Late Cretaceous carbon isotope stratigraphy in Europe: Correlation and relations with sea level and sediment stability

PALAEO ELSEVIER

Palaeogeography, Palaeoclimatology,Palaeoecology134 (1997) 39 59

Late Cretaceous carbon isotope stratigraphy in Europe: Correlation and relations with sea level and sediment stability S. Voigt a, H. Hilbrecht b a Geological Institute, Freiberg University of Mining and Technology, Bernhard-von-Cotta Strasse 2, D-09596 Freiberg, Germany, e-mai# svoigt@geo, tu-freiberg, de. b Geological Institute, E T H Ziirich, Sonneggstrasse 5, CH-8092 Ziirich, Switzerland, e-mail: hilbrecht@erdw, ethz. ch. Received 8 May 1996; received in revised form 16 December 1996; accepted 16 December 1996

Abstract

Upper Cretaceous (Upper Cenomanian to Lower Coniacian) stable carbon isotope stratigraphy of German hemipelagic marls (borehole Dresden-Blasewitz), pelagic carbonates (quarries Salzgitter Salder and S6hlde), and of two Turonian to Santonian pelagic sections in the northern Alpine Helvetic Zone, supplemented by published carbon isotope data (Kent, southern England; Gubbio, Italy), are used for correlations of northern temperate (boreal) and Tethyan sections. General carbon isotopic trends are different in pelagic and hemipelagic carbonates, probably in response to the input of terrestrial organic carbon to the inner shelf carbon reservoir (including sediments). The global component in the carbon isotope stratigraphy is best recorded in pelagic carbonates. Sufficient biostratigraphic control is present to correlate all sections across facies boundaries and between the two biogeographic provinces. Hiatuses produce breaks in the gradual carbon isotopic trends and their duration can be estimated relative to complete sections. A broad 6'3C minimum straddles the Turonian Coniacian boundary at the proposed boundary stratotype Salzgitter-Salder, with its center about 0.5-1 m below the biostratigraphic reference level (first occurrence of C. rotundatus Fiege sensu Trbger non Fiege). Increases and maxima of pelagic 613C values occur during phases of sediment accumulation. Decreasing pelagic 6'3C values and minima characterise phases of sediment erosion. The amplitudes of these stratigraphic fluctuations may indicate the intensity of sediment reworking. Changes in the sediment accumulation/erosion ratio and accompanied carbon isotopic variation may be related to short-term sea-level fluctuations and their effect on fine-grained sediment stability. © 1997 Elsevier Science B.V. Keywords: carbon isotopes; sea level; epicontinental; pelagic; redeposition; Cretaceous

I. Introduction

Biostratigraphy needs independant stratigraphic control when migration, biogeographic limitations, variation with facies, sporadic occurrences of fossils and other problems inherent in biostratigraphic methods occur. Three different methods have been applied in Western Europe as additional stratigraphic tools in the mid-Cretaceous quiet zone: 0031-0182/97/$17.00 © 1997Elsevier Science B.V. All rights reserved. PH S0031-0182(96)00156-3

event stratigraphy (Ernst et al., 1983; Mortimore, 1983; Dahmer and Ernst, 1986; Ernst and Wood, 1995), correlations based on Milankovitch cycles (Schwarzacher, 1994; Cotillon, 1995; Gale, 1995) and stable carbon isotope stratigraphy (Uli6ny et al., 1993; Jenkyns et al., 1994; Mitchell et al., 1996). Event stratigraphy is basically limited to regional correlations. Milankovitch cycles can be reliably detected in basinal successions with

40

S. l/oigl. H. Hilhrechl Pa&eogeography, PahteoclimaloloKv, &lhwoeco/ogy 134 (1997) 39 59

I]TFITi land area hemipelagic facies I

I pelagic facies • studied sections 0

sections from literature

Fig. 1. Paleogeographic m a p with geographic positions of the sections discussed in this paper. Paleogeography afler Ronov et al. (1989).

continuous sedimentation (Gale, 1995). Their recognition and correlation becomes uncertain in dynamic parts of basins and at basin margins with discontinuous sedimentation. Carbon isotope stratigraphy requires the presence of organic or skeletal carbon in the rocks, as carbonate or organic matter and is limited by diagenetic modification. Late Cretaceous pelagic carbonate deposition was widespread in Europe and makes it possible to establish carbon isotope stratigraphy in a wide range of facies and under various regimes of basin dynamics. Early work on Cretaceous carbon isotope stratigraphy revealed reproducible stratigraphic trends which were interpreted to reflect global changes in the carbon cycle (Scholle, 1974; Scholle and Arthur, 1980). Later research concentrated on the most spectacular carbon isotope event near the

Cenomanian Turonian boundary (Arthur et al., 1987: Schlanger et al., 1987). Rigorous biostratigraphic testing revealed the isochrony of the stable carbon isotope excursion at this level (Hilbrecht and Hoefs, 1986). Detailed carbon isotope stratigraphy in Cenomanian successions of the AngloParis Basin demonstrated the inter-regional and inter-facies correlation potential of stable carbon isotopes (Gale et al., 1993; Paul et al., 1994; Mitchell et al., 1996). Model calculations and field observations demonstrated a relation of maxima in the carbon isotope trends with phases of sediment accumulation and minima with phases of erosion in Late Cenomanian Late Turonian pelagic carbonates in G e r m a n y (Hilbrecht et al., 1986: Arthur et al., 1987). Subsequently phases of redeposition were linked to physical effects ot" sealevel falls on sediment stability, suggesting a relation of carbon isotope stratigraphy, sea-level fluctuations and sediment dynamics (Hilbrecht, 1989). Recently stable carbon isotope stratigraphy has been applied for the correlation of a composite Upper Cretaceous section in East Kent (southern England) with the Bottacione Gorge section near Gubbio (Italy) which are both composed of pelagic facies (Jenkyns et al., 1994). In this paper we present correlations of three German Upper Cenomanian Lower Coniacian sections of hemipelagic marls and pelagic carbonates, In the Lower Saxony Basin we selected sections in the quarries Salzgitter Salder and S6hlde, in Saxony we used cores from the borehole Dresden Blasewitz (Fig. 1). With these sections we are testing for the applicability of stable carbon isotope stratigraphy for correlation between different facies, and influences of diagenetic alteration. In addition we will present data fi'om two Turonian Santonian pelagic limestone sections with low sedimentation rates in the Alpine Helvetic Zone. These data and the published carbon isotope stratigraphy of Jenkyns et al. (1994) are the basis

Fig. 2. Stable carbon and oxygen isotope stratigraphy in the quarry of Salzgitter Salder (proposed Turonian Coniacian boundary stratotype). Stratigraphic scheme modified after Wood et al.. 1984. TC was previously correlated with the bentonite TC of S6hlde, but TC is a detrital marl in the Salder section (Wray, 1995: Wray and Wood, 1995). Abbreviations: M and "T": marls: T: bemonites: M. labiatoid.: ~l/lytiloides labiatoidi/ormis: C, rolundat.: Crenmoceramus rotundatus: cost./p/ana: costelhm4s/plana event: Di I, Di I1: Didymotis I a n d 1I e v e n t ; m . cortest, event: micraster cortestudinarium event: lso,ficr./('renmoc, event: Lsomicraster/('renmoceramus event. Event Stratigraphy after Ernst et ah (1983).

S. Voigt, H. Hi~brecht / Palaeogeography, Palaeoclimatology, Palaeoecology 134 (1997) 39-59

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S. l)~igt, H. Hi~brecht / Palaeogeography, Palaeoclimatology, Palaeoecology 134 (1997) 39 59

for correlations of northern temperate ("boreal") and Tethyan sections.

