Geological Society of America Bulletindglg/Web/Claeys/pdf/De_Vleeschouwer-2012.pdf · 1.0 m. Magnetic susceptibility sampling proce-dures entailed avoiding weathering rinds or rock

Geological Society of America Bulletin

doi: 10.1130/B30547.1 2012;124, no. 5-6;928-942Geological Society of America Bulletin

David De Vleeschouwer, Michael T. Whalen, James E. (Jed) Day and Philippe Claeys (western Alberta, Canada)Cyclostratigraphic calibration of the Frasnian (Late Devonian) time scale

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928

Cyclostratigraphic calibration of the Frasnian (Late Devonian) time scale (western Alberta, Canada)

David De Vleeschouwer1,†, Michael T. Whalen2, James E. (Jed) Day3, and Philippe Claeys1

1Earth System Sciences and Department of Geology, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium2Department of Geology and Geophysics, University of Alaska, Fairbanks, Alaska 99775-5780, USA3Department of Geography-Geology, Illinois State University, Normal, Illinois 61790-4400, USA

ABSTRACT

Until now, the duration of the Frasnian Stage has remained very poorly constrained, hampering a detailed understanding of sedi-mentation processes and environmental and evolutionary change. In this study, time-series analyses of high-resolution (10–20 k.y.) magnetic susceptibility data identify sixteen 405 k.y. eccentricity cycles in the magnetic sus-ceptibility stratigraphy of the Frasnian (Late Devonian), derived from carbonate-platform and surrounding slope and basin deposits in western Alberta, Canada. Previous studies demonstrated the generally consistent pattern of magnetic susceptibility change across the Alberta basin and thus demonstrated the util-ity of magnetic susceptibility stratigraphy as a refi ned regional correlation tool compared to biostratigraphy. In the present study, we show that the magnetic susceptibility stratigraphy of the Frasnian interval in western Alberta has been signifi cantly infl uenced by astro nomical forcing. Using the sixteen 405 k.y. eccentricity cycles as a geochronometer, we constructed a Frasnian astronomical time scale. This time scale indicates a duration of 6.5 ± 0.4 m.y. for the Frasnian. Calibrating this duration to the best available Devonian chronology, the abso-lute age of the Givetian-Frasnian boundary is recalculated to 383.6 ± 3.0 Ma, and the age of the Frasnian-Famennian boundary is recalcu-lated to 376.7 ± 3.0 Ma. These new absolute ages take into account the astronomically de-rived duration of the Frasnian, but they also yield a narrowing of the error margins of the absolute ages by several hundreds of thou-sands of years.

INTRODUCTION

The greenhouse Devonian Period (ca. 418–361 Ma; Kaufmann, 2006) is a key interval in the evolution of life on Earth. Plants radiated

rapidly and colonized numerous terrestrial habitats (Algeo and Scheckler, 1998). The fi sh-tetrapod transition occurred during the early Middle Devonian (Eifelian Stage; Clack, 2009; Niedźwiedzki et al., 2010). Long et al. (2009, 2008) demonstrated that the origin of internal fertilization and live-birth in vertebrates prob-ably took place during the Frasnian. One of the “Big Five” mass extinctions in Earth history occurred just prior to the Frasnian-Famennian boundary (Late Devonian), and extinction rates appear to have remained high through the Devo-nian-Carboniferous boundary (McGhee, 1996). Rapid climatic changes, with potential short-term glaciations, and anoxic events occurred repeatedly from the Givetian to the Famennian (Algeo and Scheckler, 1998; Lüning et al., 2003, 2004; Streel et al., 2000). Moreover, the Devo-nian System of Euramerica records at least 14 transgressive-regressive cycles of eustatic origin (Johnson et al., 1985). Several extraterrestrial impact events occurred during the middle and late Frasnian and early Famennian (Claeys and Casier, 1994; Claeys et al., 1992; Wang, 1992; Warme et al., 2002). Two distinct Late Devonian sequences of fl ood basalt volcanism are reported (Courtillot and Renne, 2003). Understanding and correlation of these events require high-reso-lution stratigraphy and dating as well as well-defi ned stage and substage duration estimates. Unfortunately, a robust high-resolution absolute time scale is not yet available (error bars on Devonian stage boundaries vary between ±2.6 and ±3.8 m.y.), and the existing conodont bio-stratigraphy, with a varying resolution between several hundred thousand to several million years (Kaufmann, 2006), is not fully capable of resolving these events. Besides chrono- and biostratigraphy, a cyclostratigraphic approach offers the opportunity to refi ne the Devonian chronology. Cyclostratigraphy has been shown to result in the most precise time scales available in the Neogene (Lourens et al., 2004) and has been extensively applied within the Cenozoic and Mesozoic (e.g., Meyers, 2008). Only few astrochronological studies address Paleozoic

greenhouse periods such as the Devonian (Chen and Tucker, 2003; Ellwood et al., 2011). How-ever, provided a thoughtful study site selection, the application of cyclostratigraphic methods in the Paleozoic can lead to signifi cant refi nements of the geological time scale. The present study provides valuable improvements to the Devo-nian geological time scale by recognizing astro-nomical cycles in the Frasnian geological record from western Canada.

MATERIALS AND METHODS

Study Site Selection

For this study, we used magnetic susceptibil-ity data from previously measured stratigraphic sections that provide some of the best exposed carbonate platform margins of Devonian age (Whalen et al., 2000a, 2000b). The studied sections were deposited at low paleolatitude along the western continental margin of Lau-russia. Magnetic susceptibility sampling was undertaken by Whalen and Day (2008, 2010) in continuous or near-continuous sections, and the high-resolution sequence stratigraphic framework provides a check on the overlap of magnetic susceptibility data sets from differ-ent sections. Magnetic susceptibility samples for this study were collected from measured stratigraphic sections that begin near the western margin of the Southesk-Cairn carbonate platform (section KC) in the east and extend along the southeast margins of the Miette (section AB/W4) and Ancient Wall platforms (sections A/C and MC) exposed farther to the west (Fig. 1). A de-tailed facies analysis is beyond the scope of this paper, and the reader is referred to Whalen et al. (2000a, 2000b) and Whalen and Day (2008, 2010) for more detailed stratigraphic infor-mation. The bulk of all sections record slope facies ; however, sections KC, AB, MC, and A/C contain the platformal Flume Formation at their base, and section W4 is within the plat-formal Ronde Formation. As demonstrated by Whalen and Day (2010), slope facies are usually

For permission to copy, contact [email protected]© 2012 Geological Society of America

GSA Bulletin; May/June 2012; v. 124; no. 5/6; p. 928–942; doi: 10.1130/B30547.1; 8 fi gures; 4 tables.

