Determining the Origin of Karst Fill at the Sub-Mesozoic Unconformity, Southeastern Saskatchewan

Gemma S. Bates 1, Alan C. Kendall 1, and Ian L. Millar 2

Bates, G.S., Kendall, A.C., and Millar, I.L. (2008): Determining the origin of karst fill at the sub-Mesozoic unconformity, southeastern Saskatchewan: in Summary of Investigations 2008, Volume 1, Saskatchewan Geological Survey, Saskatchewan Ministry of Energy and Resources, Misc. Rep. 2008-4.1, CD-ROM, Paper A-6, 9p.

Abstract Mississippian carbonates in southeastern Saskatchewan are unconformably overlain by clastic and evaporitic rocks of the Watrous Formation. Karst features are present at the sub-Mesozoic unconformity, but determining the origin of these features can sometimes prove challenging. Was karsting active during formation of the unconformity or did these karst features originate in the Mississippian and subsequently become exhumed during processes associated with development of the sub-Mesozoic erosion surface?

This study aims to distinguish sediments and sulphates of Mississippian origin from those in the Watrous Formation. Clay mineralogy, together with strontium isotope data, identify the provenance of redbed clays in the Watrous as being the Precambrian Shield, to the northeast of the Williston Basin. These clays have a clearly different signature from Mississippian sediments. Dolomite stoichiometry also allows a clear distinction to be made between Mississippian dolomites, and Watrous and alteration-zone dolomites. This combination of methods has allowed the origin of siltstones and sulphates in some karst features within close proximity to the unconformity to be determined as Watrous in age. There is also evidence of sulphates from hydrothermal waters in some karst features. These data demonstrate that karsting was active during the formation of the unconformity, but that the karst features were still open after the onset of Watrous sedimentation.

Keywords: Watrous Formation, Williston Basin, strontium isotopes, clay mineralogy, anhydrite, dolomite stoichiometry, Mississippian, Mesozoic.

1. Introduction Mississippian strata in southeastern Saskatchewan are unconformably overlain by the Watrous Formation, the lower part of which consists of reddish brown, dolomite-cemented siltstones and mudstones with interbedded sandstone and scattered anhydrite nodules. The clastics exhibit wavy- to lenticular-bedding, parallel- and cross-laminations, scattered siltstone lenses, and locally preserved mud cracks (Husain, 1990). The Watrous has been notoriously difficult to date. It rests unconformably on Mississippian carbonates of Kinderhookian to Chesterian age within the study area (Tps 1 to 13, Rges 30W1 to 11W2), and is overlain by the Gravelbourg Formation of Middle Jurassic age (Figure 1).

In contrast to the situation where redbed clastics of the Middle Devonian Ashern Formation unconformably rest upon Lower Palaeozoic carbonates that are invariably leached and contain downward-transported Ashern sediments within cavities (Kendall, 2001), karst features are rare in uppermost Mississippian carbonates. Where present, they appear to be very localized (Kent et al., 1998; Gatenby and Carter, 2001) and their origin is difficult to determine. Karst features that formed in the Mississippian may have been exhumed by erosion and undergone alteration associated with the unconformity, including precipitation of sulphate and infilling of open voids by Watrous sediments. Caves may also have originated during development of the unconformity. Dolomitic siltstones present within the caverns generally range in colour from deep red-brown (Munsell colour 10R 4/6 to 10R 3/4) to pink (5R 6/4; see Appendix for Munsell colours). The former resemble Watrous sediments, whereas the latter appear to be more typical of Mississippian lateritic siltstones. Anhydrite nodules are also associated with the clastic cavity-fills.

Here we provide evidence from clay mineralogy and geochemistry that can be employed to differentiate between Mississippian and Watrous clastic sediments within karstic cavities. Caves containing at least some sediment of Mississippian origin are considered to have formed during the Mississippian and have been exhumed and altered by unconformity-related processes, whereas caves with only Watrous sediment are considered to have formed post-

1 School of Environmental Sciences, University of East Anglia, Norwich, NR4 7TJ, UK; E-mail: g.bates@uea.ac.uk; a.kendall@uea.ac.uk. 2 NERC Isotope Geosciences Laboratory, Keyworth, Nottingham, NG12 5GG, UK; E-mail: ilm@bgs.ac.uk.

