Milankovitch-scale palaeoclimate changes in pale–dark bedding rhythms from the Early Cretaceous (Hauterivian and Barremian) of eastern England and northern Germany

ELSEVIER

Palaeogeography, Palaeoclimatology, Palaeoecology 154 (1999) 133–160

Milankovitch-scale palaeoclimate changes in pale–dark bedding rhythms from the Early Cretaceous (Hauterivian and Barremian) of eastern England and northern Germany Jo¨rg Mutterlose a,Ł , Alastair Ruffell b a

Institut fu¨r Geologie, Ruhr Universita¨t Bochum, Universita¨tsstraße 150, 44801 Bochum, Germany b School of Geosciences, Queen’s University, Belfast, BT7 1NN, N. Ireland, UK Received 28 October 1998; accepted 26 May 1999

Abstract Early Cretaceous (Valanginian–Albian) successions in northwestern Europe occupy an ideal palaeoceanographic location in which to study the interplay between the Arctic–Boreal and Tethyan seaways or realms. Marine Early Cretaceous sediments of the North Sea borderlands (north Germany, eastern England) are dominated by fine-grained siliciclastic deposition with carbonates. Occasionally this succession is characterised by visually striking bedding rhythms, comprising calcareous and less calcareous clays or pale–dark alternations on a scale of 0.5–1 m. Such rhythmic sedimentation becomes most dominant in the early to late Hauterivian, parts of the early Barremian and in the Aptian. This study concentrates on late Hauterivian sections in England and Germany, which have remained relatively unaffected by deep burial, tectonism or diagenesis. Changes in geochemistry are related to floral and faunal alternations and thus directly to palaeoenvironment. The wide range of palaeontological and geochemical analyses show conclusively how bedding rhythms, developed in the Milankovitch band, are the result of palaeoclimate change. Pale layers were ‘Tethyan influenced’ in their microflora and fauna, indicating warm surface waters that were poor in nutrients and developed during warm, seasonally arid hinterland climates. Dark layers were ‘Boreal influenced’, displaying features consistent with cooler waters, rich in nutrients. Such dark layers reflect cooler, more humid hinterland conditions. The coincidence of Tethyan taxa with bundles of pale layers displaying high gamma-ray emission and elevated radioactive element contents suggests transgressive or condensed beds. The bundling of pale–dark rhythms coincides with the long-term Milankovitch eccentricity cycle. This paper documents direct links between the reaction of hinterland weathering, marine floras and floras to Milankovitch-scale changes in bed lithology and thickness.  1999 Elsevier Science B.V. All rights reserved. Keywords: Lower Cretaceous; bedding rhythms; Milankovitch cyclicity; geochemistry; spectral gamma-ray; calcareous nannofossils; foraminifera

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author. Fax: C49-234 7094571; E-mail: [email protected]

0031-0182/99/$ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 1 - 0 1 8 2 ( 9 9 ) 0 0 1 0 7 - 8

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1. Introduction Early Cretaceous sedimentation in northwestern Europe was confined to a number of predominantly marine and non-marine basins. The basins show temporally episodic marine connections, reflecting the eustatic and tectonic processes that caused obstruction to marine circulation. The North Sea and subsidiary basins in eastern England and northern Germany all had marine connections with the Boreal–Arctic Sea to the north (Fig. 1). In addition, there also existed seaways that provided marine connections with the Tethys Ocean to the south. This palaeogeographic position makes northwestern Europe, and most especially the North Sea borderlands, an ideal location in which to study the effects of the interplay between two ocean masses and two

floral=faunal realms. An indication that the North Sea really does represent the area where two distinct ocean masses met, is provided by the lithofacies, which show interdigitation of the Tethyan carbonate-rich and Boreal clay-rich deposition (Mutterlose, 1989). Marine sedimentary successions both in northeastern England and northwestern Germany contain pale–dark bedding rhythms throughout the Hauterivian. These bedding rhythms, which have a thickness of 0.5–1 m for each pale–dark alternation, become most visible near the Hauterivian–Barremian boundary interval in basin-margin successions (Mutterlose, 1989). In a comprehensive study of pale– dark bedding rhythms in the Early Cretaceous of the northwestern German Basin, Schneider (1963, 1964) compared the variation of calcium carbonate content

Fig. 1. Palaeogeographic map for the Hauterivian of northwestern Europe, showing the studied localities (Speeton, Frielingen, Gott).

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with the light reflectivity in the alternating lighter and darker bedded sediments. Kemper (1987) used the results of this study to show the cyclic character of Early Cretaceous sediments, which he thought might represent climatic variation in one of the Milankovitch bands. Nebe and Mutterlose (1999) have conducted Fourier analysis on such rhythms, finding unequivocal proof of a Milankovitch control on deposition. Similar pale–dark bedding rhythms have been described from northeastern England (Speeton) by Rawson (1971) and Rawson and Mutterlose (1983). ‘Marl–limestone’ bedding rhythms of comparable age described by Cotillon (1984, 1987) and Cotillon and Rio (1984) from the Vocontian Basin (southeast France). These successions are even more visually striking than the North Sea counterparts studied herein, being composed of alternating calcareous black shales and clay-rich light-grey or cream limestones. The origin of bedding rhythms in mixed carbonate–clay successions is contentious and discussed by Einsele and Ricken (1991), who suggest that such alternations may be explained by primary autocyclic and=or allocyclic control or diagenesis. The arguments for primary controls (sealevel, climate) and secondary control (diagenesis) are summarised in Weedon (1986), Hallam (1986) and Einsele and Ricken (1991). It is the carbonate-rich depositional environments of the Early Jurassic of the U.K. and the Cretaceous of southeast France that have been studied most intensively with regard to changing mineral populations (Artru et al., 1969; Deconinck and Chamley, 1983; Deconinck, 1987) and palaeoecology (Sellwood, 1970). The problem of diagenetic overprinting is greatest in these limestone– shale successions, thus we chose the more subtle pale–dark beds for our study. The pale–dark rhythms that are developed over a wide geographical area in the Hauterivian (and to a lesser degree in the Barremian) of northern Germany and the North Sea (including eastern England) are ideal candidates for assessing depositional controls as here, early diagenesis is likely to be less pervasive than in shallow-water carbonate-dominated rhythms. The Hauterivian throughout this area is dominated by clay deposition in gradually subsiding basins with little post-depositional fluid movement other than expulsion under compaction. The rocks are generally thermally immature, and have

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not been buried to more than 2 km depth. This paper aims to decipher the origin of pale–dark bedding rhythms in northwestern Europe by an integrated study of geochemistry (clay mineralogy, gamma-ray spectroscopy) and micropalaeontology (calcareous nannofossils, foraminifera, mesofauna).

