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MANUAL OF SEDIMENTARY PETROGRAPHY

The Century Earth Science Series

KiRTLEY F. Mather, Editor

MANUAL OF SEDIMENTARY PETROGRAPHY

I. Sampling, Preparation for Analysis, Mechanical Analysis, and Statistical Analysis

BY

W. C. KRUMBEIN

II. Shape Analysis, Mineralogical Analysis, Chemical Analysis, and Mass Properties

BY

F. J. PETTIJOHN

Department of Geology, University of Chicago

D. APPLETON-CENTURY COMPANY

INCORPORATED

NEW YORK

LONDON

COPVRIGHT, 1938, BY

D. APPLETON-CEXTURY COMPANY. IN'C.

All rights reserved. This book, or parts thereof, must not be reproduced in any form without permission of the publisher.

PEIXTED IX THE UNITED STATES OF AMERICA

EDITOR'S PREFACE

When Sorby and his contemporaries were laying the foundations for modern petrographic research, back in the middle of the nineteenth century, the importance of microscopic examination of sedimentary rocks was stressed on at least one occasion. Nevertheless, for more than half a century the scientific study of that type of rocks received rela- tively slight attention. Many petrographers peered through their micro- scopes at thin sections of igneous rocks; many paleontologists measured minutely the shape of fossils; many stratigraphers debated the sys- tematic position and correlation of sedimentary formations ; but few geologists gave any really serious thought to the accurate and detailed study of the mineralogic characteristics of sediments and the rocks formed therefrom.

During the last twenty years, however, there has been a notable in- crease in interest iji this phase of geology, with a very gratifying ex- pansion of knowledge as a result of extraordinary improvements in the techniques of study. This is in part a by-product of the a]:)plication of geology in the petroleum industry, in part a result of the discovery that sedimentary rocks provide a field for pure research unexcelled by any other field within the broad area of earth science.

The closely related sciences of sedimentary petrography and sedi- mentary petrology are to-day established on a firm base of technical procedure and deductive theory. They have become worthy of a life- time of specialization which will well repay the devotion of a consider- able minority of geologists.

Most of the data which such specialists should use are widely scat- tered through a large number of memoirs and periodicals which have been published within the last fifteen years. Many of these are to be found only in the journals devoted to physics, soil science, statistical method, and colloidal chemistry, and in other sources which like those journals are not ordinarily available in geologic laboratories. No ade- quate handbook of sedimentary petrography has hitherto been published in this country. Drs. Krumbein and Pettijohn have therefore rendered a signal service to the student and worker in this field by preparing this very useful volume.

V

vi EDITOR'S PREFACE

It is primarily concerned with the methods of petrographic analysis of the sedimentary rocks, including the unconsolidated sediments. It covers every step of the process, from the field sampling to the final graphic and statistical analysis, with due regard for theory as well as method. It will serve admirably not only as a textbook for students but also as an indispensable aid for the professional worker dealing as in petroleum geology with sedimentary rocks and the valuable resources which they contain.

KiRTLEY F. Mather.

PREFACE

I often say that if you measure that of which you speak, you know something of your subject; but if you cannot measure it, your knowledge is meager and unsatisfactory. Lord Kelvin.

The recognition, long overdue, of the value of laboratory analysis in the study of sediments is beginning to be apparent. From the colloidal chemist, the ceramist, the ore-dressing engineer, the pedologist. the mineralogist, the statistician, and others the geologist has adapted methods which will eventually go far toward making the study of sedi- ments a more exact science.

The writers of this volume do not depreciate field study. Such studies are a necessary prelude to the laboratory work and are fundamental to the science of geology. Realization, however, that sediments, like all other rocks, are a product of definite physical and chemical processes and are capable of definite analysis on the basis of carefully gathered quantitative data has been very slow indeed among geologists as a group. We believe that any consideration of the origin of a deposit which neglects the analysis of the material itself is quite incomplete.

It has been pointed out by some that our ability to interpret analytical data lags greatly behind our ability to make the analyses and that there- fore further refinement in technique and greater accuracy in description are superfluous. It is even argued that laboratory analyses only confirm what the field geologist already knows, and such work is therefore regarded only as a refinement and not as a new contribution. The au- thors are not in sympathy with this view. It may be evident, even to the naked eye, that the sand along a beach decreases in size in the direction of transport, but whether the rate of decrease in size is expon- ential or conforms to some other law is not evident. To discover some underlying law or relationship introduces a new element into geological theory and opens up new avenues of thought.

