O and C Isotopes of the Middle Devonian Lower Winnipegosis, Brightholme and Ratner Carbonates, Elk Point Basin, Southeastern

Saskatchewan

Hairuo Qing 1, Katherine M Bergman 1

, and Orrin Cameron 1

Qing, H., Bergman, K.M., and Cameron, 0 . (200 1): 0 and C isotopes of the Middle Devonian Lower Winnipegosis, Brightholme and Ratner carbonates, Elk Point Basin, southeastern Saskatchewan; in Summary ofl nvcstigations 2001, Volume I, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 200 1-4. 1.

Abstract

The depositional environment and stratigraphic relationships of the Brightholme and Ratner units of the Middle Devonian Elk Point Group are top ics of considerable debate. A total of 20 limestone and dolostone samples from Lower Winnipegosis, Ratner, and Brightholme strata in four wells in south-central Saskatchewan were analyzed f or t/'10 and one in order to test whether or not oxygen and carbon isotopes can be used lo characterize the depos itional environment and constrain the stratigraphic relationships of these units.

The 0180 values of both limestone and dolostone samples from the Lower Winnipegosis, Brightholme, and Ratner are lower than the estimated values f or Middle Devonian marine calcite and dolomite, which suggests that the original 81xo signatures of these carbonates have been modified by post-depositional diagenetic processes. Their 81xo values, therefore, cannot be used to characterize the depositional environments of these stratigraphic units.

The tPC values of the Ratner and Lower Winnipegosis limestones and dolostones are similar to or slightly lower than the estimated Middle Devonian seawater values. The main source of carbon f or the Ratner and the lower Winnipef.:(JSis carbonates is, therefore, inorganic carbon from Middle Devonian seawater. The 8/JC values of Brightholme limestone and dolostone. however. are lower than those of Ratner and l ower Winnipegosis carbonates. Accordingly, the low otJC values of Brightholme carbonate are interpreted to have resulted from mixing of inorganic carbon of Middle Devonian seawater with organic carbon released to the pore water from organic-rich sediments. Carbon isotopes are a p otential tool f or characterizing and differentiating the Brightholme from the Ratner.

l. Introduction

The depositional environment and stratigraphic re lationships of the Brightho lme and Ratner units of the Middle Devo nian Elk Point Group are not clearly understood.

' Depanmenl of Geology, University of Regina, Regina. SK S4S 0A2.

32

Stoakes et al. , (1987) introduced the informal name "Brig htholme member' ' for the thin organic-rich shale and/or laminated mudstone that overlies the Lower Winnipegosis carbonate. The most recently proposed deposit ional model (J in et al., 1997 ; Jin and Bergman, 1999) for the Brightho lme suggests that it is the off­reef basin facies of the Upper Winnipegosis Format ion ( Figure 1).

Shearman and Fuller ( 1969) interpreted the laminites o f the Ratner as intertidal depos its analogous to modern sabkha algal mats, imply ing rapid and comple te sea-level drawdown after the termination of Winnipegosis reef deve lopment. In contrast, Kendall ( 1975) arg ued for a deep-water orig in because of the gradational contact, lateral continu ity, and perfect preservation o f individual laminae. Davies and Ludlam ( 1973) postulated a stratified water column and an anoxic bottom environment. Jin et al. ( 1997) and Jin and Bergman ( 1998) interpreted the Ratner as the initia l deposits of a stratified water column in a restricted basin. Maiklem ( 197 I), Jin et al. ( 1997), and Jin and Bergman (I 998, 1999) proposed a partia l drawdown o f at least 30 m based on the presence of vadose diagenetic features in the Keg River and Upper Winnipegosis carbonate deposits. The stratigraphic relationship of the Ratner is not clear. Jones ( 1965) first described the laminite as an inter-reef facies o f the Upper Winnipegosis. Reinson and Wardlaw ( 1972) formally proposed the Ratner Member as a distinct stratigraphic unit o f the Winnipegosis Formation. Kendall ( 1975) fi rst arg ued that the Ratner was entire ly post-Winnipegosis and should be part of the Pra irie Formation (Figure I). Petrological and stratigraphic studies of the Winnipegosis- Prairie Evaporite transition (Jin et al. , 1997; Jin and Bergman, 1998, 1999) indicate that deposition of the Ratner was genetically re lated to intense vadose d iagenesis of the Winnipegosis reefs under fluctuating rates o f marine water seepage into the basin and therefore is post­Winnipegosis (Figure I).

