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Home , Claggett Shale, Eastend Formation, Milk River Formation, Pakowki Formation

A Major Late () Unconformity, Southeastern Saskatchewan1

J.E. Christopher 2 and M. Yurkowski

Christopher, J.E. and Yurkowski, M. (2003): A major (Campanian) unconformity, southeastern ; in Summary of Investigations 2003, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2003- 4.1, CD-ROM, Paper A-12, 7p.

Abstract Mapping of Cretaceous formations in the northern two-thirds of the area encompassed by the International Energy Agency Weyburn CO2 Monitoring and Storage Project in southeastern Saskatchewan reveals the presence of a pronounced erosional unconformity between the -Campanian Alderson (Milk River) and the Campanian Lea Park Formation. Erosional relief is about 260 m between southwestern Saskatchewan and the southeast. It is accentuated by an erosional escarpment 200 m high, aligned along the downward projection of the present-day topographic Missouri Escarpment east of the City of Weyburn. Like its present-day counterpart, the sub-Lea Park escarpment forms the eastern edge of a western upland region, and overlooks a country of low-relief (60 m) knolls and depressions to the east. Lea Park sediments fill in the underlying terrain with 25 m-thick, basal fluvial sands in thalwegs, which are partially reworked and buried under 30 m-thick, deeper water bituminous . Later sediments and their Belly River equivalent tongues bury the sub-Lea Park terrain by onlap and progradational downlap.

Keywords: Upper Cretaceous, Santonian, Campanian, Belly River, Lea Park, , .

1. Introduction This paper arose out of work done on the Mesozoic strata in southeastern Saskatchewan under Sub-task 2.1.2 of the Geological Framework component of the IEA Weyburn CO2 Monitoring and Storage Project. The study area is situated between Range 24 West of the Second Meridian and The Second Meridian (longitude 102 degrees), and between Townships 1 and 17 inclusive (Figure 1). With respect to data quality, the 1300 m-thick, post- Mississippian strata can be divided into: 1) a lower half section below the Upper Colorado Niobrara Formation, where, in response to the primary focus of petroleum exploration, log suites are adequately definitive, and previous work in formations with contrasting physical characteristics has established a standard nomenclature; and 2) an upper section where log suites that were variously designed for Paleozoic carbonates and evaporites are inconsistent from well to well in their display of Upper Mesozoic rock signatures of repetitious sandy mudstones and muddy . Accordingly the lower Mesozoic formations, represented by the Jura- Watrous evaporitic clastics; the Gravelbourg-Shaunavon evaporites, carbonates and siliciclastics, and the Vanguard calcareous siliciclastics; and the Lower and Middle Cretaceous Mannville and Coloradoan Joli Fou, Newcastle, and Belle Fourche siliciclastics, are readily identified. Far less is known on a regional basis about the stratigraphic properties of the Upper Cretaceous Niobrara, Alderson (Milk River), Lea Park, Belly River, and Bearpaw formations, the sandstones of which are best developed in western Saskatchewan where they are gas prone. The regional physical relationship of the Lea Park lithosome of and to its contiguous neighbours in the study area, and to its sandy lithosome of southwestern Saskatchewan is critical to an understanding of the Upper Cretaceous and needs to be explored.

2. Stratigraphic Nomenclature All Cretaceous stratigraphic units in the study area have been defined and studied in the contiguous region of southwestern Saskatchewan, southeastern , and northern by many researchers. An historical account of their endeavours lies beyond the scope of this paper, but is readily pursued through the works of the authors cited below. North and Caldwell (1975) set up a 16-zone faunal assemblage scheme that stratigraphically partitions the Cretaceous formations of southern Saskatchewan from Alberta to . Zones I to IV categorize

1 Support has been provided by the PTRC of Regina through the generosity of many sponsors, including SIR, to the IEA Weyburn Project. 2 James E. Christopher, Ph.D., P.Geo., 252 Coldwell Road, Regina, SK S4R 4L2; E-mail: [email protected].

