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Ostracod assemblages, palaeoenvironment and cyclicity of Purbeck-type sediments ofthe Münder Formation (Lower Cretaceous, Hils Syncline, N-Germany)

Gernot Arp a,⁎, Claas Mennerich a,b

a Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geobiologie, Goldschmidtstrasse 3, D-37077 Göttingen, Germanyb Present Address: Wintershall Aktiengesellschaft, Friedrich-Ebertstraβe 160, D-34119 Kassel, Germany

A B S T R A C TA R T I C L E I N F O

Article history:Accepted 10 January 2008

Keywords:OstracodaSedimentary cyclesStable isotopesOncoidsBerriasian

Berriasian marlstones in the upper part of the Münder Formation (“Purbeck”) of the Hils Syncline have beenregarded as freshwater deposits with minor brackish influence. However, similar to some marlstones of theEnglish Purbeck Limestone Group, it remained unclear whether these deposits rich in non-marine ostracodsare lacustrine or marine-lagoonal in origin. Facies analysis of the sediments revealed strikingly symmetriccycles composed of (1) red-brown massive marlstones of distal alluvial plains, (2) greenish-grey beddedmarlstones with microbial lamination of the supralittoral zone, (3) dark-grey charophyte marlstones and(4) light-grey charophyte limestones with scattered porostromate oncoids of the eulittoral zone, (5) poorlybedded light-grey micritic limestones and (6) clayey marlstones of the sublittoral zone. Ostracod associationsrevealed by cluster analysis and associated microfossils indicate cyclic salinity changes coincident withlithofacies alternations. A Theriosynoecum-association is restricted to eulittoral and shallow sublittorallithofacies types and reflects freshwater to oligohaline conditions. This association is accompanied by thefreshwater gastropods Gyraulus and Provalvata as well as charophytes of the genera Atopochara, Clavator andGlobator. Supralittoral marlstones are characterized by a mixed Cypridea-association, composed ofautochthonous Cypridea of ephemeral freshwater pools and transported faunal elements. Increased salinitiesof miohaline to mesohaline conditions are indicated for sublittoral lithofacies types by a Mantelliana-association. Maximum flooding is indicated by the appearance of small foraminifera. Variations ofmonospecific ostracod stable isotope ratios coincide with the cyclic salinity fluctuations interpreted fromostracod assemblages along the section. This correlation of lithofacies types and ostracod faunas implies thatsalinities increased with increasing water level and vice versa. Consequently, a coupling of the water level ofthe Hils Basin to short-term eustatic sea-level changes must be assumed. Therefore, the Hils Basin isinterpreted as a marine embayment during the Berriasian with freshwater spreading out on top of marine-brackish water, but not a lake. A similar approach combining micropalaeontological and sedimentologicalevidence may help to distinguish between marginal-marine sediments affected by cyclic sea-level changesand lacustrine sediments displaying climatically-driven cyclicity in the Purbeck-type area and elsewherein Europe.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Marginal-marine, lagoonal and lacustrine sediments formed atsalinities deviating from normal-marine conditions are widespread atthe Jurassic–Cretaceous transition in Europe. These largely marly andcalcareous sediments, collectively known as “Purbeck” facies, com-monly contain stromatolites (House, 1968; West, 1975; Francis, 1984;Perry, 1994), oncoids (e.g., Platt, 1989) and porostromate algae (e.g.,Brown, 1964). Such microbialites, if marine in origin, provideindications for reconstructing changes in seawater CaCO3 saturationand calcium concentrations (e.g., Arp et al., 2001).

One characteristic feature of “Purbeck” sediments is their pro-nounced, hierarchical cyclicity, as revealed by facies (Strasser, 1988,1994; Hillgärtner and Strasser, 2003; Anderson, 2004a), stableisotopes (Jiménez de Cisneros and Vera, 1993; Joachimski, 1994), andmicrofossils (Anderson and Bazley, 1971). Amongmicrofossils, notablyostracods show distinct, cyclic short-term changes in their assem-blages, which have been taken as indication of fluctuating salinities inthe English Purbeck (“faunicycles”, Anderson and Bazley, 1971;Anderson, 1985). According to Anderson (1985), the salinity changesreflect climatically controlled variations in freshwater influx into alagoonal setting, while more recent investigations point to Milanko-vitch-type sea-level fluctuations as the cause of cyclicity (e.g.,Anderson, 2004b). These sea-level changes are considered by someauthors to be glacioeustatic, reflecting low-volume polar ice caps(Frakes and Francis, 1988; Jiménez de Cisneros and Vera, 1993).

Palaeogeography, Palaeoclimatology, Palaeoecology 264 (2008) 230–249

⁎ Corresponding author. Tel.: +49 551 397986; fax: +49 551 397918.E-mail address: [email protected] (G. Arp).

0031-0182/$ – see front matter © 2008 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2007.02.051

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Recently, Anderson (2004b) suggested that F.W. Anderson's ostracodfaunicycles coincide with fifth-order (100 ka) Milankovitch sequencesand the S-peaks within the faunicycles correspond to fifth-order(20 ka) Milankovitch cyclicity.

Yet, the interpretation of ostracod-rich marl- and limestones of theEnglish Purbeck as marine-lagoonal deposits has been questioned inrecent times by Horne (1995, 2002). Instead, this author suggests aclimatically controlled change in the abundance of permanent andtemporary athalassic water bodies (i.e., lakes and ponds) to explainchanges in ostracod assemblages (Horne, 1995, 2002). Indeed, thereare a number of ecological parameters other than salinity that mayinfluence ostracod associations significantly. Particularly, the chemicalcomposition of waters (e.g., relative concentrations of alkalinity, SO42−

and Cl−) appears to exercise a determinative role on the ostracodassemblage composition (e.g., Forester, 1986; Carbonel, 1988; Carbo-nel et al., 1988; Radke et al., 2003).

In any case, for the use of stromatolites and calcified cyanobacteriaas potential indicators of secular changes in seawater chemistry, anunequivocal assignment to a marine or non-marine palaeoenviron-ment is crucial. One of the reasons for controversial palaeoenviron-mental interpretations may be that the Purbeck marlstones, whichcontain the ostracods, received little attention with regard tomicrofacies and sedimentological features. In this context, it has tobe mentioned that Horne (1995, 2002) questioned the validity andreplicability of F.W. Anderson's named faunicycles, because theirboundaries apparently are arbitrarily drawn and some namedfaunicycles comprise several S-phases while others only one. Whilethis objection is justified, one possible reason for the difficulties inunderstanding F.W. Anderson's named faunicycles might be that adetailed lithofacies column in parallel to the faunicycles shown inAnderson and Bazley (1971: p. 29 and 32) has not been provided.Apart from the palaeontological and sedimentological investigations,

there is also little information on elemental and isotope geochemistryof Purbeck ostracod shells, although these proxies have been widelyused in Quaternary palaeolimnological studies (e.g., Chivas et al., 1985,1986a,b; Lister, 1988; Schwalb et al., 1999; von Grafenstein et al., 1999;Ricketts et al., 2001).

Purbeck sediments largely consisting of ostracod-bearing marl-stones form a part of the Tithonian–Berriasian Münder Formation ofthe Lower Saxony Basin, N-Germany (Hoyer, 1965; Schönfeld, 1979;Schudack, 1994; Elstner and Mutterlose, 1996; Gramann et al., 1997).Subordinate evaporite, sandstone and limestone beds, some of themwith oncoids or stromatolites, are intercalated in these marlstones. Anew temporary exposure in the Hils Syncline provided access to cyclicPurbeck marlstones, which may serve as an analogue for Purbecksediments of other areas in Europe and elsewhere. The purpose of thepaper is to determine the marine or non-marine nature of thesesediments and the cause of their cyclicity, as derived from facies andostracods assemblages. In addition, an attempt is made to usemonospecific stable isotope data of ostracod shells in support of thepalaeoenvironmental model.

2. Geological setting

The investigated exposure, a temporary construction pit, is locatedin NW Germany at the NW border of the town Delligsen (TK 25 sheetnumber 4024 Alfeld, R: 35 54800, H: 57 57550; 150 m above SL),50 km south of Hannover and 45 km NNW of Göttingen. The town islocated at the NE limb of the Hils Syncline (Fig. 1), a 15×40 km largedepression with a sedimentary succession ranging from LowerPermian to Upper Cretaceous. Initiated by Late Cimmerian extensionalblock tectonics of the Hercynian basement, the syncline and itsfringing anticlines formed by halotectonic movements of the PermianZechstein Group during Late Jurassic to Late Cretaceous times

Fig.1.Geologicalmap of theHils Syncline showing the location of the studied sectionDelligsen and the drilling Duingen 1. Geological cross-section according toHerrmann et al. (1971).

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(Herrmann et al., 1971; Jordan,1994). Then, during the Late Cretaceous(Coniacian–Campanian) a compressional pulse caused reverse move-ments at the halotectonic anticlines “Leinetal Axis” and “Elfas Axis”(Jordan,1994). Finally, essentially during the Pleistocene, erosion leadsto the development of an inverted relief with resistant Jurassic–Cretaceous limestones and sandstones forming ridges and hills of thesyncline centre.

In the Hils Syncline, Purbeck-type marlstones and limestones formthe 50–185 m thick Borberg Member (Henrici et al., 1949; Wolfart inWaldeck, 1975; Dannapfel, 1983) at the top of the Münder Formation(Fig. 2). In total, the Münder Formation is a 150–800 m thickTithonian–Berriasian succession of varicoloured marlstones withintercalated evaporites and oolitic limestones. They formed underrestricted marine to evaporitic conditions when the Lower SaxonyBasinwas connected to the open ocean only by narrow seaways to theEast (Polish Trough) and Northwest (North Sea Rift; Ziegler, 1990;Gramann et al., 1997). The strong thickness variations within the Hilsarea reflect rapid synsedimentary sinking of the syncline centre andsynchronous uplift of limbs. As a result, extraclasts derived fromOxfordian to Lower Tithonian limestones occur in a local conglom-eratic bed near the base of the Borberg Member (Dannapfel, 1983,pl. 5/14).

The Borberg Member of the Hils Syncline is defined, and distinctfrom other parts of the Münder Formation, by the occurrence ofmicritic freshwater limestones intercalated in marlstones (Wolfart inWaldeck, 1975). Stratigraphic equivalents farther north in the LowerSaxony main basin are upper parts of the Katzberg Member and theSerpulit Member (Kemper, 1973; Casey et al., 1975; Schönfeld, 1979;Gramann et al., 1997). Detailed lithologic sections obtained fromincised creeks, gullies and diggings were provided by Dannapfel(1983), although cyclicity was not recognized by Dannapfel (1983,p. 99).

In contrast, the studied section, with bedding inclined with anangle of 30° towards the syncline centre (Clar value 225°/30°),exposed a clearly cyclic succession (Fig. 3A). In addition, rare oncoids,

previously unknown from the Hils Syncline, occur within the fresh-water limestones.

