Preliminary Report on Conodont and Sm
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Preliminary Report on Conodont and Sm-Nd Isotope Data from Upper Ordovician Red River Strata (Herald and Yeoman Formations) in the Williston Basin, Berkley et al Midale 12-2-7-11W2, Southeastern Saskatchewan F.M. Haidl, C. Holmden 1, G.S. Nowlan 2, and K.C. Fanton 1 Haidl, F.M., Holmden, C., Nowlan, G.S., and Fanton, K.C. (2003): Preliminary report on conodont and Sm-Nd isotope data from Upper Ordovician Red River strata (Herald and Yeoman formations) in the Williston Basin, Berkley et al Midale 12-2-7-11W2, southeastern Saskatchewan; in Summary of Investigations 2003, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003-4.1, CD-ROM, Paper A-1, 13p. Abstract Recent research indicates that conodont and Nd isotope data are useful in interpreting the depositional history of Upper Ordovician epeiric sea carbonates in Saskatchewan and Iowa. Detailed analysis of an additional core (Berkley et al Midale 12-2-7-11W2) was undertaken to further document the usefulness of these tools in the interpretation of Red River strata in Saskatchewan. This paper reports the results of 74 Sm-Nd isotope analyses and 62 conodont analyses obtained from this core. Four conodont biofacies, based on conodont genera believed to be benthic or nektobenthic, are recognized for the Late Ordovician of Saskatchewan. These are, from shallowest to deepest: 1) Rhipidognathus biofacies; 2) Aphelognathus-Oulodus biofacies; 3) Plectodina biofacies; and 4) Phragmodus biofacies. In the Midale well, Aphelognathus-Oulodus biofacies and Plectodina biofacies dominate the Yeoman Formation and Coronach Member of the Herald Formation. Rhipidognathus specimens were recovered from only the top two samples in the Redvers Unit of the Herald Formation. Panderodus bergstroemi, believed to be a pelagic organism, is the only conodont species recovered from the Lake Alma Member. Values for ε Nd in the Midale well range from -10.8 to -7.1 indicating that the Nd isotope balance in seawater was primarily controlled by the relative contributions of Nd weathered from the highlands of the Taconic Orogen (εNd = -6 to -9) and possibly the extension of the Appalachian-Caledonian Orogen in the Arctic (εNd = -6 to -9), and that Precambrian rocks of the Transcontinental Arch and the Canadian Shield (εNd = -22 to -15) were not exposed on the margins of the Williston Basin during deposition of the Red River carbonates and anhydrites sampled in this well. The two samples with values of ε Nd less than -9.5 (in the lower Herald Formation) may be indicative of a contribution of Nd from weathered older Ordovician strata which, in other Saskatchewan cores, have more negative ε Nd values. Previous work has shown that, in circumstances in which increasing distance from paleo-shoreline is associated with increasing water depth in the basin, Sm/Nd chemostratigraphy can serve as an indicator of seawater paleo- depth. Sm/Nd ratios in the Midale well range from 0.145 to 0.234. Preliminary interpretation of Midale data suggests a correspondence between conodont paleoecology, Sm/Nd ratios, and lithological data: stratigraphic increases in Sm/Nd ratios reflect increasing water depth, whereas stratigraphic decreases in Sm/Nd ratios indicate decreasing water depth. Examples of these correlations include: 1) Two major shifts associated with kukersitic beds in the Yeoman Formation are interpreted to reflect deepening- shallowing events. Sm/Nd ratios increase below the kukersitic beds and then decrease within this facies. The Plectodina conodont biofacies below the kukersitic beds is replaced by a mixed Aphelognathus-Oulodus and Plectodina fauna within and above these organic-rich beds. 2) Isotope and conodont data adjacent to the contact of lower burrow-mottled Yeoman rocks with the overlying “transitional unit” suggest that a major relative sea-level change may be reflected by this lithologic change. 3) In the Lake Alma Member, shifts in the Sm/Nd ratios, the complex distribution of dolostone and anhydrite, and the paucity of conodonts suggest several changes in paleo-water depth in a saline to hypersaline environment. Keywords: Upper Ordovician, Saskatchewan, Red River, Herald, Yeoman, carbonates, anhydrite, conodonts, Sm and Nd isotopes. 1 Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2. 2 Geological Survey of Canada, 3303 - 33rd Street NW, Calgary, AB T2L 2A7. Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1 1. Introduction The relationships between sedimentology, conodont biofacies, εNd values, and Sm/Nd ratios are the subject of earlier studies conducted by the authors in Iowa and Saskatchewan (Nowlan and Haidl, 2001; Fanton et al., 2002). In order to further document the usefulness of these tools in the interpretation of depositional environments of oil-producing Red River strata in southeastern Saskatchewan, detailed sampling of a core from the Berkley et al Midale 12-2-7-11W2 well (Figure 1) was undertaken. This paper reports the results of 74 Sm-Nd isotope analyses and 62 conodont analyses obtained from this core which encompasses the upper 38.7 m of the Yeoman Formation and all but the upper 3.5 m of the Herald Formation. 