†E-mail: [email protected]




Geological Society of America Bulletin, May/June 2012 929

characterized by higher magnetic susceptibility values than coeval platform facies. The eastern-most locality (KC) records a continuous section through the entire Frasnian (Whalen and Day, 2010). Sections AB and MC are relatively con-tinuous through about the lower two thirds of

the Frasnian, but both contain short, nonover-lapping covered intervals in the Perdrix Forma-tion (Whalen and Day, 2010). Section AB ended where the terrain became too steep to safely continue sampling, and overlying section W4, from an adjacent thrust sheet, records a con-

tinuous exposure of the uppermost Frasnian and lowermost Famennian (Whalen and Day, 2008, 2010). The lower part of section A/C contains a continuous exposure of the uppermost Give-tian and lowermost portion of the Frasnian (Day and Whalen, 2005). Above a thick covered in-

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Figure 1. (A) Location map of the overthrust belt in western Alberta, showing the locations of the Miette and Ancient Wall platforms (after Mountjoy, 1965). Lettered stars indicate the location of measured stratigraphic sections along the southeast margin of the Ancient Wall and Miette platforms. (B) Palinspastically restored middle Frasnian paleogeographic map, illustrating the locations of Upper Devonian isolated and attached carbonate platforms in the Alberta basin. The easternmost barbed line indicates the eastern limit of Laramide thrusting. Buildups located west of this line are exposed in the Canadian Rocky Mountains, while those in the east are in the subsurface (after Geldsetzer , 1989; Mountjoy, 1980; Switzer et al., 1994). (C) Upper Devonian stratigraphic nomenclature of the Rocky Mountains, western Alberta subsurface (after Switzer et al., 1994). (D–E) Cross sections of the southeast margin of the Miette and Ancient Wall platform, illus-trating lithostratigraphy and general lithofacies. Locations of measured stratigraphic sections are indicated. Key to lithostratigraphy: F—Flume Formation, C—Cairn Formation, Pe—Peechee Member, A—Arcs Member, S—Simla Member, R—Ronde Member, M—Maligne Formation, P—Perdrix Formation, H—Mount Hawk Formation. Key to stratigraphic sections: MC—Marmot Crack, A/C—Thornton Creek, AB and W4—Poachers Creek, KC—Klapper Creek.



De Vleeschouwer et al.

930 Geological Society of America Bulletin, May/June 2012

terval, the upper part of section A/C records a continuous exposure of the upper Frasnian and lower Famennian (Whalen and Day, 2008). In the studied sections, these stage boundaries are recognized based on the conodont biostratig-raphy as outlined in Klapper and Lane (1989), Klapper (1997), and Whalen and Day (2008). Whalen and Day (2008) documented the mag-netic susceptibility stratigraphy and conodont biostratigraphy of the studied sections in terms of the Frasnian Montagne Noire zonation of Klapper (1989) and refi nements to that zonation by Girard et al. (2005).

Several elements of the study site descrip-tion combine to give this data set huge cyclo-stratigraphic potential. First, the available (bio-)stratigraphic framework allows for the detailed correlation of the different sections, and, when combined with the recalibrated time scale of Kaufmann (2006), they provide a fi rst-order constraint on sedimentation rate. Second, the sampling resolution up to 0.5 m in (near-) con-tinuous sections provides a high-resolution (10–20 k.y.) uninterrupted climate signal covering the entire Frasnian, which is highly suitable for time-series analysis.

Magnetic Susceptibility

Our analysis is based on magnetic suscepti-bility values of 1655 samples from 1236 m of stratigraphic section measured with a Jacob’s staff or tape measure and compass. Conodont and brachiopod biostratigraphic samples were collected where appropriate, and magnetic susceptibility samples were collected every 0.5–1.0 m. Magnetic susceptibility sampling proce-dures entailed avoiding weathering rinds or rock material with fractures, veins, abundant calcite cement, pyrite, or other obvious diagenetic sul-fi des or oxides. Magnetic susceptibility samples were weighed to within 0.001 g and measured on a KLY-3 Kappa Bridge magnetic-susceptibility meter at B. Ellwood’s laboratory (Louisiana State University). Magnetic susceptibility val-ues represent an average of three measurements of mass-normalized bulk magnetic susceptibility with units of m3/kg.

Magnetic susceptibility represents the degree of magnetization of a material in response to an applied magnetic fi eld. The magnetic suscepti-bility of a sediment varies in proportion to the concentration of detrital-dominated paramag-netic and ferromagnetic minerals and forms a proxy for the rate of supply of impurities deliv-ered to the sedimentary environment (Ellwood et al., 2000). In the studied section, large-scale magnetic susceptibility variations can be as-cribed to relative sea-level changes and million-year-scale climate changes due to, for example,

continental drift or biosphere innovations (Algeo and Scheckler, 1998; da Silva et al., 2009; Mabille and Boulvain, 2007; Whalen and Day, 2008, 2010). On a shorter time scale, climate, through the intensity of wind and precipitation, plays a decisive role in determining the respec-tive eolian and riverine fl ux of magnetic minerals to the marine system (Hladil et al., 2006; Riquier et al., 2010), in this case, the western Canadian sedimentary basin (Fig. 1). For the Quaternary, the response of different tropical monsoon sys-tems to astronomical forcing has been clearly demonstrated (e.g., Kutzbach, 1981; Kutzbach and Liu, 1997; Tuenter et al., 2003). Compared to today, the Devonian tropical climate poten-tially exhibited an even more intense monsoonal circulation (De Vleeschouwer et al., 2011; Streel et al., 2000), characterized by seasonally wet-and-dry climates (Cecil, 1990). Consequently, it is highly probable that during the Devonian Period, Milankovitch cycles infl uenced the climatic factors controlling the detrital inputs to marine basins (i.e., wind and precipitation) and/or of carbonate productivity, both of which affect the magnetic susceptibility signal (Riquier et al., 2010). As illustrated by Whalen and Day (2010, their fi g. 14), the magnetic susceptibility signal in the studied sections is mainly driven by detrital inputs, and thus the role of diagenesis can be considered negligible.

Time-Series Analysis Procedure

The frequency composition of the studied magnetic susceptibility signals was analyzed via spectral analysis, using the multitaper method (MTM; Thomson, 1982) as implemented in the SSA-MTM Toolkit (Ghil et al., 2002). Before the implementation of spectral analysis, long-term trends were removed from the magnetic susceptibility data by subtraction of the linear trends. A constant spacing (in depth) between sample points in the series was generated by linear interpolation at intervals approximating the average sample spacing. The MTM method was performed using three tapers to compro-mise between spectral resolution and side-lobe reduction. Small variations in accumulation rate behave like phase modulations and introduce erro neous spectral peaks (Muller and Mac-Donald, 2000). We chose the MTM because it averages these side lobes into the main peak and thereby gives a superior estimate of the true spectral power. To assess whether or not the strongest spectral peaks of the magnetic susceptibility signal are statistically different from the red noise spectrum, the 90% and 99% confi dence levels (CLs) were calculated: robust AR(1) estimation, median smoothing window width = (10Δt)–1, log fi t, fNyquist = 2/Δt.

The continuous wavelet transform was used to decompose the one-dimensional time series into their two-dimensional time-frequency space. This method reveals the persistency of (astro-nomical) periodicities along the studied sections and provides a tool to detect changes in sedimen-tation rate through associated changes in cycle thickness. Wavelet software was provided by Torrence and Compo (1998), using the Morlet wavelet function with wave number 6, zero-padding to the next higher power of 2, spacing between discrete scales (Δj) = 0.25, smallest scale (S

0) = 2Δt, and the number of scales =

log2NΔtS0

Δ j+1

⎛⎝

⎞⎠

.

