Paleoenvironmental changes in the Cretaceous (Albian to Turonian) Colorado Group of western Canada: microfossil, sedimentological and geochemical evidence

Cretaceous Research (1996) 17, 311 – 365

Paleoenvironmental changes in the Cretaceous (Albian to Turonian) Colorado Group of western Canada: microfossil, sedimentological and geochemical evidence *C. J. Schro¨ der-Adams, †D. A. Leckie, ‡J. Bloch, §J. Craig, ŒD. J. McIntyre and *P. J. Adams * Department of Earth Sciences , Carleton University , Ottawa , Ontario K1S 5B6 , Canada † Geological Survey of Canada , 3303 33rd St. NW , Calgary , Alberta T2L 2A7 , Canada ‡ Scealu Modus , 2617 Cutler Ave . NE , Albuquerque , New Mexico 87106 , USA § Box 668 , Bragg Creek , Alberta T0L 0K0 , Canada Œ Geological Survey of Canada , 3303 33rd St. NW , Calgary , Alberta T2L 2A7; present address: 3503 Underhill Drive NW , Calgary , Alberta T2N 4E9 , Canada Revised manuscript accepted 14 November 1995

Paleoenvironmental interpretations presented here for a portion of the Cretaceous Colorado Group marine shale succession in western Canada are based on the synthesis of biofacies, sedimentological and geochemical data. Vertical and lateral variations in foraminiferal, coccolith and dinoflagellate assemblages, in sediment fabric, structures and grain size, and in organic matter abundance and composition indicate shale deposition in a dynamic and variable basin setting. The upper Albian to middle Turonian Colorado Group shales were deposited during an overall eustatic sea-level rise punctuated by local, tectonically-induced, relative sea-level drops and variable circulation patterns. The upper Albian Westgate Formation was deposited during the initial stage of Mowry Sea transgression under a dominantly low-salinity, cool, Boreal watermass. Up to three coarsening-up cycles identified within this unit indicate local sea-level fluctuations or changes in sediment supply and / or distribution. The exclusively agglutinated foraminiferal assemblage is Boreal in affinity and reflects changes in substrate grain-size. Sedimentary structures and generally well-bioturbated sediment indicate deposition at or above storm-wave base beneath oxygenated bottom-waters. The basal lithology of the overlying lower Cenomanian Fish Scales Formation is a regionally extensive bioclastic conglomerate interpreted as either a wave-winnowed lag formed during a relative sea-level fall and subsequent rise, or a current-winnowed lag in deeper water. Deep-water bottom currents possibly were generated by mixing of the cool, low-salinity Boreal waters with warm, normal-salinity waters of Tethyan affinity as the Mowry Sea opened to the south forming the Western Interior Seaway (WIS). Organic matter is dominantly Type II, comprising a large component of marine algal material. The overlying barren, well-laminated sediments that comprise the bulk of the Fish Scales Formation were deposited under a stratified water column with anoxic bottom-waters and are characterized as a condensed section. The middle to upper Cenomanian Belle Fourche Formation conformably overlies the Fish Scales Formation. A regional sea-level drop occurred during Belle Fourche time as indicated by the progradation of Dunvegan deltaic sediments in northwestern Alberta. Widespread dysoxic conditions persisted throughout the middle to late Cenomanian in this region as shown by the limited agglutinated foraminiferal assemblage and sparse bioturbation. Increased detrital input is evident as an increase in silt content relative to the Fish Scales Formation and a re-introduction of significant amounts of Type III organic matter. The occurrence of numerous bioclastic conglomerates throughout the upper portion of the Belle Fourche Formation is possibly the result of relative sea-level drops affecting areas of different water depth with variable erosional intensity. Maximum transgression in latest Cenomanian to early Turonian time brought fully marine conditions and planktic Tethyan fauna into the Canadian portion of the WIS. This time period is represented by the Second White Specks Formation. Productivity in the upper water column was high and anoxic bottom waters preserved abundant Type II organic matter. Lateral facies variations and a diachronous introduction of Tethyan foraminifera and coccoliths to various parts of the basin indicate pathways of oceanic circulation. The influence of major Cordilleran detrital sources limited pelagic faunal development in the west. A significant unconformity in central Saskatchewan indicates local basin floor doming and subsequent erosion in late Turonian to Santonian time. ÷ 1996 Academic Press Limited KEY WORDS: Cretaceous; Western Canada Sedimentary Basin; Albian; Cenomanian; Turonian; foraminifera, nannofossils, dinoflagellates; paleoenvironment; paleoceanography; sea-level change. 0195 – 6671 / 96 / 030311 1 55 $18.00 / 0

÷ 1996 Academic Press Limited

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1. Introduction During late Albian to early Turonian time, a major epicontinental marine transgression occurred over much of western North America (Williams & Stelck, 1975; Kauffman, 1977; 1984). A thick and extensive succession of dominantly marine shale deposited during that time comprises the lower part of the Colorado Group in the Western Canada Sedimentary Basin (WCSB). A detailed description of formation characteristics (summarized in Table 1) and a new stratigraphic framework for a portion of the Colorado Group, based on lithology, geochemistry and micropaleontological content, were recently presented by Bloch et al . (1993). Lithostratigraphic formations that comprise the lower Colorado Group are recognized in the subsurface across western Canada (Figure 1) and can be correlated with outcrop sections in the Rocky Mountain Foothills to the west (Stott, 1984; Wall, 1967) and in the Manitoba Escarpment to the east (McNeil & Caldwell, 1981; Gilboy, 1988). The shale formations are, in ascending order: the Westgate Formation of late Albian age; the Fish Scales Formation of early Cenomanian age; the Belle Fourche Formation of middle to late Cenomanian age; and the Second White Specks Formation of latest Cenomanian to middle Turonian age. The paleoceanographic setting of the WCSB was affected to varying degrees by influences of two different watermasses (Figure 2); the Cretaceous Arctic Ocean (Boreal Sea) and the proto-Gulf of Mexico (Tethyan Sea). Sediment input was controlled dominantly by tectonics of the Cordillera to the west (Stockmal et al ., 1992) and subordinately, the stable craton of the Canadian Shield to the east. Superficially, the Colorado Group shales appear to be uniform in composition and therefore have remained largely undifferentiated until recently. However,

Table 1. Characteristic features of lower Colorado Group formations (after Bloch et al . (1993). SECOND WHITE SPECKS FORMATION Uppermost Cenomanian to middle Turonian Hedbergella loetterlei Zone Dominantly planktic foraminifera Abundant coccoliths Rich dinoflagellate assemblage No bioturbation Laminated marl Bentonites Type ll organic matter High TOC (up to 12 WT%) Generally sharp lower contact

BELLE FOURCHE FORMATION Middle to Upper Cenomanian Verneuilinoides perplexus Zone Low diversity agglutinated foraminiferal assemblage Reduced dinoflagellate assemblage Laminated mudstone to siltstone Non- to slightly calcareous Abundant siderite concretions Type lll organic matter Low TOC (,3 WT%) Gradational boundary to underlying Fish Scales Formation

FISH SCALES FORMATION Lower Cenomanian Barren of foraminifera Reduced dinoflagellate assemblage Abundant fish scales and algal cysts No bioturbation Laminated claystone to mudstone Basal sandstone / conglomerate Bentonites Type ll & lll organic matter High TOC (up to 8 WT%) Sharp lower contact

