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Aluminosilicate Diagenesis in a Tertiary Sandstone-Mudrock Sequence from the Central North Sea, Uk

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Aluminosilicate Diagenesis in a Tertiary Sandstone-Mudrock Sequence from the Central North Sea, Uk Clay Minerals (1996) 31, 523–536 ALUMINOSILICATE DIAGENESIS IN A TERTIARY SANDSTONE-MUDROCK SEQUENCE FROM THE CENTRAL NORTH SEA, UK J. M. HUGGETT Department of Geology, Imperial College of Science, Technology and Medicine, London, SW7 2BP, UK (Received 24 November 1995; revised 20 March 1996) ABSTRACT: Mudrocks and sandstones from the Palaeocene of the central North Sea have been studied to assess the petrology, diagenesis and extent of any chemical interaction between the two lithologies. Authigenicand detrital minerals have been distinguished using a variety of electron microscope techniques. Small but significant quantities of authigenic minerals, which would not be detected by conventional petrographic tools, have been detected through the use of high-resolution electron beam techniques. Sandstone mineralogy has been quantified by point counting, and mudrock mineralogy semi-quantified by XRD. The detrital and authigenicmineralogy in the sandstone is almost identical to that found in the mudrock. The principal difference is in the relative proportions. Qualitative mass balance suggests that cross-formational flow has not been significant in either clay or quartz diagenesis. Researchers attempting to quantify the diagenetic SAMPLE MATERIAL relationship between mudrocks and sandstones have concluded that mudrocks act as closed The sampled interval is a cored Palaeocene systems (e.g. Hower et al., 1976; Pearson & sandstone dominated sequence from a well in the Small, 1988; Totten & Blatt, 1993) or that UK sector of the central North Sea. The overall reactants move freely between the two lithologies ratio of sandstone to mudrock is high, *84%, and (e.g. Curtis, 1978; Boles & Franks, 1979; individual mudrock beds average *0.3 m thick. A Awwiller, 1993). Until recently, X-ray diffraction single mudrock unit 30 m thick occurs near the base (XRD) has been the principal investigative tool in of the sampled interval. Although Palaeocene mudrock diagenesis. Consequently, identification sequences of the North Sea are known to have a of authigenicminerals has been by inferencerather high volcanogenic component and numerous water- than by observation. Numerous transmission lain tuffs are recorded elsewhere (Knox & Morton, electron microscopy/analytical electron microscopy 1988), in this sequence no lithologically discrete (TEM/AEM) studies have investigated illite-smec- volcanogenic beds were identified. tite diagenesis, but not in the broader context of The sequence has undergone a simple continuous mudrock diagenesis. In this study, a combination burial history since deposition and now lies of electron microscope techniques, optical micro- between 2200 and 2300 m. The present-day scopy and XRD have been used to examine the average formation temperature, based on TD mineralogy of interbedded marine sandstones and logging bottom hole temperature, is 1078C. mudrocks, buried to *2.3 km. In particular, an attempt has been made to establish whether there METHODS has been any exchange of ionic transfer between the two lithologies. Whole-rock and <4 mm clay fraction XRD analyses The sediments described are from a proximal were carried out on 31 mudrock samples and 11 turbidite fan sequence which prograded from west sandstone samples. The clay fraction samples were to east into the UK sector of the central North prepared by suction onto ceramic tiles (Shaw, Sea. 1972). Samples were scanned on a Philips 1820 # 1996 The Mineralogical Society Downloaded from http://pubs.geoscienceworld.org/claymin/article-pdf/31/4/523/3297601/523.pdf by guest on 26 September 2021 524 J. M. Huggett automated X-ray diffractometer using Ni-filtered Tessier & Pedro (1982) for the latter. The samples Cu-Ka radiation. The operating conditions used and for ion-milling were cut from 30 mm thick thin- methods for semi-quantitative determinations of sections cut normal to bedding so that the mineral components are described in Huggett phyllosilicate 001 orientations were preferentially (1992). Semi-quantitative data are accurate to parallel to the electron beam. Samples for within *10% of quoted values; this assessment of ultramicrotomy were imbedded in medium-hard accuracy is based on repeated analysis of known resin and sectioned using a Reichert Jung Ultracut mixtures of standard clays (which will not E. Duplicate samples from several depths were necessarily produce identical profiles to the prepared by the method devised by Tessier mixtures of detrital and authigenicclays in natural (described in S´ ro´don´ et al., 1990) for fixing mixtures). smectite layers at 12.5À15 A˚ . All TEM samples The abundant illite in the mudrock precluded