The Causes and Implications of Microstructures in Glacial Sediments
PhD Thesis, 1998

School of Geography, University of Leeds


Andrew Evans


This thesis examines how microstructures in glaciogenic sediments reflect the processes forming them, and how these microstructures then affect the conditions around them, through a series of field studies, laboratory tests, models, and statistical analyses. Following literature reviews, a deformational chronology is developed for diamictons at Criccieth, North Wales, and their microstructures are used to indicate the stress, hydraulic, and environmental changes the materials have undergone. Microstructures of the lowest diamict indicate clast lodgement. The processes reflected in the microstructures of this lowest diamict are built into a quantitative model that estimates its residual strength (20 - 50 kPa) and the ice velocity during lodgement (20 - 50 m a-1). The response of sediment to glacial stress is further examined by triaxial testing of diamict from Yorkshire, and the subsequent examination of its micromorphology. Shears in the material are disrupted by clasts, and this may be responsible for work hardening seen during the tests. Fabric compression, and the development of immobilised shears or hydraulic fractures buffer pore fluid pressure to ~470 kPa. The information from previous chapters is then used to analyse other material from the Yorkshire coast. This analysis confirms the presence in the area of meltout tills that have undergone low strain, as well as providing evidence for the decoupling of the ice and sediment in this region, and the nature of drainage systems within and above the diamicts during glaciation. Overall this thesis details the processes forming three ‘classic’ microstructures found in glacial sediments; omnisepic fabrics, lattisepic fabrics, and melanges, and provides evidence for the processes involved in forming diamict pebbles and skelsepic fabrics. In addition this thesis details how such structures reflect coupling and decoupling processes between glaciers and their beds, and examines the manner in which microstructures affect the response of a subglacial sediment body to stress and hydraulic conditions.

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Please note in accordance with Leeds' policy on PhD theses that by downloading this thesis you agree to give the following details in reference to any materials quoted from it.

Evans,A.J. (1998) The Causes and Implications of Microstructures in Glacial Sediments, Unpublished PhD thesis, University of Leeds, pp.236.

Chapters and synopses

Titles and Acknowledgements

Title, Abstract, Contents, Acknowledgements, Thanks, Figures and Tables.

Chapter One: Introduction

Chapter summary.

Chapter Two: Previous Studies of Microscopic Glacial Sediment Structures

Problems involved in interpretation. An introduction to glacial sedimentary microstructures. The Van Der Meer classification. Possible causes. Assigning microstructural suites to glacial and pro-glacial environments.

Chapter Three: The Development and Effect of Microscopic Deformational Structures

Outlines material learnt from other branches of geology in terms of the dynamics and effects of microstructures on sediment development. Mechanisms for the creation and reduction of porosity: electro-chemical, consolidation, shear. The devlopment of shears, their hydrological and strength effects. Speculations on this basis for shears under glaciers: shear development, disruption and recognition.

Chapter Four: Historical Reconstruction from Micromorphology: A Case Study from the Lleyn Peninsula, Wales

An examination of the use of passively considered microstructures to determine the environmental history of an area. Description of the deposits at Criccieth. Previous interpretations. The macrostructure: folding and contacts between lithofacies. The microstructure. Interpretations. History: glacial, proglacial, postglacial. Potential weathering/chemical development and environmental interpretation.

Chapter Five: The Ploughing and Lodgement of Clasts: A Field Based Case Study and Theoretical Model

An examination of the development of active microstructures, and interpretation, and computer model. The model is used to estimate the rheological properties of the sediment during deformation at the end of the last glacial period. Macrostructural traces of clast ploughing. Microstructural melanges. A Weertman - derived model of ploughing that creates the appropriate microstructures. An estimate of sediment shear strength and glacial speed using the model and the field measurements.

Chapter Six: Till Deformation with Clasts: Laboratory Experiments

An examination of the active effects of microstructures on till hydro-physical properties. Overview of previous lab-based deformation of tills/diamicts: Kamb's shear box, ring-shear; triaxial. Creating a glacial analogue. The results of triaxial tests: stress, fluid storage/porosity, pore water pressure gradients, stress paths, deformation rates and shapes. Dilation. Catastrophic pore water pressure releases and work-hardening.

Chapter Seven: The Development of Micromorphology in Laboratory Tests

An analysis of the microstructures formed in the laboratory tests and their comparison with the physical properties and stress paths seen. Sample history. Strength, hydrological and fabric relationships.

Chapter Eight: The Application of Microstructural Analysis to Macroscale Forms: The Dynamics of the UK East Coast Late Devensian (Oxygen Isotope Stage II) Glaciers

A variety of field sites representing a broad range of the East Coast sediments were sampled. Microstructural samples were taken and interpreted with reference to the macroscale features, previous studies, and the knowledge developed in previous chapters. Filey Brigg: meltout till into environment of ephemeral subglacial streams. Possible stress/effective pressure paths presented. Dimlington: Overprinting from repeated deposition episodes, fluid through-flow and winnowing subglacially. Potential intra-bed throughflow with sediment removal causing till hardening and descrete shear. Statistical analysis of till pebble development from shearing fabrics. Low effective pressures and possible shear stiffening. Reighton Sands: Saturated sediment deformation at low effective pressures with high through-flow. Potential hydro-physical history suggests two episodes of rapid flow with consolidation between matching a multiple surge terrestrial reworking of glaciolacustrine materials. Conclusions: sediment strength and fabric development, hydrology, low strain microstructures developing under ice-sediment coupling, low strain microstructures developing under meltout and decoupling.

Chapter Nine: Conclusions

The development processes for: Omnisepic fabrics, descrete shear, lattisepic fabrics, melanges, skelsepic fabics, and till pebbles. The microstructures associated with ice-bed coupling, the till response to stress and fluid storage.

Appendix One: Glossary

Description of glacial sedimentary, soil science and geological terms.

Appendix Two: Programs

Pascal code for the clast lodgement models.


Details of the works quoted.

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