J.E. Christopher 1 and M. Yurkowski

Transcription

1 An Upper Cretaceous (Milk River Formation) Analogue of the Swift Current Platform and Its Relationship to Elements of the Precambrian Basement, Southern Saskatchewan J.E. Christopher and M. Yurkowski Christopher, J.E. and Yurkowski, M. (): An Upper Cretaceous (Milk River Formation) analogue of the Swift Current Platform and its relationship to elements of the Precambrian basement, southern Saskatchewan; in Summary of Investigations, Volume, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. -., CD-ROM, Paper A-, p. Abstract In western Saskatchewan, the Milk River Formation directly overlies the Niobrara Formation. Eastward, this contact truncates underlying units such that Milk River strata mostly overlie the Carlile Formation. The Milk River Formation is unconformably overlain by the Lea Park (Pierre); the hiatus increases eastward into eastern Saskatchewan and southwestern Manitoba by loss of Milk River section. At LMLE Penzance /---W, the Milk River Formation is in sharp contact with the Carlile Formation. All but the upper. m of the. m thickness of the Milk River is displayed in core. It is predominantly medium greenish grey calcareous and noncalcareous, fossiliferous (mostly baculitids, juvenile ammonites, and inoceramids), concretionary (calcitic and sideritic) mudstone with very fine-grained quartzose sand as minor to subordinate components of poorly defined bedding cycles, and limestone as thin, capping end members of a few cycles. The section displays, from bottom to top, beds, sequentially. m,. m,. m,. m,. m,. m,. m,. m,. m,. m,. m, and >. m thick. All but the tenth and, perhaps, the eighth are upward shallowing as indicated by the increase in marl, quartzose sand, and/or cemented limestone. Calcareous white specks are abundant in sequence two and scarcer in sequence ten. Sharp contacts between cycles in the core are few, and are not considered to be regionally significant, except for the cemented algal limestone pavement on sequence, about. m below the top of the Milk River Formation and on sequences six and four. These indicate significant depositional breaks, with marine draw-downs and subsequent transgressions. Additionally, the massive mudstone bed, basal to sequence, reflects a major late transgressive phase. The Milk River Formation is at its thickest ( to m) between Rges W and W. It thins westward to m at the border with Alberta and eastward to less than m at the Manitoba border. From Tp, Rge W southeastward to Tp, Rge W, the formation thins dramatically to regional values below m across a pediment to km wide. The pediment represents a sub-lea Park erosional escarpment overlooking a stripped lowland region on Milk River strata. Milk River thickening in the central area reflects increased accommodation created by penecontemporaneous subsidence of the depositional basin. The term Swift Current Platform, as originally defined, refers to a triangular region of thinning in Devonian strata of southern Saskatchewan corresponding to the area of no salt in the Middle Devonian Prairie Evaporite. Studies of Mesozoic strata indicate this region to have undergone episodic structural downwarp that provided for increased sedimentary accommodation and subsequent isostatic uplift under an erosional regime. In this context, the Milk River depositional basin on the Swift Current Platform was succeeded by a Milk River Paleo-upland of valleys and mesas, some of which were structural block and lineament controlled. Several of these elements are spatially coincident with deeply buried Precambrian basement features as depicted on the aeromagnetic map of the province. Keywords: Upper Cretaceous, Campanian, Milk River Formation, Niobrara Formation, lineaments, Elbow- Weyburn lineament, Missouri Coteau, Swift Current Platform, sub-lea Park unconformity, paleogeomorphology, basement reactivation.. Background The term Swift Current Platform was introduced by Baillie (, p) to designate an area in south-central Saskatchewan that is a prominent feature on many of the isopach and lithofacies maps here used... The platform area projects into the main basin from the south and is approximately outlined by the -foot isopach of the total Devonian isopach map. Though manifested as isopachous thinning of the Elk Point Group from feet ( m) Geological Consultant, Coldwell Road, Regina, SK SR L. Saskatchewan Geological Survey Summary of Investigations, Volume

