Paving the way to understanding Neoproterozoic glaciation : striated pavements and associated strata of Laurentia, Tarim and the North China Craton

The Neoproterozoic Era, 1000 to c. 540 million years ago (Ma), was an important transition in Earth's evolutionary and climatic history. During its latter part, the Cryogenian (c. 720-635 Ma) and Ediacaran (c. 635-540 Ma) periods, it experienced glaciation for the first time in over a billion years. The extent, behaviour and severity of this glaciation, however, remain uncertain. The most extreme reconstructions posit thick ice from equator to pole for tens of millions of years, representing a challenge to the continuation of life on Earth. This uncertainty, therefore, represents a major barrier to our understanding of an important chapter in Earth's history, with fundamental evolutionary implications. Subglacially striated pavements offer unique insights towards resolving this problem. They confirm the existence, position, flow direction, erosional efficacy and thermal regime of past grounded ice. The latter is perhaps the single most important control on ice movement, meltwater production and routing, sediment transport and deposition, erosion and resultant geomorphology. Neoproterozoic pavements are, however, globally rare and their interpretation beset with unresolved issues. First, there is misidentification. Several purported Neoproterozoic pavements have alternatively been interpreted as non-glacial features and their status remains uncertain. Second, previous publications have only provided general descriptions of striated pavements. No publication, prior to those associated with this thesis (see Declaration of Authorship and Appendix 4), has taken advantage of recent advances in technology to present high resolution photogrammetric data of Precambrian pavements. Consequently only simple interpretations have been offered for these pavements and scrutiny of those interpretations has been impossible without individual workers visiting remote outcrops. Third, although it is clear that striated pavements and associated strata independently provide different insights into past processes, it is less clear how these data may be combined to provide further insights into larger-scale questions. For example, those relating to depositional environment, sediment erosion and provenance, palaeogeographic setting and tectonic setting. This thesis addresses those issues. Chapter 4 reinterprets the only uncontested purported Neoproterozoic pavement on the North American continent as a non-glacial feature. To do so, I develop a novel approach that uses both traditional fieldwork and detailed photogrammetric surveys to record purported Precambrian geomorphology and, counter-intuitively, the surrounding modern landscape in equal detail. Chapter 5 uses traditional fieldwork and detailed photogrammetric techniques to document Ediacaran pavements of the North China Craton, presenting geomorphic data at a level of detail far surpassing any previously attempted for a Precambrian pavement. Superficial examination shows clear signs of subglacial abrasion by warm-based ice and apparently clear signs of pervasive subglacial quarrying. However, our more detailed analysis reveals that subglacial abrasion was likely only a light erosional touch and that subglacial quarrying was minimal or absent. The stepped form of the pavement, observed today, is instead the result of later fault slip in a non-subglacial setting; it does not record subglacial quarrying. Overall, despite clear evidence for past warm-based glaciation, these findings paint a surprising picture of minimal erosion that is only revealed by our unusually detailed examination. Chapter 6 explores how a novel geochemical approach may be combined with the findings from the pavement of Chapter 5 to test this hypothesis of minimal erosion at a more regional scale. This reveals that the profound unconformity beneath the Luoquan Formation, correlated with the "Great Unconformity" documented globally, most likely already existed during Luoquan Formation deposition. It transpires that, in this instance, glacial erosion was not responsible for the disconformity. Therefore, whereas the pavement based study of chapter 5 supported minimal erosion at the local scale, the chemical study of chapter 6 confirms this minimal erosion at the regional scale. Chapter 7 combines data from pavements, analysis of overlying sedimentary facies, detrital zircon ages and published geochemical and tectonic data. With these it reconstructs Neoproterozoic erosional and depositional processes, evolving depositional environments, evolving palaeogeography and tectonic setting on the NW Tarim margin. This is the first attempt at Precambrian glaciological reconstruction at this level of detail on the Tarim palaeocontinent and reveals the potential of combining apparently disparate data types from pavements and associated strata. Chapter 8 discusses the combined findings of chapters 4-7 in relation to the aims of this study (Section 1.2) then presents recommendations to future workers based upon those findings. Chapter 9 presents brief summary conclusions in relation to Chapters 4-8.