(Zechstein carbonate), north-east

Downloaded from http://pygs.lyellcollection.org/ at University of Glasgow on October 17, 2015 P R O C E E D I N G S O F T H E Y O R K S H I R E G E O L O G I C A L S O C I E T Y , V O L . 4 8 , P A R T 4 , P P . 4 0 9 - 4 1 4 , 1991

New evidence for the origin of distinctive quartz overgrowth textures in the Raisby Formation (Zechstein carbonate), north-east England M.

S. BENZAGOUTA and M .

R.

LEE

Petrographic study of a dolomitic sandstone within the Upper Permian Raisby Formation in Northumberland has revealed distinctive quartz overgrowth textures on detrital sand grain surfaces. These textures, comprising discontinuous ridges of authigenic quartz in a regular pattern, have previously been interpreted to be the result of replacement of quartz by carbonate crystals. However, rare textural inter-relationships between dolomite and quartz in this sandstone demonstrate that the ridges result from the growth of authigenic quartz along narrow pore spaces between dolomite crystals. This process may have important consequences for the petrophysical characteristics of mixed carbonate - siliciclastic reservoir rocks. SUMMARY:

For many years, considerable attention has been paid to the role of replacement and corrosion of silicate minerals, especially quartz, by carbonate crystals, in relation to the petrophysical characteristics of reservoir sandstones (for example Dapples 1971). Burley & Kantorowicz (1986 a, b) provide scanning electron microscope (S.E.M.) petrographic evidence for replacement of detrital and authigenic quartz during the precipitation of carbonates. Following the dissolution of these pore-filling and replacive carbonate crystals, owing to their previous replacement of silicate grains, a greater secondary porosity is created than was present prior to carbonate precipitation (Burley & Kantorowicz 1986a). Thus, the identification of silicate grain replacement is potentially of importance in the understanding of porosity evolution. The aim of this paper is to re-evaluate some of the textural evidence previously cited as indicative of the replacement of silicate minerals by carbonates. 1.

submarine resedimentation which has been widely recognized close to the top of the Raisby Formation in north-east England (Smith 1970). The sand grains were

DESCRIPTION

A detailed petrographic study was made of a thin sandstone within the lower part of basal Zechstein (Upper Permian) dolostones of the Raisby Formation, north-east England. The sandstone is exposed at Tynemouth Castle Cliff, Northumberland (Fig. 1). At the north-east corner of the cliff (National Grid Reference N Z 3759 6961) the Raisby Formation is 3.8 metres thick (Smith 1970). The sandstone itself is 1.2 metres thick at this point, with its base 0.9 metres above the base of the Raisby Formation (Smith 1970) (Fig. 2), and thins in a north-west direction along the cliff (Land 1974). The sandstone is parallel laminated with alternating coarser and finer laminae, and fines upwards overall. It was deposited as a mixture of carbonate mud and detrital sand grains during an episode of large-scale

Fig. 1.

Geological map of north-east England illustrating the outcrop of the Raisby Formation and the location of Tynemouth Castle Cliff (modified from Smith 1980).

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M. S. B E N Z A G O U T A & M. R. LEE

410

UTHOLOGY

SEDIMENTARY

INTERPRETATION

STRUCTURES

16-1-

COLLAPSE BRECCIATED CONCRETIONARY LIMESTONE FORMATION (CYCLE 2 CARBONATE) RESIDE OF THE HARTLEPOOL ANHYDRITE FORMATION (ZECHSTEIN CYCLE 1 EVAPORITE) RAISBY FORMATION (ZECHSTEIN CYCLE 1 CARBONATE)

MARL SLATE FORMATION (ZECHSTEIN CYCLE 1 CARBONATE)

BASAL PERMIAN SANDS (LOWER PERMIAN AEOLIAN SANDSTONES)

WESTPHALIAN - LOWER PERMIAN UNCONFORMITY

i^S

BRECCIA

l

PLANAR BEDDING

Q

Fig. 2.

