sedimentary and petrographic studies of the mannville

Saskatchewan Geological Survey 1 Summary of Investigations 2005, Volume 1

Sedimentary and Petrographic Studies of the Mannville Group in the Saskatoon Area, Central Saskatchewan: Preliminary Results

and Implications for Sediment Sources

A.Q. Wang 1, G. Chi 1 and P.K. Pedersen 2

Wang, A.Q., Chi, G., and Pedersen, P.K. (2005): Sedimentary and petrographic studies of the Mannville Group in the Saskatoon area, central Saskatchewan: Preliminary results and implications for sediment sources; in Summary of Investigations 2005, Volume 1, Saskatchewan Geological Survey, Sask. Industry Resources, Misc. Rep. 2005-4.1, CD-ROM, Paper A-15, 12p.

Abstract Mannville Group siliciclastic rocks in the Saskatoon area of central Saskatchewan were examined for vertical and lateral changes in sedimentary facies and petrological features using two continuous cores (3-28-33-23W2 and 6-18-36-6W3) in the Saskatoon area. The Mannville Group is divided into a lower sandstone-dominated Cantuar Formation and an upper siltstone and shale-dominated Pense Formation. The Cantuar Formation is characterized by fining-upward successions indicating aggradation of distributary-channel fills and flood-plain deposits in delta-plain environments. The Pense Formation is mainly composed of marine deposits probably representing a prodelta environment, with an interval of sandstone showing a coarsening-upward trend likely indicating a delta-front progradation. In contrast to the lithology and facies characteristics of the Mannville Group in western Saskatchewan that are dominated by Cordilleran sources of detrital material and an eastward change toward marine environments, Mannville strata in central Saskatchewan are influenced more by sediment supply from the Precambrian craton to the northeast and exhibit a westward change from nonmarine to marine-influenced depositional environments. In the Cantuar Formation, the presence of large amounts of lithic grains that include volcanic and metamorphic rock fragments is also consistent with sediment sources derived from Precambrian metamorphic terranes.

Keywords: Cretaceous, Mannville Group, siliciclastic rocks, Williston Basin, sedimentary sources, sedimentary facies, central Saskatchewan.

1. Introduction Thick successions of Mesozoic (mainly Cretaceous) siliciclastic rocks occur in the Western Canada Sedimentary Basin and host large amounts of oil and gas. In Saskatchewan, most oil and gas pools hosted in Cretaceous rocks occur in the western part of the province and, consequently, most previous studies of Cretaceous siliciclastic rocks have focused on this region. Few studies have been carried out on Cretaceous rocks in central Saskatchewan, so relatively little is known about the sedimentary facies and lithologies there. The area of this study is located in central Saskatchewan, near Saskatoon (Figure 1).The primary objective of the research is to document vertical and lateral changes in sedimentary facies, lithologies, and diagenetic features in Mesozoic strata by examining continuous cores obtained from potash wells. This paper presents preliminary results of core logging and petrographic studies of the Mannville Group and discusses their relevance to sediment provenance from either Cordilleran or Canadian Shield sources.

It is generally agreed that the bulk of sedimentary fill in the Mesozoic foreland basin of the Western Canada Sedimentary Basin was derived from the Cordillera to the west, and that minor volumes of sediment may have come from the Canadian Shield to the east (Potocki and Hutcheon, 1992). In southwestern Saskatchewan, the Dimmock Creek Member of the Cantuar Formation changes from fluvial to deltaic facies of flooded valleys northward into marine sandstones of the Lloydminster Member, suggesting that the clastic detritus was from the Cordillera (Christopher, 1984). Christopher (1984) further suggested that fluvial quartzose sands of the Mannville Group in the Molanosa area in north-central Saskatchewan and in southeastern Saskatchewan were derived from the Precambrian Shield. Simpson (1975) suggested that the eastern shoreline of the Cretaceous Interior Seaway was located near the northern and eastern edge of the Williston Basin where Lower Cretaceous rocks currently crop out. This study provides sedimentary and petrographic evidence that, during deposition of Mannville Group sediments in Early

1 Department of Geology, University of Regina, 3737 Wascana Parkway, Regina, SK S4S 0A2. 2 Currently with Apache Canada Ltd, #1000, 700 - 9th Avenue SW, Calgary, AB T2P 3V4; E-mail: [email protected].