2. Materials and methods

The sections of Salzgitter Salder and S6hlde consist of coccolith/calcisphere limestones (Figs. 2 and 3) and were sampled at l m intervals. Measured sections and biostratigraphic data were established by Wood et al. (1984), Dahmer et al. (1986) and Ernst and Wood (1995). The cores of the Blasewitz borehole were depleted by previous workers in the sixties. We sampled at approximately 3 m intervals from the remaining material, which is stored in the core repository of the Geological Survey of Saxony. There are no other cored sections or surface exposures with equivalent stratigraphic length coverage. Stable carbon and oxygen isotopes in wholerock (bulk) samples from Salder, S6hlde and the Dresden-Blasewitz borehole were measured in the isotope laboratory at Freiberg University of Mining and Technology. A powdered and dried sub-sample (40 mg) was reacted with anhydrous orthophosphoric acid at 2 5 C under vacuum conditions, to release gaseous CO2, and the isotopic composition was measured with a Finnigan MAT Delta E gas mass spectrometer. An acid fractionation factor of 1.01025 (Friedman and O'Neil, 1977) and normal corrections were applied. The results are expressed as per mil (%o) deviation from the Pee Dee Belemnite (PDB) standard after calibration of our laboratory standard against NBS 19 and Carrara marble. The reproducibility of repeated analyses of standards was better than 0.1%o for carbon and 0.3%~, for oxygen isotope ratios. The samples from two sections of the Helvetic Zone of the northern Alps were measured in the laboratory of J. Hoefs (University of G6ttingen) using similar techniques (Hilbrecht, 1991). The reproducibility of these samples was 0.1%~, for carbon and oxygen isotopes.

3. Studied sections

The borehole Dresden Blasewitz 1/61 penetrated 350m of Upper Cenomanian-Lower

Coniacian hemipelagic sediments in Saxony (southeastern Germany). The lower Upper Cenomanian consists of calcareous sandstones which grade into calcareous siltstones in the upper Upper Cenomanian and Lower Turonian. The Middle Turonian-Lower Coniacian consists of weakly lithified calcareous and marly siltstones. The biostratigraphic zonation is based on inoceramid bivalves and ammonites collected from the cores by Tr6ger et al. (1962). A taxonomic revision was done for this project (K.-A. Tr6ger, I. Walaszczyk and C.J. Wood, pets. commun.). The quarries near Salzgitter Salder and S6hlde (Lower Saxony Basin, northwestern Germany) expose an Upper Cretaceous pelagic epicontinental facies which has been studied in detail by Ernst and Schmid (1975), Ernst et al. (1979a,b, 1983), Wood et al. (1984), Dahmer et al. (1986), Hilbrecht (1988) and Wood (1992). The sections consist of coccolith- and calcisphere limestones, with variable amounts of foraminifera and inoceramid fragments, and intercalated marl seams (carbonate contents of 50 95%). The limestones of the Salder section experienced a higher degree of lithification. The approximately 150 m thick section near S6hlde covers the Middle Cenomanian-lower Upper Turonian. It represents a pelagic swell facies at the margin of a primary foreland near an emerging saltdome ( Hilbrecht and Dahmer, 1994). The carbonate preservation in this section is good and the diagenetic alteration and lithification is intermediate. Stable isotopes were measured previously near the Cenomanian Turonian boundary (Hilbrecht et al., 1986). Evidence for episodic redeposition observed in numerous northwest German sections is established in Sahlde but was not observed in Salder (Hilbrecht, 1988. 1989; Hilbrecht and Dahmer, 1994). The Salzgitter Salder quarry exposes 200 m of upper Middle Turonian Lower Coniacian monotonous marl limestone rhythms that accumulated in the subsiding foreland of a salt influenced tectonic structure (Wood et al., 1984; Wood, 1992; Ernst and Wood, 1995). The carbonates are strongly cemented and the microfossil preservation is moderate to bad. The Salder section is unique for its stratigraphic completeness and has been proposed as the new stratotype for the

S. Voigt, H. Hilbrecht / Palaeogeography, Palaeoclimatology, Palaeoecology 134 (1997) 39 59

Turonian-Coniacian boundary by Birkelund et al. (1984), Wood et al. (1984), and by the 2nd International Symposium on Cretaceous Stage Boundaries (Brussels, 1995). The sections at S6hlde and Salder are close to each other but they have experienced different degrees of diagenetic alteration. Rocks at S6hlde are undeformed and less altered, while rocks at Salder are steeply inclined and intensely cemented. We use these localities for a test of the influence of diagenetic processes on the stable carbon and oxygen isotopic composition and effects on the reliability of isotope stratigraphy. The sections in the Seewen Limestone of the Helvetic zone of the Allg~iu (southern Germany) consist of pelagic coccolith and calcisphere limestone with minor amounts of planktic foraminifera (Weidich et al., 1983; Weidich, 1984; Hilbrecht, 1991). They expose deposits of the CenomanianUpper Santonian southern European shelf (northern Tethys). The Seewen Limestone is usually characterised by low sedimentation rates (<1 mm/Ka) and hiatuses. Thicknesses at the localities An der Schanz and Fugenbach are unusually large and the stratigraphic completeness is comparable with the sections near Gubbio (Italy) in some parts of their stratigraphic ranges.

4. Stratigraphic correlations The correlation of the three sections (Figs. 2, 3 and 4) is based on faunal ranges, bio-events and on lithologic marker horizons (Ernst et al., 1983). The correlation between Salder and S6hlde is well established and based on bentonites and marl layers, associated with various faunal events and biostratigraphic criteria (Ernst et al., 1983; Wood et al., 1984; Wood, 1992). Some previously published correlations have been revised herein (Figs. 2 and 3). Rare-earth-element analysis suggested a different nature and different stratigraphic positions of the upper Middle Turonian layer assigned as TC (Figs. 2 and 3) in both sections (Wray, 1995; Wray and Wood, 1995). In S6hlde it is a bentonite and in Salder it is a marl. The lower Upper Turonian can be correlated using the bentonite and marl layers TD1, TD2, TE and ME.

43

Higher in the Upper Turonian we are using the Micraster marl with the Micraster eco-event. The Hyphantoceras event, an occurrence of a diverse molluscan fauna including the heteromorph ammonite Hyphantoceras reussianum d'Orbigny, is well established in Salder and other sections, but is not seen in the quarries at S6hlde. The correlation between pelagic sections of the Lower Saxony Basin and the hemipelagic succession in the borehole Dresden-Blasewitz in Saxony, at the basin margin, presents some uncertainties. Few event-markers are available. We used inoceramid assemblage zones, eco-events, and the well established stable carbon isotope stratigraphy for the Cenomanian-Turonian boundary interval (Hilbrecht and Hoefs, 1986; Tr6ger and Voigt, 1995). The boundary between the Upper Cenomanian Calycoceras guerangeri and Metoicoceras geslinianum zones is represented by the facies change from coccolith chalks to orange red coloured marls in Lower Saxony. In Saxony a change from calcareous sandstones to calcareous siltstones occurs (Seifert, 1955; Tr6ger and Voigt, 1995; Voigt and Tr6ger, 1996). Upper Cenomanian-Lower Turonian marker horizons above the facies change include the large positive carbon isotope excursion, the first appearance of the inoceramid genus Mytiloides and the Mytiloides event. The latter is a basinwide acme occurrence of inoceramids (Ernst et al., 1983; Dahmer and Ernst, 1986; Hilbrecht and Dahmer, 1994). The Middle-Upper Turonian boundary is identified in the Strehlen Limestone in Saxony, which is correlative with the basal limestone of the Teplice Formation in Bohemia (Tr6ger and Wolf, 1960; Tr6ger, 1987; (~ech and Sv~.benick~, 1992). Occurrences of coprolites followed by a mass occurrence of the brachiopod species Terebratulina ornata are recorded from both areas. In Bohemia, the Coprolite bed contains numerous fragments of large Inoceramus cuvierii Sowerby or Inoceramus lamarcki stuemckei Heinz ((~ech and Svabenick~, 1992). The marker beds above this horizon contain the "neptuni fauna" in the Bohemian-Saxonian basin, including the ammonites H. reussianum, Subprionocyclus neptuni (Geinitz), and the inoceramids Mytiloides labiatoidiJbrmis (Tr6ger) and

