International Journal of Coal Geology 89 (2012) 41–50

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International Journal of Coal Geology

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Reflectance of dispersed vitrinite in Palaeozoic rocks with and without cleavage:Implications for burial and thermal history modeling in the Devonian of Rursee area,northern Rhenish Massif, Germany

Ralf Littke, Janos L. Urai, Anna K. Uffmann ⁎, Fotios Risvanis 1

RWTH Aachen University, Energy and Mineral Resources (EMR), Lochnerstr. 4–20, 52056 Aachen, Germany

⁎ Corresponding author.E-mail address: uffmann@lek.rwth-aachen.de (A.K. U

1 Present address: RWTH Aachen University, Internati52062 Aachen, Germany.

0166-5162/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.coal.2011.07.006

a b s t r a c t

a r t i c l e i n f o

Article history:Received 8 February 2011Received in revised form 20 July 2011Accepted 21 July 2011Available online 28 July 2011

Keywords:Vitrinite reflectanceVitrinite anisotropyHigh grade diagenesisCleavage domainMicrolithonRhenish MassifPalaeotemperatureBasin modeling

Vitrinite reflectance is used as coalification parameter in coals and as maturity and palaeotemperatureindicator in sedimentary rocks. In high grade diagenetic rocks, i.e. rocks at the boundary between diagenesisand metamorphism, vitrinites become increasingly anisotropic. Therefore, it has been suggested to studymaximum/minimum reflectance instead of mean reflectance for rocks which reached this stage.In the present study, vitrinite reflectance data from a folded sequence of the northern Rhenish Massif(Germany) are combined with microtectonic study of thin sections in samples in which both bedding andcleavage are present. We show that vitrinites in different microtectonic domains (cleavage domains andmicrolithons) show significant differences in both maximum vitrinite reflectance and vitrinite reflectanceanisotropy. This behaviour is due to a strong partitioning of deformation in cleavage domains as compared tomicrolithons, so that vitrinite reflectance is controlled by the thermal and deformation history. Accordingly,estimation of maximum palaeotemperatures during burial should be based on vitrinite reflectancemeasurements in microlithons unaffected by deformation after maximum burial.Based on this finding, burial and temperature history of the Rursee area in the eastern Rhenish Massifis reconstructed. Numerical simulations indicate a maximum burial at the Siegen/Ems boundary of about5000 m and a maximum burial temperature of about 220 °C.

ffmann).onal Office, Templergraben 57,

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1. Introduction

Sedimentary basins can be regarded as large-scale reactors inwhich a great number of important reactions take place such asthe generation of natural gas and oil, the conversion of peat into coaland anthracite and the conversion of thermally unstable mineralsinto more stable ones. All these reactions are irreversible and dependlargely on temperature and time and in particular on maximumpalaeotemperature reached during burial (Hedberg, 1974; Levine,1993; Tissot et al., 1974). Thus, the reconstruction of burial tem-peratures that affected sedimentary rocks is one of the key tasks ofgeological sciences.

Accordingly an increasing number of temperature-sensitiveparameters have been developed over the last decades, providinginsight into thermal histories of sedimentary rocks (e.g. Leischneret al., 1993; Peters et al., 2005). Clearly, one of the most widely used

parameters is vitrinite reflectance (Mukhopadhyay and Dow, 1994).Vitrinites are derived from small pieces of higher land plants suchas wood and are ubiquitous in sedimentary rocks of Devonian andyounger age (Taylor et al., 1998). In very young, near-surface sedi-ments, vitrinite reflectance is as low as 0.2%. With increasing burialand temperature, reflectance increases monotonously to values ofgreater than 4%, when the transition zone between high-gradediagenesis and very-low grade metamorphism is reached (Tayloret al., 1998). Accordingly, vitrinite reflectance is a parameter, whichprovides palaeo-temperature information for rocks at the stage ofhigh-grade diagenesis/anchimetamorphism. However, it has beennoted that vitrinite reflectance anisotropy or “rotational reflectanceof dispersed vitrinite” provides superior information about thermalmaturity and the relative timing between thermal and kinematicevents in fold belts at the transition between diagenesis andmetamorphism (Houseknecht and Weesner, 1997; Levine and Davis,1989a,b).