In addition to the establishment of new principles and the interpreta- tion of rock origins, laboratory study of sediments has important eco- nomic applications. Well known is the study of "heavy minerals," which has proved its worth in the correlation of sedimentary formations. The technologist has long recognized the necessity for physical analysis of

viii PREFACE

the materials with which he is concerned. The geologist is often called upon to prospect for and estimate the worth of pottery clays, brick earths, fire-clay, fullers' earth, molding sand, etc. He must therefore be able to use such methods of analysis as will serve to indicate the use- fulness of a deposit for the purpose intended. In fact, any one engaged in the study of the particulate substances, natural or artificial (cement, paint pigments, etc.), will find valuable the methods of study of particle size and particle shape and the optical methods of identification.

In so far as the geologist is involved in problems of petroleum pro- duction and reserves, or engaged in mapping where the soils are the only clue to the nature of the subjacent fonnations, or engaged in pros- pecting for or estimating the worth of alluvial deposits, or involved in a study of the problems of soil erosion and reservoir silting, he will find the methods described herein pertinent.

The science of sedimentary petrolog}% or sedimentology, has now reached a stage of development which involves a large number of tech- niques unique to this science and distinct from those employed in the study of the igneous and metamorphic rocks. These techniques are described in a widely scattered literature in the literature of ceramics, pedolog}', petroleum technology, hydrolog}-, etc. Growing interest in sedimentology, as evidenced by the increasing number of courses de- voted to the subject, the establishment of a journal devoted exclusively to this field, and the increasing use of its methods and principles in the exploitation of petroleum and other mineral resources, has, we believe, justified the attempt to bring together, for the benefit of the geologist and other students of sedimentary materials, methods of analysis ap- plicable to these substances.

The compilation of material from an extensive literature has raised nimierous perplexing problems. Sciences vary widely in their termi- nology and in their use of mathematics. Fundamental principles common to one field are largely unknown to other fields. The authors have ac- cordingly decided to write this book primarily from the point of view of the geologist, with the hope that it will be of value to workers in other fields, at least to the extent of marshaling some of the literature for them. Geologists as a group are not mathematically inclined, but among geologists are many who have a command of mathematics and physics. The problem of writing a volume of interest to both extreme groups has been difficult.

No pretense is made of making the volume complete or exhaustive. It is inevitable that, in a work as broad as the present one. the authors should give most space to those fields and methods with which they are

PREFACE ix

most familiar. Nevertheless we believe the allotment of space to the various techniques and fields reflects fairly well present-day interests and needs. Where methods are well established and generally familiar, as are the optical methods, summaries suffice ; where newer and less well known procedures are involved, more detail has been sui)]:)lied.

It is perhaps inevitable also that there should be some omissions of important material, owing to the wide literature involved. The authors would appreciate advices concerning such omissions. Some selection has had to be made by the authors, but as far as possible references are given to further details elsewhere. No apology is made for using per- sonal material for illustrations and examples ; the greater convenience of working with familiar material is its own justification.

The present volume is largely the joint efi^ort of the two authors, but fortunately their fields of specialization adapt themselves to a divi- sion of the book into two parts. This division is more apparent than real. Individually the authors assume responsibility for their separate portions; jointly they assume responsibility for the apportionment of space and the thread of continuity which runs through the book.

This manual is written for a person of average training in the methods of science. It is assumed only that the worker has had an elementary training in laboratory technique such that he can handle a chemical balance intelligently and that he has a working knowledge of elementary physical and mathematical theory and some knowledge of crystallography.

We believe the book will be found suitable as a textbook for courses in sedimentary petrography. We have therefore attempted to explain both principles and objectives of the various techniques of analysis and to raise in the student's mind a critical attitude toward the purposes and methods of sedimentary analysis.

The authors are indebted to many writers and workers for the final design and content of this book. Gessner's excellent treatise, Dcr Schldmmanalysc, Johannsen's Manual of Pctrographic Methods, I.arsen and Berman's Microscopic Determination of the Non-opaque Minerals. and Boswell's Mineralogy of Sedimentary Rocks have been of inesti- mable value, and numerous essays and comprehensive articles have fur- nished inspiration and information. Credit has been given in many of these cases.

The authors are indebted to numerous individuals for advice and criticism. Dr. Carl Eckart of the Department of Physics of the Uni- versity of Chicago has helped in the mathematical treatment of the theoretical parts of mechanical analysis; Dr. M. W. Richardson of the

X PREFACE

Dqjartment of Psychology has critically read the chapters on statistics ; and Mr. Paul Reiner has criticized several portions of the text. Many of the illustrations were prepared by Messrs. H. HoUoway, A. Lundahl, and W. C. Rasmussen, of the Universit}- of Chicago. Among our col- leagues in the Department of Geolog}*, Drs. J. H. Bretz, Carey Croneis, and A. Johannsen have made valuable suggestions as to style and con- tent. Among other geologists and sedimentar}- petrologists who have read portions of the text are Dr. W. W. Rubey of the United States Geological Survey, J. L. Hough and Dr. Gordon Rittenhouse of the United States Soil Conservation Service, and Mr. G. H. Otto of the Soil Conservation Laborator}-, Pasadena, California. Dr. Kirtley Mather, Editor of the Centurv- Earth Science Series, has been unfailing in his encouragement during the preparation of the text. Messrs. D. H. Ferrin and F. S. Pease, Jr., of D. Appleton-Century Company have smoothed many difficulties in the arduous task of seeing the book through the press.