Due to their mollusc fauna, these Purbeck-type marls have beenregarded as freshwater deposits (Koert, 1898), although some salinity-tolerant genera occur as well (Huckriede, 1967). It remained unclearwhether the deposits represent a marine-brackish embayment withstrong freshwater influx (Schmidt and Wolburg, 1949) or formed in alacustrine setting (lakes and ponds) separated from the sea byfluviodeltaic siliciclastics (Kauenhowen, 1927; Dannapfel, 1983).

Biostratigraphically, the investigated section is early Berriasian inage and corresponds to the local ostracod biozone 21 of Schudack(1994) and Cypridea dunkeri subzone of Horne (1995). This is indicatedby the co-occurrence of the ostracods Cypridea dunkeri inversa,Theriosynoecum forbesi and Scabriculocypris trapezoides and concur-rent absence of Cypridea sowerbyi and Cypridea granulosa. This is inaccordance with the occurrence of the charophyte Globator maillardiipraecursor (bed 8), which indicates the local charophyte biozone 7 ofSchudack (1993) (Fig. 2). Consequently, lower parts of the LulworthFormation (lower part of the Purbeck Limestone Group) in Englandincludingostracod assemblage 2 (faunicycles 5–11) of Anderson (1985)are likely time-equivalents of the section studied in this paper. Youngerparts of the Borberg Member, which are temporal equivalents of theSerpulit Member farther north, contain C. sowerbyi, Cypridea binodosaand Cypridea posticalis in the drilling Duingen 1 (Wolburg in Henriciet al., 1949). This points to a correlation of younger parts of the BorbergMemberwith the upper parts of the Lulworth Formation (lower part ofMiddle Purbeck of Allen & Wimbledon, 1991) in England (Fig. 2).

3. Materials and methods

From each bed of the investigated sections one sample (0.5–1 kg)covering 10–15 cm stratigraphic thickness was taken from the middleof the bed. Consequently, sample numbers are identical to bednumbers. For microfossil analyses 200–800 g of 16 marlstone sampleswere disintegrated by treatment with hydrogen peroxide (pH-

Fig. 2. Litho- and biostratigraphy of the Jurassic–Cretaceous transition in the Hils Syncline, Lower Saxony Basin, and Dorset. 1)Wolfart (1956) inWaldeck (1975), Gramann et al. (1975),Harms (1984), Jordan (1994); 2)Casey et al. (1975), Deutsche Stratigraphische Kommission (2002); 3)Klingler (1956), Klingler et al. (1962); 4)Schudack (1994); 5)Schudack (1993); 6)

Casey (1963), Townson (1975), Westhead and Mather (1996); 7)Remane et al. (2000). Absolute ages from Gradstein et al. (1994) and Jacquin et al. (1998).

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buffered by ammonia–water mixture) for several hours, prior towashing over 2 mm- and 90 μm sieves using tap water. Microfossilswere picked from the N90 μm fraction under a binocular microscope.Care was taken to take into account every valve fragment in theresidue. Ostracod shell abundance was quantified by counting valvesper 10 g analysed sediment (Table 1). Valve fragments were taken into

account by their ratio of fragment area to average area of a wholevalve. Assignments of Cypridea-fragments to species were madeaccording to the relative species proportion of whole valves. This hasbeen done because Cypridea species have mechanically similar shellsand only three Cypridea species are known from the investigatedsection. All samples had a similar lithology (clayey marlstones to

Fig. 3. Lithofacies types of the Borberg Member: (A) field view of the investigated outcrop showing cyclic arrangement of lithofacies types inclined towards to centre of the HilsSyncline. The outcrop wall is 5 m high. (B) Hand specimen of red-brown massive marlstone (LFT 1) with yellowish calcareous patches. Bed number 10. Scale bar is 1 cm. (C) Handspecimen of greenish-grey bedded marlstone (LFT 2) with intervals of mm-scale lamination. Bed number 9. Scale bar is 5 mm. (D) Micrograph of light-grey limestone nodulespreserving wrinkled laminated mud-/packstone alternations with mud and sheet cracks. Bed number 10. Scale bar is 5 mm. (E) Dark-grey charophyte marlstone (LFT 3) withabundant charophyte stems (1) and oogonia (2), articulated (3) and disarticulated ostracod valves, and pedogenic brecciation (4). Bed number 6. Scale bar is 1 mm. (F) Light-greycharophyte limestone (LFT 4) with abundant charophyte stems and the freshwater gastropod Gyraulus. Bed number 17. Scale bar is 1 mm.



Table 1Quantitative analyses of ostracod microfossil samples, section “Delligsen”. Number of valves per 10 g sediment (absolute abundance). % of total valves counted (relative abundance)

Bednumber

Lithofaciestype

Total number ofvalves counted

Theriosynoecumforbesi

Cypridea dunkeriinversa

Cypridea tumescenspraecursor

Cyprideaprimaeva

Mantellianapurbeckensis

Stenestroemia cf.cressida

Stenestroemiadecipiens

Scabriculocypristrapezoides

Klieanaalata

Rhinocyprisjurassica

Ostracodaindeterminate

Total

[n] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed] [n/10 g sed]

Del 1 LFT 6 66 25 19 – 7 – 2 – 1 2 – 122136 54% 21% 15% – 6% – 1% – 1% 1% – 100%

Del 2 LFT 3 713 31 13 – – – – – – 4 – 761363 94% 4% 2% – – – – – – 1% – 100%

Del 3 LFT 2 48 44 18 – 28 – – – – 1 21 160203 30% 28% 11% – 17% – – – – 0.5% 13% 100%

Del 4 LFT 1 – – – – – – – – – – 0.3 0.32 – – – – – – – – – – 100% 100%

Del 5 LFT 2 1 3 1 0.1 – – – – – 0.1 4 966 11% 33% 9% 2% – – – – – 2% 44% 100%

Del 6 LFT 3 26 8 9 – 1 – – – 2 1 4 5181 51% 16% 17% – 2% – – – 4% 1% 9% 100%

Del 7 LFT 6 221 29 11 5 7 – – – 33 9 16 330424 67% 9% 3% 1% 2% – – – 10% 3% 5% 100%

Del 8 LFT 3 71 16 – – 2 – – 1 5 1 2 96128 73% 16% – – 2% – – 1% 5% 1% 2% 100%

Del 9 LFT 2 – 5 2 – – 0.5 – – 0.1 2 1 10126 – 49% 21% – – 5% – – 1% 16% 8% 100%

Del 10 LFT 1 – 0.1 – – 0.1 – – – 0.3 – 0.1 0.615 – 13% – – 13% – – – 60% – 13% 100%

Del 11 LFT 3 14 23 – – – – – – – – – 3759 39% 61% – – – – – – – – – 100%

Del 12 LFT 6 – 20 – – 53 3 – – – – 1 77223 – 26% – – 69% 4% – – – – 1% 100%

Del 14 LFT 6 – 3 1 – 2 – 0.1 0.3 5 3 3 18146 – 18% 7% – 14% – 1% 2% 34% 5% 20% 100%

Del 16 LFT 3 49 36 9 – – – – 4 15 – 9 122112 43% 31% 8% – – – – 4% 6% – 8% 100%

Del 18 LFT 6 – 4 4 1 0.5 1 1 0.3 3 1492 – 26% – – 27% 9% 3% 5% 5% 2% 22% 100%

Del 20 LFT 6 – 7 – – 119 1 1 7 38 0.3 – 173619 – 4% – – 69% 1% 0.3% 4% 22% 0.2% – 100%

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marlstones) andwere treated in the sameway, so that potential effectsof sample processing on the assemblage composition (includingfragments) is the same for all samples. Cluster analysis includingbootstrap testing was carried out using the program BOOTCLUS freelyavailable from the US Geological Survey Great Lakes Science Center(http://www.glsc.usgs.gov/) (McKenna, 2003).

Petrographic investigations were carried out with a Zeiss Axioplanmicroscope equipped with a Peltier-cooled VISICAM-colour CCD

camera (PCO Computer Optics GmbH, Kehlheim). From 13 limestoneand 10 marlstone samples 34 large thin sections (7.5×10 cm and10×15 cm) of 80 μm thickness were prepared. Cathodoluminescencemicroscopy of polished petrographic thin sections (30 μm thickness;2.8×4.8 cm size) was carried out using a CITL CCL 8200 Mk 3A coldcathode chamber and control unit mounted (C.I.T.Ltd., Cambridge UK)on a Zeiss Axiolab microscope. Accelerating voltages of 12 kV and anelectron beam current of 500 μA were used. A SPOT camera model

Table 2Stable isotope data of ostracods, gastropods, micrites and cements, section “Delligsen”

Sample Bed Lithofacies type Material δ13C δ13C δ18O δ18O

[‰ V-PDB] [‰ V-PDB] [‰ V-PDB] [‰ V-PDB]

