paleoenvironmental variability of the lower paleozoic ...€¦ · deadwood formations in subsurface...

Paleoenvironmental Variability of the Lower Paleozoic Earlie and Deadwood Formations in Subsurface Saskatchewan:

A Preliminary Assessment

Luis Buatois 1 and M. Gabriela Mángano 1

Buatois, L. and Mángano, M.G. (2013): Paleoenvironmental variability of the lower Paleozoic Earlie and Deadwood formations in subsurface Saskatchewan: a preliminary assessment; in Summary of Investigations 2013, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2013-4.1, Paper A-3, 8p.

Abstract The middle Cambrian to Lower Ordovician Earlie and Deadwood formations are analyzed based on the study of six cores in subsurface Saskatchewan. These units display remarkable paleoenvironmental variability not only with respect to water depth, but also regarding the different roles of waves and tides. The Earlie Formation mostly records deposition in tide-influenced settings, reflecting the formation of tidal sandbodies. The Deadwood Formation records for the most part deposition in wave-dominated shallow-marine settings, ranging from the upper shoreface to the offshore. An interval of the Deadwood Formation with abundant soft-sediment deformation structures most likely records sedimentation under a deltaic influence. The switch from a tide-dominated regime to a wave-dominated regime may reflect the establishment of more protected environments prone to tidal amplification during deposition of the Earlie Formation, whereas the Deadwood Formation records more open environments exposed to wave action.

Keywords: Deadwood Formation, Earlie Formation, Paleozoic, Cambrian, Ordovician, Williston Basin, core analysis, sedimentary facies, depositional environments, Saskatchewan.

1. Introduction The middle Cambrian–Lower Ordovician succession in subsurface Saskatchewan records the earliest Phanerozoic transgression in the Williston Basin (Figure 1). This succession is represented by three units, the Basal Sandstone Unit, the Earlie Formation, and the Deadwood Formation, all forming a succession up to 500 m thick (Dixon, 2007, 2008). These units have become the focus of much recent interest, particularly in connection with their potential as a sink for carbon dioxide (CO2) sequestration (Fischer et al., 2005) and the presence of spectacularly preserved small carbonaceous fossils (SCFs) (Harvey et al., 2012a, 2012b; Butterfield and Harvey, 2012). Understanding depositional conditions during sedimentation and the ranges of sedimentary facies and environments present in these units is essential to properly evaluate sandstone geometry and connectivity, which in turn are key elements for selecting CO2 storage sites in order to minimize uncertainties in CO2 sequestration. In addition, because these deposits represent, at least in part, the proximal equivalents of those containing the world-famous Burgess Shale fauna in the Canadian Rockies, detailed facies analysis may shed light on the paleoenvironmental setting of the SCFs, which reveal a cryptic Cambrian radiation in shallow water, unrecorded by the more distal Burgess Shale fauna (Butterfield and Harvey, 2012; Harvey et al., 2012b). The aim of our study is to provide a preliminary assessment of the paleoenvironmental variability of these lower Paleozoic deposits based on the analysis of six cores in subsurface Saskatchewan (Figure 2, Table 1).

2. Stratigraphic Setting The Basal Sandstone Unit rests directly on the Precambrian basement, and consists of pebbly to fine-grained sandstone intercalated with minor shale (Dixon, 2008). This unit represents nearshore deposits, and has been considered as strongly diachronic because it occurs not only at the base of the middle Cambrian–Lower Ordovician succession, but also as proximal equivalents of the Earlie and Deadwood formations (Dixon, 2007, 2008). In this study, the Basal Sandstone Unit has not been analyzed, but the possibility that these deposits are simply a facies variation within the Earlie and Deadwood formations, thereby precluding the need to be kept as a separate lithostratigraphic unit, cannot be disregarded and remains open for further study.

1 Department of Geological Sciences, University of Saskatchewan, 114 Science Drive, Saskatoon, SK S7N 5E2.

Saskatchewan Geological Survey 1 Summary of Investigations 2013, Volume 1

Figure 1 – Cambrian–Ordovician stratigraphy in subsurface Saskatchewan (after Dixon, 2008). Fm = Formation, L = Lower, M = Middle, U = Upper.