\V. C. Krumbeix. F. T. Pettijohx. Chicago, Illinois

CONTENTS

PART I

SAMPLING. PREPARATION FOR ANALYSIS. MECfTANI- CAL ANALYSIS, AND STATISTICAL ANALYSIS

PACE

Chapter i. Introduction 3

Definitions. Properties of component ^Mviins. At- tributes of grains in the aggregate. Properties of the aggregate. Preh'minary field and laboratory sched- ules. Field observations during sampling.

Chapter 2. The Collection of Sedimentary Samples .

Introduction. Purposes of sampling. Outcrop samples, discrete, serial, channel, and compound. Sub-surface samples. Bottom samples. The problem of weathering. The problem of induration. The collection of oriented samples. Size of samples. Containers for samples. Capacities of sample con- tainers. Labeling and numbering of samples. Theory of sampling sediments.

Chapter 3. Preparation of Samples for Analysis .

Introduction. Preliminary disaggregation. Sample splitting. Preparation for mechanical, mineralogical, shape, and surface texture analysis. Physical dis- persion procedures. Chemical dispersion procedures. Theory of coagulation. General critique of disper- sion. Generalized dispersion routine.

Chapter 4. The Concept of a Grade Scale

Introduction. Modern grade scales. Problems of unequal class intervals. Functions of grade scales, descriptive and analytic. Choice of a grade scale.

Chapter 5. Principles of Mechanical Analysis

Introduction. Classification of disperse systems. Concept of size in irregular solids. Settling veloci- ties of small particles. Stokes' law and its assump-

II

43

76

91

CONTEXTS

tions. Other laws of settling velocities. Theory- of sedimenting systems. Oden's general theorv-. Prin- ciples of modem methods. Principles of older meth- ods. Theory of sie%nng. Theory- of microscopic methods of analysis. Summar}-.

Chapter 6. Methods of Mechanical Analysis .... Introduction. Sieving methods. Direct measure- ment of large particles. Decantation methods. Rising current elutriation. Air elutriation. The sedimenta- tion balance. Continuous sedimentation cj^linders. The pipette method. The hydrometer method. Photo- cell method. ^Microscopic methods of analysis. Com- parisons of methods of mechanical analysis.

Chapter 7. Graphic Presextatiox of Analytical Dat.\ . Introduction. General principles of graphs. Choice of dependent and independent variables. Graphs in- voh-ing t^vo variables. Histograms, cumulative curxes, and frequenc\- cur\'es. Graphs with distance or time as independent -v-ariable. Scatter diagrams. Graphs invohnng three or more variables. Isopleth maps and triangle diagrams. Mathematical analysis of graphic data. Linear, power, and exponential functions.

Chapter 8. Elements of Statistical An.xlysis .... Introduction. The concept of a frequenc}* distri- bution. Histograms and ctunulative cur\-es as sta- tistical devices. Introduction to statistical measures of the central tendencx', the degree of scatter, and degree of as}Tnmetr\-. Arithmetic and logarithmic frequenc)' distributions. Ouartile and moment meas- ures. The question of frequeno*.

Chapter 9. Application of Statistic.\l Measl-res to Sedi- ments

Introduction. Ouartile measures. arithmetic geometric, and logarithmic. Moment measures, arithmetic, geometric, and logarithmic. Special sta- tistical measures. Sorting indices. Choice of statisti- cal dexices. Statistical correlation. Chi-square test. Theor}- of control. The probable error.

135

182

212

228

CONTENTS

Xlll

PACE

Chapter io. Orientation Analysis of Sedimentary Par- ticles 268

Introduction. Collection of oriented samples. Laboratory analysis of particle orientation. Presen- tation of analytical data. Statistical analysis.

PART II

SHAPE ANALYSIS. MINERALOGICAL ANALYSIS,

CHEMICAL ANALYSIS, AND MASS

PROPERTIES

Chapter ii. Shape and Roundness 277

Introduction. Review of (juantitatiye methods. Choice of method. Procedure of analysis. Method for large fragments. Wadell's method for sand grains.

Chapter 12, Surface Textures of Sedimentary Fragments

and Particles 303

Introduction. Surface textures of large frag- ments. Surface textures of small fragments.