Mean Standard deviation Mean Standard deviation

1 Del 1 LFT 6 Theriosynoecum forbesi (18 valves) −3.19 0.02 −0.61 0.022 Del 1 LFT 6 Cypridea dunkeri inversa (7 valves) −2.65 0.02 0.63 0.023 Del 1 LFT 6 Cypridea tumescens praecursor (4 valves) −1.93 0.03 0.42 0.044 Del 2 LFT 3 Theriosynoecum forbesi (15 valves) −4.35 0.01 −1.00 0.035 Del 2 LFT 3 Cypridea dunkeri inversa (5 valves) −2.69 0.02 0.34 0.026 Del 2 LFT 3 Cypridea tumescens praecursor (4 valves) −2.99 0.03 −0.81 0.047 Del 3 LFT 2 Theriosynoecum forbesi (8 white valves) −4.03 0.01 −1.62 0.028 Del 3 LFT 2 Theriosynoecum forbesi (14 reddish valves) −4.39 0.03 −0.36 0.049 Del 3 LFT 2 Cypridea dunkeri inversa (9 white valves) −2.95 0.02 −0.41 0.0310 Del 3 LFT 2 Cypridea dunkeri inversa (10 reddish valves) −2.63 0.02 −0.65 0.0311 Del 3 LFT 2 Cypridea tumescens praecursor (8 white valves) −3.15 0.02 0.29 0.0312 Del 3 LFT 2 Mantelliana purbeckensis (4 white valves) −1.54 0.02 −0.20 0.0113 Del 3 LFT 2 Mantelliana purbeckensis (5 reddish valves) −2.95 0.02 −1.79 0.0414 Del 5 LFT 2 Theriosynoecum forbesi (3 white valves) −3.21 0.02 −0.38 0.0315 Del 5 LFT 2 Cypridea dunkeri inversa (3 white valves) −2.42 0.02 0.84 0.0316 Del 6 LFT 3 Theriosynoecum forbesi (7 valves) −4.21 0.02 0.31 0.0217 Del 6 LFT 3 Cypridea dunkeri inversa (4 valves) −3.48 0.03 −0.60 0.0318 Del 7 LFT 6 Theriosynoecum forbesi (11 valves) −3.65 0.02 −0.19 0.0219 Del 7 LFT 6 Cypridea dunkeri inversa (3 valves) −2.37 0.01 0.23 0.0220 Del 8 LFT 3 Theriosynoecum forbesi (15 valves) −4.61 0.01 −0.70 0.0221 Del 8 LFT 3 Cypridea dunkeri inversa (4 valves) −2.45 0.02 0.89 0.0222 Del 9 LFT 2 Cypridea dunkeri inversa (5 valves) −1.80 0.02 −0.70 0.0223 Del 11 LFT 3 Theriosynoecum forbesi (3 valves) −4.75 0.02 −0.86 0.0224 Del 11 LFT 3 Cypridea dunkeri inversa (2 valves) −3.10 0.03 −0.12 0.0425 Del 12 LFT 6 Mantelliana purbeckensis (11 valves) −2.04 0.02 −0.70 0.0226 Del 12 LFT 6 Cypridea dunkeri inversa (6 valves) −2.48 0.03 −0.72 0.0427 Del 14 LFT 6 Mantelliana purbeckensis (6 valves) −3.10 0.02 −1.14 0.0128 Del 14 LFT 6 Cypridea dunkeri inversa (5 valves) −1.94 0.02 0.20 0.0329 Del 16 LFT 3 Theriosynoecum forbesi (8 valves) −2.18 0.02 −0.16 0.0330 Del 16 LFT 3 Cypridea dunkeri inversa (4 valves) −1.66 0.03 0.52 0.0331 Del 18 LFT 6 Mantelliana purbeckensis (8 valves) −1.86 0.01 0.45 0.0232 Del 18 LFT 6 Cypridea dunkeri inversa (6 valves) −1.38 0.02 0.74 0.0233 Del 20 LFT 6 Mantelliana purbeckensis (17 valves) −2.18 0.02 −0.14 0.0234 Del 20 LFT 6 Cypridea dunkeri inversa (4 valves) −1.38 0.01 0.59 0.0335 Del 4 LFT 2 Silt-bearing micrite from limestone nodule −3.97 0.02 −1.76 0.0336 Del 9 LFT 2 Silty, wrinkled micrite laminae from limestone nodule −4.85 0.02 −2.02 0.0237 Del 9 LFT 2 Calcite spar cement from shrinkage crack in l'st nodule −3.69 0.04 −8.04 0.0638 Del 9 LFT 2 Silty-bearing micrite from laminated limestone nodule −4.72 0.03 −2.05 0.0239 Del 10 LFT 2 Silt-bearing, wrinkled micrite laminae from l'st nodule −5.17 0.02 −2.26 0.0440 Del 10 LFT 2 Calcite spar cement from shrinkage crack in l'st nodule −5.28 0.01 −8.27 0.0241 Del 10 LFT 2 Silt-bearing, wrinkled micrite laminae from l'st nodule −4.66 0.02 −1.74 0.0342 Del 13 LFT 5 Silt-bearing, marly micrite −2.42 0.02 −4.05 0.0443 Del 16 LFT 3 Inhomogenous palustrine micrite with few ostracods −3.01 0.01 −3.90 0.0244 Del 17 LFT 4 Micrite with oncoid-, charophyte and ostracod fragments −0.69 0.02 −2.54 0.0345 Del 17 LFT 4 Calcite spar cement from inside of articulated bivalve −4.20 0.03 −8.38 0.0546 Del 19 LFT 5 Homogenous micrite with few ostracod valves −1.97 0.02 −3.15 0.0247 Del 19 LFT 5 Homogenous micrite with few foraminifera and ostracods −2.01 0.02 −3.62 0.0148 Del 19 LFT 5 Homogenous micrite with few foraminifera and ostracods −1.99 0.02 −2.34 0.0249 Del 21 LFT 5 Homogenous micrite with few charophytes and ostracods −1.89 0.02 −3.40 0.0150 Del 21 LFT 5 Homogenous micrite with few charophytes and ostracods −1.91 0.03 −3.35 0.0351 Del 21 LFT 5 Homogenous micrite with few ostracod valves −1.95 0.02 −3.20 0.0252 Del 11 LFT 3 Palustrine micrite with few charophytes and ostracods −2.39 0.01 −3.33 0.0353 Del 1 LFT 6 Dark-grey shell of gastropod (Gyraulus loryi) −7.52 0.02 −3.80 0.0354 Del 8 LFT 3 Dark-grey shell of gastropod (Provalvata proavia) −2.07 0.02 −0.55 0.0355 Del 11 LFT 3 Dark-grey shell of undetermined gastropod −4.24 0.02 −1.12 0.0456 Del 14 LFT 5 White-grey shell fragment of gastropod (Gyraulus loryi) −2.04 0.02 −0.19 0.0157 Del 16 LFT 3 Dark-grey gastropod shell fragment (Provalvata proavia) −0.84 0.01 0.29 0.0358 Gier 1a LFT 4 Oncoid cortex −0.96 0.01 −3.14 0.0259 Gier 1a LFT 4 Oncoid cortex −0.88 0.02 −2.84 0.0360 Gier 1a LFT 4 Oncoid fragments with adherent palustrine micrite −1.37 0.02 −3.22 0.02

a From locality Giersbach.



1.3.0 (Diagnostic Instruments Inc.) was used for image documentation.SEM imaging of gold-coated ostracod specimens was carried out usinga Hitachi S 2300 instrument (Hitachi, Japan) operating at 10 kV.

60 stable isotope analyses were obtained from ostracodvalves, gastropod shells, associated sediments and cements (Table

2). Between 2 and 18 monospecific adult ostracod valves were usedin stable isotope analyses. The valves were mechanically cleaned,fractured, and checked with respect to overgrowth and adherentsediment by light microscopy. Only fragments free of sediment andcements were used. Random samples investigated by SEM showed

Fig. 4. Lithofacies types of the Borberg Member. (A) Oncoidal encrustation of a charophyte stem in lithofacies type 4. The cyanobacterial filament voids are highlighted by theirbrightly luminescent calcite cement fill. Bed number 17. Cathodoluminescence micrograph. Scale bar is 1 mm. (B) Cathodoluminescence micrograph showing well-preserved non-luminescent ostracod valves in lithofacies type 4. A brightly luminescent calcite cement A is followed by dully luminescent sparry calcite mosaic. Bed number 17. Scale bar is 250 μm.(C) Undescribed porostromate alga attached to a charophyte stem fragment in lithofacies type 4. Bed number 17. Transmitted light. Scale bar is 250 μm. (D) Large Atopochara oogoniasurrounded by circumgranular cracks in lithofacies type 4. Bed number 17. Transmitted light. Scale bar is 1 mm. (E) Dissolution-enlarged complex desiccation cracks (craze planes)with internal sediment. Bed number 17. Crossed Nicols. Scale bar is 1mm. (F) Light-greymicritic limestone (LFT 5) with faint bedding, ostracods and charophyte oogonia. Bed number19. Transmitted light. Scale bar is 5 mm. (G) Small miliolid foraminifera in light-grey micritic limestone (LFT 5). Bed number 19. Crossed Nicols. Scale bar is 250 μm.



Fig. 5. Lithofacies, ostracod abundances and monospecific ostracod stable isotopes of the investigated section “Delligsen”. Red-labelled data points in stable isotope graph indicate data from reddish stained ostracod valves.

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only traces of surface contaminations (scattered calcite particles, allof them less than 5 μm in size). Samples of micrites and oncoidcortices were obtained from cutting planes (slab counterpart of thinsections) using a hand-held mini-drill or a steel needle. Carbonatepowders were reacted with 100% phosphoric acid (densityN1.9;Wachter and Hayes, 1985) at 75 °C using a Kiel III online carbonatepreparation line connected to a ThermoFinnigan 252 masspect-rometer. All values are reported in per mil relative to V-PDB byassigning a δ13C value of +1.95‰ and a δ18O value of −2.20‰ to NBS19. Reproducibility was checked by replicate analysis of laboratorystandards and is better than ±0.05 (1σ). The stable isotope mea-surements were carried out at the laboratory of Michael Joachimski,University of Erlangen.

4. Results

4.1. Lithofacies types and sedimentary cycles

The investigated section is composed of 13.3 m of Purbeck-typemarlstones and limestones with several distinct lithofacies types(Fig. 3A):

Lithofacies type 1 (LFT 1) is a red-brown, mottled marlstone withyellowish calcareous patches (Fig. 3B). Centimetre-sized light-greylimestone nodules with a laminated internal fabric identical tolithofacies type 2 (see below) occur either scattered or arranged inlayers. LFT 1 is very poor in fossils with only rare and disarticulatedostracod valves. LFT 1 can be classified as massive mudstone.Lithofacies type 2 (LFT 2) is a greenish-grey, bedded marlstone(Fig. 3C andD). Bedding iswell-developedwith intervals ofmm-scalelamination (Fig. 3C). Scattered light-grey limestone nodules preservea wrinkled lamination with mud cracks, sheet cracks, and rare rootvoids (Fig. 3D). LFT 2 commonly contains silt- and sand-sized quartzgrains, some of them reddish stained by haematite veneers. Rarly,subrounded intraclasts up to 5 mm in size occur. The microfossilcontent is moderate to low, with red-stained as well as unaltered

ostracod shells and fragments. Charophyte stem fragments arecommonly corroded, while oogonia are small and flattened bycompaction. LFT2 canbe classified asbeddedmudstone tobindstone.Lithofacies type3 (LFT3) is a dark-greycharophytemarlstone (Fig. 3E).It shows a faint bedding at dm-scale or is massive with minorpedogenic slickensides. LFT3 is fossiliferous,withabundantostracods,charophyte stems and oogonia, gastropods and carbonaceous plantdebris. Subrounded intraclasts up to 1 cm in size are common, amongthemblackened palustrine limestone clasts (black pebbles). The pelo-and bioturbated fabric is mud- to grain-supported with a micro-crystalline matrix. LFT 3 can be classified as charophyte packstone.Lithofacies type 4 (LFT 4) is a light-grey charophyte limestone(Fig. 3F). The limestone is massive with no bedding visible. Thinsections show abundant ostracods and charophyte remains(Figs. 3F, 4A and B), particularly stems with nodes, internodes andbranchlets. Small and large oogonia occur as well. Locally, rarecyanobacterial oncoids up to 1 cm in size formed on charophytestem fragments (Fig. 4A). In addition, abundant planar gastropods,rare bivalves and subrounded intraclasts occur as well (Fig. 3F). Thefabric ismud- to grain-supported and strongly bioturbated. Locally,dissolution-enlarged root voids, circumgranular cracks (Fig. 4D),and other complex desiccation cracks with internal micrite aredeveloped (Fig. 4E). LFT 4 can be classified as charophyte packstone.Lithofacies type 5 (LFT 5) is a light-grey micritic limestone (Fig. 4Fand G). Locally, a faint, mm- to cm-scale bedding is visible (Fig. 4F).The microfossil content is moderate, with ostracods and char-ophyte oogonia. Charophyte stem fragments are rare. Locallytrochiform gastropods occur. The fabric is mud-supported andmoderately bioturbated. There are no root voids or desiccationcracks. LFT 5 can be classified as ostracod wackestone.Lithofacies type 6 (LFT 6) is a light-grey clayey marlstone. Faintbedding is developed thoughpoorly visible. Themicrofossil contentis moderate to high, with ostracods and charophyte oogonia.Charophyte stem fragments are rare. The fabric is mud-supported

Fig. 6. Dendrogram of ostracod assemblage similarity using unweighted pairgroup average algorithm (UPGMA), squared similarity values and Pearson's correlation, based on valveabundance per 10 g sediment. 500 bootstrap samples were calculated.



with no traces of roots or desiccation. LFT 6 can be classified asostracod wackestone. In contrast to other lithofacies types, LFT 5and 6 contain authigenic pyrite aggregates in beds 12–14.