At high reflectance values, vitrinites become increasingly aniso-tropic due to condensation and ordering of aromatic layers (Béharand Vandenbrouke, 1987; Schenk et al., 1990). Therefore in high-grade diagenetic rocks, maximum reflectance (VRmax) is commonlymeasured as temperature-sensitive parameter instead of random

42 R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

reflectance VRr. Some studies in fold belts revealed that the VRmax axisis oriented parallel to the fold axis (Hower and Davis, 1981; Levineand Davis, 1984, 1989a) and that the optical ellipsoid was trans-formed from uniaxial to biaxial depending on the stress field duringcoalification. Furthermore, laboratory experiments at high differentialstress and high temperatures demonstrate reorientation of opticalaxes and increase in anisotropy (Bustin et al., 1986). These dataindicate that tectonic stress played a crucial role in the process ofordering of aromatic layers in vitrinite during progression of vitrinitereflectance. On the other hand field data and experiments indicatethat reflectance increase is slowed down if high confining pressures orfluid overpressures act (Carr, 1999; Huck and Patteisky, 1964).

In this context it is our primary objective to provide data onmaximum and minimum reflectance (bireflectance) for differentlithologies in study areas which have reached the “anthracite” stageor “anchimetamorphic” stage (high grade diagenesis and earliestmetamorphism; note: term “anthracite” stage is commonly referredto coals and “anchimetamorphic” stage is commonly referred to illitecrystallinity). Whereas most detailed studies on vitrinite reflectanceanisotropy dealt with coals, where vitrinites are abundant and oftenlarge, we concentrated here on dispersed vitrinite particles in clasticsedimentary rocks. Often, only these rocks are available as archives forformer plant particles, because coals/anthracites are comparativelyrare, especially in pre-Carboniferous rocks. The results may give hintson sampling in future studies of sedimentary rocks at the “anthracite”stage. Our second objective is to reconstruct the burial and thermalhistory for the study area using vitrinite reflectance as calibrationdata. This study area is situated at the northern margin of the westernRhenish Massif, Germany, close to the borders towards Belgium andThe Netherlands.

Fig. 1. Map of Rhenish Massif in western-central Europe and

2. Tectonic and stratigraphic setting

The study area is situated in the Rhenish Massif (“RheinischesSchiefergebirge”), Germany, 60 km southwest of Cologne in thenorthern part of the Eifel fold belt (Fig. 1). Stratigraphically, theRhenish Massif consists mainly of Devonian rocks cropping out atthe surface. Pre-Devonian rocks are restricted to the cores of a fewlarge anticlines whereas Carboniferous rocks occur along the northerntransition zone between Rhenish Massif and the so-called Sub-Variscan foreland basin (Ruhr Basin, Wurm and Inde Syncline). Thisforeland basin contains the largest hard coal resources of WesternEurope.

Lithologically, sedimentary rocks predominate which are partlyin the zone of low grade metamorphism, especially in the southernparts of the Rhenish Massif. The majority of Lower Devonian andolder rocks are clastic sediments and the same holds true for UpperCarboniferous (Pennsylvanian) rocks. In contrast shallow marinecarbonates are more common in the upper Mid-Devonian, UpperDevonian and Lower Carboniferous sequences.

The Rhenish Massif was tectonically deformed during the Variscanorogeny (330–300 Ma). K/Ar ages decrease northward from 329 to305 million years (Ahrendt et al., 1978) indicating a slow northwardpropagation of the fold belt. Orogenic shortening was greater than50% in many areas leading to the development of numerous folds andoverthrusts (Oncken et al., 2000; von Winterfeld, 1994). Deformationoccurred at a time when rocks were close to their maximum burialand temperature as indicated by pre- or syntectonic coalification(Littke et al., 1994; Nöth et al., 2001).

In the area around the artificial lake Rur (“Rursee”), Lower Devonianclastic sediments crop out (Rurberger and Heimbacher Formation).

location of study area in the Venn Anticline at Rursee.

43R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

These rocks consist of rather uniform, interlayered shale-siltstone-greywacke sequences with a total thickness of at least 2000 m.Stratigraphically, they belong to the Siegenian and Emsian. Greywackesare usually fine- to medium grained (0.1–0.5 mm average grain size).Quartz grains and rock fragments predominate and clay minerals areabundant whereas feldspars are absent. Siltstones are of similarcomposition but smaller grain size. Shales contain more than 75% clayminerals and are characterized by well visible tectonic cleavage atvariable angle to bedding. The Lower Devonian rocks exposed atthe Rursee water reservoir are Siegenian Upper Rurberger beds andEmsian Klerfer and Heimbacher beds to the NE. They are located on theSE-flank of the NE-plunging major Venn anticline which was formedduring Variscan deformation (330–300 Ma). Both units expose shales,siltstones and greywackes deposited in the subsiding Devonian EifelBasin.

