Combined isotopic and biomarker investigations of temperature- and facies-dependent variations in the Kupferschiefer of the Lower Rhine Basin, northwestern Germany

Chemical Geology, 102 ( 1992 ) 23-40

23

Elsevier Science Publishers B.V., Amsterdam [NA]

Combined isotopic and biomarker investigations of temperature- and facies-dependent variations in the Kupferschiefer of the Lower Rhine Basin, northwestern Germany A. Bechtela and W. Piittmann b aMineralogisch-Petrologisches Institut der Universitiit Bonn, Poppelsdorfer SchloJ3, W-5300 Bonn 1, Federal Republic of Germany bLehrstuhl f~r Geologie, Geochemie und Lagerstiitten des Erdrls und der Kohle, RWTH Aachen, LochnerstraJ3e4-20, W-51 O0Aachen, Federal Republic of Germany (Received January 28, 1992; revised and accepted June 16, 1992)

ABSTRACT Bechtel, A. and Piittmann, W., 1992. Combined isotopic and biomarker investigations of temperature- and facies-dependent variations in the Kupferschiefer of the Lower Rhine Basin, northwestern Germany. Chem. Geol., 102: 23-40. Organic material in selected Kupferschiefer samples from the Lower Rhine Basin of NW Germany has been studied using gas chromatography (GC), coupled gas chromatography-mass spectrometry (GC-MS) as well as C- and H-isotopic analyses of the organic matter. O- and C-isotope compositions of carbonates have also been investigated. The study area is located at the southwest shore of the ancient Zechstein ocean. The Permian Kupferschiefer was deposited here under anoxic, lagoonal conditions. Five closely-sampled profiles were analysed, covering a total depth range of 350-760 m in order to detect temperature-sensitive variations and compositional changes during Kupferschiefer deposition. Variations in isotopic and molecular compositions of organic material, as well as in calcite ~ ~3C,within the Kupferschiefer section in proximity of the basic intrusion that forms the Krefeld High are the result of post-sedimentary thermal degradation of organic matter. Variations in the fitgO-values of carbonates are governed primarily by the mineralogical composition (CaO/MgO ratios). Outside this area the isotopic data point toward a slightly increasing salinity of the seawater during Kupferschiefer sedimentation. An input of terrigenous organic matter is indicated by the occurrence of the ring-A-degraded pentacyclic triterpenoid, des-A-arborene. The compound is thought to be generated from the related pentacyclic precursor by anaerobic bacteria. The significantdecrease of the compound towards the top of the Kupferschiefer correlates with decreasing anoxic conditions during Kupferschiefer sedimentation.

1. Introduction Hydrocarbon compositions of shales can be used to study the thermal history of sedimentary basins as well as local geothermal effects (Albrecht et al., 1976; Leythaeuser et al., 1980; Alteb~iumer et al., 1983; Gilbert et al., 1985; Clayton and Bostick, 1986; Piittmann et al., Correspondence to: A. Bechtel, Mineralogisch-Petrologisches Institut der Universit~it Bonn, Poppelsdorfer SchloB, W-5300 Bonn 1, Federal Republic of Germany.

1989). For immature sediments the configuration of acyclic isoprenoid alkanes has been used to assess thermal maturation (Patience et al., 1978; Mackenzie et al., 1980; PiJttmann and Eckardt, 1989 ). Distributions of aromatic hydrocarbons in crude oil, in extracts of sedimentary rocks and in coals are sensitive to thermal alteration (Radke, 1987). Moreover, compositions of the aromatic hydrocarbon fractions provide information about the type of organic matter, sedimentary facies and paleosalinity (Radke et al., 1986; Sinninghe

0009-2541/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

24

Damste et al., 1987; Strachan et al., 1988; P~ittmann and Kalkreuth, 1989; Schwark and PiJttmann, 1990). Isotopic compositions of hydrocarbons and of carbonate minerals provide information on variations of temperature, depositional environments, and origins of organic material (Keith and Weber, 1964; Craig, 1965; Hodgson, 1966; Degens, 1970; Brown et al., 1972; Ohmoto and Rye, 1979; Deines, 1980; Hoefs, 1980; Veizer, 1983; Arneth and Matzigkeit, 1986; Ohmoto, 1986). In addition, isotopic data can provide insights into the role of organic matter in metal accumulation, the environment of metal deposition, and ore formation processes (Marowsky, 1969; Hammer et al., 1989; Bechtel and Pfittmann, 1991 ). Interpretations of ~13C determinations of organic carbon and carbonates as well as of 6180-values from carbonate minerals are often limited because of the effect of various parameters (Eh, pH, salinity, temperature) on these values, particularly for Type-II kerogens. This type of kerogen is often observed in marine sediments which contain contributions from land plant detritus added to the primarily marine organic matter in near-shore areas and lagoons (Tissot and WeRe, 1984). Beside faciesdependent variations, the temperature history (Clayton and Bostick, 1986 ) and post-depositional events such as oxidation by migrating brines (Bechtel and Pfittmann, 1991 ) can substantially influence the isotopic composition of organic matter and carbonates. In previous studies of the Kupferschiefer from the Lower Rhine Basin, the temperature history and facies variations during deposition of the shale have been described (Piittmann and Eckardt, 1989; Piittmann et al., 1989; Schwark and Piittmann, 1990), but these parameters have not been considered in relation to isotopic data so far. The earlier organic geochemical investigations of the Kupferschiefer from the Lower Rhine Basin revealed the importance of the proposed basic intrusion (Buntebarth et al.,

A. BECHTEL AND W. PLITTMANN

1982) near Krefeld (NW Germany), the socalled Krefeld High, in modifying the maturity of organic matter present in the Kupferschiefer. Common maturation parameters based on the measurement of meso-pristane formation as well as the hopane and sterane distributions were used to determine the strength and the regional spreading of the thermal influence, caused by the intrusion (Piittmann and Eckardt, 1989; Piittmann et al., 1989). Aromatic hydrocarbon compositions indicated euhaline to mesohaline (30-40%o) conditions during Permian sedimentation of the Kupferschiefer (Schwark and Piittmann, 1990 ). The results of previous inorganic geochemical studies (Diedel, 1986) demonstrate that base-metal mineralization reflects the metal distribution of the underlying strata. Basemetal enrichment in the basal Kupferschiefer unit is most pronounced in those parts of the basin, that were affected by heat flow of the Krefeld High (Diedel and Ptittmann, 1988 ). The present study provides a combined isotopic and organic geochemical investigation of samples from drill cores from the Kupferschiefer of the Lower Rhine Basin. Variations in facies, and in temperature histories, as deduced from organic geochemical data, are shown to correlate with variations of the isotopic compositions of organic matter.

