Cements and clays:

The main cements are quartz, anhydrite and carbonate (Plates 7.1, 7.3, 7.4, 7.7). Overgrowths also occur on some K-feldspar grains and rarely on microcline (Plates 7.13, 7.16).

Some of the samples from the Watt Mountain Formation also show pyrite. The identities of the authigenic clays, illite, chlorite, kaolinite, corrensite (mixed layer illite-chlorite) and smectite, have been confirmed by XRD (Table 7.7). In samples from the Keg River Formation and Chinchaga Formation, with increasing cement percentage in the rock, the amount of carbonate decreases while the content of anhydrite and clay increases. Granite Wash samples show a linear increasing relationship of carbonate, anhydrite and clay individually with the the percentage of cement. In samples from the Watt Mountain Formation with Increasing amounts of cement, the carbonate percentage increases while clay and anhydrite decrease.

Silica, as syntaxial overgrowths, sometimes as massive cementing medium (chert/opal), and exceptionally as small authigenic crystals is the most abundant authigenic mineral

although it is Impossible to measure its actual amount by optical microscopy (Plates 7.4, 7.18, 7.18-7.20). Occasional 'dust lines’ around original grains reveal overgrowth rims (Plate 7.8). Overgrowth has penetrated and filled interstices amongst quartz and feldspar grains as well as forming scalloped boundaries with corroded feldspars. Where clay is present overgrowths seem to have grown compressing the clay. Sometimes nonporous quartz mosaics can also be seen severely reducing the porosity of the rock. Under SEM many euhedral faces can be seen (Plates 7.18, 7.20) although growth of completed faces has sometimes been inhibited by the presence of fringing chlorite (Plates 8.21, 7.22). Overgrowth may also take the form of numerous small crystals 'budded' on to the surface of detrital grains.

Anhydrite is present in six samples from the Granite Wash and almost every sample of the Chinchaga, Keg River and Watt Mountain Formations. The rare exceptions amongst the three formations are those in which carbonate is the all-pervasive cement (Table 7.1-7.7).

Anhydrite occurs as blocky crystals, poikilotopic with respect to quartz and feldspar, sometimes enclosing as many as ten grains (Plates 7.23-7.28). Crystals reach a maximum of 10 mm in length, although most are between 1

PETROLOGY & DIAGENESIS 7.12

and 3 mm. The vast majority are tabular in habit; a smaller proportion are highly elongate prismatic crystals forming sheaves sometimes filling enlarged pores (Plate 7.25) and up to 2 mm long. Under SEM anhydrite laths are seen to be coated with clay. Most of the poikilotopic crystals fill large pores amongst loosely packed quartz and feldspar grains, but in those sands where quartz overgrowth has been abundant the anhydrite although poikilotopic is made up of 'skeletal' portions occupying smalt triangular and lamellar pores between the clastic grains.

The anhydrite also occurs as fillings along enlarged cleavages in feldspar (Plate 7.27) and one example of the sulphate in the Watt Mountain Formation was noted in pores of a fish-scale. Anhydrite may surround biotite but it may also penetrate along cleavages as though the biotite was forcibly displaced into partly or wholly separate flakes. In other cases (Plate 7.27) anhydrite has penetrated and disrupted polycrystalline quartz grains. Anhydrite along with quartz may completely occlude all pores but in many cases cement is restricted and the margins of the anhydrite may take on a distinctive outline. This consists of a series of narrow elongate remnants of the crystal projecting into the pore rather like a series of piers built out into the sea (Plates 7.25, 7.28). the crystals have evidently

suffered corrosion controlled by their two sets of cleavage. Under cathodoluminescence anhydrite glowed in orange (Plate 7.14).

Carbonate is less widespread than anhydrite. It is usually in smaller amounts in the Watt Mountain and Chinchaga Formations than in the Keg River Formation and Granite Wash lithology where it is present in almost all the samples with exceptions of those in which anhydrite is the all pervasive cement. Many samples have traces or low percentages of carbonate, while in others it may form 100% of the cement (Table 7.1-7.7).