WESTGATE FORMATION Upper Albian Miliammina manitobensis Zone Agglutinated foraminiferal assemblage Diverse dinoflagellate assemblage Bioturbation Laminated claystone to siltstone Non-calcareous Type lll organic matter Low TOC (,2 WT%) Sharp lower contact

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Figure 1. Sketch map of study area showing location of: 1, cores (black dots) listed in Table 2; 2, reference cores (black squares); and 3, cross-section A-B (Figure 7).

integrated data mainly from Alberta show a complex and variable depositional history representing about 8 my (Bloch et al ., 1993). Microorganisms are sensitive to changing habitats and are an excellent tool to monitor paleoecological changes. Planktic microfossils provide supporting evidence for interpretation of sea-level changes. Agglutinated foraminifera are sensitive to substrate variations and are therefore valuable facies indicators (Schro¨ der-Adams & McNeil, 1994). Benthic communities may also signal bottom-water oxygen levels (Sageman, 1989) and help to identify oxygen depletion events. This paper adds new paleontological (foraminifera, nannofossils, dinoflagellates), sedimentological and geochemical data from Saskatchewan, incorporates the previously published data from Alberta (Bloch et al ., 1993) and describes paleoenvironmental changes and depositional controls that formed the four distinct formations. The multidisciplinary approach shows that the various signals in a natural system are closely linked. The large quantity of descriptive data for each formation cannot be repeated here and the reader is referred to Bloch et al . (1993).

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Figure 2. Sketch map of the Western Interior Seaway showing the Mowry, Tethyan and Boreal seas; modified after Stelck (1975) and Kauffman (1984).

2. Stratigraphic position and age The Colorado Group was deposited over a period of 25 to 30 my, from Albian through Santonian time, when sea level was globally high. Specific sea-level maxima occurred during the late Albian, early Turonian and middle Santonian (Caldwell & North, 1984; Haq et al ., 1988). During this interval of the Cretaceous, deposition was coincident with a regional tectonic downflexing of the western margin of the North American craton (Lambeck et al ., 1987) that resulted in the development of an extensive north – south trending foreland basin (see summary in Leckie & Smith, 1992). Major marine inundations are separated by four major regressive pulses represented by the Peace River-Viking, Dunvegan, Cardium-Bad Heart and Milk River Formations (Figure 3). The Colorado Group (Figure 3) unconformably overlies the Mannville Group in the subsurface of central and southern Alberta (Figure 3) and Saskatchewan (Figure 3). The strata are overlain conformably to disconformably by the Milk River and Lea Park Formations. In the central and southern Foothills, equivalent Alberta Group strata unconformably overlie the Crowsnest Volcanics and Blairmore Group and are unconformably overlain by the Belly River Formation (Figure 3). In the Northwest Plains, equivalent strata are the upper part of the Fort St. John Group and the lower part of the Smoky Group (Figure 3). In

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Figure 3. Stratigraphic nomenclature for the Colorado Group and equivalent strata in the Western Canada Sedimentary Basin. Nomenclature for columns 1 – 5 after Stott (1963, 1982, 1984); column 6 after Simpson (1982); column 7 after McNeil & Caldwell (1981); and column 8 (this study and Bloch et al ., 1993).

Manitoba equivalent strata are the Westgate and Belle Fourche members of the Ashville Formation and the Favel Formation (Figure 3). This paper focuses primarily on the lower portion of the Colorado Group from the upper Albian Westgate Formation to the lower Turonian Second White Specks Formation, which covers a time period from about 100 to 92 Ma. A cursory summary of the formation characteristics is given in Table 1.

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The lower Colorado Group is ,700 m thick in southwestern Alberta and thins eastwards toward the Manitoba Escarpment where it is ,200 m thick. In northwestern Alberta, the lower Colorado Group exceeds 1500 m in thickness, where it overlies the Peace River Arch (Figure 1)—an area of subsidence throughout the Cretaceous (O’Connell et al ., 1990). Our biostratigraphic observations are based on foraminifera, nannofossils and dinoflagellates. The existing foraminiferal zonation (Caldwell et al ., 1978; Caldwell & North, 1984) is related to the molluscan zonal scheme of Jeletzky (1971). One specimen of Collignoniceras woollgari (Mantell) recovered from the upper Second White Specks Formation at 14-29-11-28W4 (2508.8 m), is early middle Turonian in age (R. Hall, pers. comm., 1993). Detailed nannofossil and dinoflagellate correlation charts do not exist for the WCSB. Nannofossil zonal markers of the Greenhorn Limestone of Colorado (Watkins, 1985) have not been recognized in Canada. Absolute ages are based on data from Obradovich (1991). The biostratigraphic zonation contains three foraminiferal zones (Table 2). The upper Albian Miliammina manitobensis Zone is present in the Westgate Formation. Extrapolation of the Obradovich (1991) radiometric time scale indicates that the Westgate Formation was deposited over about 1.5 my between approximately 98.7 and 97.2 Ma. A detailed biostratigraphic correlation of this zone is provided by Stelck (1975). No subzonal scheme is proposed at this time. The Fish Scales Formation is barren of foraminifera and its Cenomanian age determination relies on the dinoflagellate assemblage (Singh, 1983; Bloch et al ., 1993) and its stratigraphic position (Leckie et al ., 1992). The Fish Scales

Table 2. Foraminiferal zones in the Western Canada Sedimentary Basin. Northwest Plains

Rocky Mountain Foothills

Southern Alberta Northern Montana

6

7

8, 9

10

11

Hedbergella loetterlei

Assemblage V

Hedbergella loetterlei

Hedbergella loetterlei

Reference 1, 2, 3, 4, 5

Saskatchewan Manitoba Escarpment

This Study

Age late Hedbergella lower Cenomanian loetterlei pelagic to early ————— microfauna middle Flabellammina Turonian gleddiei

middle Verneuilinoides Verneuilinoides Verneuilinoides Assemblage Verneuilinoides Verneuilinoides Cenomanian perplexus kansasensis * perplexus IV perplexus perplexus early Cenomanian —————— late Albian 1 2 3 4

-

Texularia alcesensis

barren

barren

not defined

?

barren

——————–——————— —————— —————— —————– —————– Miliammina Miliammina Miliammina Assemblage Miliammina Miliammina manitobensis manitobensis manitobensis llb and lll manitobensis manitobensis

Gleddie, 1954 Stelck & Wall, 1954 Stelck & Wall, 1955 Stelck et al ., 1958

5 6 7 8

-

Leckie et al ., 1992 Wall, 1967 Lang & McGugan, 1988 North & Caldwell, 1975a

9 - North & Caldwell, 1975b 10- McNeil & Caldwell, 1981 11 - Bloch et al ., 1993

* Verneuilinoides kansasensis is conspecific with Verneuilinoides perplexus .