2. Analytical methods 2. i. Carbon and hydrogen isotope measurements in organic matter

Bitumen was extracted over 24 hr from portions of the pulverized and homogenized samples using dichloromethane in a Soxhlet apparatus. Parts of the extracts were used for C- and H-isotope investigations. The extracted samples were treated with 6 N HC1 and subsequently with concentrated HF to remove carbonate from the samples and to isolate kerogen concentrates. After ultrasonic treatment with dichloromethane-methanol

TEMPERATURE- AND FACIES-DEPENDENT VARIATIONS IN THE KUPFERSCHIEFER OF THE LOWER RHINE BASIN

mixture (1:1), kerogen isolates were washed with distilled water, dried and homogenized. Samples were prepared for mass spectrometric analyses of C and H isotopes by sealing with CuO in evacuated quartz glass tubes and heating to 1050°C to produce CO2 and H20. The H20 was trapped together with CO2 at liquid nitrogen temperature. Carbon dioxide was subsequently separated from H20 at dry icemethanol temperatures and was frozen into glass tubes for mass spectrometric analyses. The remaining H20 was reduced to H2 by passing it over hot uranium (Bigeleisen et al., 1952). The hydrogen gas was trapped in a sample container with activated charcoal at liquid nitrogen temperature. C-isotope determinations were carried out on VG ® SIRA-9 triple-collector, 90 °, 9-cm-radius instrument. Mass spectrometric analyses of the hydrogen gas were performed at the Max Planck Institute for Chemistry (Mainz, Germany) using a VG ® PRISM, 60 ° instrument. The overall reproducibility of the total analytical procedure was in the range of _+0.1-0.2%o for d ~3C and generally between _+ 1 and _+2%o for 8D. The results are reported relative to the PDB standard for d13C, and in relation to SMOW for riD. 2.2. Carbon and oxygen isotope investigations o f carbonates Decomposition of carbonate minerals for mass spectrometric analyses was done by reaction of H3PO4 with the untreated, pulverized samples in evacuated glass tubes (McCrea, 1950). The reaction temperature was 25°C, kept constant by a thermostat. C- and O-isotope compositions of coexisting calcite and dolomite were obtained from one sample by mass spectrometric analyses of the resulting CO2 after different reaction times ( 1 hr for calcite; overnight for dolomite). The reproducibility of results was in most cases better than _+0.1%o. The results are reported in relation to

25

PDB for (~13C, and relative to the PDB as well as the SMOW standard for d ~80. 2.3. Organic geochemical investigations Finely ground shale samples ( < 0 . 2 m m ) were Soxhlet-extracted for 24 hr using dichloromethane. The total extracts were separated into five fractions by low-pressure column chromatography over silica gel ( Merck ®, 230400 mesh, 20 cm × 1.25 cm). The solvent flow was kept constant at a rate of 5 ml m i n - ~. Saturated hydrocarbons were eluted with 30 ml of hexane. Aromatic hydrocarbons were collected as three sub-fractions. The first subfraction including a red band of Ni-porphyrins was obtained using 35 ml of hexane-dichloromethane mixture (6:1), the second sub-fraction with a red band of V = O-porphyrins was obtained by subsequent elution with 30 ml of hexane-dichloromethane ( 1:1 ) and finally the third sub-fraction was eluted with 30 ml of dichloromethane. The elution of the heterocomponents was carried out with 30 ml of methanol. For the present study only saturated hydrocarbon fractions were further investigated by GC on a Carlo Erba ® 5160 Mega Series gas chromatograph equipped with a 25 m SE 54 ® fused silica capillary column (i.d. 0.25 m m ) . The oven temperature program was operated from 80-300°C at a rate of 4°C min -1 followed by an isothermal period of 20 min. Hydrogen was used as carrier gas. Individual compounds were quantified by adding internal standards prior to GC analysis; squalene was added to the alkane-alkene fractions. G C - M S analyses were performed using a Varian ® 3700 gas chromatograph coupled to a Finnigan ® MAT 8200 mass spectrometer using the same column type and oven temperature program as described for GC analysis. The mass spectrometer was operated in the EI (electron impact) mode at 70-eV electron energy, an emission current of 1 mA and a scan range (1.1-s total scan time) from 50 to 700

26

daltons. Data were processed with a Finnigan ® Incos data system. The total carbon content was determined on a Leco ® CR12. The organic carbon content (Corg) was measured with the same instrument on samples pre-treated with concentrated hydrochloric acid. The amount of carbonates (Ccarb) was calculated from the difference between the total and the organic carbon content. 3. Results and discussion

3.1. Sample material

A. BECHTEL AND W. PUTTMANN

lected for isotopic and organic geochemical studies. Four of the selected drill sites are located in an area where maturation of the organic matter was influenced solely by burial. The profiles were selected at different distances from the former Zechstein sea shoreline. Samples of Kupferschiefer from drill site No. 132 are included to study the possible thermal influence of the "Krefeld High" on the isotopic composition of the organic matter and carbonate minerals. Nineteen of the 25 samples provided sufficient material to carry out biomarker investigations.

3.2. Organic geochemical investigations The study area in the Lower Rhine Basin is situated at the southwest border of the former Zechstein sea. In this part of the basin the Kupferschiefer was deposited during the Permian within a lagoon under anoxic conditions. In previous studies, regional temperature effects acting upon the Kupferschiefer since its deposition were investigated by analyses of the biological marker composition in 30 core samples originating from the bottom section of Kupferschiefer in each drill site (Piittmann and Eckardt, 1989; Piittmann et al., 1989). Proportions of the meso-pristane revealed a general depth-related increase of rank of the organic matter towards the north (Fig. 1 ). In addition, in the western part of the study area a zone of maturation higher than expected for its present depth is detected (Fig. 1 ). This enhanced maturation has obviously been created by a heat flow associated with the intrusive body of the "Kxefeld High". Variations of hydrocarbon compositions within the Kupferschiefer profiles have so far been investigated only with respect to porphyrins and aromatic hydrocarbons (Eckardt, 1989; Schwark and Piittmann, 1990). For the present investigation 25 core samples within closely-sampled profiles from five drilling locations (marked by triangles in Fig. 1 ) in the area of the Lower Rhine Basin have been se-

3.2.1. Facies variations. Organic carbon concentrations generally decrease towards the top of the Kupferschiefer profiles, whereas carbonate contents remain either largely constant or increase towards the top (Table 1 ). Some exceptions (i.e. 49/21) represent samples containing clay lenses. The extract yields in relation to the organic carbon contents (mg Ext. / g Corg) do not reveal significant tendencies within the profiles (Table 1 ). A detailed study of the variation of the organic matter composition was done using four equidistant samples from the 1.9-m-thick profile in drill hole 134. Table 1 shows that organic carbon content remains almost constant around 5% in 3 of the 4 samples. At the top of the profile a significant drop in organic carbon to less than 1% occurs. The largely homogeneous carbonate content as well as the almost constant CaO/MgO ratio (Table 1 ) throughout the total profile argue for uniform conditions of sedimentation in this part of the basin. However, analyses of the saturated hydrocarbon compositions of the four samples indicate major variations in the polycyclic hydrocarbon composition. Fig. 2 shows sections of the higher boiling point range in the gas chromatograms of the saturated hydrocarbon fractions.