44

S. I)~i~zt. H. Hilhrech¢ Pahwo,~eo£,raphy. Palaeoclimalology. Pahteoecolojzy 134 (1997) 39 59

Inoceramus striatoconcenlricus striatoconcentricus Giimbel. This assemblage indicates a Late Turonian age and suggests a correlation of this horizon with the Hyphantoceras event in Lower Saxony. The equivalent of the Strehlen Limestone in the Blasewitz borehole is a marlstone interval with an about 20% increased carbonate content (Fig. 5). A major hiatus exists at the base of the Strehlen Limestone equivalent, which correlates with the Coprolite bed at the base of the Teplice Formation in Bohemia (Krhovsk~, 1991" Cech and Svfibenickfi, 1992; K.-A. Tr6ger and M. Wejda, pers. commun.0 1995). The hiatus includes the uppermost part of the L lamarcki L apicalis - L cuvierii Zone and probably the whole L costelkaus L cuvierii L lanlarckistuemckei L inaequivah, is Zone. The first occurrence of Inoceramus costellatus pietzschi Tr6ger at 155 m in the Blasewitz borehole indicates the L costellatus - L striatoconcentricus Zone (Fig. 5). The occurrence of the heteromorph ammonite Eubosto, choceras saxon icum (Schliiter) (144 m) suggests a correlation with the Hyphantoceras event but it is not an evidence for it. Inoceramus striatoconcentricus slriatoconcentricus occurs between 140m and 107m and records the M. labiatoidiJbrmis - L striatoconcentricus Zone. The zonal boundary between the L costellatus - L striatoconcentricus Zone and the M. labiatoidiJbrnlis L striatoconcentricus Zone is not well defined and a hiatus is possible in this part of the section. The L scupini Zone is evident from specimens of Inoceramus scupini Heinz between 116 and 105 m in the Blasewitz section ( Fig. 5). The specimens from l 1 6 m are associated with isolated Hyphantoceras sp. and ttyphantoceras flexuosum Schlfiter (123 l 1 6 m ) . This zone is significantly reduced in thickness compared to sections in Lower Saxony. A specimen of the Coniacian ammonite genus Phtcenticeras (determination K.-A. Tr6ger and H. Summesberger, 1994, pets. commun.) was recorded at 107 m. The co-occurrence of L scup#li and Placenticeras sp. indicates a hiatus in the higher L .s'cuphTi Zone. The Phwenticems occurrence marks the Turonian-Coniacian boundary.

CFCtllllOCC1YIHllAA' I'OILItld(IIHS s e n s u Tr6ger n o n Fiege (=Cremnoceramus tarlovensis Walaszczyk et al., after results of the 2nd workshop on inoceramids, 1996 in Freiberg) occurs together with ()'emnoceramus waltersdor/ensis waltersdor/c, nsis Andert at 100 m and indicates the basal Coniacian. A specimen of Inoceramus inconstans B6hm is recorded from 55 m (C.J. Wood and 1. Walaszczyk, 1996, pers. commun.). It may probably relate with the L inconstans event at Salzgitter Salder ( Fig. 2).

5. Carbon and oxygen isotope stratigraphy 5.1. Sahter

This section (Fig. 2 is unusually expanded and complete compared to other localities in Lower Saxony, The Middle Turonian (1. h u , arcki L apicalis I. cuvieri Zone) (~13C values increase from 2.3%o to 2.7%,, followed by a strong shortterm positive fluctuation, a maximum of 3.2%o and a rapid decrease to a minimum near the marl " T " C (Fig. 2). The carbon isotope values rise above " T " C and decrease sharply below the costellatus/phma event. Near this event and below TD1 there is a smooth maximum with an amplitude of 0.5%o. A new prominent minimum occurs between TD1 and " T " D 2 in the L coste/latus L cuvierii L lamc,'c'ki stuemckei 1. inaequivalvis Zone, followed by a continuous increase to a broad peak (3.1%,,) in the late L coste/lallts L slrialoconcentricus M. labiatod(/ormis Zone. The 513C values approach a maximum of 3.1%,, at the level of the Hyphalllocercls e v e n t and decrease between the II17)hantoceras event and the base of the succession of TF, " T " G and the Micraster Marl. A lower 6~3C maximum occurs between TF and " T " G , a decline in the L scupbli Zone, and a broad minimum (2.0%.) at the Turonian Coniacian boundary. In the Lower Coniacian the carbon isotope values rise toward a maximum (2.6%o) at the inconstans event and then decrease above this interval. The 6 1~O values fluctuate more between samples than the carbon isotopic composition, and are apparently more altered by diagenesis (Scholle, 1977: Scholle and Arthur, 1980). However. even

S. Voigt, H. Hilbrecht / Pa&eogeography, Palaeoclimatolo~,,y, Polaeoecology 134 (1997) 39 59

in this most lithified limestone section (Salder) the stratigraphic trend in the oxygen isotopes resembles that in the carbon isotopes (Fig. 2). The long term trend shows nearly constant values in the Middle Turonian which is followed by an increase of about 1.0%0 in the lower Upper Turonian. From the upper Upper Turonian to Lower Coniacian the 6 ~80 values slightly decrease. 5.2. S6hlde

This section expands our stratigraphic record to the basal Upper Cenomanian (Fig. 3). In the Calycoceras guerangeri Zone and the early M. geslinianum Zone, 613C values increase continuously up to and across the facies change. This increase culminates in the global positive carbon isotope excursion in the upper M. geslinianum Zone. The sharp peak at S6hlde is caused by hiatuses in intervals where the 613C values rise and decline. A widely observed minor 6~3C maximum at the Cenomanian-Turonian boundary is absent and probably falls in the upper hiatus at this locality (Hilbrecht and Dahmer, 1994; Hilbrecht et al., 1996). A minor minimum of the stable carbon isotope values coincides with the Mytiloides-event. The Lower Turonian 613C values fluctuate around a relatively high background value of 3.3%o. A continuous long term decrease from 3.0 to 2.3%0 starts in the upper Lower Turonian and ends in the upper I. lamarcki - I. apicalis - I. cuvieri Zone. It is interrupted by a positive excursion in the upper to middle /. lamarcki L apicalis - I. cuvierii Zone. A broad weak stable carbon isotope maximum straddles the costellatus-plana event (2.3%o). It starts below the flint layer F23 and ends between TD1 and " T " D 2 . The subsequent minimum in the middle I. costellatus I. c u v i e r i i - I. l a m a r c k i s t u e m c k e i I. inaequivalvis Zone (2.0%,0 is the starting point

for a continuous increase of the stable carbon isotope values until the middle I. costellatus - I. striatoconcentricus Zone (lower Upper Turonian). The late L costellatus I. striatoconcentricus Zone is characterised by 613C values fluctuating around 2.7%,~. The sharp decline below " T " G coincides with a hardground at this level (Ernst and Wood, 1995). The corresponding hiatus is additionally