Figure 1 - Map of south-central Saskatchewan showing locations of study wells in the vicinity of Saskatoon. The distribution of Saskatchewan oil and gas pools is also shown.

Cretaceous time, the present-day Saskatoon area was located on the eastern slope of the Western Canada Sedimentary Basin.

2. Stratigraphic Divisions The two wells examined, 3-28-33-23W2 and 6-18-36-6W3, have continuous core coverage of Cretaceous strata from, in ascending order, the Lower Cretaceous Mannville Group, the Lower to Upper Cretaceous Colorado Group, and the Upper Cretaceous Montana Group. With the exception of the Mannville Group, the Viking Formation, and some sandy intervals in the Belly River Formation in the Montana Group, the majority of these strata are composed of shales. The Mannville Group represents the thickest sandstone interval within the Cretaceous succession and is the focus of this paper.

The Mannville Group includes the Cantuar and Pense formations (Christopher, 2003). Depositional environments of Cantuar Formation rocks were mainly continental and coastal, whereas those of Pense Formation strata were marginal marine (Christopher, 1984).

3. Lithofacies of the Mannville Group Lithofacies characteristics of the Mannville Group as described below are based on core logging of wells 6-18-36-6W3 and 3-28-33-23W2. The Cantuar and Pense formations are described separately.

a) The Cantuar Formation

Well 6-18-36-6W3

In well 6-18-36-6W3, the Cantuar Formation is divided into three lithofacies associations (Figure 2).

Lower Lithofacies Association

569.37 to 576.99 m: Very fine-grained sandstone, argillaceous, friable, cross-bedded, with 0.46 m (1.5 ft) thick coal


Figure 2 - Lithofacies of the Mannville Group in well 6-18-36-6W3. The gamma-ray log signature of a nearby well (1-16-36-6W3) is shown for reference (depths in feet).


(1868 to 1893 ft) seam in the upper part (Figure 2); sharp contact with carbonates of the underlying Birdbear Formation.

565.10 to 569.37 m : Characterized by very fine-grained sandstone with burrows, more argillaceous upwards; planar (1854 to 1868 ft) and trough cross-bedding common in lower part, lenticular bedding and planar lamination

dominant in the upper part; syneresis cracks present in the upper part (Figure 3A).

552.30 to 565.10 m: Five beds of white, very fine-grained sandstones interbedded with thin dark mudstones and (1812 to 1854 ft) coal seams, upward increase in bioturbation (Figure 3B) except in the basal sandstone; ripple

cross-bedding, ripple cross-lamination, and lenticular bedding common (Figure 3C).

Petrography: Quartz, feldspar, mica, and chert grains dominate the detrital minerals in the sandstones. Some glauconite crystals and abundant heavy minerals also occur. The quartz grains show straight contacts (Figure 3D) due to compaction, and compaction dissolution effects and cementation by quartz overgrowths were seen locally. The matrix consists of clay.

Figure 3 - Photographs showing various sedimentary structures and petrographic features of the lower lithofacies association of the Cantuar Formation in well 6-18-36-6W3. (A) Syneresis cracks (566.99 m [1860.2 ft]); (B) Intensely bioturbated muddy sandstone (553.52 m [1816 ft]); (C) Ripple cross-lamination (558.58 m [1832.6 ft]); (D) Straight grain-to-grain contact due to compaction (564.86 m [1853.2 ft]).


Figure 4 - Photographs showing various sedimentary structures and petrographic features of the middle lithofacies association of the Cantuar Formation in well 6-18-36-6W3. (A) Low-angle planar lamination (549.25 m [1802 ft]); (B) Sharp basal contact of the middle lithofacies association (552.30 m [1812 ft]); (C) Plant fragments on bedding plane (536.45 m [1760 ft]); (D) Volcanic fragments (V) (545.90 m [1791 ft]); (E) Metamorphic fragments (M) (545.90 m [1791 ft]); (F) Dissolution features (Dis) (545.90 m [1791 ft]).


