Journal of the Geological Society, London, Vol. 146, 1989, pp. 345-350, 8 figs. Printed in Northern Ireland

Petrographic examination and re-interpretation of concretionary carbonate horizons from the Kimmeridge Clay, Dorset

K. W. A. FEISTNER Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB23EQ, UK

Abstract: Sections through two concretionary carbonate horizons (Yellow Ledge and Maple Ledge) from the Kimmeridge Clay type locality of the Dorset coast have been examined using back-scattered imaging electron microscopy, X-ray microanalysis and X-ray diffraction. The Yellow Ledgehas experienced a complex cementation history involving precipitation of several carbonate phases. Samples from both the centre and margins of this ledge contain juxtaposed low Mg, low Fe calcite and ferroan dolomite. This indicates that the ledge did not simply grow from the centre outwards during progressive burial. By contrast,the MapleLedge was found tobe compositionally and texturally homogeneous, being cemented throughout by interlocking subhedral ferroan dolomite crystals. The observations made in this study indicate that a detailed petrographic examination of concretionary layers must be undertaken if geochemical and isotopic data obtained from the analysis of bulk samples are to be interpreted correctly.

The Kimmeridge Clay type section of the Dorset coast (Fig. outcrops on the foreshore below Cuddle, approximately 600 m east 1) contains numerousindurated cabonate-rich horizons, of the old stone jetty onthe eastern arm of Kimmeridge Bay knownlocally as the Kimmeridge Ledges. Irwin(1980) (Ordnance Survey grid reference SY 912782). It is not of constant undertook a detailed study of one of these ledges, the thickness; in section it can be seen to pinch and swell from 25 cm to Yellow Ledge, and proposed a model for its formation 45cm in thickness over a distance of a metre,and a plan view which has since become accepted as a general model for the reveals a generally hummocky appearance. Due to its great strength origin of those of the ledges thatare of concretionary the band was most readily sampled where it was thinnest, yielding a carbonate composition (e.g. Leddra et al. 1987). However, specimen spanning its entire 25 cm thickness. A complete vertical the marked variation in the weathered appearance of some section was cut from this specimen and then subdivided into nine of these ledgesimplies textural and compositional chips (l-top to 9-bottom). Thesewere then embedded in resin, highly polished using 1 pm diamond abrasive, coated with 10 nm of differences not readilyreconcilable with a single common carbonand mounted on aluminiumstubs ready for BSEM origin. Irwin's model proposes early cementation (at 10 m or examination. Nine equivalent samples were crushed and powdered less depth within the sediment) at a specifichorizon by for X-ray diffraction (XRD) analysis. ferroan dolomite to form a concretionary layer which grew Another ledge, the Maple Ledge, was also sampled. This ledge more or less symmetrically by accretion to both upper and occurs towards the middle of the autiwiodorenris Zone (Cox & lower margins during continued burial to about 500 m (Irwin Gallois 1981) and outcrops on the foreshore below the car park at 1980). Kimmeridge Bay (OS ref. SY 909789). A 12cm thick vertical The purpose of this paper is to examine the growth section was divided into four chips which were prepared as above. model proposed by Irwin(1980) in the context of new The specimens wereexamined using a Phillips 501B scanning textural and mineralogical data obtained by back-scattered electron microscope equipped with an annularfour-element imaging electron microscope (BSEM) studies of two of the solid-state back-scatteredelectron detectorand a Link Systems concretionary layers. This growth model hinges upon the AN 10050 energy-dispersive X-ray (EDX) analyser. The great interpretation of carbonand oxygen stable isotope data advantage of back-scattered electron imaging is that it can provide from the carbonatephase, as proposed by Irwin et al. mineralogical as well as textural information (Pye & Krinsley 1984). (1977). Thus positive 613 C values were interpreted as Phases with different mean atomic number can be distinguished by evidence forcarbonate precipitation in the methanogenic differences in brightness ('2-contrast'). The phaseswere then zone, and the progressive decrease in these values from identified by EDX analysis. Since the EDX analyser is only centre to margins was interpreted as evidence for the qualitative, an electron microprobe was used to provide quantitative increasing importance of CO, produced by abiotic reactions data for the composition of the carbonate phases. as a source for the precipitating carbonate. Since the methanogenic and abiotic reactions are depth related(Curtis Results and discussion 1977), estimates could thus be made for the depth at which the carbonate precipitated. The 6" 0 values were used to estimate temperatures of crystallization which were in turn The Yellow Ledge related to depth using geothermal gradient estimates. The XRDdata and examination of stained thinsections led two sets of depth estimates were consistent but not Irwin (1980) to the conclusion that ferroan dolomite was the independent. only carbonate phase present in the centre of the band and that, although calcite appeared towards the margins, it was Sampling and methodology <7 mol% of thetotal carbonate present, wasprimary in The Yellow Ledge (Fig. 2) is a yellow-weathering indurated horizon origin, and would thus have a knowngeochemical and at the base of the scitulus Zone (Cox & Gallois 1981), which isotopic signature. However, XRD results obtained in this 345