C

1

§ ,

NORMALLY GRADED BEDDING

=j

HORIZONTAL LAMINATION

>^

CROSS BEDDING

Summary stratigraphic and lithological log of Permian strata exposed at Tynemouth Castle Cliff (compiled from Smith 1970 and Land 1974).

derived by reworking of the earlier, predominantly aeolian, Basal Permian Sands following deep scouring during resedimentation (Smith 1970). The sandstone at Tynemouth Castle Cliff is as a whole both texturally and mineralogically mature, and dominated by very coarse sand-grade quartz grains in a groundmass of dolomite. Some parts of the sandstone also contain dolomitized carbonate mudstone intraclasts and scarce dolomitized bryozoan fragments and foraminifera. Coarse calcite cements now occupy the centres of many of the larger carbonate mudstone intraclasts and irregular patches throughout the dolomite groundmass. The quartz is mostly monocrystalline, although some polycrystalline grains have been recorded. Grains of both feldspar and mica are absent, although scarce rock fragments containing quartz, feldspar and mica do occur. Dolomite forms 50/xm to 70fxm sized sub-euhedral crystals which are zoned both in cathodoluminescence, and with regard to inclusions. The dolomite crystals have bright orangeluminescent inclusion-rich cores and dull redluminescent inclusion-free rims. Electron microprobe analysis also shows that they are nearly stoichiometric in

composition, with insignificant concentrations of iron and manganese (Lee 1990). Most quartz grains are supported within, and completely surrounded by dolomite, although a few do have grain-to-grain contacts at which points some pressure solution has occurred. In most cases, the junction between dolomite and quartz is delineated by the original shape of the detrital grain. There is some evidence for corrosion and pitting of grain surfaces on a scale of 10/utm to 15/Ltm, before or during dolomite precipitation. However, at some points the surrounding dolomite is absent, or has spalled off into adjacent pore-space which is now cemented by large, non-luminescent calcite crystals (Fig. 3). In these areas where quartz is not encrusted by dolomite or in contact with other quartz grains, syntaxial overgrowths of authigenic quartz have developed on the detrital grains (Figs 3, 4). In some examples, the quartz overgrowths have formed around isolated dolomite crystals, partially or completely enclosing them (Fig. 4). The precipitation of quartz overgrowths therefore postdates dolomitization. When seen under S.E.M., the surfaces of many detrital quartz grains are covered by discontinuous, interconnecting ridges of authigenic quartz (Figs 5, 6). The distribution of these ridges defines irregular to vaguely rhomb-shaped areas whose size and shape suggests that formation of the ridges was controlled by dolomite crystals.

2.

MINERAL PARAGENESIS

Petrographic study of the Tynemouth Castle Cliff sandstone has revealed a complex diagenetic history. Following deposition, the sandstone comprised quartz grains supported within a matrix of fine-grained carbonate mud. This carbonate mud then underwent dolomitization. Evidence that the dolomite is replacive in origin comes both from the anhedral shape of individual dolomite crystals, and their inclusion-rich cores. At some time after dolomitization, syntaxial cements of authigenic quartz precipitated on detrital quartz grain surfaces. Precipitation of significant amounts of authigenic quartz was only possible where the dolomite, previously surrounding detrital quartz grains, was no longer present. The last episode of diagenesis saw the precipitation of coarse pore-filling calcite cements which occluded secondary porosities within the dolomite groundmass. 3.

DISCUSSION

The quartz surface textures illustrated here (Figs 5, 6), are very similar to some described by Burley & Kantorowicz (1986a), and interpreted by them as being indicative of the replacement of quartz overgrowths by authigenic carbonates. They suggested that the replacement took place along a thin fluid film between precipitating carbonate and dissolving quartz, and was controlled by the reaction rate at the solid - fluid interface (surface reaction controlled replacement). Such replacement has been described as 'reversed


ZECHSTEIN Q U A R T Z

OVERGROWTHS

411

Fig. 3. Photomicrograph of the contact between a detrital quartz grain (DO), quartz overgrowth (QO), dolomite (D) and calcite cement (C). In the lower centre of the field of view, room for a quartz overgrowth has been provided by spalling off to dolomite into pore-space, now cemented by a single coarse calcite crystal (which is non-luminescent when seen in cathodoluminescence). Thin section, plane-polarised light. Arrow indicates way up.

