This study analyzes detrital zircon samples from the Upper Devonian Imperial Formation and Upper Devonian-Lower Carboniferous Tuttle Formation in the northern Canadian Cordillera to better understand mid-Paleozoic sediment dispersal patterns and paleogeography. Laser ablation mass spectrometry was used to date zircons from the formations. Zircons from the Imperial Formation indicate derivation from northern Laurentia, including the Canadian Shield, Arctic Islands, and possibly Greenland. Zircons from the Tuttle Formation show a distinctive Paleoproterozoic population suggesting sediment sources shifted to predominantly Laurentian by the Mississippian.
1. 515
Detrital zircon geochronology and provenance of
Devono-Mississippian strata in the northern
Canadian Cordilleran miogeocline1,2
Yvon Lemieux, Thomas Hadlari, and Antonio Simonetti
Abstract: U–Pb ages have been determined on detrital zircons from the Upper Devonian Imperial Formation and Upper
Devonian – Lower Carboniferous Tuttle Formation of the northern Canadian Cordilleran miogeocline using laser ablation –
multicollector – inductively coupled plasma – mass spectrometry. The results provide insights into mid-Paleozoic sediment
dispersal in, and paleogeography of, the northern Canadian Cordillera. The Imperial Formation yielded a wide range of de-
trital zircon dates; one sample yielded dominant peaks at 1130, 1660, and 1860 Ma, with smaller mid-Paleozoic
(*430 Ma), Neoproterozoic, and Archean populations. The easternmost Imperial Formation sample yielded predominantly
late Neoproterozoic – Cambrian zircons between 500 and 700 Ma, with lesser Mesoproterozoic and older populations. The
age spectra suggest that the samples were largely derived from an extensive region of northwestern Laurentia, including the
Canadian Shield, igneous and sedimentary provinces of Canada’s Arctic Islands, and possibly the northern Yukon. The pres-
ence of late Neoproterozoic – Cambrian zircon, absent from the Laurentian magmatic record, indicate that a number of
grains were likely derived from an exotic source region, possibly including Baltica, Siberia, or Arctic Alaska – Chukotka. In
contrast, zircon grains from the Tuttle Formation show a well-defined middle Paleoproterozoic population with dominant
relative probability peaks between 1850 and 1950 Ma. Additional populations in the Tuttle Formation are mid-Paleozoic
(*430 Ma), Mesoproterozoic (1000–1600 Ma), and earlier Paleoproterozoic and Archean ages (>2000 Ma). These data lend
support to the hypothesis that the influx of sediments of northerly derivation that supplied the northern miogeocline in Late
Devonian time underwent an abrupt shift to a source of predominantly Laurentian affinity by the Mississippian.
´ ´ ˆ ´´ ´ ´ ´ `
Resume : Des ages U–Pb ont ete determines par spectrometrie de masse a plasma inductif avec multicollecteur apres abla-`
´ ´ ´
tion au laser sur des zircons detritiques provenant de la Formation Imperial (Devonien superieur) et de la Formation Tuttle
´ ` ´ ´ ` ´
(Devonien – Carbonifere inferieur) du miogeoclinal de la Cordillere canadienne septentrionale. Les resultats fournissent des
´ ´ ¨ ´ ´ `
apercus de la dispersion des sediments au Paleozoıque moyen et de la paleogeographie de la Cordillere canadienne septen-
¸
´ ´ ´
trionale. La Formation Imperial a donne une grande plage de dates sur des zircons detritiques; un echantillon a donne des´
` ´ ¨
pics dominants a 1130, 1660 et 1860 Ma ainsi que des populations moindres datant du Paleozoıque moyen (*430 Ma), du
´ ´ ¨ ´ ´ ` ´
Neoproterozoıque et de l’Archeen. L’echantillon le plus a l’est de la Formation Imperial a donne des zircons datant surtout
´ ´ ¨ ´
du Neoproterozoıque tardif – Cambrien, soit entre 500 et 700 Ma, avec des populations moindres datant du Mesoprotero- ´
¨ ˆ ` ´ ´
zoıque et plus anciennes. Les plages d’ages suggerent que les echantillons proviennent surtout d’une region extensive dans
le nord-ouest de la Laurentie, incluant le Bouclier canadien, les provinces ignees et sedimentaires des ˆles de l’Arctique ca-
´ ´ ı
´ ´ ´ ¨
nadien et possiblement du nord du Yukon. La presence de zircons datant du Neoproterozoıque tardif – Cambrien, lesquels
´
sont absents des donnees magmatiques laurentiennes, indique qu’un certain nombre de grains proviennent sans doute d’une
´ ´
region source exotique, possiblement de Baltica, de la Siberie ou du terrane Arctic Alaska – Chukotka. Cependant, les zir-
´ ´ ´ ¨
cons provenant de la Formation Tuttle montrent une population bien definie du Paleoproterozoıque moyen avec des pics de
´ ´ ¨
probabilite relative entre 1850 et 1950 Ma. D’autres populations dans la Formation Tuttle datent du Paleozoıque moyen
´ ´ ¨ ´ ´ ¨ ´ ´
(*430 Ma), du Mesoproterozoıque (1000–1600 Ma) ainsi que du Paleoproterozoıque inferieur et de l’Archeen (>2000 Ma).
´ ` ´ ´
Ces donnees supportent l’hypothese que l’influx de sediments provenant du Nord qui a fourni le miogeoclinal septentrional
´ ´
au Devonien tardif a subi un changement abrupt vers une source d’affinite surtout laurentienne vers le Mississippien.
´
[Traduit par la Redaction]
Received 23 November 2009. Accepted 26 May 2010. Published on the NRC Research Press Web site at cjes.nrc.ca on 9 February 2011.
Paper handled by Associate Editor W.J. Davis.
Y. Lemieux and T. Hadlari.3,4 Northwest Territories Geoscience Office, Box 1500, 4601-B, 52 Avenue, Yellowknife, NT X1A 2R3,
Canada.
A. Simonetti.5 Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, AB T6G
2E3, Canada.
1This article is one of a series of papers published in this Special Issue on the theme of Geochronology in honour of Tom Krogh.
2Northwest Territories Geoscience Office Contribution 0047. Geological Survey of Canada Contribution 20100432.
3Corresponding author (e-mail: thomas.hadlari@nrcan-rncan.gc.ca).
4Present address: Geological Survey of Canada, 3303, 33rd St. NW, Calgary, AB T2L 2A7, Canada.
5Present address: 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, IN 46556.
Can. J. Earth Sci. 48: 515–541 (2011) doi:10.1139/E10-056 Published by NRC Research Press
2. 516 Can. J. Earth Sci. Vol. 48, 2011
Introduction folded miogeoclinal strata are exposed at the mountain front.
The area preserves a relatively complete Cambrian to Dev-
Despite an increasing number of U–Pb geochronology and onian, rift to post-rift passive-margin succession that lies
Nd isotopic studies that provided new perspectives on re- with a pronounced unconformity on a thick succession of
gional patterns of sediment dispersal in northwestern Canada Proterozoic sedimentary rock (Fig. 3; Aitken et al. 1982). A
and adjacent Arctic region in Paleozoic time (e.g., McNicoll wedge of Cretaceous siliciclastic strata, interpreted to have
et al. 1995; Garzione et al. 1997; Gehrels et al. 1999; Patch- been deposited in a foreland basin setting, overlies the Pale-
ett et al. 1999; Miller et al. 2006), the tectonic setting and ozoic succession.
paleogeography during deposition of the Devono-Mississip-
In Cambrian to Middle Devonian time, the northern Cana-
pian succession in the northern Canadian Cordilleran mio-
dian Cordilleran miogeocline was a continental margin
geocline is not well understood.
marked by deposition of extensive carbonate platform and
Prior to the late Devonian, the northern Cordilleran mar- minor associated siliciclastic rocks (Mackenzie Platform;
gin was dominated by an extensive shallow-water carbonate Fritz et al. 1991), and, to the west, deeper water siliciclastic
platform thickening markedly westward toward a fine- and minor carbonate succession (Fig. 3; Pugh 1983). Detrital
grained basinal succession (e.g., Fritz et al. 1991). The plat-
zircon ages from Cambrian sandstone in east-central Alaska
form was flanked to the east by the Laurentian Precambrian
suggest provenance largely from regions of the Canadian
Shield, which provided most of the sediment for the clastic
Shield (Gehrels et al. 1999).
deposits (Gordey et al. 1991). By the late Devonian, an in-
In the late Devonian, passive-margin sedimentation was
flux of fine siliciclastic sediment blanketed the northern
interrupted by a major change in tectono-sedimentological
shelf and platform, marking a profound change in depositio-
elements with uplift of clastic sourcelands north and west of
nal regime and tectonic setting along the Cordilleran margin
the platform and influx of thick wedges of coarse and fine
(Morrow and Geldsetzer 1988). In northern Yukon and
clastic sediments (Pugh 1983; Gordey et al. 1991). The
Northwest Territories, the Middle Devonian to Early Car-
Upper Devonian Imperial Formation (Bassett 1961), mark-
boniferous Imperial Assemblage, including the Hare Indian,
ing the transition from carbonate to sand-grade siliciclastic
Canol, Imperial, Tuttle, and Ford Lake formations, was in-
deposition in the northern miogeocline (e.g., Gordey et al.
terpreted by Gordey et al. (1991) to have been derived from
1991), includes a thick sequence of marine shale, siltstone,
an uplifted region in northern Yukon. On the basis of Nd
and very fine- to fine-grained sandstone that overlies black
isotopic constraints, Garzione et al. (1997) and Patchett et
siliceous shale of the Canol Formation. Within the study
al. (1999) argued that the Imperial Assemblage was likely
derived from Ordovician to Early Carboniferous orogenic area, the Imperial Formation has been interpreted as Fras-
systems in Greenland and the Canadian Arctic, as proposed nian–Famennian shelf sandstones and basinal turbidites with
by Embry and Klovan (1976). More recently, detailed sedi- an eastward or northeastward sediment source inferred from
mentology of the Upper Devonian Imperial Formation and outcrop studies (Braman and Hills 1992), and as a west- to
Tuttle Formation (Hadlari et al. 2009) indicated derivation southwestward-prograding submarine slope and fan complex
from a northeastern and eastern source region. (Hadlari et al. 2009). To the north and west, the Imperial
Formation is composed of turbidites interpreted to have
Paleozoic sediment dispersal in the northern Cordillera
been derived largely from northern source regions (Gordey
can be better constrained with detrital zircon geochronology
et al. 1991; Braman and Hills 1992). Seventeen single zircon
data. Few U–Pb detrital zircon studies have been carried out
dates from the Imperial Formation in northwestern Yukon
in strata of the miogeocline in the northern Canadian Cordil-
(C. Garzione, G. Ross, J. Patchett, and G. Gehrels unpub-
lera (e.g., Beranek et al. 2010). In this paper, we present
lished data, discussed in Gehrels et al. 1999) indicated a
new U–Pb detrital zircon dates obtained using laser ablation
dominance of >1.8 Ga detritus, consistent with derivation
– multicollector – inductively coupled plasma – mass spec-
from Canadian Shield sources, with a subordinate population
trometry (LA–MC–ICP–MS) from the Upper Devonian Im-
of mid-Paleozoic (400–450 Ma) grains. Single zircon grains
perial Formation and Upper Devonian – Lower
Carboniferous Tuttle Formation exposed in the northern from a Late Devonian sandstone-bearing unit in east-central
Mackenzie Mountains (Figs. 1, 2). The purpose of the study Alaska yielded chiefly 430 Ma, Paleoproterozoic (>1.8 Ga),
is to constrain the provenance of sandstones within these and Archean grains, consistent with an influx of detritus, in
two units to better understand regional patterns of mid-Pale- part, from Laurentian source regions (Gehrels et al. 1999).
ozoic sediment dispersal in the northern Cordillera and to Similar results have been reported from Devonian to Car-
draw conclusions regarding paleogeography of northern boniferous sandstones of northeastern Yukon Territory (Be-
Laurentia. As our data indicate, the zircons from the Impe- ranek et al. 2010).
rial and Tuttle formations were likely derived from an ex- East of Arctic Red River (see Fig. 2 for location), Impe-
tensive region of northern Laurentia, including the rial Formation is unconformably overlain by wedge of Cre-
northwestern Canadian Shield, provinces of Canada’s Arctic taceous clastic sediments deposited in a foreland basin
Islands, and Greenland. setting (Aitken et al. 1982); west of Arctic Red River, how-
ever, the Imperial Formation is conformably overlain by the
Upper Devonian – Lower Carboniferous Tuttle Formation, a
Geological setting thick succession of alternating conglomerate, coarse- to fine-
The study area lies along the northern margin of the grained sandstone, siltstone, and shale. The contact with the
Mackenzie Mountains and encompasses the southern Peel Imperial Formation is interpreted as a facies boundary and,
Plateau and Plain (Peel Region) of the northern Interior therefore, is diachronous (Pugh 1983). Hills and Braman
Plains (Figs. 2); it occupies a region where imbricated and (1978) and Braman and Hills (1992) interpreted the Tuttle
Published by NRC Research Press
3. Lemieux et al. 517
Fig. 1. Tectonic assemblage map of Yukon Territory, Northwest Territories and Nunavut showing location of study area and Fig. 2. Geol-
ogy after Wheeler et al. (1996), paleocurrent data from Embry and Klovan (1976) and Hadlari et al. (2009).
Formation as a southward-advancing turbidite succession. In The sub-Cretaceous unconformity marks a hiatus in the
contrast, Pugh (1983) viewed the unit as a deltaic depositio- sedimentary record as Late Carboniferous to Jurassic strata
nal system and interpreted the shale-out to the west as indi- are absent from the northern Interior Plains. The end of con-
cating southwest-prograding deposition despite a progressive tinental margin sedimentation and beginning of widespread
southward decrease in grain size and trend to better sorting. compressional deformation in the northern Cordillera in
By the mid-Mississippian, marine clastic and carbonate dep- Mesozoic time (e.g., Berman et al. 2007) influenced the de-
osition with sediment derivation from the craton to the east velopment of foreland basins adjacent to the mountain front
was re-established (Gordey et al. 1991). (Dixon 1999). Clastic sedimentation in the northern Interior
Published by NRC Research Press
4. Fig. 2. Simplified geological map (modified after Hadlari et al. 2009) of southern Peel Plateau and Peel Plain, and the northern Mackenzie Mountains showing seismic line traces and
518
clinoform progradation directions, turbidite paleocurrents, and locations of the detrital zircon samples. Geology after Wheeler et al. (1996).
Published by NRC Research Press
Can. J. Earth Sci. Vol. 48, 2011
5. Lemieux et al. 519
Fig. 3. Schematic stratigraphic section for the southern Peel Plateau and Peel Plain and northern Mackenzie Mountains. Modified after
Morrow et al. (2006).
Published by NRC Research Press
6. 520 Can. J. Earth Sci. Vol. 48, 2011
Plains in the Late Cretaceous was controlled largely by, and large enough (i.e., > 40 mm across, see as follows) for laser
derived from, the active Cordillera to the south and west ablation analyses. In contrast, the much finer grained Impe-
(Dixon 1999). rial samples yielded a smaller fraction of zircons that were
sufficiently large for analysis. Selected grains were, as
U–Pb geochronology much as possible, free of fractures, inclusions, and altera-
tion. U–Pb geochronology of zircons was conducted by
Sample description and analytical procedures LA–MC–ICP–MS at the Radiogenic Isotope Facility at the
This study presents U–Pb geochronological results for University of Alberta, Edmonton, Alberta, using analytical
four samples from the Devono-Mississippian clastic succes- procedures described by Simonetti et al. (2005). The analy-
sion in the Peel Region, two from the Imperial Formation ses involved ablation of zircons using a 40 mm diameter la-
(samples 07TH33B and 07WZ020A), and two from the Tut- ser spot size for 30 s. A ‘‘standard-sample-standard’’ method
tle Formation (samples 07WZ019A and 06YHL046B); their was used to correct instrumental drift during a single laser
geographic locations are shown in Fig. 2 and given in ablation session and involved analysis of an internal stand-
Table 1. ard after every 12 unknown grains; this protocol was devel-
Sample 07TH33B was collected at the type section of Im- oped for provenance studies focusing on the dating of a
perial Formation (Fig. 2). Located near the eastern erosional large number of detrital zircon grains (Simonetti et al.
edge of Imperial Formation, the type section preserves the 2005) The collector configuration allows for the simultane-
oldest strata of Imperial Formation, which were deposited ous measurement of ion signals ranging in mass from 238U
as a shallow shelf-like accumulation of sediment that pro- to 203Tl. Periodically, a 30 s blank measurement was per-
graded southwestward into a generally westward-deepening formed, which included correction for the 204Hg contribu-
basin (Hadlari et al. 2009). The sampled interval consists of tion; ion-counter bias was also determined using a mixed
fine-grained cross-stratified sandstone from the locally de- solution of Pb and Tl. Common Pb correction was applied
veloped shallow-marine facies. using an initial Pb composition taken from Stacey and
West of Imperial River, the Imperial Formation is inter- Kramers (1975).
preted as a succession of submarine fan and slope sand-
stones and shales exceeding 500 m in thickness that, based Results of U–Pb analysis
upon paleocurrent and seismic data, are interpreted to have The results are presented in Table 1 (with uncertainties at
been deposited by a system that prograded in a west-south- the 2s level) and shown in relative age–probability diagrams
west direction (Hadlari et al. 2009). Tuttle Formation is gen- (from Ludwig 2003) in Fig. 5. The diagrams present the sum
erally Famennian to Tournasian in age (Allen et al. 2009), of all ages from a sample as a normal distribution based on
overlies the Imperial Formation, is partly defined by me- the age and uncertainty of each analysis, the areas under
dium sand and coarser grain sizes, and represents a rejuve- each curve are equal. Interpretations for <1000 Ma grains
nation of sand-grade siliciclastic input to the basin from the are based on 206Pb/238U ages, which yield more precise re-
northeast (Hadlari et al. 2009). Based upon the depositional sults given the low concentration of 207Pb in younger zir-
system, the three remaining samples are significantly cons. For grains >1000 Ma, analyses are based on 207Pb/
younger than 07TH33B because they are located over 206Pb ages. To reduce the effect of discordance, possibly re-
100 km west of Imperial River. Considering their mutual sulting from isotopic disturbance and (or) inheritance, analy-
proximity, we have placed the three western samples in or- ses that are >5% discordant or >5% reverse discordant
der by the stratigraphic level that was sampled within the (italics in Table 1) have been excluded from further consid-
combined Imperial–Tuttle formation section. Sample eration.
07WZ020A, a massive fine-grained sandstone, was collected A total of 67 zircons were analyzed from sample
a few metres above the Canol–Imperial contact along a trib- 07TH33B (Imperial Formation), of which 54 were consid-
utary of the Snake River near the western edge of the study ered (Fig. 5; Table 1). A significant number of zircons
area. From the middle part of the section, sample yielded age clusters between 380 and 711 Ma (n = 26),
06YHL046B was collected from a *150 m high bluff of with peaks evident at 390, 555, and 670 Ma. Twenty-five
conspicuously hard and massive, medium- to coarse-grained, zircons yielded Proterozoic dates, with the most dominant
quartz-rich sandstone sharply overlying shale and siltstone peaks at 1100, 1350, 1670, and 1950 Ma; only one zircon
west of Cranswick River (Fig. 4). Although the bluff was falls in the interval 2100–2500 Ma. Three Archean zircons
mapped as Imperial Formation by Norris (1982), the grain were documented (i.e., 2625, 2796, and 2820 Ma).
size is clearly atypical of Imperial Formation, and is inter- Ninety-one zircons extracted from the westernmost sam-
preted here as part of Tuttle Formation. Sample 07WZ019A ple of the Imperial Formation (07WZ020A) were analyzed,
is conglomerate of Tuttle Formation from the top of the Im- of which 61 are within 5% of the concordia. This sample
perial–Tuttle section at Flyaway Creek; the section exposes has a dominant zircon age population (n = 46) between
a thin interval, *50 m thick, of sandstone and conglomerate *1000 and 2100 Ma, with major peaks at 1130, 1660, and
overlying *600 m in thickness of Imperial Formation (Ha- 1860 Ma, and subordinate age groups at 1300–1550, 1730–
dlari et al. 2009). The entire sample of quartz clast conglom- 1800, 1900–2100 Ma. The sample also includes (i) four
erate with sandstone matrix was crushed for analysis. lower Paleozoic zircons at 424, 432, 438, and 505 Ma; (ii)
Each sample yielded euhedral to anhedral, colourless to four Neoproterozoic grains at 570, 597, 645, and 855 Ma;
pink or yellow, generally well-rounded zircons consistent and (iii) seven Archean zircons between 2614 and 2806 Ma.
with a detrital origin. Samples from the Tuttle Formation There is a gap between 2100 and 2600 Ma.
were sufficiently coarse grained to yield abundant zircons Ninety-one zircon were analyzed from sample
Published by NRC Research Press
7. Lemieux et al. 521
Fig. 4. (a) Field photograph showing the contact between the Imperial Formation and overlying Tuttle Formation in the Cranswick River
area. The bluff (in the middle ground) was mapped as Imperial Formation by Norris (1982). View to northeast; field of view in middle
ground is *4 km. (b) Close-up of approximate contact (dashed line) between the Imperial and Tuttle formations. Geologist for scale. See
Fig. 2 for location of photographs.
06YHL046B (Tuttle Formation); 18 zircon have not been 1512 Ma (n = 2), and 1597–1731 Ma (n = 5). One grain
considered further. A 440 Ma Silurian-age peak is evident, yielded a Neoproterozoic date (940 Ma).
represented by seven grains between 428 and 444 Ma. The A total of 112 zircons were analyzed from sample
sample yielded 45 grains between *1800 and 2800 Ma, 07WZ019A (Tuttle Formation), of which 99 were consid-
with dominant age peaks at 1860 and 2780 Ma, and subordi- ered. A 1937 Ma Paleoproterozoic age peak is evident in
nate age groups between *2000 and 2700 Ma. Twenty zir- the zircon population, with 50 grains falling in the interval
cons fall in the intervals 1038–1366 Ma (n = 13), 1494– 1792–2000 Ma; a subordinate age cluster occurs at 430 Ma,
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