The lithofacies types described above are arranged in a strikinglysymmetric cyclic pattern (Figs. 3A and 5): The investigated sectionstarts with light-grey marlstones (LFT 5) followed by dark-greyfossiliferous marlstones (LFT 3), greenish-grey bedded marlstones(LFT 2) and finally red-brown mottled marlstones (LFT 1). Then, areverse succession follows, with greenish-grey bedded marlstones(LFT 2) overlain by dark-grey fossiliferous marlstones (LFT 3) andlight-grey marlstones (LFT 5). Again, lithology switched back to dark-grey fossiliferous marlstones (LFT 3) overlain by greenish-grey beddedmarlstones (LFT 2) and red-brown mottled marlstones (LFT 1). Then,greenish-grey bedded marlstones (LFT 2) occur again, followed bydark-grey fossiliferous marlstones (LFT 3) and light-grey marlstones(LFT 5). Finally, an interval of dark-grey fossiliferousmarlstones (LFT 3)with intercalated light-grey charophyte limestones (LFT 4), which isoverlain by light-grey micritic lime- and marlstones (LFT 5 and 6)forms the top of the section (Figs. 3A and 5).

4.2. Ostracod associations and associated microfossils

Ostracod faunas were recovered from marlstone samples covering10–15 cm thickness from the centre of each bed and thus represent“time-averaged” assemblages. The samples were analysedwith regard

to (1) relative abundance (% of total valves) and (2) absolute abun-dance of ostracod valves (valves per 10 g sediment; Table 1).

Absolute abundance of ostracods varies strongly from bed to bed,between less than 1 and more than 750 valves per 10 g marlstonesediment (Fig. 5, Table 1). Highest absolute valve abundances wereobtained from dark-grey fossiliferous marlstones (LFT 3), while light-grey clayey marlstones (LFT 6) show moderately high contents. Green-ish-grey bedded marlstones (LFT 2) are generally poor in ostracodsexcept for bed 3, which has moderately high contents. Very few andfragmented valves were obtained from red-brown mottled marlstones(LFT 1).

Three associations can be distinguished based on relative abun-dances and cluster analysis of absolute abundances of raw data (Fig. 6,Table 1). No interpretative modifications such as the distinction ofautochthonous and allochthonous valves were made prior to calcula-tions. Samples with very low numbers of valves (beds 4 and 10) wereomitted from the calculations. Also, unidentifiable ostracod valves andfragments were not taken into account in the cluster analysis.

The Theriosynoecum-association is characterized by Theriosynoecumforbesi (Fig. 7A), which is the most abundant species, with adult andjuvenile, articulated and disarticulated specimens. Cypridea dunkeriinversa (Fig. 7B) is the second most abundant, with articulated anddisarticulated adult specimens. Juvenile specimens occur aswell, but arerather rare. Accessory faunal elements areCypridea tumescens praecursor(Fig. 7C), Cypridea primaeva (Fig. 7D) and Rhinocypris jurassica (Fig. 7E).The percentage of unidentifiable ostracod fragments is low.

Fig. 7.Ostracods from the upper Münder Formation, section Delligsen, Germany. (A) Theriosynoecum forbesi (Jones,1885). Female, left valve. Bed 7 (sample Del 7). (B) Cypridea dunkeriinversa (Martin, 1940). Carapace, left side. Bed 12 (sample Del 12). (C) Cypridea tumescens praecursor (Oertli, 1963). Left valve. Bed 7 (sample Del 7). (D) Cypridea primaeva (Anderson,1941). Right valve. Bed 7 (sample Del 7). (E) Rhinocypris jurassica (Martin 1940). Right valve. Bed 9 (sample Del 9). (F) Mantelliana purbeckensis (Forbes, 1855). Male, carapace, rightside. Bed 20 (sample Del 20). (G) Klieana alataMartin 1940. Female, left valve. Bed 20 (sample Del 20). (H) Stenestroemia decipiens Anderson 1941. Left valve. Bed 20 (sample Del 20).(I) Stenestroemia cf. cressida Anderson 1971, left valve. Bed 12 (sample Del 12). (J) Scabriculocypris trapezoides Anderson 1941. Right valve. Bed 20 (sample Del 20).



Most samples of this association come from dark-grey fossiliferousmarlstones (LFT 3). Only two samples (beds 1 and 7) were obtainedfrom light-grey clayey marlstone (LFT 6). These samples alreadycontain Mantelliana purbeckensis (Fig. 7F), Klieana alata (Fig. 7G) andStenestroemia decipiens (Fig. 7H), thereby indicating a transition tothe Mantelliana-association (see below). Sample 3 was obtained fromgreenish-grey bedded marlstone (LFT 2) and contains a large numberof Cypridea specimens (more than Theriosynoecum) and a relativelyhigh percentage of unidentifiable ostracod fragments (Fig. 6; Table 1).This points to a transition to the “mixed Cypridea-association” (seebelow).

Associated microfossils include abundant charophyte stem frag-ments and abundant charophyte oogonia (among them Globatormaillardii praecursor, Clavator grovesi, Aclistochara bansoni, andAtopochara trivolvis horrida as a characteristic species). Scatteredoncoids (Fig. 4A) and rare other porostromate algae have also beenobserved (Fig. 4C).

Reworked serpulid tube fragments are continuously present,though at low numbers. Molluscs are represented by completespecimens and fragments of the gastropods Gyraulus loryi (Fig. 3F),Provalvata proavia, Provalvata varicosa and Hydrobia sp. In addition,articulated bivalve shells have been observed in thin sections of light-grey charophyte limestone (LFT 4), which also includes abundantTheriosynoecum sp.

The mixed Cypridea-association is characterized by well-pre-served, commonly-articulated specimens of Cypridea dunkeri inversaand Cypridea tumescens praecursor. Theriosynoecum forbesi is present,although commonly disarticulated and at lower numbers comparedwith the Theriosynoecum-association (Table 1). Rare, isolated valves ofStenestroemia cf. cressida (Fig. 7I), Klieana alata and Rhinocyprisjurassica occur as well. The association is characterized by a highpercentage of unidentifiable ostracod fragments and the co-occur-rence of white-grey and red-stained valves. Thus, this association

represents a mixture of autochthonous and allochthonous faunalelements and is consequently termed “mixed” Cypridea-association(Fig. 6). This is in agreement with the occurrence of corrodedcharophyte stem fragments. In addition, small oogonia, all of themflattened by compaction, have been observed. Reworked serpulid tubefragments are sporadically present. All samples of this associationhave been recovered from greenish-grey bedded marlstones (LFT 2).

The Mantelliana-association is characterized by the predomi-nance of Mantelliana purbeckensis, which occurs as articulatedand disarticulated, juvenile and adult, female and male specimens.Cypridea dunkeri inversa is the second most abundant species.Other Cypridea species are rare. Only sporadically, in bed 20, doesthe abundance of Klieana alata exceed that of C. dunkeri inversa(Table 1). Accessory faunal elements are Stenestroemia cf. cressida,Scabriculocypris trapezoides (Fig. 7J) and Stenestroemia decipiens.In addition, Rhinocypris jurassica is present as few isolated valves.The percentage of unidentifiable ostracod fragments is low.

All samples of this association come from light-grey clayeymarlstones (LFT 6). The Mantelliana-association is also present withinlight-grey micritic limestone beds (LFT 5) intercalated in the clayeymarlstones, with Mantelliana sp. identified in thin sections.

One of these beds, bed 19, contains strikingly-abundant smallforaminifera (Fig. 4G), which are absent from all other beds of theinvestigated section. The 70–190 μm-sized tests with chambersarranged in a quinqueloculine manner probably belong to the poly-morphinid genus Eoguttulina. Other associated microfossils includemoderately abundant charophyte oogonia, almost all of thembelonging to the species Porochara raskyae. Reworked serpulid tubefragments have not been detected in associationwith theMantelliana-association.

Cluster analysis provides a clear distinction of Theriosynoecum-and Mantelliana-association, with a sufficiently high bootstrapsupport (Fig. 6). Within the Theriosynoecum-association, assemblages

Fig. 8. Cross-plot of stable carbon and oxygen isotope data of ostracods, associated micrites and cements from the Berriasian upper Münder Formation. For comparison, data ofBerriasian marine and freshwater-lacustrine micrites from Spain (Platt, 1989; Jiménez de Cisneros and Vera, 1993) and marine low-Mg shells (Veizer et al., 1999) are shown, too.



from beds 1, 2, 6, 7 and 8 appear closely related, while assemblagesfrom beds 11 and 16 form a separate branch although with onlymoderate bootstrap support. Furthermore, Theriosynoecum- andmixed Cypridea-association are distinguished, though with lowbootstrap support (Fig. 6).

4.3. Oncoids

Oncoids were previously unknown from the Münder Formation ofthe Hils Syncline, although they have been mentioned from manyPurbeck-type deposits in Europe (e.g., Strasser, 1988; Platt, 1989).Small oncoids up to 0.8×2.2 mm in size have been detected only inbed 17, which is of lithofacies type 4 (Fig. 4A). Slightly larger oncoidsup to 7 mm diameter were found in light-grey charophyte limestones(LFT 4) from Giersbach, 7.1 km SE of the investigated section“Delligsen”. Cyanobacterial encrustations of 400 to 800 μm thicknesson charophyte stem fragments were also detected from dark-greycharophyte marlstones (LFT 3) at Hollingskopf, 6.6 km SE of Delligsen.It is the same lithofacies and locality from which the famousfreshwater gastropod fauna of Koert (1898) and Huckriede (1967,p. 92–94) has been recovered. However, oncoids are generally rare inthe upper Münder Formation of the Hils Syncline.

All oncoids at Delligsen formed as 25 to 250 μm thick “poros-tromate” encrustations on charophyte stem fragments (Fig. 4A). Theconcentric lamination of the oncoids results from the alternation of175 to 250 μm thick, dense and porous laminae composed of radiating,erect micritic tubes. Two filament tube types occur: Type 1 ischaracterized by entangled, irregularly dichotomous, branchingfilaments with 8–16 μm inner tube and 3–4 μm thick micritic tubewall, while type 2 filaments are straight, erect, cyanobacterial micritetubes with branching at acute angle, 5 μm inner tube diameter and2–3 μm thick microcrystalline tube walls. Type 1 filament tubes aremost common in porous laminae, while the smaller type 2 filamenttubes are dominant in dense laminae. An as yet undescribed poro-stromate alga, probably green algal in origin, has also been observed(Fig. 4C). Its stubby thallus is up to 300×500 μm in size, composed ofstraight, erect tubes (50–60 μm inner diameter, 7–9 μm thick micriticwall) and shows a regular dichotomous branching.

