(Zechstein carbonate), northeast England - Science Direct

Sedimentary Geology, 65 (1989) 285-305

285

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Dolomite calcitization and cement zonation related to uplift of the Raisby Formation (Zechstein carbonate), northeast England MARTIN

R. L E E a n d G I L L M . H A R W O O D

*

Department of Geology, Newcastle University, Newcastle upon Tyne, NEI 7RU (U.K.)

Received February 8, 1989; revised version accepted June 28, 1989

Abstract Lee, M.R. and Harwood, G.M., 1989. Dolomite calcitization and cement zonation related to uplift of the Raisby Formation (Zechstein carbonate), northeast England. In: B.W. Sellwood (Editor), Zoned Carbonate Cements: Techniques, Applications and Implications. Sediment. Geol., 65: 285-305. The Raisby Formation is the basal Zechstein carbonate of northeast England. It has undergone a complex diagenetic history during burial, including multiphase dolomitization and replacement by sulphates. Upon uplift, meteoric-derived fluids have dissolved this replacive sulphate and precipitated calcite cements in the resultant porosity. Study of these cements by cathodoluminescence and trace element geochemistry shows that they are zoned, and different cathodoluminescence zones correlate well with iron and manganese geochemical changes, demonstrating a gradual increase in oxygenation of the precipitating fluids through time. Major diagenetic processes during cement growth include precipitation of anhydrite, fluorite and barite, calcitization of dolomite and internal sedimentation. Relating these processes to cement zonation allows this complex diagenesis to be explained in terms of uplift under the influence of meteoric-derived groundwaters of constantly changing geochemical and electrochemical characteristics.

Introduction The cathodoluminescence characteristics of calcite cements, in conjunction with trace element and stable isotope analysis, have been increasingly used in recent years to help identify and define the composition of pore fluids from which cements were precipitated. This has led in turn to the delineation of sequences of luminescence characteristics which reflect larger-scale processes in the diagenetic evolution of a given b o d y of rock (Meyers, 1974, 1978; Amieux, 1 9 8 2 ; F r a n k et al., 1982; G r o v e r and Read, 1983; Machel, 1985; Meyers and L o h m a n n , 1985; Dorobek, 1987; K a u f m a n et al., 1988; N i e m a n n and Read, 1988; Searl, 1988). In this paper, the diagenesis of the

Raisby Formation, the basal Zechstein c a r b o n a t e of northeast England (Fig. 1) is examined, with reference to controls on the precipitation of late diagenetic calcite cements. It is d e m o n s t r a t e d that the study of zonation patterns within the cements can help elucidate h o w carbonate systems containing evaporites evolve when subjected to meteoric diagenesis. N o previous detailed diagenetic work has been carried out on the Raisby F o r m a t i o n although the diagenesis of the overlying F o r d Formation reef (Fig. 2), h a s been studied b y Aplin (1985), Tucker and Hollingworth (1986), and Hollingworth and Tucker (1987). A more general account of Zechstein diagenesis is given by Clark (1980).

Methods of study * Present address: School of Environmental Sciences, UEA, Norwich, NR4 7TJ, U.K. 0037-0738/89/$03.50

© 1989 Elsevier Science Publishers B.V.

Samples were examined in the field, as polished slabs, and petrographically, using stained acetate

286

M.R. LEE AND G.M. HARWOOD

peels (after Dickson 1965), polished thin sections and etched polished chips mounted on stubs for scanning electron microscope (SEM) examination. Polished thin sections were further studied firstly using cathodoluminescence (CL) with a Technosyn 8200 luminoscope. The cold cathode gun was used at a current of 130-190 microamps, with a 10 mm spot, and a vacuum of 0.05-0.065 Torr. The vacuum chamber was mounted on a Nikon Labophot microscope. All photographs were taken

with an Olympus OM-4 camera, using black and white Ilford XP-1 35 mm ASA 400 film, and colour Fuji H R ASA 200 film, with automatic 20-50 second exposure. Both calcite and dolomite were stable, although most barite rapidly changed from bright to very dull blue luminescence under the beam. Fluorite had a stable bright blue luminescence, but after approximately ten minutes observation in cathodoluminescence, its colour in plane light had changed from colourless to light

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287

DOLOMITE CALCITIZATION AND CEMENT ZONATION

Geological settingof the Raisby Formation

purple, a phenomenon also described by Dickson (1980) in fluorites from Alston, Cumbria. Microprobe analyses were carried out using a Cameca Camebax electron microprobe. Calcite resuits were optimised with a 15 pm rastered beam, at 8 kV accelerating voltage, and a 10 nA beam current. Count times for calcite were 30 seconds on peak and 15 seconds on background. Detection limits were: Ca, 210 ppm; Mg, 135 ppm; Fe, 405 ppm; Mn, 420 ppm; and Sr, 510 ppm. Larger-scale geochemical sampling was done by inductively coupled plasma (ICP) analysis, for which 20 mg samples were extracted from polished slabs using a fine-scale dental drill. The powders were dissolved in 10% hydrochloric acid and analyzed for 22 dements. Detection limits were 0.01 mole% for Ca, Mg and Fe, and 0.0005 mole% for Mn and SrCO 3. Precision of Ca values was not good ( + 0.5 mole%). No stable isotope data for these samples are yet available, although other data from Zechstein carbonates (Harwood, 1981; Harwood and Coleman, 1983; Aplin, 1985) will be used where relevant.