TEMPERATURE- AND FACIES-DEPENDENT VARIATIONS IN THE KUPFERSCHIEFER OF THE LOWER RHINE BASIN

27

Fig. 1. Map showing locations of the drill sites and the main tectonic structures in the Lower Rhine Basin (modified after PiJttmann et al., 1989). Analyses presented in this study were carried out on samples from wells marked with triangles. Iso-maturation lines were obtained from measurement of the proportions of meso-pristane (PiJttmann and Eckardt, 1989 ).

Three major sets of compounds have been identified using GC-MS analyses. Quantification of individual compounds was done in order to evaluate possible variations in organic matter inputs. Among a set of tetracyclic hydrocarbons one compound (A) dominates in the basal sample. Based on the mass-spectral data, the compound is tentatively identified as des-A-arbor-8,9-ene, which most likely originates from bisaccate conifer pollen, previously identified by microscopy as the dominant morphologically-preserved organic matter in the near-shore Kupferschiefer (Wolf et al., 1989). Correlation of a related monoaromatic tetracyclic hydrocarbon in the aromatic hydrocarbon fraction with the presence of bisaccate pollen has previously been suggested (Schwark and Piittmann, 1990). The pollen were transported from the surrounding forests into the lagoon of the Zechstein sea (Grebe,

1957), presumably both by wind and by rivers. Grebe (1957) reported that the highest pollen concentration is observed in the upper section of the Kupferschiefer and not in the bottom part. Table 2 shows that the amount of the tetracyclic alkene (A) decreases significantly from the bottom to the top of the Kupferschiefer profile (Fig. 3 ). The ring-A-degradation of pentacyclic hydrocarbons is thought to be mediated either by bacteria or by photochemical activity (Corbet et al., 1980). However, recent experiments have shown that ringA-degradation of pentacyclic triterpenoids is mediated by microbial activity rather than by photochemical activity (Wolff et al., 1989 ). It is well established that during Kupferschiefer sedimentation anoxic conditions decreased constantly (Paul, 1982). Therefore, the observed high quantity of desA-arborene in the bottom section of the Kup-

A. BECHTEL AND W. POTTMANN

s TABLE 1 General geochemical features of Kupferschiefer samples Sample No.

Depth (m)

Co~8

Cc.rb

(wt%)

(wt%)

mg Ext g Co~s

41/13 41/16 41/27 41/32

738.30 738.72 739.03 739.13

1.1 3.7 6.5 7.7

8.3 6.5 3.3 4.3

34.8 48.1 42.2 46.4

1.76 1.61 1.70 1.51

49/13 49/21 49/34 49/51 49/66

748.78 749.11 749.59 750.26 750.76

0.5 1.5 1.0 2.8 5.5

6.8 1.9 8.8 8.7 6.2

26.6 30.7 55.6 57.5 52.2

1.56 1.53 1.77 n.d. n.d.

127/18 127/28 127/32

769.19 769.71 770.00

1.7 6.0 3.7

6.3 5.6 5.8

56.4 57.3 47.7

3.70 15.25 14.52

132/3 132/13 132/19 132/21

383.05 383.83 384.11 384.34

0.8 1.3 n.d. 4.4

5.9 5.3 n.d. 3.8

32.9 49.5 n.d. 38.4

1.83 1.92 4.33 4.43

134/10 134/14 134/19 134/29

595.21 595.56 595.84 596.62

0.9 4.6 5.2 4.9

8.8 5.1 7.0 5.9

57.2 40.8 39.8 52.2

1.40 1.49 1.54 1.45

CaO. MgO

Samples are labelled according the drill hole No. in combination with the core No. (increasing with depth), n.d. = not determined. Ext = extract. *Data from Diedel ( 1986 ).

ferschiefer most likely is caused by the activity of anaerobic bacteria, which were able to degrade the related pentacyclic precursor primary under the more anoxic conditions at the beginning of Kupferschiefer sedimentation. Under presumed more oxic conditions with ongoing sedimentation, the activity of anaerobic bacteria decreased significantly, as is reflected by decreasing amounts of des-A-arborene. In order to verify this assumption, further investigations were concentrated on the analyses of polar fractions, which should contain the potential precursors of the des-A-arborene. Four diasterenes (B-E) are present in the saturated hydrocarbon fractions (Fig. 2 ). Table 2 and Fig. 3 show the variation of Corg-norrealized amount of the compounds within the profile. The concentration of the two C27-dias-

terenes remains largely constant. In contrast, both C29-diasterenes show a parallel variation with the highest concentration in the upper part of the center of the Kupferschiefer and decreasing amounts towards the bottom and the top. This pattern suggest that the organisms which produce predominantly C29-sterols underwent a maximum of their productivity in the center of the shale. Remarkably, the diasterenes are accompanied by only minor amounts of regular steranes. The favoured backbone rearrangement of the steroids argues for strong catalytic activity of clay minerals on the organic matter transformation (Siskind and Albrecht, 1985 ). Hopanes also contribute significantly to the cyclic hydrocarbon compositions. The very immature state of the organic matter is indi-

TEMPERATURE-AND FACIES-DEPENDENTVARIATIONSIN THE KUPFERSCHIEFEROF THE LOWERRHINE BASIN 134/lo

s

0

E

c '

B

134/14

F

G

F

G

S

i

B

c

134/19 D

BC

F G

A

134/29

i i

i c

Fig. 2. Sections of the higher boiling point range in the gas chromatograms of the saturated hydrocarbon fractions from Kupferschiefer samples of well 134. The quantified compounds are marked by letters A - G , listed in Table 3. S = internal standard: squalene

cared by the high concentration of the 17fl(H),21fl(H)-22R-homohopane (G; Fig. 2). The amount of this compound decreases substantially from the bottom to the top of the Kupferschiefer profile from drill hole 134 (Table 2; Fig. 3). At the same time the 17a(H), 21fl(H)-22R-homohopane decreases. The

29

concentration ratio of both hopanes remains almost constant. Approximately 40% of the originally existing flfl-homohopane has been converted to the aft isomer except for the bottom sample, where the conversion rate is higher. The decreasing Corg-normalized concentrations of the hopanes towards the top of the shale (Fig. 3 ) argues for a decreasing activity of bacteria having hopanoids as membrane constituents. Hopanoids were found in many procaryotes including some that were grown anaerobically (Ourisson et al., 1979 ). From the results given in Table 2 it can be recognized that the sterene/hopane ratio increases significantly during Kupferschiefer sedimentation, indicating an increasing marine influence in the lagoon during the initial stage of Zechstein transgression. This corresponds with the increase in the 613C-values of the carbonates towards the top of the shale (see Table 5 on p.36).