2 . Study Area and Methodology

The study area is conta ined between Townships 22 and 50, Ranges 20W2 to IOW3 (Figure 2). Samples for oxygen and carbon isotope analysis were taken from

Summary of lnvesti~ations 200 I. Vo!llme I

UTHOSffiATiGRAPHY CHRONOSIRATIGRAPHY

four wells in south-central Saskatchewan (Figure 2). Wells were chosen for sampling on the bases of: I ) the completeness and quality of core in the Brightholme unit, and 2) composition of the sediment. Five samp les were collected from each well (Table I ): two from the Lower Winnipegosis IO cm and I m bel ow the contact with the Brightholme; two samples in the Brightholme IO cm and I m above the Lower Winnipegosis contact, and one sample from the Ratner. They were obtained using a drill press equipped with a small masonry bit. Oxygen and carbon isotopic analyses were perfonned at the University of Saskatchewan .

I ~ ~ s. s ,~

8 I ~ c ,2 ~

it l f :: * B-

3. Results and Discussions Figure I - Comparison of litho.\'trutigmphic and chronostmtigraplric relationships between Winnipegosis carbonate und Briglrtholme .~Jillie, Ratner /11minite, and Wltitlww anhydrite (from Jin anti Bergman, 1999). The measured oxygen and carbon

isotopes are reported in Table I and plotted in Figure 3. The 8 180 and o';C values of marine calcite precipitated from Middle Devonian seawater are est imated to be in the ranges -5 to -3 %o PDB and 0.5 to 2.5 %o PDB (Figure 3) respectively. based on the analyses of well preserved brach iopod she lls and marine cement (Popp e1 al .. 1986). The 8 180 values of brachiopod shells from the Midd le Devonian Presqu" ile barrier at Pine Point vary between -3.8 to -4 .2 %o PDB and 813C values are a round 1.9 o/oo PDB (Qing, 1998). If a mean value of2 .5 %o is taken as the fractionation between do lomite and calcite (Major el al.. 1992), then the primary isotopic signatures of dolomite precipitated from normal Middle Devonian seawater shou Id have 8 1KO values fro m -2.5 to -0.5 o/oo PDB and 8 13C from 0.5 to 2.5 %o PDB (Figure 3). Dolomite formed by slight ly evaporated Middle Devonian seawater prior to gypsum precipi tat ion should have 8 180 values higher than -2.5 to -0.5 %0

PDB. /

/ /

/

.. -

10W3

-- . ·-

'"" ...... '" ...

5W3 25W2

' ' ' • - ,: .. • ru ,.,

4 1H·1

TWP50

TWP46

TWP42

TWP38

TWP34

Figure 2 - Schematic map of the Elk Poi11t Basi11, showing the study area and locatio11s of wells sump/ed for isotopic analyses.

Saslw1che wan Geological Survey

Most limestone samples from Lower Winnipegosis, Brightholme, and Ratner strata have a similar range of 8 180 values. from -5 .6 to -7 .3 %0 PDB (Figure 3). These values are slightly lower than that of Middle Devonian marine calcite (-5 to -3 %0 PDB). One Ratner sample, however, has a 8180 value (-4.9 o/oo PDB) close to the lower end value for Midd le Devonian marine calcite. This suggests that the orig inal 8 1KO s ignatures of these Middle Devonian marine limestones have been modified and/or overprinted by post-depositional diagenet ic processes. Their present 8 180 values, therefore, cannot be used as a parameter to differentiate the Brightholme from the Ratner. The 8 180 values for the Brightholme dolomite (-7.6 to -8.8 %0 PDB) and the Ratner dolom ite (-7 .5 to - 11.0 % 0