2 in the basin to to feet ( to m), a structurally positive Middle Devonian Swift Current Platform is assumed. Subsequent refinement in the literature has focused on the setting of its structural boundaries. In general, the platform has been referred to as an area outlined by the triangular region of no salt at the level of the Middle Devonian Prairie Evaporite (Holter,, Figure ). The platform, in this sense, extends west along the border with the United States of America, i.e., the th parallel, from about Rge W into the Jurassic oilfield trend at about W in southwestern Saskatchewan, and centrally from the border to a trapezoidal apex near Elbow at Tp, Rges W to W. The western flank of the Swift Current Platform has been more precisely located through recognition of the northoriented Shaunavon Graben between Rges W and W. South of Tp, the graben terminates Middle Jurassic shoreline sandstones blanketing the Shaunavon Monocline to the west against deeper water shales on the Swift Current Platform to the east. In the Upper Jurassic, the graben terminates Roseray shore-face sandstones on the Swift Current Platform to the east against deeper water shales over the Shaunavon Monocline to the west (Christopher,, p, Figure ). It apparently served as a hinge for structural blocks moving in opposition to one another. The Swift Current Platform, which as a Middle Jurassic depositional basin lay more than m below the escarpment of the Shaunavon shelf, reversed its position in the Late Jurassic by some m to command the Early Cretaceous Mannville topography as a dissected upland with relief of more than m. Uplift appeared to have been greatest in Tps to, Rges W to W, northwest of Swift Current. Regional dip was to the southeast into the Williston Basin. The eastern region of the Swift Current Platform features the northwesterly trending Elbow-Weyburn lineament belt (Christopher, ) here expanded to incorporate the western edge of the Middle Devonian Winnipegosis shelf as delineated by the m isopach of the Upper Winnipegosis Member of Jones (), the in-lying (by to km) eastern edge of the Prairie Evaporite salt dissolution front, and the present day topographic Missouri Coteau, km to the west. The lineament belt spans the eastern half of Jones similarly aligned Elbow-Weyburn basin facies, in which the regional Winnipegosis thickens abruptly to m against the scarp of the regional to m-thick eastern shelf facies. The keel of this basin coincides with the Missouri Coteau. Westward of the keel, the Upper Winnipegosis broadly thins to less than m, and abruptly to zero along Rge W southeast of Swift Current. This edge, however, lies east of the Middle Jurassic Shaunavon shelf (Christopher, ), and may indicate that the intervening region of the Shaunavon Graben in Winnipegosis time was structurally positive and, therefore, an extension of the Shaunavon Monocline to the west. Thus the eastern half of the Elbow-Weyburn Winnipegosis basin in this context appears to have been a northwesterly trending eastern analogue of the north-trending Shaunavon Graben. The northwestern and southern boundaries of the platform remain poorly delineated. The locations of these geological features are shown in Figure.. The Milk River Formation a) Stratigraphic Fabric The Milk River Formation of southeastern Alberta and southwestern Saskatchewan represents the basal portion of the Montana Group. It is traceable into the Telegraph Creek Formation and overlying Eagle Sandstone in northcentral Montana, and the Telegraph Creek and overlying Gammon Shale of Wyoming (Ridgley,, ). In southeastern Alberta and southwestern Saskatchewan, the members of the Milk River Formation are (oldest to youngest) Telegraph Creek, Virgelle, and Deadhorse Coulee, representative of an upward progradation from offshore shale, to shoreface sandstone, to fluvial sandstone and mudstone. These deposits taper eastward across southwestern Saskatchewan as upward-coarsening clinoforms of mudstone and sandstone. Pederson () described three facies in the Hatton-Lacadena area of southwestern Saskatchewan comprising: a) silty shale interbedded with <. mm-thick quartzose siltstone and very fine-grained sandstone, moderately bioturbated; b) silty shale interbedded with <. m-thick siltstone and well sorted, parallel-bedded and locally cross-bedded, very fine-grained quartzose sandstone with moderate to rare bioturbation; and c) well sorted, diversely bioturbated, ripple-laminated and hummocky cross-stratified quartzose sandstone with <% mudstone. All facies are indicative of an open-marine, storm-influenced basin below fair-weather base. In western Saskatchewan, the Milk River Formation overlies the Niobrara Formation. Eastward, the contact truncates underlying units so that Milk River strata mostly overlie the Carlile Formation. The top of the Milk River is in unconformable contact with the Lea Park (Pierre); the hiatus increases eastward into eastern Saskatchewan and southwestern Manitoba by loss of Milk River section (Figure ). The overall format of the foregoing description, except for the expected seaward fining of the formation, holds true for the study area. In the LMLE Penzance /---W well, the Milk River Formation lies at depths. to. m (as picked on the geophysical log, Figure ) and is in sharp contact with the Carlile Formation. All but the upper. m of the. m thickness of the formation is displayed in core, which in quality of recovery and in length is unique to central southern Saskatchewan. Here, the Milk River Formation is a variable mixture of medium greenish grey mudstone that is calcareous to noncalcareous, fossiliferous (mostly baculitids, juvenile Saskatchewan Geological Survey Summary of Investigations, Volume

3 Saskatchewan Geological Survey Summary of Investigations, Volume Alberta Burstall Macklin Assiniboia Redvers A Eastend Ogema A Leader Luseland Eatonia Maple Creek Shaunavon Monocline Unity Kerrobert Kindersley Wilkie Scott Eston Shaunavon Cabri Gull Lake Shaunavon Graben Kyle Biggar SWIFT CURRENT Asquith Zealandia Rosetown SASKATOON Osler Warman Vonda Wakaw Colonsay Allan Bruno m Raymore Imperial Springside Govan Davidson Elrose Ituna YORKTON B Craik Strasbourg B Herbert Ponteix Radisson m Dalmeny Morse Langham Delisle Outlook Central Butte Waldheim Dundurn Swift Current Platform Hanley Gravelbourg Lafleche m Willow Bunch Mossbank Rockglen Cudworth Watrous Humboldt Lanigan Nokomis Leroy Regina B. Lumsden Naicam Watson Southey Wynyard Cupar Wadena Qu'Appelle Pilot Butte Balgonie Wolseley MOOSE JAW REGINA Rouleau Ft. Qu'Appelle Milestone Rose Valley Francis Kelvington Foam Lake Balcarres Lemberg Montana W North Dakota Range Coronach Bengough Radville Yellow Grass WEYBURN Legend Porcupine Plain Shaunavon Monocline Swift Current Platform Middle Devonian Prairie Salt Edge Preeceville Winnipegosis Elbow Weyburn Sturgis Sub-Basin Axis (Jones, ) Missouri Coteau on DEM N W Upper Winnipegosis m isopach LMLE Penzance ---WM Midale Grenfell Stoughton ESTEVAN MELVILLE Kipling Lampman Arcola Saltcoats Langenburg Wawota Kamsack Esterhazy Rocanville Whitewood Wapella Alameda Oxbow W Fleming W Kilometres Figure - Map of southern Saskatchewan showing locations of the Swift Current Platform and selected associated geological features, reference well LMLE Penzance /- --W, and geophysical log cross-sections A-A, and B-B of Upper Cretaceous formations (after Christopher, Figure ). * m m * Carnduff Miles Township Manitoba

4 A A /---W/ /---W/ ----W/ ----W/ /---W/ /---W/ /---W/ /---W/ /---W/ /---W/ CUBE BAY SHORE CYPRESS L --- HOME BANFF SHAUNAVON --- EOG MCCORD --- HB MONTAGUE --- VINTAGE ET AL HARPTREE --- HUSKY CEYLON --- NORCEN NEPTUNE --- SEARCH STEELMAN --- NAL ALIDA EAST --- CANADA WEST SOUTH PIPESTONE PROV. ---.m.m.m.m.m.m.m.m.m Lea Park Lea Park Second White Specks Colorado Montana Niobrara Milk River Carlile White Specks Medicine Hat Sands Govenlock Assiniboine Belle Fourche x (U) (M) BF D BF D BF C BF A x (L) BF B Milk River K C K C BF D x x x x K A K A x x x BF A BF A K B Ardmore (U) (M) (L) K A Lea Park bentonites Pembina x K A x K B off scale K C K A x x x K C K B K A x Lea Park Pembina Gammon Boyne Morden Carlile Assiniboine Keld Lithological Keld base Belle Fourche Fish Scales Favel FishScales sp rt γ rt γ rt γ rt γ rt γ rt γ rt γ rt γ rt γ rt - Cored Interval Log Signatures γ - gamma log Swift Current Platform rt - resistivity log sp - sp log Stratigraphic datum - Top Second White Specks Figure - West-to-east geophysical log stratigraphic cross-section A-A of Upper Cretaceous formations, from the Alberta border to Manitoba across southern Saskatchewan (modified from Christopher et al., ; used with permission of the Saskatchewan Geological Society). Saskatchewan Geological Survey Summary of Investigations, Volume