Middle Lithofacies Association

545.59 to 552.30 m: White, medium-grained massive quartzose sandstone with grains of dark chert, poorly sorted, (1790 to 1812 ft) hard, fining-upward, planar bedding (Figure 4A) and ripple lamination in upper portion; basal

contact sharp and angular (Figure 4B).

535.84 to 545.59 m: Light grey and dark grey shale interbedded with argillaceous siltstone, abundant burrows, and (1758 to 1790 ft) plant fragments (Figure 4C); planar lamination and ripple cross-bedding, and syndepositional

deformation structures.

529.44 to 535.84 m: Very fine-grained sandstone, uncemented, massive, grading upward to light grey mudstone (1737 to 1758 ft) interbedded with thin argillaceous siltstone.

Petrography: Quartz, feldspar, mica, and lithic grains are the main detritus in the sandstones. Quartz grains contain abundant microfractures. The lower part (545.59 to 552.30 m [1790 to 1812 ft]) displays abundant mud matrix and some volcanic and metamorphic fragments (Figures 4D and 4E). Quartz dissolution resulting from compaction is common (Figure 4F).

Upper Lithofacies Association

523.65 to 529.44 m: Calcareous, interbedded siltstone and mudstone fining upward to light grey mudstones; pyrite (1718 to 1737 ft) concretions, syndepositional deformation structures, lenticular bedding and ripple lamination

common in the lower sandy portion.

515.11 to 523.65 m: Several thin, argillaceous siltstone beds in the base grading to light grey silty mudstone; almost (1690 to 1718 ft) all of the beds in the interval are intensely biodisturbed; cross-bedding common in the lower silty

part, and planar lamination and ripple cross-stratification dominant in the upper part.

509.63 to 515.11 m: Two fining-upward successions (each 0.61 m [2 ft] thick) consisting of very fine-grained (1672 to 1690 ft) sandstone grading to siltstone or silty claystone, followed by an upward-coarsening succession

(0.49 m [1.6 ft] thick) from claystone to siltstone and then by an interval of dark grey mudstone; planar lamination, bioturbation, carbonaceous fragments; strongly distorted bedding (Figure 5A) in the middle interval.

498.04 to 509.63 m: Calcareous, very fine-grained sandstones, thick-bedded, with oxidized iron strips and ripple (1634 to 1672 ft) cross-bedding.

490.73 to 498.04 m: Fining-upward succession with the basal sandstone containing lots of sphaerosiderite (Figure (1610 to 1634 ft) 5B) overlain by weakly indurated, argillaceous, very fine-grained sandstone, grading into grey

mudstone, accompanied by an upward increase in bioturbation.

Petrography: The framework grains are mainly quartz, feldspar, mica, and chert. There is little matrix, but carbonate cements are common (Figure 5C). The basal sandstone bed contains abundant rock fragments including chert and some volcanic fragments.

Well 3-28-33-23W2

In the 3-28-33-23W2 well, the lower lithofacies association presented in the 6-18-36-6W3 well is missing, and the other two equivalent lithofacies associations exhibit distinct characteristics (Figure 6), which are described below.

Middle Lithofacies Association

472.44 to 502.31 m: Pale, fine-grained sandstone, massive, friable and well sorted, interbedded with several thin (1550 to 1648 ft) beds of oxidized mudstone (Figure 6); sedimentary structures dominated by low-angle to high-

angle planar cross-bedding; ripple cross-lamination (Figure 7A) in the upper part of the sandstone interval. The basal sandstones are composed of three upward-fining sandstone successions, each about 0.46 m (1.5 ft) thick, grading from medium-grained sandstone to very fine-grained sandstone; sharp contact with underlying coal seam.

Petrography: The common sand-sized detrital minerals are quartz, chert, and minor feldspar. The quartzose grains show straight contacts and compaction dissolution effects, and quartz overgrowths are the


Figure 5 - Photographs of various sedimentary structures and petrographic features observed in the upper lithofacies of the Cantuar Formation at 6-18-36-6W3. (A) Deformation structure (512.67 m [1682 ft]); (B) Sandstone with abundant sphaerosiderite (493.93 m [1620.5 ft]); (C) Carbonate-cemented sandstone (505.97 m [1660 ft]).


