Carbon Isotope Chemostratigraphy of Frasnian Sequences in
Western Canada
C. Holmden 1,2, W.K. Braun 2, W.P. Patterson 1,2, B.M. Eglington 1,2, T.C. Prokopiuk 1,2, and S. Whittaker
Holmden, C., Braun, W.K., Patterson, W.P., Eglington, B.M., Prokopiuk, T.C., and Whittaker, S. (2006): Carbon isotope
chemostratigraphy of Frasnian sequences in western Canada; in Summary of Investigations 2006, Volume 1, Saskatchewan
Geological Survey, Sask. Industry Resources, Misc. Rep. 2006-4.1, CD-ROM, Paper A-8, 6p.
Abstract
We present a δ C profile for the Frasnian succession of the eastern part of the Western Canada Sedimentary Basin
as a tool for proposing and justifying regional stratigraphic correlations. Eight positive δ 13C excursions are
identified that permit detailed correlations of Frasnian sequences for these eastern areas. In particular, this
composite δ 13C profile may be used to compare ‘restricted’ and sparsely fossiliferous, or non-fossiliferous
Frasnian deposits of Saskatchewan with more open-marine deposits of eastern Alberta and the southwestern Great
Slave Lake region. Chemostratigraphic correlations were found to agree with those previously inferred on the basis
of ostracode biostratigraphy. This includes the postulation that, in Saskatchewan, a major regional unconformity
caused the omission of Cooking Lake–Leduc oil-bearing strata, which explains the absence of a major positive δ 13C
excursion that is prominent in northeastern Alberta. The most probable explanation is that the Leduc reefs and
associated carbonate platform in Alberta formed during a time of decreasing sea level, resulting in no sediment
deposition and/or possible subaerial exposure and erosion of Leduc-equivalent strata in the shallower water
environment of Saskatchewan.
13
Keywords: carbon isotopes, chemostratigraphy, Devonian, Frasnian, ostracodes, correlation, sea level.
1. Introduction
Carbon isotope profiles reconstructed from epeiric sea carbonates have been demonstrated to be useful tools for
stratigraphic correlation and interpretation. Correlation between stratigraphic sections is based on matching the
peaks and troughs in δ13C profiles that are assumed to represent globally synchronous events driven by
perturbations in the ocean C-cycle. Efficient mixing of carbon within the ocean surface layer and atmosphere, and
its rapid exchange across the sea-air interface, will ensure that any changes in the carbon isotope balance in the
oceans is quickly propagated globally through the ocean-atmosphere system, and recorded in the sedimentary
deposits of oceans and epeiric seas.
Findings over the past decade have, for several reasons, raised questions about the original premise that carbon
isotope excursions are synchronous across all contemporaneous marine reservoirs (Patterson and Walter, 1994;
Holmden et al., 1998; Immenhauser et al., 2002, 2003; Noble et al., 2005; Panchuk et al., 2005a, 2005b; Melchin
and Holmden, 2006). First, local-scale C-cycling processes in epeiric seas may overprint the contemporaneous
ocean carbon signature. This would create dissonance between local and global secular records that makes it
difficult to correlate δ13C profiles based on frequency and magnitude of the excursions. Second, because sea level
has been identified as a driver of perturbations in local C-cycling and because sea-level change is itself a ubiquitous
feature of stratigraphic architecture, chemostratigraphic records of secular variation in δ13C may simply track
lithological changes. Consequently, the local interplay between subsidence and global eustacy will create different
secular records of relative sea-level change across epeiric seas that, in turn, may result in variably shaped profiles of
δ13C stratigraphic changes (Noble et al., 2005; Panchuk et al., 2005a, 2005b; Melchin and Holmden, 2006). For
example, the frequency and magnitude of δ13C excursions in the stratigraphic record may be greater in proximal
settings of basins compared to their distal ones because of the greater sensitivity of shallower water environments to
sea-level–driven perturbations of local C-cycling (e.g., Fanton and Holmden, 2001). Other complicating factors
include the role of changing sedimentation rates and cryptic unconformities in shaping δ13C profiles, and the
presence of δ13C gradients across epeiric seas that affect baselines and peak magnitude values of stratigraphically
equivalent δ13C excursions (Holmden et al., 1998; Fanton and Holmden, 2001; Ludvigson et al., 2004; Young et al.,
2005). Carbon isotope excursions may pinch out laterally on disconformity surfaces (Ludvigson et al., 2004), or
1
2
Saskatchewan Isotope Laboratory, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2.
Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, SK S7N 5E2; E-mails:
[email protected]; [email protected]; [email protected]; [email protected].
Saskatchewan Geological Survey
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Summary of Investigations 2006, Volume 1
may not be preserved due to sediment nondeposition or subaerial erosion. In addition, modeling of the epeiric sea
C-cycle itself has led to some surprising predictions. For instance, it was recently shown that a periodic sea-level
forcing can induce local C-cycle shifts across an epeiric sea shelf (from shallower to deeper environments) that are
out of phase with each other such that a positive δ13C excursion in the shelf proximal setting might actually correlate
in time with a negative one in its distal counterpart (Panchuk et al., 2005a).
Without further empirical evaluations, it is therefore difficult to judge the degree to which complications such as
these may compromise applications of carbon isotope chemostratigraphy. Alternatively, it is conceivable that
strong, local C-cycling may actually improve regional stratigraphic correlations within the same basin because
local-scale C-cycle complexity may translate into more detailed secular records that can be correlated with higher
precision.
way
a
e
S
Pale
o -e
qu
ato
r
We evaluate this idea by reconstructing a high-resolution δ13C profile from open-marine Frasnian deposits along the
eastern margin of the Western Canadian Devonian Seaway (Figure 1). The composite profile was developed from
stratigraphic sections in the Northwest Territories (southwestern Great Slave Lake area), northeastern Alberta, and
central Saskatchewan. In this profile (Braun, 1968; Braun and Mathison, 1982), the most ‘fully open-marine’
sections are Hay River in the northwest Great Slave Lake region and the Bear Biltmore section in northeast Alberta;
the Duval section in central Saskatchewan represents
the most ‘restricted’ marine setting. The continuous
Bear Biltmore core drilled in northeastern Alberta was
chosen as the lower to mid-Frasnian subsurface
reference section for this study. This core provides a
lithostratigraphic type section for the interval
encompassing the Waterways, Cooking Lake, and
Ireton formations. These strata were deposited in
relatively open-marine conditions and contain diverse
macro- and microfaunal assemblages. The δ13C profile
for this core is exceptionally detailed with at least eight
δ13C excursions that may be used as a basis for
comparisons and correlations throughout the eastern
region of the Western Canadian Sedimentary Basin.
Great Slave
Samples from outcrop sections of open-marine and
Lake
fossiliferous ‘Hay River Shales’ exposed along the Hay
NUNAVUT
60
River before it empties into southwestern Great Slave
Hay River
YUKON
NWT
Lake and from the nearby Frobisher #4 subsurface core
Bear Biltmore
provide some data for the mid- to upper Frasnian part
of the profile not represented at Biltmore.
MANITOBA
BRITISH
COLUMBIA
ALBERTA
50
CANADA
USA
WASHINGTON
SASKATCHEWAN
Devonian
The correlation potential of the δ13C reference profile is
then tested in relation to the ostracode biostratigraphy
using two additional sections. To the south, a
continuous profile of lower to upper Frasnian strata
could be obtained from the Duval Potash Mine of
central Saskatchewan covering the poorly fossiliferous,
sometimes evaporitic Souris River Formation, the
variably ‘restricted’ Duperow, and the more openmarine Birdbear sequences. To the north, a profile was
reconstructed from the open-marine deposits of the Hay
River Shales of the southwest Great Slave Lake region.