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'L

3

4 L

U L-- 1 km St Alban's Head

Fig. 1. Location map.

study(Fig. 3) show that calcite is present throughout the band and that it predominates over dolomite in four of the nine samples, nos 3, 7, 8 and 9. This major discrepancy with Irwin's data can only be reconciled by suggesting strong lateral variation within'the band. This may possibly be the cause of the hummocky nature of the band as described above. The petrographic fabric of the Yellow Ledge shows great heterogeneity (Fig. 4) over very short distances (e.g. Figs 4d and e are within millimetres of each other). If growth took

9

.i:::: .i:::: 4 DC Q Fig. 3. Profile of XRD data from the Yellow Ledge. Numbers refer to samples. Major peaks arrowed are D, dolomite; C, calcite; Q, Fig. 2. The Yellow Ledge at its foreshore outcrop. quartz.

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(e) (f) Fe. 4. Back-scattered electron photomicrographs showing examplesof some of the different fabrics shown by the Yellow Ledge. White, pyrite; dark grey/black, organic matter and clay matrix; light grey, carbonate.

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place from thecentre towards the margins of theledge during burial from 10 m to 500 m depth, onemight expect to find a simple relationshipbetween apparent degree of compaction and position within the band. For example one might expect theorganic component of theband to be progressively moreflattened and distorted from centre to margins. However no evidence for such a relationship was observed. This may be due to primary lithological variation, but too much alteration has occurredfor the primary sedimentary fabric of the band to be recognized with any certainty.It is therefore also impossible to determine the extent towhich the original lithological variation of the band has dictated the morphologyand composition of the diagenetic carbonate. The carbonatelenses in Fig. 4f suggest that there is at least some lithological control, regardless of whether they are interpreted as recrystallized faecal pellets (Irwin 1980) or as the recrystallized products of differential loading on alternate coccolith-rich and organic-rich laminae (Downie 1955). The diagenetic carbonate may show euhedral form and cross-cutting relationships (Fig. 4e, large calcite rhomb in uppercentre) which both indicate replacive and/or displacive growth. Furthermore,although fine-grained sediments may have high porosities,their verylow permeabilities are evidence of the very small dimensions of theirpore systems. Aggregates of diageneticcarbonate larger than 100 pm (Figs 4a,d & f) aretherefore very unlikely to representoriginal pores of a similar size. There is thus substantial evidence that the diagenetic carbonate is not a passivepore-filling cementand therefore estimates for depth of precipitation made using weight percent carbonate and compactional pore occlusion models (e.g.Raiswell 1971; Oertel & Curtis 1972) are onlylikely to bevery approximate. The model proposed by Irwin (1980) is founded on the data provided by chemical and isotopic analysis of bulk samples. Unambiguous interpretation of such data requires that only one phase of unknown composition is present. However this is not always easy to ascertain. Detailed XRD analysis can distinguish discrete carbonatecompositions, but compositional variations within a solid solution continuum will result in abroad peak whichmay not be readily Fig. 5. Back-scattered electron photomicrographs showing ex- resolvable. Isotopic values vary between phases, not only amples of complex carbonate zonation within the Yellow Ledge. dueto the different fractionationfactors for different White, pyrite; black, organic matter and clay matrix; light grey, compositions, but also due to the evolution of the isotopic calcite; darker grey, ferroan dolomite. composition of thepore fluids (from which the phases precipitate) with increasing age and burial depth(Curtis 1977). Thus even a single phase system that precipitated ferroan dolomite (Fig. 6). The exact sequence of over a prolonged time and depth interval could result in precipitation is difficult to determine but atleast some of the significant isotopic variations with anabsence of any calcite clearly postdates the ferroan dolomite as euhedral textural distinction. Clearly the unambiguous interpretation calcite rims to ferroan dolomite crystals can be found. of isotopic data from bulk samples is only possible if each Sixty quantitativeelectron microprobe analyses of the sample consists only of a single phase precipitated over a carbonate phase wereobtained from transects across suitably short time anddepth interval, such that isotopic alternate chips. The analyses were random in that without a variations are minimized. BSE detectorno brightness contrast was discernible and However, BSEM examination of the Yellow Ledge has thus there was no samplingbias relatedto composition. revealed thatthe carbonate phase within any one bulk There mayhowever be an inbuilt bias towards the larger sample consists of more than one mineral. Figs 5a and 5b crystals due to poor imaging and thus the results (Fig. 6) show juxtaposed occurrences of calcite and ferroan dolomite should not be taken to indicate the actual proportions of (the two extreme carbonatecompositions commonly found), different compositions present. The absence of a high- whose compositions have been confirmed by EDX analysis. resolution BSE detector on the microprobe means that it Further slight differences in the composition of the cannot be said with certainty that each analysis represents a carbonate phase can be recognized as intermediate ‘shades single phase but it is believed that the analysed specimen of grey’ corresponding to compositions between calcite and volumeissufficiently small (approximately three cubic