PDB) overlap (Figure 3) and a re much lower than the expected value for Middle Devonian marine dolomite (-2.5 to -0.5 %o PDB), ind icating later diagenetic

33

Table J - Sample locarion, formation, location relative to the Lower Winnipelf.osis (LWJN)-Brightholme (BRHM) contact, depths, composition, and ?51NO and 6 1C vnllles (%0 PDB),· RTNR, Ratner.

Location relative to LWIN/BRHM

Well Location Formation contact

16-13-42-19W2 LWIN Im below 16-l3-42- I 9W2 LWIN IOcm below l-l5-48-17W2 LWIN Im below l-15-48-1 7W2 I.WIN 10 cm below

12-2-44-7W3 LWIN 1.2 m below 12-2-44-7W3 LWIN 20 cm below 13-3-42-l6W2 LWIN Im below 13-3-42-16W2 LWIN 10 cm below

l 6-13-42- I 9W2 BRIIM 25 c m above l 6-IJ-42-19W2 RRHM 1 m above I-15-48-17W2 13 RHM IO cm above 1- l 5-48-l 7W2 BRHM I rn above

I 2-2-44-7W3 13 RIIM 10 cm above 12-2-44-7W3 RRI IM Im above 13-3-42- 16W2 BRHM 10 cm above 13-3 -42-16W2 BRIIM I rn a bove

16-13-42-19W2 RTN R 4.9 m above 1-l 5-48-l 7W2 RTNR 7.9 m above

12-2-44-7W3 RTNR 2.8 m above 13-3-42-l 6W2 RTNR 3.5 m above

Depth (m) Compos ition

844 .l Dolostone 843.2 Dolostone 522.2 l)olostone 52 1.3 Dolostonc

Avg.

940.7 Limestone 939.7 Limestone 996.1 Lime; lonc 995.2 Limestone

Avg.

842.8 Dolostonc 842.J Dolostone 52 1.1 Dolostonc 520.2 Dolostone

Avg.

939.4 Limestone 938.5 Limestone 995 Limestone 994. 1 Limestone

Avg.

838.2 Dolostone 513.3 Uolostone

Avg.

936.7 Limestone 991.6 l .imestonc

Avg.

c'"o -6.46 -7.24 -6 .23 -6.76 -6.67

-6.2 1 -6.10 -5.94 -5 70 -5.99

-7.76 -8. 18 -7.60 -8.80 -8.09

-6.24 -5.60 -5.59 -6.45 -5.97

-7.50 - 10.99 -9.25

-4.88 -7.33 -6.1 I

1.39 0.57 1.33 0 .68 0.99

0.77 -0.59 0.69 -1.59 -0.18

-0.05 -0.73 -0.39 -1.86 -0.76

-0.95 -2.45 - 1.13 -3 .28 -1 .95

0.74 0 .96 0 .85

0.21 0.62 0 .42

The 8 13C values o f the Ratner limestone (range 0.2 to 0 .6 %0 PDB, avg. 0.4 %o PDB) are s imilar to the est imated values of Middle Devonian marine calcite (0 .5 to 2 .5 %0 PDB). The Lower Winn ipegosis limestone 813C values range from - 1.6 to 0.8 %0 PDB, and average -0.2 %o PDB, s lightly lower than that of Middle Devonian marine calcite (Figure 3). This suggests that the main source of carbon for the Ratner and Lower Winnipegosis limestones was inorganic carbon derived from Middle Devon ian seawater. Contrary to oxygen isotopic values, post-deposit ional diagenetic processes, including dolomitization, have little effect on the orig inal o13C signatu res of Ratner and Lower Winnipegosis limestone deposits. Th is interpretation is supported by the 813C values of the Ratner dolomite (0.7 to 1.0 %0 PDB, avg. 0.9 %o PDB) and the Lower Winnipegosis dolomite (0.6 to 1.4 %0 PDB, avg. 1.0 %0 PDB), which a lso fall w ith in the range for Middle Devonian marine dolom ite (Figure 3).

