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 200 1, 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 ofthese units.
The 0180 values of both limestone and dolostone
samples from the Lower Winnip egosis, 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
relationships of the Brightho lme and Ratner units of
the Middle Devo nian Elk Point Group are not clearly
understood.
Stoakes et al. , ( 1987) introduced the info rmal name
" Brig htholme member'' for the thin organic-rich shale
and/or laminated mudstone that overlies the Lowe r
Winnipegos is 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 de pos 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-wa ter orig in beca use of the
gradational contact, lateral continu ity, and perfect
preservation o f individual laminae. Da vies and Ludlam
( 1973) postulated a stratified water column and an
anoxic bottom environm ent. Jin et al. ( 1997) and Jin
and Bergma n ( 1998) interpreted the Ratner as the
initial de posits 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 partial
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 Ratn er is not clear. Jones ( 1965)
first described the laminite as an inte r-reef facies o f th e
Uppe r Winnipegosis. Re inson and Wardlaw ( 1972)
formally proposed the Ratner Membe r as a distinct
stratigraphic unit o f the Winnipegosis Form ati on.
Kenda ll ( 1975) fi rst arg ued that the Ratner was entirely
post- Winnipegosis and should be part of the Pra irie
Formation (Figure I). Petrological and stratigra phic
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 inte nse vadose d iagenesis of the
Winn ipegosis reefs under fluctuating rates o f ma rine
water seepage into the basin and there fo re is postWinn ipegosis (Figure I).
2 . Study Area and Methodology
The study area is conta ined between Townsh ips 22 and
50, Ranges 2 0W2 to IOW3 (F igure 2). Samples for
oxygen and carbon isotope analysis were taken fro m
' Depanmenl of Geology, University of Regina, Regina. SK S4S 0A2.
32
Summary of lnvesti~ations 200 I. Vo!llme I
CHRONOSIRATIGRAPHY
UTHOSffiATiGRAPHY
I
~
~
ss.
,~
I
8
~
c
,2 ~
it
l*
::
B-
f
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).
''
/
/
/
/
-
..
- --
.
.... . '" ...
'""
·-
,:
..
'
TWP50
•
TWP46
TWP42
1
H·1
TWP38
TWP34
10W3
5W3
25W2
Figu re 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
3. Results and Discussions
The measured oxygen and carbon
isotopes are reported in Table I
a nd 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 (F igure 3) respective ly.
based on the analyses of well preserved brach iopod
she lls and marine cement (Popp e1 al.. 1986). T he 8 18 0
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 8 13 C 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
do lomite precipitated from normal M idd le Devonian
seawater shou Id have 8 1KO va lues fro m -2.5 to -0.5 o/oo
PDB and 8 13 C 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 18 0 values higher than -2.5 to -0.5 %0
PDB.
• ru ,.,
4
fou r wells in south-central
Saskatchewan (Figu re 2). Wells
were chosen for samp ling on th e
bases of: I) the completeness and
quality of core in the Brightholme
unit, and 2) composition of the
sediment. Fi ve samp les were
c ollected 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 samp le 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 .
Most limestone samples from Lower Winnipegosis,
Brighth olme, and Ratner strata have a similar range of
8 180 va lues. 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 Ratn er
sample, however, has a 8 180 value (-4.9 o/oo PDB) close
to th e lower end va lue fo r M idd le Devonian marine
calcite. This suggests that the orig inal 8 1 KO s ignatures
of th ese Middle Devonian marine limestones have been
modified and/ or overprinted by post-depositional
diagenetic processes. Their presen t 8 180 va lues,
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)-Brig htholme (BRHM) contact, depths, composition, and ?5 1NO and 6 1C vnllles
(%0 PDB),· R TNR, Ratner.
Well Location
Location relative
to LWIN/BRHM
Formation contact
Depth (m)
16- 13-42-19W2
16-l3-42- I 9W2
l-l5-48-17W2
l-15-48-1 7W2
LWIN
LWIN
LWIN
I.WIN
Im below
IOcm below
Im below
10 cm below
844 .l
843.2
522.2
52 1.3
12-2-44-7W3
12-2-44-7W3
13-3-42-l 6W2
13-3-42- 16W2
LWIN
LWIN
LWIN
LWIN
1.2 m below
20 cm below
Im below
10 cm below
l 6-13-42- I9 W2
l 6-IJ-42-19W2
I- 15-48- 17W2
1- l 5-48-l 7W2
BRIIM
RRHM
13 RHM
BRHM
I 2-2-44-7W3
12-2-44-7W3
13-3-42- 16W2
13-3 -42- 16W2
c'"o
Compos ition
Dolostone
Dolostone
l)olostone
Dolostonc
Avg.
-6.46
-7.24
-6 .23
-6.76
-6.67
1.39
0.57
1.33
0 .68
0.99
940.7
939.7
996.1
995.2
Limestone
Limestone
Lime; lonc
Limestone
Avg.
-6.2 1
-6. 10
-5.94
-5 70
-5.99
0.77
-0.59
0.69
-1.59
-0.18
25 c m above
1 m above
IO cm above
I rn above
842.8
842.J
52 1. 1
520.2
Dolostonc
Dolostone
Dolostonc
Dolostone
Avg.