IDAHO
Duval
Potash
Mine
NORTH
DAKOTA
MONTANA
WYOMING
OREGON
2. Sampling and Analytical Procedures
40
The Bear Biltmore reference section (7-11-87-17W4) is
a continuous, large-diameter core stored at the Core
Research Centre in Calgary, Alberta. Small aliquots of
powdered sample were collected from the core using a
power drill equipped with a bone-crusher bit. This
sampling strategy is relatively non-destructive to the
core, leaving only a 1 cm diameter hole that also serves
as a marker for follow-up investigations. Carbonate
samples from the Hay River outcrops were obtained
from previously collected microfossil samples,
supplemented by the Frobisher #4 core taken from the
Saskatchewan Geological Survey
CALIFORNIA
NEVADA
0
500
kilometres
UTAH
COLORADO
1000
120
110
Figure 1 - Map showing the outline of the Devonian Seaway
in western Canada and the locations of the three Frasnian
sections for which carbon isotope profiles were
reconstructed. Modified from Braun (2001); position of the
paleoequator is from Heckel and Witzke (1979).
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Summary of Investigations 2006, Volume 1
same outcrop sections. Each sample represents material taken over approximately three metres (ten feet). Samples
collected during the sinking of the shaft for the Duval Potash Mine near Saskatoon, Saskatchewan, and from a core
taken in the vicinity of the shaft (Price and Ball, 1971) were used to reconstruct the Duval profile.
Carbonate samples were powdered and reacted at 70°C using a Kiel III carbonate device connected to a Finnigan
253 isotope ratio mass spectrometer. Each set of six samples was bracketed by an internal calcite standard calibrated
against the (NIST) NBS-19 standard. Conversion to the VPDB scale was performed using the values -2.20‰ and
1.95‰ for δ18O and δ13C, respectively, for (NIST) NBS-19. The uncertainty (±1σ) is based on the reproducibility of
NBS-19, which is 0.10‰ for δ18O and 0.05‰ for δ13C.
3. Results
The reference δ13C profile reconstructed from the Biltmore core yielded eight positive δ13C excursions (Figure 2).
The bulk of the Frasnian succession is covered by this profile, with the exception of the upper strata with the
Frasnian-Famennian boundary for which the Hay River outcrops serve as a reference. The eight positive δ13C
excursions are clearly resolvable against the baseline δ13C trend with peak-magnitude values ranging from 1 to 5‰.
The ostracode zonal boundaries are also shown (DFR 1 to DFR 5) within the lithostratigraphic framework.
Positive δ13C excursions in the Biltmore core are associated with lithological transitions that mark formation and
member boundaries, and with the ostracode-microfaunal zonal boundaries. This implies that the secular record of
δ13C fluctuations is tracking paleoenvironmental changes that also influenced litho- and biofacies. In the Waterways
Formation, the multiplex excursion 2 begins at the very base of the Calumet Member, and the base of the Christina
marks the transition between excursions 2b and 2c, also coincident with the DFR 1–DFR 2 ostracode zonal
boundary. The boundary separating the lower and upper DFR 2 subzones is coincident with the base of excursion 5.
The DFR 2–DFR 3 change occurs at the Waterways–Cooking Lake boundary marking the onset of excursion 7,
which itself has two prominent peaks (7a and 7b). The separation between these peaks occurs precisely at the
subzonal boundary separating the lower and upper assemblages of the DFR 3 ostracode zone. The lithology hosting
the 7a excursion is crinoidal limestone, characteristic of the Cooking Lake platform and 7b is stratigraphically
equivalent to the lower half, approximately, of the ‘Ireton’ shales. The last positive excursion 8 coincides with the
transition from the ‘Ireton’ shales to the Grosmont Dolomite following a distinctive negative δ13C interval starting at
about the DFR3–DFR4 boundary, reminiscent of the Moberly-Mildred negative deflection and low δ13C values that
characterize most of the upper Waterways sequence.
Some of the larger Biltmore excursions are replicated in the outcrop sections along Hay River in the Northwest
Territories and the Frobisher #4 core from the same area. The chemostratigraphy is not as detailed in the Hay River
succession, however, due to a coarser sampling grid, and much of the detailed structure observed in the δ13C profile
of the Biltmore core is therefore not observed. In addition, there is a covered section at the base of the Hay River
outcrop which precludes the possibility of making a seamless reconstruction.