5 Montana Group Colorado Group Carlile WS LMLE PENZANCE W/ Lea Park Formation Milk River Formation (Alderson) Boyne Morden Assiniboine Keld Belle Fourche KB =. Figure - Geophysical log of the Milk River Formation at LMLE Penzance /---W, south-central Saskatchewan. WS = Second White Specks. γ B rt ammonites, and inoceramids), and concretion-bearing (calcitic and sideritic), very fine-grained quartzose sand is present as a minor to subordinate component of poorly defined bedding cycles, and limestone as a thin, capping end member of some cycles. The section displays, bottom to top, beds (. m,. m,. m,. m,. m,. m,. m,. m,. m,. m,. m, and >. m thick), all of which, except for the tenth and, perhaps, the eighth are upward shallowing as indicated by increase in marl, quartzose sand, and/or cemented limestone. Calcareous white specks are abundant in sequence two and scarcer in sequence ten. Although the presence of calcareous white specks in the latter is of interest in that these coccoliths are the stratigraphically highest reported for Saskatchewan and Manitoba, their occurrence in the Milk River Formation is not unique in the study area as noted below. Sharp contacts between cycles in the core are few and are not seen as regionally significant, except for the cemented algal limestone pavement at. m on sequence, about. m below the top of the Milk River Formation and on sequences six and four. The exceptions indicate significant depositional breaks, with marine draw-downs and subsequent transgressions. Additionally, the massive mudstone bed, basal to sequence, suggests a major late transgressive phase of the Milk River Formation. The sequences are summarized as follows (bottom to top): Carlile contact:. m ). to. m Shale: dark greenish grey, biotitic, ca. % quartzose fine-grained sand, abundant biotite.. to. m No core.. to. m Mudstone: sharp, basal contact; greenish grey, about % lenticular interbeds of friable, glauconitic, weakly bioturbated, fine-grained quartzose sand. ). to. m Mudstone: medium greenish grey, interbedded with very fine-grained, quartzose sand, scattered boulder-sized sideritic concretions above. m, variously calcareous; scattered layers with abundant calcareous, white specks, abundant pyritic filaments; and Inoceramus at. m. ). to. m Mudstone: medium greenish grey, interbedded with dark greenish grey calcareous shale, grades upward into noncalcareous quartzose sandy concretionary mudstone. Saskatchewan Geological Survey Summary of Investigations, Volume

6 ). to. m Mudstone: medium greenish grey, noncalcareous to calcareous upward, ca. % very finegrained quartzose sand; capped by dark grey, cryptocrystalline, argillaceous limestone,. m thick. ). to. m Mudstone: sharp basal contact; medium greenish grey, noncalcareous to calcareous upward, abundant brown, concretionary nodules; capped by. m thick, argillaceous, very fine-grained quartzose sandstone. ). to. m Mudstone: medium greenish grey, abundant nodular concretions, scattered shell fragments; grades upward into sandy mudstone, capped by dark grey cryptocrystalline limestone. m thick. ). to. m Mudstone: medium greenish grey, noncalcareous, subordinate very fine-grained quartzose sand, faintly current bedded, rare pelecypods. ). to. m Sandstone: sharp basal contact; quartzose, very fine grained, subordinate silt, medium greenish grey argillaceous matrix, calcareous over upper two metres. ). to. m Mudstone: medium greenish grey, calcareous, capped by. m of sandstone, quartzose, fine grained, laminated with very fine-grained sand and silt, argillaceous, calcite cemented; includes baculitid fragment. ). to. m Mudstone: medium greenish grey, quartzose sandy, very calcareous with about % white specks at top, very fossiliferous (abundant juvenile ammonites) below; grades into basal sandstone, quartzose, very fine grained, subordinate fine, noncalcareous to calcareous upward. ). to. m Mudstone: medium greenish grey, calcareous, abundant pyritic filaments and centimetrelength rods, brown sideritic, concretionary nodules, lenticular sideritic ironstone layer; rare baculitid and pelecypod shells; yellowish green bentonite,. m thick, beneath capping dark grey, dense, cryptocrystalline, algal-laminated limestone with syneresis cracks extending downward from sharp, irregular upper contact. ). to. m Mudstone: medium greenish grey, noncalcareous, abundant centimetre-length pyritic filaments, nodular sideritic concretions, scattered juvenile ammonites; calcareous toward top of core. The distal beds of the Milk River Formation at several sites display thin bands of calcareous coccolithic shale interbedded with noncalcareous shale and ironstone concretionary argillaceous sandstone; e.g., ) Imperial Findlater ---W where the basal. m section of Milk River Formation, in contact with the underlying calcarenitic Boyne Member at. m, consists of dark grey, noncalcareous mudstone with boulder-sized sideritic concretions and concretionary beds interbedded with calcareous, fossiliferous (baculitids and pelecypods, including Inoceramus) mudstone containing scattered layers of concentrated coccoliths; ) SWP Bredenbury ---W, where, at. m, a. m-thick, dark grey, noncalcareous mudstone that grades downward into marlstone with about % very fine-grained, calcareous white specks, underlies the Pembina Member of the Lea Park Formation, and rests on a pavement of Boyne coccolithic calcarenite; ) Alwinsal Lanigan ---W, which, at. m depth, includes a similar coccolithic mudstone,. m thick, between the Pembina Formation and the Boyne calcarenite; and ) NAL Rosebank /---W, where, at. m depth, m of dark greenish grey, calcareous, coccolithic mudstone superjacent to the Boyne Member were penetrated. In contrast, at NAL Alida East /---W, white specks are apparently absent from the. m of pumicitic mudstone and marlstone attributable to the Milk River Formation, and from the upper m of the underlying Boyne limestone. Farther east, into Manitoba, the Milk River correlative is the Gammon Member of the Pierre Formation. It is essentially dark grey shale with reddish brown ferruginous concretions in contact with the Boyne below and the Pembina above. At its sole outcrop in the Riding Mountain area of Manitoba, the Gammon is. m thick and includes Baculites minerensis Landes (McNeil and Caldwell,, p). b) Depositional Setting The Milk River Formation is at its thickest ( to m) between Rges W and W. It thins westward to m at the border with Alberta and eastward to less than m at the Manitoba border. From Tp, Rge W, southeastward to Tp, Rge W, the formation thins dramatically to regional values below m across a pediment to km in width. The pediment represents a sub Lea Park erosional escarpment overlooking a stripped lowland region on Milk River strata (Figure ). It is apparent that the depositional basin was widespread, extending well beyond the study area, but the isopach map reveals little about this setting. Cross-section A-A' (Figure ) shows significant truncation of the Milk River Formation under the overlying Lea Park Formation. By changing the stratigraphic datum of the cross section from the base of the Second White Specks to one in the post Milk River Lea Park Formation (see Figure ), the deduction can be made that Milk River thickening on the central area was accomplished by increased accommodation created by penecontemporaneous subsidence of the Saskatchewan Geological Survey Summary of Investigations, Volume