The area has been intensively studied, and overview of thestructural evolution is given in Van Noten et al. (2008; see alsoHilgers et al., 2004; Holland et al., 2006; Kenis et al., 2005; Trautwein-Bruns et al., 2010; Urai et al., 2001). The rocks contain two typesof veins which are pre- to early-Variscan age. The first generationof veins is subperpendicular to bedding, clearly predating Variscandeformation and associated to a regional high pressure cell. Thesecond generation of veins is bedding-parallel and is related tobedding-parallel shear in the early stages of Variscan deformation.Folding and the evolution of axial plane cleavage are after this secondgeneration of veins (Van Noten et al., 2008). In several of the smalloutcrops of the area, the axial plane cleavage is discontinuous(Passchier and Trouw, 2005). Cleavage domains and microlithonsoccur in clay-rich lithologies, whereas there is no cleavage in directlyadjacent quartz-rich rocks. It is one of our goals to describe differencesbetween these microlithons and the cleavage domains with respectto vitrinite reflectance and vitrinite anisotropy.

Several maturation studies have been published on the northernRhenish Massif (e.g. Büker et al., 1995; Kalkreuth, 1979; Karg, 1998;Nöth et al., 2001; Oncken, 1982; Paproth and Wolf, 1973; Wellens,1997; Wolf and Braun, 1994). Some of these studies are based onmeasurements of maximum reflectance, whereas others used randomreflectance measurements. Recently, a new vitrinite reflectance mapof this area (Drozdzewski et al., 2009) was compiled for the topPennsylvanian, taking into account all available vitrinite reflectancedata and converting VRmax values into VRr values whenever necessary

Fig. 2. Cross section through the western Rhenish Massif; arrow

using standard algorithms (Taylor et al., 1998; Ting, 1991 andreferences therein). In most cases, there was a good fit betweendata measured by different authors; however, in some high maturityareas, where the low grade metamorphic stage has almost beenreached and cleavage is visible, VRmax values converted into VRr

values indicate higher maturity than directly measured VRr values.In this paper, we present data on vitrinite reflectance anisotropy

from different microtectonic domains, i.e. cleavage domains anddirectly adjacent microlithons. These rocks were taken from outcropsaround Rursee situated in Devonian strata in which cleavage is visibleindicating that the transition zone between high grade diagenesisand earliest metamorphism has been reached (Fig. 2).

3. Samples and methods

More than 100 samples from more than 80 outcrops were sampledfor vitrinite reflectance analysis. These rocks consist of interlayeredgreywacke, siltstone, and shale. Most samples contain enough vitrinitegrains (N50) for microscopic analysis. For detailed studies on micro-structure and vitrinite reflectance anisotropy 16 rocks were selected.Special care was taken to avoid samples with strong weathering. Thedip direction and angle of dip of the bedding, cleavage, and δ-lineation(this is the intersection lineation between bedding and cleavage) weremeasured in the field.

The rocks were cut both perpendicular and parallel to cleavageand bedding. From these rock pieces, polished blocks and polishedthin sections were prepared. Transmitted light microscopy was usedto identify the microtectonic domains and reflected light microscopyfor vitrinite reflectance measurements in these domains. This analysiswhich combines transmitted light and reflected light microscopyand the techniques of microtectonics and organic petrology, allowedto simultaneously measure vitrinite reflectance and determine themicrotectonic position in one sample.

Analytical methods for organic petrology closely follow theguidelines published in Taylor et al. (1998) but vary in some points.Polished sections were prepared from pieces of core which have a sizeof approximately 1–6 cm². The rock samples were embedded in amixture of epoxy resin (Araldite® XW396) and hardener (Araldite®XW397) at the rate of 10:3 and hardened in an oven (37 °C) forapproximately 12 h. Then the samples were ground and polishedusing an automated Struers Tegra Pol 21, with a Tegra Force 5 head,

indicates location of Rursee (after von Winterfeld, 1994).