Chapter 13. Preparation of Sample for Mineral Analysis 309 Introduction. Disaggregation. Clarification of grains. Special preparation problems.

Chapter 14. Separation ]\Iethods 319

Preliminary concentration of heavy minerals. Separation on basis of specific gravity. Heavy liquids. Standardization of heavy liquids. Separa- tion apparatus. Use of centrifuge. Analytical procedure. Separation on basis of magnetic per- meability. Separation on basis of dielectric proper- ties. Separation on basis of electrical conductivity. Separation on basis of visual properties. Separation on basis of shape. Separation on basis of surface tension. Separation on basis of chemical proper- ties. Errors in separation. Systematic schemes of separation.

Chapter 15. Mounting for Microscopic Study .... 357 Splitting. Mounting. Preparation of thin sections. Film method of study.

XIV

Chapter

Chapter

Chapter

Chapter

Chapter

Chapter

Author Subject

CONTENTS

PAGE

i6. Optical Methods of Identification of Min- erals 366

Introduction. The polarizing microscope. Meas- urements of small particles. Fundamental optical constants. Observations in ordinary light. Observa- tions in plane polarized light (crossed nicols). Observations in convergent light. Special methods for the study of clays. Preparation. Identification.

17. Description of Minerals of Sedimentary Rocks 412

Introduction. Mineral descriptions. Determina- tive tables. Miscellaneous tables. Record forms.

18. Mineral Frequencies and Computation . . 465

Pebble counts. Thin-section analysis. Mineral frequencies. Presentation of results. Calculation of mineral frequencies based on analysis of several fractions. Statistical methods. Mineral variations. Statistical correlation.

19. Chemical Methods of Study 490

Introduction. Quantitative analysis. Methods. Computations based on quantitative analysis. Mi- crochemical methods. Organic content. Insoluble residues. Staining methods.

20. Mass Properties of Sediments 498

Introduction. Color of sedimentary materials. Specific gravity of mineral grains and of sedimen- tary rocks. Porosity. Definitions. Determination of porosity. Methods of porosity measurement. Per- meability. Plasticity. Definitions. Methods of measurement. Hygroscopicity. Miscellaneous mass properties.

21. The Laboratory, Equipment, and Organiza- tion OF Work 522

The laboratory. Apparatus. Reference books. Or- ganization.

Index 533

Index 539

PART I

SAMPLING, PREPARATION FOR ANALYSIS,

MECHANICAL ANALYSIS, AND

STATISTICAL ANALYSIS

CHAPTER 1

INTRODUCTION

SCOPE OF SUBJECT

The study of sediments is concerned with Xl) the physical conditions of deposition of a sediment, whether glacial, fluvial, marine, etc.; (2) thTTime ojTormatjon or age of the deposit; and (3) the provenance^' or^area of denudation that furnished the material composing the sedi-"" ment. All of the analytical methods described in this volume have as their^ common aim the elucidation of these points.

^^arious names have been applied to the detailed study of sediments, ranging from sedimentation through sedimentary petrology to sedimen- tology. The latter word has not come into general use, despite its con- ciseness and clear meaning ; it may be said that usage favors the second term. Whatever name may be ultimately chosen, there is no doubt that the subject involves a complete study of sediments from the point of view and with the methods of pure science. Here are included not only geological methods of study, as typified by field work, but also the methods of the chemist, the physicist, and the statistician. In short, the complete study of sediments must make use of any and all devices which lead to an understanding of the nature and origin of the sedi- ment in question.

This broad viewpoint means that the study of sediments may be approached from various angles. PYom one angle it may be a study of the size attributes of sediments as physical mixtures of particles ; from another it may be a study of mineral suites which by depositional con- ditions have been united into a single deposit ; or, the sediment may be considered as a composite of sizes, shapes, and minerals controlled by complex environmental conditions, and the investigation may seek to evaluate the conditions of that environment. All of these points of view are related, and in their ultimate end are directed to the elucidation of geological problems, many in direct connection with historical geology.

Whatever the point of view applied to sedimentary investigations, laboratory studies will become an increasingly important source of data.

3

4 SEDIMENTARY PETROGRAPHY

Not alone do laboratory analyses supplement and refine field observa- tions, but often they afford data which cannot be gleaned by field meth- ods alone. Criticisms are often leveled against the application of refined methods of analysis to geological problems, either on the ground that they give a specious air of preciseness to fundamentally approximate data or on the ground that geolog}' is completely studied in the field and laboraton.- studies should be left to chemists and physicists. The first criticism has been more pertinent in the past than it is now, because even the rather poor data afforded by early laborator}- studies of sedi- ments have paved the way for improvements in technique and interpre- tation, as well as for tests to determine the degree of accuracy of the data. The second criticism needs no answer : tEe world is the geologist's domainpand he is justified in using whatever techniques he requires to solve problems fundamentally geological. True, much remains to be im- proved in the laboratory study of sediments, but there can be no doubt that interest is growing in the subject, and will continue at an accelerated pace during the next decade at least.