Six positive δ13C excursions in the reference section of the Waterways Formation in eastern Alberta are correlative
with excursions in the Souris River Formation of central Saskatchewan, despite the fact that the seaway over
Saskatchewan was more restricted and prone to evaporation, often reaching saturation with respect to anhydrite. The
uppermost anhydrite in the Duval section was deposited in the thin (~23 m) and largely non-fossiliferous Saskatoon
Member just below a reddish oxidized zone that may represent a subaerial exposure surface (Price and Ball, 1971).
This suggests that the sea in Saskatchewan, which had been restricted and was probably very shallow throughout
most of the early Frasnian, temporarily drained away. Indeed, most of the DFR 3 ostracodes are missing in the
Duval section (Braun and Mathison, 1982).
In order to compare the Duval δ13C profile with the Biltmore reference core, the Saskatchewan section was divided
at the exposure surface noted by Price and Ball (1971). The lower and upper Duval sections are then aligned using
the DFR 2–DFR 3 and DFR 3–DFR 4 ostracode zonal boundaries, respectively. Using this reconstruction, it is clear
that most of excursion 7 is missing in the Duval profile, supporting the ostracode evidence for a regional
unconformity associated with the Saskatoon Member. Only the rise of excursion 7a and the fall of excursion 7b are
recorded in central Saskatchewan.
Although there is a relatively strong relationship between lithological contacts and the δ13C chemostratigraphy in
the Duval shaft section, the lower resolution of the profile precludes a more detailed analysis at this time. We did
not find any consistent relationship between the δ13C excursions and the anhydrite layers.
Saskatchewan Geological Survey
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Palmatolepis gigas Zone
Ancyrognathus triangularis Zone
Mesotaxis asymmetrica Zone
Figure 2 - Carbon isotope profiles for three Frasnian sections from the eastern part of the Devonian Seaway in western
Canada. Prominent positive δ 13C excursions are labelled 1 to 9. Excursion 9 appears only in the uppermost Hay River
section, as sediments of this age are absent in the other sections. Some excursions with multiple peaks are divided further
using letter designations a, b, and c. The placement of the ostracode assemblage zones (denoted DFR) is discussed in Braun
and Mathison (1982). The DFR 2–DFR 3 boundary is used as a datum for aligning the three sections. The Duval section is
divided at an unconformity in the Saskatoon Member so that the upper part of the Duval may also be aligned to the Biltmore
section using the DFR 3–DFR 4 ostracode zonal boundary. Prominent positive δ13C excursions are interpreted to reflect
regional sea-level regression, whereas lower δ 13C values defining ‘troughs’ in the carbon isotope trend reflect more ‘openmarine’ conditions consistent with transgressions. In the Biltmore profile, the prominent transgressive and regressive
intervals are denoted T and R, respectively. The abrupt negative spike in δ 13C values between peaks 7a and 7b is interpreted
to represent an abrupt decrease in sea level and potential subaerial exposure. Lower δ 13C values occur along with lower δ 18O
values for whole rock carbonates (unpublished data), and together these findings are interpreted to reflect the growth of new
calcite and the alteration of pre-existing marine calcite, in the presence of fresh to brackish waters in a marginal-marine
setting that also experienced episodes of subaerial exposure and pedogenesis. By analogy, the negative spikes punctuating
excursion 2 may also reflect rapid shallowing and subaerial exposure events occurring between peaks 2a/2b and 2b/2c.
Abbreviations: Fm., Formation; Mbr., Member; Alex., Alexandra; Cook. L., Cooking Lake; Calum., Calumet; Chris.,
Christina; and Sewa., Seward.