Fig. 4. Backscattered electron micrograph of a polished surface of the Tynemouth Castle Cliff sandstone. The image shows the outer margin of a detrital quartz grain (DQ) partially overgrown by authigenic quartz (QO). The remainder of the pore is cemented by calcite (C). Dolomite (D) has inhibited the growth of authigenic quartz in some places, whereas in others it has been completely engulfed (arrowed). This relationship gives definitive evidence that quartz overgrowths post-date dolomitization in this case.


412


Fig. 5. Scanning electron micrograph of a fracture surface of the Tynemouth Castle Cliff sandstone. A detrital quartz grain lies within a groundmass of dolomite (D). Quartz overgrowths (QO) occur over part of the detrital grain, often as discontinuous ridges (arrowed).

Fig. 6. Scanning electron micrograph showing detail of ridges of authigenic quartz over the surface of a detrital grain illustrated in Figure 5.

stability' (Walker 1962) and is commonly considered to be controlled by pH fluctuations. Although Burley & Kantorowicz (1986b) had some difficulty in describing fluid geochemistry during replacement, especially with regard to the high pH required, Maliva & Siever (1988) have suggested that replacement of quartz by carbonate can be due to pressures exerted by carbonate crystals growing against quartz (force of crystallizationcontrolled replacement), rather than due to corrosion of the quartz by carbonate-precipitating fluids. The etching

of detrital quartz during cementation by calcite, pressure solution at grain contacts and the precipitation of quartz overgrowths have been described from the Basal Permian Sands of north-east England by Krinsley & Smith (1981). However, there is no evidence for abrasion of quartz overgrowths in the Raisby Formation sandstone at Tynemouth Castle Cliff, thus suggesting that the authigenic quartz precipitated following redeposition of the Basal Permian Sands during resedimentation of the Raisby Formation.


ZECHSTEIN Q U A R T Z

The Raisby Formation sandstone at Tynemouth Castle Cliff has no evidence for corrosion or replacement of quartz overgrowths by dolomite or nonluminescent calcite. Thin-section petrographic evidence (Figs 3 , 4) shows conclusively that quartz overgrowth precipitation post-dated the removal of inhibiting surrounding dolomite. Thus, distinctive grain surface textures (Figs 5, 6) are interpreted to result from quartz overgrowths precipitating within available pores, such as those in between dolomite crystals (interboundary sheet pores). Apart from the definitive textural evidence concerning the timing of diagenesis, it is very unlikely that quartz overgrowths could have precipitated within the Tynemouth Castle Cliff sandstone before dolomitization. Precipitation of dolomite most likely took place at the same time as, or shortly after, deposition of the overlying Hartlepool Anhydrite Formation (English Zechstein Cycle 1 evaporite) (Lee 1990). During this time, the Raisby Formation at Tynemouth Castle Cliff was probably buried to a maximum of a couple of hundred metres depth. At such depths, lithostatic pressures would probably have been insufficient to supply significant quantities of silica in solution via chemical compaction. However, silica could have been sourced intraformationally by pressure solution during later Mesozoic burial, or may have been derived from outside the sandstone, and introduced by meteoric-derived fluids during Tertiary uplift of the Raisby Formation. The latter alternative would accord with the evidence that dolomite spalled off into large pores before quartz overgrowth precipitation (Fig. 3). These pores were probably produced by the dissolution of gypsum which had previously replaced some of the dolomite, and were cemented by non-luminescent calcite after quartz overgrowths had formed. Both the dissolution of replacive gypsum within dolostones and occlusion of resultant pores by non-luminescent calcite within the Raisby Formation, were suggested by Lee & Harwood (1989) to be achieved by meteoric-derived groundwaters. These groundwaters penetrated the Raisby Formation during the Tertiary, and precipitated a characteristic sequence of calcite cements, whose carbon and oxygen stable isotopic composition is indicative of precipitation from fluids of a meteoric derivation (Lee & Harwood 1989; Lee 1990). A t a number of other localities within the Raisby Formation at outcrop, non-luminescent calcite-cemented pores after gypsum are also partially occluded by booklets of authigenic kaolinite (Lee 1990). The presence of this clay mineral provides further good supporting evidence for meteoric invasion as a significant source of silica (in addition to aluminium) from groundwaters during late stage uplift diagenesis. 4.

