Red River Carbonates (Saskatchewan): Major Facies, Lithological, and Spatial Controls on Rock Magnetism – Preliminary Observations Erika Szabo 1 and Maria T. Cioppa 2 Szabo, E. and Cioppa, M.T. (2003): Red River carbonates (Saskatchewan): Major facies, lithological, and spatial controls on rock magnetism – preliminary observations; in Summary of Investigations 2003, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.1, CD-ROM, Paper A-2, 9p. Abstract Preliminary results from ten wells in the Saskatchewan portion of the Williston Basin reveal significant lithological, facies, and spatial controls on the natural remanent magnetization (NRM) intensity in Red River carbonates. High NRM intensity values are recorded within the limestone sequences, intermediate values are seen in burrowed/fossiliferous dolostone, and the lowest values of NRM intensity are measured in laminated/bedded dolostones associated with anhydrite. The NRM intensity tends to increase towards the centre of the Williston Basin suggesting that the most probable variables affecting NRM intensities are: 1) depth of sampling and 2) proximity to the North American Central Plains Conductivity Anomaly. With a few exceptions, the magnetite grain-size distribution determined from partial anhysteretic remanent magnetization (pARM) spectra seems to be unaffected by facies and other lithological variations, or by spatial position. Well 0210, in the northwestern part of the Williston Basin, however, has magnetite grain-size values that are predominantly in the pseudosingle domain range whereas samples from the other wells display a mixed pseudosingle and single domain grain-size distribution. These findings suggest that fluid flow from the Alberta Basin may have influenced this portion of the Williston Basin. Keywords: Upper Ordovician, Williston Basin, Red River, Yeoman Formation, Herald Formation, Saskatchewan, rock magnetism, carbonates, facies control, lithological control, spatial control. 1. Introduction Paleomagnetic studies of several Paleozoic formations in the Alberta Basin indicate that paleomagnetic components of different ages can be correlated with lithological facies, and that detailed paleomagnetic and rock magnetic analyses can determine the order and timing of diagenetic events (Cioppa et al., 2001). Information on paleomagnetism in the Williston Basin is limited (e.g. Enkin et al., 2001; Cioppa, 2003); however, reliable paleomagnetic data obtained from preliminary studies in the basin (Cioppa, 2002, unpubl. data) suggest that further research is warranted. This paper presents preliminary results from a detailed paleomagnetic and rock magnetic investigation of Red River carbonates in Saskatchewan. We examine the potential for facies or spatial controls of paleomagnetic patterns in these rocks which are characterized by several different lithological facies and dolomitization events. 2. Geology The Williston Basin is ~800 km in diameter. It covers much of southern Saskatchewan and Manitoba in Canada and extends southward through Montana, North Dakota, and South Dakota in the United States (Figure 1). The Precambrian basement, which is reached at a maximum depth of ~5 km in North Dakota, features several structures that influence the generation and migration of hydrocarbons in the Williston Basin (Osadetz et al., 1992). The North American Central Plains Conductivity Anomaly (NACPCA, Figure 1), a 2000 km long and 80 km wide structure at a depth of 10 to 20 km, is characterized by basal heat flow that is ~20% higher than elsewhere in the basin (Jones and Craven, 1990; Jones and Savage, 1986). 1 Earth Sciences, University of Western Ontario, London, ON N6A 5B7; E-mail: [email protected]. 2 Department of Earth Sciences, University of Windsor, Windsor, ON N9B 3P4. Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1 Figure 1 - Distribution of Ordovician and Silurian strata within the Williston Basin with locations of sampled wells (NACPCA- North American Central Plains Conductivity Anomaly) (after Norford et al., 1994 and Morel-a-l’Huissier et al., 1990). Upper Ordovician Red River carbonates comprise the Yeoman and Herald formations (Norford et al., 1994). Within conformable limits, these formations are underlain by clastics of the Winnipeg Formation and overlain by carbonates of the Stony Mountain Formation. The Yeoman Formation is the thickest formation of the Red River carbonates. Various fossils and traces of different burrows are found in the Yeoman. The larger burrows are Thalassinoides-like structures that are thought to have been re-burrowed, as indicated by the smaller included burrows (i.e. Planolites, Chondrites) (Kendall 1976, 1977; Haidl et al., 1997; Canter, 1998; Pak et al., 2001). Several types of dolomitization are present in the Yeoman Formation. The burrows are commonly dolomitized, and may be associated with either a limestone or dolostone matrix. Also, minor saddle dolomite cement has been precipitated in vugs, fractures, and geopetal structures (Carroll, 1979; Qing et