Frequency-selective fi lters or Gaussian band-pass fi lters are able to isolate and extract the components of signals associated with a specifi c range of frequencies. Here, we employed band-pass fi lters to assess the behavior of a specifi c range of frequencies in a studied signal using the Analyseries software (Paillard et al., 1996).

Possible Distortions of the Astronomical Frequencies

When interpreting cyclic sedimentary alter-nations as a consequence of periodic changes in the orbital parameters of Earth, one should be aware of the fact that the assumption of a 1:1 record of astronomical signals in carbonate sed-iments is questionable (Westphal et al., 2004). The original astronomical signal can be altered by the climatic system, responding in a non linear way to the applied forcing. Moreover, a sedi-ment column recording climatic changes never represents these temporal changes in a linear manner. For example, variations in sedimenta-tion rate, bioturbation, differential compaction, and pressure dissolution can be responsible for distortions of the orbital frequencies in sedi-mentary records (Herbert, 1994; Meyers et al., 2001; Ripepe and Fischer, 1991; Schiffelbein and Dorman, 1986). To account for these possi-ble overprinting factors, some assumptions and restrictions must be incorporated. On carbonate platforms and ramps, sediment production rate is, above other factors, controlled by carbonate productivity. The main trend is a decrease in the carbonate productivity with increasing depth, with maximum carbonate production in reefal environments (da Silva et al., 2009; James, 1997). Given that each of the studied sections contains at least one transition between basinal and platform lithologies, the accumulation rate can certainly not be considered to be constant throughout the different sections. In depositional environments with different accumulation rates, cycles of equal duration will be expressed by a





sediment package of different thickness. This phenomenon makes the application of spectral analysis on a distance-series including different depositional settings impossible. However, if one assumes that accumulation rate within the same sedimentary environment is semiconstant, MTM-spectral analysis can be carried out for the different sedimentary environments in the studied sections separately. This assumption can be made, since Whalen and Day (2010) demon-strated the absence of signifi cant unconformities and only slight variations in sedimentation rate.

RESULTS AND DISCUSSION

Marmot Crack (MC Section)

Section MC is a relatively thick section (248 m, N = 228; Fig. 2E) from the southeast margin of the Ancient Wall platform. The section consists of platform sequences in the lower part (Thorn-ton Creek Member and the remainder of the Flume Formation). The progressive decrease in carbonate content throughout the lower part of the Maligne Formation is interpreted to indicate a deepening event, causing a transition from reefal stromatoporoid buildups to slope facies in this section. However, physical continuity was observed between platform and slope facies: Carbonate-rich background sediments charac-terize most of the Maligne Formation, although coarse-grained redeposited carbonates in the uppermost Maligne Formation signal the onset of slope facies. The Perdrix and Mount Hawk Formations are characterized by slope/basin se-quences, which are detached from the platform (Whalen et al., 2000b). Section MC spans the uppermost Givetian norrisi zone through part of Montagne Noire (MN) zone 12 (Whalen and Day, 2008). According to the recalibrated Devo-nian time scale of Kaufmann (2006), the com-bined duration of these biozones is ~5.5 m.y. Consequently, an average accumulation rate of ~4.5 cm/k.y. can be calculated for section MC.

The wavelet-analysis time-frequency space reveals a persistent low-frequency (<0.07 m–1) cycle (Fig. 2A). The 3 MTM-spectral plots (one for the platform facies of the Flume For-mation; one for the Maligne Formation; one for the slope facies of the Perdrix and Mount Hawk Formations) also indicate the presence of a low-frequency cycle by means of a spectral peak near the 90% CL. Although this is not confi rmed by the MTM-spectral plots, the wavelet analysis time-frequency space seems to suggest thinning of the cycles toward the top of the section (Fig. 2A). This pattern is consistent with the geology of the section: A relatively higher sedimenta-tion rate is expected in the platform facies of the

Flume Formation compared to the slope facies of the Perdrix and Mount Hawk Formations. Also, for higher frequencies, this pattern can be recog-nized: 0.12–0.18 m–1 and 0.28–0.35 m–1 cycles in the Flume Formation are associated, respectively, with 0.2–0.27 m–1 and 0.37–0.45 m–1 cycles in the Perdrix and Mount Hawk Formations.

Considering the biostratigraphically derived average sedimentation rate (~4.5 cm/k.y.), the low-frequency (<0.07 m–1) cycle (a few deca-meters thick) was deposited over at least a few hundred thousand years. Assuming that these decameter-scale cycles are all of (nearly) equal duration and caused by astronomical forcing, they are most likely the result of the 405 k.y. eccentricity cycle (E1). The application of a band-pass fi lter for all periodicities thicker than 10 m (Fig. 2B) fi ltered out the decameter-scale magnetic susceptibility variations.

The higher-frequency cycles that have been noted between frequencies 0.12 and 0.18 m–1 (Flume Formation) and 0.2 and 0.27 m–1 (Perdrix and Mount Hawk Formations) are 4–5 times thinner than the decameter-scale cycles that are interpreted to be the result of 405 k.y. eccentricity forcing. This frequency ratio suggests that these 3.7–8.33 m cycles are the result of 100 k.y. ec-centricity (E2) forcing. The highest-frequency cycles characterized by frequencies between 0.28 and 0.45 m–1 cannot be associated to an astronomical forcing factor. However, given the uncertainties on the Kaufmann (2006) time scale, we investigated a range of plausible sedimenta-tion rates to check for alternative astro nomical interpretations (Table 1). Since doubling or halv-ing the average sedimentation rate does not offer an alternative astronomical interpretation, high confi dence can be given to the astrochronology of section MC.

Magnetic susceptibility variations in the fre-quency range of 100 k.y. eccentricity (E2) were fi ltered out by applying a band-pass for periods between 3.7 and 8.33 m (0.12 m–1–0.27 m–1 fre-quency range; Fig. 2C). We calculated the ampli-tude envelope of these E2 cycles (based on the Hilbert transform), and it shows an extensive similarity to the decameter-scale magnetic sus-ceptibility variations that are associated with the 405 k.y. eccentricity cycle. Figure 2D shows the fast Fourier transform (FFT) of this ampli-tude envelope, with a clear spectral peak around 0.05 m–1, corresponding to the 405 k.y. eccen-tricity cycle. In other words, the 100 k.y. cycles seem to be modulated by the 405 k.y. eccentricity cycles, as is expected from astronomical theory (Laskar et al., 2004). The >10 m band-pass fi l-ter output, together with the amplitude envelope of the 100 k.y. eccentricity cycles, leads to the differentiation of thirteen 405 k.y. eccentricity cycles in section MC (Fig. 2B).