The 811c values of Brightholme limestones (- 1.0 to -3.3 o/oo PDB, avg. -2.0 o/oo PDB) are lower than

alteration of their o rig ina l marine 8180 values. The 8180 values from the measured Devon ian dolomites, therefore, cannot be used to cha racter ize the depositiona l environments of the Brightholme or the Ratner.

those of Ratner and Lower Winnipegosis limestones, as well as lower than M iddle Devon ian marine calcite (Table 1; Figure 3). Thus. in addi tio n to inorganic carbon from Middle Devonian seawater, another source of lighter carbon has been incorporated with t he Brightholme limestone. Organ ic

3 2 1

- 0 al c -1 D.

0 -2

;:,!? -3 0 -(.) -4 .., (-0

-12 -10

Estimated M Dev Marine calc ite

.. ~. <>o

0

... ... 0

0

!).

!).

-8 -6 -4 -2

Estimated M Dev. Marine dolomite

0

• LWIN-DOL

o LWIN-LS

,. BRHM-DOL

D. BRHM-LS

• RTNR-DOL

o RTNR-LS

Figure 3 - Cross plot of 'f:/"O and o11C values (%0 PDB). L WJN , the Lower Win11ipego.~is; BRIIM, Brightholme; RTNR, Ratner; LS, limesto11e; and DOL, do fostone (see text f or discussions).

34

matte r in marine sed iments has 813C values from -10 to -20 %0 PDB (Anderson and Arthur, 1983). The 8 13C value of pore water from organic-rich sediments can decrease rapidly and overprint the 8 13C signature of the origina l seawater. This process is re lated to the release of organic carbon into pore water from organic-rich sediments during decomposit ion of organ ic matter (Anderson and Arthur, J 983 ). The low &13C values of Brightholme limestones can, the refore, be attributed to the mixing of inorganic carbon from M iddle Devonian seawater w ith carbon derived from organic matter. Th is is consiste nt with the dark black colour of the rock and high TOC values (avg. -4%. maximum - 31 %) for the

Summary of Investigations 2001. Volume 1

Brightholme (Osadetz and Snowdon, 1995). Carbon isotopes thus provide a potential means of characterizing the Brightholme and possibly differentiating it from the Ratner.

4. Conclusions

The 8 180 values of both limestones and dolostones from the Lower Winnipegosis, Brightholme, and Ratner are lower than those of Middle Devonian marine calcite and dolomite, suggesting that the orig inal 8 180 signatures of these carbonates have been modified and/or overprinted bl,: post-depositional diagenetic processes. Their 81 0 values, therefore, cannot be used to characterize the depositional environments of the Brightholme or the Ratner members.

The o13C values of the Ratner and Lower Winnipegosis carbonates are similar to, or slightly lower than, the estimated values of Middle Devonian marine calcite. The main source of carbon for the Ratner and the Lower Winnipegosis carbonates, therefore, is inorganic carbon derived from Middle Devonian seawater. Post­depositional diagenetic processes, including dolomitization, have had little effect on their or iginal one signatures.

The o13C values of Brightholme limestones and dolostones are lower than those of the Ratner and Lower Winnipegosis carbonates and can be attributed to the mixing of inorganic carbon from Middle Devon ian seawater with carbon from organic sources. Carbon isotopes, therefore, provide a potential method to characterize and differentiate the Brightholme from the Ratner.

5. Acknowledgments

We thank Saskatchewan Energy and Mines (SEM) Subsurface Laboratory for access to the facility and core sampling, and Dr. Chris Holmden at the University of Saskatchewan for the isotope analyses. This project was funded by a grant from the Potash Corporation of Saskatchewan (PCS) and with matching support from an NSERC IOR grant.

