Spectral Analysis Interpretation

The spectral analysis results are summarized in Table 5-1. For comparison purposes, the spectral analysis results as originally published by Hughes et al. (2003) are also shown in Table 5-1. With one exception, the MTM Toolkit identifies similar peaks in the Hughes data to those published in Hughes et al. (2003).

If the deposits of Outcrop A are considered to represent annual rhythmites, as proposed by Hughes et al. (2003), and a similar interpretation is extended to Outcrop B, then the results of this study can be compared to both the Hughes et al. (2003) results and similar results obtained from Holocene and Pleistocene varve studies. Two spectral analysis studies have been conducted in the Great Lakes region of North America recently: Godsey et al. (1999) on Lake Huron varves, and Breckenridge (2007) on Lake Superior varves. In addition, Rittenour et al. (2000) conducted a spectral analysis study of the varves in glacial Lake Hitchcock, a proglacial lake that existed in the Connecticut River valley of the New England region of the United States during the late Pleistocene/early Holocene.

A cluster of periodicities within the range of 2.2 – 2.9 years was determined for all four rhythmite records in this study. Cycles within this range are reminiscent of the modern day quasi-biennial oscillation (QBO), periodic changes in stratospheric equatorial wind

Table 5-1: Summary of spe ctral analysis re sults from this study, and the Hughe s e t al. (2003) study

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Outcrop A (lower) Outcrop A (upper) Outcrop B (lower) Outcrop B (upper) Hughes (MT M)1 Hughes (published)2

n=229 n=585 n=216 n=119 n=256 n=256

period (couplets) period (couplets) period (couplets) period (couplets) period (couplets) period (couplets)

__________________________________________________________________________________ 56.9 31 30.1 22.8 13.3 14.3 9.2 8.8 10.2 10.7, 9.9 6.6 6.9 6.1 4.4 3.0 - 4.9 3.3 - 4.4 3.0 - 3.4 3.0 - 3.2, 5.1 3.0 - 5.0 2.2 - 2.9 2.3 - 2.9 2.3 - 2.9 2.8 - 2.9 2.3 - 2.9 2.3 - 2.9 __________________________________________________________________________________

Results are shown for detected signal periods (in couplets).

directions that cyclically vary with a period of 22 to 34 months (Baldwin et al., 2001). The QBO impacts the stratospheric polar vortex, which in turn influences weather patterns on the surface of the Earth. Godsey et al. (1999) also attributed periodicities within the range of 2 - 3 years to the Southern Oscillation Index.

A number of low periodicity events within the range of 3.0 – 6.9 years were also determined from the four rhythmite records. Godsey et al. (1999) and Rittenour et al. (2000) attribute periodicities of three to seven years to climatic fluctuations associated with ENSO, the El Niño – Southern Oscillation. Hughes et al. (2003) and Breckenridge (2007) attribute periodicities of three to six years to ENSO, and periodicities of six to seven years to NAO, the North Atlantic Oscillation. A summary comparison of the results of this study and the results of similar studies is provided in Table 5-2.

Table 5-1 shows that some periodicities appear in more than one record. A periodicity of 9.2 years was identified in the lower record of Outcrop A, which is similar to the

periodicity of 8.8 years found in the upper record of Outcrop A. This periodicity, however, was not evident in the results from Outcrop B. The periodicity of 31 years determined from the upper record of Outcrop A is similar to the periodicity of 30.1 years determined from the lower record of Outcrop B. This periodicity, however, was not characteristic of the lower record of Outcrop A or the upper record of Outcrop B. Periodicities of 56.9 and 22.8 years occur only once, in the upper record of Outcrop A and in the lower record of Outcrop A, respectively. Thus, the four records measured, as well as the Hughes et al. (2003) data, reveal periodicities in the order of 3 years

(couplets). This suggests that climate systems such as QBO and ENSO may have been operational in the Paleoproterozoic, and supports the hypothesis that the rhythmic couplets were deposited on an annual basis.

Table 5-2: Comparison of this study with similar studies conducted near the Great Lakes region.

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Study

Deposits

ENSO

NAO

Godsey et al. (1999)

Lake Huron sediments

3 - 7 years

n/a

Rittenour et al. (2000)

Lake Hitchcock sediments

3 - 7 years

n/a

Breckenridge (2007)

Lake Superior sediments

3 - 6 years

6 - 7 years

Hughes et al. (2003)

Gowganda Fm sediments

3 - 6 years

6 - 7 years

This study (2015)

Gowganda Fm sediments

3 - 6 years

6 - 7 years

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If the period of deposition is interpreted to be one year then spectral signals in the 3 - 6 year range may result from oceanic oscillations similar to the El Niño Southern Oscillation (ENSO) and periods of 6 – 7 years may indicate the influence of an oceanic oscillation similar to the North Atlantic Oscillation (NAO).

6

Discussion

The sedimentological, geochemical and spectral analysis results from the studied deposits enable interpretation of the paleoenvironmental conditions operating during deposition of the upper Gowganda Formation. The geochemical results indicate that the minerals that compose the rhythmites have not undergone significant post-depositional alteration and do not contain a significant contribution from pre-existing supracrustally modified source rocks. Thus, the geochemical results are consistent with what would be expected from sediment that was glacially eroded, transported within ice, and then deposited without being subjected to great transport distances or subaerial exposure. The presence of clasts interpreted to be dropstones throughout many of the lithofacies also imply a depositional environment that is proximal to glacial ice and a body of water (Benn and Evans, 2010). Therefore, a description of key glacial landforms near the land-water interface is

important, and is provided below. The focus is mainly on the two varve-like rhythmite- containing lithofacies: the interbedded coarse-grained siltstone and fine-grained siltstone lithofacies, and the interbedded siltstone and claystone lithofacies.

6.1

Deltas and Fans in Glacially-Influenced Environments