Occurrence of Reefal Facies in Red River Strata (Upper Ordovician), Subsurface Saskatchewan

Brian R. Pratt 1, Lawrence M. Bernstein 2, A.G. Kendall 3, and F.M. Haid/

Pratt, B. R., Bernstein, L.M., Kendall, A.G., and Haidl, F.M. (1996): Occurrence of reefal facies in Red River strata (Upper Ordovician), subsurface Saskatchewan; in Summary ol Investigations 1996, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 96-4.

Upper Ordovician carbonates of the Red River have produced significant hydrocarbons in Montana and North Dakota, but production in Saskatchewan has been limited to just a few localities largely because relatively few wells have been drilled to these depths (Haidl, 1995). The recent discovery of prolific reservoirs below the Mississippian oil pools in the Midale area of southeastern Saskatchewan confirms that the Red River is an attractive exploration target (Haidl et al., this volume). Moreover, the scheduled reversion of non-producing deeper oil and gas rights to the Crown begin-ning in April 1998 is expected to encourage interest in the Lower Paleozoic succession.

Red River strata in the Williston Basin are characterized by cyclic sedimentation. Each complete cycle begins with a fossiliferous, burrow-mottled mudstone and wackestone unit, passes through a middle laminated dolomite unit, and culminates in basin-centred evaporites. In Saskatchewan, the Red River has been subdivided into the Yeoman Formation and the overly-ing Herald Formation; the latter includes the Lake Alma and Coronach members and Redvers unit (Kendall, 1976). The Yeoman mudstone and wackestone and the Lake Alma dolomite and anhydrite represent the earliest cycle. The succeeding cycle is the succession embraced by the Coronach. The Redvers is composed primarily of laminated to thin-bedded mudstone and represents a third cycle which is incomplete in Saskatchewan. The anhydritic dolomite portions of these units have been interpreted as tidal flat deposits produced by cyclic shallowing (e.g. Ruzyla and Friedman, 1985). An alternative suggestion that the entire basin simply became episodically hypersaline is based on the 'layer-cake' character of the stratigraphy and the basin-centred distribution of the evaporites (e.g. Kendall, 1976; Longman and Haidl, 1996).

The purpose of this brief report is to record the presence of a thin reefal unit in the Yeoman-Herald transition interval near the Saskatchewan-Manitoba border (Figure 1 ). This facies represents a departure from the standard lithological succession encountered in the Red River, indicating a greater complexity of the depositional setting than hitherto believed. Furthermore, the microbial character of the biolithile contrasts with

other reported Upper Ordovician reefs from North America.

1 . Stratigraphic Setting

a) Standard Succession

Red River strata in the Williston Basin range in age through the Edenian, Maysvillian, and early Richmondian (Bolton, 1988; Elias et al., 1988). They are underlain by interbedded shale, siltstone, and sandstone of the Winnipeg Formation (Middle to Late Ordovician), and overlain by middle and late Richmondian fossiliferous mudstones, wackestones, and locally packstones of the Stony Mountain Formation.

The typical Yeoman in the subsurface of southern Saskatchewan is a massive, variably dolomitized, bioturbated mudstone and wackestone with distinct, sediment-filled Tha/assinoides (=Spongeliomorpha) burrow systems, themselves hosting a second genera-tion of Planolites burrows (Kendall, 1977; Myrow, 1995). II is locally cherty and commonly fossiliferous, with crinoid ossicles, nautiloids, brachiopods, rugose corals, and planispiral and turbiniform gastropods. Scattered organic-rich interbeds (kerogenite or kukersite - see Osadetz et al., 1992; Longman and Haidl, 1996), con-taining the microfossil Gloecapsomorpha prisca, occur in the upper third of the formation. The Yeoman grades into the basal Lake Alma Member as the bioclastic content disappears, bioturbation is restricted to scat-tered layers, thin and lenticular bedding becomes distinct, and the gamma log shows a high-value deflec-tion. In some wells there is a bed of pelletal and intraclastic grainstone locally with oncoids. The Lake Alma below the anhydrite consists of dolomitized, wavy, microbial laminites and planar- and cross-laminated and locally lenticular-bedded mudstone, with sporadic contorted and microfaulted zones.

b) Reef-bearing Succession

The cored interval in the LVR et al Steelman 7-28-4· 4W2 well (2574.5 to 2593.1 m; all measurements adjusted to log depths) can be subdivided into seven units equivalent to the upper Yeoman-lower Lake Alma,

(1) Department of Geological Sciences. University of Saskatchewan, 114 Science Place, Saskatoon. SK S7N 5E2. (2) Talisman Energy Inc. , 2400, 855 · 2nd Street SW, Calgary, AB T2P 4J9. {3) School of Environmental Sciences. University of East Anglia, Norwich NR4 7TJ. United Kingdom.

