The Neoproterozoic Tillite Group from Ella Ø, East Greenland: Carbon isotope signatures and correlations

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Buchardt, Bjّrn۱; Kristiansen, Kasper K.۲; Houmark, Michael۱
Holding Date 08 September 2008

The Neoproterozoic sediments in East Greenland are found in an N-S trending belt in the Fjord Region. Overlying the Eleonora Bay Supergroup is the Tillite Group, which for long has been interpreted as a series of glacially derived deposits including two diamictitic units. The Tillite Group itself is disconformably overlain by sediments of Cambrian age. Fieldwork by us on Ella Ø has questioned the glacial origin of at least the lower diamictitic unit (Ulvesø Fm), while the upper unit (Storeelv Fm) shows unequivocal evidence of glacial activities at the base.
On Ella Ø, the thickness of the Tillite Group is ~1200 m. The Ulvesø Fm rests conformably on shales and stromatolithic calcareous rocks of suggested warm water origin, and the Storeelv Fm is conformably overlain by clastic rocks showing pseudomorphoses after halite in the uppermost unit. The two diamictitic units are separated by the ~250 m thick Area Fm consisting of organic-rich black shales and fine-grained sandstones. The age of the Tillite Group has not been established with certainty, but an Ediacaran age has been suggested. Based on correlations to similar deposits on Svalbard the lower Ulvesø Fm has been assigned a Sturtian age while the Storeelv should be of Marinoan age.
We have performed carbon isotope analyses of carbonates and organic carbon throughout the Tillite Group at Ella Ø and from the uppermost 600 m of underlying carbonates (Bed Group 18) and shales (Bed group 19) of the Eleonora Bay Supergroup. The carbon isotope compositions of the stromatolithic carbonates of Bed Group 18 are significantly enriched compared to modern values with a mean of +7‰ V-PDB. The shift from calcareous rocks of Bed Group 18 to shaly and cherty sediments of Bed Group 19 is accompanied by a negative shift in δ13C of more that 15‰, and average values of carbonates in Bed Group 19 are close to -10‰. Organic carbon from Bed Group 19 has δ13C values close to -34‰ indicating a carbonate to organic carbon fractionation of ~25‰ similar to present-day conditions. Similar depleted carbon isotope compositions are found in the Arena Fm between the two diamictitic units. The Canyon Fm overlying the Storeelv diamictite shows a gradual shift in δ13C(carbonate) to values close to 0‰ as also found in the Cambrian rocks above.
Comparison to published carbon isotope curves from other Neoproterozoic glacial events makes it tempting to correlate the marked negative shift over Bed Group 19 to the isotopic shift predating the Marinoan glacial event. The overlying Ulvesø Fm could thus be of Marinoan age (~650 Ma), presupposing that the Ulvesø diamictite does represent a world-wide glaciation. In this scenario, the Storeelv Fm could represent the Gaskier glaciation, but our carbon isotope values do not correlate to the significant negative anomaly below and before this glacial event. The Storeelv Fm may therefore represent a local glacial event.