Geobiology (2004), 2, 21–30 ConceptualBlackwellORIGINAL Publishing, models ARTICLE for Ltd. burrow-related dolomitization models for burrow-related, selective dolomitization with textural and isotopic evidence from the Tyndall Stone, Canada MURRAY K. GINGRAS, S. GEORGE PEMBERTON, KARLIS MUELENBACHS AND HANS MACHEL Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada ABSTRACT The formation of dolomite is generally explained using models that reflect larger-scale processes that describe the relationship between the supply and transport of Mg, and geochemical conditions that are amenable to the formation of dolomite. However, heterogeneities in the substrate, such as those made by bioturbating infauna, may play a more important role in dolomitization than has been previously considered. Burrow-facilitated dolo- mitization is evident in the Ordovician Tyndall Stone (Red River Group, Selkirk Formation) of central Canada. The diagenetic fabrics present are attributed to dolomitizing fluids that both flowed through and evolved within burrow networks. Petrographic analysis suggests that two phases of dolomite formation took place. The first formed a fine-grained, fabric-destructive type that probably accompanied early burial; the second is a fine- to medium-grained, locally sucrosic dolomite that is interpreted to have precipitated during later burial. Isotopic analysis supports the proposed paragenetic history: (1) an apparent linking of the stable isotopes 13C and 18O strongly suggests that the micrite matrix formed during very early diagenesis and was derived from seawater; (2) the initial phase of dolomitization is potentially microbially mediated, as evidenced by the enrichment of 13C; and (3) isotopic values for the second generation of dolomite reflect the mixing of ground water and resorbed early dolomite. This paper conceptualizes the physical and chemical conditions required for the formation of dol- omite in association with burrow fabrics. The proposed model reveals a composite of biological and inorganic reactions that demonstrates the interdependence of sediment fabric, organic content and microbial interactions in the development of burrow-mottled dolomitic limestone. It is suggested that where burrow-associated dol- omite occurs, it is most likely to develop in two stages: first, the byproducts of the degradation of organic mate- rial in burrows locally increase the permeability and porosity around burrow fabrics in shallow diagenetic depositional environments; and, second, the passing of burrowed media into deeper dysaerobic sediment is accompanied by the establishment of fermenting micro-organisms whose byproducts mediate dolomitization. Received 17 November 2003; accepted 29 January 2004 Corresponding author: Dr Murray Gingras. E-mail: [email protected] example of this is the Ordovician Tyndall Stone of the Red INTRODUCTION River Group in Manitoba, Canada: Tyndall Stone is discussed As discrete structures in calcareous strata, burrows effect in detail below. changes in many of the physical parameters manifested in Many models exist for the large-scale processes required to carbonate strata. These include permeability (Gingras et al., form dolomite in carbonate strata. These include seepage/ 1999), porosity, fabric and texture. Such changes are reflux, freshwater/seawater mixing, various burial settings, attributed to dissolution and precipitation of sedimentary sabkha dolomite and the Coorong model (e.g. Machel & minerals within and adjacent to trace fossils. Thus, Mountjoy, 1986; Morrow, 1990). On a much smaller scale, diagenetically altered zones commonly surround burrows in there are physicochemical heterogeneities that appear to calcareous – and siliceous – media. Dolomitization, for influence the distribution of ‘patchy’ dolomite. Variability on instance, can occur in diagenetic halos several millimetres in the centimetre scale is strongly influenced by the activities thickness around and within the burrows. An excellent of bioturbating infauna. © 2004 Blackwell Publishing Ltd 21 22 M. K. GINGRAS et al. Many studies have suggested that primary fabrics and Kendall, 1977; Morrow, 1978; Jones et al., 1979; Chow & textural properties strongly influence common dolomitization Longstaffe, 1995). The objective of this paper is to discuss this patterns observed in the rock record (Beales, 1953; Murray & premise using the example of a bioturbated carbonate wackes- Lucia, 1967; Kendall, 1977; Morrow, 1978). As diagenesis tone, the Ordovician Tyndall Stone of Manitoba Canada. proceeds, the original texture of the rock deteriorates and becomes subordinate to the diagenetic texture; however, at GEOLOGICAL AND STRATIGRAPHIC SETTING the onset of diagenesis the primary sedimentary character of