4. Discussion and Implications
The highly resolved δ13C stratigraphy of the Biltmore core with its eight positive δ13C excursions serves as a
benchmark for testing and advancing regional correlations based on profile matching. The greatest strength of the
technique, however, lies in correlating sedimentary strata and comparing depositional settings where fossils are
either absent, rare, non-diagnostic or unstudied. This is notably the case with the Souris River Formation of
Saskatchewan where ostracodes and other index fossils are mostly absent. Yet it is observed that all of the peaks in
the reference δ13C profile from the Waterways Formation of Alberta have counterparts in the Souris River profile of
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Summary of Investigations 2006, Volume 1
Saskatchewan. This also implies relatively continuous deposition during the Souris River interval even though the
presence of numerous anhydrite units points to possible periods of desiccation and interruptions to sediment
deposition. The first major hiatus evidently did not occur until the end of Souris River deposition, in the Saskatoon
Member, as indicated by the omission of most of the DFR 3 ostracode zone and excursion 7 of the δ13C reference
profile. Extending the profile-matching technique, the Firebag would then be equivalent to the lower Davidson, and
the Calumet and Christina to the upper Davidson. The DFR 1–DFR 2 ostracode boundary would be located in the
upper Davidson strata (just below the Davidson evaporite) if microfossils could be found. The upper/lower DFR 2
assemblage change would be at, or close to, the upper Harris boundary and the Hatfield section would be equivalent
to most of the Moberly and Mildred members in terms of the Alberta section.
The Saskatoon ‘unit’ in the Duval section is seemingly highly condensed and possibly represents the entire Cooking
Lake platform, Leduc reefs, and lower part of the ‘Ireton Shales’ as reconstructed by Braun and Mathison (1982) on
the basis of microfaunal correlations. This implies that the ‘Leduc interval’ is missing in Saskatchewan as a result of
non-deposition or erosion. This interpretation is supported by the carbon isotope chemostratigraphy and the
omission of excursion 7. The most plausible explanation is that the ‘reefs’ formed during a major regional sea-level
drop when the seabed in Saskatchewan was exposed, thereby excluding Saskatchewan from the development of one
of the richest oil reservoirs in the Western Canada Sedimentary Basin.
The C-cycle perturbation implied by excursion 7 may be related to a sea-level drop that would have exposed vast
quantities of carbonate sediment to subaerial weathering (Noble et al., 2005; Melchin and Holmden, 2006), thus
shifting the δ13C values of the terrestrial dissolved inorganic carbon weathering flux in the mid-Frasnian seaway to
much higher levels. This, in turn, would have forced the isotope balance of carbon in seaway waters towards higher
values if all other fluxes of carbon remained the same. Alternatively, increased organic carbon burial during the
lowstand may have also driven δ13C values to higher values, but this hypothesis is difficult to evaluate without first
documenting the stratigraphic patterns of organic carbon distribution across the entire Devonian Seaway of Western
Canada.
A preliminary analysis of sea-level variations in the Biltmore reference section indicates that positive δ13C
excursions are coincident with lower sea levels and that (negative) troughs in the δ13C profile reflect times of
relatively higher sea levels. The abrupt negative spikes in δ13C values that occur between excursions 7a and 7b,
however, are interpreted as extreme sea-level lowstands associated with potential subaerial exposure and
pedogenesis. If this is correct, negative δ13C spikes reflect carbonates that formed, or were altered, in the presence
of brackish or freshwater fluids with low δ13C values. Lower δ18O carbonate values (unpubl. data) indicative of
contemporaneous meteoric waters support this interpretation. We predict, therefore, that high sea levels were
characteristic for the late Waterways (Moberly and Mildred members) and late Ireton times, and that lower sea
levels characterized most of the early Waterways, Cooking Lake, and early Ireton. It is not known, however,
whether the inferred sea-level changes were local or global in nature.
5. Conclusions
A detailed chemostratigraphic (δ13C) reference profile for Frasnian strata has been reconstructed to aid correlations
and stratigraphic comparisons for the eastern parts of the Western Devonian Seaway. The reference profile contains
eight prominent δ13C excursions. The curve is particularly useful for testing correlations between open- and
restricted-marine deposits such as in the Frasnian deposits of Saskatchewan that were mostly deposited in restricted
settings where diagnostic fossils may be absent or rare. Improved local and regional correlations across the
subsurface of Saskatchewan are now possible with the help of the δ13C reference profile, thereby providing a better
assessment of the similarities and differences with Alberta and other regions of western Canada.
