cyclicity of cenomanian-turonian organic-carbon-rich sediments in the tarfaya atlantic coastal basin...

Cretaceous Research (1997) 18 , 587 – 601

Cyclicity of Cenomanian – Turonian organic-carbon- rich sediments in the Tarfaya Atlantic Coastal Basin (Morocco)

*Wolfgang Kuhnt , † Alexandra Nederbragt and ‡ Louis Leine

* Geologisch - Pala ̈ ontologisches Institut der Christian - Albrechts - Universita ̈ t zu Kiel , Olshausenstr . 4 0 , D - 2 4 1 1 8 Kiel , Germany † Marine Aardwetenschappen Centre , Vrije Universiteit , De Boelelaan 1 0 8 5 , 1 0 8 1 HV Amsterdam , The Netherlands ‡ Shell International Exploration & Production BV , Postbus 6 6 3 , 2 5 0 1 CR Den Haag , The Netherlands

Revised manuscript accepted 2 7 January 1 9 9 7

Density logs from six completely cored and logged exploration wells across the Cenomanian – Turonian boundary in the Tarfaya Basin (Morocco) reveal cyclic sedimentation patterns of dif ferent frequencies . The cyclicity is mainly expressed as fluctuations in organic carbon and pelagic carbonate content . Two large scale cycles are observed , one from the top of the Rotalipora cushmani planktic foraminiferal biozone to the upper part of the Whiteinella archaeocretacea Zone and one from the top of the W . archaeocretacea Zone to the upper part of the Helvetoglobotruncana helvetica Zone . The large scale trends are related to major changes in organic carbon burial and probably reflect 3rd order sea-level fluctuations . Maximum organic carbon burial corresponds to benthos-free , laminated sediments , indicating bottom water anoxia . These periods coincide with sea-level highstands of the Vail-Juignet sea-level chart . Milankovitch-scale cyclicity is present , superimposed on these large scale cycles . The obliquity (39 ky in the Cretaceous) signal is the most pronounced cyclicity and allows the duration of the W . archaeocretacea Zone to be estimated as 720 ky . Individual obliquity cycles have a pronounced signature in the well logs and can be used as correlation horizons within the entire basin .

÷ 1997 Academic Press Limited

K E Y W O R D S : organic-rich sediments ; cyclicity ; Cenomanian / Turonian boundary ; Morocco ; Tarfaya Basin .

1 . Introduction

The stratigraphic importance of sedimentary cycles has been successfully demonstrated for Pleistocene marine sequences , some Tertiary sequences and various Mesozoic sediments (Einsele et al ., 1991 ; Weedon , 1993) . Even if the precise duration of a cycle is unknown , cyclostratigraphy is a powerful tool for the identification of the horizontal and vertical variability of sedimentation rates . If it is possible to associate a sedimentary cycle with an orbitally controlled climatic cycle of the Milankovitch frequency band , absolute time measurements can be made (Weedon , 1991 ; Schwarzacher , 1993 ; Herbert et al ., 1995) . The Milan- kovitch theory is well supported for the Pleistocene by evidence from deep sea sediments and ice cores (Berger et al ., 1984) . Older stratification cycles were interpreted in terms of the Milankovitch theory using their spectral composition (i . e ., Fischer et al ., 1990) . However , the causal link between the stratigraphic record and climate is still dif ficult and especially poorly understood for the Cretaceous Greenhouse world , where no polar ice caps could act as a link between orbital parameters and the global water cycle . Eicher & Diner (1991) , 0195 – 6671 / 97 / 040587 1 15 $25 . 00 / 0 / cr970076 ÷ 1997 Academic Press Limited

W . Kuhnt et al . 588

in their discussion of environmental factors controlling Cretaceous cyclic sedimentation , proposed that rhythmic fluctuations in marine carbonate during the Cenomanian – Turonian were mainly the result of fluctuations in marine productivity . These authors speculated that Milankovitch cycles left such a powerful imprint on middle Cretaceous oceans because a delicate balance between two contrasting circulation modes (i . e ., saline driven vs . temperature driven) was maintained within these oceans . This situation may have intensified fluctuations in marine productivity and acted as the catalyst for the extraordinary episode of carbon burial during the Cenomanian / Turonian boundary event (Eicher & Diner , 1991) .