Figure 2 – Location map of the six cores studied (based on Harvey et al., 2012a). RW = Ceepee Reward, CS = Canadian Seaboard Divide No. 2, RL = Ceepee Riley Lake, UR = University of Regina, FH = CPEC et al Hartaven, and GE = Husky Glen Ewen. See details in Table 1.


Table 1 – Details of well localities and cored intervals. Locations of wells are shown on Figure 2.

Well Name Label on Figure 2 Locality Well Licence Core Interval

Ceepee Reward RW 101/04-28-038-24W3 58EO63 6,192 to 6,227 ft 6,018 to 6,043 ft 5,808 to 5,833 ft 5,565 to 5,590 ft 5,384 to 5,409 ft

Canadian Seaboard Divide No. 2

CS 101/07-14-056-17W3 56AO41 3,787 to 3,806 ft

Ceepee Riley Lake RL 101/03-04-39-13W3 58H014 5,724 to 5,754 ft 5,365 to 5,390 ft 5,065 to 5,095 ft

University of Regina UR 131/03-08-017-19W2 78LO10 2067 to 2085 m

CPEC et al Hartaven FH 142/12-01-010-09W2 98E189 2443 to 2450 m

Husky Glen Ewen GE 111/16-23-002-01W2 97I438 2775 to 2793 m

The Earlie Formation consists predominantly of interbedded fine- to very fine-grained sandstone and shale (Dixon, 2007, 2008). Although some authors (e.g., Greggs and Hein, 2000) regarded this unit as indistinguishable from the Deadwood Formation, detailed mapping by Dixon (2008) argued in favour of its usefulness, although he does indicate that on geophysical logs it is often difficult to differentiate between the Earlie and Deadwood formations in southwestern Saskatchewan and towards the eastern and northern margins of the Williston Basin. The Deadwood Formation consists of interbedded shale, siltstone, sandstone, flat-pebble conglomerate, and limestone (Slind et al., 1994; Greggs and Hein, 2000). The formation is unconformably overlain by the Middle Ordovician Winnipeg Formation in eastern Saskatchewan (Greggs and Hein, 2000; Dixon, 2007, 2008). According to Dixon, the whole middle Cambrian–Lower Ordovician cycle can be subdivided into two main transgressive-regressive cycles, one comprising the Basal Sandstone Unit and most of the Earlie Formation, and the second mostly comprising the Deadwood Formation.

Although conodont and trilobite data provide a middle Cambrian (Series 3) to Lower Ordovician age for this cycle in Alberta, precise dating of these units in Saskatchewan is hindered by the lack of direct paleontologic evidence. SCFs provide new insights, albeit preliminary, on the age of these deposits (Harvey et al., 2012a). According to their dataset, the Earlie Formation is regarded as middle Cambrian in age, whereas the Deadwood Formation contains two assemblages. The “lower Deadwood assemblage” is most likely late middle Cambrian or, perhaps, ranges into the Furongian, and the “upper Deadwood assemblage” is Furongian (Harvey et al., 2012a).

3. Sedimentary Facies Variability Both physical (lithology, bed boundaries, and mechanical sedimentary structures) and biogenic attributes were considered in the facies analysis herein. Degree of bioturbation is assessed following Taylor and Goldring (1993), based on a previous scale by Reineck (1963). A sedimentary fabric characterized by no bioturbation (0%) corresponds to a bioturbation index (BI) of 0. Deposits that show sparse bioturbation with few discrete traces correspond to a BI of 1 (1 to 4%). Low bioturbation in sediment that still has preserved sedimentary structures corresponds to a BI of 2 (5 to 30%). A BI of 3 (31 to 60%) refers to sediment with discrete trace fossils, moderate bioturbation, and still distinguishable bedding boundaries. BI 4 (61 to 90%) is typified by intense bioturbation, high trace-fossil density and common overlap of trace fossils, and primary sedimentary structures are mostly erased. Sediment with completely disturbed bedding and showing intense bioturbation corresponds to a BI of 5 (91 to 99%). Completely bioturbated and reworked sediment, related to repeat overprinting of trace fossils, corresponds to a BI of 6 (100%). Our analysis suggests that the Earlie and Deadwood formations display remarkable paleoenvironmental variability not only with respect to water depth, but also regarding the different roles of waves and tides.