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Formation was deposited over ,1.4 my, between approximately 97.2 and 95.8 Ma. The Verneuilinoides perplexus Zone of middle to late Cenomanian age is present in the Belle Fourche Formation, deposited over ,2.5 my between approximately 95.8 and 93.3 Ma. The Hedbergella loetterlei Zone occurs in the Second White Specks Formation, deposited during latest Cenomanian to middle Turonian time. In eastern Saskatchewan, this zone is divided into the Clavihedbergella simplex and Whiteinella aprica subzones (Figures 4, 5). The base of the C. simplex Subzone is placed in the late Cenomanian based on bivalve occurrences (McNeil & Caldwell, 1981) whereas Caldwell et al . (1978) originally had placed it in the lower Turonian. The Second White Specks Formation was deposited over ,2.1my between approximately 93.3 and 91.2 Ma. 3. Methods The data presented herein are from samples collected from seven reference wells located in southeastern Alberta and southern Saskatchewan (Figure 1). Additional lithostratigraphic and geochemical information was extracted from another 20 wells covering an area from the central Alberta Foothills to southeast Saskatchewan (Table 3, Figure 1). Samples were taken in 5- or 1-m intervals. At lithological boundaries and other critical intervals sampling frequency increased. Micropaleontological material is available at the Geological Survey of Canada, Calgary, Alberta. Samples for foraminiferal analysis were prepared according to standard methods and washed over a 63 m m sieve. In most samples, microfossils were picked for identification. In samples of the Second White Specks Formation, containing abundant planktic foraminifera, at least 500 specimens were counted. In samples of all other formations the entire foraminiferal assemblage was counted. Foraminiferal listings (Appendices 1 – 7) reflect total number of specimens per species in each sample. Sample volumes vary. However, due to low total numbers, no relative abundances for a fully quantitative analysis were calculated. Preservation of foraminifera was generally moderate to poor allowing in many cases generic identifications only. In addition, different species concepts among previously published studies hinder regional correlations and somewhat distort paleontological, and consequently, paleogeographic reconstructions. Approximate abundance estimates of bivalve shell prisms, algal cysts and fish remains in the sand-sized fraction, which are important for environmental interpretation, are listed under ‘Miscellaneous Components’. Permanent slides of nannofossils were examined under a light microscope. Slide preparation techniques were modified from those of Edwards (1963), Stover (1966), Gartner (1968) and Smith (1981). Counting techniques varied with the abundance of coccoliths noted during preliminary scans. If coccoliths were abundant, a half-slide scan was performed; otherwise a full slide scan was used. Total numbers of coccoliths per sample are listed in Appendices 8 – 14. Palynological samples were prepared from the 10-35-45-2W4 well. Standard acid-processing techniques were used followed by heavy liquid flotation and sieving to concentrate dinoflagellates in the 145 m m and 120 m m fractions. Despite the abundant amorphous organic material that commonly obscured the dinoflagellates, identifications were possible in all samples. Rock-Eval pyrolysis was performed in duplicate on pulverized shale samples using standard procedures on a DELSI Rock-Eval II / TOC apparatus (Espitalie´

Figure 4. Total number of agglutinated and planktic foraminiferal specimens and number of species per sample at 11-36-22-1W2 in eastern Saskatchewan. The lower Cenomanian Textularia alcesensis Zone of Caldwell et al . (1978) was not found in this study.

318 C. J. Schro¨ der-Adams et al.

Figure 5. Biostratigraphic ranges of foraminiferal species with common occurrences at 11-36-22-1W2 in eastern Saskatchewan. All species of the upper Albian Westgate Formation disappear at the contact to the Fish Scales Formation. All planktic species except Heterohelix globulosa (Ehrenberg) disappear in the middle of the Second White Specks Formation showing that the biofacies boundary is not coinciding with the lithofacies boundary. This is the only location where the Clavihedbergella simplex Subzone could be recognized. The assemblage contains numerous rare species which are not shown in this figure (see Appendix 7).

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Table 3. Cores used in this study.

Core Location 11-36-22-1W2 16-36-27-10W2 16-12-31-21W2 5-22-34-1W3 5-22A-34-1W3 16-26-36-4W3 11-16-35-8W3 4-16-37-13W3 2-20-40-18W3 10-25-1-27W3 06-18-45-1W4 10-35-45-2W4 07-14-01-5W4 06-22-11-6W4 06-34-30-8W4 11-12-06-16W4 07-12-43-21W4 06-16-06-22W4 06-16-11-22W4 10-34-42-22W4 11-21-12-23W4 06-21-55-25W4 06-29-13-27W4 06-30-13-27W4 06-07-12-28W4 04-13-54-18W5 09-09-56-19W5 10-05-53-20W5 10-25-65-20W5 05-01-77-20W5 16-01-61-22W5 05-09-72-8W6

Name S.W.P. Bredenbury *1 Tide Water Flint1 Shell Quill Lake1 Elstow 5-22 US Borax & Chemical P.C.A. Saskatoon1 C.M.S. Vanscoy*1 C.M.S. Asquith1 PEX Cathkin1 Saskoil Willow Creek* Anderson et al . Ribstone* Anderson Husky Roros*1 Pacific Amoco Sapphire G. Basin Bux Medicine Hat Amoco B1-Youngstown* Amoco Conrad* LCM et al . Buff Lake N Gulf Mohawk Blood Canhunter Keho LCD et al . Bradshaw Melaar Barons Ajax Morinville Can Hun et al . Claresholm MLC Dekalb Claresholm Canadian Superior Oxley Texex Edson HCS et al . Beaver Creek H. B. Galloway Sabine et al . Iosegun Imperial Kathleen Amoco Bigstone Imperial Wembley

Logged Interval (m) Top Bottom 195.4

392.2

381.0 464.8

396.2 518.0

523.2

698.2

836.0 398.4 408.4 578.5 516.6 691.2 548.6 1009.0 970.0 1110.1 1053.0 1268.0 853.7 2080.0 2120.0 2572.0 2098.5 2237.2 2655.4 1317.4 393.2 1868.0 1246.6

854.4 461.2 499.9 605.8 553.2 831.2 630.8 1020.3 988.3 1126.5 1071.3 1286.3 871.5 2088.0 2124.6 2606.8 2116.5 2273.2 2685.6 1335.6 454.2 1904.0 1507.1

* Reference cores with detailed micropaleontological data. 1 Used in cross-section (Figure 7).

et al ., 1977). The information obtained includes the type of organic matter (oxygen and hydrogen indices), and the amount of total organic carbon (TOC). Tmax is the temperature at which maximum hydrocarbon generation occurs during pyrolysis and is an indication of the maturity of the organic matter contained in the sample. For a detailed discussion of the generation and interpretation of pyrolysis data, the reader is referred to Peters (1986) and references therein. Shales are classified after the method of Potter et al . (1980) as siltstone (,32% clay-sized material), mudstone (32 – 65% clay) and claystone (.65% clay) where clay-sized material is ,4 m m in size (Potter et al ., 1980). Silt content is estimated from core observations, textural analysis and examination of clay-separate residues. 4. Results This paper presents new data from Saskatchewan and from the 11-12-6-16W4 well in southeastern Alberta (Figure 1, Appendices 1 – 14). These observations, in addition to data presented in Bloch et al . (1993), provide the basis for