2. Lithostratigraphy of Red River Strata in Berkley et al Midale 12-2-7-11W2 Middle to Upper Ordovician strata in southeastern Figure 1 - Map of southeastern Saskatchewan showing Saskatchewan comprise a basal clastic unit (Winnipeg locations of the Berkley et al Midale 12-2-7-11W2 well Formation) and an overlying sequence characterized by (star) and of four wells (solid circles) reported on in repetition of carbonate and evaporite lithologies Nowlan and Haidl (2001) and Fanton et al. (2002). (Yeoman, Herald, Stony Mountain, and Stonewall formations, Figure 2). The stratigraphic nomenclature established for Red River strata in the study area (Figures 2 and 3) reflects the interpretation that the repetition of carbonate and evaporite strata represents three brining-upward cycles. The Yeoman Formation and the lower Herald (Lake Alma Member) form the oldest cycle, the middle part of the Herald Formation is the middle cycle (Coronach Member), and the Redvers Unit encompasses the youngest cycle (Kendall, 1976). The evaporite unit at the top of the youngest cycle is present only in North Dakota (Longman and Haidl, 1996). New core and geophysical log data from the more than 300 wells drilled into the Red River since 1976 suggest a more complex depositional history than the current stratigraphic breakdown implies (Pratt et al.,1996; Haidl et al., 1997; Canter, 1998; Kent and Kissling, 1998; Kreis and Kent, 2000). Conodont and isotope data from previous studies by the authors and from this study also support the interpretation of a complex depositional history (Nowlan and Haidl, 2001; Fanton et al., 2002). a) Yeoman Formation Burrow-mottled Unit The Yeoman is 90.9 m thick in the Berkley et al Midale 12-2-7-11W2 well. Only the uppermost 38.7 m of this formation are cored of which the lower 35.4 m are composed primarily of fossiliferous lime mudstones and wackestones characterized by abundant trace fossils and varying degrees of dolomitization (Figure 3). The mottled texture that characterizes much of the Yeoman in southeastern Saskatchewan and in stratigraphically equivalent rocks in adjacent areas dominates this core from 2570 m to the base (2605.4 m) (Figure 4). This mottling is primarily the result of Thalassinoides burrows that commonly contain secondary Planolites, Paleophycus, and Chondrites burrows (Pak and Pemberton, this volume). Macrofossils identified include corals, brachiopods, bryozoans, echinoderms, stromatoporoids, and gastropods. Kukersitic laminae/beds are preserved in several limestone intervals, the thickest of which occurs at 2577.5 to 2578.8 m (Figures 3 and 5). The micro-alga Gloeocapsomorpha prisca, of probable cyanobacterial origin (Stasiuk and Osadetz, 1991; Stasiuk et al., 1993), is the primary component of these organic-rich rocks. “Transitional” Unit The uppermost 3.32 m of the Yeoman in this well are composed of interbedded dolowackestone, dolopackstone, and dolograinstone with abundant skeletal material, coated grains, and peloids (Figures 3 and 6). Lithologic variations at this stratigraphic interval in other wells in southeastern Saskatchewan include stromatoporoid dolorudstone, microbial doloboundstone, oolitic dolograinstone, and dolomudstone/wackestone (Pratt et al., 1996; Haidl et al., 1997; Kreis and Kent, 2000) The contact of this “transitional unit” with the overlying Lake Alma Member of the Herald Formation is difficult to pick in core and on geophysical logs. In this well, the contact is Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 1 placed at the base of a peloidal unit that is interpreted to correlate with a bed characterized by high gamma reading on geophysical logs. Other workers place the “transitional unit” in the Lake Alma (e.g., Kreis and Kent, 2000). b) Herald Formation Lake Alma Member A thin (0.2 m) peloidal bed at the base of the Lake Alma is overlain by a unit of laminated to bedded dolomudstones. The overlying Lake Alma Anhydrite comprises a lower unit dominated by nodular anhydrite (Figure 7), a middle unit of dolomudstone with anhydrite laminae, and an upper unit of laminated to bedded anhydrite with abundant dolostone beds and laminae (Figure 3). The contact with the overlying Coronach Member is placed above a thin (1.0 cm) bed of intraclasts. Coronach Member Figure 2 - Stratigraphic correlation chart of Lower Paleozoic strata in southeastern The Coronach comprises, in Saskatchewan. Numeric values for isotopic ages from Okulitch (2003). ascending order: 1) laminated argillaceous dolomudstone (0.2 m thick); 2) fossiliferous burrow- mottled limemudstone/wackestone; 3) mottled dolomudstone/dolomitic lime mudstone; 4) lime mudstone/ wackestone with abundant skeletal debris including laminar stomatoporoids; 5) laminated lime mudstone; laminated dolomudstone with stromatolitic intervals (Figure 8); and 6) an anhydrite bed (Figure 3). The contact with the overlying Redvers Unit is placed at the top of the Coronach Anhydrite.