al., 2001). The Herald Formation is divided into the Lake Alma and Coronach members, and the overlying Redvers unit (Kendall, 1976). Laminated to bedded dolostones and calcareous dolostones, sometimes interbedded with burrowed or fossiliferous dolostones, lie beneath nodular, bedded, and laminated anhydrite with anhydritic dolostone Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 1 interbeds in the Lake Alma Member (Nowlan and Haidl, 2001). From bottom to top, the Coronach Member is composed of argillaceous dolostones, fossiliferous and burrowed limestones and dolostones, laminated dolostones that are locally limestones, and an upper anhydrite (Kendall, 1976; Nowlan and Haidl, 2001). The base of the Redvers unit is argillaceous dolostone, which is overlain by an upper laminated dolostone or limestone (Nowlan and Haidl, 2001). In the Lake Alma Member of the Herald Formation, dolomitization is recognized in the form of non- porous cryptocrystalline and porous crystalline dolostone (Qing et al., 2001). 3. Methodology Systematic sampling, utilizing the paleomagnetic sampling procedure described by Lewchuk et al. (1998) and Cioppa et al. (2000), was carried out on core from ten wells in southern Saskatchewan (Table 1). Nine wells are located along two cross-section lines: one east-west (A-A’) and the second (B-B’) north-south (Figure 1). The tenth well (0210) was sampled in the west-central part of the province, removed from the other sampling locations (Figure 1). Detailed macroscopic lithological observations were made of each core. A total of 152 plugs, each 2.5 cm in diameter, were drilled from the well cores. From the core plugs, 434 specimens were cut. As the first phase of this project, we measured the natural remanent magnetization (NRM) of all specimens using a vertical- configuration 2G cryogenic magnetometer with a lower sensitivity limit of ~2x10-6 A/m. Thirty pilot specimens (2 to 4 specimens per well) were subjected to alternating field (AF) demagnetization before any rock magnetic experiments were performed. Results of these measurements will be presented in the near future. We also measured stepwise partial anhysteretic remanent magnetization (pARM) on the pilot samples by treating the specimens in steps of 10 mT up to 100 mT using the procedure introduced by Everitt (1961) and a Sapphire Instruments SI-4 Alternating Field (AF) demagnetizer. The AF and pARM demagnetization procedures differ in that the pARM procedure has a small biased direct circuit (DC) field applied over a section of the AF demagnetizing field (i.e. DC between 10 and 20 mT over AF demagnetization from 100 mT). The procedure is performed because coercivity is a function of grain size and by doing a step sequence of DC biases (0 to 10 mT, 10 to 20 mT, ... , 90 to 100 mT) an estimate is obtained of the relative amounts of magnetic grain sizes. The percentage of the different ranges of magnetic grain sizes is approximated by the measured pARM values and their corresponding grain size ranges as defined by Jackson et al. (1988). Thus, the magnetization intensity values for the untreated specimens are considered to represent the amount of multidomain (MD) size grains, the sum of pARM values obtained over the intervals 0 to 10 mT to 30 to 40 mT represent the amount of pseudosingle domain (PSD) particles, and the sum of the pARM values for the remaining intervals (40 to 50 mT to 90 to 100 mT) represent the amount of single domain (SD) magnetic particles. All experiments were done in a magnetically shielded room in the Paleomagnetics Laboratory at the University of Windsor (Canada). 4. Results and Preliminary Observations a) Natural Remanent Magnetization (NRM) The NRM intensities vary from well to well across the study area and also within individual wells with values ranging from 10-6 A/m to 10-2 A/m. NRM intensities of specimens cut from the outside of most plugs were systematically higher than those cut from the interior of the plug (e.g. for ~12% of the plugs, the outside specimens exceeded twice the NRM values from specimens from the middle of the plug). It is considered that the outer surface of a core, which would have been in direct contact with the core barrel, is much more likely to acquire a drilling- induced viscous magnetization than the interior of the core. Therefore, the analysis of the NRM intensity distribution was carried out only on specimens cut from the middle of the core plugs. Table 1 - List of sampled cores. Well Well Name Location Logged Interval (m) Formation Sampled 0201 Pan American White Bear Cres. 5-15-10-2W2 2148.1 to 2482.6 Herald and Yeoman 0202 Ceepee Annette 10-25-36-5W2 3083.0 to 3103.0 and Yeoman 3154.0 to 3164.0 0203 Esso Bromhead 6-28-3-12W2 2875.6 to 2893.6 Yeoman 0204 PCP Scurry et al Lake Alma 5-29-01-17W2 3071.0 to 3089.5 Herald and Yeoman 0205 Trilink Hazelwood 4-14-11-5W2 2225.0 to 2243.0 Yeoman 0206 Mark Saskoil Minton 1-10-3-21W2 2856.0 to 2874.0 Yeoman 0207 T.W. Wapella Cr. 12-34-14-1W2 1677.5 to 1692.8 Herald and Yeoman 0208 Imp.