4.4. Stable isotopes

Stable carbon and oxygen isotope ratios of ostracod valves havebeen analysed from 14 marlstone beds of the studied section. Inaddition, gastropod shells (5 samples), oncoid cortices (2 samples),micrites of LFT 2, 4 and 5 (16 samples), and blocky spar cement(3 samples) have been measured (Table 2).

The covariance plot (Fig. 8) demonstrates that each of the ostracodgenera form their own field, thoughwith some overlap. Theriosynoecumforbesi shows themost negative δ13C values of the four species analysed,with δ13C values between −4.8 and −2.2‰. Its δ18O values range from−1.6 to +0.3‰. Cypridea dunkeri inversa shows δ13C values between −3.5and −1.4‰ and δ18O values between −0.7 and +0.9‰. The few dataavailable for Cypridea tumescens praecursor plot in the same field asC. dunkeri inversa. Mantelliana purbeckensis shows similar δ13C ranges(−3.1 to −1.5‰), however, their δ18O data are shifted towards slightlymore negative values (−1.8 to +0.5‰). Gastropod shells show a widerange of isotope ratios, with δ13C values between −7.5 and −0.8‰, andδ18O values from −3.8 to +0.3‰.

Sediments associated with the ostracods show isotope ratiosclearly distinct from the ostracod valves (Fig. 8). In comparison topublished data from other micrites of the same age (Platt, 1989;Jiménez de Cisneros and Vera, 1993), micrites of the investigatedsection are located between open-marine and freshwater-lacustrinemicrites (Fig. 8). Compared to ostracod valve values, micrites of theinvestigated section are generally shifted towards more negative δ13Cvalues.

Micrites of lithofacies type 5 form a distinct field at −2.4 to −1.9‰for values δ13C and −4.0 to −2.3‰ δ18O (Fig. 8). In comparison,micrites of lithofacies type 3 are slightly more negative with regard toδ13C (−3.0 to −2.4‰) and δ18O (−3.9 to −3.3‰). However, oncoidcortices and oncoid-bearing micrites of lithofacies 3 show muchhigher δ13C values, ranging from −1.4 to −0.7‰, at similar δ18O values(−3.2 to −2.5‰). By contrast, micrites of lithofacies type 2 exhibit themost negative δ13C data (−5.2 to −4.0‰) with δ18O values shiftedtoward less negative values (−2.3 to −1.7‰).

Temporal trends in stable isotope ratios of ostracod valves alongthe investigated section are shown in Fig. 5. This graph shows that

Fig. 9. Ostracod associations, suggested salinity stratification and facies model of the upper Münder Formation using Walther's Law. Palaeogeographic map based on Hoyer (1965)shows the position of the narrowHils embayment between ESE–WNW trending halotectonic anticlines during the early Berriasian. Small island within the Hils embayment indicatesposition of the Weenzen diapir.



Theriosynoecum forbesi valves generally have more negative δ valuescompared to Cypridea dunkeri inversa, which provides the mostcontinuous record. In general, ostracod valves (T. forbesi, C. dunkeriinversa, Cypridea tumescens praecursor) from lithofacies type 6 arecommonly more positive in δ13C and δ18O values compared to valuesof the same species from lithofacies type 3 above or below (Fig. 5).Mantelliana purbeckensis, abundant only in lithofacies types 5 and 6,shows lower δ13C and δ18O values in beds 12 and 14 compared to beds18 and 20 at the top of the section. Valves from lithofacies type 2,which is characterized by a high percentage of fragmented as well asreddish stained valves, exhibit a wide scatter of δ13C and δ18O valueswith average values commonly similar to valves from lithofacies 6(Fig. 5).

5. Discussion

5.1. Depositional zones and facies model

The Münder Formation, traditionally called “Münder marl”, islargely composed of marlstones with only minor evaporite andlimestone intercalations. This is especially true for its younger sectionin the Hils Syncline, the Borberg Member, whose marlstones werepreviously collectively regarded as freshwater to brackish-waterdeposits. However, from lithofacies, microfossil assemblages andstable isotope signatures it is suggested that these marlstones repre-sent a number of different depositional zones in lateral continuity toone another (Fig. 9).

Distal alluvial plains are represented by red-brown mottledmarlstones (lithofacies type 1). This interpretation is supported bythe absence of bedding, the reddish colour reflecting disseminatedhaematite and pointing to prolonged periods of subaerial exposure.The marlstones may partially represent primary mudstones fromflooding events, where rapid settling of clay aggregates can result inmacroscopically massive fabrics (Schieber, 1999). However, relicticcarbonate nodules of lithofacies type 2 within these red-brownmarlstones indicate that they largely resulted from pedogenictransformation. Stable isotope data have only been obtained fromcalcite spar cementing shrinkage cracks of the carbonate nodules(Fig. 8). They show negative δ18O values consistent with formationfrommeteoric water. However, the δ13C result of these cements is onlymoderately negative (−4.4‰) and may reflect a limited source of soil-derived inorganic carbon during their formation.

The supralittoral zone (Fig. 9) is represented by greenish-greybedded marlstones (lithofacies type 2). The mud-supported fabricindicates a prevailing low-energy environment, while silt- and sand-layers as well as the few intraclasts point to temporary floodingevents. A wrinkled lamination is considered to reflect former micro-bial mats or biofilms stabilizing the muddy sediment surface (Fig. 9).Evidence of temporary subaerial exposure comes from mud- andsheet cracks, while rare root traces indicate a scattered vegetation onthe supralittoral flat (Fig. 9). Haematite-stained ostracod valves andhaematite-stained quartz grains indicate influx of componentsreworked from alluvial plains, while corroded charophyte stem frag-ments are derived largely from eulittoral environments (Fig. 9). Theδ13C values of supralittoral micrites are as low as in meteoric sparcements (Fig. 8), and thus may indicate an influx of inorganic carbonfrom respiration of organic compounds. Surprisingly, the δ18O valuesare less negative compared to lithofacies types 3, 4 and 5, and may beexplained by an evaporation effect on the supralittoral flats.

The eulittoral zone (Fig. 9) corresponds to dark-grey fossiliferousmarlstones (lithofacies type 3) and light-grey charophyte limestones(lithofacies type 4). Generally low-energy conditions are reflected bymud-supported fabrics, while black pebbles and scattered intraclastspoint to episodic wave agitation. Abundant charophyte stems andoogonia, ostracods and molluscs reflect a well-oxygenated, denselyvegetated, submerged shallow-water environment (Fig. 9). Carbonac-

eous plant debris probably accumulated at the immediate shoreline.The rare occurrence of oncoids (Fig. 4A), which is in contrast to someother lacustrine–palustrine Purbeck sediments in Europe, may reflectlimited areas of sufficient water agitation. Short-duration subaerialexposure and weak pedogenesis is indicated by pedogenic slicken-sides (peloturbation) in marlstones as well as circumgranular cracks,complex desiccation cracks and root voids in charophyte limestones(Fig. 4D and E). Thus, all characteristics of a palustrine environmentare present (e.g., Freytet and Plaziat, 1982; Platt, 1989; Arp, 1995). Thisis in accordance with the moderately negative δ13C and δ18O values ofthe micrites, intermediate between brackish-marine and freshwater-lacustrine micrites from other Purbeck carbonates (Fig. 8). Comparedto the micrites, oncoids and samples rich in oncoid fragments show aclear shift towards more positive δ13C values at similar δ18O results(Fig. 8). Such 12C depletion in oncoids may reflect effective photo-synthetic carbon isotope fractionation in settings low in dissolvedinorganic carbon (i.e., poor pH buffering) and high in Ca2+ concentra-tions (Arp et al., 2001).

The sublittoral zone (Fig. 9) is represented by light-grey micriticlimestones (lithofacies type 5) and light-grey clayey marlstones(lithofacies type 6). Low-energy conditions are reflected by mud-supported fabrics. Partial preservation of primary lamination indicates aless intense bioturbation, possibly due to low levels of dissolved oxygen.Also, the microfossil content is lower than in the eulittoral lithofaciestypes. Ostracods and charophyte oogonia dominate, while charophytestems are rare. Also, gastropods are less abundant, now represented byconical shells. There is no evidence of subaerial exposure and no rootvoids occur. Consequently, a subaquatic environment below zone ofrooted vegetation is assumed (Fig. 9). The sublittoral micrite δ13C andδ18O data are less negative compared to eulittoral micrites, thus, show astronger tendency towards marine values (Fig. 8).

While a clear facies zonation can be established by applyingWalther's Law (Fig. 9), a distinction between a lacustrine and amarginal-marine origin of the Borberg Member cannot be made onlithofacies criteria alone. Similar difficulties exist with regard to theEnglish Purbeck (see below). At first glance, palustrine limestones(LFT 4) as well as abundant charophytes point to a lacustrine setting(hardwater lake), in which the lithofacies alternations would reflectflooding-evaporation cycles. However, there is only aweak covariationin the δ13C and δ18O data, although an evaporative lowering of waterlevel in hydrologically closed, shallow basins commonly implies aconsiderable loss of water and CO2. Also, the symmetric nature of thecycles remains unexplained. Consequently, only ostracod assemblagesand associated microfossils may provide further arguments withregard to salinity and hydrology.

5.2. Salinity and hydrology

5.2.1. Salinities derived from ostracod assemblagesAnalysis of ostracod assemblages is a valuable tool for the

reconstruction of palaeosalinities, hydrology and climatic changes(e.g., Keen, 1977; Neale, 1988; Mette, 1997). Cyclic changes in ostracodassemblages have been well-known from the English Purbeck forseveral decades (Anderson et al., 1967; Anderson and Bazley, 1971).Since that time these “faunicycles” have been attributed to salinityfluctuations in a marginal-marine to marine-lagoonal setting, becausesome of the assemblages are associated with marine molluscs, whileothers are not. Nonetheless, there is still no common agreement onthe salinity preferences of many Purbeck ostracod genera and species(Bate, 1965; Anderson and Bazley, 1971; Anderson, 1985; Schudack,1993; Schudack, 1994; Horne, 1995, 2002; Elstner and Mutterlose,1996). A detailed discussion on the ecological requirements of anumber of Jurassic ostracod genera that range into the Purbeck isprovided by Weiβ (1995) and Mette (1997).