44 R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

polishing system. It is a two stage process under wet conditions:at first the samples were ground sequentially using two differentcarborundum papers and H2O as lubricant. The first paper is aDiamante MD Piano 120 plate (Struers GmbH), the second is a SiCpaper 1200 (grain size 15 μm, Struers GmbH). After each grindingstage samples were washed with H2O to remove debris and preventswelling. The next step was polishing the sample block to obtain aclean, uniformly flat and scratch- free surface. Three polishing lapscovered with short-nap cloth (MD Plan (Struers GmbH), MD Dac(Struers GmbH) and Billard OP-U (Buehler) were loaded subsequent-ly with suspensions of decreasing grain size and used in the followingsequence; 9 μm (DP-Plan), 1 μm (DP-Nap) and 0.5 μm (Feinpol OP-U).Each step lasted 2 to 4 min, depending on the kind of rock, but shouldbe kept to a minimum to avoid the development of a relief. Waterwas used as a lubricant during polishing. After each polishing stepthe sample was washed with water to remove debris. Finally, theblock was hand-buffed to remove fine smears and checked under amicroscope for polishing quality and particle relief.

Vitrinite reflectance analysis was performed on the individualsamples at a magnification of 500× in a dark-room using a Zeiss AxioImager microscope for incident light equipped with a tungsten-halogen lamp (12 V, 100 W), a 50×/0.85 Epiplan-NEOFLUAR oil im-mersion objective and a 546 nm filter. Zeiss immersion oil (ne=1.518;23 °C) and mineral standards of known reflectance were used forcalibration; namely Klein and Becker® leuco-saphire (0.592%),yttrium–aluminum–garnet (YAG; 0.889%), gandolinium–gallium–

garnet (GGG; 1.721%) and cubic zirconium (3.125%). The standardsare kept in dust free boxes at constant temperature and humidity. Inthis study the GGG and the cubic zirconium standards were used.

Fig. 3. Geologic map of Devonian strata around lake Rur (“Rursee”) in the Eifel mountains,numbers. Map modified after Geologisches Landesamt NRW (1980).

Initially vitrinite reflectance measurements (VRr) at random orienta-tion of grains were made (i.e., no rotation of the microscope stage)in non-polarized light. In order to reach sufficient accuracy ofvitrinite measurements, at least 50 points were measured on eachsample whenever possible. Data are processed using the DISKUS Fossilsoftware (Technisches Büro Carl H. Hilgers).

To obtain rotational reflectance data, polarised incident lightwas used and the microscope table was rotated by hand in 10° steps.For all measurements in sections perpendicular to cleavage, in the 0°position the axis of cleavage (which is usually well visible in thesamples in transmitted light) runs parallel to the vertical axis of themicroscopic image. From this position, the microscopic stage wasrotated in 10° intervals. Besides the reflectance data, the position ofthe long axis of each particle relative to foliation was recorded.

Numerical 1D modeling of burial and temperature history wasperformed for a pseudo-well at the eastern margin of Rursee accordingto the methods described by Nöth et al. (2001). The method of 1Dmodeling using PetroMod software and in particular calibration of themodels has beendescribed in Petmecky et al. (1999) andNelskampet al.(2008).

4. Results and discussion

4.1. Sample quality and maturity pattern

Care was taken to obtain non-weathered samples in order to avoidany influence of weathering on vitrinite reflectance. Typical featuresof weathered vitrinite are discussed in Taylor et al. (1998: 529; seereferences therein), but are not observed in our sample set. An even

western Germany. Mean vitrinite reflectance values of outcrop samples are shown as

Fig. 4. Microlithon (green) and Cleavage Domain (yellow) regions in a thin section of agreywacke of the Rursee area. SS: Bedding, sf: Foliation cleavage.

45R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

more sensitive parameter on degree of weathering is pyrite oxidation(Littke et al., 1991). Good pyrite preservation as observed in oursamples indicates that organic matter is not weathered significantly.

Mean random vitrinite reflectance (VRr) data were obtained formore than 100 samples of about 80 locations around Rursee coveringan area of about 30 km2. Although there is some scatter in the data, anoverall tendency of decreasing maturity from west to east is visible(Fig. 3). As the axis of the large Venn anticline and of adjacentstructures dip towards northeast (Figs. 1, 2 and 3), younger strata arefound at the eastern flank of the anticline towards the east. This dataindicates an increase in maturity with stratigraphic age. Highest

Fig. 5. a: Reflected light view of vitrinite particle from a cleavage domain. Bedding (ss) androtations of the microscope stage (C, in °). b: Reflected light view of vitrinite particle froReflectance is displayed for different rotations of the microscope stage (C, in °).

values reach more than 5.5% VRr and minimum values are at 2.5% VRr

(Fig. 3) Thus the data are in accordance with VRmax data publishedearlier on the same area byWellens (1997). The scatter in the VRr datais partly due to an intense folding and faulting of the strata leadingto many smallscale anticlines and synclines. Another reason for thescatter is the difference in microstructural domains, which will bediscussed in detail below.

