INVESTIGATING POTENTIAL CLIMATIC CYCLES IN GLACIALLY-INFLUENCED RHYTHMITES OF THE UPPER GOWGANDA FORMATION

The Gowganda Formation is a glacially-influenced unit of the Huronian Supergroup, Canada. The 2.45-2.2 Ga Huronian Supergroup is divided into five groups, three of which contain well-defined tectono-climatic cycles. The cycle represented by the Cobalt Group is composed of the Gowganda and Lorrain formations. The Gowganda Formation contains a basal tillite-rich unit and an overlying siltstone-mudstone unit that is locally composed of rhythmically layered couplets disrupted by dropstones. Earlier workers have suggested that the rhythmites may represent glacial varves, but the general lack of normal grading and the presence of multiple sub-laminations in the mudstone layers do not support this interpretation. Lenticular bedding and wavy bedding are common throughout the rocks, suggesting that the rhythmites may represent prodelta deposits. Locally interbedded orthoconglomerate and slumped sandstone support this interpretation. Two presumed contemporaneous outcrops in the Elliot Lake region, denoted Outcrop A and B and located approximately 4 km apart, were investigated by examining the deposits on macroscopic and microscopic scales, and by measuring couplets from the base of each coarser grained bed to the top of the overlying finer grained bed. Two rhythmite records were measured in Outcrop A: the lower record is 1.8 m thick and the upper record is 5 m thick, with the rhythmites averaging 7 mm and 9 mm thick, respectively. Similarly, two records were measured in Outcrop B: the lower record is 3 m thick and the upper record is 1.5 m thick, with the rhythmites averaging 13 mm and 12 mm thick, respectively. The data were analysed using the SSA-MTM Toolkit for Spectral Analysis to test for depositional pattern periodicity. If the rhythmites represent annual cycles, periodicities of 2-3 years and 4-7 years were determined from both Outcrops A and B. These periodicities may be analogous to the quasi-biennial oscillation found in Pleistocene/Holocene varves, and climatic fluctuations associated with ENSO, the El Niño – Southern oscillation, respectively. If the rhythmites represent daily tidal cycles, we would expect the spectral analysis to yield periodicities around the Precambrian tidal cycle of 13 or 26 days, but these periodicities were not found in our results.