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depths in the sulphate-reduction zone where available iron wasbeing scavenged by reactive sulphur species and thus was not available for incorporation into the calcite lattice. If the euhedral calcite crystals were indeed formed at shallow burial depths, one would expect that thefine-grained 'matrix' material would have deformed around them giving a rather obvious compactional fabric. However there is very little, if any, evidence for this and some crystals(e.g. the large crystal in the upper centre of Fig. 4e) certainly seem to be the product of post-compactional, later diagenetic growth. If Ca l00 this is the case then another reason must be sought for the lack of iron in the calcite lattice. Perhaps all the reactive iron was consumed during the precipitation of the pyrite. The composition of the carbonate in samples 1, 3 and 5 varies between low Mg, low Fe calcite and ferroan dolomite yL5Awith an average composition of 32-34%Mg and 11- 13% Fe. This variation isdefined by the plotted analyses which form a line between these two 'end-member'

..e" .- .. 2- M950 - - " . " " -50 %o - " " - Fe50 Fig. 6. Carbonate phase compositions from electron microprobe analysis. Small dots represent one ortwo analyses; figures indicate the numbers of analyses represented by the largergroupings.

microns) to minimize compositional mixing,given that analyses were taken from the centres of crystals and that within-crystal compositional zoning of the kind seen in Figs 5a and 5b is relatively rare. All the samples analysed contain lowMg (1-3%), low Fe (l-3%) calcite. In sample 9, in which this composition is predominant, the calcite euhedra (Fig. 7) indicate thatat least some of this phase is authigenic inorigin and not purely recrystallized primary carbonate. The low Fe content of this calcite could suggest that it formed at shallow burial

(b) Fig. 8. Back-scattered electron photomicrographs showing the typical fabric of the Maple Ledge. White, pyrite; black, organic matter; light grey, ferroan dolomite; fine-grained dark grey, clay minerals; coarse-grained dark grey, quartz. Note: in (b) the Fig. 7. Back-scattered electron photomicrograph showing secon- inclusion of quartz grains within ferroan dolomite crystals (e.g. just dary euhedral calcite growth. White, pyrite; black, organic matter over half-way up theright-hand side) indicating the replacive and clay matrix; light grey, calcite; dark grey, quartz.. growth of the ferroandolomite.