2. Geological setting 2.1. General Our study centres on the Hauterivian stage of the Early Cretaceous in northwestern Europe (Fig. 2). The early Hauterivian and preceding latest Valanginian were characterised by a marine transgression which caused expansion of the internal seaways across the North Sea and north Germany (Mutterlose, 1996). This transgression began in the latest Valanginian and reached its maximum in the early Hauterivian Endemoceras amblygonium ammonite Zone. Thereafter, Hauterivian palaeogeography remained fairly stable, with a sea-way towards the Tethys via Poland (Carpathian sea-way) allowing floral and faunal exchanges for most of the time. The latest Valanginian and the earliest Hauterivian are characterised by a strong influx of Tethyan nannofloras and faunas, which must have used this Carpathian sea-way. The regression of the late early Hauterivian (Endemoceras regale ammonite Zone) was followed by an extensive mid-Hauterivian transgression (lower Simbirskites (M.) staffi ammonite Zone and beds immediately below). After a regression in the mid-late Hauterivian (upper S. (M.) staffi ammonite Zone and Simbirskites (C.) gottschei ammonite Zone) another transgressive peak is to be observed in the latest Hauterivian (Simbirskites (C.) discofalcatus ammonite Zone). Significant palaeogeographic and palaeoceanographic changes occurred at the Hauterivian–Barremian boundary; the overall regressive nature of the Barremian is often quoted (Rawson and Riley, 1982; Ruffell and Batten, 1990). A regression in the earliest Barremian created brackish–lacustrine conditions in central and southern Poland. Finely laminated sediments, known as the Hauptbla¨tterton and various thin Bla¨tterton horizons, are typical of the Barremian of northwestern Germany. These sediments are enriched in organic matter (6–8% TOC).

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Fig. 2. Lithology and biostratigraphic ranges of the sections at Speeton, Frielingen and Gott.

2.2. Specific locations (England and Germany) The Speeton section is situated in northeast England about 15 km southeast of Scarborough (Fig. 1). The succession consists of approximately 115 m of clays and calcareous clays of the Ryazanian–Albian. About 17 m of latest Hauterivian (i.e. Simbirskites discofalcatus ammonite Zone) and early Barremian (i.e. Hoplocrioceras rarocinctum and Hoplocrioceras fissicostatum ammonite Zones) sediments were considered in this study (Fig. 2). The uppermost Hauter-

ivian consists of about six calcareous clay–poorly calcareous clay rhythms, with an average thickness of about 0.5 m. The early Barremian comprises six similar rhythms, each having an average thickness of about 1.4 m (Fig. 3). The carbonate content ranges from 2.2 to 13.6%. The dark beds show a variation of carbonate values from 2.2 to 6.8%, the pale beds from 2.9 to 13.6%. The stratigraphy and lithology of the Speeton section are described by Rawson (1971) for the Hauterivian and by Rawson and Mutterlose (1983) for the Barremian. The Speeton section lies on

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Fig. 3. Profiles of the sections at Speeton, Frielingen and Gott.

the western margin of the Cleveland Basin, thus representing a more marginal setting comparable to the Gott section in northern Germany (Fig. 1).

The Frielingen section lies about 20 km northwest of Hannover in northern Germany (Fig. 1). Approximately 20 m of clays and calcareous clays

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Fig. 4. Photograph of the Frielingen section, western wall of pit, showing typical pale–dark rhythms.

of latest Hauterivian (Simbirskites discofalcatus ammonite Zone; compare Fig. 2) are exposed. Dark to medium-grey clays alternate with pale grey to white clays. The succession consists of 22 pale–dark rhythms, each having an average thickness of about 0.9 m (Fig. 3). Carbonate values vary between 5 and 53%. Dark beds show a variation in carbonate contents between 5 and 23%, whereas the pale beds range between 12 and 53%. Organic carbon varies from 0.3 to 1.7% Corg, with most samples ranging from 0.5 to 0.9% Corg. The Corg values for the dark beds are in general slightly higher (0.2–0.3%) than those of the directly under- and overlying pale beds. The pale–dark bedding rhythms are extremely well exposed in the Frielingen pit and offer a good possibility for detailed studies (Fig. 4). A comprehensive description of the lithology, stratigraphy, flora and fauna for this outcrop is given by Mutterlose and Ruffell (1997). Frielingen lies in the central part of the northwestern German Basin, approximately 50 km from both the Hildesheimer Halbinsel in the south and the Pompeckj’s Swell in the north (Fig. 1). The Gott section is situated about 30 km south of Hannover (Fig. 1). About 85 m of clay and calcareous clays are exposed. The late Hauterivian (Simbirskites discofalcatus ammonite Zone; Fig. 2), which is 11.4 m thick, is characterised by obvious bedding rhythms. These consist of 13 pale–dark rhythms, with

an average thickness of about 0.9 m (Fig. 3). Carbonate content varies between 4 and 27% in the pale beds, and between 6 and 15% in the dark beds. Rich and diverse micro- and macrofaunas occur, representing a well-oxygenated environment (Mutterlose, 1989, 1991). The early Barremian (Praeoxyteuthis pugio belemnite Zone) is 8.3 m thick and similar in lithology to the late Hauterivian. It comprises visually striking pale–dark rhythms, deposited in a well-oxygenated nearshore environment (DChondrites Beds). The early Barremian consists of thirteen pale–dark rhythms, with an average thickness of about 0.6 m (Fig. 3). The Gott section has been described in some detail by Mutterlose (1997); it is located in the eastern part of the northwestern German Basin, about 20 km north of the Hildesheim palaeo-peninsula. It thus reflects a marginal marine setting in comparison to the Frielingen section.

3. Methods 3.1. Geochemistry Two aspects of the geochemistry of the Early Cretaceous succession in northern Germany and eastern England are studied. First, long-term (stage-length) changes in lithology and geochemistry are assessed

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using either previously published work and accumulating new background spectral gamma-ray data. Thereafter, detailed bed-by-bed analyses of the late Hauterivian pale–dark rhythms were made in an attempt at correlating faunal changes with geochemistry. The three sections under consideration were logged in detail with accompanying outcrop measurement of spectral gamma-ray data taken at 30-cm intervals. In total, some 200 samples of clay and calcareous clay were analysed by X-ray diffraction and X-ray fluorescence with every tenth sample examined under the scanning-electron microscope. The results of some 72 analyses are discussed in detail here. Sampling for this work was concentrated on the late Hauterivian sections exposed at Speeton on the Yorkshire coast between Filey and Flamborough Head (uppermost C beds) and in the Frielingen and Gott claypits near Hannover. High sample densities were also obtained for preceding studies (Ruffell and Batten, 1995) from borehole sections across the Barremian– Aptian boundary drilled around Hannover and from British Geological Survey boreholes drilled in the mid-North Sea. At Frielingen pale–dark rhythms are particularly well-developed and thus sample locations can be easily related to lithological logs (see Fig. 4 and Mutterlose and Ruffell, 1997). 3.1.1. X-ray diffraction for clay mineral analysis The methodology employed here is comparable to that described by Tucker (1988). This stage-bystage analysis of clays is largely based on techniques developed by Gibbs (1967) and Brindley (1980). In summary, each clay sample was analysed for bulk mineralogical content by X-ray diffraction analysis of powdered sample initially on a Phillips PW1170 and subsequently on a Siemens D5000 diffractometer. To make semi-quantitative estimates of the proportions of the clay minerals, the sample was disaggregated, the <2-µm fraction separated and mounted on a glass or tile slide by rapid air-drying or vacuum suction. The glass slide method was utilised in the laboratories of Birmingham and Belfast Universities, the ceramic tile method was used at Imperial College. Sets of samples were re-run at each laboratory in order to provide internal standards to the preparation process. Both methods gave similar results; the glass-mounted samples generally gave sharper