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The Raisby Formation is an almost entirely carbonate sequence, pervasively dolomitized, with thin limestones near the base. It is 20-50 m thick and in many places overlies the Upper Permian Marl Slate, the initial deposit of the Zechstein sea (Smith, 1980). Where the Marl Slate is absent, the Raisby Formation lies unconformably on lower Permian aeolian sands (Rotliegendes) or on eroded Carboniferous. The Raisby Formation is itself overlain by reef and lagoonal carbonates of the Ford Formation (Fig. 2) (Smith, 1980; Smith et al., 1986). The contact between the two is not well exposed, but may well have been related to a basin sea-levd fall, corresponding to the Hampole Beds of Yorkshire (Smith, 1968). Depositional environments of the Raisby Formation range from outer shelf to lower carbonate slope (Smith, 1970a), with the shallowest water sediments being present in the south of the outcrop. Most of the sediments

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288

were originally fine-grained carbonate muds, with rare intercalated turbidites, debris flows and slumps.

Diagenesis The Raisby Formation is overlain by carbonates and evaporites of the first and later Zechstein cycles (Fig. 2). It was the reflux of fluids related to the deposition and diagenesis of overlying formations, which led both to pervasive dolomitization and to replacement by sulphates within the Raisby Formation. Both these events are considered as being of Upper Permian age. Thin primary limestones near the base of the Raisby Formation have largely resisted replacement by dolomite and sulphate. They are well cemented microsparitic mudstones/wackestones and contain rare hardgrounds. Limestone preservation was due to this early micrite/microspar marine cementation. Dolomitization of the Raisby Formation, although multiphase, was due to reflux through the formation of magnesium-rich fluids associated with deposition of the overlying Hartlepool Anhydrite Formation. Replacive sulphates, which postdate dolomitization, vary from isolated millimeter-size euhedrai crystals, to nodules tens of centimeters in diameter (e.g. Harwood, 1981). Replacement was multiphase and due either to calcium sulphate-rich fluids derived from burial dewatering of the Hartlepool Anhydrite Formation (Clark, 1980), or to calcium sulphate formation closely associated with dolomitization. There is no firm evidence for the uplift and exposure of the Raisby Formation after deposition, prior to the Late Cretaceous/Early Tertiary (Smith, 1972). Braithwaite (1988), however, invokes an uplift and emergence of the Permian sequence in northeast England during the Triassic to explain the formation of calcite concretions in Zechstein Cycle 2 carbonates. Similar concretions have been recorded from Zechstein carbonates in northern Europe which show no evidence of uplift, and are still buried to considerable depths; thus the importance of Triassic uplift to the Permian of northeast England is considered negligible. Following a Late Cretaceous/Early Tertiary uplift, the formation was exposed to the influence


of meteoric-derived fluids, which firstly hydrated, then eventually dissolved the sulphates from within the dolostones. Sulphates are almost completely absent from the dolostones at outcrop, and gypsum in general is not present within 100 m of the present-day surface. Anhydrite has not been recorded by us within the Raisby Formation from onshore cores, although anhydrite, in the process of being rehydrated to gypsum, is present in cores from the North Sea 13 km east of the Durham coast at 200 m below the sea floor. Where sulphates have been dissolved from within the Raisby Formation, coarse zoned calcite cements partially or completely fill the resultant cavities. In some cases calcite has partly replaced the precursor sulphate phase, but in most situations at outcrop it is just a passive cement. Study of the late calcite cements will be concentrated on those occurring at Raisby Quarry, the type locality of the formation (Smith et al., 1986). In Raisby Quarry, the cements fill both cavities after sulphates and inter-clast porosity within dolostone breccias. Cement sequences filling cavities after sulphates at other localities within the formation will be referred to for comparison.

The Raisby Formation at Raisby Quarry The Raisby Formation, as exposed in Raisby Quarry,comprises a thin-bedded sequence of carbonates. A thick limestone, nodular in its upper part, lies between a thin basal dolostone, and a much thicker dolostone unit which comprises the rest of the Raisby Formation (Fig. 3). The dolostones are structureless, and unfossiliferous, whereas the limestone has been strongly bioturbated, and contains a sparse fauna of brachiopods, bryozoans, foraminifers and ostracodes. Coarse, angular, clast-supported dolostone breccias occur in places between the dolostone and the top of the underlying limestone in Raisby Quarry (Woolacott 1919a, b) (Figs. 3 and 4). These breccias can be classified as a mosaic packbreccia (Morrow, 1982). Clasts vary from centimeters to meters in diameter, and have had minor relative displacement. The breccias extend for 5 m vertically, but may persist for tens of meters laterally. Both vertically and laterally they

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grade into fractured bedded dolostones. Their contact with the limestone below is sharp; the limestone is neither fractured nor brecciated. Diagenetic fabrics in the Raisby Quarry breccias Most dolostone breccia clasts have been partially or totally calcitized. They are composed of an interlocking mosaic of calcite crystals, 200 # m in diameter, which have inclusion-rich cores showing a mottled CL appearance "and zoned inclusion-free overgrowths (Fig. 5). Most inclusions are dolomite relicts less than 10 #m diameter, although small aggregates of iron oxides/hydroxides such as goethite needles, are common. The calcitized clasts also contain cavities after sulphates. These are ovoid (3-10 cm long) and partially to completely filled by coarse (4000 #m) calcite spar cement (Fig. 6). The dolostone adjacent to this cement has been partially calcitized (Fig. 7). This replacement has selectively affected certain zones within the dolomite crystals, and is

Fig. 4. A typical hand specimen of the Raisby Quarry breccia, with calcitized clasts (C), internal sediment (I), equant (E), and columnar cements (CO). Arrow shows way up.

