The Union Island Group of the Great Slave Lake, NWT, Canada: A Perspective on the Aftermath of the Lomagundi Carbon Isotope Excursion
- Author(s): Magad-Weiss, Logan Kiel
- Advisor(s): Bekker, Andrey
- et al.
The Lomagundi Event (LE) was a period in Paleoproterozoic Earth history during which carbon isotopic composition of carbonates deposited globally record highly positive isotopic values (averaging at +8 ‰ VPDB). Current geochronological constraints from basins worldwide have bracketed the LE between ~2.22 and 2.06 Ga (Karhu and Holland, 1996; Bekker et al., 2003; Bekker and Holland, 2012; Bekker, 2014). The termination of the LE is commonly associated with the deposition of organic-rich lithologies, a return of carbon isotope composition of carbonates to ~0 ‰ VPDB.
The East Arm basin of the southeastern Slave craton contains an extensive Paleoproterozoic cover sequence of sedimentary and volcanic strata overlying the Archean granitic basement. Conventionally, it has been inferred that the 1.93 Ga Wilson Island Group is the base of the Paleoproterozoic succession based on the higher degree of metamorphism compared to the structurally isolated Union Island Group (Sheen et al. 2018; Bowring et al. 1984). Recent U-Pb zircon dating of the lower basalt unit of the Union Island Group has constrained its depositional age to 2045.8 ± 1.0 Ma (Sheen et al., 2018), placing this group at the base of the Paleoproterozoic supracrustal succession of the East Arm basin.
The Union Island Group contains carbonates, mafic volcanics, and mudrocks unconformably overlying an Archean granitic basement of the Slave craton with locally developed quartz pebble conglomerate and paleosols. The five stratigraphic units (inferred from regional mapping) in ascending order are: 1) lower massive dolostone locally underlain by quartz pebble conglomerate and paleosols developed on the Archean granitic basement, 2) organic-rich black shale with carbonate interbeds, 3) lower basalt package composed of alkaline to sub-alkaline basalt flows, flow breccia, pillows, and pillow breccia, 4) finely laminated, upper dolostone with thin red mudstone beds and locally present sub-alkaline pillow basalts, and 5) a laminated red and green mudstone (Thorstad, 1976; Hoffman et al., 1977; Sheen et al., 2018). Carbon isotope values of carbonates from the lower unit are highly positive (with the average of +7.4 ‰ VPDB), while the upper dolostone unit C isotopic values are near 0 ‰ (with the average of +0.5 ‰ VPDB). The highly positive C isotope values of carbonates from the lower dolostone unit are similar to those of carbonates deposited globally during the LE whereas the near-to-zero C isotope values of the upper dolostone unit are similar to those of carbonates deposited after the LE. 87Sr/86Sr values are highly radiogenic, with the lowest values of 0.7084 and 0.7095 for the lower and upper dolostone units, respectively, suggesting that the carbonates of the Union Island Group do not record a primary marine signal. This could be an indication of deposition in a closed to semi-closed basin, where a lacustrine setting is intermittently subjected to marine incursions. Organic carbon isotope values for the black shale unit separating the dolostone units are highly negative (with values as low as -43.1 ‰ VPDB), indicating oxidative methane cycling, while nitrogen isotopic values are positive (averaging +5.0 ‰), consistent with nitrification-denitrification. Carbon isotope values of carbonates associated with the black shale are slightly negative (with values averaging -1.9 ‰ VPDB) indicating diagenetic origin. Deposition of the Union Island Group black shales is proposed to have occurred in a redox-stratified water column beneath the redoxcline, on the basis of Fe-speciation data which suggests deposition under anoxic ferruginous conditions.
Deposition of the lower dolostone unit of the Union Island Group might correspond to the final stage of the LE, or a short-lived highly positive carbon isotope excursion in the aftermath and preceding the 2.03 Ga (Bekker et al., 2016) Wooley Dolomite excursion. However, the 2045.8 ± 1.0 Ma age seems to contradict with the accepted age for the end of the LE at 2.06 Ga. Combined with the earlier inferred similarly short-lived carbon isotope excursion at ~2.03 Ga (Bekker et al., 2016) in the Wooley Dolomite, the Union Island Group record points to a volatile state of the biogeochemical carbon cycle in the aftermath of the LE, which until recently was not appreciated.