3.2.2. Maturation assessment. The temperature history of sedimentary organic matter can influence the C-isotopic composition (Ohmoto and Rye, 1979; Hoefs, 1980; Ohmoto, 1986). Moreover, post-depositional oxidation of buried organic matter in sediments results in the formation of carbonates having isotopic compositions different from carbonates precipitated during sedimentation (Ohmoto, 1986; Hammer et al., 1989; Bechtel and Piittmann, 1991 ). Therefore, recognition of maturity differences of the Kupferschiefer in the five drill holes is of importance for interpretation of the isotopic data. Variations regarding the temperature influence can be recognized from the degree of racemization of biomarker hydrocarbons. We have selected three parameters from the variety of molecular parameters commonly used for maturation assessment. The high percentage of the meso-isomer of pristane indicates low maturation of the Kupferschiefer in the study area. The lowest degree of racemization is observed in the sample from drill hole 134 which retains 70% of meso-pristane (Ta-

30

A. BECHTEL AND W. PUTTMANN

TABLE 2 (#g g - ] Con) of individual biomarker compounds

Quantities

Compound

of Kupferschiefer

samples from drill site 134

Sample No.

A des-A-arbor-8, 9-ene B C27-diasterene (20S) C C:7-diasterene (20R) D C29-diasterene (20S) E C29-diasterene (2OR)

F 17a(H),21fl(H)-homohopane (22R) G 17fl(H),21fl(H)-homohopane (22R)

depth (ml

sample no.

134/14

134/19

134/29

2.28 9.52 10.69 31.54 32.27 5.80 8.39

3.13 8.38 11.25 36.34 46.32 7.17 9.91

6.57 9.40 13.38 28.95 35.91 13.05 18.87

22,20 10,23 11,91 25,96 32,90 18,41 21,93

borehole 13/,

tetracyclic alkene

£el

134/10

diasterene

hopane

• £27-diasterene (20 S}

• 17~(H},21/HH)- homohopane 122 R )

o C27-diasterene (20 R)

17/HH},21/sIH)-homohopaneq22 R)

• [29-diosterene 120 S) C29-dia~terene (20 R)

\\

• °

13¢/10 13&/1~

i •

':! / /

'//

T1

e•

•

•

v

tJ

596.93' "

Icl [

I S

I 10

I 15

I 20

I 10

I 20

I 30

I z,0

I 50

I S

I ~0

I 15

I 20

(Fg/gEorg)

Fig. 3. C o n c e n t r a t i o n s (pg g - ~ Corg) o f individual biomarker compounds determined within the extracts of Kupferschie-

fer samples f r o m d r i l l site ]34.

ble 3 ). The near-shore type of Kupferschiefer is represented by this well. The equilibrium value of ~ 50% is reached only in the sample from well 132 (Table 3). The proximity of this well to the Krefeld High has previously been used to explain the low meso-pristane proportion despite the shallow depth of the Kupferschiefer in the area of this well (Piittmann and Eckardt, 1989 ). The three

samples originating from areas more distant from the former shore-line and separated from the Krefeld High show intermediate values with respect to the degree of pristane racemization. In mature sediments and crude oils, pristane racemization is uniformly completed, yielding 50% of meso-pristane (Shlyakhova and Volkova, 1977). Incomplete pristane racemiza-

TEMPERATURE-AND FACIES-DEPENDENTVARIATIONSIN THE KUPFERSCHIEFEROF THE LOWERRHINE BASIN TABLE3 Values of maturation parameters in the bottom samples of Kupferschiefer from each well Well No.

Pri Phy

% Meso-Pri

Hop 1

134 41

0.55 0.77 0.75 1.03 1.09

70.0 57.0 54.0 53.0 50.5

0.54 0.17 0.12 0.09 0.05

49

127 132

Abbreviations: Pri=pristane; Phy=phytane; Hop l=flfl/ (Bfl+ aft)-22R-homohopanes.

tion has only been observed in recent sediments and immature ancient sediments such as the Messel shale (Germany), the Green River shale (Wyoming, U.S.A.) and shallow Toarcian shales of the Paris Basin (France) (Patience et al., 1978; Mackenzie et al., 1980). The variation of the maturation of the organic matter in the bottom samples from five of the wells is additionally reflected by the pristane/phytane ratio. The increase of the value of this ratio from 0.55 to 1.09 in the five well samples (Table 3) is related to the increasing maturation probably caused by the selective generation ofpristane from an individual and still unknown precursor (Pfittmann and Eckardt, 1989). Therefore, in the present case the pristane/phytane ratio was not solely controlled by the paleoenvironmental conditions during Kupferschiefer sedimentation. Additionally, the degree of isomerization of the flfl-22R-homohopanes into the afl-22Rhomohopanes has been measured in the bottom samples of the Kupferschiefer in the wells (Table 3). As with Mackenzie et al.'s (1980) study of the Toarcian shales of the Paris Basin this parameter proved to be useful for recording differences in the maturation of Kupferschiefer at the five well locations. Again, the lowest degree of racemization is observed in drill hole 134, whereas in 132 the conversion is almost complete. In drill holes 41, 49 and 127 intermediate racemization values are observed. These values reflect a constant deple-

31

tion offlfl-homohopanes due to the increasing burial depth going from drill hole 41 over No. 49 to No. 127. The maturation study by biomarker analysis reveals that in drill hole 134 the maturation of the organic matter of the Kupferschiefer is far less than that determined in the other samples. The maturity range of samples from drill holes 41, 49 and 127 is relatively narrow based on the data derived from hopane and pristane racemization (Table 3 ). The highest maturation is observed in the Kupferschiefer from drill hole 132 although at present the Kupferschiefer is here at its most shallow depth. This variation in the inferred temperature history is clearly reflected by variations in the isotopic composition as discussed in the following sections.