7 Datum: Lea Park Formation Niobrara E Ardmore Bentonite km m Pembina Pembina ale h ds e ton es Medicine Hat an Boyne Morden Carlile Gammon st White Specks Carlile Millwood Milk River Lea Park W Favel Lim Govenlock Carlile Shale nd White Specks Saskatchewan Monocline Swift Current Platform Figure - Panel diagram of geophysical log cross-section A-A with datum in the Lea Park Formation illustrates down-set of the Swift Current Platform with respect to the Shaunavon Monocline to the west and the Saskatchewan Monocline to the east. x, bentonites. depositional floor. Thus, the morphology of the Milk River lithosome, as simulated in the isopach map of Figure, is shaped by geometry of the depositional basin and by Campanian pre Lea Park erosional agents. The central mass of the formation is largely confined to the region between Rges W and W, and the international border (Tp ) and Tp, in which it thickens to more than m. Both geophysical logs and core reveal a cyclic stratigraphic layout which is similar in style to the Albian Pense Formation and is attributable to episodic subsidence of the Swift Current Platform (Christopher,, p).. Post Milk River Landscape a) Geomorphology Isostatic rebound of the Swift Current Platform at the end of Milk River sedimentation, with accompanying marine regression, created an eastward gradient by which Milk River strata were back-stripped westward to a low-relief veneer across the eastern third of southern Saskatchewan. At the approach to the Swift Current Platform from the northeast and east, the landscape acquired escarpment-like characteristics by which it rose onto the plateau of thickened sediments corresponding to a pre Lea Park Swift Current Paleo-upland. The escarpment on the isopach map is depicted as a low-gradient pediment (between the m and m contours) m high and km wide, trending easterly for km from Rge W along Tp, and at the Third Meridian (longitude ) southeasterly km to Regina from Tp. South of Regina (Tp, Rge W) the scarp of the pediment turns westward along Tp into the mouth of the km-wide, southeasterly opening Moose Jaw Paleo-amphitheatre. The pediment, which fronts a high-elevation terrace at the to m contours, declines southeastward as a low-relief ramp delimiting the eastern flank of the amphitheatre, into a terraced scarp, m high, between Rges W and W, and Tps and. Southeastwardly, the ramp forms a divide, to km wide and m high, into the Weyburn oilfield district, and beyond Rge W, Tp W it forms a ridge, less than m high, to Rge W, Tp near Estevan. The central mass of the paleo-upland forms a broad divide marked by traces of shallow drainage systems radiating off to the flanks, into steepened stepped valley-in-valley terraces. Toward the east, four terraces are identified: Terrace is reflected by the narrow, m-deep Herbert Paleovalley, trending east from the m contour to the m contour on a north swinging arc through Tps and on the crown of the paleo-upland between Rges W and W. The valley appears to be part of an early drainage system developed on the upland prior to uplifts indicated by the succeeding terraces two to four of the Moose Jaw Paleo-amphitheatre. The latter is recessed into the upland as a three-stage erosional basin that slopes gently southeastwardly from a m-high back wall along Rge W at Tps to. Saskatchewan Geological Survey Summary of Investigations, Volume

8 Alberta A Kilometres SPV SPV Miles B Montana m McPV MPV m HPV W Range m Legend Wascana Creek N W Arm River N W Thunder Creek N W Moose Jaw Creek N W Avonlea Long Creek N W Qu Appelle River N W Missouri Coteau on DEM N W Winnipegosis Elbow Weyburn Sub- Basin Axis, N W (Jones, ) Figure - Isopach map of the Milk River Formation, southern Saskatchewan which includes an overlay of the trends in the present-day drainage system and of the Middle Devonian Winnipegosis Formation. Terrace is delineated by the m descent of the floor of the amphitheatre between the and m contours. The south wall of the amphitheatre, as taken along the m contour west of Rge W, Tp, rises m above this floor at the m isopach. Terrace ( to m) shapes the floor of the amphitheatre at Tp, Rge W,where it is entrenched along the south wall, here m high, by an inner valley, km wide and m deep. This base served as a collecting basin for a tributary system entering from off the eastern pediment of the paleo-upland near Regina. Terrace ( to m contours) is represented by the break in slope along the south wall at Tp, Rge W, initiating an incision m deep. At the international border, the south wall of the amphitheatre rises to a height of m. m Middle Devonian Prairie Evaporite Upper Winnipegosis m isopach (Jones, ) Paleovalleys HPV = Herbert Paleovalley MPV = Mankota Paleovalley McPV = McMahon Paleovalley SPV = Shackleton Paleovalley Contour Interval = m m North Dakota W B Township W Manitoba A Saskatchewan Geological Survey Summary of Investigations, Volume