Figure 6 - Lithofacies of the Mannville Group in well 3-28-33-23W2.

main cement. The sandstones are very clean, containing almost no matrix except sporadic sericite crystals. The basal sandstone contains some volcanic (Figure 7B) and metamorphic fragments.


Figure 7 - Photographs showing various sedimentary structures and petrographic features of the Cantuar Formation in well 3-28-33-23W2. (A) Ripple cross- lamination (474.34 m [1559.5 ft]); (B) Volcanic fragments (V) (502.01 m [1647 ft]); (C) Faulted sphaerosideritic sandstone (458.42 m [1504 ft]).

Upper Lithofacies Association

441.96 to 472.44 m: Composed of five fining-upward cycles, each consisting of sandstone in the lower part and 1450 to 1550 ft mudstone in the upper part. Bioturbation is rare (only faint signs of bioturbation in the first

sedimentary cycle). Plant fragments, iron stains, and sphaerosiderite, as well as small-scale faults were observed (Figure 7C).

Petrography: The detrital grains consist of quartz, chert, and some feldspar. Compared with the middle lithofacies association, the upper lithofacies association contains much more clay matrix, and the porosity is lower. Glauconite and carbonate cements are developed locally.


b) The Pense Formation The Pense Formation is less than 15 m (50 ft) thick in the studied cores. It is characterized by fissile dark shale with interbedded calcareous sandstone (6-18-36-6W3) or siltstone and mudstone (3-28-33-23W2). The sediments are intensely bioturbated in both wells. The basal contact is marked by oxidized mudstone in well 6-18-36-6W3, and is overlain by a lag of siderite concretion clasts in well 3-28-33-23W2.

4. Interpretations and Discussion Sediments of the Lower Cretaceous Mannville Group were deposited over wide regions of the Western Canada Sedimentary Basin following a long period of exposure and erosion of upper Jurassic and older strata. In Saskatchewan, the Mannville Group thickens from the southwest, south, east, and northeast toward northwest (Christopher, 2003), suggesting drainage systems from both the southwest and the northeast. The depositional environments of the Mannville Group in the study area and their significance with regard to sediment sources are discussed below.

a) Facies Analysis The Mannville Group in the study area is interpreted as a deltaic depositional system. Sedimentary facies in the western core show more evidence of marine influence than in the eastern core. The lower lithofacies association in well 6-18-36-6W3 is interpreted as recording the transition zone from subaerial delta plain to subaqueous delta plain. Distributary-channel sandstones, barren of trace fossils and containing coal seams, indicate subaerial environments, whereas distributary-channel sandstones having dark shales and abundant burrows represent subaqueous environments. This lower lithofacies association is not present in the eastern well 3-28-33-23W2 (Figure 8).

The middle lithofacies association in well 3-28-33-23W2 shows typical fluvial-channel deposits. The three upward-fining successions are interpreted as point-bar deposits separated by interfluve mudstones, where the red colour indicates subaerial exposure. The succession shows an upward decrease in scale of sedimentary structures, from cross-bedding to ripple-bedding, and a decrease in thickness of individual sandstone beds. In well 6-18-36-6W3, the middle lithofacies association is interpreted as representing delta-plain deposition. The basal sandstone bed is interpreted as a distributary-channel deposit that is overlain by flood-plain deposits. The flood-plain deposits are characterized by mudstone and silty mudstone, with intercalated current ripples, and thin sandstone beds interpreted as crevasse-splay deposits (Figure 8).

The upper lithofacies association in both wells exhibits characteristics of delta-plain deposits with each fining-upward succession interpreted as distributary-channel deposits overlain by fine-grained inter-distributary deposits (Figure 8). The western well shows more evidence than the eastern well of deposition within a marine environment. The intense bioturbation indicated by marine trace fossils such as Teichichnus in well 6-18-36-6W3 is evidence of a more prospering marine fauna than present in well 3-28-33-23W2. The coarsening-upward succession is interpreted as distributary mouth-bar deposits, indicating that the western area was located on the updip edge of the delta front.