An area of special sensitivity to climatically driven fluctuations in marine productivity is the NW African Atlantic margin where numerical circulation models predict unusually high mid-Cretaceous coastal upwelling intensities (Kruijs & Barron , 1990) . Close to the northern edge of this potential upwelling zone is the Moroccan Tarfaya coastal basin which contains a complete and expanded sedimentary sequence across the Cenomanian / Turonian boundary . Sediments consist almost exclusively of pelagic carbonates with a high content of organic carbon and disseminated biosiliceous material ; terrigenous input was very low (Leine , 1986) . The pelagic deposits accumulated at rates of 64 – 71 m / my (wells S05 , S21 , S25 , and S75) , 87 m / my (well S20) and up to 118 m / my in the depocenter of the basin (well S13) . Molluscs and microfaunal assemblages indicate a depositional environment in a deep shelf sea at a depth of the order of 200 – 300 m (Einsele & Wiedmann , 1982 ; Kuhnt et al ., 1990) . Upwelling- induced high primary productivity resulted in a well developed and persistent oxygen minimum zone that may have impinged on the NW African shelf at a water depth of between 100 and 500 m , analogous to the present day Peruvian continental margin upwelling system (Thurow et al ., 1992) . Sediments deposited within the range of this oxygen minimum zone were largely protected from bioturbation and thus provide ideal records of past oceanic and associated climatic signals .

Organic carbon-rich biosiliceous pelagic chalks spanning the Cenomanian / Turonian boundary of the Tarfaya Basin show cyclic variation in organic carbon content and carbonate . In this paper we consider this cyclicity in six exploration wells along a transect across the Tarfaya coastal basin (Figure 1) . We correlate cycles with the Tethyan standard planktic foraminiferal zonation (Caron , 1985) . Within this biostratigraphic framework we (1) analyse the spectral trends of the cyclicity of density logs using time series analysis , (2) estimate a total duration for the Whiteinella archaeocretacea Zone from the number of obliquity cycles , and (3) discuss the relation of this pronounced cyclicity to climatically driven productivity changes .

2 . Methods

Cyclostratigraphy is based on density logging data of the Cenomanian / Turonian boundary interval within six exploration wells in dif ferent parts of the Tarfaya Basin . All boreholes have been logged completely from bottom to surface by BPB Instruments Ltd , East Leake , UK . The logging suite comprised the following logs : Long Spacing Density , Bed Resolution Density , natural Gamma Ray , Long Spacing and Short Spacing Neutron Neutron , Multi-Channel Sonic , the Focus- sed Electric Resistivity and the Caliper . Apart from the last two , all logs showed

Organic carbon-rich sediments in Morocco 589

Figure 1 . Exploration wells studied in the Tarfaya coastal basin .

cyclicity in the bituminous chalk sections . Cyclicity was found to be best expressed by the Long Spacing Density log , hereafter referred to as the density log . Consequently , this type of log has been chosen for our cyclicity analysis .

Density logs are generally sensitive to borehole washout ef fects . Fortunately , the caliper logs showed the boreholes to be in good condition , so that a correction for borehole rugosity was not required . The density logging tool actually measures the electron density of the formation , which is related in a complex way to bulk density . The logging tool was regularly calibrated in the field by insertion into cylinders of concrete and water of known density . A strong correlation was found between bulk densities measured on wet core samples and densities derived from the logs . Within the framework of the oil shale campaign of Shell International Exploration and Production , Fischer Assay analyses were carried out on numerous core samples . A clear correlation was found between bulk density and oil yield as well as between organic carbon content and oil yield (Leine , 1986) , with low bulk density values corresponding to high organic carbon content .

Core material from two exploration wells (S13 and S75) was available for a micropalaeontologic study , to calibrate the logging-based cyclostratigraphy to a standard chronostratigraphic timescale using planktic foraminiferal biozonation (Figure 2a , b) . The samples from well S13 were also used for Rock-Eval analysis of organic matter and carbonate content . Density logging data were compared to these Rock-Eval data for well S13 and to Fischer Assay data for the other wells . This comparison indicated a fairly good correlation of the logging data with the organic carbon and carbonate content of the sediment .