Thornton Creek (Section A/C)

Section A/C (469 m, N = 598, Fig. 3E) is also from the southeast margin of the Ancient Wall platform. The Marmot Crack (MC) section is 2 km to the south, so the stratigraphy of sec-tion A/C is similar to that of section MC. The fi rst 82.5 m consist of platform sequences of the Flume Formation (including the Thornton Creek Member). Because of structural compli-cations, only the lowermost 10 m of the over-lying Maligne Formation was sampled. For this reason, the Maligne Formation was added to the Flume platform facies to carry out MTM-spectral analysis. This part of the section was sampled at 0.5 m resolution. Between 92.5 and 279 m, the section was largely covered, and only a handful of magnetic susceptibility measure-ments were carried out and are useless for time-series analysis. The upper interval from 279 to 469 m of the section was sampled at 0.5 m resolution. As in section MC, the Mount Hawk Formation consists of slope and basin facies. The lower part of the overlying Simla Member (Southesk Formation) includes carbonate-poor argillaceous and silty facies, while the middle and upper part of the Simla Member records the fi nal stage of Frasnian platform prograda-tion. The overlying early Famennian Sassenach Formation records an infl ux of siliciclastics into the basin (Mountjoy and Savoy, 1995) and thus records slightly higher magnetic susceptibility signatures than underlying Simla facies. The fi rst 92.5 m of section A/C span the upper part of the Givetian disparilis zone, the Givetian norrisi zone, and Frasnian MN zones 1 through 3, and part of MN zone 4 (Whalen and Day, 2008). According to the recalibrated Devonian time scale of Kaufmann (2006), these biozones have a combined duration of ~1.7 m.y., and an average accumulation rate of ~5.4 cm/k.y. can be calculated. The upper part of section A/C (279–469 m) spans the uppermost part of MN zone 11, MN zones 12 and 13, and the early Famen nian lower and middle triangularis zones (Whalen and Day, 2008). According to the re-calibrated Devonian time scale of Kaufmann (2006), these biozones have a combined dura-tion of ~4.5 m.y., and an average accumulation rate of ~4.2 cm/k.y. is calculated for this part of section A/C.

The MTM-spectral plot of the lower part of section A/C (0–92.5, N = 186; Fig. 3A) shows a strong and distinct low-frequency peak between frequencies 0.07 m–1 and 0.1 m–1. Although this spectral peak corresponds to a zone of high spectral power in the wavelet-analysis time-frequency space, this period seems not to be persistent between 50 and 70 m. Also, between frequencies 0.16 m–1 and 0.32 m–1 and between





Figure 2. Frasnian periodicities in the magnetic susceptibility (MS) signal of section MC. (A) Time-frequency space resulting from wavelet (Morlet) analysis, and MTM-power spectra showing distribution of spectral power. Frequency bands characterized by an increased spec-tral power are delineated. The two lowermost frequency bands are identifi ed as the result of 405 k.y. eccentricity (E1) and 100 k.y. eccentricity (E2) forcing. (B) The E1-fi ltered magnetic susceptibility signal (dark-gray dotted line). (C) The E2-fi ltered magnetic susceptibility signal (light-gray solid line) and its amplitude envelope (dark-gray dashed line). (D) Fast Fourier transform (FFT) of the E2-amplitude envelope. (E) Magnetic susceptibility (MS) signal of section MC (black solid line). CL—confi dence level, Th.—Thornton Creek Member.

TABLE 1. ALTERNATIVE ASTRONOMICAL INTERPRETATIONS FOR SECTION MC

Half sedimentation rate

Biostratigraphically derived sedimentation rate

Double sedimentation rate

Astronomical interpretation

Sedimentation rate: 2.25 cm/k.y. 4.5 cm/k.y. 9 cm/k.y.Frequency range

<0.07 m–1 >635 k.y. >317 k.y. >158 k.y. E10.12–0.27 m–1 164–368 k.y. 82–184 k.y. 41–92.2 k.y. E2





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ne).





0.6 m–1 and 0.82 m–1, both wavelet analysis and MTM-spectral analysis suggest an elevated spectral power. In the upper part of section A/C (279–469 m, N = 365; Fig. 3A), both wave-let and spectral analyses reveal a strong low-frequency (<0.11 m–1) cycle that is persistent throughout the Mount Hawk Formation and the Simla Member, but it loses spectral power in the Famennian Sassenach Formation. The time-frequency plot suggests a thickening of the cycles between 280 and 400 m. Also, higher-frequency cycles (0.2–0.4 m–1 and 0.4–0.6 m–1) show a thickening of the cycles throughout these late Frasnian and earliest Famennian deposits. Moreover, just as for the low-frequency cycles, high-frequency cyclicity abruptly stops around 400 m, suggesting that the input of coarser-grained siliciclastic sediments during the early Famennian hindered the development of astro-nomical cycles in the Famennian magnetic sus-ceptibility stratigraphy of section A/C. Also, the thickening of cycles is in line with geological expectations: A relatively higher sedimenta-tion rate is expected in the prograding platform facies of the Simla Member compared to the slope facies of the Mount Hawk Formation.

Considering the biostratigraphically derived average sedimentation rate (~5.4 cm/k.y. and ~4.2 cm/k.y. for the lower and upper part of section A/C, respectively), the low-frequency cycle (between 9 and 25 m thick in both parts of the section) was deposited over at least a few hundred thousand years. Assuming that these decameter-scale cycles are all of (nearly) equal duration and caused by astronomical forcing, they are most likely the result of the 405 k.y. eccentricity cycle (E1). The application of a band-pass fi lter for all periods between 9 and 25 m (0.04–0.11 m–1 frequency range; Fig. 3B) fi ltered out decameter-scale magnetic suscepti-bility variations.

The higher-frequency cycles, which have been noted in the lower part (0.18–0.32 m–1) and upper part (0.2–0.4 m–1) of section A/C, are 3–5 times thinner than the decameter-scale cycles that we interpret to be the result of 405 k.y. eccentricity forcing. This frequency ratio suggests that these 3.1–5.55 m (lower part) and 2.5–5 m (upper part) cycles are the result of 100 k.y. eccentricity (E2) forcing. The high-frequency cycles in the lower part of section A/C, with frequencies between 0.6 m–1 and 0.82 m–1, have a frequency ratio close to 1:3 when compared to the frequency of the 100 k.y. eccentricity cycles. Therefore, these high-frequency cycles are interpreted as the ex-pression of 33 k.y. obliquity forcing (perio dicity of Devonian obliquity cycle after Berger and Loutre, 1989; Berger et al., 1992). The high-frequency cycles in the upper part of section A/C (0.4–0.6 m–1) cannot be associated with an astro-

nomical forcing factor. Table 2 indicates that if the average sedimentation rate is doubled, an al-ternative astronomical interpretation is possible for section A/C, where the lowest-frequency cycle would correspond to 100 k.y. eccentricity and the meter-scale cycle to obliquity. However, since stratigraphy and formation thicknesses of sections A/C and MC are very similar, it seems highly unlikely that sedimentation rate is signifi -cantly higher in section A/C compared to section MC. Therefore, the original astronomical inter-pretation is favored.