9 The divide of the eastern pediment separates the southeasterly trending, deep-valley entrenched system of the Swift Current Paleo-upland from the headwater basin of a north and northwesterly trending, m-deep, shallow valley system in the outlying paleo-lowland to the east. This valley flanks the pediment and is traceable northward from Tp, Rge W to Tp, Rge W, but its relationship to the upland valleys was not determined. Along the international border, the Campanian pre Lea Park lowland extends more than km to the Cenozoic erosion edge in southwestern Manitoba. The western flank of the Milk River Swift Current Paleo-upland is delineated by the m contour marking the eastern edge of the low-relief Shaunavon Paleo-lowland along Rge W south of Tp, and north-northwestward to Rge W, Tp. It rises to the foot of the western divide at Rge W, steepening at the m contour into an irregular escarpment fronting the paleo-upland at the m contour. The four re-entrant valleys dissecting the front are short in length and westward directed from the divide between Rges W and W. The herein-named Mankota Paleovalley along Tp is entrenched to a depth of m along a digitated back-wall that in Rge W forms an amphitheatre on an erosionally breached dome. The m-deep McMahon Paleovalley heads in Rge W and trends westerly in Tp and southwesterly at Rge W. The Shackleton Paleovalley, though relatively shallow at to m, is the most prominent in length ( km) and expression. From a head in Tp, Rge W it bears north and west to Tp W, and northwest to Tp, Rge W, where it changes direction to the east and northeast to Rge W, Tp, only to disappear at the northern escarpment of the Swift Current Paleo-upland. From Tp, Rge W the valley is essentially flat floored at the m isopach and exhibits a gradient of only m over a downvalley length of km. This segment of the Shackleton drainage indicates a perched base-level on the Shaunavon Monocline when the Swift Current Platform was still relatively depressed at about the base-level associated with the floor of terrace level. The headward-incised segment, like those of the McMahon and Mankota valleys, would thus be evidence of erosion that accompanied the initial isostatic uplift of the platform. Accordingly, the Shackleton Paleovalley demarcates not only the Shaunavon Monocline to the south, but also encloses the area of the Kindersley structural block, incising it similarly to the Lower Cretaceous, sub-cantuar Plato Paleovalley basin in Tps,, and (Christopher,, Figure ). The Swift Current Platform, immediately to the southeast, is delineated by the m contour. Thus, the Shackleton Paleovalley circumscribes a northwestern extension of the Swift Current Paleo-upland onto the Kindersley structural block. A comparison with the Lower Cretaceous pre-cantuar Swift Current Paleo-upland shows the latter to rise to a divide along Rges W and W south of Tp, and the heights of the sub-cantuar paleo-upland to span the region between Rges W and W. Thus, pre Lea Park erosional agents have back-stripped eastward the upper level of the Campanian paleo-upland to a divide along Rge W, Tps and. The region between the two divides appears to have behaved differently from that east of Rge W in response to differential movement controlling the Shaunavon Graben. b) Tectonic Fabric Episodic repetition of Phanerozoic structural anomalies from one stratigraphic level to another can commonly be attributed to underlying movement of basement blocks transmitted by way of fractures and faults, even when modified by secondary salt-dissolution effects in the Devonian evaporitic units. On a larger scale, periodic expansion of the Williston Basin into southern Saskatchewan accompanied relative depression of the Swift Current Platform, whereas, contraction of the Williston Basin, when accompanied by its deepening, apparently placed the platform in a state of relative uplift. The greater of these uplifts created topographic relief which, on tectonic relaxation, was encroached by sediments prograding from the Rocky Mountain uplifts into easterly migrating foreland basins. The Campanian pre Lea Park contraction of the Williston Basin initiated the last of the Mesozoic uplifts of the Swift Current Platform. Elements of the sub Lea Park topography related to the platform are replicated at the present day topographic surface by the gross dichotomy into eastern lowland and western upland prairie, and by lineament and fault-line control on the directions of drainage in the broad zone linking them. These linear forms, by their abundance and regional persistence on the landscape and in the depositional fabric of the underlying formations, have caught the attention of geologists from the early years to the present and are reviewed in Mollard (). The eastern pediment terminating the pre Lea Park Swift Current Paleo-upland reflects a bevel of Milk River strata across a km-wide belt trending N W between the Third Meridian and Regina. Other similarly aligned trends shown on the isopach map (Figure ) include segments of the Middle Devonian Prairie Evaporite salt dissolution edge, the southeasterly course of the Qu Appelle River between Tp, Rge W and Tp, Rge W, the Arm River, and Wascana Creek southeast of Regina. The Qu Appelle River bears northeast across the strike of the buried pediment and east on the pre Lea Park paleo-lowland. The Moose Jaw Paleo-amphitheatre appears to have been carved by stream action along a dominant northwesterly lineament system and subsidiary easterly and northerly subsets. This is suggested by the alignment of the contemporary Thunder and Moose Jaw creeks with respect to the northeastern side, and Avonlea and Long creeks located along the keel, km to the east of the Missouri Coteau. The four knick-points, i.e., break in slopes, in the associated entrenched Herbert Paleovalley system indicate Swift Current Platform uplifts of the order of m, m, m, and m, for a total of m. Thus, entrenchment exceeds the m height of the eastern pediment and reflects the state of an off-loading platform undergoing isostatic uplift on a southeastward tilt toward the Williston Basin. Whereas this entrenched Saskatchewan Geological Survey Summary of Investigations, Volume