Time-series analyses of the observed cyclicity were carried out using the density logs of wells S05 , S13 , S20 , S21 , S25 , and S75 . Paper copies of the density logs were scanned on a flat bed scanner , and resulting bitmaps were converted to numerical logs with a resolution of one density value per 0 . 2 m of section . Logs were smoothed by repeating a three point moving average 20 times for well S13 , 10 times for well S20 and 6 times for the other four wells , as an aid


Figure 2a , b . Correlation of density logs , planktic foraminiferal ranges and zonation , and inferred sequence stratigraphy for Tarfaya wells S13 and S75 . R . 5 Rotalipora ; H . 5 Helvetoglobotruncana ; W . 5 Whiteinella ; D . 5 Dicarinella ; M . 5 Marginotruncana ; TST 5 Transgressive Systems Tract ; HS 5 Highstand . Shaded areas in the density logs indicate proposed position of sequence boundaries .


in visual correlation of the logs . The number of smoothing repetitions was varied based on the average thickness of cycles in each well , to obtain the same degree of smoothing for all wells . All sections had clear non-linear long term trends which completely overshadowed higher frequency cycles when time series analyses were run on the data with subtraction of linear trends . Therefore , long term trends were approximated by repeating a five point moving average 500 times for well S13 and 400 times for all other wells . Residuals from this trend were used as input for time series analysis .

As well as long term trends in density values , changes in the cyclicity pattern occurred which we believe to correspond not only to changes in amplitude of dif ferent orbital cycles but also to changes in accumulation rates . The latter caused low signal to noise ratio spectra when thick intervals of the wells were analysed in one go . Intervals for final time series analysis were selected in the thickest section of well S13 , mostly by trial and error , repeating a Fast Fourier Transform (FFT) on successively thinner intervals until spectra with well defined narrow peaks were obtained . The Maximum Entropy method was used to calculate final power spectra for two intervals (middle of cycles 1 to 8 , and 8 to 16) in each of the wells , using a 35% autoregressive order throughout . As the intervals are relatively thin , the advantage of better resolution at low frequencies far outweighs any disadvantages of the method . However , results were compared to the more commonly used Blackman & Tukey method , to check that no spurious peaks had appeared at middle to high frequencies . More detailed discussions of smoothing as well as spectral analysis methods are given by Hinnov & Goldhammer (1991) and Sprenger & ten Kate (1992) .

3 . Biostratigraphic results

Our study concentrates on the chronostratigraphic interval between about 95 . 8 and 90 . 5 my according to the timescales of Obradovich (1993) and Gradstein et al . (1994 , 1995) , which spans three global planktic foraminiferal zones (Figure 2a , b) : (1) the late Cenomanian Rotalipora cushmani Zone , (2) the Whiteinella archaeocretacea Zone , which covers the Cenomanian / Turonian boundary , and (3) the early to middle Turonian Helvetoglobotruncana helvetica Zone .

Within the thick pelagic sequence of the Tarfaya Basin several bio-events allow a further subdivision of these three standard planktic foraminiferal zones . The last occurrence (LO) of Rotalipora greenhornensis is a distinct datum in the upper part of the Rotalipora cushmani Zone , about 3 . 3 m below the final extinction of R . cushmani in well S13 . Immediately above the LO of R . greenhornensis a strong increase in radiolarian abundance and the occurrence of ‘atypical’ specimens of R . cushmani with a high trochospire and weakly developed keels is observed . These changes in the zooplankton community structure is accompanied by a pronounced positive d 1 3 C excursion in both carbonate and organic carbon (Kuhnt et al ., 1990) . The extinction of R . cushmani s .l . at 183 . 2 m in well S13 is followed by a Whiteinella -dominated assemblage with the first occurrence of typical W . archaeocretacea .

The base of the Helvetoglobotruncana helvetica Zone is defined by the first appearance of H . helvetica (Wonders , 1980) . In the Tarfaya sequence a transitional interval is observed at the base of the H . helvetica Zone . This interval is transitional in two aspects : (1) taxonomically by the occurrence of forms transitional between Whiteinella spp . of the W . aprica group , Whiteinella


praehelvetica and H . helvetica ; (2) quantitatively by the extreme rarity of typical H . helvetica in the lowermost part of the zone , which may bias the definition of the lower zonal boundary (depending on the richness and preservation of the faunal assemblages) . Although a few transitional forms between W . praehelvetica and H . helvetica already occur at 98 . 4 m in well S13 , we define the base of the H . helvetica zone with the common occurrence (at least more than 1% of the planktic assemblage) of typical H . helvetica at 90 m , resulting in a thickness of 93 m for the W . archaeocretacea Zone . In well S75 the last occurrence of R . cushmani is observed at 86 . 5 m in the upper part of cycle 0 (sample 77 K) . The first common occurrence of H . helvetica is at 35 . 8 m (sample 19) . The thickness of the W . archaeocretacea Zone is 50 . 7 m in well S75 .