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compositions. These linescan eitherbe interpreted as requires a knowledge of the phases present and BSEM mixinglines formed byanalysing a mixture of different examination has revealed that these phases are in fact more proportions of the two end-member compositions, or as diverse than had formerly been supposed. Further isotopic evidence for continued crystal growth as the pore-water investigation of the origin of the Yellow Ledge must composition evolved. Qualitative EDX analyses obtained therefore await the development of a suitable in situ isotopic using the BSEM suggest that single phase crystals of ratio measurement technique. However, if homogeneity of a intermediate composition do exist, lending weight tothe horizon can be shown, as in the case of the Maple Ledge, second interpretation. This interpretation would, however, there is no reason why isotopic data provided by bulk tend to predict the existence of zoned crystals rather than methods of analysis should not be used. The strong lateral single phase crystals of intermediate composition. Such variationsuggested by the discrepancy between the XRD zones crystals have not been observed in significant data obtained in this study and those reported by Irwin numbers. wouldalso imply that more than one section should be Sample 7 contains a wide range of carbonate examined before a model is proposed. compositions that do not appear to be readily explicable by The verydifferent textureand mineralogy of the two either of the above interpretations, thus indicating an even ledges so far examined suggests that their mode of origin more complex cementation history. must differ quite significantly and thus no onemodel is likely Even though the microprobe data raise yet more to be ableto explain the origin of all the concretionary questions, they do confirm that the carbonate mineralogy is Kimmeridge Ledges. indeed too diverse for simple interpretation of bulk analytical data. I would like to thank K. Pye for his encouragement and criticism of this manuscript which has also benefitted greatly from comments by The Maple Ledge R. Raiswell and J. Bellamy. I am also indebted to S. Adlington and Murphy for their support. This work has been carried out during The fabricof the Maple ledge is totally different from that of tenure of a NERC Research Studentship at Cambridge University. the Yellow Ledge. It consists throughout of subhedral interlocking ferroan dolomite crystalswhich give rise to a sucrosic texture (Figs 8a and 8b). The composition of the References carbonate phase has been determined solely on the basis of Cox, B. M. & GALLOIS,R. W. 1981. The stratigraphy of the Kimmeridge Clay qualitative EDX data obtained using the SEM. The textural of the Dorset type area and its correlation with some other Kimmeridgian and compositional homogeneity across the ledge suggests sequences. Report of the Institute of Geological Sciences., 80/4. that itmay have formed over a relatively short time and CuRrrs, C. D. 1977. Sedimentary gemhemistry: environments and processes dominated by involvement anof aqueous phase. Philosophical depth interval. Again reliable estimates fordepth of Transactions of the Royal Society of London, A, 286,353-372. formation cannot be made using weight percent carbonate DOWNIE,C. 1955. The Kimmeridge Oil Shale. PhD Thesis,University of and compactional pore occlusion models (Raiswell1971; Sheffield. Oertel & Curtis 1972), since textural evidence clearly shows IRWIN,H. 1980. Early diagenetic precipitation and pore fluid migration in the Kimmeridge Clay of Dorset, England. Sedimentology, 27, 577-591. thatthere has been replacive growth of ferroan dolomite -, CURTIS,C. D. & COLEMAN,M. 1977. Isotopic evidence for source of (Fig. 8b). It is also hard to imagine the primary porosity of diagenetic carbonates formedduring burial of organic-richsediments. this very fine grained sediment being the shape and size of Nature, 269, 209-213. the present carbonate crystals (as would have to be the case LEDORA,M. J., YASSIR,N. A., JONES,C. & JONES, M.E. 1987. Anomalous if they are indeed a passive pore-filling cement). compressional structures formed during diagenesis of a dolostone at Kimmeridge Bay, Dorset. Proceedings of rhe Geologists’ Association, 98, 145-155. Conclusions OERTEL,G. & CURTIS,C. D. 1972.Clay-ironstone concretion preserving fabrics due to progressive compaction. Bulletin of the Geological Society The model for the formation of the Yellow Ledge (and by of America, 83, 25972606. implicationalso theother concretionary Kimmeridge PYE, K. & KRINSLEY,D. H. 1984. Petrographic examination of sedimentary rocksin the SEM usingbackscattered electron detectors. Journal of Ledges) proposed by Irwin (1980)must be treated with Sedimentary Petrology, 54, 877-888. caution, since it is based on data provided by chemical and RAISWELL,R. 1971. The growth of Cambrianand Liassic concentrations. isotopic analysis of bulk samples. Interpretation of such data Sedimentology, 17, 147-171.

Received 27 January 1988; revised typescript accepted 7 November 1988.

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