45

evident from the absence of the H y p h a n t o c e r a s event and the bentonite T F in S6hlde. Relative to the Salder section, about 15 m of section may have been eroded at S6hlde. The amplitude of fluctuations (Fig. 3) in 61~O values is large, and is likely caused by diagenetic alteration. The long-term trend resembles that in the carbon isotope stratigraphy. The absolute 61sO values are approximately 1.0%0 heavier than those of Salder reflecting the lesser degree of lithification in this section. In the lower Upper Turonian, the oxygen isotope ratio increases like in Salder. The similar stratigraphic trend in both sections argues for a strong primary component in the oxygen isotope signal. 5.3. D r e s d e n - B l a s e w i t z

The stable carbon and oxygen isotope stratigraphy of the Dresden Blasewitz section represents hemipelagic sediments at the basin margin ( Fig. 4). The long term trend in the stable carbon isotopes is characterised by a steady decrease from the Lower Turonian to the Lower Coniacian. The carbonates of the plenus Zone (= I. pictus bohemicus Zone) record the known positive carbon isotope anomaly of the Cenomanian-Turonian boundary event. The broad peak indicates a section with high sedimentation rates with no evidence of hiatuses. In the Lower Turonian the 613C values steadily decrease with a minimum in the upper Lower Turonian (1.7%0). After an increase of 613C values at the Lower Middle Turonian boundary we observe a continuous decrease during the Middle Turonian 1. apicalis - I. cuvierii and the lower L l a m a r c k i - L apicalis L cuvierii Zones and a minor positive excursion in the higher /. l a m a r c k i - I. apicalis - L cuvierii Zone. The section is incomplete in the Upper Turonian, reflecting the general situation in the Bohemian Saxonian Basin (Vale6ka and Sko6ek, 1990; KrhovskS~, 1991; (~ech and Svfibenickfi, 1992). The macrofossil ranges indicate a stratigraphic gap in the L costellatus L c u v i e r # - L l a m a r c k i s t u e m c k e i - L inaequivalvis Zone and the basal /. costellatus - I. striatoconcentricus Zone (Fig. 5). Near this horizon the 613C values increase toward a maximum in the higher /. costellatus I.

S. Voigt, H. Hi~brecht/Pa/aeogeog;raphv Pa/aeoc/imatolog~y,Palaeoecoh)gy134 (1997) 39 59

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Metoicoceras geslinianum; M. labiat.: Mytiloides labiatus; M. here.: Mytiloides herc3'nicus; L lain. stuemckei: lnoceramus lamarcki stuemckei; L inaequival.: lnoceramus #~aequivalvix: M. lab.: Mvtiloides labi~ttoid(/i)rmis;cost./plana: costellatus/plana event" M and "T": marls; T: bentonites.

S. Voigt, H. Hi~brecht/Palaeogeographv Palaeoclimatologv, Palaeoecology 134 (1997) 39 59

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Fig. 4. Stable carbon and oxygen isotope stratigraphy in the borehole Dresden-Blasewitz. Note different scale for 61~O values (change in the 1. pictus bohemicus Zone)• Below the split point a strong negative fluctuation is caused by cementation under the influence of isotopically "light" groundwater in the calcareous sandstone aquifer• Abbreviations: M. her(Tn.: Mvtiloides hercvnicus: L apical.: lnoceramus apicalis; L costell.: lnoceramus costellatus; L striat.: Inoceramus striatoconcentricus, M. labiatoid.: Mytiloides labiatoid(#)rmis; C rotund.: Cremnoceramus rotundatus.

48

S. l~fig/, H. Hilhrecht / Palaeo,geograp]O', Pa/aeoclimatology, Pa/aeoecolo~y 134 ( 1 9 9 7 ) 3 9

~

Lower Turon. ~

g- J pictus II+~c~= I E - - -

i

M--~;er.I

a

~

n

;

59

Upper Turon_

M

Lower Coniacian

I~- I~ Ic ~ot,~ C =,~t,------~

6o ifPennrich fauna Inoceramus pictus et ssp. SOW. Mytiloides ssp. Mytiloides hercynicus PETRASCHEK Inocerarnus apicalis WOODS Inoceramus lamarcki et ssp. PARK. Inocerarnus costellatus pietzschi TROGER Inoceramus costellatus costellatus WOODS I. stnatoconcentricus striatoconcentricus GOMBEL Mytiloides (?) scupini (WALASZCZYK & TROGER) Cremnoceramus rotundatus TROGER non FIEGE * Inoceramus dresdensis dresdensis TROGER Cremnoceramus waltersdorfensis waltersdorfensis (AND.) Cremnoceramus waltersdorfensis hannovrensis (HEINZ) Inoceramus inconstans BC)HM Calycoceras sp. Lecointriceras sp. Collignoniceras cf. carolinum (D'ORB.) Nostoceras (Eubostrychoceras) saxonicum (SCHLOTER Hyphantoceras sp. Hyphantoceras flexuosum (SCHLUTER) Scaphites sp. ] Placenticeras sp. i Micraster sp.

1i

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Fig. 5. Vertical ranges of b i o s t r a t i g r a p h i c a l l y i m p o r t a n t bivalves and a m m o n i t e s in cores of the borehole Dresden Blasewitz. Pcnnrich fauna indicates an occurrence of HG~teris .YL'])I¢:IllMI/¢'~IIH Roemer. R/ll'tl¢'/lO,~'IrClHi1 ~ld~orhiculalun7, and Lima ,~ramdata Nilss. * The species C)'emnoceranu~.f rotlttldolus T r 6 g c r non Fiege was n~.tllled as ('I'UIIIIIOCCI'HllllI,~ garloven,~iv Walaszczyk et al. d u r i n g the Second l n o c e r a m i d W o r k s h o p in Freiberg, 1996. A b b r e v i a t i o n s : I. apic. : lm*ceranms apicalis: I. costel/. : Inoceramtts ~'o.~lc[]~tllts: I. MIiHlo~'.: L .slriatocom'eHlricu,s': M. Ird~hTloid.: ,'l(vti/oidcs lahialoicl(/i~rmi.< 4. i'ollllldHllls: ('I'¢'IlllH~('L'I'HlllltS I'O[lllld~llllA.

striatoconcentricus Zone (2.0%o at 155m) and decrease rapidly at the base of the M. ktbiatodiJbrmis L striatoconcentricus Zone. Above this level the rapid or sudden changes in 61-sC values of adjacent samples suggest the existence of numerous hiatuses or were caused by diagenetic alteration. The upper maximum coincides with the occurrence of H3"phantoccras .t:lexuostml and hToceramus scupini in the Blasewitz cores ( Figs. 4 and 5 ) which may indicate the base of the 1. sctq)ini Zone. The 6~3C values decrease through the L scupini Zone toward a minimum of 1.0%o below the Turonian Coniacian boundary. The Lower Coniacian starts with ~-SC values fluctuating around 1.3%,,. A short negative excursion at 55 m depth coincides with the occurrence of hToceramus inconstans in the borehole. The stratigraphic trend in the oxygen isotope

values differs from all previously studied sections and does not correlate with the carbon isotope stratigraphy ( Fig. 4). The Upper Cenomanian (C. guerangeri o1" L piclus pictus Zone) 61sO values of - 1 3 . 0 to 9.0%, were measured in a calcareous sandstone. This "Unterquader" member is an aquifer, and the low values are most likely caused by recent cementation in the presence of groundwater. The 6~so values increase sharply across the facies change from calcareous sandstones to less permeable calcareous siltstones in the lower part of the pletms Zone up to values of 6.0 to 5.5%,. The sudden increase indicates the upper limit of meteoric cementation. The upper Upper Cenomanian to Lower Turonian consists of lithified calcareous siltstones with oxygen isotope values between - 6 . 0 and -5.5%,,. The oxygen isotope signal is probably