a) Earlie Formation The Earlie Formation consists of: parallel-stratified to planar crossbedded, very coarse- to medium-grained sandstone with rare mudstone partings; planar crossbedded to current-ripple, cross-laminated, medium- to fine-grained sandstone with abundant mudstone layers, forming heterolithic intervals characterized by wavy, flaser and lenticular bedding; and mudstone intervals with thin, very fine-grained sandstone intercalations, displaying lenticular and wavy bedding (Figures 3A to 3C). Sharp-based, parallel-laminated, very fine-grained sandstone occurs locally interbedded with the fine-grained heterolithic intervals (Figure 3D). Mudstone drapes on cross-laminated and crossbedded sets are extremely abundant, as are syneresis cracks (Figure 4). Fluid-escape structures and convolute lamination are present locally.

Figure 3 – Sedimentary facies of the Earlie Formation, Ceepee Reward 101/04-28-038-24W3, licence #58EO63. A) General view of deposits; note fining-upward trends with sand-dominated interval representing a tidal sandbody passing upwards into heterolithic facies (yellow line defines the boundary); base is on the lower left and top on the upper right; depth 6,227 to 6,192 ft (1897.0 to 1887.3 m). B) Close-up of sandstone-dominated interval; note mudstone partings near the base; depth 6,227 ft (1897.0 m). C) Close-up of sandstone-dominated interval showing abundant syneresis cracks (arrows) and mudstone partings; depth 6,220 ft (1895.9 m). D) Close-up of heterolithic interval showing syneresis cracks (yellow arrow) and moderate bioturbation intensities in the fine-grained deposits, and intercalations of sharp-based, unbioturbated parallel-laminated, very fine-grained sandstone interpreted as distal tempestites (white arrow); depth 6,222 ft (1896.5 m).


Bioturbation is highly variable. In general, sandstone-dominated units tend to display lower degrees of bioturbation (BI 0 to 2), containing Planolites in mudstone drapes and Palaeophycus in the sandstone beds. Only locally does bioturbation intensity tend to be higher (BI 3 to 4) in these sandstone-dominated deposits, and the ichnofauna contains not only Planolites and Palaeophycus, but Teichichnus and Chondrites(?) as well. With increased mudstone interbeds, the degree of bioturbation displays an overall increase (BI 0 to 4), and the ichnofauna contains Planolites, Palaeophycus, Teichichnus, and Diplocraterion. The interbedded sharp-based, parallel-laminated sandstone is unburrowed.

Tidal influence is indicated by the recurrence of mud-draped foresets in the crossbedded, sandstone-dominated packages. In addition, alternation of thin, current-generated sandstones and mudstone layers, together with mudstone drapes on ripple foresets, suggest tidal influence during deposition. Evidence of storm deposition is sparse, essentially restricted to the sharp-based parallel-laminated sandstones, which are interpreted as distal tempestites. The Earlie Formation records deposition in tide-influenced settings, as evidenced by the formation of tidal sandbodies.

b) Deadwood Formation The Deadwood Formation consists of a wide variety of deposits. Trough and planar crossbedded very coarse- to medium-grained sandstone, hummocky cross-stratified fine- to very fine-grained sandstone, interbedded microhummocky cross-stratified and wave-rippled, cross-laminated, very fine- to fine-grained sandstone and siltstone, and parallel-laminated shale are the dominant facies (Figures 5, 6, and 7). Locally, flat-pebble conglomerate (Figure 6D) and cobble and pebble conglomerate are present. Current-ripple cross-lamination is relatively uncommon. Small-scale soft-sediment deformation structures, such as convolute lamination and load casts, are present locally. For example, in the core from the Ceepee Reward well, soft-sediment deformation structures are extremely abundant in the interval from 5,590 to 5,565 ft (1703.8 to 1696.2 m), and include large-scale features such as slump structures (Figure 8). Syneresis cracks are present, but typically not abundant. Trace fossils are unevenly distributed in these deposits.