4 . Sedimentological and geochemical characteristics of the bituminous limestones

The Cenomanian / Turonian sequence of the Tarfaya basin consists of dark brownish-grey laminated kerogenous chalks (Figure 3) , alternating with non- laminated lighter coloured limestones containing a lower kerogen content . Sediments consist almost exclusively of organic matter-rich biogenic carbonates . Main components of the kerogen-rich chalks are faecal pellets containing abundant coccoliths , tests of foraminifera , kerogen-flasers , and a carbonate- matrix mainly composed of coccoliths and micrite . Phosphate is common , and can form nodules measuring up to 3 cm across (Leine , 1986) . Biogenic silica occurs finely disseminated in the matrix of the chalk ; within the W . archaeocretacea Zone well-preserved radiolarian tests are observed . Occasionally calcispheres occur as well as shell fragments of bivalves (both concentrated in light limestone layers or nodules) , fish remains , sparry carbonate cement (only in kerogen-poor layers) , and disseminated pyrite .

Ricken (1993) examined depositional dilution processes causing variations in carbonate , silicate , and organic carbon inputs . Variations within this three component system can be related to distinctive organic carbon (C o r g )-CaCO 3

patterns : variation in carbonate flux is characterized by a distinctly negative correlation of organic carbon and CaCO 3 , while variation in organic carbon flux would result in organic carbon contents fluctuating largely independent of the carbonate content . According to this model , the cyclicity of the organic carbon / carbonate ratio in the upper Cenomanian of the Tarfaya Basin is mainly

Figure 3 . Small scale lamination in Tarfaya Core S13 at 172 m and 170 m (base of the W . archaeocretacea Zone) . The relative proportion of light and dark laminae is the controlling factor on average density measured in the density logs . 1 : Thin section photograph (transmitted light) of lamination at 172 m core depth ; magnification 3 18 (the photograph represents a 1-cm interval of the core) . 2 : Intercalation of a carbonate-rich lamina at 170 m depth ; SEM photograph ; scale bar 5 1 mm . 3 : Detail of the carbonate-rich layer of Figure 2 ; foraminiferal / coccolith packestone ; scale bar 5 100 m m . 4 : Composition of a typical light layer at 170 m 5 coccolith ooze ; note the good preservation of the coccoliths ; etched or broken specimens are extremely rare ; scale bar 5 10 m m . 5 : Dark layer at 170 m ; flaky laminated zone , characterized by heavily etched and broken coccoliths , indeterminate carbonate particles , and finely disseminated silica—probably material transported by faecal pellets ; scale bar 5 10 m m .


Figure 4 . Regression plot of CaCO 3 and Total Organic Carbon (TOC) values within the uppermost Cenomanian to middle Turonian (30 – 185 m) of Tarfaya well S13 .

caused by fluctuations of the carbonate flux related to the productivity of organisms with carbonate skeletons such as coccoliths , planktic foraminifera and calcispheres (Figure 4) .

5 . Cyclostratigraphy

The cyclicity of organic matter and carbonate accumulation in the Tarfaya Basin allows the correlation of well logs from proximal to distal parts of the basin (Figure 5) . Although the sediment thickness changes considerably , the number of individual cycles remains constant with a maximum thickness in well S13 in the most distal part of the basin . We use the density logs of six exploration wells in the Tarfaya Basin to establish a cyclostratigraphic framework for the timespan from the last occurrence of R . cushmani to the first abundant occurrence of H . helvetica s .s . ( W . archaeocretacea Zone) (Figure 6) . The rock densities are mainly a function of the organic carbon vs . carbonate contents in each layer .

Two large scale cycles (A and B in Figure 2) are observed with a wavelength in the range of 120 m (Cycle A) and 40 m (Cycle B) in the most distal well S13 , and only 60 m and 20 m in the proximal wells S05 , S21 , S25 , and S75 (see Figure 5) . The well S20 holds an intermediate position with around 90 m (Cycle A) and 30 m (Cycle B) . The lower cycle has its maximum organic carbon content (i . e ., minimum rock density values) at the base of the W . archaeocretacea Zone ; the peak of the upper cycle is just above the first occurrence of H . helvetica .

