Chapter 11. Glacial Lithofacies and Stratigraphy

CHAPTER GLACIAL LITHOFACIES AND STRATIGRAPHY 11 J. Lee British Geological Survey, Nottingham, United Kingdom 11.1 INTRODUCTION Reconstructing the environments, dynamics, and record of past glaciation requires a detailed knowledge of both the products of glaciation—effectively the sediments, landforms, and glacitectonic structures that we see in the geological record, and the genetic processes that formed them (Fig. 11.1). Such an understanding of glacial deposits, landforms, and processes underpins our understanding of glacier behaviour, the links between ice masses, climate change, and other feed- back mechanisms, and the applied significance of glaciated terrains from the perspective of resources and geohazards (Fig. 11.1). Building all of this knowledge into a robust geological model requires the employment of a systematic methodology for describing, recording, and interpreting geological evidence. For glacial sediments the principal method, and one routinely employed elsewhere in sedimentology, is the hierarchical lithofacies approach. In turn, understanding how these lithofacies and other glacial features (e.g., landforms and glacitectonic structures) fit together and correlate in both time and space is called stratigraphy. Stratigraphy is a key concept within geology. It enables the development of a framework of events and features that describe both the evolution of a geological succession and how that succession fits into the wider palaeoenvironmental picture or Earth system. Within the context of glacial geology, e.g., a succession of glacigenic sediments and landforms in the Great Lakes region of Canada might document the repeated glaciation of the area. Studying the sediments (lithofacies) and landforms plus their temporal and spatial relationships (stratigraphy), would enable the number of ice advances, their flow directions, and associated geological processes to be reconstructed. These could then be linked-in, perhaps with geochronology, to the larger- scale behaviour of different lobes of the last Laurentide Ice Sheet and ultimately an ice sheet model. Such an approach is equally valid fromanappliedapproachwithlevelsofgenetic interpretation replaced by applied interpretation for the purpose of developing applied thematic models. This chapter will review and illustrate the principles behind lithofacies analyses and stratigraphic approaches within glacial successions. It will outline the appropriate techniques and methods that can be employed as well as the relevant complexities and challenges that may be encountered when applying them. Past Glacial Environments. DOI: http://dx.doi.org/10.1016/B978-0-08-100524-8.00011-7 © 2018 Elsevier Ltd. All rights reserved. 377 378 CHAPTER 11 GLACIAL LITHOFACIES AND STRATIGRAPHY Geological Lithofacies model Geological Stratigraphy interpretation Applied Landforms model Description Interpretation Application FIGURE 11.1 The hierarchical approach to glacial geology encompassing description, interpretation, and application. 11.2 GEOLOGICAL COMPLEXITIES IN GLACIAL SEQUENCES The lithofacies approach (Section 11.3), together with many of the stratigraphic techniques outlined (Section 11.4) are commonly used within a range of geological contexts. However, their successful application to glacial sequences requires due care and consideration of a number of factors that control both preservation potential and the spatial and temporal complexity encountered within the geological record (Rose and Menzies, 1996). The preservation potential for continental (as opposed to marine) glacial sediments and landforms is generally much lower than many other sedimentary environments. This is due to their susceptibility to reworking or removal on land during subsequent glacial events and by the vast range of glacial (e.g., mass-wasting, outburst floods) and nonglacial (e.g., periglacial and slope) processes that can operate within or adjacent to glaciated environments. Preservation is particularly low in upland areas but, by contrast, areas that lie to the periphery of major ice masses tend to act as sediment sinks with improved preservation of both sediments and landforms. Despite this, even in these peripheral areas, glacial sequences are highly fragmented and disparate in appearance, reflecting the range of active processes that form and degrade them. The concepts of both time and space within geology are probably the most important yet chal- lenging aspects of geology to rationalize and are also highly relevant to glacial geology. Episodes of glaciation during the Earth’s history have occurred back as far as the Neproterozoic Era some 750 million years ago. However, the geological time units (e.g., thousands of years to hundreds of millions of years) employed for much of the geological