6. References

Anderson, T.F. and Arthur, M.A. ( 1983): The carbon cyc le and carbon isotopes; in Stable Isotopes in Sedimentary Geology, SEPM Short Course No. 10, pl.80-1.151.

Davies, G.R. and Ludlam, S.D. (1973): Origin of laminated and graded sediments, Middle Devonian of western Canada; Geol. Soc. Amer. Bull. , v84, p3527-3546.

Jin, J. and Bergman, K.M. ( 1999): Sequence stratigraphy of the Midd le Devon ian Winnipegosis carbonate-Prairie Evaporite transition, southern

Saskatchewan Geological Survey

Elk Point Basin; Carbonates and Evaporites, v/4, p64-83.

-~--~(1998): Occurrences of Middle Devon ian Ratner laminite in the distal part of the Elk Point Basin; in Kreis, L.K. (ed.), Eighth International Williston Basin Symposium, Core Workshop, Sask. Geol. Soc., N. Dakota Geol. Soc., Montana Geo!. Soc., Regina, p89-104 .

Jin, J., Bergman, K., Haid!, F., Blair, M., and Ricci, A. ( 1997): Vadose diagenesis of the Winnipegosis carbonate and the origin of the Ratner laminite and Whitkow anhydrite, Middle Devonian, southern Saskatchewan; in Summary of Investigations 1997, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 97-4, p l97-2 12.

Jones, L. ( 1965): The Middle Devonian Winnipegosis Formation of Saskatchewan; Sask. Dep. Miner. Resour., Rep. 9&, IO Ip.

Kendall, A.C. (1975): The Middle Devonian Winnipegosis and lower Prairie Evaporite formations of the commerc ial potash areas; in Summary of Investigations 1975 by the Saskatchewan Geological Survey, p6 l-65.

Maiklem, W.R. (197 1): Evaporative drawdown - a mechanism for water-level lowering and diagenesis in the Elk Point Basin; Bull. Can. Petro l. Geo!. , v 19, p487-503 .

Major, R.P., Lloyd, R.M., and Lucia, F.J. (1992): Oxygen isotope composit ion of Holocene dolomite formed in a humid hypersaline setting; Geol., v20, p586-588.

Osadetz, K.G. and Snowdon, L.R. ( 1995): Significant Paleozoic Petroleum Source Rocks in the Canadian Williston Basin : Their Distribution, Richness, and Thermal Maturity Southeastern Saskatchewan and Southwestern Manitoba; Geol. Surv. Can., Bull. 487, 60p.

Popp, B.N., Anderson, T.F., and Sandberg, P.A. ( 1986 ): Textural, elemental, and isotopic variations among constituents in Middle Devonian limestones, North America; J. Sed. Petrol., v56, p715-727.

Qing, H. ( 1998): Petrography and geochemistry of early-stage, fine- and medium-crystall ine dolomites in the Middle Devonian Presqu' ile at Pine Point, Canada; Sediment. , v45 , p433-446.

Reinson, G.E. and Wardlaw, N.C. (1972): Nomenclature and strat igraphic relationships, Winnipegos is and Prairie Evaporite formations, central Saskatchewan, Bull. Can. Petro l. Geol. , v20, p30 1-320.

Shearman, D.J. and Fuller, J.G.C. M. ( 1969): Anhydrite d iagenesis, calcitization, and organic laminites, Winnipegosis Formation, Middle Devonian,

35

Saskatchewan; Bull. Can . Petrol. Geol., v 17, p496-525.

Stoakes, F., Cambell, C., Hassler, G., Dixon, R. , and Forbes, D. (1987): Sedimentology and Hydrocarbon Source Potential of the Middle Devonian Winnipegosis Formation of South­eastern Saskatchewan; Stoakes Campbell Geoconsulting, unpubl. rep., 98p.

36 Summary of Investigations 200/, Volume I