Superimposed on these large scale fluctuations of organic carbon content are higher frequency cycles with a maximum wavelength of about 5 – 6 m in well S13 , 4 – 5 m in well S20 , and 3 – 4 m in the other wells . Individual higher frequency cycles can be recognized by their distinct amplitude and shape . In total , 16 cycles are suf ficiently distinct to serve as correlation tools within the Whiteinella archaeocretacea Zone (Figure 5) . Above Cycle 16 the frequency of the cyclicity increases and the thickness of each cycle is significantly lower . This change is probably the result of a decrease in sedimentation rate as well as a change in expression of orbital cycles .


Figure 5 . Density logs of Tarfaya wells S13 – S75 . Cycles with pronounced signature that allow basin-wide correlation are marked .

Very characteristic is the cycle directly at the base of the Whiteinella archaeocretacea Zone (Cycle 1 in Figure 5 , 170 – 175 m in well S13) , which generally is the only cycle where rock density values are below 1 . 80 g / cm 3 . Rock Eval data of samples within this cycle give TOC values of up to 17 . 4 % and indicate a pronounced S2-pyrolysis peak of 111 mg hydrocarbons per g sediment (Kuhnt et al ., 1990) . The thickness of Cycle 1 is about 3 m in wells S05 , S21 , and S25 , approximates 3 . 5 m in S75 , reaches 4 m in S19 , and has a maximum of 5 . 5 m in well S13 (Figure 5) . Other easily recognizable cycles are numbers 5 , 10 , 14 and 16 (96 – 99 m in well S13) . Cycle 16 consistently shows densities lower than 2 . 0 g / cm 3 and Rock Eval values reach 9 . 4% TOC and a S2-pyrolysis peak of 63 . 8 mg hydrocarbons per g sediment (Kuhnt et al ., 1990) .


Figure 6 . Smoothed density logs of Tarfaya wells S13 – S75 and their correlation with biostratigraphic zones .

An estimate of the duration of observed high frequency cycles can be given using the most recent Cretaceous timescale of Obradovich (1993) and Gradstein et al . (1995) . This timescale , for its Cenomanian / Turonian interval , is largely based on 4 0 Ar / 3 9 Ar laser fusion dating of bentonites in the Western Interior Basin of the United States . The estimated duration of the oceanic anoxic event at the Cenomanian / Turonian boundary ( Sciponoceras gracile to Nigericeras scotti ammonite biozones , corresponding to the lower three quarters of the W . archaeocretacea Zone ; Wiedmann & Kuhnt , 1996) is 0 . 5 million years as a maximum value . A lower limit of 0 . 3 my was estimated by Arthur & Premoli Silva (1982) . This would approximately correspond to our cycles 1 – 10 , resulting in a maximum duration of less than 50 000 years for each cycle and a minimum duration of more than 30 000 years .

Time Series Analysis of the logging data in wells S13 to S75 has been used to better understand the cyclicity within the Cenomanian / Turonian boundary interval . The interval was subdivided into two parts , corresponding to Cycles 1 – 8 and 8 – 16 (Figure 5) , which give power spectra with narrow , well defined peaks . Maximum Entropy power spectra were calculated for those two intervals in all six wells (Figure 7) . Seven characteristic frequencies labelled A – F could be identified in most or all wells for both intervals . Frequencies of these cycles within the analysed intervals are similar in all wells , and the ratios of their wavelength relative to the other peaks are similar to the expected Milankovitch ratios (Table 1) . The most clearly expressed cycle overall , Cycle C , is identified as the main obliquity cycle (39 ka) , with B and D as the other two obliquity components (54 and 30 ka respectively) . Identification of Cycle B as an obliquity component is tentative ; it could also be part of the eccentricity cycle . Cycle A is weakly developed and not present in all spectra , but it is more clearly representative of the 100 ka eccentricity cycle . Cycles E and F are interpeted as the two precession cycles with a duration of about 20 ka .

A change can be observed in the power of individual cycles of the


Figure 7 . Maximum Entropy spectra (35% autoregressive order throughout) for (a) Cycles 1 – 8 and (b) Cycles 8 – 16 in all six wells . Wells are identified to the right of each power spectrum , with depth interval for which the spectrum was calculated . Horizontal scale in cycles per metre ; amplitude on the vertical scale not specified , but the vertical scale is linear and the same for all spectra of the same cycle . Peaks are letter coded , with all letters indicating the same cycle in both intervals in all wells (see Table 1) . Letters between brackets are used to indicate weakly developed cyclicities (for high frequencies those that are not significant using Blackman & Tukey method) . Identification of peaks and correlation between wells is based on comparison of wavelength relative to other peaks and frequency within the analysed interval .

Milankovitch frequency band between the lower part (Cycles 1 – 8 , Figure 7a) and the upper part (Cycles 8 – 16 , Figure 7b) of wells S05 , S75 , S21 and S25 , the short holes in the shallow part of the basin . In the lower part of each well , obliquity cycles (C) are very characteristic , whereas in the upper part precession


Table 1 . Wavelengths of peaks identified in Maximum Entropy power spectra of Figure 7 , their frequency within the interval (thickness of the interval for which the power spectrum was calculated divided by the wavelength) and their calculated duration in ky assuming that Cycle C corresponds

to the main obliquity cycle

1 to 8 Wavelength in metres Frequency in interval Duration in ky (C 5 obliquity)

well A B C D E F A B C D E F A B C D E F

S75 3 . 9 2 . 5 1 . 9 1 . 4 4 . 4 6 . 8 8 . 9 12 . 1 61 39 30 22 S5 4 . 3 2 . 6 2 1 . 5 4 . 2 6 . 9 9 . 0 12 . 0 65 39 30 23 S21 4 . 4 2 . 7 2 1 . 5 3 . 9 6 . 3 8 . 5 11 . 3 64 39 29 22 S25 3 . 9 2 . 8 1 . 9 1 . 5 1 . 3 5 . 1 7 . 1 10 . 5 13 . 3 15 . 4 54 39 26 21 18 S20 11 5 3 . 4 2 . 4 1 . 9 2 . 5 5 . 4 7 . 9 11 . 3 14 . 2 126 57 39 28 22 S13 13 6 . 3 4 . 6 3 . 4 2 . 5 2 . 2 2 . 8 5 . 9 8 . 0 10 . 9 14 . 8 16 . 8 110 53 39 29 21 19

8 to 16 Wavelength in metres Frequency in interval Duration in ky (C 5 obliquity)

well A B C D E F A B C D E F A B C D E F

S75 9 4 . 6 3 1 . 8 1 . 3 2 . 3 4 . 6 7 . 0 11 . 7 16 . 1 117 60 39 23 17 S5 4 2 . 6 1 . 7 1 . 2 5 . 0 7 . 7 11 . 8 16 . 7 60 39 26 18 S21 7 5 2 . 4 1 . 5 1 . 2 2 . 8 4 . 0 8 . 3 13 . 3 16 . 7 115 81 39 24 20 S25 8 4 . 6 2 . 5 1 . 8 1 . 3 1 . 1 2 . 3 3 . 7 6 . 8 9 . 4 13 . 1 15 . 5 117 72 39 28 20 17 S20 7 . 1 3 . 9 2 . 8 1 . 9 3 . 8 6 . 9 9 . 6 14 . 2 71 39 28 19 S13 13 9 . 1 5 3 . 6 2 . 7 2 . 3 2 . 8 4 . 0 7 . 2 10 . 0 13 . 3 15 . 7 101 71 39 28 21 18

cycles (E) dominate . Within the deeper , more distal wells (S20 and S13) obliquity cycles are the main element throughout the entire interval .

We used the main obliquity cycle to estimate the duration of the W . archaeocretacea Zone from well S13 . The results of the time series analysis indicate that the interval between the middle of Cycle 1 to the middle of Cycle 16 (172 m to 98 m in well S13 ; Figure 5) corresponds to 15 obliquity cycles , or 585 ky , using a duration for the main obliquity period of 39 ky for the Late Cretaceous (Berger & Loutre 1994) . Calculating accumulation rates and extra- polating to both the base and the top of the W . archaeocretacea Zone (181 m and 90 m respectively in well S13) adds another 135 ky . This gives an estimate of 720 ky for the duration of the W . archaeocretacea Zone .

6 . Discussion

The cycles in the Tarfaya Basin are mainly caused by changes in the carbonate – organic carbon ratio in the sediment . This relationship is quite obvious from regression plots of carbonate against organic carbon from random samples of Cenomanian sediments where no pronounced cyclicity is observed , and from Lower Turonian sediments with obvious cycles . There is no apparent correlation of carbonate and organic carbon throughout the Cenomanian , but within the lower-middle Turonian a distinct negative correlation between carbonate and TOC is observed (Figure 4) . There are two possible factors which could have influenced this ratio : (1) a constant export flux of organic matter and carbonate , modulated by periods of enhanced preservation of organic matter on the sea-floor because of fluctuations in bottom water oxygenation (i . e ., caused by periodic


formation of warm saline intermediate waters) ; (2) fluctuations in both carbonate and / or organic matter export fluxes related to changes in primary production .