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peaks (corresponding to the 001 [basal] reflection of the clays), but on heating the glass preparations often desiccated. Proportionally (both in terms of semi-quantitative analyses and estimates of ratios) no significant difference was detected between ceramic and glass mounts. Samples were glycolated and heated to 490–550ºC in order to estimate the relative quantities of smectite, chlorite and kaolinite in convention with the methods of Tucker (1988). 3.1.2. X-ray fluorescence and scanning-electron microscope All samples were analysed by X-ray fluorescence and atomic absorption spectroscopy for trace-elemental content. Varying trace elements can reflect accurately the changing proportions of detrital clays within a succession, and serve as a useful guide to radioactive element content within organic-rich shales, condensed beds etc. Generally, organic-rich sediments are enriched in radioactive elements and heavy metals as these form complex metallo-organic compounds. This is also true of some of the dark clays of the Hauterivian and especially Barremian sections studied here. A total of twenty representative samples were examined under a scanning-electron microscope in order to determine the physical nature of the clays within the pale and dark beds. Most showed amorphous patterns of a mixed clay population; some crystalline forms of kaolinite were noted in microfractures and inside microfossils. Samples from beds 116 at Frielingen and 207 (Aptian) at Gott showed smectite in flocculated form, suggesting a bentonite component. Although volcanic clays are well-known from the Aptian of Germany, (Kemper, 1982), they are not recorded in the Hauterivian. Volcanic clays are known in the Hauterivian of the North Sea=Yorkshire (northeast England: Knox, 1991) and from the Berriasian of the Bentheim area (Zimmerle, 1979). 3.1.3. Outcrop measurements of gamma-ray radiation It is widely recognised that increased gammaray emission from sedimentary successions (whether recorded in borehole or at outcrop) is the result of concentration, usually by reworking or by condensation. In mud-dominated depositional sequences out-

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crop gamma-ray peaks are associated with ‘hot’ or transgressive shales (Van Wagoner et al., 1990). The same is true of borehole measurements from gammaray logging: gamma-ray ‘spikes’ are commonly used to define flooding surfaces (Van Wagoner et al., 1990). A hand-held gamma-ray spectroscope (Scintrex GS5) was used to make measurements of total gamma-ray count; potassium, uranium and thorium. The measurements were conducted stratigraphically (i.e. with only vertical spacing), using the methodology of Slatt et al. (1992) whereby five readings of 10 s duration each were taken, the lowest and highest discarded and the remainder averaged. The results from gamma-ray logging conducted in Germany comprise a high-resolution study of the pale– dark rhythms exposed at Frielingen. In England, two sets of data were collected: (1) ‘background’ measurements from the main outcrop of the Speeton Clay Formation; (2) ‘high-resolution’ measurements from the topmost 3 m of the C Beds. The background measurements were taken before the high-resolution study to provide a safeguard against the high-resolution measurements being accidentally made in a uniquely radioactive (or non-radioactive) part of the succession. The high-resolution dataset was obtained by sampling across the same interval, and at the same scale to that conducted for the studies of micropalaeontology (20–50 cm). This enables direct comparison of results for both the German and English sections. 3.2. Micropalaeontology The same, or immediately adjacent samples to those taken for geochemical analysis were also analysed for their micropalaeontological content, with specific regard to palynomorphs, calcareous nannofossils, foraminifera and ostracods. Three aspects were studied: (1) long-term variations throughout the Early Cretaceous of both the northwestern German Basin and the English Cleveland Basin (eastern North Sea); (2) a low resolution analysis of the late Hauterivian pale–dark rhythms was made in an attempt to correlate faunal changes with varying geochemical contents; (3) high-resolution studies of pale and dark beds were performed on the German and the Speeton sections to analyse the internal structure of individual rhythms. In this paper we only

present the data relevant to pale–dark rhythms, i.e. the results of the low- and high-resolution studies. 3.2.1. Calcareous nannofossils A total of 161 samples from the Frielingen, Gott and Speeton sections were examined for calcareous nannofossils. For the low-resolution study 26 samples were analysed from 22 pale–dark rhythms of the Frielingen and 27 samples from 13 pale–dark rhythms of the Gott section. Thus about 1–2 samples were studied from each bed, with a sample spacing of 20–50 cm (approximately the same as in the gamma-ray logging). In the high-resolution study a total of 108 samples at intervals of 2–6 cm were evaluated from three rhythms (C2F–LB6; 3.25 m) of the Speeton section (55 samples) and one rhythm (beds 111–113; 1.10 m) from the Frielingen section (53 samples). This resulted in 5 to 19 samples for each lithological unit, depending on the thickness of individual beds. This sample density was significantly higher than the gamma-ray logging, but the same as the high-resolution study of clay minerals. Simple smear-slide preparations were examined under the light microscope using a magnification of 1500ð. The abundance of calcareous nannofossils in the Lower Cretaceous material is variable. Calcareous nannofossils may constitute anywhere from 0% to 50% of the rock. Abundances for each sample were gained by counting at least 300 specimens or all specimens in at least 200 fields of view. For examination under the scanning electron microscope, samples were prepared following the procedures outlined by Wise and Kelts (1972). 3.2.2. Foraminifera and ostracods The study is based on a total of 114 samples from all three localities. For the low-resolution study 23 samples were analysed from 22 pale–dark rhythms of the Frielingen section and 34 samples from 13 pale–dark rhythms of the Gott section. 1–2 samples were studied from each bed at a vertical spacing of 20–50 cm. In the high-resolution study 57 samples were evaluated from one to four pale–dark rhythms of the Speeton (30 samples) and Frielingen (27 samples) sections; distances between samples are 10 cm. For each sample 250 g of raw material were processed. Samples were dried overnight in a 60º oven before washing. The material was disaggregated in

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hot water and H2 O2 then washed over a 63 µm sieve. The dry residue was split into five different fractions: >630 µm, 315–630 µm, 200–315 µm, 100–200 µm and <100 µm. The faunas (foraminifera, ostracods, mesofauna) of the fractions >630 µm, 315–630 µm and 200–315 µm were completely picked. The fraction 100–200 µm, characterised by high abundant assemblages in all samples, was split down to 1=8, 1=16 or 1=32 (when needed). For each sample at least 300 specimens were picked in order to gain a representative picture of diversity and abundance of foraminifera. The residues were examined under an Olympus SZ 60 light binocular microscope with light intensification of 140ð. The taxonomy of Meyn and Vespermann (1994) was found to be highly suitable for this study. In order to show major trends in the vertical fluctuation of foraminifera the most common taxa were grouped. Species which make up less than 2% of the fauna were neglected. Ostracods and mesofaunas were only studied qualitatively. Carbonate measurements were made by using atomic absorption spectrometry (AAS). Material and samples are deposited in the Geological Department in Bochum.