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321

paleoenvironmental reconstructions discussed here. Additional Rock-Eval data from Alberta and Saskatchewan are given in Bloch & Leckie (1993). 4.1. Micropaleontology Foraminiferal zones in Saskatchewan are the same as those in Alberta (Figures 5, 6). The planktic foraminiferal fauna in Saskatchewan are characterized by higher numbers of well-preserved specimens and slightly higher diversity compared to Alberta where no species of Clavihedbergella Banner & Blow occur (Figure 4). Coccolith assemblages in Saskatchewan are comparable to those in Alberta. Coccoliths occur only in the Second White Specks Formation and resemble the assemblage of the lower Turonian Quadrum gartneri Zone (Doeven, 1983), although the zonal marker species has not been recognized in our samples. Species diversity is higher in Saskatchewan compared to Alberta (Appendices 8 – 14). The Second White Specks Formation takes its name from the abundance of calcareous nannofossils that are typically concentrated in white, fine to very-finegrained, sand-sized fecal pellets. The pellets have been reworked into parallellaminated and rippled beds 0.25 to 2 cm thick; starved ripples and millimeterthick laminae are common. Muddy laminae occur on foreset drapes. Most of the rippling is unidirectional, with rare possibly wave-reworked upper surfaces. Fish debris is locally concentrated and associated with the rippled beds. 4.2. Unconformities In central Saskatchewan two unconformities are prominent in core 11-16-358W3. A thin interval of the Second White Specks Formation, identified in only one sample, is preserved (Appendices 6, 13). This sample contains foraminifera of the Whiteinella aprica Subzone of early Turonian age and overlies directly the Verneuilinoides perplexus Zone of middle to early late Cenomanian age. The Clavihedbergella simplex Subzone of latest Cenomanian to earliest Turonian age (McNeil & Caldwell, 1981) is missing. Also absent is a transitional zone including the benthic calcareous taxon Neobulimina albertensis (Stelck & Wall), that is observed in the section at 6-18-45-1W4. These missing subzones are interpreted to indicate a local unconformity spanning the Cenomanian / Turonian boundary. The second unconformity is indicated by the occurrence of Santonian sediments of the First White Speckled Shale (Figure 3) directly overlying the Second White Specks Formation. The Santonian age is determined by the occurrence of the Globigerinelloides sp. Zone (Caldwell et al ., 1978) and the nannofossil assemblage with common occurrences of Kamptnerius magnificus Deflandre and Marthasterites furcatus (Deflandre). The unconformities are mapped in a WE cross-section of well logs across Saskatchewan (Figure 7). To the south at 10-25-1-27W3, Santonian nannofossil assemblages occur in an interval just above the Second White Specks Formation, indicating that the second unconformity may extend regionally to the southwest. At this locality, it cannot be determined if Santonian sediments directly overlie lower Turonian sediments because of a barren zone of 3 m between the Second White Specks Formation and the First White Speckled Shale (Appendix 12). The regional cross-section through the Colorado Group in Saskatchewan (Figure 7) demonstrates the relatively flat, conformable stratigraphy of the lower

Figure 6. Biostratigraphic ranges of foraminiferal species with common occurrences at 6-34-30-8W4 in southeastern Alberta. Primitive agglutinated taxa such as Placentammina sp. 1 and Saccammina alexanderi (Loeblich & Tappan) range through several formations, but disappear in the Fish Scales Formation. The assemblage contains numerous rare species which are not shown in this figure (see Appendix 2).

322 C. J. Schro¨ der-Adams et al.

Figure 7. East – west cross section across Saskatchewan (see Figure 1 for locality) using the Viking Formation as datum. Note thinning due to erosional unconformities in 11-16-35-8W3. The lower unconformity in core 11-16-35-8W3 is based solely on micropaleontological observations and cannot be traced to adjacent wells.

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Figure 8. Pseudo Van Krevelen diagrams for four formations of the lower Colorado Group. Data are from Bloch & Leckie (1993).

Colorado Group except for the area in east central Saskatchewan (11-16-138W3) where marked unconformities occur over an apparent structural high. 4.3. Geochemistry Pseudo Van Krevelen diagrams for the four formations are shown in Figure 8 and TOC and HI profiles from three wells are shown in Figure 9. Both the Fish Scales and Second White Specks Formations show increased HI and TOC values across the basin, relative to the Westgate and Belle Fourche Formations. The Westgate Formation generally has the lowest HI and TOC values but the Belle Fourche Formation has a large number of samples with very high OI values (.75 mg CO2 / g OC) as well as low HI values. These samples come from the northwest part of the study area where Dunvegan pro-deltaic sediments interfinger with more distal Belle Fourche shales. Belle Fourche samples with high HI values (.250 mg HC / g OC) are primarily from the eastern part of the study area (Figure 1). At 11-16-35-8W3, the Second White Specks Formation and the unconformably overlying First White Speckled Shale have similar TOC and HI values and therefore cannot be distinguished on geochemical signatures alone. 4.4. Sedimentation rates Minimum and maximum (compacted) sedimentation rates, based on observed thicknesses and radiometric dates (Obradovich, 1991) are shown in Table 4. Sedimentation rates are generally low with maximum rates ranging from 2 to 6 cm / ky. Isopachs through four slices of the lower Colorado Group in Alberta are shown in Figure 10. The slices do not conform specifically to our formation boundaries due to the nature of the log-derived, digital data base from which they are generated (see Mossop & Shetson, 1994). However, the isopachs indicate how basement subsidence and Cordilleran tectonics affected sedimentation. Isopachs

Figure 9. TOC (open circles) and HI (crosses) profiles from three reference wells (see Figure 1) showing the abundance and type of organic matter for the studied interval.

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Table 4. Sedimentation rates for lower Colorado Group formations. Formation

Westgate Fish Scales Belle Fourche Second White Specks

Age*

99-97 97-96 96-93 93-91

Thickness (metres)

Compacted Sedimentation Rate (cm / 100 y)

Min

Max

Min

Max

20 10 20 25

120 20 150 90

1 1 0.7 1.25

6 2 5 4

* from Obradovich (1991).

of the Westgate Formation (Figure 10a) show maximum thickness of up to 400 m in northeastern British Columbia with thinning towards the southeast. The isopach from the base of the Fish Scales Formation to the base of the Dunvegan Formation (Fish Scales and lowest Belle Fourche Formations; Figure 10b) shows a similar southeastwards thinning with maximum thicknesses over the Peace River Arch in northeastern British Columbia and northwestern Alberta. The isopach from Dunvegan to Second White Specks Formation (most of the Belle Fourche Formation) (Figure 10c) shows a thick area on the Peace River Arch but the thinning trend has shifted and is northeastwards. Isopachs of Fish Scales and Belle Fourche Formations (Figure 10d) show a general eastwards thinning. Isopach maps from above the Second White Specks Formation (Leckie et al ., 1994) show a general southwards shift of the depocentre and basin, with thinning towards the northeast.

4.5. Bioclastic conglomerates The Fish Scales and Second White Specks Formations contain horizons of disarticulated bioclastic conglomerate, a few centimeters to a few decimeters thick, which occur discontinuously across the basin (e.g., Leckie et al ., 1992; Bloch et al ., 1993). Bioclastic conglomerate associated with the Second White Specks Formation consists of fish and shark teeth, fish scales, disarticulated vertebrate remains, angular to subangular clasts of mudstone and a green, chloritic, marine clay. Rare extraformational pebbles (commonly chert or quartz clasts) to 2 cm diameter and coalified wood debris may be present. Bed thicknesses vary from 2 to 20 cm. The conglomerates are generally massive, although crude subhorizontal bedding occurs locally. On exposed surfaces of the conglomerate, tapered shell or bone bioclasts are aligned indicating current activity. Alignment in overlying laminae may be in the opposing direction, suggesting the influence of wave action. A bioclastic conglomerate or coarse-grained sandstone, a few centimeters to decimeters thick, commonly occurs at the base of the Fish Scales Formation forming a sharp contact with the underlying Westgate Formation. This conglomerate is more regionally persistent than that at the base of Second White Specks Formation and appears similar to that of the Second White Specks Formation, although locally it may contain a larger siliciclastic component. Detritus consists primarily of vertebrate remains with minor amounts of chert or quartz, intraformational shale clasts and siderite clasts. Other components include belemnites and phosphatized bioclastic debris.