290

M.R. L E E A N D G.M. H A R W O O D

Fig. 5. Thin section Of a calcitized clast in cathodoluminescence. Each crystal has an inclusion-rich core (C) and inclusion-free,zoned overgrowth (O). equant crystals, three separate zones can be distinguished on the basis of cathodoluminescence characteristics (Fig. 8); zone 1 cements have bright orange luminescence, with darker subzones. This generation of calcite was also responsible for the replacement of dolomite; zone 2 cements have a darker orange luminescence and are sector zoned, and zone 3 is non-luminescent, with some hairline bright orange-luminescent subzones. All columnar crystals (stage 2) are non-luminescent, with no zonation in cathodoluminescence. Layers of stage 2 cements alternate with internal sediments (Fig.

4).

Fig. 6. Hand specimen showing cavities after sulphate from within the Raisby Quarry breccia, now filled with calcite cements. The adjacent dolostone to these cavities is calcitized (C). Arrow shows way up.

Within zone 1 of the equant cements, large square-ended laths are commonly seen both under luminescence and in stained acetate peels (Figs. 7 and 9). These average 400 /~m long, and commonly radiate away from their substrate. These laths closely resemble structures that have been interpreted as pseudomorphs after replacive gypsum (Hollingworth and Tucker, 1987) but their rectilinear outlines, scale and arrangement in radiating aggregates (Fig. 9), suggest pseudomorphs after pore filling anhydrite (A.C. Kendall, personal communication, 1988). The square-ended terminations of the pseudomorphs show that they could not be after another orthorhombic mineral such as barite. The pseudomorphs are composed of calcite which luminesces more brightly than the host cement (Figs. 7 and 9) although is in optical


291

Fig. 7. Cathodoluminescence of the bases of the cavities shown in Fig. 6, where the dolomite ( D ) has been partly caleitized (arrowed), and the cement hosts pseudomorphs after anhydrite (A).

continuity with it. Zoning of the calcites filling and surrounding the pseudomorphs suggests that the anhydrite cements were dissolved and the resultant voids filled after they had been completely enclosed by the host cement. The luminescence of the calcite filling anhydrite pseudomorphs suggests that it was precipitated during zone 1, although the exact timing is uncertain. Fractures filled by both calcite and anhydrite are present within Zechstein Cycle 1 carbonates in offshore

cores, but there is no previous record of anhydrite as a cement from nearshore and onshore exposures in the English Zechstein. Such anhydrites have, however, been recorded from Devonian carbonates of western Canada (Machel, 1986).

Cement geochemistry The individual cement zones have distinctly different trace element geochemistries. Zone 1

Fig. 8. A typical stage 1, equant, clast rimming cement, with all three luminescence zones (1, 2, and 3), and sector zones in zone 2 (arrowed). The cement is overlain by internal sediment ( I ) incorporating clasts of the stage 1 cement. Small squares are sites of microprobe analysis.

292


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Also the orange-luminescing cements (zones 1 and 2) contain much more Sr than zone 3 or stage 2 calcite (Figs. 10 and 11). If the geochemistry of the associated diagenetic components (dolomite clasts, calcitized clasts and internal sediments) is also plotted, clear trends are discernable (Fig. 11). The clasts, from their Mn

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F i g . 11. Trace element geochemistry from ICP analysis, of the major components within the breecias. Insets show in detail differences between luminescent ( z o n e s 1 a n d 2) and non-luminescent, columnar c e m e n t s . Z o n e 3 cements were too narrow to be sampled for I C P analysis.

and Sr contents, were most likely altered during stage 1, zones 1 or 2, whereas the internal sediments were cemented by stage 2 calcite. The high FeCO 3 values for clasts reflect the abundant iron oxide/hydroxide associated with them. However, the clasts are commonly enriched by twice as much Fe as that in the host dolomite, suggesting that the iron was not solely derived from this source.

Interpretation of diagenetic sequences within Raisby Quarry breccias

Sulphate dissolution Dissolution of sulphate within the dolostones was by meteoric-derived fluids undersaturated with respect to calcium sulphate, following uplift of the formation in the Late Cretaceous or Early Tertiary

294

(Smith, 1972; Aplin, 1985; Tucker and Hollingworth, 1986). There is no evidence of any sulphates remaining within the dolostones at Raisby Quarry, although some undissolved gypsum has been recorded from the limestone. Brecciation The structure of the breccias at Raisby Quarry suggests formation by brittle fracture of the dolostones. Brecciation was more or less synchronous with sulphate dissolution, as both cavities after sulphates and inter-clast pores are partly filled by zone 1 calcite cement containing pseudomorphs after anhydrite laths. This occurrence of anhydrite is problematical in that it postdates gypsum dissolution. Possible reasons for anhydrite precipitation will be discussed in the next section. Cavities after sulphate within the breccia fragments all show calcitization below their original bases (i.e. geopetally arranged calcitization; Figs. 6 and 7), regardless of clast displacement. This again suggests to us that at least the main phase of sulphate dissolution must have predated (or was synchronous with) brecciation. The absence of any large interbedded masses of sulphates, apart from minor replacive sulphates in laterally equivalent non-brecciated dolostones, precludes formation of the breccias by solution collapse. However, the margins of some brecciated areas are constrained laterally by major joint sets, showing a degree of control by jointing and/or faulting. Major joint sets are related to the Butterknowle fault, situated a few hundred meters south of the brecciated areas (Smith and Francis, 1967). Brecciation was probably related to fluid flow along fracture sets produced during reactivation of the Butterknowle fault in the Tertiary. The fractured rock may have been more susceptible to dissolution by the calcium-rich, but magnesium-poor fluids, which formed when meteoric-derived groundwaters dissolved gypsum. Preferential late-diagenetic dolomite dissolution along fractures in the Ford Formation was also noted by Aplin (1985). The limestone, which directly underlies the breccias, probably focused the dissolution in this area as it is very well cemented and impermeable.


