A Revised Lithostratigraphic Framework and Characteristics of the Upper Devonian Duperow Formation, Southeastern Saskatchewan X.C. Cen 1 and O. Salad Hersi 1 Cen, X.C. and Salad Hersi, O. (2006): A revised lithostratigraphic framework and characteristics of the Upper Devonian Duperow Formation, southeastern Saskatchewan; in Summary of Investigations 2006, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2006-4.1, CD-ROM, Paper A-9, 17p. Abstract The Duperow Formation in southeastern Saskatchewan forms the lower part of the Upper Devonian Saskatchewan Group and consists of three members: Saskatoon, Wymark, and Seward, in ascending order. The Saskatoon Member (17 to 37 m thick) consists of stromatoporoid floatstone, bioclastic wackestone/mudstone and subordinate dolomudstone, and anhydrite. Bioclasts include globular stromatoporoids, corals, brachiopods, bivalves, and crinoids. The Wymark Member (77 to 145 m thick) is also dominated by stromatoporoid/Amphipora floatstone/bafflestone and bioclastic packstone to mudstone with minor dolomudstone and anhydrite interbeds. A thin (0 to 6 m thick), discontinuous halite unit (the Flat Lake Evaporite) locally caps this member. Bioclasts in the carbonate portion of the member include globular stromatoporoids, Amphipora branches, corals, brachiopods, gastropods, and crinoids. The Seward Member (31 to 78 m thick) contains burrow-mottled bioclastic rudstone, packstone, wackestone, mudstone, dolomudstone, and rare anhydrite layers. Gastropods, bivalves, brachiopods, and crinoids form the most abundant framework grains in the member. The three members are internally organized in a shallowing-upward rhythmic sedimentation defined by recurring intervals of bioclast-rich, normal-marine, subtidal to inter-tidal lithofacies grading upward into more restricted lime-mudstone and dolomudstone, and evaporites. The restricting-upward rhythmic sedimentation is attributed to be the result of high-order, relative sea- level fluctuations affecting a broad lagoonal basin (the Duperow Embayment) separated from the open-marine Alberta Basin by coeval carbonate build-ups (e.g., the Leduc reefs). Keywords: southeastern Saskatchewan, Upper Devonian, Duperow Formation, Saskatoon Member, Wymark Member, Seward Member, Flat Lake Evaporite, lithostratigraphy, carbonate-evaporite cycles, Williston Basin. 1. Introduction The Duperow Formation is a Late Devonian carbonate-dominated stratigraphic unit that accumulated in the Williston Basin and occurs in the subsurface of southern Saskatchewan, North Dakota, and Montana. Despite the formation’s chronostratigraphic equivalence to strata that host and produce significant amounts of hydrocarbons (e.g., the Leduc reefs of Alberta and the Duperow Formation of North Dakota and Montana), no substantial petroleum-producing reservoirs have yet been recognized in Saskatchewan. Also, as the stratigraphic, sedimentological, and diagenetic characteristics of the Duperow Formation in Saskatchewan are poorly known in comparison to those of equivalent intervals in the nearby regions, our studies are expected to improve our understanding of the reservoir potential of the formation, and may eventually kindle exploration interest. This paper presents the results of a detailed stratigraphic subsurface investigation of the Duperow Formation of southeastern Saskatchewan. The study area occupies the north-central and northeastern parts of the Williston Basin. It comprises a rectangular area bounded to the east by the Saskatchewan-Manitoba border, to the west by the Third Meridian, to the south by the Canada-U.S.A. border, and to the north by Township 20 (Figure 1). Our findings are based on data collected from 27 cores and 66 wireline logs (Table 1 and Figure 2). More detailed sedimentological information and microfacies examination of the Duperow Formation in southeastern Saskatchewan are provided in another paper in this publication (see Cen and Salad Hersi, this volume). 1 Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2; E-mail: [email protected] or [email protected]. Saskatchewan Geological Survey 1 Summary of Investigations 2006, Volume 1 0 km 300 2. Geological Setting n The Duperow Formation and the a w e conformably overlying Birdbear h Alberta tc Formation form the Upper a k Devonian Saskatchewan Group. s Calgary a Manitoba n S The Duperow conformably i overlies the Souris River s a Regina Formation of the Manitoba B Group (Figure 3). It was Study area deposited during the Late BC a t Devonian in the southeastern r e Western Canada Sedimentary b Duperow Basin (WCSB) in a backreef l setting southeast of the Leduc A Embayment reef barrier of the Alberta Basin. North Dakota The WCSB, a large, northwest- Montana southeast–oriented basin extending from the northwestern South Northwest Territories to Dakota southwestern Manitoba, was, in Idaho Devonian