Sea level was high during deposition of the Pense Formation and delta-front and prodelta marine conditions prevailed in the study area.

b) Petrographic Evidence Sandstones in foredeep settings typically contain moderate to high proportions of recycled sedimentary debris, but little metamorphic and volcanic material and feldspar (Dickinson and Suczek, 1979). Within the studied sandstones of the Mannville Group, volcanic and metamorphic rock fragments are present, especially within the basal sandstones. The presence of these lithic grains is consistent with a source from the Precambrian craton located to the northeast of the study area, although lithic grains are common in Mannville rocks elsewhere.

The westward facies change from more continental to more marine conditions, as discussed above, is consistent with the proposal that the detritus forming the Mannville succession in central Saskatchewan was sourced primarily from the east. However, it is worth noting that deposition of the Mannville Group was topographically controlled (Christopher, 2003); thus whether a well has a marine or more fluviatile facies is largely a result of its position relative to Mannville paleotopography. Considering the position of central Saskatchewan in the Western Canada Sedimentary Basin, it is possible that the study area was located on the west-dipping eastern margin of the basin, which later became the interior seaway in post-Mannville time (Williams and Stelck, 1975).


Figure 8 - Facies correlation between the two wells studied. The datum used is the first flooding surface within the Joli Fou Formation. More marine influenced deposits are present in the western core.


5. Summary The Mannville Group in the Saskatoon area of central Saskatchewan is divided into a lower sandstone-dominant Cantuar Formation and an upper siltstone- and shale-dominant Pense Formation. Facies analysis indicates a deltaic system controlled deposition, with the Cantuar Formation having mainly formed in delta-plain environments and the Pense Formation mainly in marginal marine settings. Facies change towards more marine influence from east to west, coupled with the occurrence of volcanic and metamorphic rock fragments in the sandstones, indicate that the sediments of the Mannville Group in the study area were primarily derived from the Canadian Shield to the east.

6. Acknowledgments We acknowledge the support of Profico Energy Management Ltd. and NSERC for this study. QW is particularly indebted to Drs. J.E. Christopher and D.M. Kent for help in core logging and for stimulating discussion. Gratitude is

also expressed to the staff of the Subsurface Geological Laboratory for logistical support for core logging. Critical reviews by Drs. J.E. Christopher and O. Salad Hersi have improved the paper.

7. References Christopher, J.E. (1984): The Lower Cretaceous Mannville Group, northern Williston Basin region, Canada; in

Stott, D.F. and Glass, D.J. (eds.), The Mesozoic of Middle North America, Can. Soc. Petrol. Geol., Mem. 9, p109-126.

__________ (2003): The Jura-Cretaceous Success Formation and the Lower Cretaceous Mannville Group of Saskatchewan; Sask. Industry Resources, Rep. 223, CD-ROM, p4-15.

Dickinson, W.R. and Suczek, C.A. (1979): Plate tectonics and sandstone compositions: AAPG Bull., v63, p2164-2182.

Potocki, D. and Hutcheon, I. (1992): Lithology and diagenesis of sandstone in the Western Canada Foreland Basin; in Macqueen, R.W. and Leckie, D.A. (eds.), Foreland Basins and Fold Belts, AAPG Mem. 55, p229-257.

Simpson, F. (1975): Marine lithofacies and biofacies of the Colorado Group (Middle Albian to Santonian) in Saskatchewan; in Caldwell, W.G.E. (ed.), The Cretaceous System in the Western Interior of North America, Geol. Assoc. Can., Spec. Pap. 13, p553-585.

Williams, G.D. and Stelck, C.R. (1975): Speculation on the Cretaceous Palaeogeography of North America; in Caldwell, W.G.E. (ed.), The Cretaceous System in the Western Interior of North America, Geol. Assoc. Can., Spec. Pap. 13, p1-19.

sedimentary and petrographic studies of the mannville

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