Saskatchewan Geological Survey 1 Summary of Investigations 2003, Volume 1 Figure 1 - Location map of study area. Path of stratigraphic cross-section A-A’ (Figure 3) is also illustrated. Albian and Cenomanian formations below the Second White Speckled Formation, and thus lay outside the scope of this paper. Stratigraphically higher formations include Turonian, Coniacian, and Santonian assemblage zones V, VI, and VII, and Campanian zones VIII to XI inclusive (Figure 2). a) Upper Colorado Group: Second White Specks, Carlile, and Niobrara Formations (Faunal Zones V to VII)

Second White Specks Formation – Turonian

The Second White Specks Formation consists of laminated, calcareous claystone and siltstone, generally with abundant white to grey, coccolith debris flakes up to several millimetres across. Bentonitic marker beds at the top, and benthic and planktonic foraminifera, inoceramid and other pelecypod fragments, fish scales and bones, and ammonites are common. Basal contact on the Belle Fourche Formation is marked in places by a bed of coarse bioclastic debris up to 5 cm thick. Thickness of this formation is about 25 m.

Carlile Formation – Turonian

The Carlile Formation comprises mainly dark grey, noncalcareous shale, mudstone, and siltstone and commonly includes fish debris; sandstone increases upward in relative proportion. Thickness is 40 to 60 m and the formation may terminate at a disconformity.

Niobrara Formation – Coniacian to Santonian The Niobrara Formation is predominantly laminated, calcareous, coccolithic shale, and mudstone interbedded with shaly chalk, noncalcareous shale, siltstone, and sandstone. There are also beds of inoceramid prisms, foraminiferal tests, bentonitic layers, and concretionary calcite, siderite and phosphorite. Thickness of the formation ranges up to 140 m. There are three informal members: the basal argillaceous Govenlock, about 60 m thick; the sandy and shaly medial Medicine Hat of similar thickness, and the capping 30 m-thick First White Specks. The gas-productive “Medicine Hat Sandstone”, up to 25 m thick, is present at the top of the Medicine Hat Member. b) Alderson (Milk River) – Santonian to Campanian The Alderson (Milk River) unit is about 110 m thick and is composed of highly bioturbated, medium grey, very fine- to medium-grained, muddy sandstone interbedded with siltstone and dark grey, silty shale. are predominantly pelecypodal and baculitid. Its complex stratigraphic relationships, which require ongoing detailed analysis, are summarized in Pedersen (this volume). With respect to the presence of a Late Santonian to Early Campanian regional unconformity, North and Caldwell (1975, p321), on the basis of foraminiferal data, speculated that disconformities (hiati) are extant in southwestern Saskatchewan at the base of the Alderson (Milk River), and in eastern Saskatchewan at the base of the Pembina Formation. They did not, however, recognize any age gap between

Saskatchewan Geological Survey 2 Summary of Investigations 2003, Volume 1 the Eagle (Milk River) and Claggett (Pakowki) in western Saskatchewan. This contrasts with observations made by Ridgley (2000), Payenberg (2002), and Shurr and Ridgley (2002) that an unconformity on the Alderson (Milk River) lies below the Pakowki and its correlative Claggett Shale in the contiguous region of Saskatchewan, Alberta, and Montana.

c) Lea Park (Pakowki), Belly River and Bearpaw Formations (Faunal Zones VIII to XVI)