X Voigt, H. Hilbrecht / Palaeogeography, Palaeoclh~latology, Palaeoecology 134 (1997) 39 59

influenced by diagenetic alteration in this part of the section. From the Middle to Lower Coniacian the section consists of less lithified marls and calcareous silts and the microfossil preservation is moderate to good. The oxygen isotopes may reflect a more primary signal in this interval. In the Middle Turonian, the 61sO values show an overal increase of about 0.5%,. Across the Middle-Upper Turonian a significant sudden increase occurs which correlates with the hiatus in the lower Upper Turonian which is indicated by macrofossil ranges (Figs. 4 and 5). Through the Upper Turonian and Lower Coniacian (up to 80 m) the oxygen isotope values remain nearly constant around -4.5%o. Above 80 m the oxygen isotopic composition fluctuates and shows a trend toward more negative 61~O values. This trend can be explained by groundwater infiltration and contribution of meteoric carbonate cements in the weathered marls.

6. Discussion

6.1. Correlation o f stable carbon isotope stratigraphy Fig. 6 shows the correlation of the carbon isotope stratigraphies of Salder, S6hlde and Blasewitz. The sections represent different intensities of diagenetic alteration, different sedimentary facies, and palaeogeographic positions in the northern European epicontinental sea in Late Cretaceous time. The correlation is based on biostratigraphic data, lithologic marker beds (bentonites, marls) and stable carbon isotope stratigraphy. Fig. 7 shows a correlation with the published carbon isotope stratigraphy of pelagic sections in southern England (Kent), Italy (Gubbio) and of two German sections in the Helvetic Zone of the Alps (Hilbrecht, 1991; Jenkyns et al., 1994). These correlations are based on biostratigraphic data. Fig. 6 and 7 show long-term trends in the carbon isotope stratigraphy of the pelagic carbonate sections, with maxima in the Upper Cenomanian, middle Middle Turonian, Upper Turonian and the Lower Coniacian. Distinct minima occur near the Lower Middle Turonian boundary, the late Middle Turonian, the basal Upper Turonian, and

49

at the Turonian Coniacian boundary. In the Blasewitz section the Turonian and Lower Coniacian carbon isotope values show a linear decline (correlation coefficient with depth r2=0.71 ), with superimposed fluctuations. Under biostratigraphic control, however, these fluctuations can be correlated with the carbon isotopic trends in the pelagic sections. The detailed structure of the stable carbon isotope stratigraphies in various sections demonstrates a good correlation potential across Europe, including the correlation of the northern temperate ("boreal") and the Tethyan realm. In sections from all depositional environments of the Cenomanian Turonian boundary interval we observe very similar carbon isotopic trends (Fig. 6). Discrepancies can be readily explained with the existence of hiatuses, evident from independant biostratigraphic and sedimentological data. This accords with previous observations in globally distributed sections (e.g. Hilbrecht and Hoefs, 1986; Arthur et al., 1987; Schlanger et al., 1987; Gale et al., 1993). In the uppermost Lower Turonian (M. Ilercynicus event) a widespread hiatus exists in most European sections (Ernst et al., 1983; Ernst and Wood, 1995). The 613C values decrease rapidly in S6hlde, whereas in the Blasewitz section there is a more continuous minimum. The rapid change at S6hlde occurs at a hiatus. In the Gubbio section the sharp decline of the 613C values is comparable and may be related to a correlative stratigraphic gap or condensation. The Kent section is more expanded in this part and shows a continuous decrease. The 613C values tend to decrease in all sections during the Middle Turonian. This decline is interrupted by a positive excursion in the /. lamarcki - L apicalis L cuvierii Zone (Figs. 6 and 7). The position of the Middle-Upper Turonian boundary is recognised by the first appearance of Inoceramus costellatus near the costellatus/plana event (Ernst et al., 1983; Wood et al., 1984, K.-A. Tr6ger and I. Walaszczyk, pets. commun., 1995). All carbon isotope curves show a low maximum at this level, which can be used as an additional criterion for the Middle-Upper Turonian boundary.

50

S. l'~)i~zt,H. Hilhrecht ,; Paldeogeo~mphv, Palaeoc'limatolo~y, Palaeoec'olo~y 134 (1997) 39 59 ~3C (%o PDB)

~3C (%0 PDB) i

2

rn

3

4

m 200

o 50

--~

o o

<-

o ...i

6~3C (%o PDB)

2.0

if

•~,

c 2

. . . . . . . .

3.0

~ 40

~00-I

~ =

/ /

/

/

= ~ = o_

350

~ •

,

Dresden-Blasewitz

Salzgitter-Salder

SShlde

Fig. 6. Correlation of stable carbon isotope stratigraphy in the sections at Salzgitter Salder, SOhlde and Dresden-Blasewitz, based on biostratigraphic criteria, faunal and lithological marker horizons (lull lines), and on carbon isotopes (dashed lines). Bentonite TC is not identified in Salder. Abbreviations: L apical.: hloc'eramus apicalis; L cost.: Inoceramus costellalus: 1. cuvier.: Inoceramus cuvierii; I.[. stuemckei: ]noceramus [amarcki stuemckei; L inaequ.: hloceramus inaequivah,is; L striat.: hToceramus s/riatoconcentricus: M. labiatoid.: Mytiloides labiatoid([brmis; C. rotumk,.: Cremnoceramus rotundatus: M. cortest, e.: Micraster cortestudinarium event: ttvphantoc, ev.: ttvphantoc'eras event; cost./p]cma or c./p.: coste[]cstus,plana event; Pvc. ev.: Pycnodonte event: M and "T": marls: T:

bentonites.

In the lower L lamarcki

stuemckei

costellatus L c u v i e r i i - I. L i n a e q u i v a l v i s Zone, we

observe a p r o n o u n c e d 613C m i n i m u m a n d a subseq u e n t rise toward a b r o a d high m a x i m u m . Salder exposes an e x p a n d e d section of this interval, with a c o n t i n u o u s isotopic trend a n d with the t l y p h a n t o c e r a s r e u s s i a n u m event in the u p p e r part of this m a x i m u m . In the SOhlde a n d Blasewitz sections the b r o a d m a x i m u m is t r u n c a t e d in its upper part. This a n d the absence of the Hyphantoceras r e u s s i a n u n 7 event in both sections

suggests a hiatus. The sharper peaks at Kent, G u b b i o , F u g e n b a c h and A n der Schanz occur in sections where the U p p e r T u r o n i a n is less e x p a n d e d or c o n t a i n s hiatuses. The I. s c u p i n i Z o n e is an interval of widespread hiatuses a n d c o n d e n s a t i o n . The ~13C values decrease c o n t i n u o u s l y in Salder a n d correlative intervals at Kent, G u b b i o , a n d in the Helvetic zone. Two m i n o r positive c a r b o n isotope events, however, can be traced between the sections. The lower peak occurs above b e n t o n i t e T F in Salder,

S. Voigt, H. Hilbrecht / Palaeogeography, Palaeoclimatology, Palaeoecology 134 (19971 39-59

51

13C (%. PDB) 1.0

2.0

3.0

4.0

5.0

scale (m) 0 20 40. 60 80. 100

......