Anhydrite cementation The first precipitate after brecciation was the anhydrite cement. The precipitation of anhydrite is problematical, given the fact that it fills cavities after dissolved gypsum/anhydrite. High temperatures and/or salinities are recognised as conditions for precipitation of anhydrite (Hardie, 1967; Schreiber et al., 1986; Langbein, 1987). Fluids within the breccias would have been enriched in calcium and sulphate from gypsum dissolution, with some contribution from dolomite and, to a lesser extent, limestone dissolution. In certain microenvironments, concentrations could have been raised enough to precipitate anhydrite due to the common ion effect (Freeze and Cherry, 1979). The more normal precipitation of gypsum or bassanite in this situation could have been inhibited by organic macromolecules derived from the dissolved gypsum, vegetation at the surface, or both (Cody and Hull, 1980; Sonnenfeld, 1984). Maliva (1987) described anhydrite nodules formed by the interaction of sulphate-rich fluids with calcium released during dolomitization of the host carbonate. His anhydrite was precipitated at both low temperatures (25°C) and shallow burial depths (30-40 m). An alternative and perhaps more attractive hypothesis is that anhydrite was precipitated from calcium- and sulphate-rich fluids at elevated temperature. These fluids were probably associated with nearby epigenetic fluorite and barite mineralization in the Ferryhill area (6 km southwest of Raisby Quarry) and along the Butterknowle fault, immediately south of Raisby Quarry (Fig. 1) (Hirst and Smith, 1974; Harwood, 1983). In these localities, where both minerals occur, fluorite always predates barite. Fluid inclusions in fluorite from Chilton quarry give a temperature of around 100°C, whereas barite from Ferryhill was formed at temperatures of 70°C or less (Hirst and Smith, 1974). Sulphur isotope analyses from the barite gives 634S of + 9.3%o (Solomon et al., 1971), similar to values obtained from barites in the Zechstein of Yorkshire (Harwood, 1981; Harwood and Coleman, 1983). These values are very close to both those of Permian evaporites and Permian


295

Fig. 12. Cathodoluminescence photographs from the same cavity formed after sulphate dissolution from Chilton Quarry (see also Emery and Marshall, 1989). Fluorite ( F ) luminesces bright blue. Photograph (b) demonstrates the relative timing of mineralization. A geopetal internal sediment of fluorite overlies the initial dull orange-luminescing cement zone. The trend of zonation is from dull orange- (D) to non-luminescence. Here, the bright orange zone, normally overlying dull zone at other localities, is represented by closely spaced orange-luminescing hairline zones (H), overlying the dull orange-luminescing zone. Note that the dominantly non-luminescent calcite is a columnar crystal, and the change in luminescence and crystal habit occurs at the point of internal sedimentation of fluorite, suggesting either a hiatus in calcite precipitation, or a major change of fluid flow within the cavity at this point.

296

seawater (Harwood and Coleman 1983). Hirst and Smith (1974) suggested that the fluorite formed from the interaction of basinal-derived fluorine with local calcium, and that the barite originated from the interaction of hot BaC12-bearing fluids (derived from the underlying Carboniferous), with sulphate from the dissolution of gypsum or anhydrite within the Raisby Formation. At Chilton Quarry (Fig. 1) fluorite occurs within cavities formed by the dissolution of sulphates. There mineralization predates, or is synchronous with, the initial, dark orange-luminescing cement zone (Fig. 12a, b), suggesting that it was broadly contemporaneous with the anhydrite cementation in Raisby Quarry. It is, therefore likely that anhydrite, fluorite and barite were all partially sourced by the same fluids of elevated temperature interacting with sulphate-rich fluids (or the sulphates themselves) within the Raisby Formation. Calcite cementation and dolomite calcitization

Using trace element data in conjunction with luminescence characteristics, it is possible to interpret the geochemical conditions of the environments in which the cements were precipitated, specifically the redox conditions (pc (or Eh)), which affect the amount of iron and manganese incorporated into the cements (cf. Evamy, 1969, Meyers, 1974, Oglesby, 1976, Amieux, 1982). The amount of Mn and Fe within the calcite lattice is considered to be the major control on luminescence intensity, although the exact controls are still poorly understood (Machel, 1985; Mason, 1987). Recent research has shown that the incorporation of trace elements into a fluid, or into cements derived from that fluid, may be controlled by numerous factors apart from pc. These include temperature and pH of the fluids (Harris et al., 1985), diagenesis of clays and organic matter scavenging trace elements (Machel, 1985; Searl, 1988), and distribution coefficients of trace elements within the fluids (assuming equilibrium conditions exist) (Harris et al., 1985). The relative magnitudes of the distribution coefficients themselves may depend on crystal growth rate, coupled substitution, the nature of the overall reaction, and temperature (Given and Wilkinson, 1985;