10 valley indicates an early breach of the upland region in North Dakota and drainage toward a depositional centre beyond, the low-relief drainage from off the Weyburn-Midale divide, directed northward along the base of the eastern pediment is in opposition to, but congruent with, the axis of the succeeding basal Lea Park (Pembina) depositional basin on the paleo-lowland (Christopher and Yurkowski, ). No topographic contradiction exists if the valley in the amphitheatre which underlies the course of the present day Avonlea and Long creeks, took a similar course to the present day adjacent Souris River by making a U -shaped bend through North Dakota en route to the Lea Park basin in Manitoba. Elements of the Milk River basin have Paleozoic antecedents in the context of the Devonian Elk Point Basin, in particular the Winnipegosis. These are displayed, for example, in the report on the Middle Devonian Winnipegosis Formation by Jones (). He named and described the Elbow-Weyburn facies as a northwesterly trending trough of Upper Member deeper water carbonates. From an axial thickness of. m, the facies thins abruptly at the western edge of an eastern shelf blanketed by thin carbonates, but to the southwest thins gently to zero, to define a sub-basin, wedge-like in cross section, corresponding in area to the Milk River Swift Current Paleo-upland. As delineated by the m Upper Winnipegosis isopach (Figure ), the keel of the Elbow-Weyburn sub-basin coincides with the location and bearing of the present-day Missouri Coteau. The eastern m contour is sinusoidal and southeasterly striking below, and in alignment with, the upper scarp of the Milk River pediment. It strikes easterly along Tp and southerly in Rges to W to incorporate an area corresponding to the eastward-expanded Milk River ramp at the Moose Jaw Paleo-amphitheatre. The western Winnipegosis m isopach coincides with the crown of the Milk River Paleo-upland which trends northwesterly between Rges W and W, and lies northerly along the divide to Rge W, Tp. The zero Winnipegosis contour lies along Rge W between Tps and, and beyond northwest, along the trend of the Milk River Basin. In the east, the axial coincidence of the Winnipegosis Elbow-Weyburn sub-basin and the present day Missouri Coteau, and the parallel proximity of the present day Avonlea Long Creek lineament, with the deep incision in the Milk River Moose Jaw Paleoamphitheatre, indicate fundamental fault control. The eastern pediment therefore appears to be a hinge zone on which westerly retreat of the Milk River escarpment stalled by virtue of the thickened sedimentary pile on the rising Swift Current Platform, and the northwesterly and southeasterly trends of the present drainage in this region are a Cenozoic reiteration of Mesozoic re-activated joint and fault control. These faults have been the subject of lively debate with respect to their origin by tectonic activity in the basement or in Prairie Evaporite salt (Bishop, ; Edie, ; Woodward, ; Kupsch, ; Binnert Haites, ). The best evidence of a genetic relationship between the Precambrian Shield and the Swift Current Platform is seen in the aeromagnetic map of Saskatchewan. This map, produced by the Geological Survey of Canada and included in the Geological Atlas of Saskatchewan (Slimmon, ) shows the basement to comprise distinct magnetic domains and trends. Lithological data for the sub-phanerozoic basement are sparse, so delineation of the domains is based on various extrapolations, as outlined in Christopher (, p), from regions of exposed Precambrian Shield. The basement nomenclature adopted in this paper is after Kreis et al. (). By overlay of the Milk River Formation isopach map (Figure ) on the First Vertical Derivative, Aeromagnetic Map of Saskatchewan (see Figure ), the Swift Current Platform area is seen to be centrally cored by a triangular high-magnetic (> nt) block, fronted to the east by the western flank of the Moose Jaw Paleo-amphitheatre and the axis (keel) of the Winnipegosis Elbow- Weyburn sub-basin (Missouri Coteau), and in the west by an irregular, steepened north-south front along Rge W coincident with the re-entrant valleys from off the Milk River Paleo-upland divide. The enclosed region corresponds to the Assiniboia Magnetic Domain (region A, Figure ). The truncated apex of the domain in Tp coincides with the east-trending headwater Herbert Paleovalley of the Moose Jaw Paleo-amphitheatre. East of the Assiniboia Magnetic Domain lies the km-wide magnetic-low region (- to - nt) named the Reindeer Magnetic Quiet Zone (RMQZ, Figure ). It underlies the high terrace on the Milk River eastern pediment, as bounded by the m contour to the east and the Winnipegosis Elbow-Weyburn Axis to the west. South of Tp, the area of the RMQZ expands in conformance with the eastward spread of the Winnipegosis Elbow-Weyburn sub-basin and the ramp to the Milk River pediment east of the Moose Jaw Paleo-amphitheatre. The RMQZ includes all the active lineaments and faults in the eastern region of the Milk River Paleo-upland. However, the Milk River pediment itself slopes northeastward onto the medium high magnetic Humboldt Domain (region H, Figure ). The western flank of the Assiniboia Magnetic Domain as defined by Kreis et al. () is shown as trending northeasterly from Tp, Rge W to Tp, Rge W (Figure ). However, south of Tp, Rge W, this linear forms the western flank of a triangular magnetic low, which at its apex encounters the irregular front of the Assiniboia magnetic high. The latter conforms inversely to the general north-aligned sigmoidal front presented by the westward-opening, short-ranged paleovalley systems, such as the Mankota, off the divide of the Milk River Swift Current Paleo-upland. The triangular magnetic low apparently corresponds to the Val Marie Arch, which enters Saskatchewan as a northern spur of the Bowdoin Dome of Montana. North of Tp, Rge W, the apparent linear has been linked to another magnetic low positioned on the western side of its projection. However, that low is medium intensity ( to nt), sigmoidal in shape, being northeasterly and northerly between Rges W and W, westerly along Tps and from Rge W to W, and northeasterly on the crown of the paleo-upland as defined by the m isopach. It is probably not a northeastern extension of the triangular magnetic low. We therefore suggest re-defining the western front of the Assiniboia Magnetic Domain to be north trending to Tp Saskatchewan Geological Survey Summary of Investigations, Volume

11 nt Legend Precambrian Magnetic Domains (Kreis et al., ) A = Assiniboia H = Humboldt B = Battlefords RMQZ = Reindeer Magnetic Quiet Zone E-REIND EER-SA SK B H m QZ RM m e Gr A m m W Montana at s ll Fa ne Zo c i on ct Te Alberta m Upper Winnipegosis m isopach (Jones, ) m Milk River isopach Contour Interval = m HE ARN Township Manitoba SUPERIOR-REINDEER BOUNDARY ZONE Wascana Creek N W Arm River N W Thunder Creek N W Moose Jaw Creek N W Avonlea Long Creek N W Qu Appelle River N W Missouri Coteau on DEM N W Winnipegosis Elbow Weyburn SubBasin Axis, N W (Jones, ) SASK-REINDEER BO UNDARY ZONE W W North Dakota Kilometres Range Miles Figure - First vertical derivative aeromagnetic map of southern Saskatchewan (data to generate the map courtesy of the Geological Survey of Canada, Natural Resources Canada) showing selected magnetic-domain boundaries from Kreis et al. (), with overlay of the isopach map of the Milk River Formation as in Figure. along Rge W, where it is more-or-less in conformance with the topographic divide on the Milk River Formation. Its distinctive character notwithstanding, the Assiniboia Magnetic Domain is not homogeneous, but is fronted at the second terrace step ( to m) of the Moose Jaw Paleo-amphitheatre by a palmate area of medium magnetic intensity and, at the border with Montana between Rges W and W, is cored by a triangular magnetic-low region that corresponds to the highest part of the Milk River Paleo-upland as defined by the m Milk River isopach. The region to the west of the re-defined Assiniboia Magnetic Domain is marked by linear belts of magnetic highs and lows striking northeasterly, northerly, and northwesterly. Some correspond to Mesozoic and Cenozoic isopachous and structural features. For instance, a to km-wide belt of medium and high intensity, trending N E from Rge W, Tp, intercepts the Assiniboia Magnetic Domain at Rge W, Tp, and is flanked on Saskatchewan Geological Survey Summary of Investigations, Volume