~--~'~~

~

/

813c (~ PDB)

1ZO

/ _~ //,13CI.PDB)FugenbacA hnderschanz

140. 160180-

200220 " 240 • 260. 280-

~I

~

8 c 1'toPDBI so.,oo

300320 -

Fig. 7. Correlation of published stable carbon isotope stratigraphy (Jenkyns et al. (1994): southern England (Kent), Italy (Gubbio); Hilbrecht (1991): An der Schanz and Fugenbach in the Helvetic Zone), and data presented in this paper (Figs. 2, 3, 51. Correlation based on biostratigraphic criteria and bentonites (TDI, TE, TF). Thicknesses of sections to scale.

while the second is more marked by a plateau in Salder before the 813C values decline towards the Turonian Coniacian boundary. The Turonian Coniacian boundary (first appearance of C. rotundatus sensu Tr6ger non Fiege = Cremnoceramus tarlovensis Walaszcyk et al. (in prep.) above Didymotis event II ) is well marked by a minimum in the ~13C values below the boundary at Salder (Figs. 2, 6 and 7). At Blasewitz the carbon isotope minimum lies below the first record of C. rotundatus (limited macrofossil record in the cores) but is coincident with an occurrence of the Coniacian ammonite genus Placenticeras. Jenkyns et al. (1994) reported on discrepancies at the Turonian-Coniacian boundary between the isotope stratigraphy and biostratigraphic ages in English sections and at Gubbio. They suggest a mismatch of boundary definitions by microfossils and macrofossils, based on biostratigraphic data (planktic foraminifera) for the Gubbio section (Premoli-Silva, 1977). We discuss this case in greater detail to demonstrate the high potential of

the carbon isotope stratigraphy for long-range correlations. Following accepted definitions at this time Premoli-Silva (1977) identified the T u r o n i a m Coniacian boundary at the base of the Dicarinella concavata Zone and this boundary was used subsequently by Jenkyns et al. (1994). Later progress in the correlation with the macrofossil record, however, lead to define the boundary with the first appearance of Dicarinella primitiva at a lower stratigraphic level (Robaszynski et al., 1979; Caron, 1985). In the sections An der Schanz and Fugenbach (Helvetic Zone) the 613C minimum was consistently identified near the first appearance of D. primitiva (Hilbrecht, 1991). These observations and our correlations (Figs. 6 and 7) suggest a good match of the Turonian-Coniacian boundary definitions based on inoceramids (C. rotundatus FAD) in northern Europe and planktic foraminifera (D. primitiva FAD) in the Tethys. In the Lower Coniacian Cremnoceramus erectus Zone of Salder we observe two 613C maxima that

52

S. ['~(g'l, H, Hilhrecht

Pahwogeo~ral)hy, Puhwoclimalology, Pahteoe¢'oh~gU' 134 / 1997) 39 59

can be correlated between sections, followed by a marked decrease in 6~3C. The decrease in i5~3C is associated with the inconstans event in Salder and the occurrence of L inconstans in the Blasewitz borehole. The correlation with Kent, however, is problematic based on the inconsistent biostratigraphic data applied for the same British carbon isotope data (compare Jenkyns et al., 1994 and Gale, 1996). The section at Gubbio may be condensed or contains hiatuses in this interval based on available stratigraphic data from planktic foraminifera and the carbon isotopic trend. 6.2. D([Ji,rences between peht,gic and hemipelagic sections

The differences in the general stratigraphic trends of 1~13Cvalues between pelagic and hemipelagic sections across Europe have been demonstrated by Hilbrecht et al. (1992) and with higher stratigraphic resolution in this paper. The 613C values of the carbonates at Blasewitz and two other hemipelagic sections in Germany and Poland are generally lighter than northern German pelagic sections and overall stratigraphic trends are linear, rather than sinuous as in pelagic sections. These differences may be attributed to diagenetic alteration of an original "'pelagic" carbon isotopic trend in hemipelagic marls. The decrease of ~513C values in the Blasewitz section may suggest some relation with the depth of burial. This is not supported by observations in the Radbod 6 borehole (northwestern Germany) and the Puwawy borehole (Poland) by Hilbrecht et al. (1992) who observed a linear increase of i513C values in the Cenomanian and a linear decrease in the Turonian. The inflexion point is at the late Cenomanian 613C spike. Diagenetic alteration is also ill conflict with the 6~80 record of the Blasewitz section. The data from the Upper Cenomanian sandstones is clearly overprinted by meteoric cements. The influence of meteoric cements finished in the 1. pictus bohemicus Zone by reaching constant background values around - 5.5%,,. The oxygen isotope stratigraphy in the Middle and Upper Turonian is similar to the pelagic sections. The rise toward more positive 8~so values in the lower Upper Turonian and the return to constant background

values in the L costellatus L striatoconcentricus Zone is punctuated as expected from the hiatus in the Blasewitz section. The changes in the oxygen isotopes are associated with paleontologic events that demonstrate relationships with cooling events. We consider it unlikely that the carbon isotope signal is altered in comparison with pelagic sections while the oxygen isotope record preserved the widespread stratigraphic variation. It is more likely that the differences in the carbon isotope stratigraphies of pelagic and hemipelagic sections reflect primary facies differences. We identify two possibly interrelated causes for the isotopic differentiation between pelagic and hemipelagic depositional environments: (A) The influx of isotopically light terrestrial organic material and its oxidation in nearshore marine environments can lead to a negative shift in the carbon isotopic composition of the inner shelf water masses. This isotopic composition may be recorded by the carbonate producers. (B) Isotopically light terrestrial organic matter is oxidised in the sediments and leads to the generation of isotopically light CO, in the pore waters with a corresponding shift of 613C values of benthic carbonate producers and diagenetic cements. The isotopic effect on pore waters has been observed in modern marine sediments (McCorkle et al., 1985). In late Quaternary pteropod-foraminifer marls of the lower Amazon fan, the isotopic effect on benthic carbonate may be up to 4%o (Showers and Bevis, 1988). Local palaeoenvironmental effects on the carbon isotope stratigraphies may be also expressed in the different amplitudes of 613C curves and absolute values in pelagic sections (Hilbrecht et al., 1992), e.g. at Gubbio in comparison with northern European sections (disputed by Jenkyns et al. (1994) but obvious in their data, see Fig. 7 in this paper). This implies that carbon isotope stratigraphy derived from a local section contains signal fi'om the local carbon reservoir. Local processes affecting the carbon reservoir and the resulting carbon isotope stratigraphy could be studied relative to other sections. In the Cenomanian Turonian boundary interval the similarities in the stable carbon isotope stratigraphies of various facies may reflect effects of the

S. Voigt, H. Hilbrecht / Palaeogeography, Palaeoclimatology, P~daeoecology 134 (1997) 39-59

maximum sea-level highstand during this time (Haq et al., 1988): widespread pelagic sedimentation, reduction in land area and reduced facies differences between sections (Hilbrecht and Dahmer, 1994). Hilbrecht et al. (1996) suggested an increased mobilisation of nutrients through transgressive erosion during maximum long-term sea-level rises that exceed the amplitudes of previous maximum sea-level highstands. They link the transgressive phases during maximum highstands with oceanic anoxic events. The massive introduction of nutrients through erosion and subsequent marine productivity masked the isotopic effects of detrital terrestrial organic material.