Among soil scientists there is a fairly standardized routine of analysis, but this stage of development has by no means been reached in sedi- mentary studies. One finds in a single year papers prepared on methods of analysis or presentation of data as remote as the poles, and it is no small problem to determine the relative dependability of the methods or the degree to which the results are comparable. It is perhaps too early to advocate the adoption of standardized routines for sedimentary analysis, inasmuch as it is not clear in all cases whether current data are the most valuable for the ends toward which they are directed. Natural phenomena are exceedingly complex when examined in detail, and anahtical procedures must be developed which do not destroy the very data being sought. It is at least fortunate that much current work is done \\-ith modem techniques, based on sound theory, but there still are numer- ous aspects of the subject where a state bordering on chaos prevails.

The scientific study of sediments may be divided into two broad divi- sions. The first of these is the field and laboratory investigation of sedi- ments, which yields data that lead to their description and classification. The second part of the subject is concerned with the laws of sedimenta- tion and the origin of sedimentar}'^ deposits. To the first aspect may be applied the term sedimentary petrography or sedimentography. The second division is properly designated as sedimentary petrology or sedi- vientology.

The distinction between petrography and petrology is, according to

INTRODUCTION 5

T3'rrell,^ that petrography is the study of rocks as specimens, whereas petrology is the science of rocks, that is. of the more or less detinite units of which the earth is huilt. These general terms may apply equally well to igneous, sedimentary, or metamorphic rocks. Specifically, Mil-

ner - has defined sedimentary petrology as follows :

[Sedimentary] petrolojry connotes something: more than mere description of rock-types based on microscopical analysis, and in its wider sense embraces comprehensive investijrations of their nature, origin, mode of deposition, inherent structures, mineralogical composition, mechanical constitution, tex- tural analysis, various chemical and physical properties, in short, all data leading to an understanding of the natural history of the formations under review.

In practice, one seldom distinguishes between sedimentary petrography and sedimentary petrology. JMost studies of sediments, perhaps, are di- rected toward the petrological aspects of the problem: the clarification of details of origin, transportation, deposition, or diagenesis. Actually, of course, the petrographic aspects precede the petrological. because it is first necessary to assemble facts about the sediments, both from the field and the laboratory.

It is only with the methods of petrographic analysis that this volume is concerned. The purpose of the book is to present theories and methods of examining sediments, from the field sampling to the final graphic and statistical analysis. Petrological aspects are only touched upon as they apply to certain details of analytical methods and to indicate the underlying purposes of the laboratory investigations.

The point of view which this book presents is based on the premise that every sediment is a response to a defiiiite^et of environmental con- ditions. WHiatever the conditions may be, there are characteristics of the sediment which may be measured in the laboratory and which reflect the environmental factors that produced them. A change in the environ- ment (pressure, temperature, chemical associations) results in a corre- sponding adjustment of the rock to its new conditions. Owing to incomplete adjustments, however, the rock materials may have some characteristics inherited from previous states and some due to conditions existiiig at the moment. Refinements of technique and interpretation, however, make possible the unraveling of even such complexities. This point of view requires a careful consideration of all techniques used, a critical evaluation of data, and a reliance on the methods of pure science.

^ G. W. Tyrrell, The Principles of Petrology (London, 1926). p. i. - H. B. IMihier, Report of researcii on sedime<itary rocks by British petrologists for the year 1927: Rely. Com. Sed., Nat. Research Council, 1928. p. 0.

6 SEDIMENTARY PETROGRAPHY

Mineralogical, as well as size and shape studies, throw light on the physical conditions of deposition, but it is largely on minerals alone that we have to rely for the determination of age (petrographic correlation) and for knowledge of the provenance of the deposit. The problem of provenance or of source of the sediments involves a knowledge of the composition of the parent rock tvpes, a knowledge of mineral stability in respect both to climate and to mechanical wear ; and an understanding of the relations between mineral frequencies and the transportation of the sediment.

Twenhof el ^ has defined sedimentation as follows :

Sedimentation includes that portion of the metamorphic cyclefromthe de- struction of the parent rock, no maner what its origin or constitution, toOie consolidation of the products derivedTfrom that destruction (with any addi- tions from other sources) into pother rock.

The term sedivieutatioji thus connotes a process and is to be distin- guished from the products of that process, the sedimentar)- rocks. Twen- hofel - has also defined a sediment as

... a deposit of solid material (or material in transportation which may be deposited; made from any medium on the earth's surface, or in its outer crust imder conditions of temperature approximating those normal to the surface.