Saskatchewan Geological Survey 147

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Figvre 1 · Map of the northern portion of the Williston Basin showing location of the Steelman area cross-section (Figure 2), structural contours on the top of the Red River, and Red River hydrocarbon shows (adapted from Haid/ et al., this volume).

but different from the typical sequence. Tentative corre-lations suggest that the reef described here developed during early Lake Alma time (Figure 2).

The basal 1 O m (2583.5 to 2593.1 m) are thin- and nodular-bedded, locally burrowed, cherty dolomitic wackestones containing crinoid ossicles and scattered gastropods and rugose corals. This facies contrasts somewhat with the typical Yeoman in which bedding is largely absent due to pervasive bioturbation and Thalassinoides galleries. A bioturbated kukersitic bed marks the top of this unit. The rest of the cored Yeoman-Lake Alma interval is dolomitized.

The overlying metre (2582.5 to 2583.5 m) is composed of locally burrowed, bioclastic packstone and rudstone also containing rugose corals and gastropods. Above this is 3.4 m (2579.1 to 2582.5 m) of compacted rudstone exhibiting planar and cross lamination, abun-dant laminar and scarce domical stromatoporoids (Figure 3A), crinoid ossicles, trilobite sclerites, rugose corals, ostracode and brachiopod valves and pellets. Many of the stromatoporoids are fragments. They are mostly up to a few centimetres wide and less than half a centimetre thick, but in places are 8 cm wide and 3.5 cm thick; preservation is poor, but they appear to belong to Cystostroma. Locally there are zones of isopachous bladed cement. Capping this rudstone is

148

0.4 m (2578.7 to 2579.1 m) of thin-bedded and lami-nated mudstone and finely intraclastic grainstone.

The 2.8 m thick reefal boundstone unit (2575.9 to 2578.7 m) overlies this. Above the boundstone is 0.8 m (2575.1 to 2575.9 m) of compacted grainstone and sub-ordinate wackestone containing peloids, small microbial intraclasts, ostracode valves, ramose bryozoans up to 2.5 mm long, and fragments of clotted micrite and Ortonella 0.3 to 3 mm across. Wackestone laminae are riddled with Planolites burrows. Overlying this is laminite with abundant stylolites, a typical Lake Alma facies. All three of these units are heavily replaced by anhydrite laths, and patches of 'microsparry' anhydrite are abundant in the boundstone.

c) Reef Composition

The boundstone is thrombolitic, i.e. composed of digitate and locally crust-like 'thromboids' of microbial origin (see Pratt, 1995). Digitate thromboids are several millimetres to a centimetre wide (Figure 38) and consist of clotted micrite with tiny fenestral pores (Figure 4A). They are coated in places by 'stromatoids', in this case microbial rinds up to several millimetres thick which have a laminated and outwardly radiating dendritic internal fabric of micrite clots in microspar (Figure 48). In some zones, the thromboids and coatings are neomorphosed into microspar and pseudospar.

Summary of Investigations 1996

A VISTA STEELMAN LVR ET AL STEELMAN SCEPTRE STLMN 5 UNIT A' 2-14-5- SW2 -- 9km - - 7-28-4-4W2 - - 8km-- 9-34-3-4W2

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c OHON.t,CH

HERALD •""•"'11s LAKFAJMA --1

YEOMAN

WINNIPEG

l3 MICROBIAL BOUNDSTONE ~ STROMATOPOROID RUDSTONE

I CORED INTERVAL [ DST ,ecovered muo-and m~ s.anwtltr

Figure 2 - Stratigraphic cross-section showing relationships between the 'anomalous' reef-associated sequence in L VR et al Steelman 7-28-4-4W2 and the 'normal' Lake Alma-Yeoman transition.