Factor 1 may have played a certain role because occasional findings of benthic foraminifera , which indicate a slightly better oxygenated sea-floor , generally occur within the carbonate-rich cycles . Light carbonate layers and nodules with abundant calcispheres and rare microbenthos are interpreted to indicate short-term oxic bottom water conditions . However , many of these cycles are still finely laminated , and benthic foraminifera are rare and restricted to forms well adapted to oxygen deficient environments such as Gabonita levis , G . obesa , Lingulogavelinella sp . and Gavelinella ex gr . dakotensis .

Factor 2 can be related to changes either in carbonate or in organic matter production or both . A possible cause of a fluctuation in relative production of calcareous nannoplankton and organic-walled marine algae or diatoms may have been water temperature ; i . e ., organic-rich layers may correspond to periods of cooler surface water masses and carbonate-rich layers may represent warmer periods .

However , a major problem arises with this interpretation . The main forcing mechanism on fluctuations in the composition of primary producers in the Tarfaya Basin coincides with obliquity cycles , which generally have more significant ef fects on climatic conditions in higher latitudes . Studies on Milan- kovitch scale cyclicity in mid-Cretaceous low latitude pelagic sediments generally have not recorded major sedimentological changes coinciding with obliquity cycles (Herbert & Fischer , 1986 ; Melnyk & Smith , 1989) . Only in boreal settings , such as for the Turonian of southern England , has a similar dominance of the obliquity cycle been observed (Cottle , 1989) . This study was based on abundance plots of the benthic foraminifera Gyroidinoides nitida and Gavelinella emscheriana . The abundance of these forms may have been controlled mainly by the availability of food as export fluxes of organic matter to the sea-floor , and thus reflect fluctuations in surface productivity coinciding with temperature changes . The record of high latitude obliquity cycles in the low-latitude basin of Tarfaya can be explained , however , if sediment composition was controlled by the temperature record of upwelling mid-water masses , which may have formed in high latitude areas .

Another possible mechanism was suggested by Tiedemann et al . (1994) to link Milankovitch climate cycles in the obliquity frequency band with fluctuations in the organic carbon content of marine sediments for the Pliocene of the North Atlantic : obliquity cycles cause changes in the intensity of trade winds , leading to fluctuations in wind-driven coastal upwelling and resulting in enhanced primary production . Wind-driven coastal upwelling may also have been the major controlling factor of the unusually high productivity in the Tarfaya Basin during the mid-Cretaceous .

An enhanced seasonal contrast during the accumulation of the organic-rich part of the cycle may be concluded from a very distinct fine lamination which can be observed on a millimeter scale and most probably reflects annual cycles (Figure 3) . A bloom in calcareous nannoplankton produced a very thin light layer , which was followed by a thicker organic matter-rich layer consisting of faecal pellets , phosphate , kerogen , and amorphous biogenic silica sedimented during the rest of the year .

A correlation of the large scale cyclicity with the latest revision of the Eustatic Sea-level Timescale of Haq et al . (Juignet & Breton , 1992) shows a match of


sea-level highstands with the organic-rich peaks in the Tarfaya Basin , whereas lowstands coincide with carbonate-dominated biogenic sedimentation (Figure 2a , b) . This obvious correlation of Milankovitch-modulated productivity cycles and 3rd order sea-level fluctuations are further evidence of a close generic link between Cretaceous sea-level fluctuations and climate .

Acknowledgements

We acknowledge the permission given by Shell International Exploration and Production BV in The Hague to publish the density logs used in this paper . Jean-Paul Herbin (Institute Franc ̧ ais du Pe ́ trole , Rieul Malmaison) provided Rock Eval analyses of well S13 . WK acknowledges the financial support of the Deutsche Forschungsgemeinschaft (grant KU 649 / 3) . Many aspects of this paper have benefited from various discussions with Jean-Paul Herbin , Ju ̈ rgen Thurow (London) , Andreas Prokoph (Zittau) , Florian Luderer (Kiel) and the late Jost Wiedmann (Tu ̈ bingen) . Helpful comments of two anonymous reviewers and the enormous editorial ef forts of D . J . Batten are highly appreciated .

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