HIGH MANGANESE CONCENTRATION IN THE SEDIMENTARY RECORD OF THE GOWGANDA FORMATION, THE HURONIAN SUPERGROUP

The Huronian Supergroup in Ontario, Canada, comprises of sedimentary rocks recording Paleoproterozoic glaciations (2.2 ~ 2.4 Ga). The youngest and most extensive glacial unit, the Gowganda Formation, is up to 1400 m in thickness, and is divided into a lower diamictite (the Coleman Member) and an upper argillite (the Firstbrook Member) (e. g., Rainbird and Donaldson, 1988). The Gowganda Formation provides an opportunity for investigation of the continuous sedimentary record during and after the Paleoproterozoic glaciation because of its large thickness and continuity. In this study, we conducted high-resolution analyses of color spectra, and grain, mineral and chemical compositions using continuous drilling core samples covering the entire Gowganda Formation (CO0338). The Coleman Member in the core is 250 m in thickness and consists of repetition of diamictite, laminated argillite with dropstones, and normally graded sandstone beds. On the other hand, the Firstbrook Member is 860 m in thickness and consists of thick laminated argillite, with frequent intercalations of thin wavy sandstone, normal-graded sandstone, and parallel- and cross-laminated sandstone beds. The color of the argillite bed is green (a* < 0) in the Coleman Member, but abruptly changes to red (a* > 0) just above the uppermost diamictite bed of the Coleman Member. The color of the argillite bed changes to neutral from 70 to 600 m above the base of the Firstbrook Member, and then, changes to red again from approximately 600 to 1000 m. The color of argillite beds probably reflects an abundance of hematite. Fe-rich black argillaceous laminae are observed in reddish argillite from the base to 70 m above the Firstbrook Member. We also found that Mn is concentrated about 60 times the background level in argillite between 250 and 650 m above the base of the Firstbrook Member. These results suggest that deposition of Fe- and Mn-rich sediments occurred immediately after the end of the Gowganda glaciation.