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Mississippian. These data can also be used to provide information on the origin of Watrous clastics and their subsequent alterations.

2. Clay Mineralogy Lower Watrous sediments are dominated by a detrital clay assemblage of illite and iron-rich chlorite (data from X-Ray Diffraction (XRD), shown in Table 1 and Figure 2). This agrees with earlier data from Le Nindre and Gaus (2005). The assemblage is consistent with the dominant detrital clay minerals of the Triassic in North America, sourced from granitic and gneissic rocks (Weaver, 1989). The Canadian Precambrian Shield is the most likely source for this detritus (Figure 3). Illite is associated with the degradation of granitic rocks in arid climates, where the degree of soil leaching is limited (Hardy and Tucker, 1988), with chlorites typically forming under the same conditions as illite (Weaver, 1989).

Palygorskite and kaolinite also occur as neoformed and transformed clays. Kaolinite forms in heavily leached soils and was identified in a red, dolomitic Watrous sample (XC1; Table 1 and Figure 2) and in some Le Nindre and Gaus (2005) samples. Formation of kaolinite is facilitated by a lack of plant life, a feature that is commonly associated with arid, hypersaline lake basins (Weaver, 1989). Palygorskite also forms in (semi-)arid environments, either through transformation of smectite or by neoformation in aluminium-rich soils and/or shallow water. Palygorskite is present in both of the calcitized Watrous samples (Table 1), suggesting an association of palygorskite and calcite.

Figure 1 - Stratigraphic correlation chart (modified from Saskatchewan Industry and Resources, 2004).

In contrast to the Watrous sediments, Mississippian siltstones lack chlorite and palygorskite. Mississippian sediments appear to have a predominantly detrital character; weathered minerals entered the area and transformation occurred under the influence of marine conditions leading to the reconstruction of well-crystallized illite from mixed layers (Millot, 1970). Both chlorite and palygorskite are present in siltstone from the karst feature (sample XC6, Table 1, Figure 2), consistent with the cavity fill being of Watrous origin and the cave being related to the sub-Watrous unconformity.

Table 1 - Clay mineralogy and dolomite percentage of total carbonate from representative samples.

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Sample Well LocationWell License Depth (m) Description Origin

Detrital Clays

Neo/transformed Clay

Dolomite as % of total carbonate

XC1 101/4-34-8-33W1 66D008 1056.27 Red, dolomitic Watrous

Lower Watrous Formation

Illite and chlorite

Kaolinite 100

XC2 101/12-16-9-33W1 68D011 1056.12 Green, dolomitic Watrous

Lower Watrous Formation

Illite and chlorite

100

XC3 101/16-16-11-10W2 59J008 1201.81 Red, calcareous Watrous

Lower Watrous Formation

Illite and chlorite

Palygorskite 32

XC4 101/16-16-11-10W2 59J008 1205.78 Green, calcareous Watrous

Lower Watrous Formation

Illite and chlorite

Palygorskite 17

XC5 101/14-16-9-33W1 60C006 1056.85 Mississippian red siltstone

Mississippian (Souris Valley)

Illite 58

XC6 101/16-30-9-33W1 60I047 1052.46 Red siltstone from karst

Karst fill Illite and chlorite

Palygorskite 100

3. Origin of Sulphate-precipitating Brines

Figure 2 - XRD plots identifying clay minerals in samples XC1 (Lower Watrous) and XC6 (karst fill). X-axis represents degrees of 2θ (360°) relating to the angle of incidence of the X-ray beam.

Figure 3 - Principal post-Palaeozoic drainage from the Canadian Shield (adapted from Martin, 1966). The easternmost valley is filled with Triassic and younger sediments; the westernmost valley is hypothetical. The Williston Basin is outlined in green and the study area in blue (Tps 1 to 13, Rges 30W1 to 11W2).

Strontium isotope ratios of Lower Watrous anhydrites are more radiogenic than if the strontium was sourced entirely from Triassic marine seawater (Bates et al., 2007). Strontium isotope values obtained from 13 anhydrite nodules in the Lower Watrous have a mean 87Sr/86Sr ratio of 0.708475 ±0.000098 (1σ; n=13), (see Bates et al., 2007, for data table and analytical methods; data reported relative to a value of 0.710250 for NBS987 standard). The lowest value is most likely to approach marine strontium, with the more radiogenic values representing an increasing influence of continental strontium in the depositional basin. Radiogenic strontium is most commonly sourced from old, acidic, continental shields, which is in agreement with clay provenance from the Canadian Shield. The radiogenic strontium can enter the basin either by subsurface flow of waters sourced from the Shield and/or the radiogenic strontium can be absorbed and transported on the surface of the fine terrigenous clay-size and silt-size particles introduced during deposition of the redbed sediments (Chaudhuri and Clauer, 1992).

Bates et al. (2007) identified a mixing of radiogenic Watrous brines with less radiogenic groundwaters immediately beneath the unconformity. The resulting alteration-zone sulphates have 87Sr/86Sr ratios more radiogenic than Mississippian sulphate values, but less than Watrous sulphates; the mean 87Sr/86Sr ratio is 0.708286 ±0.000113 (1σ; n=7; Table 2) for alteration sulphates after satin-spar veins, which are the only sulphate source that can be proven to have precipitated solely from the mixed brines.

Strontium isotope ratios were obtained from sulphates in a minor karst feature in the Rosebank Pool, 101/6-33-4-32W1 (Figure 4). Dolostone within the alteration zone contains fenestrae that are filled by anhydrite and red and green siltstone (Figure 4b and 4c). This porosity-occluding anhydrite is isotopically similar to alteration-zone anhydrite, whereas the anhydrite nodules within the karst feature, surrounded by red siltstone (Munsell colour 10R 4/6), have a clear Watrous isotope signal (Figure 4a). This suggests that the karst feature was present prior to redbed deposition and that alteration-zone formation was coeval with deposition of the Watrous sediments. Anhydrite blebs that formed in the redbeds within the open karst feature precipitated from Watrous brines, whereas the alteration anhydrite has a mixed isotope signal where more radiogenic Watrous brines mixed with less radiogenic groundwaters already present within the carbonate pore system.

Kent et al. (1998) suggested that all sulphates infilling cave systems in this region may not be of the same origin, and the geochemistry of one sample analyzed in this study supports that statement. Anhydrite in red siltstone (Munsell colour 10R 6/2 to 10R 5/4), sampled from 14 m beneath the unconformity within a major karst in well 101/11-22-2-34W1 in the Carnduff Pool (Gatenby and Carter, 2001), gives an 87Sr/86Sr ratio of 0.708130 ±0.000010. This value suggests formation

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Table 2 - 87Sr/86Sr isotopic ratios and core depths of anhydrite veins within the alteration zone.

Figure 4 - Minor karst feature in Rosebank Pool, well 101/6-33-4-32W1. a) 87Sr/86Sr isotopic ratios of sulphates versus depth, with error bars based on external precision (±0.000010, 2σ), with red and blue arrows representing standard deviation of Watrous and alteration-zone sulphates, respectively; b) core photograph with unconformity highlighted by green dashed line, and sample locations identified by arrows (scale bar in centimetres); and c) photomicrograph of dolostone with anhydrite infilling porosity (1090.9 m; cross polars; scale bar 1 mm).

from less radiogenic brines than either the Watrous or the mixed alteration-zone fluids. The sulphates are most likely to be of hydrothermal origin where the waters have come into near equilibrium with Mississippian (Kinderhookian/Osagian) sulphate (87Sr/86Sr ±0.70765 to ±0.70810; values from McArthur et al., 2001).

It is important when sampling sulphates not to confuse proto-karst fissures with near-horizontal anhydrite veins after gypsum ‘satin-spar’ (Kendall, 1975). The latter formed throughout the alteration zone, subjacent to the unconformity, by a succession of ‘crack-seal’ increments (Ramsay, 1980), and therefore must not be mistaken for sulphate fills of open karst features.