Figure 3 - Polished core slabs: A, rudstone with thin laminar stromatoporolds and one large domical one (2581 .1 m); 8, brown-coloured, mottled thromboidal boundstone overlying buff-coloured laminated mudstone: internal sediment filling large growth-framework cavity is replaced by anhydrite microspar (2577.8 m).

Saskatchewan Geological Survey 149

lntergrown in some zones are straight to sinuous, mi-crite-walled tubules 70 to 150 µm in diameter (Figure 4C). These are similar to Terebel/a polychaete worm tubes known from Mesozoic reefs (Pratt, 1995). Also in some zones are millimetre-sized, knob-like Ortonella hemispheroids composed of radiating, juxtaposed tubes (Figure 4C). Both in the reef and in the grainstone above, two populations are exhibited where tubes in some are 20 to 50 µm whereas in others they are 50 to 70 µm wide. We refer to these as Ortonella white

recognizing that taxonomic assignment is problematic, for this 'genus' belongs to a morphologically variable group of what are probably calcified microbial filament clusters (Dragastan, 1988; Riding, 1991) rather than cal-careous green algae (Roux, 1985).

Cream-coloured , isopachous, fibrous cement occurs locally on thromboids. Fine pyrite is disseminated along the margins of thromboids in some areas. Petsparite with microbial intraclasts and micrite with Planolites

Figure 4 - Vertically oriented thin section photomicrographs of boundstone (7-28-4-4W2): A, thromboidal fabric of ramose bryozoans encased in clotted microbial micrite with intergrown micrite-walled Terebella-like 'worm' tubes (mostly as ring-like cross-sections), round white areas are small vugs many of which are filled by anhydrite, and replacive anhydrite laths are in lower left (2576.55 m); 8, partly recrystallized thrombolds (right side and bottom) coated by stromatoidal laminated micrite, remaining growth-framework cavity is filled by micrite pellets and replaced by anhydrite microspar and small laths (2577.2 m); C, Ortonel/a hemispheroid encrusted by clotted and peloidal micrite with micrite-walled 'worm · tubes (ring-like cross-sections), upright ramose bryozoan at left (2576.55 m).

150 Summary of Investigations 1996

burrows, themselves fil led with pelmicrite, occur in some of the larger cavities and as scattered thick lami-nae and thin beds. Other cavities around the thromboi-dal framework are filled with pelmicrite or laminated micrite. ·

Articulated ostracodes are common. Scattered ramose bryozoans, 2 to 5 mm long, occur in cavities and incor-porated into the framework by microbial micrite en-crustation (Figure 4A). Also present are rare macluritid gastropods and th in-shelled bivalves 3 to 5 mm across and 50 µm thick with distinct ridges parallel to the growth lines.

2. Discussion

The sedimentological implications of the reefal unit in the basal Lake Alma Member are unclear, given that it has been encountered in only one well. Nonetheless, it does point to a more complex facies pattern than hith-erto believed for the upper Yeoman and lower Herald formations. The detailed stratigraphic and sedimentologi-cal relationships of these facies to the 'normal' Herald-Yeoman succession are the subject of ongoing investigations.

The stromatoporoidal rudstone and grainstone above the Yeoman wackestone were probably deposited as a carbonate sand bank or biostrome. The composition of the reefal boundstone is markedly different from that documented in other Upper Ordovician examples which exhibit a more diverse, normal-marine fauna and flora (Lake , 1981; Harland and Pic kerill , 1989). Still, micro-bial structures such as thromboids and Ortonella are known to have formed in normal seawater, and the resemblance of the Lake Alma boundstones to some of those in the Middle and Upper Triassic of the Dolomites of northern Italy is striking (for a summary see Pratt, 1995). Water energy was probably sufficient to oxygen-ate the reef site, as suggested by the locally winnowed nature of internal sediment. Nutrient supply was also probably adequate because a benthic fauna and micro-bial flora did develop. However, the low-diversity fauna of ostracodes, ramose bryozoans, bivalves, gastropods, and encrusting and burrowing worms suggests an envi-ronment stressed in some way. There are, in general, members of these animal groups that are tolerant of brackish or slightly hypersaline conditions, or those of fluctuating salinity, and it may be in this area that the limiting factor is to be found. The isopachous submarine cement argues for supersaturated normal-marine or hypersaline water rather than uniformly brackish water whose saturation state might have been less conducive to cementation. Temperature may have been involved if it exceeded the tolerance limits of typical Ordovician benthic fauna! components.