Initially, Anderson and Bazley (1971) and Anderson (1985) groupedthe Purbeck ostracods into C-phase species (i.e., Cypridea species),



S-phase species (i.e., species belonging to genera other than Cypridea)and rare E-phase species (i.e., species associatedwith evaporites). Theyconsidered C-phase species to indicate less saline brackish watersthat probably never were completely fresh (Anderson, 1985, p. 6),while S-phase species should reflect more saline to almost marineconditions. Later publications questioned the assignment of some S-phase species (such as Bisulcocypris/Theriosynoecum, Darwinula) tomore saline conditions (e.g., Kilenyi and Allen, 1968). Furthermore,Horne (1995, 2002) focused on other environmental factors such ashabitat stability as an alternative interpretation of faunicycles. Heargues that the majority of the Purbeck ostracods are best regardedas non-marine, probably inhabiting fresh to saline athalassic lakes.Cypridea species may have colonized ephemeral pools because theyprobably had desiccation-resistant eggs. Consequently, alternations ofC- and S-phase assemblages might reflect changing relative abun-dances of ephemeral and permanent water bodies in conjunctionwithsalinity variations, similar to observations frompresent-day Australianathalassic saline lakes (Horne, 1995, 2002). For the ostracod generaoccurring in the investigated section of the Hils Syncline, the likelypalaeoenvironmental preferences are summarized as follows:

Theriosynoecumwas a freshwater genus of permanent water bodies,possibly tolerating oligohaline conditions (Bate, 1965; Weiβ, 1995;Mette, 1997; Horne, 2002). The genus Cypridea inhabited freshwaterto oligohaline, permanent as well as ephemeral water bodies(Schudack, 1993; Schudack, 1994; Horne, 1995, 2002). Rhinocyprisjurassica (Fig. 7E) is considered by many authors to have preferredfreshwater to oligohaline waters, but occurred in miopliohaline andhypo- to hypersaline settings, too (Kilenyi andAllen,1968; Schudack,1993; Schudack, 1994; Weiβ, 1995). Therefore, it appears to havebeen an opportunistic species with broad salinity tolerances,however, preferring lower salinities (Weiβ, 1995, Table 18). Klieanaalso tolerated a broad salinity spectrum, almost continuously fromfreshwater to pliohaline waters (Schudack, 1993; Weiβ, 1995),although some authors consider it as an opportunistic brackish-water genus (Schudack, 1994; Elstner and Mutterlose, 1996). In theinvestigated section,Klieana is characterized by sporadic occurrenceswithout clear relation to cyclicity (Fig. 5), and may have been anopportunist avoiding freshwaters. Mantelliana, a typical S-phasegenus of Anderson (1985), is considered by Schudack (1993) to havepreferred plio- and brachyhaline conditions. Schudack (1994) listedMantelliana as marine, whereas Elstner and Mutterlose (1996)regarded it as a brackish-water genus. For the investigated section,lacking overlap with Theriosynoecum occurrence and co-occurrencewith miliolid foraminifera suggests at least mio- to mesohalineconditions forMantelliana purbeckensis (Fig. 5). Scabriculocypris, alsoa typical S-phase genus, apparently preferred brackish (Schudack,1994; Elstner andMutterlose,1996), oligo- tomesohaline conditions(Schudack, 1993), while Stenestroemia had its maximum inmeso- tobrachyhaline waters (Weiβ, 1995). Thus, information on palaeoen-vironmental preferences of ostracod genera suggest the followinginterpretation of ostracod associations of the investigated section ofthe Hils Syncline.

The Theriosynoecum-association represents a freshwater associa-tion, living submerged at the eulittoral shoreline and shallowsublittoral zones that were vegetated with abundant charophytes,cyanobacteria and some green algae (Fig. 9). Temporary oligoha-line conditions might have been tolerated by this association,which is largely autochthonous as indicated by the co-occurrenceof adult and juvenile, articulated specimens. The occurrence of thefreshwater gastropod genera Gyraulus and Provalvata as well asthe charophyte genus Atopochara, which is restricted to fresh-

water–oligohaline conditions, strengthen this interpretation(Huckriede, 1967; Bandel, 1991; Schudack, 1993). The gastropodgenus Hydrobia, which prefers brackish to euryhaline waters buthas some present-day endemic freshwater relatives, too (Kaestner,1982; Miller, 1990), may point to temporary oligohaline conditions(see also Huckriede, 1967, p. 94).The mixed Cypridea-association includes a high percentage ofallochthonous and reworked valves, as evidenced by commonunidentifiable valve fragments and haematite-stained valves. Thelatter reflect a high influx of valves from sediment reworked fromadjacent alluvial plains (Fig. 9). By contrast, non-stained Theriosy-noecum specimens (e.g., in bed 3)were probably introduced fromtheeu- or sublittoral into the association by flooding events. Auto-chthonous faunal elements are well-preserved articulated Cyprideaspecimens, which probably inhabited ephemeral pools (cf. Horne,1995, 2002) between the biofilm-covered supralittoral mud flats(Fig. 9).

The Mantelliana-association largely consists of autochthonousvalves as indicated by abundant articulated, adult and juvenile,female andmale specimens ofMantelliana. This association reflectsmio- to mesohaline, permanently subaquatic conditions with onlyscattered charophyte vegetation (Fig. 9). Accessory faunal ele-ments, such as Scabriculocypris and Stenestroemia, which had theirmaximum occurrence at mesohaline and higher saline conditions,support this view. Cypridea dunkeri inversa is still abundant, andtherefore apparently tolerated comparatively broad salinity ranges.In turn, freshwater indicators such as Theriosynoecum, the gastro-pods Gyraulus, Provalvata and the charophyte Atopochara are lackingcompletely from this association. However, true marine ostracodtaxa (e.g., Paranotacythere) are still absent.

5.2.2. Model of a salinity-stratified marine embaymentRegarding the temporal succession of ostracod assemblages, their

likely salinity requirements, and the lithofacies types hosting theassemblages, the following environmental history is suggested for theinvestigated section (Fig. 10). After an initial development from oligo-to miohaline sublittoral conditions (bed 1) to freshwater–oligohalineeulittoral (bed 2) and freshwater supralittoral conditions (bed 3), thesediments were subaerially exposed to form a distal alluvial plain (bed4). From freshwater supralittoral conditions in bed 5, salinity andwater depth successively increased again to freshwater–oligohalineeulittoral (bed 6) and oligo- to miohaline sublittoral conditions (bed7). Then, regression caused a backward shift to freshwater–oligohalineeulittoral (bed 8) and freshwater supralittoral conditions (bed 9),followed by a second prolonged subaerial exposure (bed 10). Astronger transgressive pulse re-established freshwater–oligohalineeulittoral (bed 11), followed by miohaline sublittoral conditions (beds12–14). A stronger influx of seawater sulphate might also be indicatedby the sudden occurrence of authigenic pyrite in beds 12–14 (Fig. 10).A minor regression towards oligohaline conditions is recorded in bed16, followed by a further increase in water depth and salinity to mio-and mesohaline sublittoral conditions at the top of the section.Maximum salinities of 5–10‰ probably were reached in bed 19, whenabundant small miliolid foraminifera appeared (Figs. 4G and 9).

In summary, during deposition of the Borberg Member (“Pur-beck”), salinity and water level fluctuations were positively correlated(Fig. 10). This implies that the Hils Syncline was not a lacustrine basinbut an embayment directly linked to the sea. In this embayment afreshwater layer of lower density spreading out on top of brackish-marine water (Fig. 9; cf. Jordan, 1971, p. 234) must be assumed,because the impermeable basin floor did not permit the formation of agroundwater seepage zone at the eulittoral. Fluvial delta sedimentshave not been found in the Borberg Member, so that a number of



smaller creeks or a broader meteoric water surface influx along theshoreline is assumed. Consequently, a freshwater eulittoral shiftinglandwards upon sea-level rise, and seawards upon sea-level drop, isfavoured as an explanation of the observed facies pattern (Figs. 9and 10).

Further information to support this model can be derived frommonospecific stable isotope data of the ostracod shells, which mayindicate salinity changes that arise frommixing of marine and meteoricwaters if a number of conditions are fulfilled (Holmes and Chivas, 2002).In general, δ13C in ostracod valves is considered to reflect the isotopiccomposition of the dissolved inorganic carbon (DIC) in the host water,while δ18O depends on the isotopic composition of the host water, thetemperature of formation and the vital effect of the ostracod species(Lister, 1988; von Grafenstein et al., 1999; Holmes and Chivas, 2002;Anadón et al., 2002). Also, δ13C values can be biased by microhabitateffects and vital offsets (Anadón et al., 2002).

Nonetheless, idealized mixing of two endmembers (marine andmeteoric water) results in a linear regression line for δ18O, and forδ13C if the DIC of both waters is the same (Anadón et al., 2002).Indeed such linear gradients can be observed in nature when

residence times are short, while longer residence times cause δ13Cand δ18O to behave non-conservatively due to a number of effects(Ingram et al., 1996; Sackett et al., 1997; Anadón et al., 2002; Holmesand Chivas, 2002): In particular, these are isotopic exchanges withatmospheric CO2 and H2O, and isotopic fractionation due to evap-oration and/or biotic activity (photosynthesis, respiration). Finally,if freshwater input comes from two or more sources, each onedisplaying a different isotopic composition, no correlation of ostracodassemblages and isotope ratios in their shells can be expected(Anadón et al., 2002).

In any case, the application of stable isotopes in fossil case studiesis crucially dependent on the lack of diagenetic alterations of thevalve minerals. First studies on δ13C and δ18O ratios in ostracods ofthe Upper Jurassic Morrison Formation have demonstrated thepreservation of primary signals (Schudack, 1999). For the presentstudy, cathodoluminescence imaging of ostracods in thin sectionsdemonstrate their good preservation (i.e., non-luminescent valveswith well-preserved microstructure; Fig. 4B).

The analyses from the Borberg Member show that each of theostracod genera analysed (Theriosynoecum, Cypridea, Mantelliana)

Fig. 10. Cyclicity of lithofacies and ostracod assemblages of the investigated section “Delligsen”, and inferred sea-level and salinity changes.



forms its own field in the δ13C–δ18O covariance plot (Fig. 8), separatedfrom values of associated micrites by a positive offset in δ18O, which isinterpreted as a vital effect. Monospecific δ13C and δ18O of ostracodvalves obtained from sublittoral light-grey marlstones (LFT 6) arecommonly less negative than those from dark-grey charophytemarlstones (LFT 3) (Fig. 5). This is in accordance with the interpreta-tion of cyclic salinity changes between more saline LFT 6 (sublittoral)and less saline to fresh LFT 3 (eulittoral). Furthermore, thewide scatterof ostracod δ13C and δ18O values from supralittoral greenish-greybeddedmarlstones (LFT 2; Fig. 5) is consistent with the view that LFT 2contains autochthonous valves (recording freshwater conditions) aswell as allochthonous valves (reflecting higher salinities of underlyingbeds or penecontemporaneous sublittoral environments). Also, thestable isotope trend towards more positive values from bed 11 to bed20 at the top of the section, including an intermediate decrease in bed16 (Fig. 5), is consistent with the interpretation of a general tendencytowards higher salinities (Fig. 10).