12 the southeastern side by the triangular magnetic low area corresponding to the Val Marie Arch. The belt is interpreted by Kreis et al. () to be a northeasterly extension of the Late Cretaceous Laramide Great Falls Tectonic Zone of Montana. Jura-Cretaceous elements are seen in the geographic correspondence of: ) the north-oriented, high-magnetic linear belt in Rge W, Tp, with the sub-cantuar Delta Paleovalley off the pre-cantuar Swift Current Paleo-upland onto the eastern divide of the west-opening Plato Paleovalley at Tp on the Unity structural block (Christopher,, Figure ); and ) the medium-high intensity magnetic belt trending N W from Rge W, Tp, to the interprovincial boundary at Tp in near conformance with the sub-cantuar Eastend Maple Creek Paleovalley and faultcontrolled lineament. Both elements above are related to the north-trending Shaunavon Graben along Rge W; the one by its northaligned course, the other by headwater location. The relationship is attributed to Late Jurassic uplift of the Swift Current Platform on the eastern side of the Shaunavon Graben relative to the Shaunavon Monocline on the west (Christopher, ). As indicated by the Upper Jurassic Roseray clinoforms (Christopher, ) and the pre- Cantuar topographic drainage patterns, the platform was initially tilted toward the Williston Basin in southeastern Saskatchewan, but mild uplift there deflected the pre-cantuar drainage to the north along the Jurassic escarpment at the Second Meridian (between Rges W and W) (Christopher,, Figure ). However, the eastward translation of the topographic divide from Rge W on the pre-cantuar Swift Current Paleo-upland to Rge W on the pre Lea Park Swift Current Paleo-upland south of Tp reflects a significant uplift in the Late Cretaceous of the structural block related to the Assiniboia Magnetic Domain relative to the Shaunavon Monocline. A further consequence would have been the spread of the plateau representative of the paleo-upland to its eastern pediment. The Swift Current Platform thus incorporates a central basement block defined by the Assiniboia Magnetic Domain and the flanking peripheral RMQZ, which terminates on its easterly track from the north, and deflects in the east the predominantly north-oriented Precambrian tectonic terrains as exhibited on the Shield and extrapolated south beneath the Phanerozoic strata. In the west, the Swift Current Platform includes the general north-oriented belt of smaller magnetic elements featuring strong northeasterly and northwesterly subsets associated with the Shaunavon Graben along Rge W. Most of the southern basement samples dated as Early Proterozoic and allocated to the Swift Current Anorogen by Collerson et al. () were taken from this southwestern region. Nonetheless, the tectonic activity indicated in the Mesozoic and Cenozoic is linked directly to orogenic events of the Laramide Rocky Mountain foreland basin to the south and southwest in central Montana. The eustatic movements indicated for the eastern side of the platform reflected the major orogenic episodes of the Paleozoic, Mesozoic, and Cenozoic, but as outlying effects in the Proterozoic deeper crust of the Williston Basin. c) Post-Laramide Structure Surface The post-laramide Swift Current Platform (Figure ) lies depressed below outlying monoclines to the west, north and northeast as an adjunct of a structurally deepened Williston Basin centred in northwestern North Dakota. The keel of the platform lies along the Moose Jaw Paleo-amphitheatre as extended to the northwest, and plunges southeastward from m msl at Rge W, Tp, to - m at Rge W, Tp, for a decline in elevation of m. The post-laramide plunge is thus twice that of the m decline in Campanian slope. The northern front of the platform is delineated by the m structural contour as an eastward projecting anticline along Tp between Rges W and W, and as knobs at Rges W to W and Rges W to W, where it turns southeasterly to Tp W, Rge W. The eastern front is a km-wide terrace that corresponds in layout to the Campanian Milk River pediment, but without the eastward slope, now eliminated by down-set of the Milk River Formation. A large dome between Rges W and W, Tps and, at elevation m, i.e., m above the terrace, is a structural remnant of the Campanian plateau. It overlooks to the southwest and west the post-campanian edge of the Middle Devonian Prairie Evaporite, as delineated by the salt-dissolution collapse Elbow sub-basin (Goudie, ; Christopher, ; Christiansen, a; Holter, ), the western edge of which is defined by the north-south dissolution front along Rge W. The relationship between the Campanian Milk River erosional surface and its post-laramide structural configuration along Tp is depicted in Figure. It shows that subsidence of the intact Milk River Formation to form the trough of the sub-basin in Rges W to W was in the order of m, well within Holter s estimate of. m thickness of the Prairie Evaporite section at the outlying White Rose et al St. Denis ---W well. Christiansen (a, b) deduced the time of salt removal in the north of the sub-basin to have been Cenozoic and Pleistocene. Along the line of section in Figure, the structural profile rises gently m eastward between Rges W and W and crosses the falling profile of the Milk River Campanian landscape onto the structural terrace. At about Rge W, the structural surface rises eastward counter to the descent of the Campanian surface. Presumably the terrace represents a tectonic hinge between the Swift Current Platform and the Saskatchewan Monocline which, in the figure, has risen on the up-tilt by m at Rge W. As displayed on the structure map (Figure ), the terrace itself is also pocked by post-laramide salt dissolution sinks, as at Tp, Rge W, Tp, Rge W, and Tp, Rge W with relief of m, m, and m, respectively. The implied great Saskatchewan Geological Survey Summary of Investigations, Volume

13 Legend Milk River Gas wells Middle Devonian Prairie Evaporite Wascana Creek N W Arm River N W Thunder Creek N W Moose Jaw Creek N W Avonlea Long Creek N W Qu Appelle River N W Missouri Coteau on DEM N W Winnipegosis Elbow Weyburn SubBasin Axis, N W (Jones, ) m Upper Winnipegosis m isopach Contour Interval = m Township m B B m m Alberta A Manitoba - m - Montana A - North Dakota W Range W W Kilometres Miles Figure - Structure map on the Milk River Formation (sub Lea Park erosion surface), southern Saskatchewan; overlay as for Figure. magnitude of salt dissolution at these sites indicates incremental decreases of Upper Devonian salt thickness as well as that in the Prairie Evaporite, and an inter-connection probably related to basement re-activation. On the other hand, the structural dome aforementioned geographically corresponds to one of similar dimensions on the Neocomian sub-cantuar erosion surface (Christopher,, Figure ) and may reflect drape of a mesa on the Paleozoic limestone. The regional post-laramide salt dissolution of the Milk River Formation extends southward to Tp as indicated by the nearly uniform blanket of overlying Lea Park strata that mirrors the topographic forms on the Milk River Paleo-upland. Beyond, in the Moose Jaw Paleo-amphitheatre to the international border, the Lea Park isopachs are inversely faithful to the underlying topography, but display a relief one-third of the latter and thereby indicate strong compaction of the Lea Park. The foregoing, added to a reduced basin-ward isopachous gradient accompanying Lea Park thickening, indicate a resumption of penecontemporaneous down-warp in the Williston Basin. The Moose Jaw Paleo-amphitheatre, more properly referred to as a structural trough, is not a product of salt dissolution per se in that its elements, including lineaments, continue southeasterly into the Williston Basin across the salt dissolution edge of the Prairie Evaporite without deviation in bearing or decrease in relief. Salt-dissolution activity in the Late Paleozoic and Early Mesozoic had already delineated the Hummingbird Trough in Rges W and W, Tps to Saskatchewan Geological Survey Summary of Investigations, Volume