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Figure 10. Isopach maps of four time slices within the upper Albian Viking Formation to uppermost Cenomanian base of Second White Specks Formation; E and C indicate the location of Edmonton and Calgary. Modified from Leckie et al . (1994).

5. Paleoenvironmental changes Paleoenvironmental changes in the WCSB are evident on local and basin-wide scales. Basin-wide indicators include biostratigraphic zonations, organic-matter type and lithological features that permit differentiation at the formation level. Local scale changes result in intraformational variations that are not observed regionally. 5.1. Basin -wide environmental changes Westgate Formation . The major transgressive phase, which begins at the base of the Westgate Formation, follows the Viking sea-level lowstand (Beaumont, 1984; Leckie, 1986; Stelck & Koke, 1987; Leckie & Reinson, 1993). During late Albian time, this sea-level rise culminated in the expansion of the Mowry Sea

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which extended from Wyoming to the Arctic and from the Rocky Mountain Foothills to eastern Manitoba (Stelck, 1975; Williams & Stelck, 1975). The abundance of benthic foraminifera, the degree of bioturbation and the presence of horizontal trace fossils such as Planolites , Terebellina , Helminthopsis , and Muensteria indicate that moderately to well-oxygenated bottom-waters were present (Ekdale et al ., 1984). A diverse foraminiferal assemblage that represents numerous benthic foraminiferal feeding strategies in intervals of high bioturbation also suggests the presence of a well-oxygenated benthic environment (Jones & Charnock, 1985; Koutsoukos et al ., 1990). Foraminiferal composition, including taxa of Miliammina Heron-Allen & Earland, Trochammina Parker & Jones, Ammobaculites Cushman and numerous ataxophragmiids, as well as wave rippled and hummocky cross stratified fine-grained sandstone, suggest a shallow, wave-influenced sea of inner neritic to possibly middle neritic water depth. The formation is characterized by low TOC values (,2wt%) and dominantly Type III organic matter (Figure 8); the latter indicates a terrestrial origin as the primary sediment source. Vertical variations in grain size, indicated by changes from mudstone to siltstone (coarsening- or fining-upward cycles), may be linked to fluctuations in relative sea level or changes in sediment source and / or supply. Changes in grain size, based on gamma ray log signatures (Figure 7) and visual core descriptions, are detected in the texture of agglutinated foraminifera tests which reflect the grain size of the substrate (Figure 11). In some wells, two coarsening-upwards cycles correspond to a concomitant change in grain size of agglutinated foraminiferal tests. Towards the top of the formation, a relative shallowing of the basin is indicated by a widespread coarsening upwards trend (Figure 7), an increase in foraminiferal species with coarse-grained, robust tests (Figure 11) and an increase in bioturbation. Evidence that waves affected the sea floor include hummocky cross-stratified, wave-rippled, and combined flowrippled thin sandstone beds. Sharp-based, graded siltstone beds in this interval may be the result of distal or low-intensity storm events. The foraminiferal species composition shows an affinity to Arctic agglutinated assemblages of the Albian Tuktu and Grandstand Formations of northern Alaska described by Tappan (1962), Bergquist (1966) and Sliter (1979). Taxa such as Verneuilinoides borealis Tappan, Textularia topagorukensis Tappan, Psamminopelta browsheri Tappan, Ammobaculites fragmentarius Cushman, Miliammina manitobensis Wickenden, and Haplophragmoides topagorukensis Tappan in both areas indicate a path of faunal exchange. Dinoflagellate assemblages also suggest the migration of plankton between the Boreal and interior Mowry seas. Among the numerous species in common are Batioladinium jaegeri (Alberti), Chichaouadinium vestitum (Brideaux), Ellipsoidictyum imperfectum (Brideaux & McIntyre), Florentinia cooksoniae (Singh), Gardodinium trabeculosum (Gocht), Luxadinium propatulum Brideaux & McIntyre, Odontochitina singhii Morgan, Oligosphaeridium totum Brideaux, Ovoidinium verrucosum (Cookson & Hughes), Palaeoperidinium cretaceum Pocock, and Pseudoceratium expolitum Brideaux (Brideaux & McIntyre, 1975; Doerenkamp et al ., 1976; May, 1979; May & Stein, 1979; Dixon et al ., 1989). The exclusively agglutinated foraminiferal assemblage over such a wide shallow basin is unique and lacks modern analogs. During the latest Albian, the Mowry Sea had no connection with Tethys to the south and therefore probably had a cool, low salinity watermass of Boreal affinity (Stelck, 1975). Brackish water conditions have been inferred from oxygen isotope data from middle Albian

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Figure 11. Environmental changes in the Colorado Group at 6-34-30-8W4 in southeastern Alberta. Sandy (Viking) to silty (Westgate) substrates show increased bioturbation and contain diverse coarse grained foraminiferal assemblages, whereas fine substrates have a diminished fine grained fauna or are barren of benthic taxa. Belle Fourche and Second White Specks assemblages signal dysoxic to anoxic bottom water conditions. During the major third-order transgression of late Albian to Turonian age, several smaller sea-level oscillations took place.

marine sediments (Bloch, 1990) and similar conditions in the late Albian may have limited foraminiferal populations during Westgate deposition. The predominance of agglutinated foraminifera in the fossil record of the Mowry Sea assemblage might also be the result of preferential preservation. Terrigenous input provided sufficient iron for the ferric oxide-organic cement of

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agglutinated species. Refractory Type III organic matter may have limited extensive sulphate reduction thereby reducing iron demand (Gautier & Claypool, 1984). This, in turn, would limit the disaggregation of agglutinated tests (Schro¨ der-Adams & McNeil, 1994).