PROPERTIES OF SEDIMENTS

Every- sedimentar)' deposit has certain fundamental characteristics or properties, some of which are associated with the individual panicles and others \\-ith the aggregate of all the particles. In some cases there is an overlap, but the following classification indicates the principal character- istics.

Properties of component grains. The fundamental properties of the component grains of a sediment are (i) sizes, (2) shapes. (3^ surface textures, and (4) mineralogical composition. The last characteristic de- termines such attributes as densit)-. hardness, color, and the like, of each grain. Each of these four fundamental properties may be examined in the laborator}-.

The f imdamental properties of the particles are important because they reflect either directly or indirectly many of the vicissitudes through which the sediment has passed. Size is related to the medium of transportation

iW. H. Twenhofel, Treatise on Sedimentation, 2nd ed (Baltimore, 1932), P- xxvii.

- hoc. cit.

INTRODUCTION 7

and its velocity ; shape is related in part to the medium of^jtrans]2ortation and lo the distance and rigor of transport ; surface texture may reflect sul)sc(|ucnt clKingcs due lo sohilion, or il nia_\- furnisli clues ti> \\\c luclliod of transijurtutiuji. iMnally, the mincralogical c<^)_mi" '^'' '' '" indi rales pos- sihle source rocks, as well as any post-depositional chaiiL^es tliat may have occurred.

^Attributes of component grains in the aggregate. Interest in the compo- nent grains of a sediment often involves the frequency distributions of grain properties in the aggregate ; for example, size is expressed in terms oTTsize frequency distnbufion (mechanical analysis) rather than by cataloging the individual size of each i:)article. In similar manner, shape, mineral composition, and other properties may be considered statistically as distributions of grain properties. Each of these distributions may then be studied in terms of their own characteristics, such as average size, average density, average degree of sorting or sizing, and the like.

Another important attribute of the component grains in terms of their aggregate properties is the orientation of the particles in space (the "fab- ric" of the rock). The orientation of the particles, considered statistically, may indicate among other things whether deposition was subaqueous or subaerial.

PKopcrties of thejwgregate. In addition to the attributes of the indi- vidual particles, there are various aggregate properties of the sediment which are important. These include (i) the ccnienlation of the jjarticles in the specimen, (2) structures, such as bedding, concretions, and the like, and (3) the color of the sednriciit. These properties also furnish information about the history of the sediment. The color of the sediment in the aggregate, including the nature of the cement, may help determine conditions of deposition, or post-depositional changes. Some of these aggregate properties may be controlled in large part by projK^rties of the component grains. The orientation of the particles helps determine such structures as bedding and in addition may be a factor in such aggregate properties as porosity and permeability.

To a large extent the aggregate properties, as they are defined above, may be studied in the field, whereas the properties of the comi)onent grains and their distribution in the sediment may best be considered in the laboratory. A thorough examination of sediments therefore involves a combination of field and lal.ioratory work, and in the modern develop- ment of the science neither is comjilcte without the other. Laboratory methods must be quantitative, inasmuch as quantitative data are necessary to the development of complete theories of sediment transportation and

8 SEDIMENTARY PETROGRAPHY

deposition. In the future development of the science there can be little doubt that this quantitative and theoretical aspect of the science will be increasingly emphasized.

PRELIMINARY FIELD AND LABORATORY SCHEDULE

It is appropriate that a schedule outlining the examination of sediments be given here as a preliminary outline by which the scope of the present book may be indicated. The schedule is divided into two parts : the first lists the features of the rock that may be observed in the field by ordinary' geological methods, and the second includes those characteristics which are best determined quantitatively in the laboratory. This volume is directly concerned with the details of the second section of the schedule.

The following organization of data on sedimentary' rocks is adapted, with some changes, from the excellent report on the field description of sedimentary rocks by Goldman and Hewett.^

Field Schedule

External form of the rock unit

Dimensions, persistence, regularitj-

Color

\\"et or dn,', on basis of accepted color scheme -

Bedding

Sharp or transitional

Plane, midulatorj-, or ripple-marked

Thickness

Constant or variable

Rhythmic or random Attitude and direction of bedding surfaces

Horizontal, inclined, or curved

Parallel, intersecting, or tangential to other beds

Relation of particle properties to attitude and direction ^ Markings of bedding surfaces

Mudcracks, rain prints, footprints, etc. Disturbances of bedding

Folding or crumpling

Intraformational conglomerates

1 M. I. Goldman and D. F. Hewett, Schedule for field description of sedimentary rocks : National Research Council, Committee on Sedimentation, Washington, D. C.

2 M. I. Goldman and H. E. Merwin, Color chart and explanation of the color chart for the description of sedimentary rocks, prepared imder the auspices of the Divi- sion of Geologv and Geography, of the National Research Council, Washington. D. C, 1928.