times, dominated by Wyoming accumulation of thick carbonate deposition (Moore and Richards, Figure 1 - Map showing location of the study area in southeastern Saskatchewan 1989; Wendte, 1992a, 1992b). (boxed area) (modified from Ehrets and Kissling, 1985). The strata are made up of thick, carbonate-dominated rhythmic Table 1 - Well localities in the study area in southeastern Saskatchewan referred to megasequences that indicate in this paper. large-scale sea-level fluctuations. These megasequences include, in Core ascending order, the Upper Elk Location Well Number Wymark Point (Gedinnian to Givetian), Sasktoon Seward lower middle upper the Beaverhill Lake (Givetian to Lower Frasnian), the Woodbend 1 131/11-36-22-1W2 x x x x x (Lower to Middle Frasnian), the 2 16-10-3-25W2 x x x x Winterburn (Upper Frasnian to 3 15-28-12-2W2 x lowermost Famennian), and the 4 6-33-7-8W2 x x x Wabamun (Lower to Upper 5 9-32-7-6W2 x Famennian) (Campbell, 1992; Stoakes, 1992a; Wendte, 1992a, 6 1-8-2-6W2 x x x 1992b, among others). They are 7 8-30-7-8W2 x well developed in the Alberta 8 2-11-10-9W2 x Basin (Figure 4) and become 9 16-34-6-11W2 x x thinner eastward into 10 12-2-7-11W2 x x Saskatchewan. This eastward 11 7-29-6-11W2 x x thinning is accompanied by pronounced lithological changes 12 3-26-4-20W2 x x that warrant application of 13 1-10-3-21W2 x different lithostratigraphic terms 14 11-19-5-28W2 x x x for strata in the eastern regions, 15 16-11-6-25W2 x x x x e.g., Saskatchewan (North 16 7-36-7-25W2 x American Commission on 17 11-14-22-6W2 x x Stratigraphic Nomenclature, 1983). The Beaverhill, 18 6-29-22-6W2 x Woodbend, and Winterburn 19 6-34-14-12W2 x megasequences correlate with, 20 1-28-24-10W2 x respectively, the Manitoba 21 9-20-20-25W2 x x x Group, the Duperow Formation 22 14-10-17-22W2 x x x and the Birdbear Formation of 23 16-10-21-25W2 x x southern Saskatchewan (Figure 4; Stoakes, 1992b). The 24 1-24-20-33W1 x x x x x Beaverhill, Woodbend, and 25 4-16-18-23W2 x Winterburn megasequences of 26 4-21-7-22W2 x 27 141/11-36-22-1W2 x x x x Saskatchewan Geological Survey 2 Summary of Investigations 2006, Volume 1 104 102 106 1 33 32 31 30 29 28 27 26 25 24 23 22 21 20 19 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 25 20 24 23 18 1 27 22 17 23 21 21 24 20 19 18 25 22 17 MOOSE JAW REGINA 16 2 15 1 re 1 14 u 19 ig d 1 F n 13 ,a 0 3 12 , 1 11 , 9 s 8 8 10 re u 9 ig F WEYBURN 8 10 4 5 7 16 26 7 6 15 9 11 14 5 4 12 2 3 13 6 2 ESTEVAN 1 49 Figure 2 - Location of the study wells (filled circles represent wells with cores; open circles represent wells with no cores; both types of wells have wireline logs that are examined in this study). the Alberta Basin are dominated by reefs, open-marine carbonates, and deep-marine shales (Campbell, 1992; SOUTHERN PERIOD STAGE/ Stoakes, 1992b, 1992c). Toward the east, typical AGE SASKATCHEWAN depositional sites include reef-sheltered, shallow- BAKKEN marine to lagoonal settings where cyclic sedimentation THREE of open to restricted carbonates and evaporites FORKS BIG VALLEY FAMENNIAN GROUP predominate. The Duperow Formation, which is the TORQUAY focus of this study, is characterized by rhythmically R BIRDBEAR E bedded, shallow-marine limestones, dolostones, and P P SASKATCHEWAN evaporites organized in a shoaling-upward pattern U GROUP DUPEROW N FRASNIAN (Andrichuk, 1951; Wilson, 1967; Kent, 1963, 1968; A I Dunn, 1975; Wilson and Pilatzke, 1987). The formation N SOURIS RIVER MANITOBA was deposited in a south-southeast–oriented, shallow- O GROUP marine, inner platform setting termed here the V DAWSON BAY E “Duperow Embayment” (Figure 1). Besides E GIVETIAN L PRAIRIE D D ELK EVAPORITE correlations with known Late Devonian strata (e.g., D I POINT Stoakes, 1992a), recent biostratigraphic conodont M EIFELIAN WINNIPEGOSIS GROUP ASHERN dating (McCracken and Kreis, 2003) has indicated that DALEJAN the Duperow Formation is Late Devonian (Frasnian) in R EMSIAN E age. W PRAGIAN O L LOCHKOVIAN Figure 3 - Chart showing Devonian stratigraphic succession of southwestern and southeastern Saskatchewan (after Sask. Industry Resources, 2003). Saskatchewan Geological Survey 3 Summary of Investigations 2006, Volume 1 Saskatchewan Geological Survey 4 Summary Northwest Alberta Southeast NORTHEASTERN SOUTHERN PEACE STURGEON BRITISH COLUMBIA SASKATCHEWAN RIVER LAKE LEDUC KOTCHO THREE FORKS BIRDBEAR WABAMUN GROUP TETCHO,TROUT RIVER,KAKISA REDKNIFE RN GROUP DUPEROW WINTERBU IRETON LEDUC JEAN MARIE WOODBEND FORT SIMPSON GROUP IVER RIS R LEDUC LEDUC SOU ROUP GRANITE AKE G RHILL L WASH BEAVE LINE OF ofInvestigations 2006,Volume1 CROSS SECTION SHALLOW WATER CARBONATE OPEN MARINE BASINAL RESTRICTED CARBONATE- EVAPORITIC SUCCESSION Figure 4 - Schematic cross section showing the lateral relationship between Devonian strata of southern Saskatchewan and those of the Alberta Basin to the west (modified from Stoakes, 1992b).