M.R. L E E A N D G.M. H A R W O O D

Reeder and Grams, 1987). Mason (1987) added pressure and composition of the host crystal to this list, and also suggested that crystal growth rate and redox chemistry may interact to vary trace element incorporation, as both are partly pH dependent. This study is concerned with generalized cement evolution over a large extent of outcrop, in contrast to most of the above-mentioned factors which control precipitation only over a small area, such as within a pore or between growth sectors of a crystal (Reeder and Prosky, 1986; Reeder and Grams, 1987). Here, the major factor controlling Fe and Mn incorporation we consider to be pe, as this variation would probably have the largest and most significant affects on meteoric-derived cement precipitation in the Raisby Formation. In a similar manner, Grover and Read (1983), Dorobek (1987) and Barnaby and Rimstidt (1988) concluded that redox conditions are the dominant factor controlling iron and manganese contents within calcite cements in carbonate aquifers. Luminescence characteristics suggest that calcitization of dolomite was synchronous with cement zone 1. These calcites contain relatively high manganese, but low iron (Fig. 10). The precipitating fluid may have been oxidizing, with iron precipitating out as iron oxides/hydroxides. These are common by-products of dolomite which has been calcitized within an oxidising environment (A1-Hashimi and Hemingway, 1973; Frank, 1981). Frank et al. (1982), Harris et al. (1985) and Dorobek (1987) showed that Mn 2÷ can be incorporated into calcite synchronous with iron oxidation, as Mn 3+ is more soluble over a wider range of pe conditions than Fe 3+. However, iron may also have been removed from the fluid by H2S-scavenging, to form pyrite, or marcasite, in a reducing environment (Frank et al., 1982). With a change to higher pe conditions iron sulphides would have oxidized to form the iron oxides/hydroxides seen at outcrop. This latter possibility is considered more likely. Within the Raisby Formation, calcitization of dolomite was achieved by calcium-enriched fluids, as described both from first-cycle Zechstein carbonates in Yorkshire (Smith, 1976; Harwood, 1986; Kaldi, 1986), and the Ford Formation

DOLOMITE

CALCITIZATION

AND CEMENT

ZONATION

(Smith, 1981; Aplin, 1985). Calcitization in such a relatively near-surface environment would also be in accordance with the views of De Groot (1967), who stressed the importance of high fluid flow, low partial pressure of CO 2 and temperatures less than 50°C. Fluid evolution during dolomite calcitization was probably similar to that described by Hanshaw and Back (1979) and Back et al. (1983). Their models show that, in a carbonate aquifer containing calcite, d o l o m i t e and gypsum, groundwaters evolve to the point where they are simultaneously supersaturated with respect to calcium and carbonate (from dissolution of calcite, dolomite and gypsum), but are undersaturated with respect to magnesium. In order to maintain equilibrium, dolomite is dissolved and calcite precipitated due to the common-ion effect. This precipitation lowers pH, calcium and carbonate concentrations, thus causing further dolomite dissolution. Thus, there is an incongruent dissolution of dolomite (simultaneous dissolution of dolomite and precipitation of calcite), i.e. calcitization. In this model calcium availability is the controlling factor of calcitization, a view which is in agreement with the hypotheses of Kastner (1982), Back et al. (1983), Budai et al. (1984), and Stoessell et al. (1987). The stage 1 zone 2 cements are darker orange luminescing than zone 1, corresponding to a greater Fe 2÷ (although similar Mn 2+) concentration, and thus a higher Fe2÷:Mn 2÷ ratio (Fig. 10). This indicates a change from zone 1 to less reducing conditions. The reduction of p H accompanying calcite precipitation may also have led to incongruent dissolution of more dolomite, thus liberating more Fe 2÷ and Mn 2÷ into the system, and making both available for incorporation into calcite. The outer parts of zone 2 cements are commonly sector zoned (Fig. 8). All of the sector zones luminesce darker orange than the host cement, correlating with depletion in both magnesium and manganese. All stage 1 zone 3, and stage 2 cements are non luminescent, containing little Fe, and negligible Mn (Figs. 10 and 11). Trace element variations thus suggest change to a higher fluid pc, preventing incorporation of Mn3÷/Mn4+ and Fe3+into the calcite lattice. Such a pattern of orange- to

297

non-luminescent cement zones is termed a "positive sequence" by Amieux (1982). Within zone 3, and stage 2 cements however, hairline brightly luminescent zones are common (Fig. 12a, b). Dorobek (1987) suggested such subzones reflect fluctuations in pe related to temporary aquifer stagnation. However, as the pores were gradually being occluded, the system may, at times, have become sufficiently dosed enough to produce pe decrease due to iron and manganese oxidation, pyrite/marcasite oxidation or oxidation of organic matter introduced into the pores. Commonly there is evidence of corrosion at the contact of zone 2 with zone 3 or stage 2 cements, suggesting fluctuating levels of calcite saturation during precipitation of non-luminescent calcite. Internal sedimentation within the breccias terminated precipitation of zone 3 (Fig. 8). The change from dull orange-luminescent calcite of zone 2 to non-luminescent calcite of zone 3 prior to the onset of internal sedimentation, perhaps indicates a genetic association with a major change in fluid input through the breccias, which was responsible for triggering internal sedimentation. These fluids were of higher pe than those which precipitated the zone 2 cements, resulting in the precipitation of a non-luminescent calcite (zone 3). Such an abrupt change in pore waters could help explain why the zone 2 cements are not overlain by a bright orange-luminescing zone (low Fe, high Mn), the expected sequence to follow on from zone 2 if there was a gradual increase in oxygenation of the fluids (Frank .et al., 1982).

Magnesium enrichment. One anomalous feature in the Raisby Quarry breccias, and in cements elsewhere in the formation, is that most nonluminescent cements have a higher MgCO 3 content than earlier orange-luminescent calcites, commonly by 3 mole% (Fig. 11). Such enrichment is normally restricted to columnar cements (stage 2), and is not common in non-luminescent parts of the equant calcite (zone 3). There has been little previous description of magnesium content of calcites influencing luminescence. Mason (1987) found no relationship between magnesium and luminescence. However, Harwood (1981) suggested that, in late-diagenetic

