INVESTIGATING THE SEDIMENTARY EVIDENCE FOR THE SNOWBALL EARTH HYPOTHESIS: MODERN ANALOGS AND IMPLICATIONS FOR NEOPROTEROZOIC CLIMATE MODELS

The Neoproterozoic era (c. 750–570 Ma) saw the break-up and assembly of supercontinents and the evolution of multi-cellular life; however, what is less certain is the nature of the Neoproterozoic climate. The Snowball Earth model posits that glaciations during this era resulted in a completely ice covered Earth; a theory based on interpretations of Neoproterozoic sedimentary rocks (diamictites), found on all present-day continents, as glacial tillites. However, others contend that the sedimentology and depositional setting of these rocks suggests that deposition was influenced by active tectonics associated with supercontinent break-up. A key diamictite deposit in this debate is the Squantum ‘tillite’ (c. 595-570 Ma), which accumulated in a volcanically active arc-basin setting during the Neoproterozoic. Our investigations of the Squantum in 2007 revealed that it records the downslope transfer of large volumes of unstable volcanic and sedimentary debris from basin margin slopes due to basin rifting. To better understand the depositional origin of the Squantum this study conducted a detailed sedimentological analysis of the modern glacial deposits of Mount Rainier, a large stratovolcano in Washington, USA. This volcano has undergone extensive glaciation and volcanism since the end of the last ice age, and thus serves as a unique modern analog setting for the Squantum deposit, as well as other Neoproterozoic diamictites (NDs) that accumulated in active volcanic settings. The goal of this study was to determine whether the remobilization of glacial sediment downslope obscures the glacial signature of the deposit. This was achieved by determining whether typical glaciclastic features (i.e. striated stones, strong clast fabrics, etc.) could be identified in both the proximal and distal components of the deposits. The results of this study revealed that the glacial signature of a deposit is indiscernible in distal deposits as many of the tillites are transformed downslope into debris flows. Given the volcanic setting of many NDs, a broader understanding of modern volcanically and glacially influenced sediments is fundamental to the accurate identification of ‘true’ NDs, and thus the true characterization of the Neoproterozoic palaeoclimate.