4. Results 4.1. Geochemistry Analysis of the clay mineralogy of a sedimentary rock can yield much information on the tectonic provenance, climate (and weathering regime in the sediment source-lands) and sea-level history of an area. Original clay mineralogical contents can be significantly altered by diagenesis related to fluid movement and low-grade metamorphism. The formation of clay minerals by weathering processes reflect the physio-chemical climatic conditions in the sediment source-lands. Clay minerals are rarely altered during transport or deposition (Gibbs, 1967), and thus a sedimentary succession that has not been buried significantly may preserve clay mineralogical variations representing original changes in climate, sea level or tectonics. 4.1.1. Proxies (1) Illite and chlorite are common products of the physical weathering of medium-grade metamor-

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phic rocks (often called ‘basement’ in undeformed basins). (2) Illite may also form when clays are subjected to periodic intense heat under evaporitic conditions (‘illitisation’). (3) Kaolinite is a chemical weathering product of acidic–intermediate igneous rocks, arkoses and feldspathic sediments, formed under humid and warm climatic conditions. (4) Kaolinite is commonly larger, heavier and more susceptible to flocculation than other clays, and thus makes a good indicator of ‘nearshore’ conditions when found in abundance in sediments (Gibbs, 1967). (5) Kaolinite is also a common diagenetic mineral, forming through the action of acidic groundwaters. Well-crystallised kaolinite is often found filling void spaces and it commonly reduces porosity in reservoir sandstones. (6) Smectite is a term used for a group of expandable clay minerals, including calcium or sodium montmorillonite. Smectite is a common alteration product of alkaline volcanic rocks and is usually found as the major constituent of tuff or bentonite (tonstein) layers. (7) Smectite may also form by the weathering of basic–alkaline igneous rocks under hot and humid climatic conditions and has a wide distribution in the oceans of the world, being found in deep seas (unlike kaolinite). (8) Interstratified illite and smectite may form mixed- layer clays indicative of very early diagenetic reactions (Deconinck and Chamley, 1983). Such mixed-layer minerals commonly form through the wetting and drying of soil profiles in seasonally arid climates. 4.1.2. Clay mineralogy The results of a semi-quantitative analysis of the clay mineralogy of the pale–dark rhythms show that pale (calcareous) beds contain abundant mixed-layer illite–smectites and always some chlorite. The pale beds contain crystalline smectite, illite and kaolinite in lesser amounts than the dark beds. By contrast, the dark beds contain illite and kaolinite in abundance with lesser amounts of smectite and mixed-layer minerals. The variation in clay mineral abundances between dark and pale beds is rarely more than 10% in any one

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mineral species, and is significantly lower than the changes in the abundance of different clays throughout the upper Hauterivian. It is important to place bed-by-bed changes in clay mineralogy in the regional context in order to understand the likely mechanisms controlling their abundance. The error margins involved in semi-quantitative analysis of clay mineral populations are potentially very high. In reconnaissance studies of the clays of the Lower Cretaceous it was found that different methods of calculating mineral abundances could lead to a 5–10% variation in results. As bed-by-bed variations rarely exceed this figure, overall trends in the clay mineral population are taken as being more reliable than absolute values. In addition, due to the different origin of clay mineral species, relative proportions are also thought to reflect more accurately the changes in the amount of detrital clay minerals. The variations between pale and dark layers observed here are consistent throughout the Hauterivian sections studied. 4.1.3. X-ray fluorescence-based analysis of major and minor elements X-ray fluorescence analysis for major and trace elements was performed on a Siemens SRS 303 AS at Queen’s University Belfast. Clay samples from sites analysed at outcrop were crushed and pelleted with scans made for major and minor elements. Table 1 comprises a summary of the main hosts for different elements in marine mudrocks. The results from Germany and England are plotted in Tables 2 and 3. There are minor, yet consistent differences in elemental variation between pale and dark beds. The

noteworthy chemical variations include higher Si, Ba and Rb in the pale beds and higher Fe, Ca, and Zr in the dark beds. 4.1.4. Gamma-ray logging ž Gamma-ray logs displaying the total count of emissions from all minerals are similar to those used in boreholes and may be used to differentiate lithologies such as quartz sandstones or limestones from feldspathic sands or shales (Hurst, 1990). ž In heterolithic successions, gamma-ray peaks may be interpreted as representing flooding surfaces. ž Spectral gamma-ray logs allow differentiation of the radiation source (K, U or Th). ž Ratios of Th to K or U have been used to further help identify flooding surfaces in sequence stratigraphy (Davies and Elliott, 1996) ž Ratios of Th to K or U have also been used to assess the intensity of hinterland weathering (Parkinson, 1996) as Th is considered relatively insoluble compared to U and K which are leached in soils of a humid climate. Before commencing our bed-by-bed measurements of the pale–dark rhythms, we took 29 measurements of total count, K C U C Th, U C Th and Th from the entire Speeton Clay Formation, in order to assess the inherent variability through the succession (Table 4). This also provided a crude plot of gamma-ray variation for the whole section, in order to place the detailed study in the context. A similar background study was not possible in the German successions (Table 5), due to a lack of continuous

Table 1 Main mineralogical and rock hosts for the elements discussed in this study Si quartz, clay, mica, feldspar, biogenic

Ti clays, micas, rutile

Al clays, micas

Fe clay, micas, carbonate and oxide diagenesis

Mn pyroxenes, diagenesis

Mg carbonate, clays, micas

Ca carbonate

Na salts, feldspathoids, amphiboles, feldspars

K salts, micas, glauconite

P fossil bones and teeth, diagenesis

Cr clays

Cu diagenesis

Nb various, inc. organic matter

Th heavy minerals, clays

U clays, organic matter

V clays

Y various, clays

Zr zircons

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Table 2 Element variations between pale and dark beds at Frielingen analysed by XRF (AAS and gamma-ray data omitted) Major (%)

SiO2

TiO2

Al2 O3

Fe2 O3

MnO

MgO

CaO

Na2 O

K2 O

P2 05

Pale [13] Dark [22]

44.0 53.0

0.6 0.8

16.0 21.0

7.00 8.00

0.15 0.03

1.20 1.50

19.00 6.0

0.23 0.30

2.0 3.2

1.5 0.04

Minor (ppm)

Ba

Cr

Cu

Nb

Ni

Pb

Rb

Sn

Sr

Th

U

V

Y

Zn

Zr

Pale [13] Dark [22]

303 364

108 155

11 26

12 18

43 71

22 40

86 148

0 2.7

432 222

6 7.5

6 3

267 350

81 26

67 82

13 24

Table 3 Element variations between pale and dark beds at Speeton analysed by XRF (AAS and gamma-ray data omitted) Major (%)

SiO2

TiO2

Al2 O3

Fe2 O3

MnO

MgO

CaO

Na2 O

K2 O

P2 O5

Pale [22] Dark [13]

55.0 53.0

1.0 0.8

22.0 20.0

5.00 7.00

0.02 0.02

2.0 2.0

6.0 6.5

1.0 0.80

1.4 5.5

0.05 0.05

Minor (%)

Ba

Cr

Cu

Nb

Ni

Pb

Rb

Sn

Sr

Th

U

V

Y

Zn

Zr

Pale [22] Dark [13]