Lea Park (Pakowki) Formation – Campanian The Campanian Faunal Zone VIII occurs in the largely marine Lea Park dark grey shale with subordinate sandstone beds that increase upward in number and thickness. The formation is 120 to 170 m thick. In southwestern Saskatchewan it overlies the Alderson (Milk River) on a 0.15 m- to 0.30 m-thick bed containing abundant chert pebbles (Crockford and Clow, 1965) indicative of an unconformity. This contact is taken to be the Eagle or Milk Figure 2 - Schematic diagram of the stratigraphic relationships of Upper Cretaceous River “shoulder” in the general formations between southwestern and southeastern Saskatchewan. Faunal assemblage usage of workers in the region. zones V to XVI adapted from North and Caldwell (1975, Figure 3). The Pakowki is depicted as passing eastward into the Lea Park shale lithosome in southern Saskatchewan (Figure 2). North and Caldwell (1975) showed the Pembina Formation as a correlative of the basal Lea Park and regarded its assemblage VIII fauna as succeeding assemblage VII after a significant hiatus. By virtue of the indicated unconformity, it is a post-Alderson (Milk River) development, not a shale facies of the Alderson (Milk River).

Belly River Formation – Campanian The Belly River Formation of southeastern Alberta (Crockford and Clowe, 1965) consists of two members. The Foremost, 75 to 140 m thick, forms the lower half of a section constructed of interbedded carbonaceous shale, light grey sandstone and and bentonite, representative of continental to marine conditions eastward into Saskatchewan. The overlying Oldman, 90 to 180 m thick, is largely sandstone and argillaceous sandstone, coaly, nonmarine but grading into marine eastward in Saskatchewan, where both members combine to form the Belly River Formation. McLean (1971) displayed by means of geophysical log-sections the transition of the Belly River sandstones across southern Saskatchewan as diachronous tongues sloping into the Lea Park shale lithosome. This relationship, as traced informally in the course of this work, appears to be sandstone tongues of the Foremost Member (assemblage zone IX) which slope as clinosequence sets into the Lea Park shale, and the Oldman (assemblage zones X and XI) which carries across the basin to southeastern Saskatchewan as parasequence sets.

Saskatchewan Geological Survey 3 Summary of Investigations 2003, Volume 1 Bearpaw Formation – Campanian This 360 m-thick body of calcareous and fossiliferous, medium grey shale and fine-grained quartzose sandstone is extensively described in Caldwell (1968) and the ten units (Outlook, Broderick, Matador, Sherrard, Demaine, Beechy, Ardkenneth, Snakebite, Cruikshank, and Aquadell) have been tracked across the present study area (Figure 3). These were assigned faunal designations XI to XVI by North and Caldwell (1975).

3. The Late Cretaceous (Campanian) Unconformity a) General Payenberg (2002) in his re-examination of the relationship between the of and the overlying terminated the former at the end of the Santonian (ca. 83.5 Ma), i.e., above the Deadhorse Coulee Member, and placed the base of the latter at 81 Ma in the Campanian (ammonite zone Baculites obtusus). He thereby indicated a hiatus of 2.5 Ma between these two formations in Alberta. In southwestern Saskatchewan, he recognized the Alderson as younger than the Deadhorse Coulee but older than the Pakowki, and equivalent to the Upper Eagle in Montana. Thus, the post-Alderson hiatus is dated as in early Campanian (ca. 81 Ma). In terms of the regional geophysical log correlations of the pertinent formations into southeastern Saskatchewan, the associated unconformity appears to be erosional with overall relief greater than 200 m. The regional stratigraphic relationships of the Upper Cretaceous formations are generalized in the schematic diagram of Figure 2. b) Southeastern Saskatchewan The geophysical log cross-section of Upper Cretaceous formations (Figure 3) is anchored in the west to Crockford and Clow’s (1965, Figure 2) easternmost well in the line of section initiated in southeastern Alberta. The log for this well, Delhi et al Kealey Spring 10-30 (Lsd 10-30-8-22W3), in this paper is utilized as a reference log for the west- to-east line of cross-section into southeastern Saskatchewan. Most formations decrease only slightly in thickness from west to east, as if the line of section is aligned with the regional depositional strike. The Lea Park and Belly River formations, however, undergo a combined 50 percent attenuation from 259 to 131 m over the 283 km distance between Delhi et al Kealey Spring 10-30 and the Imperial Oil Garville 8-23 (8-23-3-24W2). If one were to select a stratigraphic datum on one of the units in the overlying Bearpaw Formation, the formations beneath the Lea Park would be displaced downward at the Kealey Spring 10-30 well by 137 m, reflecting the fact that the Lea Park and Belly River formations at that site lay at the eastern end of an active depocentre different from those of the Colorado-Alderson (Milk River) formations below and the Bearpaw Formation above. Along the 372 km west-to- east line of section, thickness of the Bearpaw hardly varies. It is 393 m at Kealey Spring 10-30, 378 m at White Rose et al. Lake Alma 8-22 (8-22-2-17W2), 396 m at Erskine West Coast Beaubier 7-30 (7-30-2-16 W2), and 387 m at Bell W. Oungre 14-7 (14-7-3-15W2). In general, the pre-Lea Park formations thin gently to the east and northeast, partly in response to regional attenuation by deposition, and partly due to internal disconformities. The most persistent of the minor discontinuities are those basal to the Second White Specks and the Medicine Hat Member of the Niobrara Formation, although severe local truncations, such as that of the Aquadell Member of the Bearpaw Formation, are also present; for instance, at Shell Bergfield A10-15 (10-15-2-21W2) 54 m has been removed under the Formation.