4.2. Influence of lithology and cleavage

In the study area, coarse grained sedimentary rocks are usuallygreywackes, rich in rock fragments, quartz, and chert, whereas fine-grained rocks are shales/claystones and siltstones. Mostly both rocktypes are interlayered on a centimetre tometer scale in the study area.Cleavage is developed only in fine-grained rocks having vitrinitereflectance values (VRr) greater than 2.5–3%. In some cases, there aresignificant differences in reflectance in the same outcrop although anyinfluence from magmatic dikes or sills can be excluded (for examplessee Figs. 4 and 5b)

Interestingly, we found in the same samples strong discrepanciesbetween VRr and VRbi (bireflectance: difference between Rmax

and Rmin during stage rotation) of individual particles, especially infine-grained rocks. Thin section microscopy clearly revealed thatwithin the same layer these rocks are strongly heterogeneous ona millimetre-centimetre scale, divided into cleavage domains andmicrolithons (Passchier and Trouw, 2005). An example is shown inFig. 4.

Vitrinites in both domains are shown in Fig. 5a and b. Twomicrographs from the same outcrop are represented in reflectedlight, one of a microlithon and the other of a cleavage domain, cut

cleavage foliation (sf) are marked in the picture. Reflectance is displayed for differentm a microlithon. Bedding (ss) and cleavage foliation (sf) are marked in the picture.

Fig. 7. Angle between direction of foliation and the long axis of the particles.

46 R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

perpendicular to bedding and cleavage foliation. It can be seen thatthe shape of the vitrinite particles and anisotropy differs greatly. Thesample from the microlithon shows a difference between minimumand maximum reflectance of 1.7%, whereas that from the cleavagedomain is 5.7%. Also, the maximum reflectance differs greatly and isat 4.7 and 7.6%, respectively. This difference cannot be attributed tothe general temperature history, but must be related to the differencein deformation.

The same analysis was performed systematically for a total of 48particles frommicrolithons and 73 particles from cleavage domains ofdifferent outcrops from the Rursee area. Results are plotted in Fig. 6.The mean bireflectance for vitrinites from microlithons is about 2%with maximum values not exceeding 3.6% and minimum values ofonly 0.2%. In contrast, samples from cleavage domains have a meanbireflectance of about 5% with maximum values greater than 12% andminimum values of 2% (Fig. 6).

In microlithons, vitrinite particles with a low length/width ratiopredominate. In contrast, in cleavage domains, minerals and vitriniteparticles are aligned parallel to cleavage foliation, i.e. generally thereis only a small angle between the direction of foliation and the longaxis of the particles (Fig. 7) which varies between 0 and 48° and isusually less than 20°.

4.3. Vitrinite particles of different elongation

In order to assess systematic difference in vitrinite shape betweenparticles in cleavage domains and in microlithons, the aspect ratio(length/width: L/W) of the particles was measured; results are shownin Fig. 8. In the microlithons, the L/W ratio is on average at about 2,reaching maximum values of about 5. In cleavage domains, the meanvalue is about 4, and there are several particles characterized by

Fig. 6. Histograms of bireflectance values (Rmax−Rmin) for vitrinite particles fromcleavage domains (a) and microlithons (b).

Fig. 8. Length/width ratio (see text for explanation) of vitrinite particles from cleavagedomains (a) and microlithons (b).

Fig. 9. Plot of L/W ratio versus bireflectance (Rmax−Rmin) for samples frommicrolithonsand cleavage domains.

47R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

values greater than 10. However, both groups of particles also containequiaxed particles, and the minimum of both populations is 1.

The clear and systematic difference in the properties (both shapeand optical) of particles from microlithons and cleavage domainsis evident. Because the cleavage is a feature which is not present inthe rocks during deposition but forms much later during tectonicdeformation this difference is clearly related to the formation ofcleavage. To illustrate this further, bireflectance was plotted againstL/W ratio (Fig. 9). The results reveal clear differences betweencleavage domains and microlithons with respect to both properties,indicating that not only bireflectance but also shape (L/W ratio) isaffected by cleavage.