343 278

130 134

21 21

21 23

50 50

24 24

141 129

2.5 7

302 329

4.2 3.1

1.8 2.6

196 192

35 31

94 90

15 18

Table 4 Summary of gamma-ray emissions (Speeton, eastern England)

Dark (e.g. LB5E, C2A) Dark grey (e.g. C2D, C2F) Light (e.g. LB6, C1B) Light grey (e.g. C2C, C2E)

Total count (average)

U C Th C K (average)

U C Th (average)

1760 (7) 1595 (9) 1833 (7) 1721 (5)

50 (7) 50 (9) 55 (7) 56 (5)

25 (7) 20 (9) 25 (7) 27 (5)

Table 5 Summary of gamma-ray emissions (Frielingen, northern Germany; all values are derived from five measurements of 10 s each, the highest and lowest discarded and the remainder averaged)

Dark (e.g. 126, 128) Pale (e.g. 127, 129)

Total count (average)

U C Th C K (average)

U C Th (average)

1683 (30) 1703 (24)

196 (30) 220 (24)

152 (30) 160 (24)

sections. The high gamma-ray emission recorded from the basal D Beds of the Speeton Clay is from the lowest bed of the succession (a phosphate nodule bed: the ‘Coprolite Bed’) known for its high U and Th content (Scott et al., 1987). Low gamma-ray

emissions were recorded in the upper D Beds (Fig. 5) with high gamma-ray counts noted from the C Beds. These may be interpreted as broadly ‘transgressive’ signatures (based on the gamma-ray response alone. The upper C Beds spike in the gamma-ray profile is the subject of the high-resolution study. The uppermost C Beds were selected for detailed analysis as they clearly display the pale– dark rhythms under consideration: in addition, the horizon is in the upper parts of one of the three gamma-ray ‘spikes’ observed in the low-resolution analysis (Fig. 5). The overall high gamma-ray signatures of the low-resolution analysis form a slightly asymmetrical curve in beds C2D and C2C. In addition, certain bed junctions show abrupt increases in gamma-ray radiation. Such gamma-ray log motifs

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Fig. 5. The stratigraphic location and ‘background’ spectral gamma-ray recordings made for the Speeton section, eastern England.

are interpreted by Slatt et al. (1992) as representing flooding surfaces. A test of the interpretations made on this high-resolution scale was conducted by also logging a pale–dark interbedded section in the B Beds. Elevated gamma-ray emissions occur within a pale bed and groups of pale beds: the drop in gamma-ray emission observed in the B Beds from the low-resolution work (Fig. 5) appears quite

abruptly at the base of Bed LB5E. On average, the light beds are more radioactive and this is clearly seen in the gamma-ray peaks from individual beds. There is rarely an abrupt switch in gamma-ray emission from individual pale to dark bed. Instead the counts tend to fluctuate between beds as part of a longer-term change in emission (Fig. 6). The results of the gamma-ray logging exercise in Germany are

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Fig. 6. Comparison of selected geochemical data acquired in the topmost Hauterivian C Beds at Speeton (eastern England). Note the correlation between high gamma-ray emission and pale beds; high kaolinite contents and elevated Th=K and Th=U ratios. Key for clay minerals in Fig. 7.

broadly similar, with the same caveats on interpretation (Fig. 7). A detailed log was constructed for Frielingen (Fig. 8). Both show consistent peaks in the pale beds with gamma lows in the dark beds and fluctuating signals from the pale to dark and dark to pale transition. Ratios were also calculated from the results of X-ray fluorescence (in some cases, no spectroscope was available during sampling, e.g. for the lower beds at Frielingen and at Gott), although these were less convincing than from the outcrop measurements of spectral gamma-rays, probably as a result of the different sensitivity of each method. 4.2. Micropalaeontology Groups of organisms commonly used for deciphering palaeoecological and palaeoclimatic questions include palynomorphs, calcareous nannofossils and foraminifera. Apart from the radiolaria, siliceous microorganisms (diatoms, silicoflagellates) did not yet exist in abundance in Hauterivian times.

4.2.1. Calcareous nannofossils This group of phytoplankton reflects the ecological conditions of the surface water, in particular availability of nutrients, temperature and salinity. A total of 55 species has been observed and documented from the Hauterivian (Mutterlose, 1991). Only a few taxa make up the 95% of the overall abundance and these are commonly used for palaeoecological interpretations. The following proxies, based on various sources, are used in this study: (1) A high diversity is indicative of stable conditions, in particular oligotrophic warm surface waters (McIntyre and Be´, 1967; McIntyre et al., 1970; Brand, 1994). (2) Low-diversity assemblages are considered to be typical for unstable conditions, cool surface waters enriched in nutrients (Okada and Honjo, 1973; Brand, 1994). (3) Watznaueria barnesae is an eurytopic species, the first to invade new niches (Mutterlose, 1991). (4) Biscutum constans and Zeugrhabdotus spp. are considered to be taxa indicative of a high nutrient supply (Roth and Bowdler, 1981; Roth, 1986; Roth

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Fig. 7. Comparison of selected geochemical data acquired in the uppermost Hauterivian succession exposed at Frielingen (northern Germany). Note the correlation between high kaolinite contents and elevated Th=K or Th=U in the dark beds. No outcrop spectral gamma-ray data were acquired below beds 122=121 as such units were not exposed when the gamma-ray spectroscope was being operated in the early 1990’s.

and Krumbach, 1986; Watkins, 1986, 1989; Erba et al., 1989, 1992). (5) Crucibiscutum salebrosum is a cold water species (Mutterlose, 1992). (6) Rhagodiscus asper, Nannoconus spp., Micrantholithus spp. and Conusphaera spp. are interpreted as thermophile warm water taxa (Erba, 1987; Mutterlose, 1991, 1996; Erba et al., 1992). Low resolution. The low-resolution study analysed 26 samples from 22 pale–dark rhythms of the Frielingen and 27 samples from 13 pale–dark rhythms of the Gott section. Thus 1–2 samples were studied from each bed, samples are spaced at distances of 20– 50 cm. Calcareous nannofossils are abundant and diverse throughout the sections, with both being highest in the pale beds (Fig. 9). Most common species include Watznaueria barnesae, Rhagodiscus asper, Vekshinella spp., Corollithion spp., Biscutum constans, Crucibiscutum salebrosum and Sollasites horticus. Diversity and abundance of calcareous nannofossils correlate directly to lithology. Pale beds yield

abundant and diverse floras, while the dark beds have a lower diversity. The average value of species diversity is 29.5 (Frielingen) and 30.2 (Gott) for pale layers, 25.3 (Frielingen) and 26.7 (Gott) for dark layers. Exceptions are beds with a very high CaCO3 content (>50%), which have extreme low abundance and diversity. These horizons are thought to be of diagenetic origin and are not considered in the present study. W. barnesae (the most common species in the Cretaceous) is more common in dark layers than in pale and more common at the basin margin than at the basin centre. B. constans, the species second in abundance, varies from 26% (sample 123=1) to 2% (sample 111=1) for Frielingen respectively from 38% (sample 69=1) to 3% (sample 59=1) for Gott. The abundance of this species does not show any correlation with bed colour. There is a steady decrease in the lower and middle part of the Frielingen section (samples 101=1–118=2), while the upper part (samples 119=1–127=1) is characterised by high values (>20%). Horizons with the lowest abundance of B. constans show an inverse correlation with relatively high abundances for Nannoconus spp. The