Figure 5 – Sedimentary facies of the Deadwood Formation, Husky Glen Ewen 111/16-23-002-01W2, licence #97I438. A) General view of shoreface deposits; base is on the lower left and top on the upper right; depth 2791 to 2782 m. B) Close-up of upper-shoreface through cross-stratified sandstone; depth 2791.9 m. C) Trichophycus (arrow) in cross-section view; depth 2779.9 m. D) Trichophycus (arrow) in bedding-plane view of sandstone bed with mudstone parting; depth 2779.9 m.

Figure 4 – Earlie Formation, Ceepee Riley Lake 101/03-04-39-13W3, licence #58H014; syneresis cracks on bedding-plane view of heterolithic facies; depth 5,744 ft (1750.8 m).


Figure 6 – Sedimentary facies of the Deadwood Formation, Ceepee Reward 101/04-28-038-24W3, licence #58EO63. A) General view of offshore-transition deposits; base is on the lower left and top on the upper right; depth 5,833 to 5,808 ft (1777.7 to 1770.3 m). B) Close-up of hummocky cross-stratified sandstone; depth 5,806 ft (1769.7 m). C) General view of fair-weather mudstone interbedded with thin tempestite sandstone; depth 5,815 ft (1772.4 m). D) Flat-pebble conglomerate; depth 5,390 ft (1642.9 m).

Figure 7 – Deadwood Formation, CPEC et al Hartaven 142/12-01-010-09W2, licence #98E189; close-up of intensely bioturbated sandstone with Teichichnus (Te) and Asterosoma (As); depth 2450 m.

No bioturbation is recorded in the interval characterized by intense synsedimentary deformation. Except for the presence of Skolithos, the trough and planar cross-stratified facies are unburrowed. The hummocky cross-stratified sandstone typically lacks bioturbation. The interbedded microhummocky cross-stratified and wave-rippled, cross-laminated sandstone and siltstone display variable bioturbation intensities, with relatively thick intervals virtually unbioturbated (BI 0) and others showing sparse to moderate (BI 1 to 3) and, more rarely, intense (BI 5) bioturbation. The assemblage includes Planolites, Palaeophycus, Teichichnus, Asterosoma, Diplocraterion, and Trichophycus. The shale contains few distinctive trace fossils; only tiny Planolites and Helminthoidichnites have been detected in bedding planes.

The Deadwood Formation mostly records deposition in wave-dominated shallow-marine settings, ranging from the upper shoreface to the offshore. The trough and planar cross-stratified sandstone facies records migration of two- and three-dimensional dunes in the upper shoreface, whereas the hummocky cross-stratified sandstone represents repeated storm action in the lower to middle shoreface. The interbedded microhummocky cross-stratified and wave-rippled, cross-laminated sandstone and siltstone record the recurrent alternation of quiet-water sediment fallout, with combined and pure oscillatory flows during storm events in an offshore transition. The shale-dominated intervals essentially represent suspension fallout sedimentation in an offshore environment. The interval with abundant soft-sediment deformation structures most likely records sedimentation under deltaic influence.


Figure 8 – Sedimentary facies of the Deadwood Formation from Ceepee Reward core 101/04-28-038-24W3, licence #58EO63. A) General view of deposits showing large-scale soft-sediment deformation structures; base is on the lower left and top on the upper right; depth 5,590 to 5,565 ft (1703.8 to 1696.2 m). B) Close-up showing intense soft-sediment deformation; depth 5,582 ft (1701.4 m). C) Convolute lamination; depth 5,577 ft (1699.9 m). D) Slump structure; depth 5,567 ft (1696.8 m).

4. Concluding Remarks Although the middle Cambrian–Lower Ordovician sedimentary cycle has been traditionally interpreted in terms of the classic wave-dominated environmental zonation (e.g., Dixon, 2008), our preliminary study suggests a more complex facies pattern, with the Earlie Formation recording tide-dominated settings, and intervals of the Deadwood Formation representing deltaic environments. Tidal sandbodies have been extensively described from the lower Cambrian Gog Group, demonstrating that apparently monotonous sandstone units in fact display considerable environmental variability and complex geometries (Desjardins et al., 2012a, 2012b). The cause for the switch from a tide-dominated regime in the Earlie Formation to a wave-dominated regime in the Deadwood Formation is unclear at present, requiring further study, but preliminary work suggests that the Earlie Formation may have recorded more


protected environments prone to tidal amplification, whereas the Deadwood Formation represents more open environments exposed to wave action.