use were examined in a Jeol 4000FX TEM. Lattice of the precise method of S´ ro´don´ (1980) for fringe measurements were made in the Scherzer identification of the proportion of smectite in underfocus condition to obtain structural informa- illite-smectite. The presence of low-angle illite- tion (Guthrie & Veblen, 1989). Semi-quantitative smectite superlattice reflections precluded use of EDS analyses were obtained from single particles the method of S´rodon´ (1981) for determining the using a Philips T200 with an Oxford Instruments proportion of smectite in illite-smectite because the lithium drifted silicon detector and AN10000 method is only applicable to smectite-rich randomly software. interstratified illite-smectite. Estimates of the illite- smectite composition were therefore obtained from SANDSTONE PETROLOGY measurement of the glycol-treated 002/003 reflec- tion (where it could be clearly identified) after peak Detrital mineralogy deconvolution to separate out the discrete illite component (Moore & Reynolds, 1989). The sandstones are quartz-dominated with subor- Forty-five sandstone samples were examined using dinate amounts of feldspar, rock fragments and clay an optical microscope. Proportions of major compo- matrix. Other detrital components include biotite nents were estimated but no point count data are (variably altered to chlorite), muscovite, heavy available. Samples for petrological analysis in back- minerals, glauconite, organic matter, plus very scattered electron imaging (BEI) mode were prepared rare bioclasts and chlorite grains. by gently polishing rock chips which had been The proportion of mica decreases with depth, vacuum impregnated with low viscosity araldite. The from <5% to <1%. Detrital clay matrix (probably polished samples were examined in a Hitachi S2500 mostly illite and illite-smectite) forms a variable SEM with an Oxford Instruments 860 EDS analyser. proportion of the sandstones, but is typically <5%. Semi-quantitative and quantitative energy dispersive The XRD data show no depth trend in the X-ray analyses were carried out to characterize silt- proportions of illite or illite-smectite (Fig. 1). size particles. Ten sandstone samples were examined Visible porosity is 5À20% and predominantly by fracture-surface SEM, with qualitative EDS to intergranular, some of which may be secondary, confirm mineral identifications. The relatively low generated through leaching of ferroan calcite. On resolution of backscattered electron microscopy average, an estimated 1% of the porosity was (BSEM) is typically x3000, which precludes the generated by feldspar leaching (mostly K-feldspar), obtaining of much useful morphological data on and <0.5% through rock fragment leaching (princi- clays in mudrocks. However, with a field emission pally tuff and chert). Organic content is typically SEM (FESEM), magnifications of 20,000–50,000 are 41% (mean = 0.4%), but ranges up to 3%. routine. Fresh fracture surfaces were platinum coated for FESEM examination. Qualitative EDS analyses Authigenic non-clay minerals were carried out in the FESEM using an Oxford Instruments high-purity germanium detector with The parageneticsequence,so far as it has been Moran Scientific software. established, is summarized in Fig. 2. The principal Mudrock samples for TEM were prepared by ion- authigenicminerals in the sandstones are ferroan beam milling and by ultra-microtomy; see Phakey calcite and quartz overgrowth. The total proportion et al. (1972) for details of the former technique, of non-clay authigenic minerals is typically <3%. Downloaded from http://pubs.geoscienceworld.org/claymin/article-pdf/31/4/523/3297601/523.pdf by guest on 26 September 2021 Aluminosilicate diagenesis in a sandstone-mudstone sequence 525 FIG. 1. (a) Depth plotted against semi-quantitative percentages for illite (solid squares) and illite-smectite (open squares) in the <4 mm fraction of selected sandstone samples. (b) Depth plotted against chlorite semi-quantitative percentages (<4 mm fraction XRD data). Mudrock (solid squares), sandstone (open squares). (c) Depth plotted against semi-quantitative percentages of plagioclase and K-feldspar in the mudrocks. Solid squares = K-feldspar , open squares = plagioclase. (d) Depth plotted against semi-quantitative percentages for illite (solid squares) and illite-smectite (open squares) in the mudrock <4 mm fraction. Data points are accurate to within 10% of the plotted values. (e) Depth plotted against semi-quantitative percentages for kaolinite (solid squares) and chlorite (open squares) in the mudrock <4 mm fraction. Data points are accurate to within 10% of the plotted values in 1aÀe. From textural data the quartz and carbonate packed, finer (5À10 mm) kaolinite platelets occur in cements are inferred to be relatively early, with some samples. These may represent a separate the quartz predating the carbonate. episode of
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