298

cements in cavities after sulphates in first-cycle Zechstein carbonates of Yorkshire, MgCO3 contents greater than 2.5 mole% inhibit luminescence. Within the Raisby breccias, MgCO3 contents reach 3 mole%, and similar values are recorded within non-luminescent cements elsewhere in the formation. The reciprocal relationship of Mg to combined Fe and Mn concentrations (Figs. 10 and 11) is not solely due to Mn2÷ and Fe 2÷ substituting in the Mg 2+ sites within the calcite lattice. Increase in magnesium could also be due to the cessation of some process which acted to remove Mg 2÷ from the precipitating fluids. Drever (1971) suggested that Mg 2÷ could be removed from interstitial seawater by cation exchange with iron in clay minerals. The released iron then combines with sulphide generated during sulphate reduction. Sulphate reduction may have been active during early stages of cement precipitation at Raisby Quarry, but this is not considered important during non-luminescent cement precipitation. Magnesium could have also been removed by late diagenetic dolomite precipitation (Folk, 1974), but there is no unequivocal evidence for this. Magnesium enrichment is thus due to some factor which controls its partition from the fluid into growing columnar calcite crystals, and which is coincident with increasing pe acting to inhibit Fe and Mn incorporation into the calcite. Published literature on the incorporation of magnesium into calcite in seawater solutions identifies controls which include; the Mg : Ca concentration ratio of the solution, and surface effects (Mucci and Morse, 1983), inhibition due to sulphate (Mucci et al., 1989) and phosphate (Moiler and Werr, 1972). Folk (1974) suggested that magnesian calcites commonly have acicular habits because of magnesium poisoning, inhibiting sideways crystal growth. Given and Wilkinson (1985), suggested that the magnesium content of certain varieties of calcite, including those precipitated from meteoric-derived fluids (similar to those within the Raisby Formation paleo-aquifer), is a product of both concentration of magnesium in the fluid and crystal growth rates along the c-axis direction. Magnesium incorporation into the crystal lattice is


enhanced by rapid c-axis growth, even with relatively low concentrations of magnesium in the fluid. They state that, in diagenetic environments where Ca 2+ is in excess of COg- within the precipitating fluid, growth of calcite in the c-axis direction is a function of COg- concentration and supply. With a relatively high rate of COg- supply, c-axis growth rates will be increased, and so elongate crystal habits (i.e., columnar) result. To produce columnar crystals, which are the last precipitates where they occur (both in the Raisby Quarry breccias, and elsewhere in the formation), rates of COg- supply must have been high in the upper parts of the Raisby Formation paleo-aquifer. The isopachous nature of the columnar crystals also suggests precipitation in the phreatic zone (Longman, 1980). Calcite precipitation in the lower parts of the freshwater vadose/upper phreatic zone is commonly due to rapid CO2 degassing adjacent to the water table (Harris et al., 1985; Searl, 1988). In this environment the supply of COg- was enhanced by disassociation of HCO 3 due to CO2 degassing (Given and Wilkinson, 1985). These findings are supported by data from a modern aquifer containing dolomite, limestone and gypsum (Back et al., 1983). In this example both CO3: and M g : C a ratios are highest in the recharge area, decreasing down-flow. Therefore, magnesium-enriched columnar cements in the Raisby breccias, and at other localities in the formation, imply an enhanced COg- :Ca 2÷ ratio, relative to conditions prevailing during earlier, equant cement precipitation. They are probably non-luminescent because, in the upper phreatic levels of the aquifer, where COg- supply was highest, fluids would have been oxidizing, and thus Mg enrichment was coincident with iron and manganese oxidation (Table 1). Discussion

Comparison with sequences in other parts of the Raisby Formation Cement sequences within cavities after sulphates in other parts of the formation are variable and differ from those developed in the Raisby Quarry

299


TABLE 1 Correlation between the cement sequence in the Raisby Quarry breoeias, and other localities in the formation (Chilton, Houghton-leSpring and Sherbum Hill Quarries) RAISBY BRECCIAS PRECIPITATE STAGE 2

RELATIVE pe

CALCITE CEMENT (Non-luminescent) (Ca, Mg) CO3

OTHER LOCALITIES (COMPOSITE) PRECIPITATE

CRYSTAL HABIT

HIGH

CGL~

.

.

CALCITE CEMENT (Non-luminescent with bright orange hairline subzones) (Ca, Mg (Mn)) CO3

RELATIVE pe

CRYSTAL HABIT

HIGH

COLUMNAR

(Internal sedimentation) ..... STAGE 1 ZONE3

.

CALCITE CEMENT (Non-luminescent with bright orange luminescent subzones) (Ca (Mn)) CO3 ....

STAGE 1 ZONE2

(corrosion).

.

.

.

HIGH

.

(corrosion)_ CALCITECEMENT (Non-luminescent with bright orange hairline subzones) (Ca (Mn)) CO3

(corrosion). . . . .

CALCITE CEMENT (Dull orange luminescent) (Ca, Fe, Mn) CO3

.

.... LOW

EQJN~

HIGH

(corrosion). . . .

CALCITE CEMENT (Bright orange luminescent) (Ca, Mn) CO3

HIGH

Goethite & Limonite

STAGE 1 ZONE1

CALCITE CEMENT (Bright orange luminescent) (Ca, Mn) CO3

VERY LOW

EQUANT

CALCITE CEMENT (Dull orange luminescent) (Ca, Fe, Mn) CO3

?