-7.76
-8. 18
-7.60
-8.80
-8.09
-0.05
-0.73
-0.39
-1.86
-0.76
13 RIIM
RRI IM
BRHM
BRIIM
10 cm above
Im above
10 cm above
I rn a bove
939.4
938.5
995
994. 1
Limestone
Limestone
Limestone
Limestone
Avg.
-6.24
-5.60
-5.59
-6.45
-5.97
-0.95
-2.45
- 1.13
-3 .28
-1 .95
16-13-42-19W2 RTN R
1-l 5-48 -l 7W2
RTNR
4.9 m above
7.9 m above
838.2
513.3
Dolostone
Uolostone
Avg.
-7.50
- 10.99
-9.25
0.74
0 .96
0 .85
12-2-44-7W3
13-3-42-l 6W2
2.8 m above
3.5 m above
936.7
991.6
Limestone
l.imestonc
Avg.
-4.88
-7.33
-6.1 I
0.21
0.62
RTNR
RTNR
The 8 13 C 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 ipegos is limestone 8 13 C
values range from - 1.6 to 0.8 %0
PD B, and average -0.2 %o PDB,
slightly lower th an that of Midd le
Devonian marine calcite (Figure
3). Th is suggests that the main
source of carbon fo r the Ratner
and Lower Win nipegosis
limes to nes was inorganic carbon
derived from Midd le Devon ian
seawater. Contrary to oxygen
isoto pic va lues, post-deposit ional
diagenetic processes, including
do lomitization, have little effect
o n the or ig ina l 13 C signatu res of
Ratner and Lower Winn ipegosis
limestone deposits. Th is
interpretation is supported by the
8 13 C va lues of the Ratner
dolomite (0.7 to 1.0 %0 PDB, avg.
0.9 %o PDB) and the Lower
Winnipegos is dolo mite (0.6 to
1.4 %0 PDB, avg. 1.0 %0 PDB),
wh ich a lso fall w ith in the range
for Middle Devonian marine
dolom ite (Figure 3).
o
The 8 11c values of Brighth ol me
limestones (- 1.0 to -3.3 o/oo PDB,
0 .42
avg. -2.0 o/oo PDB) are lower than
those of Ratner and Lo wer
alteration of their o rig ina l marine 8 180 values. The
Winni pegos is limestones, as well as lower than M iddle
18
8 0 val ues from th e measured Devon ian dolomite s,
Devon ian marine calcite (Table 1; Figure 3). Th us. in
therefore, cannot be used to c ha racter ize the
addi tio n to inorganic carbon from Midd le Devonian
depositiona l environments of the Brightho lme or the
seawater, another source of lighter carbon has been
Ratner.
incorporated with t he Brightho lme limestone. Organ ic
matte r in marine sed iments has
8 13 C va lues from
Estimated M Dev Estimated M Dev.
-10 to -20 %0 PDB (A nderson
Marine calc ite
Marine dolomite
and Arthur, 1983). T he 8 13 C
3
value of pore water from
organic-rich sediments can
2
decrease rapidly and overprint
• LWIN-DOL
1
the
8 13 C signature of the orig ina l
~ . <>o
o LWIN-LS
0
seawater.
This process is re lated
0
of o rganic carbon
to
the
release
0
al
... ...
,. BRHM-DOL
-1
into pore water fro m organic-rich
0
D.
D. BRHM-LS
sediments during decompos it ion
-2
0
!).
of organ ic matter (Anderson and
;:,!?
• RTNR-DOL
0
-3
!).
Arthur, J 983 ). T he low &13C
values
of Brightholme limestones
o RTNR-LS
..,( .) -4
can,
the
re fore, be attributed to
(-0
the mixing of inorganic c arbon
-12 -10
-8
0
-6
-4
-2
from M iddle Devonian seawater
w ith carbon derived from organic
matter. Th is is consiste nt w ith the
dark black colour of the rock and
Figure 3 - Cross plot of'f:/"O and o11C values (%0 PDB). L WJN , the L ower
high TOC values (avg. -4%.
Win11ipeg o.~is; BRIIM, Brightholme; RTNR, Ratner; LS, lim esto11e; and DOL,
do fostone (see text f or discussions).
maximum - 31 %) for the
-c
..
-
34
Summary of Investigations 2001. Volume 1
Brig htholme (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 a nd dolomite, suggesting that the
orig inal 8 180 signatures of these carbonates have been
modified and/or overprinted bl,: post-depositional
diagenetic processes. Their 8 1 0 values, therefore,
cannot be used to characterize the depositional
e nvironments of the Brightholme or the Ratner
me mbers.
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 Devonia n seawater. Postdepositional diagenetic processes, including
dol omitization, have had little effect on their or iginal
one signatures.
The o13C values of Brightholme li mestones and
dolostones are lower than those of the Ratner a nd
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 pote ntial method
to characterize and diffe rentiate the Brightholme from
th e 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 w ith matching
su pport from an NSERC IOR grant.
6. References
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cyc le and carbon isotopes; in Stable Isotopes in
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( 1997): Vadose diagenesis of the Winnipegosis
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Reinson, G.E. a nd Wardlaw, N.C. (1972):
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36
Summary of Investigations 200/, Volume I