3.3. Carbon and hydrogen isotope investigations of organic material C-isotopic compositions were determined on total organic matter and on solvent-extracted residues. Additionally, ~13C-values of the bitumen were determined in those samples for which sufficient solvent extract was available. The data are summarized in Table 4. The d~3C-values of the total organic matter show a variation from - 28.7 to - 24.2%0 (Table 4 ). These values are in the range of ancient marine organic carbon (Deines, 1980; Hoefs, 1980; Schwarcz, 1969). Compared with modern organic matter in marine sediments, which show average ~13C_values near -22%0, the Cisotopic data are relatively light (Ohmoto, 1986 ). This might indicate a contribution of a terrigenous component to the organic material. However, isotopic composition of marine organic carbon is controlled by the kinetic fractionation factor between dissolved HCO£ in the ocean and organic matter as a result of photosynthesis. Furthermore, ~3C-values of

32

A. BECHTEL AND W. PUTTMANN

TABLE4

Carbon and hydrogen isotope composition of the organic compounds Sample

Depth

TOC

(m)

(~13C

Kerogen ~13C

Bitumen

No.

(°/oo vs. P D B )

(°/oo vs. P D B )

~3 C

O'D

(%o vs. P D B )

(%0 vs. S M O W )

41/7 ( C a t ) 41/13 (T~) 41/16 ( T I ) 41/27 ( T ) ) 41/32 (T~) 41/39 (C~) 49/13 (T~) 49/21 ( T I ) 49/34 (T~) 49/51 ( T t ) 49/66 (T~)

738.05 738.30 738.72 739.03 739.13 739.66 748.78 749.11 749.59 750.26 750.76

-28.0 -27.6 -27.1 n.d. n.d. -25.1 n.d. n.d. n.d. n.d. n.d.

n.d. -27.5 -27.0 - 26.2 -25.9 n.d. -27.6 -27.9 -28.3 -28.5 -28.5

n.d. -28.5 -27.6 - 26.7 -26.1 n.d. n.d. n.d. -29.8 -29.5 -28.9

n.d. - 125 - 118 n.d. - 109 n.d. n.d. n.d. - 132 - 130 - 128

127/18 ( T l ) 12 7/28 ('1"1) 127/32 (T~)

769.19 769.71 770.00

n.d. n.d. n.d.

- 28.4 - 28.4 -28.7

- 30.2 - 29.8 -29.4

- 137 - 133 - 129

132/3 (T~) 132/13 (T~) 132/19 (T~) 132/21 (TI)

383.05 383.83 384.11 384.34

- 26.5 -26.2 -25.3 -24.2

n.d. n.d. n.d. n.d.

- 27.5 -27.4 n.d. -25.0

- l 15 - 110 n.d. -92

134/10 134/14 134/19 134/29 134/33

595.21 595.56 595.84 596.62 596.93

n.d. n.d. n.d. -28.3 - 27.8

- 27.6 -27.9 - 28.0 -28.2 n.d.

- 29.8 -29.5 - 29.1 -28.9 n.d.

- 135 - 132 - 130 - 125 n.d.

('l'l) (TI) (Tl) (TI) (C1)

T O C = total organic carbon; Ca~ = Zechsteinkalk; T t = Kupferschiefer; C ~ = Zechstein conglomerate. Numbering system of the samples is explained in Table I. n.d. =not detected.

marine organic matter depend on the isotopic composition of dissolved carbonate in seawater, which is controlled by ~13C of atmospheric CO: and the equilibrium isotope effect between dissolved HCO~- and atmospheric CO2. These complex relationships are very sensitive to changes in the carbon cycle during Earth's history. Therefore, these facts must be taken into account when making interpretations based on comparison with modern data. Because organic material of the samples consists mainly of kerogen ( > 94 wt%), the d ~3C-values of total organic carbon within the Kupferschiefer unit are nearly identical with those of kerogen, as shown for samples 41/13, 41/16 and 134/29 (Table 4). Therefore, they

will be discussed together. In comparison to the d~3C-values determined on the kerogen, the extracted bitumens yield lower values in the range of - 30.2 to - 25.0%0 (Table 4). Plots of the ~13C-values of organic carbon vs. depth (Fig. 4) indicate that the investigated Kupferschiefer profiles can be subdivided in two groups. Low dl3C-values of kerogen ( - 2 8 . 7 to - 27.6%o) were obtained from samples of drill sites 49, 127and 134 (Fig. 4a). A slight depletion in 13C towards the basal Kupferschiefer unit is detected in these profiles (Fig. 4a). The extracted bitumens show variations from - 3 0 . 2 to -28.9%o. Within these profiles the differences between d~3C-values of the bitu-

TEMPERATURE-

AND FACIES-DEPENDENT

VARIATIONS IN THE KUPFERSCHIEFER

Ligend

x%

[Ol[ --~

OriH sites • 132 x t. 1

x

~9 127

T 1

?

'

i

I

13.

,',d \ ' \

"\-

~-"

c1 O -31

I

I

I

I

-29

I

-27

I

I ~

-25

-23

d'13C (POB} a.

Kerogen

Legend:

Coi

Orill sites X

\2

o

L -]I

I

-2g

i

132

x

~.1

+ ~9 • 127 0 1]L.

TI

£i

•

I

-2?

I

I

-25

I

I

-23

a'~3C (POB)

b Bitumen Fig. 4. Isotopic composition of: (a) insoluble organic carbon (kerogen); and (b) extracted bitumens within the Kupferschiefer drill hole profiles.

men and the corresponding kerogen vary from 2.2 to 0.4%0 and generally decrease with increasing depth. These results are comparable to those of Marowsky ( 1969 ) from the Kupferschiefer in NW Germany (&13C of organic carbon between 28.5 and - 26.0°/0o). The trend towards isotopically lighter organic carbon with depth might be due to changes in the primary composition of the organic material during sedimentation, as suggested by the results of organic geochemical investigations discussed on pp. 27-29. As compared with these data, samples from drill sites 41 and 132 yield higher 8~3C-values -