However, as shown in Figure 3, the Milk River “Shoulder”, prominently displayed as an outward shift of the spontaneous potential, resistivity, gamma-ray, and sonic logs, and used to define the top of the Alderson (Milk River) in the west, actually truncates this unit in the study area. Between Ranges 15W2 and 14W2 along Township 2, this erosion surface has truncated 150 m of Alderson (Milk River) and First White Specks. Eastward across the study area, it variously truncates the Medicine Hat Member. The low-lying relief is blanketed with sandstones and shales of the Lea Park Formation as depicted in Figure 2. This basal Lea Park unit is commonly characterized by two sandstone beds, generally upward fining and about 20 to 30 m thick in total, that underlie a radioactive shale, 15 to 20 m thick, as shown at the Bell W. Oungre 14-7 well (Figure 3). Pre-Belly River sandstone-shale beds, most likely equivalent to the Pakowki Formation farther west, onlap the unconformity surface onto the Alderson (Milk River), as distinguished from the Lea Park shale-sandstone sets equivalent to the Belly River Formation. These are relationships to be further explored in the progress of this study. Another way of viewing the sub-Lea Park unconformity is from above. The Niobrara units are regionally uniform, though thinning to the east and northeast at a rate far less than the high relief exhibited on the sub-Lea Park unconformity. Thus an isopach map taken of the interval between the base of the Second White Specks and the unconformity is, in effect, a topographic analogue of the sub-Lea Park erosion surface. Figure 4 illustrates an isopach map of this interval in the western portion of the study area between Ranges 24 W2 and 14W2 inclusive, and Townships 1 and 17 inclusive. The isopachytes are a measure of the topographic relief, which falls from highs above 340 m in the southwest to below 120 m in the northeast, i.e., some 200 m. The topography is marked by a

Saskatchewan Geological Survey 4 Summary of Investigations 2003, Volume 1 y area in southeastern Saskatchewan y area linking western Saskatchewan to the stud ss-section of the Upper Cretaceous formations Cretaceous Upper the of ss-section (location of line is illustrated in Figure 1). in illustrated is of line (location Figure 3 - West to eastcroFigure stratigraphic 3 - West