14 Structural Elevations B W TRW m m TRW m + m m + m TRIIW m + m - m m TRW m + m Post-Campanian Elbow sub-basin salt dissolution controlled subsidence - ca. m - m - m TRW m + m TRW m - m - m m TRIW m - m - m m TRW m - m - m m Profile TRW m - m Milk River - m m - m TRW m - m m Campanian Erosional Truncation Landscape TRW m - m (pre-lea Park) m - m (Campanian) TRW m - m m - m Post-Campanian tectonic uplift TRW m - m E - m B m m m Legend Planimetric line of section Structure cross section Milk River Formation Isopach cross section of Milk River Formation Campanian erosional profile of Milk River Formation Reference well Relative elevation difference to m datum Relative isopach difference to m datum Sample reading: well # Structural elevation: m msl (actual) Relative to datum ( m - m) = Figure - West to east panel diagram, B-B of the Milk River Formation along Tps and, Rges W to W, southern Saskatchewan, illustrating the relationship between the Campanian morphology and the post-laramide structural expression of the Swift Current Platform (location of wells are plotted on Figures,, and ). m Thickness of Milk River Formation: m Relative Campanian ground elevation of Milk River surface ( m - m) = m Saskatchewan Geological Survey Summary of Investigations, Volume

15 , and in the easterly embayment of the salt-dissolution front to Rge W, Tps to, i.e., in the South Regina Trough of McTavish and Vigrass (). Reflections of the latter feature have been shown to be present in the Winnipegosis and in the Precambrian basement as exhibited on the aeromagnetic map. On the other hand, Maastrichtian salt-dissolution activity created the relatively minor Coronach Trough delineating the western flank of the Roncott high in Rge W, Tps to (Broughton, ). The Val Marie Arch, (i.e., the northward projection of the Laramide-rejuvenated Bowdoin Dome into Saskatchewan to Tp, between Rges W and W), with its highest structural elevation at m, commands the structural topography of the study area. It rises some m above the flanking Shaunavon Graben in Rges W to W, and m above the keel of the Swift Current Platform at Rge W, Tp. To the west and northwest of the Val Marie Arch lies the broad Shaunavon Monocline, floored at elevations of m to m msl and having less than m in local relief. It slopes into a) the depressed Swift Platform along a front trending northeast from Tp, Rge W, to Tp, Rge W, and b) the Kindersley block, north along Rge W to Tp. Most of this scarp is geographically coincident with the western salt-dissolution front of the Prairie Evaporite. The monocline, though structural, is also elevated stratigraphically by the eastward truncated strata of the ca. m-thick sub Milk River Niobrara Formation and is host to nearly all the known Milk River natural gas pools of southwestern Saskatchewan (Figure ).. Economic Aspects The Milk River Formation is a major Saskatchewan gas producer in the western part of the study area, where it is at relatively high structural elevations and features tectonic structures including the Shaunavon Monocline and the Kindersley structural block. In spite of voluminous production over a wide geographical area (Figure ), controls on the economic accumulations of gas within the Milk River are not fully understood. Given the broad geographical area (> km, including Alberta production) in which these accumulations occur, the assumptions that numerous trapping mechanisms and a variety of productive facies are in play are reasonable. Ridgley (), O Connell (), Payenberg (), Pedersen (), Ford et al. (), Young et al. (), and Yurkowski () have reviewed the pools and the various depositional environments of the reservoir rocks. Five wells (/---W, /---W, /---W, /---W, and /--- W) on the structurally depressed saddle linking the Val Marie Arch and the Shaunavon Monocline show that economic production occurs east of the Hatton and Abbey-Lacadena pools. Commercially these wells are strategically located along the Swift Current oil-field trend and are, therefore, easily tied in to the pipeline infrastructure. The most productive to date is the ---W well, which after eight months of production, has yielded on average mcf/day (. x m /d), and a cumulative,. mcf (. x m ). The other wells have produced to mcf/day (. to. x m /day) over limited time periods. Rice and Claypool () first identified the gas reservoired in Upper Cretaceous strata of Montana, South Dakota, and Alberta as biogenic in origin. Fishman et al. () confirmed a similar origin for Saskatchewan s Milk River gas by isotopic fingerprinting of the gases and indicated that methane generation began soon after deposition and continued to be generated for well over million years. They also theorized that the gas only started to accumulate in economic volumes after the area was uplifted so that the gas could exsolve due to the reduction of hydrostatic and lithostatic pressures. Given the early methanogenesis, methane may have been generated wherever the Milk River was present provided that sufficient organic material was available. Robinson s () analyses of the Milk River core at ---W indicated the presence of thermally immature Type III kerogen (terrestrial-derived plant and woody materials) with TOC values averaging % which was considered as a fair to good value for source potential. As source does not appear to be a restriction, the transmissibility of Milk River strata as well as trapping mechanisms must be considered. Cored sections of the formation are sparse on the Swift Current Platform, but the type well described herein is a good sample of the facies, which is predominantly mudstone intercalated with subordinate quartzose sand and minor argillaceous limestone (i.e., it is a deeper water facies, but is not unlike the facies of the main producing region). Gas is produced from similar lithologies in North Dakota. The Little Missouri Field lies in the extreme southeastern corner of North Dakota (~ km due south of Weyburn, Saskatchewan). Production is from the Gammon (Milk River equivalent) Formation of thin, discontinuous lenses and laminae of siltstone interbedded with shale. The gas is trapped in an anticline and production is associated with, and strongly influenced by, fractures (Gautier, ; Shurr, ). The Little Missouri Field is situated more than km from the approximate eastern edge of the Eagle Sandstone and, therefore, serves in part as a model for exploration of Saskatchewan s deeper water Milk River facies to the north and east of current production trends. Saskatchewan Geological Survey Summary of Investigations, Volume