Carbon isotope stratigraphy using carbonate cements in the Triassic Sherwood Sandstone Group: Corrib Field, west of Ireland
Introduction
Stratigraphic analysis of sandstones has typically involved a range of tools and approaches including lithostratigraphy, biostratigraphy, heavy mineral analysis, magnetostratigraphy and chemostratigraphy (e.g. Nichols, 1999). Some forms of isotope stratigraphy have proved to be useful although a fundamental rule is that the primary stratigraphic isotope pattern must not have been significantly affected by secondary, i.e. diagenetic, processes (Emery and Robinson, 1993). Thus strontium isotope stratigraphy has proved to be successful but only in rocks with low concentrations of rubidium (e.g. K-rich clays and feldspars) since over time, during diagenesis, 87Rb undergoes fission to 87Sr and thus affects the 87Sr/86Sr signal (e.g. Elderfield, 1986). Carbon isotope stratigraphy has been shown to be a valuable approach but only for systems that have not been dominated by secondary sources of carbon. Thus some limestones have a remnant primary δ13C stratigraphic signal since any influx of carbonate in a fluid form during diagenesis has not swamped the primary carbonate present in rock form (e.g. Lambert et al., 1987). Carbon isotope stratigraphy based on carbonate minerals in sandstones has not been developed as a stratigraphic correlation tool since it is widely assumed that the carbon isotope ratios in carbonate minerals will have been dominated by the influx of carbonate during burial (e.g. Morad et al., 1990). For this reason, most studies of carbon isotopes from carbonate minerals in sandstones have not presented the data in a stratigraphic context. In this paper, it will be demonstrated that carbon isotope stratigraphy can be a useful correlation tool in unfossilferous sandstones although admittedly under possibly unusual circumstances.
The occurrence of carbonate cements in sandstones is not specific to any particular depositional environment; they are a common, and often the predominant, mineral cement in sandstones (Morad, 1998). They can also form at just about any stage of diagenesis. Early diagenetic (eogenetic) carbonate mineral cements such as calcite and dolomite are, in many cases, one of the dominant cements in redbed continental sandstones deposited under arid to semi-arid conditions (Worden and Burley, 2003). Carbonate cements in redbed sandstones form either in soil profiles (in the unsaturated zone, i.e. pedogenic), or at or below the water-table (in the saturated zone, i.e. phreatic) (e.g. Khadkikar et al., 1998, Wright, 1990).
The data presented in this paper for the Triassic Sherwood Sandstone Group rocks of the Corrib Gas Field, west of Ireland, will show that oxygen isotopes are altered during early and burial diagenesis as a result of recrystallisation. However, carbon isotope ratios in the pedogenic and phreatic dolomite cements seem to be unchanged despite a ∼ 250 Myr diagenetic history, burial to depths of ∼ 4000 m and heating to temperatures of ∼ 165 °C. The carbon isotope trend through > 300 m of sandstones in the Corrib Field spans a time interval of several million years and is consistent over an area of at least 60 km2 and therefore can be used to help in the correlation in these redbed sandstones.
Section snippets
Geological background
The Corrib Gas Field is in the Slyne Basin that sits on the continental shelf, offshore western Ireland on the western European margin (Fig. 1). The Slyne Basin is ∼ 40 km wide and has the general characteristics of a half-graben (Fig. 2). This basin is filled with Mesozoic and Tertiary sedimentary rocks (from 2.5 to 4.0 km in thickness) that have a predominant easterly dip towards the major eastern boundary fault (Fig. 2; Dancer et al., 1999). The structure of the basin was initially controlled
Stratigraphic framework
The Triassic succession in the Corrib Field has here been grouped into fluvial channel, floodplain and playa-lacustrine facies associations. Well-sorted, fine-to medium grained, well-cemented sandstone with dispersed mudstone intraclasts represents the dominant fluvial channel facies sedimentary rocks (81% of the cored interval). Based on the dominance of sand-grade sediment, the relative lack of fine-grained sedimentary rocks, the presence of thick-stacked fluvial sediment packages with
Methods
Samples for petrography and then stable isotopes analysis were collected from cored sections of the Sherwood Sandstone Group from the six exploration and appraisal wells in the Corrib Field between 1 and 3 m intervals.
Polished blocks, thin sections stained for carbonates and stubs of broken surfaces were examined using secondary electron (SE), CL and backscattered electron (BSE) microscopy. Samples were examined with a CAMSCAN CS44 SEM equipped with a SE detector, a high-resolution solid-state
Carbonate cements
As in many semi-arid fluvial and alluvial sedimentary rocks in general (e.g. Khadkikar et al., 1998) and in the Sherwood Sandstone in particular (e.g. Burley, 1984, Purvis and Wright, 1991, Ruffell and Shelton, 1999, Strong et al., 1994) there is abundant carbonate cement in the Triassic Sherwood Sandstone Group at Corrib (Schmid et al., 2004). The predominant carbonate cement in the SSG in the Corrib Field is non-ferroan dolomite and there is only minor ferroan dolomite/ankerite. Calcite
Stable isotope results
Primary isotope patterns from carbonate minerals in modern redbed environments record information about both the isotopic composition of meteoric water and temperature of mineral growth as well as the amount of biomass present in the ecosystem. However, isotopic analysis of pedogenic or phreatic carbonate cements in ancient sandstones may only be useful for palaeoenvironment reconstruction or correlation if they have remained unaltered by any post-formational processes. There are two main
Mechanism of carbonate cement accumulation in continental sedimentary rocks
There are a number of possible primary sources of the components (mainly Ca2+, Mg2+ and bicarbonate) in eogenetic carbonate cements in continental sedimentary rocks. The main mechanisms for near-surface carbonate accumulation can be distinguished between organic and inorganic processes (Talma and Netterberg, 1983). Organic sources of the components of carbonate cements include: (a) accumulation of skeletal and protective parts of organisms, (b) transpiration of CO2 by plants and increased
Conclusion
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The Lower/Middle Triassic redbeds of the Sherwood Sandstone from the Corrib Gas Field, west of Ireland, contain dolomite, in the form of dolocrete, that developed very early in the paragenetic sequence either as pedogenic or phreatic cement.
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The Sherwood Sandstone has undergone a lengthy burial and thermal evolution (max. temperature of about 165 °C).
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Despite burial diagenetic processes, there is a strong stratigraphic pattern in the carbonate δ13C signature. δ13C variations of 5‰ in the Sherwood
Acknowledgments
The authors would like to thank Enterprise Ireland (now Shell Ireland) for sponsoring this research project, providing the necessary core material as well as providing stimulating discussion of the results. We are particularly grateful to Steve Kenyon-Roberts and John Downey. Neil Meadows and Kelly McGuire are acknowledged for providing elementary petrographic data from the Sherwood Sandstone. Thanks also to the isotope lab staff in Earth and Ocean Sciences at Liverpool University. Comments and
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