Fish Scales Formation. The boundary between the Westgate and Fish Scales Formations (Figure 3) is a sharp lithological contact, expressed as a thin, parallel or cross-laminated sandstone or pebble layer with abundant disarticulated fish and other vertebrate remains. Leckie et al . (1992) suggested three possible mechanisms to explain deposition of the Fish Scales Formation. The first is that a sharp drop in sea level at the end of the Albian resulted in submarine erosion and the formation of a lag deposit. In the Peace River area, Stelck et al . (1958) described the Textularia alcesensis Zone from the uppermost Albian to lower Cenomanian. The absence of this zone over the broad area of the WCSB (Caldwell et al ., 1978; Caldwell & North, 1984; Bloch et al ., 1993; this study) may be explained by a disconformity, suggesting that the basal deposit of the Fish Scales Formation is a lag. This faunal assemblage is reported only in northwestern Alberta and northeast British Columbia and the limited distribution of the T. alcesensis Zone may also reflect localized facies development. The second proposed mechanism is that the Fish Scales Formation was deposited during continued transgression. Minor changes in the dinoflagellate assemblage at this boundary (Figure 12) indicate no significant change in surface waters. Sea-level rise resulted in the deposition of a condensed interval comprising concentrated fish debris (Leckie et al ., 1992; Leckie & Smith, 1992). The Fish Scales Formation has numerous criteria of a condensed section as described by Loutit et al . (1988). It is relatively thin; contains considerable bioclastic debris indicating low sedimentation rates; has a high Type II organic matter content; and contains numerous bentonite layers. A third mechanism, which is also based on continued sea-level rise, suggests that the bioclastic debris was winnowed from the substrate by vigorous bottom currents (Leckie et al ., 1992). These currents may have been generated when two separate watermasses (Tethyan and Boreal) joined to form the Western Interior Seaway (WIS), sometime in the early Cenomanian (Hay, 1989). The submarine erosion caused by these currents may also have removed sediments of the T. alcesensis Zone. The presence of warm Tethyan waters in the northern region of the seaway, however, is problematic as nannofossils and planktic foraminifera, such as Hedbergella Bro¨ nnimann & Brown and Heterohelix Ehrenberg, are not found. Both genera are known to be the hardiest planktic foraminifera first occurring in the WIS during the Greenhorn transgression (Leckie et al ., 1991) and rare occurrences of these taxa are reported from lower Cenomanian sections only as far north as Colorado (Eicher & Diner, 1985; Caldwell et al ., 1993). In Canada, at the Albian / Cenomanian boundary, drastic environmental changes occurred which resulted in the loss of benthic foraminiferal assemblages (Figures 5, 6), increased organic-matter content (Figure 9), a change in organic matter type (Figure 8), and widespread bottom-water anoxia. Density stratification may have resulted from the incomplete mixing of Tethyan and Boreal waters. Alternatively, stratification may have been caused by the introduction of a meteoric, low density, low-salinity surface watermass. A brackish water lid produced by increased runoff and precipitation has been described as a cause for

Figure 12. Biostratigraphic ranges of dinoflagellate species at 10-35-45-2W4 in southeastern Alberta. Note the numerous taxa which range across the boundary between the Westgate and Fish Scales Formations, and the abundant first appearances at the base of the Second White Specks Formation.

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middle Cenomanian to lower Turonian sections depauperate of benthic organisms (Kauffman, 1975; Arthur et al ., 1985; Arthur et al ., 1987; Pratt et al ., 1993; Kyser et al ., 1993; Glancy et al ., 1993). The increased freshwater flux is interpreted as a signal of climatic change. The Albian / Cenomanian boundary interval is not preserved in the US section which does not permit a direct comparison. Our data are compatible with the hypothesis that a climatic change, possibly connected with transgression, took place at the beginning of the Cenomanian, but additional work on time equivalent, non-marine sediments is necessary to determine a direct relationship between climate change and brackish water conditions in the WIS. Regardless of the causal mechanism(s), the sharp basal lithological contact, the occurrence of bioclastic conglomerate and the abrupt disappearance of agglutinated species (Figures 5, 6) suggest a period of marine erosion at the Albian – Cenomanian boundary. This boundary in the WCSB coincides with an unconformity in Arctic Canada that is associated with sea-floor spreading in the Amerasia Basin (Embry & Dixon, 1990; Dixon, 1993). It also approximately coincides with, or just postdates, an episode of kimberlite emplacement in central Saskatchewan (Gent, 1992). A possible link between these events invites further investigation. Belle Fourche Formation. The contact between the Fish Scales and Belle Fourche Formations is conformable. A slight and gradual improvement in conditions for benthic organisms during Belle Fourche time is indicated by low species diversity and strongly fluctuating numbers of agglutinated foraminiferal specimens (Figures 4 – 6). Cold and low-salinity waters of Boreal origin continued to dominate the WIS (Eicher & Diner, 1985, Cadrin, 1992, Caldwell et al ., 1993). A remnant connection to the southern part of the seaway and the wide extension of the Boreal watermass are implied by distributions of the same agglutinated taxa over the entire WIS (e.g., Loeblich, 1946; Eicher, 1967; Eicher & Worstell, 1970). South of the present-day Canada / US border, marine conditions continued to support rare occurrences of planktic foraminifera in the partially time-equivalent Graneros Shale in Colorado (Eicher & Diner, 1985, 1989), South Dakota and Wyoming (Eicher & Worstell, 1970). In Canada, the prevalence of Boreal waters prevented planktic foraminifera and coccoliths from colonizing northern regions during Belle Fourche time. Bottom oxygen levels changed from anoxic at Fish Scales time to dysoxic at Belle Fourche time, which allowed a few opportunistic agglutinated foraminiferal species to colonize the substrate (Figure 11). Unstable environmental conditions are reflected by sudden, abrupt fluctuations in total numbers of one or two dinoflagellate (Leckie et al ., 1992) and foraminiferal species (Appendices 1 – 7). A similar pattern was observed in the Belle Fourche Member of the Ashville Formation in the Manitoba Escarpment (McNeil & Caldwell, 1981) and in Wyoming and Montana (Eicher, 1967). Numerical modeling of paleoceanic circulation in the WIS suggests that circulation was generally storm-dominated with currents affecting the sea floor down to 200 m (Ericksen & Slingerland, 1990). Brief increases in oxygen levels at the sediment / water interface can be achieved by the breakdown of watermass stratification during storms. Stormrelated oxygenation events have been invoked as the mechanism of colonization by macrofaunal communities in the Cenomanian Hartland Shale Member (Greenhorn Formation; Sageman, 1989).

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In northwestern Alberta progradation of the Dunvegan Delta is attributed to a middle Cenomanian lowstand (Bhattacharya & Walker, 1991a). A lowering of sea level may destabilize a stratified water column by reducing water depth, intensifying storm-related oxygenation or changing circulation patterns. A relatively rapid sea-level rise after the middle Cenomanian lowstand is indicated by nannofossil and planktic foraminiferal occurrences in the uppermost Belle Fourche at 11-12-6-16W4 and at 11-36-22-1W2. These occurrences signal the first presence of Tethyan-sourced waters in the Canadian part of the WIS. Second White Specks Formation. During the latest Cenomanian to early Turonian, sea level in the WIS reached a maximum (Kauffman, 1977, 1984; Haq et al ., 1988, Kauffman et al ., 1993, Caldwell et al ., 1993). The Second White Specks Formation was deposited at this time. Warm Tethyan waters of normal salinity entered the northern parts of the basin and fostered the northern migration of planktic foraminifera and coccoliths (Figure 13). Dinoflagellate assemblages also became more diverse around the Cenomanian / Turonian boundary (Figure 11). The thin, rippled, nannofossil-bearing sand laminations are inferred to be the result of bottom-flowing, unidirectional currents in the seaway. Sedimentary structures do not indicate significant wave activity or influence and this is evidence for deposition primarily below storm wave base. The abundance of pelagic organisms, coupled with widespread bottom water anoxia, resulted in high TOC values (up to 12 wt%) of dominantly Type II organic matter (Figure 8). A northwards decrease in surface water temperature is indicated by the progressively smaller number of planktic foraminiferal and coccolith species in the Second White Specks Formation in Canada compared to the Greenhorn Formation of the United States (Eicher, 1969; Eicher & Worstell, 1970; Eicher & Diner, 1985). No keeled planktic foraminifera, with the exception of one questionable specimen of Praeglobotruncana delrioensis Loeblich & Tappan in the most southern well at 10-25-1-27W3, are found in the WCSB. The most northern reported occurrence of late Cenomanian keeled foraminifera are from the Cone Member of the Marias River Formation in northern Montana and southernmost Alberta (Lang & McGugan, 1988). Keeled genera are the deepest-dwelling mid-Cretaceous planktic foraminifera and are regarded as being the most sensitive to environmental change (Leckie, 1987). Persistent bottom-water anoxia is indicated by a lack of benthic foraminifera at most localities (Figure 5) and the widespread occurrence of well laminated, non-bioturbated sediments. During the latest Cenomanian to early Turonian sea-level highstand, warm, normal-marine waters from Tethys entered the WCSB and extended to northern Canada and Alaska. Planktic foraminifera in the Turonian Seabee Formation of the Arctic north slope of Alaska (Tappan, 1962) indicate the northernmost extent of Tethyan waters. The total lack of calcareous and agglutinated benthic foraminifera in most cores indicates persistent bottomwater anoxia in the northern part of the sea, in contrast to the south (Arthur et al ., 1985; Eicher & Diner, 1985). A lower Turonian zone, rich in benthic species, was described from the Great Plains (Eicher & Worstell, 1970) and from the Black Mesa Basin in Arizona (Leckie et al ., 1991), but was not found in the WCSB. It was implied by these authors that increased benthic species diversity was the result of more vigorous circulation between the WIS and the Tethys during peak transgression. However, by the time these waters reached the