3. Acknowledgments

The Natural Sciences and Engineering Research Council of Canada funds B.A.P.'s reef research. F.M.H. and L.M.B. publish with the permission of the Chief Geologist . Saskatchewan Geological Survey, and Talisman Energy Inc., respectively.

Saskatchewan Geological Survey

4. References Bolton, T.E. (1988): S1romatoporoidea from the Ordovician

rocks of central and eastern Canada; Geel. Surv. Can., Bull. 379, p17-45.

Dragastan, 0 . (1988): Some "Porostromata" algae, an attempt toward their classification : Revista Espanola de Micropaleontologia, v20, p251 ·272.

Elias, R.J., Nowlan, G.S., and Bolton, T.E. (1988): Paleontol· ogy of the type section. Fort Garry Member, Red River Formation (Upper Ordovician), southern Manitoba; in Wohlberg. D. L. (ed.), Contributions to Paleozoic paleontol-ogy in honor of Rousseau H. Flower, N. Mex. Bur. Mines Miner. Resour., Mem. 44, p341 -359.

Haidl, F.M. (1995): Hydrocarbon potential of Lower Paleozoic carbonate strata in southeastern Saskatchewan; in Summary of Investigations 1995, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 95·4, pl 18-128.

Harland, T.L. and Pickerill, R.K. (1989}: Patch reefs in Ordovician limestones. St. Honore, Quebec; In Geldsetzer. H.H.J., James, N.P., and Tebbutt. G.E. (eds.), Reefs, Canada and adjacent areas, Can . Soc. Petrol . Geol ., Mem. 13, p201·207.

Kendall , A.C. (1976): The Ordovician carbonate succession (Bighorn Group) of southeastern Saskatchewan; Sask. Miner. Resour. , Rep. 180, 185p.

_ _ _ _ (1977): Origin of dolomite mottling in Ordovician limestones from Saskatchewan and Manitoba; Bull. Can. Petrol. Geel., v25, p480·504.

Lake, J.H. (198 1 ): Sedimentology and paleoecology of Upper Ordovician mounds of Anticosti Island, Quebec; Can. J. Earth Sci. , vl 8, p1562-1571 .

Longman , M.W. and Haidl. F.M . (1996): Cyclic deposilion and development of porous dolomites in the Upper Ordovician Red River Formation, Williston Basin; in Longman, M.W. and Sonnenfeld, M.D. (eds.), Paleozoic systems of the Rocky Mountain region, Rocky Mountain Section, SEPM (Soc. Sediment. Geol.}, p29·46.

Myrow, P.M. (1995): Thalassinoides and 1he enigma of early Paleozoic open-framework bu rrow systems; Palaios, v10, p58-74.

Osadetz, K.G. , Snowdon, L.R., and Brooks, P.W. (1992): 011 families and their sources in Canadian Williston Basin (southeastern Saska1chewan and southwestern Manitoba}; Bull. Can. Petrol. Geel., v40, p254·273.

Pratt, B.R. (1995): The origin, biota, and evolution of deep· water mud-mounds; in Monty, C.L.V., Bosence, D.W.J., Bridges, P.O., and Pratt, B.R. (eds.), Carbonate mud-mounds: Their origin and evolution, Int. Assoc. Sedimen· tologists, Spec. Publ. 23, p49-123.

Riding, R. (1991 ): Calcified Cyanobacteria; in Riding, R. (ed.). Calcareous Algae and Stromatolites, Springer-Verlag. Berlin, p55-87.

Roux, A. (1985}: Introduction a l'etude des algues fossiles PaleozoTques (de la Bacteria a la tectonique des plaques); Bulletin des Centres de Recherches Exploration-Production Elf-Aquitaine, v9, p465-699.

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Ruzyla, K. and Friedman, G.M. (1985): Factors controlling porosity in dolomite reservoirs of the Ordovician Red River Formation, Cabin Creek Field, Montana; in Roehl, P.O. and Choquette, P.W. (eds.), Carbonate Petroleum Reservoirs, Springer-Verlag, New York, p41-58.

152 Summary of Investigations 1996