In the lower part of section A/C, magnetic susceptibility variations in the frequency range of 100 k.y. eccentricity (E2) were fi ltered out by applying a band-pass between periods 3.1 and 5.55 m (0.18 m–1–0.32 m–1 frequency range; Fig. 3B). Contrary to the case for the MC section, the amplitude envelope of the 100 k.y. eccentricity cycles does not show an extensive similarity with the 405 k.y. eccentricity cycles (Figs. 3C and 3D). Many reasons can be proposed to explain this mismatch, given the pathway from climate forcing to preservation in the stratigraphic rec-ord. Consequently, caution is advised when using the fi ve and a half 405 k.y. eccentricity cycles that are differentiated in the lower part of section A/C (Fig. 3B). In the upper part of section A/C, 100 k.y. eccentricity cycles were fi ltered out by applying a band-pass between periods 2.5 and 5 m (0.2 m–1 and 0.4 m–1 frequency range). In this case, the amplitude envelope of the 100 k.y. cycles does show similarity with the 405 k.y. eccen tricity cycles. The amplitude modulation of the 100 k.y. cycles by 405 k.y. eccentricity is confi rmed in Figures 3B–3D, so strong con-fi dence can be put in the astrochronology of the upper part of section A/C (Fig. 3B).

Poachers Creek (Sections AB and W4)

Section AB (Fig. 4B) is from basin-to-slope exposures along the southeast Miette platform margin, except for the lowermost 24 m of the section, which is composed of carbonate-ramp facies of the Flume Formation. Section W4

consists of the late Frasnian carbonates of the Ronde Member (= Simla Member equiva-lent), recording platform progradation, and the quartz-rich carbonates of the overlying Sassenach Formation. The Poachers Creek sec-tion was sampled at 0.5 m intervals. However, no magnetic susceptibility measurements were carried out between 122 and 149 m in section AB. This interval in the upper Perdrix Forma-tion is poorly exposed and highly condensed. Therefore, this 27-m-thick gap in the data cov-erage will defi ne the boundary between sec-tion AB1 (0–122 m, N = 235) and section AB2 (149–290 m, N = 254; Fig. 4E). Section W4 (0–67 m, N = 137; Fig. 4E) covers the young-est interval in the Poachers Creek sections and is from a thrust sheet adjacent to the one where section AB was sampled. For time-series analyses, the magnetic susceptibility signal was broken up into these three different parts (AB1, AB2, and W4), and evaluated separately.

Section AB1Section AB1 spans MN zones 1–7 and the

lower part of zone 8 (Whalen and Day, 2008). According to Kaufmann (2006), these combined biozones lasted for ~2.5 m.y., which implies an average accumulation rate of ~4.9 cm/k.y. for section AB1.

Both wavelet and spectral analyses suggest a strong and persistent low-frequency cycle (<0.08 m–1), as well as a strong, but less persis-tent, higher-frequency cycle (0.1–0.22 m–1; Fig. 4A). Considering the biostratigraphically de-rived average accumulation rate (~4.9 cm/k.y.), the low-frequency cycle was deposited in at least a few hundreds of thousands of years, while the higher-frequency cycle was depos-ited in 93–204 k.y. These durations, together with the frequency ratio between the discussed low-frequency and higher-frequency cycle (i.e., 1:3–1:5), lead to the interpretation of the low-frequency cycles as the result of 405 k.y. eccen-tricity forcing and the higher-frequency cycles as the result of 100 k.y. eccentricity forcing. The application of a band-pass fi lter between

TABLE 2. ALTERNATIVE ASTRONOMICAL INTERPRETATIONS FOR SECTION A/C





Lower part of A/CSedimentation rate: 2.7 cm/k.y. 5.4 cm/k.y. 10.8 cm/k.y.Frequency range

0.07–0.1 m–1 370–518 k.y. 185–259 k.y. 93–130 k.y. E10.16–0.32 m–1 115–232 k.y. 58–116 k.y. 29–58 k.y. E20.6–0.82 m–1 45–62 k.y. 23–31 k.y. 11–15 k.y. Ob

Upper part of A/CSedimentation rate: 2.1 cm/k.y. 4.2 cm/k.y. 8.4 cm/k.y.Frequency range

<0.11 m–1 >428 k.y. >214.3 k.y. >107 k.y. E10.2–0.4 m–1 120–238 k.y. 60–119 k.y. 30–59 k.y. E2





Fig

ure

4. F

rasn

ian

peri

odic

itie

s in

the

mag

neti

c su

scep

tibi

lity

sign

al o

f se

ctio

ns A

B a

nd W

4. (

A)

Tim

e-fr

eque

ncy

spac

e re

sult

ing

from

wav

elet

(M

orle

t)

anal

ysis

, and

MT

M-p

ower

spe

ctra

sho

win

g di

stri

buti

on o

f sp

ectr

al p

ower

. Fre

quen

cy b

ands

cha

ract

eriz

ed b

y an

incr

ease

d sp

ectr

al p

ower

are

del

inea

ted.

In

the

fi rst

122

m o

f sec

tion

AB

(i.e

., A

B1)

, tw

o fr

eque

ncy

band

s ar

e id

enti

fi ed

as th

e re

sult

of 4

05 k

.y. e

ccen

tric

ity

(E1)

and

100

k.y

. ecc

entr

icit

y (E

2) fo

rc-

ing.

In

sect

ion

AB

2 (1

49–2

90 m

), th

e in

fl uen

ce o

f ecc

entr

icit

y (E

1 an

d E

2), o

bliq

uity

(Ob)

, and

pre

cess

ion

(Pr)

can

be

delin

eate

d by

dis

tinc

t fre

quen

cy b

ands

. In

the

uppe

rmos

t sec

tion

W4

(0–6

7 m

), tw

o fr

eque

ncy

band

s ar

e as

soci

ated

wit

h E

1 an

d E

2 fo

rcin

g, r

espe

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ely.

(B) T

he E

1-fi l

tere

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agne

tic

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bilit

y si

gnal

(da

rk-g

ray

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) The

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al (

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-gra

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(D)

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FT

) of

the

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ampl

itud

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E)

Mag

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sig

nal o

f se

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B a

nd W

4 (b

lack

sol

id li

ne).





12.5–50 m and 4.55–10 m (0.02–0.08 m–1 and 0.1–0.22 m–1 frequency range) fi ltered out both low- and higher-frequency cycles, respectively (Figs. 4B and 4C). Table 3 illustrates that by halving the sedimentation rate, the higher-frequency cycles could be interpreted as 405 k.y. cycles, but this alternative astronomical inter-pretation is most unlikely since the amplitude envelope of the 100 k.y. cycles varies with the rhythm of the 405 k.y. eccentricity cycles (Figs. 4B–4D), which can be seen as a confi rmation of the original astronomical interpretation. Based on both the band-pass fi lters and their respective interpretation, four and a half 405 k.y. eccen-tricity cycles were identifi ed in section AB1.

Section AB2Section AB2 spans MN zone 9 through part

of MN zone 12 (Whalen and Day, 2008). These combined biozones span ~2.5 m.y. (Kauf mann, 2006) and have an average accumulation rate of ~5.65 cm/k.y.

The MTM-spectral plot shows a very strong low-frequency cycle (<0.06 m–1 peak; Fig. 4A). Unfortunately, a major area of this cycle in the wavelet analysis time-frequency space lies within the cone of infl uence (COI), in which edge effects become important (Torrence and Compo, 1998). However, after fi ltering out these decameter-scale magnetic susceptibility variations (16.67–50 m periodicity), it becomes clear that these low-frequency cycles are an important compo-nent of the section’s magnetic susceptibility stra-tigraphy (Fig. 4B). Also, between frequencies 0.08 m–1 and 0.16 m–1 and between 0.25 m–1 and 0.4 m–1, both spectral and wavelet analyses sug-gest an important periodic component (Fig. 4A). Considering the biostratigraphically derived average accumulation rate (~5.65 cm/k.y.), the low-frequency cycle was deposited in more than 295 k.y., the 6.25–12.5-m-thick cycle (0.08 m–1

and 0.16 m–1) was deposited in 106–212 k.y., and the 2.5–4 m cycle (0.25–0.4 m–1) was de-posited in 44–71 k.y. These durations, together with the frequency ratios between the discussed cycles (~1:4 and ~1:3, respectively), lead to the interpretation of the low-frequency cycles as the result of 405 k.y. eccentricity forcing and the higher-frequency cycles as the result of 100 k.y. eccentricity and obliquity forcing, respectively. This interpretation implies that the true average accumulation rate must have been higher than is suggested by biostratigraphy. However, Table 3 shows that if the average sedimentation rate is halved, the higher-frequency cycles can be in-terpreted as the result of 405 k.y. and 100 k.y. eccentricity forcing, respectively. Still, the origi-nal astronomical interpretation is preferred given the large degree of amplitude modulation of the 100 k.y. cycles by the 405 k.y. eccentricity cycles (Figs. 4B–4D).

The frequency band in the time-frequency space between 0.6 m–1 and 0.8 m–1 (Fig. 4A) is where one would expect the infl uence of preces-sion. Here, in the time-domain, an alternation between stronger and weaker spectral power can be observed. These alternations seem to behave rhythmically: Parts of the section with a strong spectral power in the precessional frequency domain coincide with maxima in the 100 k.y. eccentricity (6–12 m) band-pass fi lter output (dash-dot lines between Figs. 4A and 4C). This pattern indicates the amplitude modulation of precession by eccentricity and can be consid-ered a major confi rmation of the astronomical interpretation.

Section W4Section W4 spans MN zones 13a to 13c and

the early Famennian lower and middle triangu-laris zone (Girard et al., 2005; Klapper, 1989; Whalen and Day, 2008). The duration of these

biozones is ~1.5 m.y. (Kaufmann, 2006), and an average accumulation rate of ~4.5 cm/k.y. can be calculated.

Wavelet and spectral analyses indicate two strong periodicities in the magnetic suscepti-bility signal of section W4: decameter-scale cyclicity between 0.05 m–1 and 0.1 m–1 and meter-scale cyclicity between 0.22 m–1 and 0.33 m–1 (Fig. 4A). Considering the biostratigraphi-cally derived average accumulation rate (~4.5 cm/k.y.), the decameter-scale cycle was depos-ited in 222–444 k.y., and the meter-scale cycle was deposited in 67–101 k.y. These durations lead to the interpretation of the decameter-scale cycles as the result of 405 k.y. eccentricity forc-ing and the meter-scale cycles as the result of 100 k.y. eccentricity forcing. Only if the bio-stratigraphically derived average sedimentation rate is more than doubled can an alternative as-tronomical interpretation be made. Given that an average sedimentation rate of ~9 cm/k.y. is far beyond the range in the studied sections, the original astronomical interpretation is pre-ferred. Decameter-scale and meter-scale cycles were fi ltered out by applying a band-pass fi lter between 10 and 20 m (0.05–0.1 m–1 frequency range) and between 3 and 4.5 m (0.22–0.33 m–1 frequency range), respectively (Figs. 4B and 4C). The amplitude envelope of the meter-scale cycles demonstrates an extensive similarity to the decameter-scale cycles in the upper part of the section. This is not so in the lower part, where the amplitude envelope concatenates the fi rst two decameter-scale cycles (Figs. 4B–4D). Based on the decameter-scale cycles, four 405 k.y. eccentricity cycles were identifi ed in section W4 (Fig. 4B).

Klapper Creek (Section KC)

Section KC (332.1 m, N = 266; Fig. 5E) is from a thrust sheet, to the east of the thrust sheet of section AB. It is situated between the Southesk-Cairn platform to the east and the Miette plat-form to the west (Fig. 1). Since section KC is positioned only slightly more proximal to ter-restrial source areas compared to section AB, its stratigraphy is similar. Just as in section AB, the lowermost 21 m consist of the carbonate-ramp deposits of the Flume Formation, fol-lowed by the slope facies deposited during the Maligne transgression and the basinal units of the Perdrix and Mount Hawk Formations. The carbonates of the Arcs and Ronde Members (Southesk Formation) record a progradational phase. The uppermost part of the section spans the Frasnian-Famennian boundary at or below the contact with the quartz-rich shallow-water carbonates of the Sassenach Formation. For MTM-spectral analysis, the slope-to-basinal

TABLE 3. ALTERNATIVE ASTRONOMICAL INTERPRETATIONS FOR SECTIONS AB AND W4





AB1Sedimentation rate: 2.45 cm/k.y. 4.9 cm/k.y. 9.8 cm/k.y.Frequency range

<0.08 m–1 >510 k.y. >255 k.y. >128 k.y. E10.1–0.22 m–1 186–408 k.y. 93–204 k.y. 46–102 k.y. E2

AB2Sedimentation rate: 2.1 cm/k.y. 4.2 cm/k.y. 8.4 cm/k.y.Frequency range

<0.06 m–1 >590 k.y. >295 k.y. >148 k.y. E10.08–0.16 m–1 212–425 k.y. 106–212 k.y. 53–106 k.y. E20.25–0.4 m–1 88–142 k.y. 44–71 k.y. 22–35 k.y. Ob

W4Sedimentation rate: 2.1 cm/k.y. 4.2 cm/k.y. 8.4 cm/k.y.Frequency range

0.05–0.1 m–1 444–889 k.y. 222–444 k.y. 111–222 k.y. E10.22–0.33 m–1 135–202 k.y. 67–101 k.y. 34–50 k.y. E2





Figure 5. Frasnian periodicities in the magnetic susceptibility signal of section KC. (A) Time-frequency space resulting from wavelet (Morlet ) analysis, and MTM-power spectra showing distribution of spectral power. Frequency bands characterized by an increased spec-tral power are delineated. Between 20 and 250 m, two frequency bands are identifi ed as the result of 405 k.y. eccentricity (E1) and 100 k.y. eccentricity (E2) forcing. In the upper part of the section, only the infl uence of E1 forcing can be delineated by a frequency band. (B) The E1-fi ltered magnetic susceptibility signal (dark-gray dotted line). (C) The E2-fi ltered magnetic susceptibility signal (light-gray solid line) and its amplitude envelope (dark-gray dashed line). (D) Fast Fourier transform (FFT) of the E2-amplitude envelope. (E) Magnetic susceptibility (MS) signal of section KC (black solid line), giving rise to the differentiation of 12 E1 cycles in section KC.

TABLE 4. ALTERNATIVE ASTRONOMICAL INTERPRETATIONS FOR SECTION KC





Sedimentation rate: 2.25 cm/k.y. 4.5 cm/k.y. 9 cm/k.y.Frequency range

0.02–0.1 m–1 465–2326 k.y. 233–1163 k.y. 116–581 k.y. E10.11–0.25 m–1 186–423 k.y. 93–211 k.y. 47–106 k.y. E2





facies of the Maligne, Perdrix, and Mount Hawk Formations and the platform facies of the Arcs and Ronde Members were analyzed separately.

Section KC spans the uppermost Givetian, the entire Frasnian, and the lowermost Famennian. In terms of biostratigraphy, this section spans the uppermost Givetian norrisi zone, all 13 Fras-nian MN zones, and part of the early Famennian triangularis zone (Klapper and Lane, 1989; McLean and Klapper, 1998; Whalen and Day, 2008). According to the recalibrated Devonian time scale of Kaufmann (2006), the duration of these biozones is ~7.7 m.y., and an average accumulation rate of ~4.3 cm/k.y. can be cal-culated. Between 20 and 250 m in the section, wavelet and spectral analyses indicate a strong and persistent cycle with a frequency between 0.02 and 0.075 m–1 (Fig. 5A). In the upper part of the section, this decameter cycle seems to in-crease its frequency to 0.03–0.1 m–1. Moreover, between 20 and 250 m, wavelet and spectral analyses demonstrate a broad frequency range of increased spectral power between 0.11 m–1

and 0.25 m–1. Considering the biostratigraphi-cally derived average accumulation rate (~4.3 cm/k.y.), the decameter-scale cycle was de-posited in a minimum of several hundreds of thousands of years, and the meter-scale cycle (0.11–0.25 m–1) was deposited in 93–211 k.y. These durations, together with the frequency ratios between the low- and higher-frequency cycles (~1:4), lead to the interpretation of the decameter-scale cycles as the result of 405 k.y. eccentricity forcing and the meter-scale cycles as the result of 100 k.y. eccentricity. Even by doubling or halving the average sedimentation rate, no alternative astronomical interpretation can be made (Table 4). The decameter-scale cycles were fi ltered out by applying a band-pass fi lter between 13.33 and 50 m (0.02–0.075 m–1 frequency range) in the slope-to-basinal part, and a 10–33.3 m (0.03–0.1 m–1 frequency range) fi ltering in the upper part of the section (Fig. 5B). In the basinal part of the section, the 100 k.y. eccentricity cycles were also fi ltered out by applying a band-pass between periods 4 and 9.09 m (0.11–0.25 m–1 frequency range; Fig. 5C). The amplitude envelope of these cycles demonstrates clear modulation by the 405 k.y. eccentricity cycle (Figs. 5B–5D). This observa-tion reinforces the astronomical interpretation, and, together with the decameter-scale cycles, it leads to the identifi cation of 12 Frasnian 405 k.y. eccentricity cycles in section KC (Fig. 5B).

Sedimentation Rates

In all studied sections, for every formation and member in which astronomical cycles are recognized, an estimate of the average sedimen-

tation rate was made, based on the average thick-ness of the astronomical cycles in that formation or member (Fig. 6). Average sedimentation rates vary between 2.8 and 8.5 cm/k.y. and are con-siderably higher in the sections near the Miette platform in the east versus sections near the Ancient Wall platform to the west. This pattern is indicative of the relative position of the sec-tion with respect to the platform margin or other platforms. Sections MC and A/C were more dis-tal from the platform margin than AB and KC. They were also much more distal from the east-ern/northeastern siliciclastic source than Miette sections AB and KC, which explains their lower magnetic susceptibility values (Whalen and Day, 2010). Ancient Wall sections MC and A/C exhibit an average sedimentation rate pattern that meets the geological expectations: Carbon-ate platform units (Thornton Creek Member and Flume Formation) accumulate slightly faster compared to basinal units (Maligne, Perdrix , and Mount Hawk Formations). However, this model is not repeated in the Miette sections. Here, the highest accumulation rate is found in the Mount Hawk Formation, which records a major increase in magnetic susceptibility. Both the maximum sedimentation rate and high mag-netic susceptibility values in this formation are probably due to carbonate-ramp progradation from east to west, resulting in seaward migration of the shoreline and increased siliciclastic input into the sedimentary environments recorded in

the Mount Hawk Formation (Whalen and Day, 2010). In all sections, the shallow-water plat-form facies (Arcs, Ronde, and Simla Members) overlying the Mount Hawk Formation are char-acterized by a nearly uniform accumulation rate around 3–4 cm/k.y.

The average sedimentation rate pattern in Figure 6 strongly supports the astronomical interpretation because (1) the patterns of aver-age accumulation rate among sections from the same carbonate platform are very similar, and (2) the average accumulation rate pattern is con-sistent with the lithological record.

Cyclostratigraphic Calibration of the Frasnian Time Scale

In all studied sections, 405 k.y. eccentricity cycles are recognized. In most of the sections, even 100 k.y. eccentricity cycles are identifi ed, and in section AB2, the infl uence of obliquity and precession is also distinguished. The goal of this study is to correlate astronomical cycles among the studied sections so as to estimate the duration of the Frasnian. Whalen and Day (2008) established correlations among the studied sections based on their magnetic sus-ceptibility signature, conodont biostratigraphy, and sequence stratigraphy. Their investigation provided control on the relative timing of 15 magnetic susceptibility excursions. The astro-nomical cycles identifi ed in this study provide

Platform PlatformSlope-to-basin

0

2

4

6

8

10

MiettePoachers Creek (AB and W4)Klapper Creek (KC)

Thorn

tonCree

kFlu

me

Malign

e

Perdrix

Mt. Haw

kArcs

Simla

/ Ron

de

Formation / Member

0

2

4

6

8

10

Aver

age

Sedi

men

tatio

nra

te(c

m/ k

.y.)

Ancient WallMarmot Crack (MC)Thornton Creek (A/C)

Figure 6. Estimate of the evolution of average sedimentation rates throughout the studied sections, based on the average thickness of the identifi ed astronomical cycles in the different formations and members.





additional relative and absolute time control. Figure 7 shows correlations of the magnetic susceptibility signals in all studied sections and shows the band-pass fi lter output that was inter-preted as the infl uence of the 405 k.y. eccentric-ity forcing cycle. The extremes of the fi ltered

405 k.y. eccentricity cycles, deduced from the original magnetic susceptibility signals, were used as correlation anchors between the different sections. Ultimately, by correlating all 405 k.y. cycles among the different sections, a well-defi ned magnetic susceptibility stratigraphy can

be constructed for the entire Frasnian. The most valuable result of this work is not the slightly higher resolution that characterizes this correla-tion compared to Whalen and Day (2008), but the temporal information that this correlation contains. The astronomically calibrated mag-

Figure 7. Cyclostratigraphic correlation of the studied sections. Magnetic susceptibility stratigraphy (black) and band-pass fi ltered signal of the magnetic susceptibility stratigraphy at the frequencies of the 405 k.y. eccentricity cycles (E1, dark-gray dotted line). Dark/light-gray bands indicate sixteen 405 k.y. eccentricity cycles over the entire Frasnian stage, implying a duration of 6.5 ± 0.4 m.y. (the counting error is estimated to be one E1 cycle).