6.3. Sea level, sediment dynamics and carbon isotope stratigraphy Based on theoretical considerations Arthur et al. (1987) suggested a causal relationship between phases of sediment erosion with decreasing 813C values in stratigraphic sections. The observed return to more negative 8~3C values after positive excursions requires the input of 12C to the oceanic carbon reservoir. This 12C could be released by erosion and oxidation of organic matter stored in the sediments during the preceeding 813C rise. Hilbrecht et al. (1986) and Hilbrecht (1991) verified this relationship with field observations in northwestern Germany and the alpine Helvetic Zone. Seibertz (1979), Hilbrecht (1988) and Kaplan (1992b) reported on widespread effects of sediment instability and subsequent gravity transport by debris flows, slides, and turbidity currents during these erosion phases. This sediment transport occurred in a time of continuous and weak tectonic deformation in northwestern Germany (Hilbrecht, 1988) and did not relate with tectonic deformation in the Helvetic Zone (Hilbrecht, 1991). Numeric simulations and field evidence demonstrate a physical relationship between the amplitude of sea-level falls and the intensity of redeposition during related phases of sediment instability (Hilbrecht, 1989). Mitchell et al. (1996) proposed a relationship between the area of seafloor in response to sealevel changes and organic carbon burial. Their hypothesis links long-term changes toward posi-

53

tive 8t3C values with long-term eustatic sea-level rise. Problems exist in this hypothesis with nutrient availability for the increased production and deposition of organic matter. Constant influx of nutrients would lead to an expected change in the shelVocean fractionation in carbon deposition (Berger and Winterer, 1974) and no change in the oceanic carbon isotope ratio. Being aware of this problem, Mitchell et al. (1996) summarize results of other authors and suggest marine erosion in coastal lowlands to derive nutrients for minor changes in productivity and subsequent fluctuations in 813C values. Another source for nutrients would have been increased volcanism, induced by an increased rate of seafloor spreading and submarine intraplate volcanism (Pitman, 1978; Schlanger et al., 1981; Larson, 1991a,b). Changes in the long-term sea-level curve of Haq et al. (1988) are mirrored by the late Cenomanian rise in background 813C values, the 813C maximum near the Cenomanian Turonian boundary, and the Lower to Middle Turonian decrease. The maximum in the late Turonian 813C, the minimum at the Turonian Coniacian boundary, and the early Coniacian increase are seen in all pelagic sections, but do not correlate with long-term sea-level changes. Fig. 8 shows how short-term variation in the ~13C values is related with changes in sediment accumulation and erosion that were tied in to the macrofossil zonation. Phases of sediment accumulation and rises and maxima in 813C coincide with sea-level rises. Phases of sediment erosion, and falls and minima of 8a3C coincide with sea-level falls (cf. Hilbrecht et al., 1986). The Upper Cenomanian 813C rise occurred during a phase of sediment accumulation. The increase ends below the Plenus bed where a hiatus can be observed. The hiatus is indicated by intense bioturbation of Chondrites (Ernst et al., 1983; Hilbrecht and Dahmer, 1994) and corresponds to a sea-level fall that is well recorded at the basin margin (Voigt and Tr6ger, 1996). The Plenus bed, with the 613C maximum, was deposited during a major sea-level rise. The decline of the 8~3C values toward the Mytiloides event coincides with abundant storm deposits and minor debris flow deposits (Hilbrecht and Dahmer, 1994). The "white boundary bed"

54

S. Foi~,t. H. Hilbrecht / Palaeogeography, PolaeoclimaloloEy, Palaeoecolo~y 134 (1997) 39 .59

8

130 (%o PDB)

I

.~ E o (..) d

g

I

t

i

upper limit of phases of erosion and redeposition

~ ~ (J C. rotun.

_ ~

~,

sea level highstand

V

~

~

8~

.

"%: ~

~

highstand

~.~

~

oE g

~

*

=

,j

o

(E,°,t, woo,, 199,)

SalzgitterSalder ~ S~bhlde

I

~"

Fig. 8, Composite Upper Cenomanian Lower Coniacian stable carbon isotope stratigraphy based on data t¥om Salzgitter Salder and S6hlde. Decreases and minima of the 813C trend correspond with phases of sediment reworking and erosion during sea-level falls. Increases and maxima of the ~3C trend correspond with phases of sediment accumulation during sea level rises and highstands. Sea-level information after Haq et al. (1988), Ernst et al. (1983) and Voigt and Tr/3ger (1996).

S. Voigt, H. Hilbrecht / Palaeogeography, Palaeoclimatology, Palaeoecology 134 (1997) 39 59

of Lower Saxony (Fig. 3) covers upper Lower Turonian redeposited sediments (slide and debris flow deposits) at the Lower-Middle Turonian boundary, and is involved in later slides that terminated during the early Middle Turonian. The 8~3C values sharply decline below the "white boundary bed", remain constant during the deposition of "the white boundary bed" and decline further in an interval associated with slides, debris flows and turbidites (Hilbrecht, 1988). The rise of 613C values toward the Middle Turonian maximum marks the end of redeposition. The upper Middle Turonian 8~3C minimum occurs in the higher I. l a m a r c k i I. apicalis - L cuvierii Z o n e where syndepositional faults exist below the flint layer F23 at S6hlde and slides and debris flows occur in other sections (Fig. 3 and Hilbrecht, 1988). The faults disappear above the flint layer F23 and sediment accumulation became continuous near the costellatus plana event that is indicated by a minor maximum of 813C values at the Middle-Upper Turonian boundary. The wider Middle-Upper Turonian boundary interval is characterised by abundant redeposited sediments, that are indicated by debris flow and slide deposits, isolated turbidite beds, and hiatuses. This phase was recognized in northern Germany and in the Helvetic Zone of the Alps during the 813C minimum (Hilbrecht et al., 1986; Hilbrecht, 1988, 1991; Frieg et al., 1989; Ki~chler and Ernst, 1989; Kaplan, 1992a; Kaplan et al., 1994). Haq et al. (1988) recognized one of the most dramatic shortterm sea-level falls in the early Late Turonian, in an interval where we observe a major 813C minimum ( Fig. 8). The biostratigraphic ages of Upper Turonian and Coniacian sea-level fluctuations can not be extracted with confidence from the publication of Haq et al. (1988). Instead, we used Ernst and Wood (1995) and Wood (1992), who suggest three transgressive events in the Late Turonian-Early Coniacian: the H y p h a n t o c e r a s event, a minor event reflected by the M i c r a s t e r Marl, and the Early Coniacian C r e m n o c e r a m u s abundance maximum. All three events coincide with positive shifts in the carbon isotopic composition in the present carbon isotope data (Figs. 6, 7 and 8, Jenkyns et al.,