Keeping in mind that the formation of the cleavage domains is atectonicprocesswhichoccurredat the timeofmaximumburial or shortlyafter that time, the model which simply explains all these observationsconsiders the development of slightly deformed (during compaction)and anisotropic (maximum temperature) vitrinites, followed by defor-mation partitioned in the cleavage domains where pressure solutionprocesses lead to an enrichment of insoluble particles and plasticdeformation of the vitrinites in these zones (Passchier and Urai, 1988).Therefore the different properties of vitrinites in the microlithons andcleavage domains is of tectonic origin, with the microlithons showingonly minor tectonic overprint, but the vitrinites in cleavage domains(maybe at less than a mm distance in the rock) strongly deformed.

Fig. 10. Average values and standard deviations of vitrinite reflectance anisotropymeasured in the same sample, in microlithons and cleavage domains. Error bars showone standard deviation of the samples.

For several samples we found a large number of vitrinites both inmicrolithons and cleavage domains, whereas in other samples thereare vitrinites either only in microlithons or only in cleavage domains.Their bireflectance values from the same sample with their standarddeviation are plotted in Fig. 10. The average value is about 2.5 timeshigher in the cleavage domains than in microlithons, but with quite ahigh variability. The data suggest a correlation between the two setsbut more data are needed to substantiate this.

4.4. Interpretation of microstructures

It is clear from the data presented that vitrinites in the cleavagedomains are more strongly deformed. During formation of thecleavage, pressure solution processes lead to an enrichment ofinsoluble particles and plastic deformation of insoluble but weakvitrinites in these zones (Passchier and Urai, 1988). While the aspectratio and anisotropy on average is clearly higher, there are alsovitrinites in the cleavage domains, which have a low aspect ratio andanisotropy. A basic explanation of this phenomenon is shown inFig. 11. Here, the first picture shows a set of vitrinite particles aftercompaction, with a weak preferred orientation parallel to bedding, inthat part of the rock which later will become a cleavage domain. Theanisotropy which is also variable is not shown. The second pictureshows the same, but after a homogeneous strain shown by thedeformed circle, as can be the case in a cleavage domain. Although thestrain of all particles is the same (as shown by the small deformedcircles in each), their aspect ratio and orientation is quite variable(Ramsay and Huber, 1987). Using the same argumentation, andkeeping in mind the weakly crystalline structure of the deformingvitrinites, it is also quite conceivable that deformation of a particlewith the principle shortening direction parallel to the direction oflargest reflectance can lead to a reduction of reflectance anisotropy.Here, more work is needed to understand this process further.

4.5. Burial and temperature history

It was one objective of this study to reconstruct and possiblyquantify burial and temperature history of the northwestern RhenishMassif, i.e. the Rursee study area. In order to assess the maximumtemperatures reached by the rocks at Rursee, numerical 1D basin

Fig. 11. A possible explanation of the complex relationship between particle shape andvitrinite reflectance. a) shows a set of vitrinite particles after compaction, with a weakpreferred orientation parallel to bedding, in that part of the rock which later willbecome a cleavage domain. b) shows the same, but after a homogeneous strain shownby the deformed circle. Although the strain of all particles is the same, their aspect ratioand orientation is quite variable.

48 R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

models were calibrated by vitrinite reflectance values. For calibration,the Easy%Ro algorithm of Sweeney and Burnham (1990) was used. Inthis approach, VRr, i.e. the mean random reflectance, is calculatedfrom temperature history. For comparison with real data, we usedVRr data measured on the coarse grained samples (sandstones/greywackes) without cleavage following standard procedures. Thismodel is valid for the eastern margin of Rursee, where lowestvitrinite reflectance values were recorded (2.5–3.0% VRr).