3 Such properties as porosit>' and permeabilin- may be determined in the labora- tor>- from oriented field samples.

INTRODUCTION 9

I Concretions 1 Kinds, size

Condition and distribution . Orientation with respect to bedding I Form, size, composition

Internal structure

Boundary against country rock 1 Sharp or transitional

Relation to bedding

Distribution I Random or regular

Organic constituents ^

Kinds, size [ Condition Whole or broken

Distribution I Orientation with respect to bedding

Laiioratoky Schedule 2

Preparation of sample for analysis Sample splitting Disaggregation and dispersion

Particle size analysis

Shape analysis Roundness I Sphericity

Surface texture analysis

Mineralogical analysis \ Separation of heavy minerals Microscopic examination

Orientation of particles in sample

Mass properties of sediment Porosity and permeability . Specific gravity

' Chemical analysis

Graphic presentation of data

Statistical analysis of data

It may be mentioned that although the above schedules present little overlap, in actual practice some of the quantitative data arc obtained

1 This refers only to the megascopic remains. Microfossils demand specialized laboratory techniques not included in tliis volume.

- Tliis outline presupposes that samples have been collected in the field. For a field outline of sampling routine, see below; the subject of sampling forms Chapter 2 of this volume.

1

V.

lo SEDIMENTARY PETROGRAPHY

directly in the field and part of the field data are secured in the laboratory. For example, the study of pebble orientation may be conducted at the exposure, and if the rock being investigated is consolidated, some field observations, such as bedding and other structures, may be observed from the sample. On the other hand, if the material is incoherent, such features as bedding, grain orientation, and the like are not preserved during sam- pling, and steps must be taken to complete such observations in the field. It may be said as a general rule that too much data cannot be collected. This is true especially in a field such as sedimentary petrology, where research has not yet advanced to the stage where it may be predicted whether a given set of observational data are pertinent to the study or not. Schedule of field observations during sampling. In addition to the general observations to be made on the formation as a whole, as outlined above, there are several specific observations to be made in the field at the time a sample is collected. These specific data include :

1. Location of the sampling locality, either as a point on a map or with ref- erence to some easily located landmark.

2. Nature of the sampling point, as an outcrop, roadcut, or ditch.

3. Nature of the material sampled, including type of rock, portion of bed sampled, and so on.

4. Nature of the sample, as from a single point, a composite sample from several parts of the bed, as a channel througli the bed, etc.

5. Relation of sample to surrounding rock, as, for example, from just beneath a stained zone, whether cut by joints, and the like.

6. Topography of sampling site, as river bottom, terrace, top of hill, etc.

7. Depth of sample beneath immediate surface at point of sampling.

8. Zone of weathering from which sample is taken, if this can be determined.

9. A field evaluation of the total condition of the sample for the purpose desired, as excellent, good, fair, poor. This is desirable when many samples are collected and the laboratory work may involve using scattered samples to outline the scope of the study.

It cannot be emphasized too strongly that the detailed investigation of sediments should not be a hurried process. The investigator should spend adequate time in the field, examining the general set-up of the problem, locating sampling sites, measuring sections, and in general accumulating sufficient data so that work need not be delayed during the ensuing labora- tory season owing to failure to observe adequately in the field. As a gen- eral rule it is better to acquire too many samples and field observations than not enouirh of either.

CHAPTER 1 THE COLLECTION OF SEDIMENTARY SAMPLES

INTRODUCTION

The physical impossibility of analyzing an entire sedimentan' formation, or even an appreciable part of one. renders it necessary- to work \\-ith samples. A sample is assumed to be a representative part of the formation at the point of sampling, or sometimes of the entire_formation. How nearly it is representative determines in large measure the validity of the final conclusions, assuming, of course, that the methods of analysis cor- rectly describe the sample.

Interest in a sediment may arise in a \-ariet}- of ways. It may be a question of the economic exploitation of a limestone or a tire-clay ; it may be merely a desire to supplement general geological field work with some quantitative data. On the other hand, the study may involve consid- eration of the conditions of sedimentation, the agents that formed the deposit, the possible source rocks, and the like. One may thus argue that the process of sampling may be either casual or precise, depending upon the ends in view. This is an attitude with which the authors cannot wholly concur. It seems reasonable that if a sample is worth collecting, it is worth collecting well.

PURPOSES OF S .A M P I. I N G

Samples for display. In unconsolidated material a display sample may consist merely of a small vial of the material sand. silt, or clay or it may consist of a selection of pebbles in a tray. The collection of such samples affords no particular difficulties unless structures, such as bed- ding and grain orientation are to be preserved.