While our method works well for the eastern region of the Devonian Seaway, correlations and comparisons have yet
to be established across the prairie provinces, and from Biltmore to the Rocky Mountains. Several biofacies
observations point to different paleoenvironmental settings, and future studies will be concentrated on
reconstructing several δ13C profiles for central Alberta (Edmonton region) and the Rocky Mountains (Jasper Park
region). Included – and with particular emphasis – would be the attempt to decipher the ‘Leduc reef enigma’ of
western Canada and to separate worldwide sea-level changes from the known regional and local ones.
6. References
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Oswald, D.H. (ed.), International Symposium on the Devonian System, Volume 2, Calgary, Alta. Soc. Petrol.
Geol., p617-652.
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__________ (2001): Modes of extinction of two Devonian Ostracode faunas of western Canada; Can. J. Earth Sci.,
v38, p173-185.
Braun W.K. and Mathison, T.F. (1982): Ostracodes as a correlation tool in Devonian studies of Saskatchewan and
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Publ. #6, p43-49.
Fanton, K.C. and Holmden, C. (2001): Sea level forcing of Late Ordovician carbon isotope excursions; Geol. Soc.
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Heckel, P.H. and Witzke, B.J. (1979): Devonian World Paleogeography determined from distribution of carbonates
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Devonian System – a Paleontological Association International Symposium, Paleontological Association,
London, Spec. Pap. Paleont. 23, p99-123.
Holmden, C., Creaser, R.A., Muehlenbachs, K., Leslie, S.A., and Bergström, S.M. (1998): Isotopic evidence for
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Immenhauser, A., Kenter, J.A.M., Ganssen, G., Bahamonde, J.R., Vliet, V.A., and Saher, M.H. (2002): Origin and
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in shallow-marine carbonate δ13C and δ18O; Sediment., v50, p1-7.
Ludvigson, G.A., Witzke, B.J., González, L.A., Carpenter, S.J., Schneider, C.L., and Hasiuk, F. (2004): Late
Ordovician (Turinian-Chatfieldian) carbon isotope excursions and their stratigraphic and paleoceanographic
significance; Palaeogeog. Palaeoclim. Palaeoecol., v210, p187-214.
Melchin, M.J. and Holmden, C. (2006): Carbon isotope chemostratigraphy in Arctic Canada: Sea level forcing of
carbonate platform weathering and implications for Hirnantian global correlation; Palaeogeog. Palaeoclim.
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Noble, P.J., Zimmerman, M.K., Holmden, C., and Lenz, A.C. (2005): Early Silurian (Wenlockian) δ13C profiles
from the Cape Phillips Formation, Arctic Canada and its relation to biotic events; Can. J. Earth Sci., v42,
p1419-1430.
Panchuk, K.M., Holmden, C., and Kump, L.R. (2005a): Sensitivity of the epeiric sea carbon isotope record to localscale carbon cycle processes: Tales from the Mohawkian Sea; Palaeogeog. Palaeoclim. Palaeoecol., v228,
p320-337.
Panchuk, K.M., Holmden, C., and Leslie, S. (2005b): Local controls on carbon cycling in the Midcontinent Region
of North America with implications for carbon isotope secular curves; J. Sed. Resear., v76, p200-211.
Patterson, W.P. and Walter, L.M. (1994): Depletion of 13C in seawater ΣCO2 on modern carbonate platforms:
Significance for the carbon isotopic record of carbonates; Geol., v22, p885-888.
Price, L.L. and Ball, N.L. (1971): Stratigraphy of Duval Corporation Potash Shaft No. 1, Saskatoon, Saskatchewan;
Geol. Surv. Can., Pap. 70-71, 107p.
Young S.A., Saltzman, M.R., and Bergstrom, S.M. (2005): Upper Ordovician (Mohawkian) carbon isotope (δ13C)
stratigraphy in eastern and central North America: Regional expression of a perturbation of the global carbon
cycle; Palaeogeog. Palaeoclim. Palaeoecol., v222, p53-76.
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