55

1994). A decrease of 81ac values occurred between the transgressive phases. Mid of the H y p h a n t o c e r a s event and the Micraster Marl sediment reworking is evident by a hiatus indicated by a small hardground in S6hlde (Ernst and Wood, 1995). The Turonian Coniacian boundary interval is characterised by widespread erosion and hiatuses in abundant sections. Hiatuses are evident in Saxony (borehole Blasewitz) and in northern Germany (quarry Hoppenstedt, Horna, 1996). The pronounced 813C minimum is an excellent indication of the boundary. In many sections the sedimentation starts to become continuous in the C. erectus Zone (e.g. compilation by Jenkyns et al., 1994). This start of a new phase of sediment accumulation coincides with a rise and a maximum of the 613C values. The minor Middle Turonian 813C peak in the/. l a m a r c k i - I. apicalis - I. cuvierii Zone and the late Turonian maximum (Fig. 7) occur near phosphatised hardgrounds in the English Chalk (Gale, 1996). Hancock (1987) interpreted hardgrounds in pelagic carbonates as evidence of maximum sealevel fall. Gale (1996) discriminated between glauconitic hardgrounds as indicators of maximum rate of sea-level fall, and phosphatised hardgrounds as indicators of sea-level rise and increased current activity. The Upper Turonian 813C maximum in the English Chalk (Bridgewick Marls to Lewes Marl; Gale, 1996) is correlative with the the I. costellatus - I. striatoconcentricus and M. labiatoidiformis Zones (including the H y p h a n t o c e r a s event and the Micraster Marl) in Lower Saxony (Fig. 7). In England, the hardground formation (Hitch Wood Hardground; Gale, 1996) may have occurred in the regressive period between the H y p h a n t o c e r a s event and the Micraster Marl, where we observe decreasing 813C values in the expanded section of Salder. A reworked, intense bioturbated bed that comprises the interval between the H y p h a n t o c e r a s event and bentonite TF is known from SShlde (Ernst and Wood, 1995; Wray et al., in press). The formation of the Hitch Wood Hardground in England probably took place below the onset of transgression that is indicated by the M i c r a s t e r marl in northern Germany and correlates with the decreasing

56

S. Vo~,,t. H. Hilhrecht / Pahteogeograp/o'. Pa&eoclimatoh)gy. Palaeoecoh)gy 134 (1997)39 59

carbon isotope values of the Kent curve of Jenkyns et al. (1994). Jenkyns et al. (1994) identified regional or local phases of increased organic matter deposition in the Atlantic and circum-Atlantic areas during some of their carbon isotope maxima. The Upper Turonian 813C maximum did not find an explanation through this approach. At Halle (Westphalia, northwestern Germany) the occurrence of a single black shale bed in the higher Upper Turonian may suggest a possible relation with organic matter deposition in shallow environments. It consists of a basal interval with scours in the underlying sediment filled with glauconitic calcareous sandstone and an upper organic-rich black marl with gradual contact (graded bedding) between both units (Kaplan, 1986). The black shale is probably a turbidite that demonstrates deposition of organic-rich sediments together with calcareous glauconitic sands upslope and redeposition in a more basinal depositional environment. This may suggest that organic matter deposition did not occur in deeper marine or oceanic environments (in agreement with negative evidence of Jenkyns et al., 1994), but possibly in shallower water. Sediment reworking and redeposition after the Upper Turonian ~I3C maximum was caused by increased current activities during short-term regressions and created numerous hiatuses, such as in the Blasewitz section. This reworking may have exposed the isotopically light organic matter deposited during the Upper Turonian 613C maximum and may have led to the subsequent decline of the i)13C values and the minimum at the Turonian- Coniacian boundary. The Upper Turonian carbon isotope maximum has an amplitude of about 1%, at Salder. The ~13C values at the Turonian Coniacian boundary are approximately the same as in the early Late Turonian prior to the maximum. If sedimentation of organic matter leads to a rise of 8~3C values in marine carbonates and erosion and oxidation of previously deposited organic matter leads to a decrease of 813C values of marine carbonates (Arthur et al., 1987), there should be information about the quantity of preserved organic matter in the amplitudes of 8t3C minima before and alter positive 813C excursions. If organic matter, depos-

ited before the positive 613C excursion, escaped fi-om erosion, we expect 613C values in the subsequent minimum that are higher than in the minimum before the 613C rise. The 613C values before and after the Upper Turonian 613C maximum are nearly the same and, consequently, we do not expect preservation of organic matter deposited during the positive 613C excursion. On this basis the absence of major organic-rich deposits in the late Turonian (Jenkyns et al., 1994) is predictable from the stable carbon isotope stratigraphy. Our observations award with data from other stratigraphic intervals in the Cretaceous where carbon isotopes and sequence stratigraphic data has been linked directly (Gr6tsch et al., 1996; Mitchell et al., 1996). Present observations suggest that stable carbon isotope stratigraphy reflects the sea-level curve in Cretaceous sections.

7. Conclusions

Stable carbon isotope stratigraphy can be used for long-range correlations and correlations between different facies. Our observations demonstrate a high-resolution correlation potential between pelagic and hemipelagic facies even with moderate biostratigraphic control. The correlation between the northern temperate ("boreal") and Tethyan biogeographic provinces is probably more accurate and reliable than with any other stratigraphic technique. In addition, high-resolution stable carbon isotope stratigraphy can be used as an independant stratigraphic tool in the lower Upper Cretaceous, where magnetochronology can not be applied to detect diachroneity in the biostratigraphic record. On a local or regional scale, hiatuses are indicated by sudden changes in the isotopic composition of the carbonates. The duration of hiatuses and the potentially related depth of erosion can be estimated relative to complete sections. Such studies may have a significant potential when carbon isotope stratigraphy and graphic correlation are combined. Differences exist between pelagic and hemipelagic sections in the overall carbon isotopic trends and between their absolute values. They are

S. Voigt, H. Hilbrecht / Palaeogeography, Palaeoclimatology. Palaeoecology 134 (1997) 39 59

p r o b a b l y related to the i n p u t of terrestrial organic c a r b o n , or possible differences between pelagic a n d hemipelagic calcareous p l a n k t o n c o m m u n i t i e s . The terrestrial organic m a t t e r could have caused a shift t o w a r d lighter 81ac values in the nearshore water masses, or in the c a r b o n dioxide of the pore waters with s u b s e q u e n t isotopic effects o n diagenetic carb o n a t e cements. The l o n g - t e r m c a r b o n isotopic trend in pelagic sections traces first order sea-level changes d u r i n g the U p p e r C e n o m a n i a n to Middle T u r o n i a n (Mitchell et al., 1996). Short-term fluctuations in 8t3C values reflect changes in the ratio of sediment erosion a n d a c c u m u l a t i o n . The t i m i n g a n d the intensity of sediment erosion in C e n o m a n i a n a n d T u r o n i a n pelagic c a r b o n a t e s was physically related to short-term sea-level falls. Decreases in ~13C values a n d m i n i m a occur d u r i n g phases o f sedim e n t erosion, a n d increases a n d m a x i m a in 613C values occur d u r i n g phases of sediment deposition. The a m p l i t u d e of negative extremes in the 8x3C curve m a y indicate the intensity of sediment erosion a n d redeposition. S h o r t - t e r m sea-level fluctuations controled sediment stability, a n d therefore, the short-term c a r b o n isotope signal of early Late Cretaceous sediments.

Acknowledgements We t h a n k K.-A. Tr6ger, C.J. W o o d a n d I. Walaszczyk for e x a m i n a t i o n of macrofossils of the Blasewitz borehole a n d discussion o f the biostratigraphic data. S a m p l i n g in Salder a n d S6hlde benefitted from assistance by G. Ernst a n d T. Voigt. Core material was kindly provided by the Geological Survey of Saxony. This p u b l i c a t i o n presents results from the Cretaceous Epic o n t i n e n t a l G a t e w a y s Project ( C E G A P ) , supported by the Special P r o g r a m " C o n t r o l Processes on Biogenic S e d i m e n t a t i o n " of the G e r m a n Research F o u n d a t i o n ( D F G ) , u n d e r g r a n t n u m b e r Tr 309/5. We t h a n k S.F. Mitchell, H.J. H a n s e n a n d the editor for their constructive a n d helpful reviews.

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