Certainly further processes, apart from the binary mixing ofmeteoric and marine water, would have influenced the observedisotope ratios, because the water in the elongated Hils embayment(Fig. 9) probably had a fairly long residence time. For example, theoccurrence of skeletal oncoids within LFT 3 and 4 points to poorly pH-buffered, low-DIC freshwater in the eulittoral zone, which issubstantiated by the 12C-depleted oncoidal carbonate. Thus, the δ13Cof eulittoral waters likely was affected by photosynthesis not onlywithin the microenvironment of the oncoid-forming biofilms, but as awhole. Then, the initial eulittoral water was lower in δ13C thanexpected from isotopic equilibrium precipitation of the eulittoralmicrites. However, this is still in accordance with a binary mixingmodel. An alternative explanationwould be that palaeoproductivity inthe sublittoral zone and its water columnwas significantly higher thanthat of the eulittoral zone. But, eulittoral sediments show darkersediments with much more abundant charophyte remains comparedto sublittoral sediments, so that the opposite is much more likely, anda binary mixing model is adequate to explain most of the observedstable isotope patterns.

5.2.3. Alternative modelsAn alternative explanation for the observed changes in ostracod

assemblages can be proposed, reflecting variations in the solutecomposition, resulting from mixing of marine and riverine waters,chemical evolution during evaporation and mineral precipitation(Carbonel et al., 1988; Horne, 2002, p. 63). It has been shown, e.g., forAustralian salt lakes, that the chemical evolution of waters alongpathways of the Eugster–Jones–Hardie model of solute evolution(Eugster and Jones, 1979) coincides with the occurrences of differentostracod faunas (Radke et al., 2003). These pathways largely reflect theionic structure (Ca2+/alkalinity ratio, Ca2+/SO42− ratio, Mg2+/alkalinityratio; Eugster and Jones, 1979) of the initial freshwaters andsubsequent modification by a number of processes like ionicexchange, mineral precipitation/dissolution and microbial activity(e.g., Radke et al., 2002).

Fig. 11 illustrates binary mixing of regional groundwaters of thepresent-day Hils Syncline with hypothetical Berriasian seawater. Inaddition, pathways of evaporation and mineral precipitation (calcite,gypsum) are shown for endmembers and selected mixing-zonewaters. As a result, all mixing-zone waters of the Hils Syncline plotwithin the same field on a Ca2+–SO42−–Alk triplot (“1B” of Radke et al.,2003), prior to and after surpassing the CaCO3 branchpoint (Fig. 11).This is because there are no basaltic rocks available for silicateweathering and no low Ca2+/high-alkalinity riverine waters occur inthe present-day catchment area of the Hils Syncline. Only aftersurpassing the gypsum branchpoint by strong evaporation does a shifteither to pathway “1A” or “2” occur (Fig. 11). However, there is noindication of gypsum precipitation within the Borberg Member. Also,extraclasts derived fromUpper Jurassic Formationswithin the BorbergMember suggest that surface influx of meteoric water into the Hilsbasin was largely received from these formations providing Ca2+–HCO3− waters only. Therefore, changes in the ionic structure of the Hilsbasin waters during the deposition of the Borberg Member are not alikely explanation for the changes in ostracod assemblages.

A further alternative explanation of the observed lithofacies andostracod assemblage patterns to be considered is a closed-lake model.

Fig. 11. Ca2+–SO42−–Alk triplot of a hypothetical Berriasian seawater, regional freshwaters of the Hils Syncline, mixing lines and pathways upon evaporation and mineral precipitationof selected mixing ratios (50:50 and 5:95). Berriasian seawater has been calculated on basis of the Spencer–Hardie-Model (Spencer and Hardie, 1990; Stanley and Hardie, 1998) andequilibration to a 10-fold calcite supersaturation (Arp et al., 2001) at a pCO2 of 1100 ppmV (Berner, 1994). Upper right: pathways 1A, 1B and 2 within the Ca2+–SO42−–Alk triplot (Radkeet al., 2002). Note that spring water from the Malm aquifer is the likely source of freshwater in the Hils Syncline during early Berriasian time. Model calculations were performedusing the computer program PHREEQC (Parkhurst 1995).



Increasing salinities during the transgressive limbs of the cycles mayhave resulted from strong evaporation concentrating ions in a closedlake, while a minor surplus of riverine water influx relative to theevaporational loss of water still may have increased the water level. Ifso, a climatically-driven cyclicity between ephemeral pools (fresh-water, however, with highly variable conditions) and saline, perma-nent lakes (more constant conditions), in a wetland setting separatedfrom possible sea-level changes, could be suggested (Horne, 1995,2002).

However, to explain decreasing salinities during the regressivelimbs of the cycles, further additional assumptions are required. Toremove the salts during decreasing water level, either saline watersflowing off through a permeable basin floor or evaporite precipitationfollowing by clay sealing and freshwater re-floodingmust be assumed.However, there is no indication of strong evaporation from the stableisotope record (Fig. 8). Furthermore, the basin floor was impermeableand no evidence of evaporite traces exists within the marlstones andmicritic limestones. Also, while fluvial delta progradation causingsilting up and a shift towards freshwater could be proposed to explainthe regressive limbs of the cycles, no delta sequences are known fromthe Borberg Member and lateral shifts of the salinity-distributionpattern, as proposed for the hydrologically-closed Lake Bosten(Mischke and Wünnemann, 2006), are unlikely to occur in a cyclictime pattern. Consequently, the closed lake model appears inadequateto explain the observed cyclicity in lithofacies and ostracodassemblages.

Finally an open-lake model could be envisaged, with saline bottomwaters due to brine outflow from salt anticlines or diapirs. Again, thismodel does not provide a mechanism for the observed cycliccovariation of lithofacies and ostracod assemblages. On the otherhand, the model of a salinity-stratified marine embayment explainsconclusively all observations in the Borberg Member with leastnumber of assumptions.

5.3. Cyclicity

The symmetric pattern of the cycles and the coupling of salinityand water depth suggest eustatic sea-level changes at high subsidencerates during the deposition of the Borberg Member (“Purbeck”) of theHils Syncline (Fig. 10). Short-term eustatic sea-level changes inPurbeck perimarine settings are commonly interpreted to reflectorbitally-forced third- to sixth-order Milankovitch cycles (Strasser,1988, 1994; Jiménez de Cisneros and Vera, 1993; Joachimski, 1994;Hillgärtner and Strasser, 2003; Anderson, 2004a,b) and may beglacioeustatic in origin (Frakes and Francis, 1988; Jiménez de Cisnerosand Vera, 1993).

Based solely on the investigated section at Delligsen (Figs. 5 and10), neither a time estimate can be provided for the cycles, nor acyclicity of the whole Borberg Member demonstrated. However, acomplete section of the Borberg Member has been obtained by thedrilling Duingen 1 (Fig. 12; Henrici et al., 1949), which is located 9 kmNW of the section Delligsen (Fig. 1). At least 24 cycles could beidentified in the 180 m thick Borberg Member by applying lithofaciesand microfossil palaeoenvironmental interpretations of this study tocore descriptions andmicrofossil content of the drilling (Fig.12). Cyclicsedimentation already starts in top parts of the Katzberg Member, butcommonly is below the resolution of the core descriptions. Most partsof the Borberg Member show 3 to 8 m thick cycles (Figs. 10 and 12),with cycle thickness increasing to 12 m at the top, possibly reflectingan acceleration of subsurface salt movements and subsidence (Fig. 12).Together, the Katzberg and Borberg Member are 475 m thick in thedrilling Duingen 1 and may represent approximately 3.2 Ma (Fig. 2),from which an average subsidence rate of 14.8 cm/ka can becalculated. Then, each 3 to 12 m thick cycle would represent a timeinterval of 20,000 to 80,000 yr, which is in the order of magnitude offifth- to sixth-order Milankovitch cycles.

Fig. 12. Lithofacies cyclicity of the Borberg Member in the drilling Duingen 1 based oncore descriptions (Henrici et al., 1949), microfossil content (Henrici and Wolburg inHenrici et al., 1949) and few core samples stored at the Geocentre Hannover. Greenish-grey to grey calcareous sandstones are considered as equivalent of lithofacies type 2.Thick lines indicate length of cores. Ostracod zones according to U. Schudack inGramann et al. (1997). The diachronous lower boundary of the Borberg Member isdefined by the transition from predominantly red-brown mottled marlstones topredominantly grey marlstone beds with intercalated charophyte limestones. Theupper boundary is drawn at the transition to predominantly grey-brownish sandstone-claystone alternations with carbonaceous intercalations and siderite nodules.



The high subsidence rate, as a prerequisite for the complete,symmetric record of sedimentary cycles, is considered to reflect thestrong synsedimentary halotectonic down-movement of the HilsSyncline and synchronous uplift of fringing halotectonic anticlines, asdemonstrated by strong thickness variations of the total MünderFormation (Hoyer, 1965; Herrmann et al., 1971; Waldeck, 1975;Dannapfel, 1983; Jordan, 1994) and Oxfordian limestone extraclastsin a conglomerate bed near the base of the Borberg Member(Dannapfel, 1983).

Fifth-order Milankovitch cycles of the English Purbeck andcorresponding fluctuations in ostracod assemblages apparentlyshow a more asymmetric pattern (rapid transgression followed byprogradation of facies types; e.g., Anderson, 2004b) and arerepresented by less thick sediments (ca. 30 cm for Goldspur and forBurwash faunicycle, ca. 65 cm for Durlston faunicycle at Guestlingboreholes; Anderson and Bazley, 1971) if compared to the 3 to 12 mthick cycles of the Borberg Member of the Hils Syncline. This mayreflect that the English Purbeck was deposited in an orthotectonically-controlled extensional basin with episodic-active normal faulting(Underhill, 2002) and that subsidence rates (0.8 to 2.8 cm/ka) were amagnitude lower than those of the Hils Syncline.

Except for the Purbeck type-section (Clements, 1993), a directcomparison of lithofacies and ostracod assemblage cycles of theBorberg Member (“Purbeck”) of the Hils Syncline with that of theEnglish Purbeck is hampered by limited facies descriptions (e.g., withregard to emersion and pedogenesis) of the fine-grained sedimentaryrocks hosting the ostracod assemblages (e.g., Anderson, 1985, p.42)and by limited information on potential transport of valves and post-mortem mixing (Horne, 2002, p.60). Possibly as a result of that, theassignment of many of the marlstones to a marine-lagoonal orlacustrine setting is still discussed. As a consequence, cyclicity isexplained alternatively by short-term eustatic sea-level fluctuations(Anderson, 2004b), climatically-driven variations in rainfall andfreshwater influx into a lagoon (Anderson and Bazley, 1971; Anderson,1985) or climatically-driven variations in the abundance of ephemeraland permanent lake water bodies (Horne, 1995, 2002). Only anintegrated approach at English localities using palaeontological datacombined with sedimentological evidence can distinguish betweenmarginal-marine and lacustrine sections of the English Purbeck. In anycase, short-term eustatic sea-level fluctuations at high halotectonicsubsidence rates provide a consistent explanation for the observedcoupling of lithofacies and ostracod assemblages in the BorbergMember (“Purbeck”) of the Hils Syncline (Figs. 9 and 10). This does noexclude possible fluctuations in ostracod assemblages below thesampling resolution of this study. The sea-level changes probablywere orbitally-forced, as have been suggested for other Purbecksettings (Strasser 1988, 1994; Jiménez de Cisneros and Vera, 1993;Hillgärtner and Strasser, 2003; Anderson, 2004a,b).