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Fig. 8. High-resolution study of the uppermost beds exposed in the Frielingen section, made to demonstrate the correlation between pale beds and high (total) gamma-ray emission together with low Th=K and Th=U ratios, not easily observed on the section with all beds.

middle part of the Gott section (samples 57=1–65=1) has B. constans in low abundance in comparison with the lower (samples 50=1–56=1) and upper parts (samples 66=1–73=1) which are higher, in common with Frielingen. B. constans is also more common in the basin margin setting (the Gott section with an average of 18.9% for 27 samples) than in the basin centre (the Frielingen section with an average of 12.5 for 25 samples). The abundance of Rhagodiscus asper varies in the Frielingen section from 36% (sample 106=1) to 1% (sample 113=1), in the Gott section from 34% (sample 60=1) to 4.2% (sample 67=1). Both sections have the same trend of a decrease of abundance towards the top. In Frielingen bed numbers 101–120 have relatively high values and bed numbers 121–

127 have low ones. The Gott section shows similar trends with bed numbers 50–64 showing high values and bed numbers 65–73 low ones. Nannoconus spp. is restricted in both sections (Frielingen, Gott) to the lower part of the succession. In Frielingen the highest abundance of 10% is attained in sample 117=1, in Gott with 5% in sample 60=1. Different species of Nannoconus (N. bucheri, N. circularis, N. aff. circularis, N. globulus, N. kamptneri, N. minutus, Nannoconus sp.) are common in pale beds and only subordinate in dark beds. Tethyan warm-water floras consisting of nannoconids (N. kamptneri, N. circularis, N. globulus, N. minutus, Nannoconus spp.) are restricted to the pale beds and they are rare or absent in the dark beds. The relative abundance of Nannoconus varies between 0.3 and 10% in the pale beds

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Fig. 9. Fluctuations in the calcareous nannofossil content in the Frielingen section on a low-resolution scale.

(max. bed 117–9.9%). Based on a high diversity and high abundance it is possible to correlate the beds 117 of Frielingen and 58 of Gott. This sudden mass occurrence of nannoconids is designated as the Nannoconus Event, which can be followed over 50 km. It is interpreted as a major palaeoceanographic event, related to an influx of Tethyan warm-water floras. High resolution. A high-resolution study of 53 samples from the Frielingen section covered a 1.10-mthick sequence (Fig. 10), resulting in 19 samples for the lower pale bed (111), 19 samples for the intermediate dark bed (112) and another 15 samples for the upper pale bed (113). The interval under discussion is hatched in Fig. 9. The pale beds 111 and 113 have higher CaCO3 values than the dark layer 112. All three units show gradual changes (Fig. 10). CaCO3 values vary from 48% (sample 105) to 4.1% (sample 136). Highest values were obtained for the lower part

(samples 101–119), lowest for the middle part (samples 120–136). Diversity varies from 28 to 36 species. A direct correlation with lithology is not recognisable, since the dark clay of sample 112 attains a high abundance. The abundance of W. barnesae varies from 17% to 35% within both pale horizons and increases up to 37% in the dark part. The distribution of B. constans clearly correlates with lithology. The pale lower part (samples 101–119) is marked by a gradual increase in abundance from 1% (sample 101) to 5% (sample 119). The middle dark part (samples 120– 138) shows the highest values (sample 121; 6.5%) and the upper part (samples 139–153) is characterised by extreme low values around 1%. B. constans is thus more common in dark beds poor in CaCO3 . High abundances of R. asper mark the two pale beds, low values the dark bed. R. asper shows a negative correlation with B. constans. Nannoconids are only common in the lower part of the section (samples 101– 119).

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Fig. 10. Fluctuations in the calcareous nannofossils content in the Frielingen section on a high-resolution scale.

To conclude, the pale horizons are characterised by a high diversity and taxa of warm-water affinities including R. asper, Cretarhabdus spp., Nannoconus spp., Conusphaera spp. and Micrantholithus spp. (Fig. 11). Using the proxies set out above, these assemblages are believed to reflect stable conditions, with warm surface waters which were poor in nutrients. Dark, clay-rich beds are considered to be deposited under unstable conditions, with cooler or less saline surface waters, rich in nutrients. Taxa typical for this setting are B. constans, Sollasites horticus and Corollithion spp. (Fig. 12). 4.2.2. Foraminifera Ecological factors that control the abundance and diversity of benthic foraminifera include food availability (organic matter flux), temperature, substrate characteristics and oxygen content of bottom waters.

In general the benthic foraminifera have been used in the following ways as interpretive proxies. (1) The highest total diversity and a high diversity of calcareous foraminifera is indicative for well oxygenated, shallow tropical to subtropical seas (Michael, 1974; Kemper, 1987; Murray, 1991). (2) Associations of low diversity but high abundance are considered to be typical for an unstable environment, in particular low-oxygen or low-temperature conditions (Michael, 1979; Kemper, 1987; Kaminski et al., 1995). (3) A dominance of primitive agglutinated foraminifera (e.g. Trochammina spp., Haplophragmoides spp.) has been suggested to indicate cool water (Michael, 1979; Kemper, 1987). During the Cretaceous these forms are more common in the boreal and deep-sea basins such as the Norwegian Sea and the Carpathian flysch.

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(4) Meandrospira washitensis is considered to be a stenothermal warm-water species indicative for higher water temperatures (Michael and Pape, 1971; Michael, 1979). (5) Hechtina antiqua has been interpreted as a thermophile species (Kemper, 1987). However it has been observed in the Cretaceous of the Norwegian Sea (M.A. Kaminski, pers. commun.). Benthic foraminifera of the pale–dark bedding rhythms of Frielingen are generally abundant and diverse, comprising 128 species (35 agglutinated, 93 calcareous). The fauna is dominated by ten species (Epistomina caracolla, Falsogaudryinella tealbiensis, Marginulinopsis jonesi, Planularia tricarinella, Proteonina ampullacea, Trochammina depressa, Trochammina globigeriniformis, Trochammina squamata, Reophax scorpiurus, Verneuilinoides neocomiensis), which in average make up 60–80% of the abundance of each sample. Detailed studies were completed by Heinrich (1990, 1991) and Klein and Mutterlose (1997). The calcareous species Astacolus bronni, Discorbis dreheri, Epistomina caracolla, Lingulogavelinella sigmoicosta, Globulina prisca, Hechtina antiqua, Lenticulina mu¨nsteri, Lenticulina dunkeri, Marginulinopsis? gracillissima, Marginulinopsis jonesi, Meandrospira washitensis and Planularia tricarinella make up more than 2% in some samples. Foraminiferal diversity in general is higher in the pale beds than in the dark ones (Fig. 13). Pale layers are characterised by diverse faunas, dominated by calcareous foraminifera. Dark layers on the other hand show assemblages of lower diversity. In dark layers calcareous foraminifera are at least as common as agglutinated taxa; their abundance decreases, however, in comparison to the pale layers. The average diversity for pale layers is 61.9 species (10 samples from 4 pale layers) and 49.7 species (17 samples from 5 dark layers) in the dark layers. The absolute diversity of agglutinated foraminifera stays however constant in all samples. The C=A ratio (calcareous=agglutinated foraminifera) shows the same pattern. Pale layers have a higher abundance of calcareous foraminifera than dark ones. Apart from sample 102, which

