Siderite cement

Pye, K. (1984). SEM analysis of siderite cements in interstitial marsh sediments, Norfolk, England. Marine Geology, 56, 1-12. 

Fig. 4. SEM picture.s of a siderite cemented interval in the Melke Formation (well 6506/12-6 depth 4197.6 mKB). Carbonate cementation contribute to a reduction in permeability. (A) Thin section in backscattered mode. (B) Authigenic siderite.

Pye, K. (1981) Marshrock formed by iron sulphide and siderite cementation. Nature 294, 650-2. 

Unlike calcite and dolomite, siderite rarely forms as an extensive pore-filling cement, but rather as discrete fine crystals, spherules and nodules scattered in the host sediments. Nevertheless, Baker et al. (1996) found that early diagenetic siderite concretions (0.5-2 mm) form up to 30% of Triassic sandstones and mudstones from eastern Australia. Laterally continuous siderite-cemented offshore shelf sandstone sheets (15 cm thick) occur in Upper Cretaceous sequences from Canada (McKay et ai, 1995). 

Well Depth (mRKB) Formation Quartz clasts K-feldspar clasts Plagio- clase clasts Mica clasts Heavy minerals Carbon- ate fossils Plant frag- ments Clay clasts Clay matrix Pyrite cement Authi- genic kaolinite Authi- genic illite Calcite cement Siderite cement Dolomite cement Quartz cement Porosity Grain size (mm)... 

Fig. 8. Histogram of 5 C frequency distribution for (a) calcite cement in the Lower Namur Sandstone of the Gidgealpa Field (b) calcite cement in the Namur Sandstone and the Adori Sandstone, its lateral equivalent, for different petroleum fields, including the Big Lake, Kema, Marana, Moomba, Spencer, Strzelecki, Tantanna and Warana Fields (see Fig. 1 A) (c) siderite cement in Jurassic elastics (d) Cooper basin CO2 gases (e) Eromanga basin CO2 gases. Compiled from various sources (as shown). Note that the 5 C character of Eromanga basin calcite cements is similar to that of Cooper basin carbon dioxide gases. See text for explanation.

Application of quantitative back-scattered electron image analysis in isotope interpretation of siderite cement ... 

Carothers, 1992 Spiro et al., 1993 Morad et a ., 1994), The mineral is most suitable for the study of pore-water evolution during sediment subsidence because, unlike other carbonate minerals, siderite probably does not undergo recrystallization and isotope re-equilibration during burial diagenesis, as it has no unstable precursors or polymorphs (Curtis etai, 1975 Gautier, 1982 Pearson, 1985 Curtis Coleman, 1986). Therefore, stable isotope data of siderite cements can provide a powerful tool for the interpretation of diagenetic events in geological... 

Determination of quantitative elemental composition of siderite cement was carried out on polished thin sections covered with a thin layer of carbon and using a CAMECA SX 51 electron microprobe at 15 kV, with a 20 nA beam current and a 0.2 pm beam diameter. The BSE imaging system linked to the electron microprobe was used to detect zonation in the siderite cement, and composition analyses were carried out for each zone (Table 2). Results were normalized to 100 mol% Fe + Mn, Mg and Ca. The precision of the analyses was 100% 2. The standards used were MgO for Mg, wollastonite for Ca, rhodonite for Mn, and FcjOj for Fe. 

Table 2. Microprobe results (mol%) for the different siderite cement generations, which can be further subdivided on the basis of colour variations under the BSE microscope...

Table 3. Carbon and oxygen isotope data of the Tirrawarra Sandstone siderite cements. A good match is observed between measured (6 0 5as) and calculated oxygen isotope values determined from image analysis results.

SI, early siderite cement S2, middle generation siderite cement S3, late generation siderite cement BD, distal braid delta BM, medial braid delta BS, beach-barrier sandstone MBB, back-barrier marsh. 

Acquisition of the BSE image of siderite cement. Mg-poor (early-formed) siderite cement was found to be relatively light-coloured, whereas more Mg-rich siderite is darker in colour. 

Fig. 4. Petrographic, BSE and colour image characteristics of Tirrawarra Sandstone siderites. (A) Plane-polarized view of the main siderite cement generations that can be distinguished under the optical microscope in this case, which is the exception rather than the rule. SI has a brownish colour, whereas S2 and S3 are clear and colourless. SI is typically engulfed by S2. Note the concentration of fluid inclusions in S2, and the irregular serrated boundary between S2 and S3 (arrow), implying some dissolution of S2 prior to precipitation of S3. Sample Ml-9598, Moorari 1, 2925.5 m.

Fig. 6. In this BSE image a homogeneous S3 cement is the main pore-filling event however, examination of other micrographs shows that the relative proportion of the different siderite cement generations can vary with different fields of view cf. Figs 4A and 5). Again, notice the dissolution boundary between S2 and S3, the dissolution pits associated with S2, and the isolated remnants of SI (arrow) within the S2 matrix. Further observe the incipient euhedral rhombic terminations of the S3 cement that grew on the S2 dissolution surface. Sample Ml-9598, Moorari 1, 2925.5 m.

Fig. 9. BSE image of a fine-grained, moderate to poorly sorted back-barrier marsh sample completely cemented by SI (white). Note the serrated nature of some quartz grains and the very high intergranular volume (>50%), which suggest the replacement of part of the margins of quartz grains by siderite cement. Sample F4-9441, Fly Lake 4, 2877.6 m. Scale bar = 500 pm.

Fig. 10. Ternary diagrams showing the compositional ranges of different generations of siderite cement in the Tirrawarra Sandstone. The early generation of the siderite cement (SI) is very rich in Fe, whereas the middle (S2) and late generations (S3) have much higher substitution of Mg and fall within the realm of sideroplesite and pistomsite. S2 and S3 have almost identical compositions except that S2 has a slightly higher Ca content.

Fig. 11. BSE image showing the intergrowth between S2 cement (white) displaying characteristic rhombs and quartz overgrowths (medium grey). Locally the siderite is completely engulfed by the quartz cement (arrow), indicating that some siderite cementation preceded quartz (Q) cementation. Sample FI-9433, Fly Lake 1, 2875.2 m. Scale bar = 50 pm.

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