When in large amounts, the carbonate is calcite with some ferroan component affecting the margins of the pores; where only a few pores are occupied by the carbonate, it is often completely ferroan (Plates 7.29-7.32). Dolomite and ferroan-dolomite are also present as pore fillings in a number of samples (Plates 7.32-7.37). Under SEM tiny rhombs of dolomite show well developed faces and edges and show an intergrowth pattern amongst themselves (Plates 7.31, 7.32). Like the anhydrite the calcite and dolomite may be poikilotopic although the crystals are usually much smaller in size (Plates 7.29, 7.35). Millimetre-sized poikilotopic crystals are common (Plate 7.30); even where finer the calcite forms sparry crystals seldom less than 200 pm.

PETROLOGY & DIAGENESIS 7.13

In some calcite rich samples the coarser spar shows a wavy extinction. The poikilotopic habit means that a euhedral shape is rarely seen. Exceptionally rhombs (about 100 pm) are developed in clay (Plate 7.31). Some calcite also occurs in the secondary pores of skeletal feldspars. Carbonate seems to occupy some unusually large pores, suggesting that it is partly replacive or has occupied previously enlarged pores. Calcite and dolomite have irregular boundaries against quartz and feldspar (Plates 7.33, 7.35, 7.36). By contrast corrosion of the carbonate has left it with irregular margins to voids. The carbonate lies against corroded quartz overgrowths, suggesting later formation; the former position of quartz overgrowths is marked by indistinct lines through the carbonate (Plate 7.35).

Luminescence colours of orange and red were observed for carbonate under cathodoluminescence (Plates 7.12-7.14). No distinct zonation was distinguished except for a few light and dark patches. Back-scatter images also revealed no zonation amongst the carbonate cement (Plates 7.37).

Clays occur as chlorite, illite and kaolinite with minor amounts of corrensite. The clay is widespread geographically. It is more abundant in sandy bands at or near the boundary

with shales. The clay, mostly detrital, fills many pores and is usually a greenish chlorite together with illite. The pore filling clay also covers all the grains making analysis difficult under SEM (Plates 7.17, 7.22). For most of the samples the small amounts of clay appear as fringes to grains and occasionally fill pores. Chlorite is the most common and widespread; illite is fairly common in samples over the whole area; kaolinite was recognised in thin-sections from four wells (Table 7.1-7.6) but evidently is more widespread according to XRD analysis (Table 7.7).

Authigenic chlorite is seen under the SEM as blades 10-20 pm sometimes arranged in small sheaves or characteristic ’rosettes' (Plate 7.22. EDX shows it to be iron-rich with some magnesium. The crystals grow normal to grain faces of both quartz and feldspar. However, the clay seems to be sandwiched between the overgrowths and subsequent carbonate and anhydrite fillings. Chlorite-smectite mixed layer clays also occur (Plate 7.39).

Illite, like chlorite is both detrital and authigenic. The latter, often 'hairy' in habit tends to be associated with K-feldspar grains (Plates 7.15, 7.40). Some of the illite is mixed illite-smectite (Plates 7.41, 7.42). Jansa & Fischbuch (1974) have reported mainly illitic clays with small amounts of kaolinite and traces of

PETROLOGY & DIAGENESIS 7.14

chlorite from the Gilwood sandstone of Sturgeon-Mitsue area, Alberta.

Kaolinite is scarce (Table 7.1-7.6) and was only found in abundance in stubby crystals and ’books' (about 10 pm across) in well 8-12-72-5W5 (Table 7.5) where chlorite also occurs but not so abundantly.

Illite and chlorite are the only two clay members present, in Chinchaga Formation samples, with the former being more abundant than the latter. Illite does not occur as well developed fibres, but instead as anastomosing masses engulfing the primary quartz and feldspar grains. Illite is also present in some of the skeletal feldspars. Some illite-smectite is also present in some of the pores. Chlorite occurs as rims around the grains.

Tiny crystals of pyrite are widely dispersed throughout in Watt Mountain Formation samples, although concentrated near the chlorite patches.

7.3 GEOCHEMISTRY: