Geological constraints on Neoproterozoic glacial episodes
Neoproterozoic glacial episodes are amongst the most intense glaciations that the Earth has experienced, and are associated with major changes in the Earth System, such as the breakup of a supercontinent, the evolution of complex multicellular life and extreme geochemical perturbations. A Neoproterozoic stratigraphic scheme has been developed over the last thirty years that uses these major glacial episodes for correlation, yet Neoproterozoic tillites remain stratigraphically under-constrained. This thesis investigates stratigraphic and sedimentological constraints on Neoproterozoic glacial episodes through field based case studies on all Neoproterozoic purported tillites in Britain and on the namesake formations of the three classic Neoproterozoic glacial episodes. This is accompanied by an analysis of a database of all pre-Neogene purported tillites constructed from a literature survey. Together these strands of research offer insights into the constraints on Neoproterozoic glacial episodes provided by different stratigraphic techniques, and the application of those techniques to reconstruct Neoproterozoic glacial episodes. Results show that 1) age constraints on Neoproterozoic purported tillites are on average ten times less precise than on Paleozoic ones; 2) 190 tillites have age constraints that are compatible with an Ediacaran age; 3) cap carbonate stratigraphy is not applicable to 63% of Neoproterozoic tillites; 4) the presence of Neoproterozoic cap carbonates is not conclusive evidence of a Hard Snowball Earth; and 5) a single purported tillite in Great Britain has strong evidence for ice. This thesis demonstrates that glacial episodes do not provide a reliable basis for the correlation of Neoproterozoic successions.