LATH

BARITE & FLUORITE (BaSO4) (CaF2)

LOW

Pyrite, (dolomite calcilization)

ANHYDRITE CEMENT (CaSO4)

breccias. The variability is understandable considering the large area of outcrop under study, although all cavities attest to the same trend of fluid evolution. Most cavities at outcrop show a thin layer of coarse, equant calcite crystals (Fig. 13). Where cavities are incompletely filled, cements show an evolution from luminescent to nonluminescent calcite (Fig. 14). In many cavities, only the equant calcite is developed, but columnar cements overlie them at some localities. Geochemically, the cements filling cavities are very similar to those at Raisby Quarry. Orange-luminescent cements have Fe, Mn and Sr with little Mg, whereas non-luminescent columnar cements are enriched in Mg with little Fe, Mn or Sr. Invariably the orange-luminescent cements are dull orange (Fe- and Mn-rich) at the cavity margins, and pass

R(~EI'rE & CUBIC

to bright orange (Fe-poor, Mn-rich), to nonluminescent (Fe-poor, Mn-poor). This fits very well predicted and documented sequences of cement precipitation from fluids of increasing pe (Frank et at., 1982; Grocer and Read, 1983; Dorobck, 1987). In cements that occlude cavities after sulphates within the Raisby Formation at outcrop, the changes in luminescence were mainly controlled by pe, and not other factors such as supply of Fe and Mn. This is demonstrated by the occurrence of limonlte and goethite within calcites. These minerals are considered to be primary precipitates, and not weathering products after iron sulphides. They occur at the contact of dull and bright orange-luminescing cement zones (Fig. 15). Iron was not incorporated into the calcite lattice during

300

M.R. LEE A N D G.M. H A R W O O D

i

J !

Fig. 13. A typical cavity after sulphate from Houghton-le-Springat outcrop, lined by equant calcite crystals.

precipitation of the bright orange-luminescing cements, because partial oxidation had occurred. Reduced iron is present in earlier dark orangeluminescing zones (Fig. 10). The exact timing of oxidation relative to precipitation of the bright orange-luminescing zones is variable, and commonly the iron minerals encrust a corroded surface between bright orange- and non-luminescent zones. This suggests other factors in addition to pe

controlled the exact timing of iron oxidation and precipitation, perhaps pH, and concentrations of other anions and cations in solution (Stumm and Morgan, 1970). In some cavity-lining cements, notably in exposures around Sherburn Hill (Fig. 1), the initial dull orange-luminescing zone contains anhydrite relics 40 # m in length. They indicate that firstly, the initial stages of calcite precipitation predated

Fig. 14. Cathodoluminescenceof the equant cement in Fig. 13, with evolution from dull- to bright orange-, to non-luminescent zones.

DOLOMITE

CALCITIZATION

AND CEMENT

ZONATION

301

Fig. 15. Calcite cements filling a cavity after sulphate from Chilton Quarry, showing faces of a calcite crystal encrusted with goethite (G). This occurrence of goethite suggests that it grew on the calcite crystal substrate during a hiatus in calcite cement growth. The encrusted surface marks the junction between orange-luminescent and non-luminescent cement.

complete sulphate dissolution, and secondly that the calcite has partly replaced, or enclosed the remnant sulphate. The reaction m a y have been bacterially mediated (Harwood and Coleman, 1983; Pierre and Rouchy, 1988). Carbon and oxygen stable isotope data of cavity-lining calcite cements, similar to those of the Raisby Formation, from the Ford Formation reef (Aplin, 1985) typify a meteoric water with 8180 of - 6 to -5%0 and 813C of - 8 to -6%0. Aplin (1985) further describes uplift-related calcitized dolostones in the Ford Formation which are similar to calcitized dolostones in the Raisby Quarry breccias, that also have a typical meteoric signature with 81s0 of - 6 . 7 to -5.6700 and 813C of - 7.7 to - 5.4%0.

Implications for geochemical evolution The cathodoluminescence patterns and corresponding trace element sequences in the Raisby Formation are best interpreted in terms of uplift during the Tertiary under the influence of meteoric-derived groundwaters; thus, over time, any one pore was in contact with increasingly more oxygenated fluids (Fig. 16). Increase in oxygenation of groundwaters within an aquifer with proximity to the point of recharge is c o m m o n

(Champ et al., 1979; Freeze and Cherry, 1979; Drever, 1982). These changes in pe are controlled by redox reactions, and proximity to areas of oxygen input. At any one time, dolostones close to recharge will have no remaining gypsum, and their cavities after sulphates will form the site for precipitation of non-luminescent calcite cements. Down-dip in the subsurface, similar cavities will have initial phases of orange-luminescent calcite precipitated, while below these gypsum a n d / o r anhydrite will be minimally affected by meteoricderived fluids (Fig. 16). Thus, the cement evolution over time also reflects the geochemical gradient developed in meteoric-derived fluids within the formation at any one time. This agrees with models for fluid evolution in a carbonate aquifer containing gypsum, dolomite and limestone proposed by Hanshaw and Back (1979) and Back et al. (1983), where concentrations of Ca, Mg, SO 4 and CO 3 in the fluids are related to complex interactions between dissolution and reprecipitation of a number of minerals. In the Raisby Formation paleo-aquifer these processes were integrated with the evolution of electrochemical patterns. This explains the relative abundances of iron and manganese in the cements, and so, broadly, cathodoluminescence characteristics (Fig. 16). The cement sequences

302


Distribution of cement types

Variation in Ion concentration

Relative concentration of celclte, dolomlte, and gypsum In groundwater

soil zone water table

tJ

tO.. ¢J .m r._

o

stage zone

OO

t5-

Anhydrite cement

Limit of groundwater penetration

G

Gypsum

m Decreasing Ca Mg S04 Fe Mn Sr increasing Mg:Ca HC03

:_3-

I

i

'

.-" I

i /i "O 02

"10 02

-i

02 Q.

Fig. 16. Model for fluid evolution within the Raisby Formation, based on the sequence in the Raisby Quarry breccias.'The vertical axis represents distance from the surface, but may also represent evolution through time of any one cavity. The lowermost cavity is completely filled by gypsum, which is itself the product of rehydration of anhydrite. Contact with meteoric-derived fluids as the surface is approached causes dissolution of gypsum. Locally, anhydrite cements were reprecipitated in the cavities after complete gypsum dissolution. The succeeding cavities illustrate progressive occlusion of the porosity by calcite cements of changing composition, from stage 1 to stage 2. This reflects increasing oxygenation of the precipitating fluids. The expected variations in composition of the fluids, and saturation states of calcite, dolomite and gypsum is also shown, based on work from modern aquifers (Back et al., 1983).

illustrated are not isochronous between cavities within the formation. They do, however, reflect the same general pattern of fluid evolution, which in detail was variable in both space and time.

Summary Information from cathodoluminescent zoning, and trace element composition of cement crystals


has enabled the late-diagenetic evolution of the Raisby Formation to be better understood. Three main conclusions can be drawn from this study: (1) During initial phases of uplift of the Raisby Formation there was a complex interplay between sulphate dissolution and precipitation of carbonates. Locally, elevated fluid temperatures led to precipitation of anhydrite, fluorite and barite before calcite cementation began. (2) Calcitization of dolomite took place under the influence of calcium-rich and magnesium-poor meteoric-derived fluids, contaminated with calcium from gypsum dissolution. (3) If it is assumed that pe is the main control (in a carbonate aquifer) on iron and manganese incorporation into the calcite lattice, then diagenetic events in the Raisby Formation can be fitted into a chronological framework. In any one pore, the sequence of cement zones developed attests to a progressive increase in pe of the precipitating fluids. This in turn, relates to a regional variation in electrochemical characteristics of the host fluids and, broadly, proximity to the surface or recharge area. Acknowledgements We are grateful to Peter Oakley (Newcastle) for help with the ICP analyses, which were performed at Royal Holloway and Bedford New College, and Stuart Kearns and Pete Hill (Edinburgh) for their assistance with the electron microprobe. We would also like to thank Christine Jeans for drafting the diagrams, Trevor Whitfield for making the thin sections, Dr. D.B. Smith for help in the field, plus Dr. M.C. Akhurst, and Dr. A.C. Kendall for their helpful discussions. We further acknowledge constructive comments from Bruce Sellwood and the two (anonymous) reviewers. M.L. acknowledges receipt of a research studentship from the Natural Environment Research Council (No. G T 4 / 8 6 / GS/133). References A1-Hashimi, W.S. and Hemingway, J.E., 1973. Recent dedolomitization and the origin of the rusty crusts of Northumberland. J. Sediment. Petrol., 43: 82-91.

303 Amieux, P., 1982. Cathodoluminescence: method of sedimentological study in carbonates. Bull. Cent. Rech. Explor. Prod. Elf-Aquitaine, 6: 437-483. Aplin, G.F., 1985. Diagenesis of the Zechstein main reef complex, N.E. England. Ph.D. Thesis, University of Nottingham (unpublished). Back, W.M., Hanshaw, B.B., Plummer, L.N., Rahn, P.H., Rightmire, C.T. and Rubin, M., 1983. Process and rate of dedolomitization: mass transfer and I4C dating in a regional carbonate aquifer. Geol. Soc. Am. Bull., 94: 1415-1429. Barnahy, R.J. and Rimstidt, D.J. 1988. Correlation of calcite cathodoluminescence with redox conditions of carbonate cementation. Abstr., B.S.R.G. Conf., Analysis and Interpretation of Zoned Calcite Cements. Braithwaite, C.J.R., 1988. Calcitization and compaction in the Upper Permian Concretionary Limestone and Seaham Formations of North-east England. Proc. Yorks. Geol. Soc., 47: 33-45. Budai, J.M., Lohmann, K.C. and Owen, R.M., 1984. Burial dedolomite in the Mississippian Madison Limestone, Wyoming and Utah thrust belt. J. Sediment. Petrol., 54: 276-288. Champ, D.R., Gulens, J. and Jackson, R.E., 1979. Oxidation-reduction sequences in ground water flow systems. Can. J. Earth Sci., 16: 12-23. Clark, D.N., 1980. The diagenesis of Zechstein carbonate sediments. In: H. Fiichtbauer and T. Peryt (Editors), The Zechstein Basin with Emphasis on Carbonate Sequences. Contributions to Sedimentology, 9. pp. 167-203. Cody, R.D. and Hull, A.B., 1980. Experimental growth of primary anhydrite at low temperatures and water salinities. Geology, 8: 505-509. De Groot, K., 1967. Experimental dedolomitization. J. Sediment. Petrol., 37: 1216-1220. Dickson, J.A.D., 1965. A modified staining technique for carbonates in thin section. Nature, 205: 587. Dickson, J.A.D., 1980. Artificial colouration of fluorite by electron bombardment. Mineral. Mag., 43: 820-822. Dorobek, S.L., 1987. Petrography, geochemistry and origin of burial diagenetic facies, Siluro-Devonian Helderberg Group (carbonate rocks), central Appalachians. Am. Assoc. Pet. Geol. Bull., 71: 492-514. Drever, J.I., 1971. Magnesium-iron replacement in clay minerals in anoxic marine sediments. Science, 172: 1334-1336. Drever, J.l., 1982. The Geochemistry of Natural Waters. Prentice-Hall, Engiewood Cliffs, N.J., 388 pp. Emery, D. and Marshall, J.D., 1989. Zoned calcite cements: has analysis outpaced interprepation? In: B.W. Sellwood (Editor), Zoned Carbonate Cements: Techniques, Applications and Implications. Sediment. Geol., 65:205-210 (this issue). Evamy, B.D., 1969. The precipitational environment and correlation of some calcite cements deduced from artificial staining. J. Sediment. Petrol., 39: 787-793. Folk, R.L., 1974. The natural history of crystalline calcium

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