6. Conclusions

(1) Marlstones of the “Purbeck” of the Hils Syncline have beendeposited in sublittoral, eulittoral, supralittoral as well as distalalluvial plain environments.

(2) While red-brown marlstones formed in a distal alluvial plain,greenish-grey marlstones with faint lamination of formerbiofilms are deposited in the supralittoral. Dark fossiliferouscharophyte marl- and limestones represent a palustrineeulittoral, while poorly stratified light-grey marl- and lime-stones formed in sublittoral environments. In contrast to somePurbeck-type marginal-marine or lacustrine settings of thesame age, a wave-exposed eulittoral with intense oncoidformation was not developed.

(3) Ostracod assemblages indicate that salinity rose with risingwater level, and salinity decreased with decreasing water level.Monospecific stable isotope data of ostracod valves are in

agreement with this interpretation. At maximum water levelsmiliolid foraminifera occurred. This implies that the Hils basinwas not a lake but a salinity-stratified freshwater to brackishembayment directly connected to the fluctuating marine sea-level.

(4) The symmetric pattern of lithofacies cycles and correspondingostracod assemblages are inferred to reflect short-term eustaticsea-level oscillations at high subsidence rates. The latter reflecthalotectonic movements initiated by late Kimmerian blocktectonics.

(5) Stable isotope data of rare, skeletal oncoids demonstrate thatphotosynthesis could effectively cause 12C-depletionwithin themicroenvironment of the oncoid-forming biofilms. This pointsto low dissolved inorganic carbon concentrations in a calcite-supersaturated eulittoral freshwater that hosted the Theriosy-noecum-association.

(6) A similar, combined micropalaeontological and sedimentologi-cal approachmay help to distinguish betweenmarginal-marinesediments affected by cyclic sea-level changes and lacustrinesediments displaying climatically-driven cyclicity in the Pur-beck-type area and elsewhere in Europe.

Acknowledgements

The authors wish to thank Alexander Satmari and Harald Tonn forpreparing thin sections. We are grateful to Helga Groos-Uffenorde,Göttingen, for introduction to microfossil preparation. We areindebted to Andreas Reimer for support at the SEM. Franz Gramann,Burgwedel, is kindly acknowledged for comments on foraminifera andextraclast identification in thin sections. We thank ExxonMobilProduction Germany GmbH and BEB Erdgas & Erdöl GmbH for thepermission to use data of the drilling Duingen 1, and FriedrichWilhelm Luppold, LBEG, Geozentrum Hannover, for access to coresamples of this drilling. Detailed and critical reviews by David J. Horneand an anonymous reviewer as well as helpful advice by the specialvolume editors Jonathan Holmes and Steffen Mischke are greatlyappreciated. We are especially grateful to David J. Horne for his timeand efforts in improving the English of the manuscript.

References

Allen, P., Wimbledon, W.A., 1991. Correlation of NW European Purbeck-Wealden(nonmarine Lower Cretaceous) as seen from the English type-areas. CretaceousResearch 12, 511–526.

Anadón, P., Gliozzi, E., Mazzini, I., 2002. Paleoenvironmental reconstruction of marginalmarine environments from combined paleoecological and geochemical analyses onostracods. In: Holmes, J.A., Chivas, A.R. (Eds.), The Ostracoda: Applications inQuaternary Research. Geophysical Monograph, vol. 131. American GeophysicalUnion, Washington D.C., pp. 227–247.

Anderson, E.J., 2004a. The cyclic hierarchy of the ‘Purbeckian’ Sierra del Pozo Section,Lower Cretaceous (Berriasian), southern Spain. Sedimentology 51, 455–477.

Anderson, E.J., 2004b. Facies patterns that define orbitally forced third, fourth, and fifthorder sequences of sixth order cycles and their relationship to ostracod faunicycles:the ‘Purbeckian’ (Berriasian) of Dorset, England. In: D'Argenio, B., Fischer, A.G.,Premoli Silva, I., Weissert, H., Ferreri, V. (Eds.), Cyclostratigraphy. Approaches andCase Histories, SEPM Special Publication. Society for Sedimentary Geology, vol. 81,pp. 245–260.

Anderson, F.W., 1941. Ostracods from the Portland and Purbeck beds at Swindon.Proceedings of the Geologists' Association London 51, 373–384.

Anderson, F.W., 1985. Ostracod faunas in the Purbeck and Wealden of England. Journalof Micropalaeontology 4, 1–68.

Anderson, F.W., Bazley, R.A.B., 1971. The Purbeck beds of the Weald (England). Bulletinof the Geological Survey of Great Britain 34, 1–173.

Anderson, F.W., Bazley, R.A.B., Shephard-Thorn, E.R., 1967. The sedimentary and faunalsequence of the Wadhurst Clay (Wealden) in boreholes at Wadhurst Park, Sussex.Bulletin of the Geological Survey of Great Britain 27, 171–235.

Arp, G., 1995. Lacustrine bioherms, springmounds, andmarginal carbonates of the Ries-impact-crater Miocene, Southern Germany). Facies 33, 35–90.

Arp, G., Reimer, A., Reitner, J., 2001. Photosynthesis-induced biofilm calcification andcalcium concentrations in Phanerozoic oceans. Science 292, 1701–1704.

Bandel, K., 1991. Gastropods from brackish and fresh water of the Jurassic–Cretaceoustransition (a systematic reevaluation). Berliner Geowissenschaftliche Abhandlun-gen. Reihe A, Geologie und Paläontologie 134, 9–55.



Bate, R.H., 1965. Freshwater ostracods from the Bathonian of Oxfordshire. Palaeontology8, 749–759.

Berner, R.A., 1994. GEOCARB II: a revised model of atmospheric CO2 over Phanerozoictime. American Journal of Science 294, 56–91.

Brown, P.R., 1964. Petrography and origin of some Upper Jurassic beds from Dorset,England. Journal of Sedimentary Petrology 34, 254–269.

Carbonel, P., 1988. Ostracods and the transition between fresh and saline waters. In: DeDeckker, P., Colin, J.P., Peypouquet, J.P. (Eds.), Ostracoda in the Earth Sciences.Elsevier, Amsterdam, pp. 157–173.

Carbonel, P., Colin, J.-P., Danielopol, D.L., Loeffler, H., Neustrueva, I., 1988. Palaeoecologyof limnic ostracods: a review of some major topics. Palaeogeography, Palaeoclima-tology, Palaeoecology 62, 413–461.

Casey, R., 1963. The dawn of the Cretaceous period in Britain. Bulletin of the South-Eastern Union Scientific Societies 117, 1–15.

Casey, R., Allen, P., Dörhöfer, G., Gramann, F., Hughes, N.F., Kemper, E., Rawson, P.F.,Surlyk, F., 1975. Stratigraphical subdivision of the Jurassic–Cretaceous boundarybeds in NW Germany. Newsletters on Stratigraphy 4, 4–5.

Chivas, A.R., De Deckker, P., Shelley, J.M.G., 1985. Strontium content of ostracods indicatelacustrine palaeosalinity. Nature 316, 251–253.

Chivas, A.R., De Deckker, P., Shelley, J.M.G., 1986a. Magnesium content of non-marineOstracod shells: a new palaeosalineometer and palaeothermometer. Palaeogeo-graphy, Palaeoclimatology, Palaeoecology 54, 43–61.

Chivas, A.R., De Deckker, P., Shelley, J.M.G., 1986b. Magnesium and strontium in non-marine ostracod shells as indicators of palaeosalinity and palaeotemperature.Hydrobiologia 143, 135–142.

Clements, R.G., 1993. Type-section of the Purbeck Limestone Group, Durlston Bay,Swanage, Dorset. Proceedings of the Dorset Natural History and ArchaeologicalSociety, 114, pp. 181–206 (for 1992).

Dannapfel, M., 1983. Stratigraphische und mikrofazielle Untersuchungen im Serpulit(Berrias/Unterkreide) der Hilsmulde, Südniedersachsen. Unpubl. Diploma thesisTechnische Universität Clausthal-Zellerfeld, 153 pp.

Deutsche Stratigraphische Kommission (Ed.), 2002. Stratigraphische Tabelle vonDeutschland 2002.

Elstner, F., Mutterlose, J., 1996. The Lower Cretaceous (Berriasian and Valanginian) inNW Germany. Cretaceous Research 17, 119–133.

Eugster, H.P., Jones, B.F., 1979. Behavior of major solutes during closed-basin brineevolution. American Journal of Science 279, 609–631.

Forbes, E., 1855. In: Lyell, C. (Ed.), A Manual of Elementary Geology, 5th Ed., p. 296.London.

Forester, R.M., 1986. Determination of the dissolved anion composition of ancient lakesfrom fossil ostracodes. Geology 14, 796–798.

Frakes, L.A., Francis, J.E., 1988. A guide to Phanerozoic cold polar climates from high-latitude ice-rafting in the Cretaceous. Nature 333, 547–549.

Francis, J.E., 1984. The seasonal environment of the Purbeck (Upper Jurassic) fossilforests. Palaeogeography, Palaeoclimatology, Palaeoecology 48, 285–307.

Freytet, P., Plaziat, J.C., 1982. Continental carbonate sedimentation and pedogenesis —late Cretaceous and early Tertiary of southern France. Contributions to Sedimentol-ogy 12, 1–213.

Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., van Veen, P., Thierry, J.,Huang, Z., 1994. A Mesozoic time scale. Journal of Geophysical Research 99,24051–24074.

Gramann, F., Heunisch, C., Klassen, H., Kockel, F., Katschorek, T., Dulce, G., Harms, F.-J.,Mönnig, E., Schudack, M., Schudack, U., Thies, D., Weiβ, M., 1997. Das Niedersäch-sische Oberjura-Becken - Ergebnisse interdisziplinärer Zusammenarbeit. Zeitschriftder deutschen geologischen Gesellschaft 148, 165–236.

Gramann, F., Jordan, R., Kemper, E., Simon, P., Voss, H.H., 1975. Der Oberjura im RaumDeister, Süntel und Osterwald. Fauna und Sedimentation. Exkursion B, 45.Jahresversammlung der Paläontologischen Gesellschaft, Hannover, pp. 1–45.

Harms, F.J., 1984. Erläuterungen zur geologischen Karte von Niedersachsen 1: 25 000Blatt Nr. 4025 Freden. Niedersächsisches Landesamt für Bodenforschung, Hann-over, 168 pp., 1 map.

Herrmann, A., Gramann, F., Jordan, R., Schmid, F., 1971. Die Asphaltkalk-Lagerstätte beiHolze

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