Glaciations, preservation of their sedimentary record and sea level changes — A discussion based on the Late Precambrian and Lower Palaeozoic sequence in Norway

Terrestrial glacial sediments have a low preservation potential in the sedimentary record. Tillites and other glacial sediments of pre-Quaternary age are often found to represent marginal facies in subsiding marine basins or continental rift environments. Continental rifting provides an ideal tectonic setting for both accumulation of ice and preservation of glacial sediments in the rapidly subsiding rift basins. Uplifted horsts along the rifts may be sites for mountain glaciations at relatively low paleolatitude. The late Precambrian sparagmite basin of South Norway may be an example of this. Major pre-Quaternary glaciations should be expected to be accompanied by eustatic sea level changes. Records of sea level changes in tectonically stable areas may provide some constrains on the timing and magnitude of ancient continental glaciations. The Upper Ordovician sequence in the Oslo Region presents a continuous shallow marine carbonate environment which is sensitive to sea level changes. The records here suggest that the Ordovician glaciations in West Africa were either very minor glaciations compared to the Quaternary glaciation causing less than 20 m of eustatic sea level changes, or that the glaciations occurred in periods marked by tectonic unconformities.