Saskatchewan Geological Survey 5 Summary of Investigations 2003, Volume 1 pronounced northwesterly trending escarpment, the foot of which extends from Township 1, Range 13W2 in the southeast to Township 12, Range 24W2 in the northwest, and across which the erosion surface descends across a map distance of 77 km on a slope of 2.6 m/km. A 30 m-deep, 10 km-wide valley forms the base of escarpment and the western flank of a rolling terrain to the northeast, where elevation differences are of the order of 60 m or less. One such knoll west of the City of Weyburn is five or more townships in area. To the southwest of the escarpment relief is relatively low. The sub- Lea Park surface resembles a Campanian antecedent of the present-day topography in the study area, in that the escarpment corresponds to the coincident Missouri Coteau, which overlooks the first Prairie step to the east. A similar isopach map by McLean (1971) of the overlapping region to the west of the study area shows: 1) the 160 m contour continuing northwest to the region of Kerrobert in west-central Saskatchewan; and 2) the upland rising above the 360 m contour. Along the Alberta boundary, the regional strike turns north and so Figure 4 - Isopach map of the stratigraphic interval between the base of the Second delineates a basin the floor of White Specks and the sub-Lea Park unconformity, in the western portion of the study which slopes northeast toward area, southeastern Saskatchewan. Saskatoon. In the present study, reconnaissance of the region to the east indicates continuity of the lowlands beyond the Second Meridian.

The implications of the existence of this erosion surface in the middle of what has been regarded as the axial region of the major North American Upper Cretaceous marine basins provokes much thought and speculation on their tectonic history. Obviously, the 260 m sub-Lea Park topographic relief implies: 1) base-level elevation changes of similar magnitude, and 2) that at some time in the early Campanian, as intimated by North and Caldwell (1975) and Payenberg (2002), normal depositional cycles in the Rocky Mountain foreland basin were profoundly interrupted because of uplift and/or sea-level fall. The hiatus was apparently long enough for erosional forces, presumably meteoric, to alter the configuration of the basin. Recovery of the marine basin apparently occurred by transgression of the early Lea Park sea from the southeast and south reworking fluviatile sands in the thalwegs of the sea floor, and, as indicated by bituminous shale overlying the sand bodies, was followed by fairly rapid flooding to depths of 100 m or more. Younger 30 m-thick apparent channel sandstones that cap the basal Lea Park unit indicate other sea- level falls which gave rise to low-stand by-pass fluviatile deposits in the depocentre of the basin.

4. References Caldwell, W.G.E. (1968): The Late Cretaceous Bearpaw Formation in the South Saskatchewan River Valley; Sask. Resear. Counc. Rep. 8, 86p.

Saskatchewan Geological Survey 6 Summary of Investigations 2003, Volume 1 Crockford, M.B.B. and Clow, W.H.A. (1965): Upper Cretaceous formations of the Cypress Hills–Alderson (Milk River) area, southeastern Alberta and southwestern Saskatchewan; in Alta. Soc. Petrol. Geol. 15th Ann. Field Conf. Guidebook, Part 1, Cypress Hills Plateau, p184-197. McLean, J.R. (1971): Stratigraphy of the Upper Cretaceous in the Canadian Great Plains; Sask. Resear. Counc., Geol. Div., Rep. 11, 96p. North, B.R. and Caldwell, W.G.E. (1975): Foraminiferal faunas in the Cretaceous System of Saskatchewan; in Caldwell, W.G.E. (ed.), The Cretaceous System in the Western Interior of North America, Geol. Assoc. Can., Spec. Pap. 13, p303-331. Payenberg, T.H.D. (2002): Integration of the Alderson Member in southwestern Saskatchewan into a litho- and chronostratigraphic framework for the Milk River/Eagle shoreline in southern Alberta and north-central Montana; in Summary of Investigations 2002, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2002-4.1, p134-142. Ridgley, J.L. (2000): Lithofacies architecture of the Milk River Formation (Alderson Member of the Lea Park Formation), southwestern Saskatchewan and southeastern Alberta – its relation to gas accumulation; in Summary of Investigations 2000, Volume 1, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 2000-4.1, p106-120. Shurr, G.W. and Ridgley, J.L. (2002): Unconventional shallow biogenic gas systems; Amer. Assoc. Petrol. Geol. Bull., v86, no11, p1939-1969.

Saskatchewan Geological Survey 7 Summary of Investigations 2003, Volume 1