OF THE LOWER RHINE BASIN

33

of kerogen between - 27.6 and - 24.2%0 (Table 4). The extracted bitumens vary from 28.5 to - 25.0%0. In contrast to the trend in &'3C of organic carbon within drill holes 49, 127 and 134, Kupferschiefer samples from the bottom part of these profiles show a significant enrichment in ~3C (Fig. 4a). Furthermore, the differences between C-isotope composition of the bitumens and kerogen are generally smaller (between 1.2 and 0.2°/0o) and show a minor decrease with depth (Fig. 4 ). The isotopic compositions of organic compounds from these profiles are comparable to those observed in Kupferschiefer samples from the Pb/Zn-bearing zone in the Richelsdorf hills, Germany (Bechtel and Pfittmann, 1991 ). The d13C-values of hydrocarbons within drill site 132 can be related to an enhanced maturity of the organic material (Table 3 ) from localized heating by the intrusive body of the Krefeld High. The organic geochemical investigations indicate that in the area affected by the intrusion, maturation of extractable organic matter has been increased significantly (Fig. 1 ). Similar observations have been reported by Clayton and Bostick (1986) from their study of the organic matter composition in Pierre Shale (Colorado, U.S.A. ) near an igneous dike. An increase in thermal maturation results in increasing ~13C-values of organic carbon due to the preferential release of isotopically light hydrocarbons through progressive degradation of organic matter (Ohmoto and Rye, 1979; Deines, 1980; Ohmoto, 1986; Arneth and Matzigkeit, 1986 ). The smaller differences between the extracted bitumens and the corresponding kerogen can be also explained by increasing thermal maturity (Arneth and Matzigkeit, 1986 ). Saturated aliphatic hydrocarbons, which will build up in the bitumen fraction after being incorporated into the sediment, are derived from the lipid fraction of the biomass and consist of isotopically light CH bonds (Galimov et al., 1975 ). This explains the isotope difference between bitumens and -

34

kerogens of relatively immature sediments. During progressive alteration of the organic material isotopically light organic compounds were preferentially released from the bitumen fraction. Consequently the residual organic carbon is enriched in L3C, as is especially marked by variation in the isotopic composition of the extractable bitumens. At a higher maturation state, the extractable organic matter becomes isotopically heavier and the isotope contrast with the kerogen decreases (Arneth and Matzigkeit, 1986 ). This isotope trend has been attributed to increasing conversion of biological material and increasing formation of extractable organic matter from the maturation process of the kerogen (SchoeU et al., 1983). As suggested by the biomarker compositions, the divergence of ~ 13C-values within drill site 41 from the trend in Kupferschiefer profiles distant from the intrusive body cannot be explained as a result of temperature effects. The results of the organic geochemical investigations indicate that the maturation of organic matter in this profile is influenced only by the burial depth. The temperature-related data are comparable with maturation assessments from drill sites 49 and 127 (Table 3 ). So far, the only recognizable difference is the occurrence of a Rotliegend unit in the bottom part of profile 41, which is missing at the other locations. Therefore, the general tendency towards isotopically lighter organic carbon with decreasing depth within this Kupferschiefer sequence might be the result of gradual changes in the sedimentary facies. In the sequence from Rotliegend to Zechstein the environment during sedimentation generally changed from oxidizing to reducing. However, ascending formation waters might have influenced the organic matter after its deposition. Some of the extracted bitumens were selected for analyses of the H-isotope composition (Table 4). Comparison oft~13C- and 6Dvalues (Fig. 5) supports the results of the presented C isotope and organic geochemical in-

A. BECHTEL AND W. POTTMANN

vestigations. Corresponding with the C-isotope data, the extracted bitumens from drill sites 49, 127 and 134 show the lowest riD-values and a relatively small variation from - 137 to - 125%o (Fig. 5 ). A positive correlation between ~13C and O'D exists. A significant enrichment in deuterium towards the basal Kupferschiefer unit is shown by the H-isotope data of the total extracts from drill sites 41 and 132 (Fig. 5). The 6D-values vary from - 125 to -92%o (Table 4) and show a good correlation (R=0.981 ) with the corresponding ~13C-values (Fig. 5 ). Calculation by the least-squares method yields the following equation: oq)= 8.08~3C + 107.26 Comparable D / H and 13C/~2C fractionation trendlines of light hydrocarbons were obtained from laboratory simulation experiments (Sackett, 1978 ), indicating the isotopic sensitivity to thermocatalytic production. In general, the isotopic compositions of the extracted bitumens fall within the range obtained from Paleozoic and younger crude oils (Schoell and Redding, 1978). 3.4. Carbon and oxygen isotope investigations of carbonates

The fi13C-values of carbonate minerals vary from - 2 . 2 to +4.9%o (Table 5). Oxygen of the carbonates yield t~180-values in the range of +25.1 to +32.2%o, related to the SMOW standard, or with respect to PDB, from - 5 . 6 to + 1.3%o (Table 5 ). Comparable results were obtained by Marowsky (1969) from weakly mineralized Kupferschiefer samples from NW Germany and by Haranczyk (1986) on samples from the Fore-Sudetic Monocline in Poland. The carbonates from the Marl Slate in England (the lateral equivalent of the Kupferschiefer of North Central Europe) studied by Sweeney et al. (1987) contain relatively uni-

TEMPERATURE- AND FACIES-DEPENDENT VARIATIONS IN THE KUPFERSCHIEFER OF THE LOWER RHINE BASIN Kupfersch,efer

Lower

Rhine

35

Besin

Bitumen 90

I

I

I

I

I

I

I

-100

Legend: Oritl sLtes

-110

•

t,I

.

t,g

x 127

123

'.-,o -120

x

-130

• 132

jY

o 13t~

÷

/x -lkO "31

I -30

I -29

I -20

I

I

I

I

-27

-26

-25

-2~

d'13£ (PDB)

Fig. 5. Cross-plot of the 6 ~3C- vs. 07)-valuesof the extracted organicmaterial from Kupferschiefersamples. form 613C-values between + 2.6 and + 3.2%o. An overall upward enrichment in 13C of 1.32.0%o is observed with negative spikes of the 6~80-values in calcite-rich units of the profiles. These isotopic patterns appear again in our investigation of the Kupferschiefer of the Lower Rhine Basin. The CaO/MgO ratios (Table 1 ) are in the same order of magnitude within the profiles of drill sites 41, 49 and 134. On average, the lowest values ( 1.40-1.54) were determined in drill site 134, which is closest to the ancient shoreline. A similar enrichment of dolomites in the carbonates has also been observed in the Schwellen facies of the Kupferschiefer near the Harz area (Paul, 1982). Within the profiles of two drill sites ( 127, 132) a significant variation of the CaO/MgO ratio is determined. The increase of the ratio towards the bottom section is mainly due to a depletion in dolomite (Diedel, 1986). These variations of calcite/dolomite ratios are strongly reflected by the measured 6180-values. Fig. 6 shows that in drill sites 127 and 132

the depletion in dolomite is accompanied by a depletion in 1 8 0 . Thus, the mineralogical composition of the carbonates governs the variation in the 6180-values. Similar observations have been reported from isotopic investigations of the Mad Slate (Sweeney et al., 1987). A plot of the 613 C , t~18O data of the carbonates (Fig. 6) shows that most of the samples yield 613C-values in the range of +1.1 to +4.9%o. According to the classification by Keith and Weber (1964), these calcites and dolomites fall into the field of marine carbonates. Upward enrichment in ~3C and 180 is observed within all Kupferschiefer profiles (Table 5). The observed tendency in the isotopic composition of carbonate can be explained by an increasingly marine influence on the sedimentary environment, accompanied with increasing salinity in the Zechstein ocean during transgression (Paul, 1982; Schwark and Piittmann, 1990). Some of the calcites and dolomites, espe-