If the structure of unconsolidated sediments is to be preserved, and if the material is sufficiently fine-grained to be cohesive, an undisturbed sample may be collected by a routine procedure. Antevs ^ describes the process for collect- ing unconsolidated varved clay samples as follows :

1 E. Antevs, Retreat of the last ice-sheet in eastern Canada : Canadian Geol. Sur- vey, Memoir 146, p. 12, 1925.

II

12 SEDIMENTARY PETROGRAPHY

"The samples are taken in tight troughs of zinc plate, conveniently 195^ inches long, 2 inches wide, and ^ inch high. The face of the clay bank is carefully smoothed and the trough is cautiously pressed in, a knife being used to cut away the clay just outside the edges, until the trough is entirely filled with clay. The troughs are then cut out from the bank, and the projecting clay is removed."

If the rock is indurated, the display sample may consist of a chip or a trimmed hand specimen. The hand specimen should be about 3x4 in. in size and from i to 13^ in. thick. The smallest dimension is usually chosen at right angles to the bedding. The corners of the specimen should be rectangular and not rounded,^ so that they conform to the standards set for hand specimens of igneous rocks.

Samples for commercial analysis. Samples of sediments collected for com- mercial analysis present a number of problems peculiar to the purposes for which they are used. In general, however, the methods of sampling are similar to those used for the detailed laboratory study of sediments for scientific purposes.

Commercial analyses may be made for such diverse purposes as the deter- mination of the CaO or I\IgO content of limestone ; the fuel value or the determination of special constituents of coal ; the value of gravel for use as road materials ; or the value of silica sand for glass-making. Regardless of the purpose of the analysis or the state of induration of the material, the prime requisite is that the sample must be representative of the formation. A specialized aspect of commercial sampling is the prospecting of economically useful deposits. This topic does not properly come within the scope of the volume, and interested readers are referred to standard texts on the subject.^

Samples for detailed laboratory investigations. A critical choice of samples, necessary in any detailed study of sediments, should take into consideration as many elements of the problem as may be evaluated, so that the final results are not weakened by poor samples, collected without regard to the purposes of the study.

Sediments may vary in terms of the coarseness of their particles, in the degree of sorting or homogeneity, in their manner of bedding or arrangement of particles, in their degree of induration, and in their degree of alteration. In any given formation, one must also consider the vertical and lateral varia- tions in size of the formation, the presence or absence of bedding, changes in the thickness of the formation or its individual beds, and changes in the shape, size, and arrangement of its particles. Further, some sediments are exposed to view in extensive outcrops, and others are hidden witlnn drilled wells or are covered by bodies of water. Each of these cases presents its own problems, some of which are far from being solved.

1 A. Johannsen, Manual of Petrographic Methods, 2nd ed. (New York, 1918), p. 607.

-C. Raeburn and H. B. Milner, Alluvial Prospecting (London, 1927).

COLLECTION OF SAMPLES 13

Unfortunately, there is at present no general mathematical theory of sampling sediments which enables one in every case to determine the technique of sampling a priori; the science of sampling is still in the stage where "rule of thumb" procedures predominate. These practical rules are based on experience and thus are satisfactory in an em[)irical sense; happily, they are supported by favorable results, and to some extent they may be checked by statistical theory. In a later section of the chapter some of the elementary aspects of sampling theory will be dis- cussed.

OUTCROP SAMPLES

Sedimentary formations exposed in outcrops are the most convenient to sample because the sampling site may be examined in detail and some judgment may be used in choosing the particular point of sampling. Given such an outcrop, the problem involves the number of samples to be taken, the size of the samples, and the desirability of preserving structures or particle orientations.

Spot samples.'^ An isolated sample taken at a particular point_on_tiie outcrop may be termed a spot sample, or a discrete sample. Such samples are collected separately and kept separately, being thus distin- guished from composite samples.

The decision to collect a spot sample may be based on the apparent homogeneity of the deposit as exposed to view. If the outcrop represents a bank of unbedded sand or silt, or even glacial till, with no changes in composition detectable by eye, a single sample may be taken from any convenient point along the outcrop. Unless the object of the study is the investigation of weathering, the principal precaution to be followed is that no weathered or altered phases of the formation be included. This necessitates taking the sample at some distance below the soil horizon, and beneath the surface of the outcrop face.

An area on the outcrop face is first cleaned or scraped, and a sample taken by scooping out a limited amount from a square or circular zone. In general it may be desirable to have the depth of penetration about as great as the width of the face sampled, so that a roughly cubical or cylindrical volume is obtained. The sediment may be removed with a scoop, the ])oint or chisel of a hammer, or a small i)ick. A bag or other receptacle should be at hand so that none of the sample is spilled or lost.

1 The term grab sample is often used for individual samples collected at a given point.