Recognising cold-based glaciation in the rock record: striated bedrock surfaces of the > 540 million year old Luoquan Formation of China

When preserved from deep time glaciation, subglacially striated bedrock surfaces allow the interpretation of past ice characteristics that are often elusive from the study of sediments alone. Salient amongst these is the thermal regime, which has a profound influence upon ice behaviour and consequent sediment erosion, transport and deposition. Typically, striated bedrock surfaces are linked to ice at its pressure-temperature melting point, indicating a locally warm-based thermal regime. Conversely, a cold-based thermal regime is defined by ice frozen to the substrate and is linked to minimal erosion. Cold-based erosional forms have been identified in Antarctica but their recognition is next to impossible if imprinted upon a surface previously or subsequently affected by warm-based erosion (e.g. striation). In the ancient record this is especially problematic, as it is typically only through the recognition of characteristic warm-based features that a surface can be confirmed as subglacial at all. Consequently, it is likely that there is an observational bias in the rock record toward warm-based over cold-based ice. This study, through careful geomorphologic analysis of unusually well preserved striated surfaces of the North China Craton from the Ediacaran Period (c. 635 – 540 Ma), presents rare examples that record dominant cold-based and more limited warm-based erosion on the same subglacial surface. It is hoped that this approach may benefit other workers interested in identifying cold-based as well as the more obvious warm-based subglacial conditions from the record of deep time glaciation.