netic susceptibility stratigraphy consists of 16 (±1 counting error) 405 k.y. eccentricity cycles. Therefore, the duration of the Frasnian is esti-mated at 6.5 ± 0.4 m.y. A rough error estimate of plus or minus one 405 k.y. eccentricity cycle is made, since the possibility exists that a 405 k.y. cycle was either overlooked or counted twice. This study’s duration estimate is based on the correlation of the lowest-frequency cycles from four different geological sections, taking into account constraints from the available magnetic susceptibility stratigraphy and biostratigraphy. This procedure signifi cantly reduces the prob-ability of erroneous cycle counting. Moreover, sixty-three 100 k.y. cycles can be counted when the reliable astrochronologies of section MC and the upper part of section A/C are combined. This implies a duration estimate very similar to the duration estimate based on the 405 k.y. cycles and strengthens confi dence in the setting of a quite tight counting error. Errors that arise from uncertainties on the exact stratigraphic position of the Frasnian stage boundaries in the studied sections are in the order of a few meters, and are one order of magnitude smaller com-pared to the counting error. It is assumed that these uncertainties are already accounted for by the counting error.

The 6.5 ± 0.4 m.y. duration of the Frasnian is considerably different from the one given in the 2004 geologic time scale (GTS2004; 10.8 ± 5.2 m.y.; Gradstein et al., 2004), but it is still within the margin of error. However, GTS2004 is not seen as the best available time scale for the Devonian because it relies on isotopic ages derived by a variety of methods. Moreover, questions have been raised concerning the relia-bility and biostratigraphic constraints of these isotopic dates (Pogson, 2009). However, our es-timate is in good agreement with the best avail-able Devonian chronology (Kaufmann, 2006), which assigns a duration of 7.6 ± 7.2 m.y. to the Frasnian. By considering the error margins on previous estimates of the duration of the Fras-nian, it is clear that the present estimate (6.5 ± 0.4 m.y.) is characterized by unprecedented pre-cision and accuracy.

The cyclostratigraphic methods used in this study provide a better duration estimate of the Frasnian Stage and insights into the relative timing of Frasnian substages. Also, by calibrat-ing the geological time scale to the cyclostrati-graphically derived duration of the Frasnian, signifi cant improvements to the absolute time scale are made (Fig. 8). This is done by add-ing the duration of the Frasnian (including error bars) to the absolute age (including error bars) of the Fras nian-Famennian boundary (after Kaufmann’s [2006] chronology). In this way, a new estimate of the absolute age of the Givetian-

Frasnian boundary is obtained ([376.1 ± 3.6 Ma] + [6.5 ± 0.4 m.y.] = [382.6 ± 4.0 Ma]). When this Givetian-Frasnian boundary age is overlaid with the boundary age from Kaufmann (2006), one obtains a new age estimate for this bound-ary (383.6 ± 3.0 Ma), characterized by a signifi -cantly narrower error bar compared to Kaufmann (2006) (Fig. 8). The same procedure can be fol-lowed for the Frasnian-Famennian boundary. Subtraction of the duration of the Frasnian from the absolute age of the Givetian-Frasnian bound-ary (Kaufmann, 2006) leads to a new estimate of the absolute age of the Frasnian-Famennian boundary ([383.7 ± 3.1 Ma] + [6.5 ± 0.4 m.y.] = [377.2 ± 3.5 Ma]). By overlaying this abso-lute age with the Frasnian-Famennian boundary age from Kaufmann (2006), one obtains a new age estimate for this boundary (376.7 ± 3.0 Ma), characterized by a narrower error bar compared to Kaufmann (2006) (Fig. 8).

CONCLUSIONS

Time-series analysis on magnetic susceptibil-ity variations in different Late Devonian carbon-ate rocks in western Canada reveals the presence of several persistent periodicities throughout the Frasnian. Thanks to the rough estimates of

sedimentation rate (based on biostratigraphy), cyclic ity in the studied sections is ascribed to the different astronomical parameters (eccen-tricity, obliquity, and precession). The astro-nomical interpretation of observed periodicities is supported by the presence of several ampli-tude modulations consistent with astronomical theory, and by average sedimentation rate pat-terns that are in agreement with lithostratigraphy and with biostratigraphically deduced average sedimentation rates. The recognition of 16 ± 1 405 k.y. eccentricity cycles in each of the stud-ied sections entails a wealth of temporal infor-mation and leads to a highly accurate estimate of the duration of the Frasnian (6.5 ± 0.4 m.y.). Starting from the best Devonian time scale avail-able at present (Kaufmann, 2006), the Givetian-Frasnian boundary is recalibrated to 383.6 ± 3.0 Ma, and the Frasnian-Famennian boundary is recalibrated to 376.7 ± 3.0 Ma. In this study, cyclostratigraphic methods proved useful for determining durations of stages. Moreover, they proved their usefulness in recalibrating the ab-solute age of stage boundaries and in narrowing the error margins on these absolute ages in the Paleozoic. These results unambiguously dem-onstrate the enormous potential of cyclostrati-graphic methods in the Paleozoic. Especially in

Figure 8. Recalibration of the Frasnian time scale. By respectively adding and subtracting 6.5 ± 0.4 m.y. to/from the absolute age of the Givetian-Frasnian boundary and Frasnian-Famennian boundary (after Kaufmann, 2006), additional age estimates for those bound-aries are calculated. By a simple overlay between Kaufmann’s (2006) ages and the additional ages, refi ned absolute age estimates are obtained: 383.6 ± 3.0 Ma for the Givetian-Frasnian boundary and 376.7 ± 3.0 Ma for the Frasnian-Famennian boundary.





the Devonian, where the uncertainty on the time scale is enormous, these methods permit the un-certainty to be minimized. Such improvements to Paleozoic time scales will undoubtedly prove useful for a wide range of geoscientists.

ACKNOWLEDGMENTS

This work was supported by a Ph.D. fellowship awarded by the Research Foundation–Flanders (FWO) assigned to David De Vleeschouwer. Philippe Claeys thanks the Vrije Universiteit Brussel research fund for support. Acknowledgment is also made to the donors of The Petroleum Research Fund, administered by the American Chemical Society, and to the National Geographic Society–Committee for Exploration and Research for partial support of this research. Thanks go to the Department of Geology and Geophysics and the Geophysical Institute, University of Alaska–Fair-banks, for their support. This research would not have been possible without the cooperation of Parks Canada for permission to sample at fi eld localities in Jasper National Park. Special thanks go to Christian Zeeden (Utrecht University, The Netherlands), Stephen R. Meyers (University of Wisconsin–Madison) and an anonymous reviewer, whose thoughtful comments helped improve the manuscript signifi cantly. This paper is part of the United Nations Educational, Scientifi c and Cultural Organization (UNESCO) International Geoscience Program (IGCP-580), titled “Application of Magnetic Susceptibility as a Paleoclimatic Proxy on Paleozoic Sedimentary Rocks and Characterization of the Magnetic Signal.”

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Geological Society of America Bulletindglg/Web/Claeys/pdf/De_Vleeschouwer-2012.pdf · 1.0 m. Magnetic susceptibility sampling proce-dures entailed avoiding weathering rinds or rock