5. Acknowledgments Financial support for this project was provided by the Saskatchewan Ministry of the Economy. We would like to thank Patricio Desjardins and Paul Johnston for reviewing this paper, and staff of the Saskatchewan Subsurface Geological Laboratory for their assistance in core slabbing and displaying material. Special thanks to Melinda Yurkowski and Fran Haidl for all their support and help.

6. References Butterfield, N.J. and Harvey, T.H.P. (2012): Small carbonaceous fossils (SCFs): a new measure of early Paleozoic

paleobiology; Geol., v40, p71-74.

Desjardins, P.R., Buatois, L.A., Pratt, B.R., and Mángano, M.G. (2012a): Subtidal sandbody architecture and ichnology in the Early Cambrian Gog Group of western Canada: implications for an integrated sedimentologic-ichnologic model of tide-dominated shelf settings; Sediment., v59, p1452-1477.

Desjardins, P.R., Buatois, L.A., Pratt, B.R., and Mángano, M.G. (2012b): Forced-regressive tidal flats: response to falling sea level in tide-dominated settings; J. Sediment. Resear., v82, p149-162.

Dixon, J. (2007): Correlations in Cambrian and Lower Ordovician strata of Saskatchewan; Geol. Surv. Can., Open File 5523, CD-ROM, 18p.

Dixon, J. (2008): Stratigraphy and facies of Cambrian to Lower Ordovician strata in Saskatchewan; Bull. Can. Petrol. Geol., v56, p93-117.

Fischer, D.W., LeFever, J.A., LeFever, R.D., Anderson, S.B., Helms, L.D., Whittaker, S., Sorensen, J.A., Smith, S.A., Peck, W.D., Steadman, E.N., and Harju, J.A. (2005): Overview of Williston Basin Geology as it Relates to CO2 Sequestration; Energy and Environmental Research Center, Univ. North Dakota, Grand Forks, 25p.

Greggs, D.H. and Hein, F.H. (2000): The sedimentology and structures of the Lower Paleozoic Deadwood Formation of Saskatchewan; in Summary of Investigations 2000, Volume 1, Saskatchewan Geological Survey, Sask. Energy and Mines, Misc. Rep. 2000-4.1, CD-ROM, p7-13.

Harvey, T.H.P., Vélez, M.I., and Butterfield, N.J. (2012a): Small carbonaceous fossils from the Earlie and Deadwood formations (Middle Cambrian to Lower Ordovician) of southern Saskatchewan; in Summary of Investigations 2012, Volume 1, Saskatchewan Geological Survey, Sask. Ministry of the Economy, Misc. Rep. 2012-4.1, Paper A-1, 8p.

Harvey, T.H.P., Vélez, M.I., and Butterfield, N.J. (2012b): Exceptionally preserved crustaceans from western Canada reveal a cryptic Cambrian radiation; Proceed. Nat. Acad. Sci., v109, p1589-1594.

Reineck, H.-E. (1963): Sedimentgefüge im Bereich der südliche Nordsee; Abhandlungen Senckenbergischen Naturforsche de Gesellschaft, v505, p1-138.

Slind, O.L., Andrews, G.D., Murray, D.L., Norford, B.S., Paterson, D.F., Salas, C.J., and Tawadros, E.E. (1994): Middle Cambrian to Lower Ordovician strata of the Western Canada Sedimentary Basin; in Mossop, G. and Shetsen, I. (eds.), Geological Atlas of the Western Canada Sedimentary Basin, Can. Soc. Petrol Geol./Alta. Geol. Surv., p87-108.

Taylor, A. and Goldring, R. (1993): Description and analysis of bioturbation and ichnofabric; Geol. Soc. Lon. J., v150, p141-148.


paleoenvironmental variability of the lower paleozoic ...€¦ · deadwood formations in subsurface...

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