the deposit must exert a control on the patterns of remineral- Tyndall Stone is a dolomite-mottled Ordovician limestone ization and dissolution. The above studies demonstrate the that is quarried in Manitoba, Canada. The formal designation potential of determining dolomite distribution by understand- of this unit is the Selkirk Member of the Red River Formation ing rock selectivity and thereby suggest that early dolomitiza- (Cowan, 1971; Kendall, 1977). Shallow drilling indicates that tion can be related to the development of the initial porosity/ the mottled limestone occurs in the lower half of the Selkirk permeability network. Contrasts between the burrow fill and Member (Kendall, 1977). Similarly mottled limestone comprises matrix potentially develop notable anisotropic permeability the subsurface equivalent Yeoman Formation, which is present that can be exploited by dolomitizing fluids (Beales, 1953; in south-eastern Saskatchewan (Fig. 1; Kendall, 1975). Both Fig. 1 Schematic summary of the stratigraphic relationships associated with the Red River Formation. © 2004 Blackwell Publishing Ltd, Geobiology, 2, 21–30 Conceptual models for burrow-related dolomitization 23 the Selkirk and Yeoman Formations are overlain by evaporitic interpenetrating Thalassinoides-like burrows (Fig. 2A). Upon dolomicrites that are in turn overlain by evaporites (Kendall, close scrutiny it is apparent that the Thalassinoides appearance 1975, 1977). is normally due to a dolomitic halo, 2–10 mm thick, Selkirk and Yeoman Formation strata were deposited as an encompassing causative burrows 2–3 mm in diameter (locally epicontinental carbonate platform in the Williston Basin. Palaeophycus and Chondrites; Fig. 2A). The dolomite is typi- Their distinct sedimentological and ichnological signature are cally fine grained, although sucrosic crystals may be adjacent reminiscent of other Palaeozoic epicontinental carbonates, to the causative burrows. Many of the burrows have been most notably the Devonian Palliser Formation of the Western internally filled by calcite cement, which forms an interlocking Canadian Sedimentary Basin (Beales, 1953), and the Upper crystal mosaic (Fig. 2A,D). Staining from the original organic Ordovician Bighorn Facies from Williston Basin deposits in material is locally present at the causative burrow margins. the USA (Zenger, 1996a,b). Kendall (1977) noted that the degree of dolomitic mottling varies locally, but is typically confined to areas adjacent to burrows. Less commonly, dolomite has destructively replaced METHODS aragonitic allochems and infilled rare, small vugs (Fig. 2F). The lithological database consists of 15 samples from outcrop Petrographic analysis reveals that four different forms of near Tyndall, Manitoba, and eight core fragments from a calcite or dolomite are commonly observed in the Tyndall single well penetration in southern Saskatchewan. All samples Stone. These include: micrite (C1), or microcrystalline calcite were thin sectioned and stained with Alizarin red. (Fig. 2B); very fine-grained dolomite (D1), characterized by Samples for isotopic analysis were extracted from both out- subplanar subhedral crystals that form an idiotopic mosaic crop and subsurface datasets. Each sample was taken from a texture with calcite allochems dispersed throughout – D1 discrete (dolomitized) burrow-mottle and the calcite matrix domains in C1 lend the mottled appearance to the Tyndall adjacent to it. Where practical, 20–50-mg isotope samples Stone (Fig. 2C); fine- to medium-grained euhedral dolomite were extracted using small, high-speed rotary saws. Small areas (D2), which forms isopachous to undulatory rinds in and isolated within and near dolomitized trace fossils were sampled adjacent to the causative burrow structure, and within rare, using 1-mm-diameter coring bits: the drilled samples typically millimetre-scale vugs (Fig. 2D); and a calcite mosaic cement weighed between 1 and 3 mg. (C2), infilling allochems, open burrows and voids (Fig. 2E). The samples were subsequently prepared for analysis by reacting them with 100% H PO at 25 °C using the technique 3 4 Isotopic data of Epstein et al. (1964). The isotopic composition of calcite and dolomite were estimated by collecting and analysing The isotopic dataset is shown in Fig. 3. Micrite C1 plots a crudely evolved CO2 after reaction times of 1 h and 1 week, respec- linear trend, whereas calcite C2 generates a slightly enriched tively. Isotopic analyses were made at the University of Alberta cluster (Fig. 3). Isotopic ratios from C1 range between δ13C with a Finnigan-MAT 252 mass spectrometer operated in the of −2.2‰ and δ18O of −10.5‰ to δ13C of 0.7‰ and δ18O of dual inlet mode. −7.8‰. By contrast, isotope values for C2 are constrained within