connor & Walker took place in the central North Sea in the early part 1993). Most significantly, the reduction in the ben- of the Paleocene/Eocene boundary interval. Two of thic assemblages takes place at the level where pale the features described above, the reduction in greyish green, waxy, unbedded claystones (Lista benthic faunas and the influx in kaolinite, parallel Formation) are replaced upwards by medium to events described from oceanic sections. The dark grey, crudely laminated mudstone (Sele marked increase in the proportion of terrestrial Formation, base unit S la) (see O'Connor & Walker palynomorphs can be interpreted in terms of 1993, fig. 21). The boundary probably marks an increased terrestrial run-off, resulting from either increase in sedimentation rate, with greater reten- sea-level fall or climatic change. According to tion of organic matter. However, the reduction in some interpretations, the increase in relative numbers of the agglutinating foraminifera cannot abundance of Apectodinium might be an indicator be ascribed simply to a dilution effect, as it is of warming, but other factors, such as sea-level accompanied by a significant reduction in diversity. change, may also be involved (Thomas 1996). Further up the section, a second lithological However, an increase in temperature around the change takes place, marked by a progressive S 1a/S l b boundary has previously been inferred by increase in the number and thickness of turbidite Schrrder (1992) from the composition of the sandstone layers (c.859Y6" in fig. 1 of O'Connor & palynomorph assemblages, and fluctuations in the Walker 1993). An upward increase in fine-scale composition of the palynomorph assemblages lamination, and decrease in bioturbation, is reported from the remainder of the Slb section observed within this unit, indicating the onset of (Forties Sandstone) may also be of climatic origin bottom-water anoxia. The top of the turbidite sand- (Wood & Tyson 1996). stone unit is marked by a rapid upward transition to The occurrence of kaolinite in the lower part of delicately laminated mudstone. A marked high the Sele Formation is almost certainly an indicator gamma-ray wireline-log spike occurs at the top of of increased humidity in the source areas, as it is this transition (here placed at c. 8567' 6" in fig. 1 of virtually absent from the underlying Lista O'Connor & Walker 1993). This gamma-ray spike Formation, even in sandier facies lower in the is accompanied by a sharp and sustained increase in section. However, variations in the relative uranium content. Benthic foraminifera disappear abundance of kaolinite within the Forties Sandstone altogether in the uppermost part of the the turbidite are clearly related to the overall grain-size, and sandstone, and are of only rare occurrence through- cannot be taken as a direct climatic indicator. out the remainder of the Sele Formation. A combination of palynological data plus clay The changes in microfaunal abundance and mineral data thus points to at least a general climat- diversity are accompanied by changes in the paly- ic warming around the Sla/Slb boundary, close to nomorph assemblages (Thomas 1996), with the the high gamma-ray wireline-log spike, and it is upward transition from greenish claystone to grey tempting to think that various events recorded mudstone being marked by a dramatic increase in over this interval may in some way be related to the the ratio of terrestrial to marine forms. This feature broadly coeval events recorded from ocanic persists to the top of the lower division of the Sele successions. However, many other factors need to Formation (unit Sla of Knox & Holloway 1992), be taken into account in view of the progressive above which the turbidite sandstone unit is restriction and evental isolation of the North Sea Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
8 R.W. O'B. KNOX
Basin over this period. For example, there is good From these observations, it is plausible to evidence for substantial sea-level fall at the suggest that the Upnor Formation and immediately Lista/Sele formation boundary, leading to an overlying continental beds are represented in the increased land area and a sharp influx of terrestrial central North Sea by the transitional, upward- palynomorphs. A second sea-level fall may be coarsening turbidite sandstone unit. On this represented by the base of the turbidite sand- correlation, the turbidite sandstone unit would stone unit, but several features, such as the approximate to the horizon of mammal migration slight decrease in the proportion of terres- (Hooker 1996) and the negative carbon isotope trial palynomorphs, the abrupt incoming of excursion recorded by Stott et al. (1996). The Apectodinium, and the increase in fine-scale principal reduction in benthic faunas would, lamination point more to a change in basin/hinter- however, not be related to the oceanic 'benthic land configuration than to a simple sea-level fall. extinction', however, since it occurs lower in the The succeeding laminated mudstones, which are succession. associated with a reduction in kaolinite percentage, In the absence of any reliable onshore- a reduction in Apectodinium abundance, and a offshore corelation tool, such correlations must be sustained reduction in the proportion of terrestrial considered speculative, as must any connection palynomorphs, are interpreted as representing between the events recorded from the central North transgression. The maximum-flooding may be Sea and the global changes that were occurring represented by the high-gamma peak. Alternatively, at about the same time. It is quite possible to it may occur close to the kaolinite and terrestrial explain the central North Sea events purely in palynomorph minima, in which case the high- terms of regional tectonism, with the progressive gamma peak could be regarded as a purely basinal elimination of benthic faunas and the onset of basin phenomenon, perhaps related to the onset of full anoxia resulting from basin isolation and the influx anoxia. of kaolinite resulting from increased humidity and The fall in sea-level represented by the precipitation in response to changes in palaeogeog- Lista-Sele facies transition is reflected throughout raphy. We are thus left with the tantalizing situation the UK North Sea area, with an overall fall in sea- that some of the oceanic events recorded from the level of at least 100 m being inferred for southern Paleocene/Eocene boundary interval are paralleled England. In the Bradwell section (see Knox et al. in the broadly coeval sediments of the central North 1994) fine-grained mudstones of Rhabdammina Sea, but that on present evidence it is not possible biofacies (Thanet Formation, Lista equivalent) are to say whether some or all of the North Sea events overlain by pedogenically altered lagoonal or shal- are caused by local (North Atlantic) tectonism or by low marine sandy mudstones (upper division of the global climatic change. Of course, it may be wrong Upnor Formation, Sele equivalent). It is difficult to to think in terms of these two extremes, as changes explain a sea-level fall of such magnitude and in oceanic circulation pattern, global climatic rapidity other than by tectonic uplift (see also Neal change, and mammal migration could themselves 1996). all be explained in terms of plate reorganization Precise correlation of the onshore and offshore prior to the opening of the North Atlantic (Eldholm sections is uncertain. However, it seems likely that & Thomas 1993). the hiatus at the base of the lower division of the In conclusion, one of the most intriguing aspects Upnor Formation (Ellison et al. 1996), during of early Paleogene stratigraphy is the evidence in which the Thanet Formation was subjected to sediments of late NP9 age for a major but decalcification and other effects of meteoric leach- short-term interruption to the long-term trends of ing, equates with the major sea-level fall recorded oceanic warming and climate change. It must be at the base of the Sele Formation (unit S 1a). In both expected that such a profound change in the world's cases, the overlying sediments appear to have been oceans would have left its mark on the terrestrial deposited at a time of continued, if somewhat and epicontinental marine record, in which case restricted, connection with oceanic waters (allow- the onshore and offshore successions of northwest ing influx of NP9 calcareous nannofloras to south- Europe must be prime candidates for study. ern England). Further restriction of free connection Such study would not be of purely academic between the North Sea and the eastern Atlantic interest, since an understanding of the relative appears to have occurred prior to deposition of the roles of regional uplift, eustasy, and global upper division of the Upnor Formation, which is of climatic change would not only answer some restricted marine facies. The onset of continental long-standing scientific questions, but also deposition, represented by the base of the lower leaf throw new light on the origin of one of of the Reading Beds and by the base of the Argiles northwest Europe's most productive oil reservoirs, plastiques bariolres marks the complete closure of the Forties Sandstone. The elucidation of event the southwestern oceanic connection. stratigraphy within the Paleocene/Eocene Downloaded from http://sp.lyellcollection.org/ by guest on October 2, 2021
CORRELATION OF THE EARLY PALEOGENE IN NORTHWEST EUROPE: AN OVERVIEW 9 boundary interval in northwest Europe thus I am grateful to Richard Corfield, David Jolley and Andy provides a prime example of the benefits of Morton for helpful comments on the manuscript. interchange of data and ideas across the Publication is with the approval of the Director, British academic/commercial divide. Geological Survey.
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