Figure 13. Relative abundance of planktic foraminifera and coccoliths in the Second White Specks Formation in a SW/ E profile. At 11-12-6-16W4 and 11-36-22-1W2 coccoliths have migrated into the region before planktic foraminifera. In central Saskatchewan at 11-16-35-8W3 the SWSF is only represented in a thin interval due to unconformities at the top and at the base. It is directly overlain by the First Speckled Shale and underlain by the Belle Fourche Formation. The Whiteinella aprica Subzone is present whereas the Clavihedbergella simplex Subzone is missing.

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northern regions, they were oxygen depleted and did not support a diverse benthos. Hay et al . (1993) suggested that the mixing of southern and northern watermasses during the Cenomanian produced a dense, oxygen-poor intermediate body of water. The gradual northward incursion of these oxygen depleted waters into Canada may have resulted in widespread anoxic benthic waters. Outflow either to the north or to the south was balanced by the inflow of northern and southern surface waters which created major plankton kills during mixing. This introduced large amounts of organic matter into the water column creating an extensive oxygen minimum zone (Hay et al ., 1993). Characteristics of the Second White Specks Formation are consistent with this hypothesis. Faunal evidence for an oxygen minimum zone was also found in the Black Mesa Basin and linked to the incursion of warm waters from the south (Leckie et al ., 1991). In contrast to the lack of benthic foraminifera, bivalves, primarily of the genus Inoceramus Sowerby, are especially abundant throughout the Second White Specks Formation. Inoceramids are epifaunal suspension feeders which rested ‘afloat’ above the sediment – water interface without burrowing activity (Hattin, 1982). These organisms evidently survived environmental conditions, including anoxia, that were limiting factors for others (Kauffman, 1988; Sageman, 1989). The number of fragmented calcite prisms in the sand-sized fraction increases significantly at the top of the formation where planktic foraminifera and coccoliths disappear. This suggests that inoceramids continued to thrive during the initial stages of the Turonian regression. Above the Second White Specks Formation, a basin-wide sea-level drop is recorded by the change in organic-matter type (to Type III), the disappearance of planktic foraminifera and coccoliths, and the introduction of coarser-grained clastics, culminating in the deposition of Cardium sandstones and conglomerates (Figure 3). 5.2. Variations in paleoenvironment across the basin In conjunction with temporal basin-wide paleoenvironmental changes responsible for the different formations of the Colorado Group, there are also lateral variations in sedimentation patterns, sediment sources, and paleoceanographic conditions across the basin that resulted in intraformational facies. These facies variations become apparent by comparing reference wells. To the west, basin geometry and sedimentation patterns were controlled primarily by tectonics and sediment distribution (Armstrong, 1988; Leckie & Smith, 1992; Stockmal et al ., 1992). In the east, a subordinate amount of sediment was derived from the stable craton (Caritat et al ., 1994). The WIS is divisible into four structuralsedimentary zones (McNeil, 1984, fig. 3; Kauffman, 1977). Each zone is characterized by a variable but different sedimentation rate and sediment source. The western foredeep had maximum subsidence and sedimentation rates and deposition occurred in relatively shallow water. The west-median trough had high rates of subsidence and sedimentation in deep water. Subsidence and water depth decreased eastward towards a hinge zone, which was bounded by an eastern platform having low subsidence and sedimentation rates. Organic matter deposited in the western foredeep is characterized by low HI values (Figure 14) typical for Type III. Shallowing at the hinge zone, located at approximately 102-1048 longitude (Figure 1), appears to be well expressed as a decrease in HI values in the Westgate, Fish Scales and Belle Fourche formations. The hinge zone is represented by the 11-16-35-8W3 well (Figure 1) in central

Figure 14. Contour maps of averaged Hydrogen Indices (HI) for four formations in the study area. Map of the Second White Specks Formation includes data from Macauley (1984). Contours were generated using Surface III (Kansas Geological Survey, 1992) by averaging formation values for each well, gridding (using a moving average) and contouring the averaged grid nodes. Because of the small number of control points (well locations), the contours should be regarded as general in nature.

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Saskatchewan. Here, the Turonian section is marked by two unconformities so that only a thin interval of the Second White Specks Formation is preserved (Figures 7, 13). In the centre of the basin, doming, perhaps associated with the intrusion of kimberlite pipes in central Saskatchewan (Gent, 1992), may have caused stratigraphic highs (Figure 7) and subsequent erosion during sea-level lowstands. These unconformities extend through central and perhaps southern Saskatchewan (Caldwell et al ., 1978). Facies changes are also evident in the organic geochemistry of the Belle Fourche Formation. Higher sedimentation rates in the west resulted in lower TOC (,2 wt%) and HI values (,200 mg HC / g OC) compared to the more central regions characterized by greater abundances of organic carbon (up to 8 wt%) and higher HI values (up to 275 mg HC / g OC) (Figure 14). The dominant source of sediment was the Dunvegan Delta (Stott, 1984; Bhattacharya & Walker, 1991b) in the Central Foothills and Northwest Plains. These dominantly siltstone and sandstone deposits contain rich agglutinated foraminiferal assemblages (Stelck & Wall, 1954, 1955; Stelck et al ., 1958) which do not occur further south and east in Alberta and Saskatchewan. As discussed above, it is not clear if the described foraminiferal zones, based on these assemblages, are missing because of a disconformity or if they did not develop due to facies changes. During deposition of the Second White Specks Formation, different faunal distributions resulted, in part, from variable sedimentation rates. High sedimentation rates (Table 4) are inferred for an unusually thick interval of the Second White Specks Formation at 6-34-30-8W4 (Figure 13; see fig. 6 in Bloch et al ., 1993) located in the west-median trough. Planktic foraminifera did not flourish (Appendix 2), probably due to the high detrital input. Coccoliths, however, do occur in significant numbers (Appendix 8) and appear to have been less affected by the increased detrital input (Figure 13). A high sediment influx from the west also dilutes the organic matter content (Figure 8) and HI values in the Second White Specks Formation (Figure 14). Representative species of the Clavihedbergella simplex Subzone only occur in eastern Saskatchewan (North & Caldwell, 1975a; this study) and Manitoba (McNeil & Caldwell, 1981) where planktic foraminifera and coccoliths show a higher species diversity. In the Rocky Mountain Foothills, where low numbers of planktic foraminiferal species are found (Wall, 1967), a turbid water column and more rapid sedimentation most likely prevented certain species from invading the area. This is consistent with facies and strand line maps of the Western Interior of the United States (e.g., Elder, 1991). Lower surface water salinities in the west could also have limited planktic foraminiferal distribution. An alternative explanation for the missing Clavihedbergella simplex Subzone in Alberta and western Saskatchewan was proposed by Caldwell et al . (1993) who concluded that the invading Tethyan waters followed an eastern path and only later spilled westward over Saskatchewan. The earliest occurrence of pelagic organisms in all studied wells is in the upper Belle Fourche Formation at 11-36-22-1W2 located at the Saskatchewan / Manitoba border. This observation is consistent with the hypothesis of Caldwell et al . Generally, the first occurrence of planktic foraminifera corresponds with the first occurrence of coccoliths in the uppermost Cenomanian to middle Turonian interval. At 11-12-6-16W4 and 11-36-22-1W2, however, coccoliths appear slightly below the first occurrence of planktic foraminifera (Figure 13). The initial phase of northern nannofossil migration in the latest Cenomanian is occupied