timescale (see Section 11.3.2)areincompre- hensible to us as humans because often we have no standard point of reference or a limited context. Understanding the relative speed and duration of geological processes or specific events that are pre- served within the geological record, provides geologists with that crucial reference point or context. This can in-part be achieved using geochronology, but also through the study of contemporary geological (e.g., glacial) environments which can inform us of the rates and scales at which processes operate at. Some processes occur at very slow rates but may persist over a relatively long period of time. A prime example is the slow but steady accumulation of silts and clays in a glaciolacustrine basin. At the other end of the spectrum are catastrophic processes that occur rapidly, have a 11.2 GEOLOGICAL COMPLEXITIES IN GLACIAL SEQUENCES 379 3 2 8 5 1 27–26 ka 19 ka 7 2 16 ka 17 ka 1 8 17 ka 9 19 ka 16 ka 19 ka 7 6 16 ka 5 19 ka 3 16 ka 16 ka 1 2 17 ka 5 23–21 ka 2 4 19 ka 3 27 ka ? 5 4 16 ka 17 ka Be ka 1 19 ka Be Be 19 ka 19 21 17 ka 16–15 ka 17–16 ka Be – to 17 ka 25 Outer limit of FIS and BIIS Be Retreat margins of FIS and BIIS 23 – 20 ka These numbers refer to marinal positions of the 21 ka Ages on ice limits in calendar years 1 southeastern margin of the FIS and are referred to in Kalm (2012) Be Beryllium 10 ages FIGURE 11.2 Age estimates on the maximum extent and retreat positions of British Irish Ice Sheet and Fennoscandian Ice Sheet during the Last Glaciation (MIS 2) illustrating the time-transgressive and disparate nature of glacial systems. Note that the maximum spatial extent of these ice sheets varies temporally from between 21 and 26À27 ka. From Bo¨se et al., 2012. Quaternary glaciations of northern Europe. Quat. Sci. Rev. 44, 1À25. significant geomorphic impact, but only last a short period of time—e.g., a moraine dam failure and resulting outburst flood (Clague and Evans, 2000). Equally significant are phases of nondeposition, which are called a hiatus, which whilst not unique to glacial environments are nevertheless common. The occurrence of hiatuses reflects localized changes in sediment supply (e.g., cessation of sediment input into a small lake basin) or geological process (e.g., ice-bed decoupling); or much broader changes in palaeogeography (e.g., exposure of a subglacial surface during deglaciation). A spatial understanding of glacial processes is also important. This is well-illustrated by the concept of the so-called ‘Last Glacial Maximum’ (LGM)—the time during the last glacial period (Pleistocene) when ice sheets reached their greatest extent (see chapter: Quaternary Glaciations and Chronology). Global ice volume is traditionally defined as peaking approximately 21,000 years ago (Clark and Mix, 2002). However, Northern Hemisphere ice sheets and sectors of individual ice sheets reached their maximum extent at different times (Fig. 11.2). For example, the British-Irish Ice Sheet reached its maximum extent at 27 ka BP, although the Irish and North Sea Ice Streams reached their maximum limits much later (Clark et al., 2012). Similar patterns can also be recog- nized around the southern sectors of the Fennoscandian Ice Sheet (Houmark-Nielsen and Kjær, 2003; Bo¨se et al., 2012; Kalm, 2012; Marks, 2012; Rinterknecht et al., 2012) and the margins of the Laurentide Ice Sheet (Dyke et al., 2002). Whilst highlighting the caution required when using the LGM term (Bo¨se et al., 2012), this example also draws attention to the asynchronous nature of many glacial systems and their coupling to the ‘global system’. Thus, conventional stratigraphic 380 CHAPTER 11 GLACIAL LITHOFACIES AND STRATIGRAPHY correlation methods based upon ‘counting-up’/‘lithological similarity’ (lithostratigraphy) or ‘land- form preservation’/‘maximum extent’ (morphostratigraphy) are inherently weak and overlook independence (both within and between glacial systems) and the role of local processes, influences, and responses as controls within the glacial system (Rose and Menzies, 1996; Benn and Evans, 2010; Bo¨se et al., 2012). From the above outline, it can be seen that there are many factors that influence both the preservation and nature of glacial sequences. Within Section 11.3 the concept of ‘glacial lithofacies’, a key technique in describing and interpreting glacial sequences and one of the main the building blocks of stratigraphy, is introduced. 11.3 GLACIAL LITHOFACIES 11.3.1 INTRODUCTION Glacial sediments can be deposited (or tectonically accreted) in a range of different settings that may be defined by their geomorphology (e.g., glaciolacustrine, glaciofluvial) and position relative to a body of ice (e.g., subglacial, englacial, supraglacial, ice-marginal, and proglacial).

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