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Well Number Well License Sample Depth (m)Distance Beneath Unconformity (m) 87Sr/86Sr

111/8-9-10-30W1 85L054 864.50 7.20 0.708192141/15-18-10-30W1 93J091 898.30 7.53 0.708276101/12-34-3-31W1 57G047 1072.12 0.08 0.708152101/4-34-8-33W1 66D008 1057.52 1.12 0.708429101/4-34-8-33W1 66D008 1058.80 2.40 0.708215101/12-16-9-33W1 68D011 1060.08 3.08 0.708441101/14-16-9-33W1 60C006 1048.26 0.25 0.708298

4. Cementation of Watrous Redbeds The Watrous redbeds are usually cemented by dolomite and anhydrite, which precipitated from concentrated brines. Locally, calcite-cemented clastics (Kaldi and Hartling, 1982; Kendall, 2001) occur within the Lower Watrous and suggest that different formational waters were responsible for cementation in these areas.

Dolomites were analyzed using XRD (methods in Hardy and Tucker, 1988), to determine: 1) the relative stoichiometry (using the equation of Lumsden, 1979), which relates mole% calcium carbonate to the d104 peak, where a change in the calcium content affects the relative position of the peak), and 2) the degree of ordering (intensity of d015 peak over intensity of d110 peak). Stoichiometric dolomite has an equal number of calcium and magnesium ions (CaMg(CO3)2), but typically there is an excess of either calcium or, less commonly, magnesium within the mineral. Watrous dolomite cements have near-stoichiometric values, averaging 50.6 mole% calcium (Figure 5). Within the redbed sequence, precipitation of gypsum-anhydrite would have resulted in a high magnesium/calcium ratio, and subsequently near-stoichiometric dolomite precipitating from brines with an abundance of magnesium ions. Lumsden and Chimahusky (1980) attribute near-stoichiometric, evaporite-related dolomites to products of syndepositional dolomitization.

The alteration zone consists of pervasively dolomitized and anhydritized Mississippian carbonates. The stoichiometry of these dolomites average 50.2 mole% calcium and resembles that of dolomite cements in Watrous clastics as opposed to their original, less stoichiometric, Mississippian values (average 53.8 mole% calcium). This was demonstrated by data from Midale dolostones immediately overlying the Frobisher evaporite (see Kendall and Walters, 1978), with one sample from unaltered Mississippian dolostone (101/7-12-3-1W2; 1281.53 m), the other from exactly the same stratigraphic horizon, but within the alteration zone (101/9-12-3-1W2; 1266.63 m). The alteration-zone dolostone appears completely recrystallized from its original Mississippian chemistry and now resembles Watrous dolomites (see circled samples in Figure 5). This interpretation is consistent with coeval formation of both dolomites from the same brines (see Bates et al., 2007).

Figure 5 - Stoichiometry versus degree of ordering of dolomite groups. X-axis refers to ratio of XRD peaks d 015 and d 110. Circled samples and arrow represent stratigraphically similar Mississippian dolomite that has undergone alteration. Labels refer to samples analyzed for clay mineralogy (Table 1).

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Kaldi and Hartling (1982) and Kendall (2001) identified a region (Tps 10 to 12; Rges 7 to 11W2) where Mississippian anhydrites had been calcitized and the overlying Watrous was cemented by calcite instead of dolomite. This could be the result of a mixing zone where the magnesium/calcium ratio was reduced by an input of less saline brines from below. Upwelling springs, supersaturated with respect to calcite, would also calcitize the Mississippian anhydrite, and could result in overland flows upon the redbed depositional surface.

Figure 6 - Photomicrograph of calcite-cemented Watrous (calcite is stained red); calcite has also partly replaced dolomite in a detrital dolomite (DD) clast; 101/16-16-11-10W2, 1201.81 m; sample XC3 in Table 1, taken ~4 m above unconformity (cross polars; alizarin red S stained; scale bar represents 1 mm).

Lower Watrous samples from these local calcitized regions appear petrographically to have only calcite cement. XRD analysis, however, identifies up to 32% dolomite as a total percentage of the carbonate (Table 1). In their study, Le Nindre and Gaus (2005) identified >60% of carbonate as dolomite. This dolomite differs markedly from dolomite of calcite-free Lower Watrous redbeds (Figure 5), with significantly higher calcium content (>53 mole% calcium compared with calcite-free dolomite which has 50 to 52 mole% calcium) and an increased degree of ordering. This dolomite is believed to be detrital based on petrographic (Figure 6) and chemical evidence. The calcite cement appears to replace dolomite clasts (Figure 6), with stoichiometric values that more closely resemble Mississippian rather than Watrous dolomite cement.