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almost exclusively by coccolith taxa which are more resistant to calcium carbonate dissolution. More delicate coccolith species became established later, indicating an increase in surface water temperature during the early to middle Turonian. A horizon of bioclastic conglomerate has been used as a marker for the base of the Second White Specks Formation (Bloch et al ., 1993). However, at 11-12-616W4 and 6-34-30-8W4 (Figure 1), bioclastic conglomerate is overlain by an agglutinated Belle Fourche microfauna. In central Saskatchewan at 11-16-358W3, and in numerous outcrop sections along the Rocky Mountain Foothills, a bioclastic conglomerate occurs at the base of the Second White Specks Formation. In locations where the basal Second White Specks Formation conglomerate occurs, sediment has been eroded creating an unconformity (e.g., Figure 7). In contrast, at 6-18-45-1W4 and 11-36-22-1W2, where there is no conglomerate between the Belle Fourche and Second White Specks Formations, the microfauna shows a transitional zone with the occurrence of the calcareous benthic taxon N. albertensis (6-18-45-1W4) or the Clavihedbergella simplex Subzone (11-36-22-1W2) (Figure 5). In southern Saskatchewan at 10-25-127W3, a bioclastic layer occurs at the top of the Second White Specks Formation. At this locality, the bioclastic layer is probably linked to a later unconformity that places the First White Speckled Shale (Santonian) on Turonian Second White Specks Formation. Other occurrences of coarse bioclastic layers in marine shales of the Western Interior Sea are reported from the Central Great Plains and southern Rocky Mountains of the US. Extensive skeletal grainstones are described from the basal Lincoln Limestone of late Cenomanian age (Hattin, 1986). The origin of these late Cenomanian conglomerate layers associated with the organic-rich, deeper water marine shales is somewhat enigmatic. Missing faunal zones indicate a marine erosion surface. The sporadic distribution of a conglomerate at the base of the Second White Specks Formation most likely reflects varying water depths in the basin. Where the basin was shallower, such as in central Saskatchewan (Figure 7), a drop in base level resulted in winnowing and erosion of older sediments. The occurrence of Verneuilinoides perplexus (Loeblich) (Belle Fourche) above the bioclastic layers at 11-12-6-16W4 and 6-34-30-8W4 indicates this erosional event occurs at a similar stratigraphic position to that of the Dunvegan Delta of Bhattacharya & Walker (1991a). In the Pasquia Hills of east – central Saskatchewan, vertebrate faunal assemblages from a time-equivalent bioclastic conglomerate suggest reworking of sediment which was initially deposited in a nearshore to possibly estuarine depositional setting (Cumbaa et al ., 1992; Cumbaa, 1993). The disarticulated bioclastic layers are interpreted to be evidence of basin shallowing and subsequent winnowing on the sea floor. Alternatively, the conglomerates may be winnowed lags which result from storms that are able to reach the sea floor at a lower base level. Evidence for wave reworking are beds of aligned, tapered bioclasts which are opposed in succeeding laminae. However, wave activity would be expected to form coarse-grained ripples (c.f. Leckie, 1986) which we have not observed. 6. Summary The uppermost Albian to middle Turonian lower Colorado Group of the WCSB reflects a time of dominantly marine-shale sedimentation related to a global

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third-order sea-level rise. Microfossil assemblages, sedimentological records and geochemical signals, however, give indications of relative sea-level changes overprinting the general eustatic trend. Figure 15 illustrates how important characteristics of the Westgate, Fish Scales, Belle Fourche and Second White Specks Formations are related to changes in relative sea level. The figure summarizes the general regional picture of the WCSB. The upper Albian Westgate Formation was deposited in the Mowry Sea that was still relatively shallow and dominated by well-oxygenated, cold, low-salinity Boreal waters. Agglutinated foraminiferal biofacies and bioturbation patterns record two to three coarsening-upwards cycles that indicate relatively short-lived regional sea-level drops or an increase in sediment influx. The contact between the Westgate and lower Cenomanian Fish Scales Formations is a marine erosional surface expressed as a bioclastic conglomerate layer. Erosion was either the result of a sea-level drop and wave reworking or intense winnowing by unidirectional, bottom-water currents. These currents may have been generated by the opening of the WIS. Warm Tethyan waters had not reached the region by early Cenomanian time, as indicated by a lack of planktic foraminifera and nannofossils. Low-salinity surface waters still dominated the northern seaway. An increased influx of meteoric waters may have formed a brackish water lid inhibiting vertical circulation and causing bottom anoxia. In the middle Cenomanian Belle Fourche Formation characteristic features change gradually. A sea-level lowstand caused the progradation of the Dunvegan Delta and provided increased detrital input to the western portion of the basin. Dominantly Boreal waters persisted in the Canadian portion of the WIS. A restricted agglutinated foraminiferal fauna resulted from the combined effects of oxygen depletion and low salinity. Towards the end of the Cenomanian, the basin was again subjected to marine erosion. Bioclastic conglomerate layers at several stratigraphic positions in the middle to upper Cenomanian interval resulted from a sea-level drop. The uppermost Cenomanian to middle Turonian Second White Specks Formation marks the sea-level highstand in the WIS. Water in the basin was at maximum depth and warm, normal-salinity Tethyan waters reached the WCSB bringing abundant planktic foraminifera and coccoliths. Bottom-water conditions were persistently anoxic due to high bioproductivity and a lack of vertical circulation.

Acknowledgments S. Carr, S. Cumbaa, D. McNeil, R. Scammell, and J. Wall provided interesting and informative discussions during the course of this work. Technical support was provided by D. Then, J. Wong, B. Davies, A. Heinrich, R. Stewart and R. Fanjoy of the ISPG and C. Langill of Carleton University. The Natural Science and Engineering Research Council of Canada supported the research of Schro¨ der-Adams. The Panel of Energy and Resource Development (PERD), Project 6.1.1.14, funded much of this work. M. Leckie, B. Sageman and L. Leithold are thanked for their perceptive and constructive reviews of the manuscript.

Figure 15. Characteristic features of the formations in the lower Colorado Group related to sea-level changes.

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