The input data applied for the model are summarized in Table 1.Sediment–Water–Interface–Temperature (SWIT) evolution throughtime was estimated from the palaeogeographical position of the area.In addition, for calculation of palaeo-surface temperatures, palaeowaterdepths were adopted from Büker et al. (1996). Heat flow through timewas affected during Lower Devonian times by extension of thelithosphere, which led to high subsidence rates along with high heatflow(Littke et al., 2000). For the area aroundRursee, aβ-value of 1.8wascalculated based on structural modeling (Littke et al., 2000). This factorcan be converted into a heat flow during maximum extension of 90 to100 mW/m2 following the approach of Bodri and Bodri (1985). Adeclineof crustal extension andheatflow through timewas assumed forthe following Devonian/Carboniferous period. This is reflected by adecreasing sedimentation rate. Lithologies and thicknesses for thedifferent eroded stratigraphic intervals listed in Table 1were taken fromthe well-known stratigraphic record of surrounding areas, where thecomplete sequence of Devonian and Carboniferous rocks is preserved(von Winterfeld, 1994). The post-Permian sedimentation and erosionhistory was very roughly adopted from knowledge on surroundingareas (i.e. Büker et al., 1996) and from fission track data published byGlasmacher et al. (1998) and Karg et al. (2005). Based on the lithologies,petrophysical properties were assigned as described in Nelskamp et al.(2008). Post-Permian sedimentation and erosion history did not affectmaturation, because temperatures remainedmuchbelow those reachedin the Late Carboniferous. Our model includes the assumption of rapidcooling between 120 and 80 Ma as suggested by both Glasmacher et al.(1998) for an area further to the west and Karg et al. (2005) for an areafurther to the east. Late Cretaceous (Campanian/Maastrichtian) sedi-

Table 1Input of the basic geological data for modeling of temperature and burial history.

Layer Presentthickness (m)

Eroded(m)

Depositionfrom (Ma)

Depoto (M

Quarternary 0 2 0Tertiary 0 65 2Upper_Cretaceous 0 99 65Lower_Cretaceous 0 142 99Jurassic 0 200 142Keuper 0 100 235 200Muschelkalk 0 100 243 235

Buntsandstein 0 200 251 243Zechstein 0 50 258 251Permian 0 301 258Stephanian 0 305 301Westphalian 0 700 316.5 305Namurian 0 900 326.5 316.5Dinantiann 0 300 358 326.5Upper_Devonian 0 600 381 358Mid_Devonian 0 600 392 381Ems_Klerfer_und_Heimbacher_FormationB 0 800 398 392Ems_Klerfer_und_Heimbacher_FormationA 400 800 404.5 398Siegen_Upper_Rurberger_FormationC 400 405 404.5Siegen_Upper_Rurberger_FormationB 800 406 405Siegen_Upper_Rurberger_FormationA 800 407 406Siegen_Mid_Rurberger_Formation 300 409 407Siegen_Lower_Rurberger_Formation 300 411.5 409Lower_Devonian 400 417.5 411.5

Basement 1000 440 417.5440

mentation followed by erosion of the same strata was not taken intoaccount, although these sediments are well known from the northernrim of the RhenishMassif, 50 kmnortheast of Rursee. Due to their smallthickness (at best a few hundred metres), these sediments would nothave affected maturation (Fig. 12b).

Thus, it was assumed that maximum burial and maximumtemperatures occurred prior to or during Variscan orogeny and thatno deep burial occurred during post Variscan times. This is clearlyindicated by apatite fission track data (Glasmacher et al., 1998; Karget al., 2005) and low thermal maturities of post-Carboniferous rocksin the area north of the Rhenish Massif, where Permo-Triassicand younger rocks unconformably overlay the folded Devonian-Carboniferous sequence. Sedimentation occurred during the Devoni-an syn- and early post-rift stage at high subsidence rates (Littkeet al., 2000; von Winterfeld, 1994). This was followed by only slowsedimentation during the Early Carboniferous and by rapid sedimen-tation during the Late Carboniferous foreland basin stage whenmaximum burial was reached (Karg et al., 2005). Metamorphic agesbased on K–Ar and Rb–Sr ranging between 308 and 312 Ma (Krammet al., 1985) indicate cooling during the Late Carboniferous; highesttemperatures were probably reached slightly earlier as suggested bystudies on coalification andapatitefission tracks in adjacent areas (Bükeret al., 1995; Karg et al., 2005). Based on these assumptions (see also datasummarized in Table 1), burial and temperature history was calculatedfor the Upper Rurberger Formation calibrated by measured meanrandom vitrinite reflectance data (Fig. 12). It should be noted thatmajordeformation occurred during or directly after maximum burial.