36

A. BECHTEL AND W. PI~TTMANN

TABLE 5 Carbon and oxygen isotope c o m p o s i t i o n o f carbonates

Sample

Calcite

Dolomite

Jr3 C (%o vs. P D B )

Jls O (%o vs. S M O W )

J~s O (%o vs. P D B )

J13 C (%o vs. P D B )

jls O (%o vs. S M O W )

jls O (%o vs. P D B )

41/4 41/7 41/13 41/16 41/27 41/32 41/39 41/47 41/51

n.d. + 2.8 +3.0 +3.0 +2.5 +3.0 +0.4 -0.5 -0.3

n.d. + 29.2 +29.4 +29.2 +28.4 +29.1 +28.1 +27.2 +28.7

n.d. - 1.6 -1.5 -1.6 -2.4 -1.7 -2.7 -3.6 -2.1

+ 3.7 + 3.8 +3.9 +4.2 +3.5 +3.7 +1.4 +1.0 + 1.9

+ 30.2 + 30.3 +30.2 +30.0 +28.4 +29.0 +29.1 +28.8 +31.3

- 0.6 - 0.6 -0.6 -0.9 -2.4 -1.8 -1.7 -2.0 +0.4

49/13 49/21 49/34 49/51 49/66

+3.7 + 3.3 + 3.5 + 3.0 + 2.9

+29.3 + 29.2 +29.0 + 28.6 + 29.6

-

+4.6 + 4.3 +4.0 + 4.0 + 3.5

+29.7 + 30.8 +29.2 + 29.2 + 29.8

-

127/18 127/28 127/32

+2.6 + 1.1 +1.5

+27.4 +25.2 +25.4

-3.4 -5.5 -5.3

+4.9 +2.4 +2.3

+29.4 +26.3 +25.1

-1.4 -4.4 -5.6

132/3 132/13 132/19 132/21 132/27

-0.3 -1.1 -2.2 -2.2 n.d.

+28.8 +28.1 +25.9 +25.7 n.d.

-2.0 -2.7 -4.8 -5.1 n.d.

+2.6 +2.2 + 1.9 +1.1 -2.0

+28.8 +28.6 +26.7 +26.1 +24.7

-2.1 -2.2 -4.1 -4.6 -6.0

134/3 134/10 134/14 134/19 134/29 134/33 134/39 134/46

+2.8 +3.8 +3.2 +2.8 +2.1 n.d. n.d. n.d.

+30.1 +31.6 +31.5 +31.4 +31.1 n.d. n.d. n.d.

-0.8 +0.7 +0.6 +0.5 +0.2 n.d. n.d. n.d.

+4.1 +4.7 +4.2 +3.6 +2.8 + 2.1 -2.8 - 1.4

+31.3 +32.2 +32.1 +32.1 +32.0 + 30.1 +26.0 +28.4

+0.4 + 1.3 + 1.2 +1.2 +1.1 - 0.8 -4.7 -2.4

1.5 1.7 1.8 2.2 1.3

1.1 0.1 1.6 1.6 1.1

n.d. = not detected.

cially from the top of the Kupferschiefer section and from the total profile of drill site 134, fall outside the range of "normal" marine carbonates (Hoefs, 1980; Veizer, 1983). These samples show higher C- (j13C > +4o/oo) and O-isotope compositions (J180 > 0%o), which can be related to an enhanced evaporation in the lagoonal environment near the southern border of the Zechstein sea (Fig. 1 ). The carbonate minerals from the drill site

closest to the shoreline (134) yield J ~8O-values on average 2%o higher within the Kupferschiefer unit (Table 5; Fig. 6). These differences in J~so can be related either to carbonate formation from ~ 10 °C colder seawater (O'Neil et al., 1969) in this part of the Zechstein basin, or to precipitation from 1SO-enriched ocean water due to enhanced salinity. However, temperature variations in this order seem unreasonable within this regionally re-

TEMPERATURE- AND FACIES-DEPENDENT VARIATIONS IN THE KUPFE~SCHIEFER OF THE LOWER RHINE BASIN

Kupferschiefer

I

I

I

Rhine

Lower

Carbona,e

Basin

minerals

I

l

l

I

I

,o,

Legend:

+

** o o

o o

x

x

0rill sites

•

_

÷

•

x

ii

eoi x

x

[]

f~ t~

37

~.1

Cc

o /,I dol x 4.9 cc

.m

÷ 49 d01

D

~" 127 4.

I

[]

Lo

0

I

I

cc

o 127 d01

--

I

I

I

I

• 132 c c t:] 132 dot

I

• 134 cc - 13t,. dol

-1 -2 -3

i

i

z

I

I

-6

-5

.t,

-3

-2

I -I

0

I

I

I

2

or180 (POO)

Fig. 6. Cross-plot of the &~3C- vs. c~18O-values of carbonate minerals within the Kupf~rschiefer unit.

stricted sedimentary basin (Fig. 1). Broecker (1974) and Faure ( 1977 ) have presented relationships between the c~180-values and the salinity of the water in the North Atlantic and the Red Sea, respectively. Compared with the data from the Red Sea (Faure, 1977) a variation of 2%0 in ~180 of the former Zechstein seawater correlates with a salinity increase of ~ 5%0 during Kupferschiefer deposition at the shoreline (drill hole 134) due to preferential loss of H ~60 during evaporation. The C-isotope compositions of the calcites from drill site 132, located near the Krefeld High (Fig. 1), differ significantly from those of the carbonate samples from the other bore holes (Fig. 6). The J 18O-values of these calcites are comparable with the data generally obtained from the Kupferschiefer samples of the Lower Rhine Basin. The samples from profile 132 are characterized by isotopically heavy organic carbon and by an enhanced thermal maturity of organic material. Therefore, the low ~13C-values of the calcites from these samples provide evidence that isotopically light CO2, possibly generated through degradation of organic matter, may

have been a source of calcites. Thus, the strong variation in the c$13C-values, comparing calcites and dolomites, argues for post-depositional alteration of the carbonate composition. One of the common features of carbonate minerals in sediment-hosted sulphide deposits is a large variation in &13C, with a predominance of negative values (Pine Point, Northwest Territories, Canada; Red Stone River, Northwest Territories, Canada), indicating the contribution of organic carbon to carbonate formation (Ohmoto, 1986).