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has the highest carbonate content, calcareous foraminifera never exceed 50% of the abundance. The fraction 200–100 µm is always characterised by high abundances of agglutinated specimens. All fractions >200 µm are dominated by calcareous specimens. Some species of calcareous foraminifera are restricted to pale layers: E. caracolla, L. sigmoicosta, G. prisca var. fistulous, Ramulina aculeata, Bullopora laevis, Psilocitharella strigilatta bettenstaedti, M. washitensis, Lenticulina eichenbergi, F. tealbiensis and Haplophragmium aequale (Fig. 13). E. caracolla (Fig. 11) is common only in the pale beds and occurs rarely in dark beds. The mass occurrence in bed 125 (max. 18.6% in sample 101) is quite distinctive. In the dark beds the abundance of E. caracolla varies from 0 (sample 93) to 1.3% (sample 105). M. washitensis, a species of presumably Tethyan warm-water affinities (Michael, 1979); is common only in the pale layers. M. washitensis and F. tealbiensis show similar vertical fluctuations. Both species are more common in pale layers; their abundance decreases from bottom to top.M. washitensis makes up 6.1% of the abundance (F. tealbiensis 8.4%) in sample 90, and up to 2.5% (5.9%) in sample 109. The taxa Ammodiscus spp., Haplophragmoides spp., P. ampullaceae and Trochammina spp. are here considered jointly as the group of selected primitive agglutinated forams (Fig. 12). The vertical fluctuation of this group shows a negative correlation to M. washitensis and F. tealbiensis; the abundance of selected primitive agglutinated forams increases from bottom to top. The abundance of selected primitive agglutinated forams decreases in pale layers to 31.7% (sample 102), while it reaches up to 86.2% (sample 107) in dark layers. In average selected primitive agglutinated forams make up 60% of the total abundance in each sample. Ammodiscus spp. and Haplophragmoides are in general rare. Ammodiscus is most common in samples 106 and 107, both poor in CaCO3 . Abundance is low in the lower part of the section (3.8%), higher in the upper part (sample 106 20.5%; sample 107 25.4%). At the same time the

Fig. 11. Association of typical calcareous nannofossils and foraminifera for pale beds. 1–4 D Nannoconus circularis; 5 D Micrantholithus obtusus; 6 D Rhagodiscus asper; 7 D Epistomina caracolla; 8 D Falsogaudryinella sp.; 9 D Lingulogavelinella sigmoicosta; 11 D Hechtina antiqua; 12 D Psilocitharella strigillata bettenstaedti.

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Fig. 12. Association of typical calcareous nannofossils and foraminifera for dark beds. 1, 2 D Biscutum constans; 3 D Zeugrhabdotus sisyphus; 4 D Reophax scorpiurus; 5 D Verneulinoides neocomiensis; 6 D Haplophragmoides cushmanni; 7 D Haplophragmoides concavus; 8 D Ammobaculites subcretaceus; 9 D Proteonina ampullacea.

abundance of P. ampullaceae and Trochammina spp. decreases. The overall abundance of P. ampullaceae decreases towards the top of the succession, while the abundance of Trochammina spp. increases. 4.2.3. Ostracods and bryozoa Ostracod diversity and abundance both are considerably higher in the pale beds compared to the

dark ones. Species with eye tubercles and spines are more common in pale beds. The thermophile genus Cytherelloidea is recorded among the ostracods within the pale layers. Thus, evidence is provided for sedimentation in warm light-penetrated seawater. The washed residues of the pale layers yield bryozoans which are restricted to the pale layers. Their abundance varies from bed to bed, being

Fig. 13. Fluctuations in the foraminifera content in the Frielingen section on a high-resolution scale.

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absent from samples 100 and 108, and very common in 89 and 103. The most common species Berenicea tenuis and Discosparsa aff. pedunculata are encrusting taxa.

5. Conclusions 5.1. Clay mineralogy Although the margins of error are high in estimating absolute quantities of different clay mineral species in any given sample, the variations between pale and dark layers observed here are consistent throughout the Hauterivian sections studied. Pale layers contain the mixed-layer illite–smectites that may indicate more ‘offshore’ conditions, when sealevels were high, tectonic uplands were low in relief and when the diagenetic alteration of illites and smectites could occur, i.e. in a seasonally arid climate. Dark layers generally show high quantities of kaolinite and smectite, with a few samples containing mixed-layer minerals and chlorite. This assemblage indicates more ‘nearshore’ conditions, although the presence of both smectite and kaolinite may be better explained by a humid weathering regime, with hinterland areas being actively eroded. 5.2. Elemental variation Systematic variations in Si, Ba, Rb, Fe, Zr and Ca occur in the pale–dark beds. The high-resolution nature of this study, together with the stable depositional conditions described in the introduction and the low sensitivity of XRF and AAS, produces replicatable results only in Si, Ca and Zr fluctuation. Si reflects silt contents, Ca is from fossil sources and Zr may well reflect the presence of detrital zircons. The correlation between elevated Zr and kaolinite contents suggests high run-off in near-shore conditions. 5.3. Clay mineralogy, elements and gamma-ray logs compared There are significant gamma-ray peaks within the general low values of the dark rhythms. This may reflect discrete organic-rich horizons. For the most part, however, we would suggest some condensation

for the pale beds, indicative of a shut-off in clastic sediment supply or possibly where organic productivity has out-paced clastic sedimentation. Possible hosts for radioactive elements in the pale beds include glauconite (for K), associated organic matter (for U) and, illite (for Th). Given that we know from XRD, XRF and acidification that the pale beds contain more illite–smectites, glauconite and carbonate, we consider that a combination of illite–smectite and glauconite and organic matter in carbonate provides the increase in gamma-rays in the pale beds (Fig. 14). This observation has a strong bearing on the analysis of pale–dark rhythms from other successions: in the Early Cretaceous, it seems that the pale beds are broadly ‘transgressive’ with dark beds recording ‘regressive’ conditions and sluggish water-bottom circulation. On a bed-by-bed scale, low kaolinite contents correlate with low Zr (presumably from zircons) and low Th=K or Th=U ratios. Exceptions to this rule are common: the decrease in kaolinite=illite ratios around beds 108 to 109 in the Frielingen section (Figs. 7 and 8) appear to correlate with an increase in Zr. This is only apparent however, as the semi-quantitative estimate of absolute kaolinite abundance does increase with Zr, whilst measurements of Th= or Th=U at an horizon with any significant zircon content may be dubious (Hurst, 1990). Clay mineralogical variations between pale and dark beds in the Hauterivian of Germany and England may best be explained by variations in the original diagenetic formation of clays in the sediment source-lands. Comparison of clay mineralogy with Th=K and Th=U ratios shows a link between elevated Th and kaolinite. This may be the result of K and U being leached from hinterland soils, leaving Th- and kaolinite-rich soils that were eroded and deposited in dark layers. This variation most likely had a climatic control, pale beds representing warm, seasonally arid climates typical of the Tethyan realm, dark beds being indicative of warm humid hinterlands and cold seas. These short-term clay variations (bed by bed) are superimposed on a longer term cycle of variations that is marked by decimetre to metre-thick relative concentrations of pale vs. dark rhythms depicted most accurately on gamma-ray logs. A good example is the overall increase in gamma-ray emission through the pale beds marked ‘condensed section’ in Fig. 6.

