Tucker - Sedimentary rocks in the field: a practical guide

The Geological Field Guide Series

Barnes, J.W. and Lisle, R.J. (2004) Basic Geological Mapping, 4th edn. ISBN: 978-0-470-84986-6, 5th edn publishing (2011).

ISBN: 978-0-470-68634-8

Fry, N. (1991) The Field Description of Metamorphic Rocks.

ISBN: 978-0-471-93221-5

McClay, K.R. (1991) The Mapping of Geological Structures.

ISBN: 978-0-471-93243-7

Milsom, J. and Eriksen, A. (2010) Field Geophysics, 4th edn.

ISBN: 978-0-470-74984-5

Tucker, M.E. (2011) Sedimentary Rocks in the Field, 4th edn.

ISBN: 978-0-470-68916-5

This edition first published 2011 2011 by John Wiley & Sons, Ltd.

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Library of Congress Cataloguing-in-Publication Data

Tucker, Maurice E.

Sedimentary rocks in the field: a practical guide/Maurice E. Tucker. - 4th ed.

p. cm.

Includes bibliographical references and index.

ISBN 978-0-470-68916-5 (paper)

1. Sedimentary rocks. 2. Petrology-Fieldwork. I. Title.

QE471.T84 2011

552.5dc22

2010033307

A catalogue record for this book is available from the British Library.

First Impression 2011

PREFACE

The study of sedimentary rocks is often an exciting, challenging, rewarding and enjoyable occupation. However, to get the most out of these rocks, it is necessary to undertake precise and accurate fieldwork. The secret of successful fieldwork is a keen eye for detail and an enquiring mind; knowing what to expect and what to look for are important, although you do need to remain open-minded. Be observant, see everything in the outcrop, then think about the features seen and look again. This book is intended to show how sedimentary rocks are tackled in the field, and has been written for those with a geological background of at least first-year university or equivalent.

At the outset, this book describes how the features of sedimentary rocks can be recorded in the field, particularly through the construction of graphic logs. The latter technique is widely used since it provides a means of recording all details in a handy form; further, from the data, trends through a succession and differences between horizons readily become apparent. In succeeding chapters, the various sedimentary rocktypes, textures and structures are discussed as they can be described and measured in the field. A short chapter deals with fossils since these are an important component of sedimentary rocks and much useful information can be derived from them for palaeoenvironmental analysis; they are also important in stratigraphic correlation and palaeontological studies. Having collected the field information, there is the problem of knowing what to do with it. A concluding section deals briefly with facies identification and points the way towards facies interpretations, and the identification of sequences and cycles.

Maurice E. Tucker

ACKNOWLEDGEMENTS

I should like to thank the friends and colleagues who have willingly read drafts of this handbook, suggested changes for this fourth edition, and kindly provided photographs, in particular Jenny Bevan, Telm Bover-Arnal, Jim Gallagher, Annette George, Dougal Jerram, JuanCarlos Laya, Mike Mawson, Zahra Seyedmehdi and Paul Wright. Zoe took the picture on the front cover. I am indebted to Vivienne for providing support, encouragement, assistance and patience in ways that only a wife can.

INTRODUCTION

This book aims to provide a guide to the description of sedimentary rocks in the field. It explains how to recognise the common lithologies, textures and sedimentary structures and how to record and measure these features. There is a chapter explaining how fossils can be studied in the field, since they are common in many sedimentary rocks and are very useful for palaeoenvironmental analysis. A concluding chapter gives a brief introduction to the interpretation of sedimentary rock successions: facies, facies associations, cyclic sediments and sequences.

1.1 Tools of the Trade

Apart from a notebook (popular size around 20 10 cm), pens, pencils, appropriate clothing, footwear and a rucksack, the basic equipment of a field geologist comprises a hammer, chisel, hand-lens, compass-clinometer, tape measure, acid bottle, sample bags and marker pen. A Global Positioning System (GPS) receiver is most useful, and not only in remote areas (see Section 1.3). A hard hat for protection when working below cliffs and in quarries, and safety goggles for the protection of the eyes while hammering, should also be taken into the field and used; see Section 1.4 for further safety considerations. A camera is invaluable. Topographic and geological maps should also be carried, as well as any pertinent literature. If a lot of graphic logging is anticipated (see Section 2.4) then pre-prepared sheets can be taken into the field. Non-geological items that are useful and can be carried in a rucksack include a whistle, first-aid equipment, matches, emergency rations, knife, waterproof clothing and a space blanket.

For most sedimentary rocks, a geological hammer of around 0.51 kg (12 lb) is sufficiently heavy. However, do be sympathetic to the outcrop and remember that many future generations of geologists will want to look at the exposure. In many cases it will not be necessary to hammer since it will be possible to collect loose fresh pieces from the ground. A range of chisels can be useful, if a lot of collecting is anticipated.

A hand-lens is an essential piece of equipment; 10 magnification is recommended since then grains and features down to 100 m across can be observed. When holding the lens close to your eye, the field of view being examined with a 10 lens is about 10 mm in diameter. To become familiar with the size of grains as seen through a handlens, examine the grains against a ruler graduated in millimetres. For limestones, it can be easier to see the grains after licking the freshly broken surface.