4. Summary and conclusions The results of isotopic studies of organic carbon from Kupferschiefer samples outside the area which was affected by the Krefeld High show the importance of changes in the primary composition of organic matter. The results of organic geochemical analyses indicate that these compositional variations were caused by changes in the amount of terrestrial organic matter, as well as by environmental changes during deposition.

38

The isotopic analyses of the profile near the intrusive body give higher c~3C- and o'D-values, with a significant tendency towards isotopically lighter organic carbon with decreasing depth. The higher thermal maturation of the organic matter, detected by organic geochemical investigations, indicates an enhanced degradation of the organic matter. Enrichments of the organic material in ~3C and D suggest the preferential release of isotopically light hydrocarbons through progressive thermal degradation of the organic matter. The D / H and 13C/~2 C fractionation trendline suggests thermocatalytic production of the light hydrocarbon fractions in the area of wells 41 and 132. The low fi~3C-values of calcites, as well as the pronounced tendency towards isotopically lighter carbonate carbon with depth in profile 132, which is influenced by the heat flow from the Krefeld High, support the conclusions from the isotopic analyses of organic material. The occurrence of isotopically heavy organic carbon together with low c~~3C-values of calcites suggests that ~2C-rich CO2 generated by degradation of organic material may have been a source of carbonate carbon. In an area where no temperature effects can be detected, the carbonate minerals from most samples yield c~3C- and fi~SO-values which are in the range of marine carbonate (Keith and Weber, 1964). The overall upward enrichment in fil3C and fi~80 can be explained by an increasing marine control on the sedimentary environment during transgression of the Zechstein sea. Some carbonates from the top of the shale and from the most shoreward Kupferschiefer sequence yield higher t~13C- and c~80-values, compared with the results from "normal" marine carbonates (Hoers, 1980; Veizer, 1983). These values argue for an enhanced evaporation in the lagoonal environment near the southern border of the Zechstein sea and an increasing salinity of the former Zechstein seawater during Kupferschiefer deposition.

A. BECHTEL AND W. POTTMANN

Acknowledgements Support of this study by S. Hoernes (Institute of Mineralogy and Petrology, University of Bonn) and M. Wolf (RWTH-Aachen) is gratefully acknowledged. This article profited from the critical reviews by J. Trichet (University of Orlrans, France) and P.A. Meyers (University of Michigan, U.S.A.). We thank G. Friedrich and P. Redecke (Institute of Mineralogy and Mineral Deposits, RWTHAachen) for providing samples and for information about the geology, petrology and inorganic geochemistry. Thanks also go to U. Lichtenstein (Max Planck Institute for Chemistry, Mainz) for performing hydrogen isotope analyses. Financial support by the Deutsche Forschungsgemeinschaft (Ho 868/6 and Pu 73/ 2 ) is gratefully acknowledged.

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TEMPERATURE-ANDFACIES-DEPENDENTVARIATIONSIN THE KUPFE'RSCHIEFEROFTHELOWERRHINEBASIN plan, I.R., 1972. Early diagenesis in a reducing l]ord, Saanich Inlet, British Columbia, III. Changes in organic constituents of sediment. Geochim. Cosmochim. Acta, 36:1185-1203 Buntebarth, G., Michel, W. and Teichmiiller, R., 1982. Das permokarbonische Intrusiv von Krefeld und seine Einwirkung auf die Karbonkohlen am Niederrhein. Fortschr. Geol. Rheinl. Westfalen, 30:31-45 Clayton, J.L. and Bostick, N.H., 1986. Temperature effects on kerogen and on molecular and isotopic composition of organic matter in Pierre Shale near an igneous dike. In: D. Leythaeuser and J. RullkiStter (Editors), Advances in Organic Geochemistry 1985. Pergamon, Oxford, pp. 135-143 Corbet, B., Albrecht, P. and Ourisson, G., 1980. Photochemical or photomimetical fossil triterpenoids in sediments and petroleum. J. Am. Chem. Soc., 102: 1171-1173 Craig, H., 1965. The measurement of oxygen isotope paleotemperatures. In: E. Tongiorgi (Editor), Proc. Conf. on Stable Isotopes in Oceanographic Studies and Paleotemperatures, Spoleto, p.3 Degens, E.T., 1970. Biogeochemistry of stable carbon isotopes. In: G. Eglinton and M.T.J. Murphy (Editors), Organic Geochemistry. Springer, Berlin, pp. 304-329 Deines, P., 1980. The isotopic composition of reduced organic carbon. In: P. Fritz and J.Ch. Fontes (Editors), Handbook of Environmental Isotope Geology, Vol. 1: The Terrestrial Environment, A. Elsevier, Amsterdam, pp. 329-406 Diedel, R., 1986. Die Metallogenese des Kupferschiefers in der Niederrheinischen Bucht. Ph.D. Thesis, Rheinisch-Westf~ilische Technische Hochschule Aachen, Aachen (unpublished). Diedel, R. and Piittmann, W., 1988. Base metal mineralization and organic carbon maturity in the Kupferschiefer of the Lower Rhine Basin. In: G.H. Friedrich and P. Herzig (Editors), Base Metal Sulfide Deposits in Volcanic and Sedimentary Environments. Soc. G6ol. Appl., Spec. Publ. No. 6, Springer, Berlin, pp. 60-73 Eckardt, C.B., 1989. Organisch-geochemische Untersuchungen am Kupferschiefer Nordwestdeutschlands-Metallporophyrine als Reife- und Faziesindikatoren. Ph.D. Thesis, Rheinisch-Westf'~ilische Technische Hochschule Aachen, Aachen (unpublished). Faure, G., 1977. Principles of Isotope Geology. Wiley, New York, N.Y., 464 pp. Galimov, E.M., Migdisov, A.A. and Ronov, A.B., 1975. Variation in the isotopic composition of carbonate and organic carbon in sedimentary rocks durings Earth's history. Geochem. Int., 12:1-19 Gilbert, T.D., Stephenson, L.C. and Philp, P., 1985. Effect of a dolerite intrusion on triterpane stereochemistry and kerogen in Rundle oil shale, Australia. Org. Geochem., 8:163-169 Grebe, H., 1957. Zur Mikroflora des niederrheinischen Zechsteins. Geol. Jahrb., 73:51-74

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