International Geologiical Congress - Oslo 2008

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CGC-04 Neoproterozoic ice ages: Quo vadis? - Part 1

 

The Neoproterozoic Tillite Group of East Greenland revisited, in view of the snowball earth hypothesis

 

Eugene Domack, Hamilton College (United States)
Paul Hoffman, Harvard University (United States)
Galen Halverson, University of Adelaide (Australia)
 

 

We recently examined exposures of the Neoproterozoic Tillite Group of East Greenland across a ∼100 km north to south transect. Our goal was to compare this succession of diamictites (the Ulvesø and Storeelv Fms.) and associated facies (intervening Arena Fm.) to correlative units studied more extensively in Svalbard. We were able to provide new constraints on the depositional setting by mapping stratigraphic panels and ascending to previously unexamined outcrops.
We interpret the Ulvesø to Storeelev succession as representing two distinct glacial episodes separated by a protracted (high-stand) period of submarine fan or fan delta turbidite deposition (The Arena Fm). At Kap Weber we observed evidence of subglacial conditions within the Ulvesø diamictites in the form of sediment deformation and truncation of compressive features (reactivation surfaces). These are clear indications of basal meltwater activity and remobilization of basal debris beneath grounded ice. Such features are characterized by sorting of the diamictite into sandstone or siltstone (meltwater flow), partial erosion by the sorting process into the underlying horizon (diamicite), followed by shearing and truncation along upper surfaces. The Ulvesø is condensed at Brogetdal but is overlain by a few m of glacimarine mega varves which transition into the overlying Arena Fm.
Northern sections are conspicuous in the absence of rudites both within the two glacigenic sections and within intervening sandstones and shales of the Arena Fm. On Ella Ø conglomeratic intervals are common in both diamictite fms. and in the Arena which, while lacking any evidence of ice rafting, appears to be the distal equivalent of interbedded conglomerate and sandstone found within, and along the contact with, the overlying Storeelv Fm. This indicates that the Arena Fm. may have been sourced from terrain that was glaciated above sea level but not to sea level.
The Storeelv glacial succession is as follows:
• Ella Ø exposures consist of (non-systematic) interbedding of tillites and subarial outwash (glaciofluvial sandstones and conglomerates) in primarily terrestrial sections of the Storeelv.
•Kap Weber and Brogetdal, contain a systematic succession of a basal tillite into (1) overlying progradational marine based cycles of diamicite (glacial marine into tillites) (2) punctuations of well sorted sandstone channel complexes, (3) a terminal glacial diamictite , and (4) a post-glacial flooding surface and high-stand shale bed. In turn this then is overlain by 7-8 m of laminated, siliceous dolostone (cap carbonate). While the glacigenic intervals in the Tillite Group clearly record an active hydrologic component, in the form of meltwater facies and dynamic glacial oscillations, this by no means discounts the presence of Snowball Earth conditions within the stratigraphic time frame of the basin infill, which took place at a paleo latitude of about 30o.

 

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