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Fig. 14. Synthesis of geochemical and micropalaeontological data from pale–dark bedding rhythms.

5.4. Calcareous nannofossils The distribution pattern of calcareous nannofossils clearly shows two sedimentary cycles superimposed on one another. These (long- and short-term) variations are recognisable both along the basin margin (Speeton, Gott section) and the basin centre (Frielingen section). Each pale–dark bedding rhythm shows a cyclic variation in the stratigraphic distribution of calcareous nannofossils which can be compared to other measured changes (Fig. 14). The pale layers are characterised by Tethyan floras (Nannoconus spp., Conusphaera, Micrantholithus) and relatively high CaCO3 and a low Corg content, respectively. The nannofossils indicate warm surface water, poor in nutrients. Cosmopolitan floras (R. asper, Cretarhabdus spp.) of possibly warm-water affinities are more common in the pale layers (Fig. 14). Dark layers on the other hand, impoverished in CaCO3 and enriched in Corg, show a relatively high percentage of W. barnesae, Sollasites horticus, Corollithion spp.

and B. constans. The latter species is common in cooler, nutrient-rich surface water and is taken as a proxy for a slightly higher productivity (Roth, 1986; Roth and Krumbach, 1986; Watkins, 1986, 1989; Erba, 1987; Erba et al., 1992). The variation in calcareous nannofloras within each pale–dark rhythm may be best explained by climatic variation on the scale of Milankovitch cycles (precession and obliquity). These short-term variations reflect changes of surface water temperature and fertility (Fig. 15). Superimposed on these small-scale rhythms are lower-order cycles caused by longer-term fluctuations in the sea-level–climate system. A transgression in the middle part of the Simbirskites discofalcatus ammonite Zone allowed an influx of Tethyan genera and species. The regression in the late discofalcatus Zone caused a dominance of B. constans.

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Fig. 15. Model of the palaeoenvironment as reconstructed by nannofossils and foraminifera.

5.5. Foraminifera The vertical fluctuation in benthic foraminifera is clearly bound to lithology (Figs. 13 and 14). Pale layers have a high CaCO3 content, a high total diversity of foraminifera and a high diversity of calcareous foraminifera. Pale beds are characterised by both generally lower abundances of foraminifera than the dark beds, and particularly of primitive agglutinated

foraminifera. By contrast, calcareous foraminifera and ostracods are common. M. washitensis and F. tealbiensis, species of presumed Tethyan warmwater affinities (Michael and Pape, 1971; Michael, 1979), are common only in the pale layers (Fig. 13). Dark layers on the other hand yield assemblages of a high overall abundance and a higher abundance of agglutinated foraminifera (Figs. 13 and 14). These fluctuations indicate a climatic control on the depo-

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sition of the pale–dark rhythms. Pale beds reflect warmer periods, dominated by thermophile warmwater taxa. In these phases Tethyan floras and faunas migrated into the basin from the southeast via the Carpathian. Dark layers on the other hand indicate cooler conditions, partly connected to suboxic conditions of bottom waters (Fig. 15). 5.6. Combined conclusions The Hauterivian pale–dark rhythms are thought to reflect Milankovitch-type climatic cycles occurring with a frequency of 22,300 years (Nebe and Mutterlose, 1999). The alternation from pale to dark beds thus occurs at a frequency too high for most tectonic controls, excluding intra-plate stress. Discerning the effects of climate and sea-level when examining high-frequency changes such as these is a difficult task, and thus a linked sea-level climatic control may initially be invoked in the control of

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the clay mineralogical variations. The palaeoecology and geochemistry may aid in our understanding of the over-riding control; palaeoecological information leads us to believe that changing climates caused variations in circulation, absolute temperatures and productivity, leading to the pale–dark rhythmicity observed (Figs. 15 and 16). Clay mineralogical variations throughout the Early Cretaceous of the North Sea borderlands compare well with published sealevel curves and climatic changes (Ruffell and Rawson, 1994): thus while climatic change is almost certainly dominant in controlling bedding rhythms and short-term (sub-stage) changes, sea level may also influence stage-length changes in mineralogy. Our study of gamma-ray emissions also shows that individual Milankovitch-scale bedding rhythms may be compared to beds and bed-sets, with bundles of rhythms very broadly comparable to parasequences. On the scale of individual pale–dark rhythms, we may conjecture that the light beds are indicative of

Fig. 16. Model of the depositional environment of pale and dark beds.

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low levels of in-situ weathering alterations in the sediment source-lands. Such a climate would be generally warm as Tethyan faunal and floral influences are strong. Dark beds may be the results of high levels of physical and chemical weathering under humid climatic conditions when abundant chemical nutrients and organic matter would be washed into the basin and cause increased organic productivity. This organic matter would have been buried by the occasionally abundant sedimentary input. On its own, this does not imply a higher sedimentation rate for the dark layers: during pale-layer times calcium carbonate production and preservation on the seafloor may have been roughly equable to terrigenous clastic input and the preservation of organic matter. Elevated gamma-ray emissions from pale layers may reflect the K content of glauconites and U in skeletal material. Both of these reflect condensation, or slow deposition in the pale beds. From these assumptions, and from a knowledge of the floral and faunal changes, we may be able to characterise the processes controlling the development of the pale–dark rhythms. Pale rhythms are known to reflect warmer conditions when Tethyan influences were high. Further, we can suggest that this ‘Tethyan-type’ climate was seasonally arid, causing the formation of mixed-layer clays in soil profiles around the basin margins. The cut-off in kaolinite may also reflect ‘transgressive’ conditions, also indicated by Tethyan influences of biota. However, we also observe the more ‘offshore’ or transgressive smectite decreasing in abundance in pale beds, suggesting a change in the nature of clastic supply to the basin. The dark layers may reflect cooler, humid conditions typical of the ‘normal’ boreal cold sea– warm hinterland type climatic conditions (Fig. 16). Such a scenario could have promoted sluggish water-bottom circulation, leading to the preservation of organic matter in addition to higher inputs of clastic material to the basin.

Acknowledgements We gratefully acknowledge the financial assistance of the British Council and the DAAD (awards ARC 576, 989, 313=ARC-scu) and of the Deutsche Forschungsgemeinschaft (Mu 667=8, 9). Trevor El-

liott, Michael Kaminski, Jens Matthiessen, Paul Wignall and Suzanne Leroy all made very valuable comments during the writing of this paper. A. Bornemann and M.Pringle compiled most of the figures.

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