A Major Late Cretaceous (Campanian) Unconformity, Southeastern Saskatchewan1
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A Major Late Cretaceous (Campanian) Unconformity, Southeastern Saskatchewan1 J.E. Christopher 2 and M. Yurkowski Christopher, J.E. and Yurkowski, M. (2003): A major Late Cretaceous (Campanian) unconformity, southeastern Saskatchewan; 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 Santonian-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 shales. 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, Colorado Group, Bearpaw Formation. 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 sandstones. Accordingly the lower Mesozoic formations, represented by the Jura-Triassic Watrous evaporitic clastics; the Jurassic 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 shale and sandstone 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 stratigraphy 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 Alberta, and northern Montana 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 Manitoba. 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. Fossils 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 coal 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,