In ourmodel, high heat flowswere assumed for the time of Devonianrifting (90 mW/m2) decreasing thereafter to 60 mW/m2 in the Carbon-iferous. The simulation revealed a thickness of 4700 m of erodedsediments of pre-Permian age. Due to the fact that post-Permian burialtemperatures remained much below the maximum pre-Permiantemperatures, post-Permian burial history can be assumed to have onlya small effect on present-day maturity. Thus, the model is basicallycalibrated by the assumption of a Carboniferous heat flow,which is closeto the average value for continental crust and by burial by later eroded

sitiona)

Erodedfrom (Ma)

Erodedto (Ma)

Lithology Age PWD(m)

SWIT(°C)

Heat flow(mW/m2)

0 0 9 6065 0 23 6099 45 25 60

142 45 24 60200 65 20 60

200 170 Shale (typical) 235 45 26 60170 140 Limestone

(ooid grainstone)243 65 25 60

140 115 Shalesandysilty 251 30 25 60115 109 Sandstone (typical) 258 20 24 60

305 5 24 60316.5 5 24 63

300 290 Shalesandysilty 326.5 50 24 63290 275 Shalesiltysandy 345 50 24 63275 270 Limestone (shaly) 350 100 23 63270 265 Limestone (micrite) 358 100 23 65265 260 Limestone (shaly) 381 100 23 7010 65 Shalesandysilty 392 50 24 7565 0 Shalesandysilty 396 50 24 80

Shalesandysilty 400 50 24 90Shalesandysilty 404.5 50 24 90Shalesandysilty 411.5 50 24 70Shalesandysilty 417.5 50 24 65Shalesandysilty 440 0 25 65Shale (organic lean,siliceous, typical)BASEradio2_1

49R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

Devonian and Carboniferous rocks with a total thickness of 4700 m.Within the Siegen/Ems boundary layer temperatures of about 220 °Cwere achieved during maximum burial (5100 m at about 300 Ma;Fig. 12a, b) leading to vitrinite reflectance (VRr) values of 2.9%as measured in the coarse grained greywackes without any cleavage.This leads to a good fit between measured and calculated vitrinitereflectance.

Our model was based on calibration with measured vitrinitereflectance data from rockswithout cleavage. If wewould have calibratedthe model with data from rocks showing cleavage, a total thickness of6800 m of eroded Palaeozoic rocks (instead of 4700 m)would have to beassumed in order to calibrate the model. Furthermore, the calculatedtemperature during maximum burial would be at 290 °C (instead of220 °C). This comparison shows that the selection of rocks withoutcleavage is essential for calibration of burial and temperature historymodels.

5. Conclusions

At high levels of diagenesis and during earliest stages of meta-morphism, deformation influences vitrinite reflectance. Our dataindicate differences between cleavage domains andmicrolithons withrespect to vitrinite reflectance and bireflectance. Even vitrinites from

Fig. 12. a: Burial and temperature history for sedimentary rocks from outcrop

Fig. 12. b: Maturity evolution through time for Siegen/Ems boun

differentmicrotectonic domains (cleavage domains andmicrolithons)from the same sample can show significant differences in bothmaximum vitrinite reflectance and vitrinite reflectance anisotropy.

This difference is due to a strong partitioning of deformation intocleavage domains as compared to microlithons, so that in these micro-structural domains the observed reflectance of the vitrinites is caused by acombination of burial temperatures and deformation.

Accordingly, calculation of maximum palaeotemperatures duringburial should be based on vitrinite reflectance measurements in rockswithout cleavage, or - if this is not possible - on vitrinites in microlithonswhich are less affected by deformation than those in cleavage domains.This requires a combinationof transmitted and reflected lightmicroscopy.

In addition, differences between reflectance in microlithons andcleavage domainsmay provide important information on deformationin rocks that have reached the stage of late diagenesis or earliestmetamorphism.

Anumericalmodel onburial and temperaturehistorywasdevelopedfor the Siegenian/Emsian boundary layer at Rursee. According to thismodel, maximum temperature of about 220 °C was reached at about5 km burial depth during the Late Carboniferous leading to meanvitrinite reflectance values of about 2.9%. Deformation occurred duringor directly after maximum burial and greatly affected vitrinite reflec-tance in cleavage domains.

s around Rursee. The model was calibrated by VRr data from greywackes.

dary strata at Rursee based on the model shown in Fig. 12a.

50 R. Littke et al. / International Journal of Coal Geology 89 (2012) 41–50

Acknowledgement

The authors wish to thank G. Rantitsch for a thoughtful and detailedreview of an earlier version of this manuscript.

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