Petrographic Examination and Re-Interpretation of Concretionary Carbonate Horizons from the Kimmeridge Clay, Dorset

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 to be 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 613C 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 the total 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 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/2/345/4897844/gsjgs.146.2.0345.pdf by guest on 25 September 2021 346 RE-INTERPRETATION OF CARBONATEHORIZONS KIMMERIDGEIN CLAY '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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/2/345/4897844/gsjgs.146.2.0345.pdf by guest on 25 September 2021 K. W. A. FEISTNER 347 (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. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/2/345/4897844/gsjgs.146.2.0345.pdf by guest on 25 September 2021 348 RE-INTERPRETATION OF CARBONATEHORIZONS IN KIMMERIDGE CLAY 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 more flattened and distortedfrom 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

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