CONCLUSIONS

We recognize that the replacement of silicates by authigenic carbonates is clearly widespread. However, we offer an alternative explanation for the formation of

OVERGROWTHS

413

distinctive ridges of authigenic quartz on detrital grain surfaces (Figs 5 , 6 ) . W e suggest that the ridges formed not by the replacement of quartz overgrowths by dolomite, but by the precipitation of authigenic quartz along interboundary sheet pores between dolomite crystals. This is based on petrographic evidence from the sandstone that quartz overgrowths post-dated dolomitization. Recognition of this style of quartz cementation is not only significant in relation to the evaluation of diagenetic histories, but also potentially of importance in influencing porosity and permeability within carbonate-siliciclastic reservoir rocks. This is particularly significant with regard to the destruction of pre-existing pores adjacent to detrital silicate grains enclosed by carbonates. Thus, the quartz grain surface textures in Figures 5 and 6 are indicative of the destruction, not creation of porosity.

Acknowledgements. The authors would like to thank Trevor Whitfield in the Department of Geology, University of Newcastle upon Tyne for making the thin sections, and the staff of the Department of Metallurgy, University of Newcastle upon Tyne for assistance with the S.E.M. Helpful reviews were provided by R. W. O'B. Knox and G. K. Lott, with constructive criticism from M. C. Akhurst and P. Earl. This work was undertaken whilst the authors were in receipt of research grants from the Algerian government (M. Said Benzagouta) and the Natural Environment Research Council (Martin Lee).

References BURLEY, S. D. & KANTOROWICZ. J . D. 1986a. Thin section and S.E.M. textural criteria for the recognition of cement-dissolution porosity in sandstones. Sedimentology 33,587-604. BURLEY, S. D. & KANTOROWICZ, J . D. 1986b. Thin section and S.E.M. textural criteria for the recognition of cement-dissolution porosity in sandstones - Reply to discussion. Sedimentology 33,608-614. DAPPLES, E. C. 1971. Physical classification of carbonate cement in quartzose sandstones. Journal of Sedimentary Petrology 41,196-204. KRINSLEY.D. H . & SMITH, D. B. 1981. A selective SEM study of grains from the Permian Yellow Sands of north-east England. Proceedings of the Geologists' Association 92,189-196. LAND, D. H . 1974. Geology of the Tynemouth district. Geological Survey of Great Britain Memoir, England and Wales, Sheet 15. LEE, M. R. 1990. The sedimentology and diagenesis of the Raisby Formation (Zl carbonate), northern England. Unpublished Ph.D. thesis, University of Newcastle upon Tyne. LEE, M. R. & HARWOOD, G. M. 1989. Dolomite calcitization and cement zonation related to uplift of the Raisby Formation (Zechstein carbonate), northeast England. Sedimentary Geology 65, 285-305. MALIVA, R. G. & SIEVER, R. 1988. Diagenetic replacement

controlled by force of crystallization. Geology 16, 688-691.

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SMITH, D. B. 1970. Submarine slumping and sliding in the Lower Magnesian Limestone of Northumberland and Durham. Proceedings of the Yorkshire Geological Society 3$, 1-36. SMITH, D. B. 1980. Permian and Triassic rocks. Pp. 36-38 in ROBSON, D. A. (editor) The geology of north east England. Natural History Society of Northumbria Special Publication. WALKER, T. R. 1962. Reversible nature of chert-carbonate replacement in sedimentary rocks. Geological Society of America Bulletin 7 3 , 237-242.

M. S. BENZAGOUTA

Department of Geology The University of Newcastle upon Tyne Newcastle upon Tyne NE1 7RU M. R . L E E , B . S c , P h . D .

Department of Physics The University of Essex Wivenhoe Park Colchester Essex C04 3SQ Revised manuscript received: 11th March, 1991.