Although cation disorder (calcium ions substituting in magnesium sheet and vice versa) is theoretically distinct from stoichiometry, it is often observed that the highest ordered dolomites are the most stoichiometric. Dolomite within the calcite-cemented Watrous, however, exhibits high mole% calcium, but is also highly ordered. This could be a result of dolomite recrystallization by calcite-rich waters, causing an increase in calcium/magnesium ratio and a reordering of magnesium cations to the magnesium sheet and calcium cations to the calcium sheet (Malone et al., 1994).

The stoichiometry of dolomite samples XC5 and XC6 (refer to Table 1 and Figure 5) suggests a different origin for these siltstones also. Figure 5 identifies the Mississippian dolomite-cemented siltstone (XC5) as having an excess of calcium in its structure, compared with the karst siltstone (XC6) which has a more stoichiometric value, similar to Watrous dolomite cement. With the combined sedimentological and chemical evidence, it is most likely that the siltstone present within this particular karst feature is of Watrous origin, again suggesting that at least some of the karst features present in the northern Williston Basin were active during formation of the unconformity.

5. Conclusions • The Lower Watrous sediments accumulated in an arid climate with terrigenous clays, most likely sourced from

the Canadian Precambrian Shield. • The clays yield a detrital assemblage typical of the Triassic of North America, with neoformed clays indicating

formation in arid, hypersaline lakes. • The redbeds are also associated with radiogenic strontium-rich brines, with high 87Sr/86Sr sourced from the

continental input. • Formational Watrous brines were often supersaturated with respect to calcium sulphate and precipitated

gypsum-anhydrite, resulting in an increase in the magnesium/calcium ratio forming near-stoichiometric dolomite cement in the early-diagenetic environment.

• These fluids percolated through the underlying Mississippian carbonates resulting in coeval dolomitization and anhydritization of both the Watrous redbeds and alteration zone.

• The redbeds are locally cemented by calcite, providing evidence of dilution of concentrated Watrous brines by calcium-enriched waters.

• Watrous sediments can be distinguished from Mississippian in terms of colour, clay mineralogy, dolomite stoichiometry, and strontium isotopes. Data obtained from analyses of siltstone and anhydrite, present in a few

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karst features in the study area, indicate that not all karst features at the sub-Mesozoic unconformity are exhumed Mississippian caverns.

• Some Mississippian karst features may have been partially open once exhumed and Watrous sediment infilled the voids. Alteration has also occurred where sulphate with either a Watrous or alteration-zone signature is present.

• Caves with a complete absence of Mississippian material originated during unconformity formation and were infilled by Watrous sediment.

• The sub-Watrous unconformity surface is unusual in that there is a general absence of karstic developments in most areas and outliers of evaporites are preserved (Kendall, 2001); in most circumstances, the evaporites would be expected to erode more rapidly than carbonates.

6. Acknowledgments We thank the Saskatchewan Ministry of Energy and Resources for access to core, and their staff and John Lake for valuable discussions. We are grateful to the editors, Chris Gilboy and Fran Haidl, for suggestions which have led to the improvement of this manuscript. Prof. Julian Andrews is also thanked for discussions relating to XRD results. Strontium isotope data were obtained at the NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, Nottingham. This paper forms part of a NERC-funded PhD by Gemma Bates (grant number NER/S/A/2005/13587).

7. References Bates, G.S., Kendall, A.C., and Millar, I.L. (2007): Contemporaneous alteration of Lower Watrous clastics and the

sub-unconformity alteration zone in southern Saskatchewan: petrographic and geochemical evidence; in Summary of Investigations 2007, Volume 1, Saskatchewan Geological Survey, Sask. Industry and Resources, Misc. Rep. 2007-4.1, CD-ROM, Paper A-7, 10p.

Chaudhuri, S. and Clauer, N. (1992): History of marine evaporites: constraints from radiogenic isotopes; in Chaudhuri, S. and Clauer, N. (eds.), Isotopic Signatures and Sedimentary Records, 43, Springer-Verlag, Berlin, p177-198.

Gatenby, W.H. and Carter, S. (2001): Paleokarst in the Midale Beds, Carnduff Field, southeast Saskatchewan; CSPG Core Conference Abstracts, Calgary, Alberta, p118-1 to 118-22.

Hardy, R. and Tucker, M.E. (1988): X-ray powder diffraction of sediments; in Tucker, M.E. (ed.), Techniques in Sedimentology, Blackwell Scientific Publications, London, p191-228.

Husain, M. (1990): Regional geology and petroleum potential of the Lower Amaranth Formation, Coulter-Pierson area, southwestern Manitoba; Manitoba Energy and Mines, Petroleum Open File Report POF 11-90, 54p.

Kaldi, J. and Hartling, A. (1982): Calcitized anhydrite: its significance as an exploration tool in the Mississippian Tilston Beds of southeastern Saskatchewan; in Summary of Investigations 1982, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 82-4, p106-111.

Kendall, A.C. (1975): Anhydrite replacements of gypsum (satin-spar) veins in the Mississippian caprocks of southeastern Saskatchewan; Can. J. Earth Sci., v12, p1190-1195.

__________ (2001): Late diagenetic calcitization of anhydrite from the Mississippian of Saskatchewan, western Canada; Sediment., v48, p29-55.

Kendall, A.C. and Walters, K.L. (1978): The age of metasomatic anhydrite in Mississippian reservoir carbonates, southeastern Saskatchewan; Can. J. Earth Sci., v15, p424-430.

Kent, D.M., Lake, J.H., and Ware, M.J. (1998): Some paleokarst features in Mississippian carbonate rocks of southern Saskatchewan: origin, geometry and implications to petroleum exploration; in Gilboy, C.F., Paterson, D.F., and Bend, S.L. (eds.), Eighth International Williston Basin Symposium, Sask. Geol. Soc., Spec. Publ. No. 13, p72-85.

Le Nindre, Y.-M. and Gaus, I. (2005): Characterisation of the Lower Watrous aquitard as a major seal for CO2 geological sequestration, (Weyburn unit, Canada); in Wilson, M., Gale, J., Rubin, E.S., Keith, D.W., Gilboy, C.F., Morris, T., and Thambimuthu, K. (eds.), Greenhouse Gas Control Technologies: Proceedings of the 7th

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International Conference on Greenhouse Gas Control Technologies, September 5 to 9, 2004, Vancouver, p761-770.

Lumsden, D.N. (1979): Discrepancy between thin section and X-ray estimates of dolomite in limestone; J. Sed. Petrol., v49, p429-436.

Lumsden, D.N. and Chimahusky, J.S. (1980): Relationship between dolomite nonstoichiometry and carbonate parameters; in Zenger, D.H., Dunham, J.B., and Ethington, R.L. (eds.), Concepts and Models of Dolomitization, SEPM Spec. Publ. 28, p123-137.

Malone, M.J., Baker, P.A., and Burns, S.J. (1994): Recrystallization of dolomite: evidence from the Monterey Formation (Miocene), California; Sediment., v41, p1223-1239.

Martin, R. (1966): Paleogeomorphology and its application to exploration for oil and gas (with examples from western Canada); Amer. Assoc. Petrol. Geol. Bull., v50, p2277-2311.

McArthur, J.M., Howarth, R.J., and Bailey, T.R. (2001): Strontium isotope stratigraphy: LOWESS Version 3: best fit to the marine Sr-isotope curve for 0-509 Ma and accompanying look-up table for deriving numerical age; J. Geol., v109, p155-170.

Millot, G. (1970): Geology of Clays: Weathering, Sedimentology, Geochemistry; Chapman & Hall, London, 429p.

Munsell, A.H. (1923): A color notation; Munsell Color Company, Baltimore, MD.

Ramsay, J.G. (1980): The crack-seal mechanism of rock deformation; Nature, v284, p135-139.

Saskatchewan Industry and Resources (2004): Stratigraphic Correlation Chart; URL <http://www.ir.gov.sk.ca/stratchart>, accessed 25 March 2008.

Weaver, C.E. (1989): Clays, Muds, and Shales; Developments in Sedimentology, 44, Elsevier, Amsterdam, 820p.

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Appendix Rock-Color Chart (Munsell, 1923) identifying the range of colours referred to in this paper.