Balanced Geological Cross-Sections: An Essential Technique in Geological Research and Exploration

144 Pages·12.082 MB·English

by  Nicholas B. Woodward, Steven E. Boyer, John Suppe(auth.)

Balanced Geological
Cross-Sections

Short Course in Geology: Volume 6
Balanced Geological
Cross-Sections:
An Essential Technique in
Geological Research and Exploration
Nicholas B. Woodward, Steven E. Boyer
and John Suppe
@
Short Course Presented at the
28th International Geological Congress
Washington, D. C.
American Geophysical Union, Washington, D. C.
Maria Luisa Crawford and Elaine Padovani
Short Course Series Editors
Library of Conpets Cataloging-in-PublicationD ata
Woodward, Nicholas B.
Balanced cross-sections.
Bibliography: p.
1. Geological mapping. I. Boyer, Steven E.
11. Suppe, John. III. Title.
QE36.W66 1989 55lt.O22'3 89-14885
ISBN 0-87590-703-2
Copyright 1989 by the American Geophysical Union, 2000 Florida Avenue, NW, Washington,
DC 20009, U.S.A.
Figures, tables, and short excerpts may be reprinted in scientific books and journals if the
source is properly cited.
Authorization to photocopy items for internal or personal use, or the internal or personal use
of specific clients, is granted by the American Geophysical Union for libraries and other users
registered with the Copyright Clearance Center (CCC) Transactional Reporting Service,
provided that the base fee of 51.00 per copy plus $0.10 per page is paid directly to CCC, 21
Congress Street, Salem, MA 10970. 0065-&448/89/%01. + .lo.
This consent does not extend to other kinds of copying, such as copying for creating new
collective works or for resale. The reproduction of multiple copies and the use of full articles or
the use of extracts, including figures and tables, for commercial purposes requires pennission
from AGU.
CONTENTS
Preface vii
Introduction 1
Goals 1
Development of Balanced Sections 4
Fundamentals 6
parallel Folding 17
practical Techniques and Assumptions 21
Topical Sessions
Session 1: Ramps and flats 21
Session 2: 3-D Geometry 24
Session 3: Stratigraphic separation diagrams 27
Session 4: Cutoff-line maps 30
Session 5: Branch-line maps 33
Session 6: Thrust systems 35
Session 7: Sequences of deformation 44
Session 8: Projections to depth 51
Session 9: Fault-fold relationships 58
Session 10: Dip-spectral analysis 59
Session 11: Relationship between fault shape and fold shape 62
Session 12: Pin lines 66
Session 13: Sinuous-bed method 67
Session 14: Equal area method 72
Session 15: Restoration of a ramp 74
Session 16: Restoration of a section 75
Session 17: Simple duplex restoration 77
Session 18: Multiple working hypotheses 80
Session 19: 3-D Models and strike sections 82
Session 20: Seismic interpretations 84
Session 21: Loose lines 94
Session 22: Testing published sections 97
Session 23: Retrodeformation of penetratively deformed rocks 99
Session 24: Computer Applications 101
Session 25: Non-layercake stratigraphy 104
Session 26: Grandfather Mountain window restorations 106
Session 27: Canadian Rockies section 108
Session 28: Wyoming section 110
Session 29: Appalachian section 113
Session 30: Extensional terranes 115
Session 31: Application to petroleum exploration 116
Discussion and Summary 119
Balancing other people's sections 119
Drawing your own section 121
Summary 126
References 127
PREFACE
The first edition of these short course notes Woodward, Boyer and Suppe to present a two day
was prepared by Woodward and Boyer for the SE short course at the National Geological Society of
section meeting of the Geological Society of America meeting in Orlando, Florida, November 1985
America held in Knoxville, Tennessee, March 1985, (v. 11, 2nd edition). We are extremely pleased to
and published as Dept. of Geol. Sci. Studies in be able to participate in the International
Geology, v. 11. The Structural Geology and Geological Congress and have completely revised
Tectonics Division of the GSA subsequently invited the present course notes.
We would like to express our thanks to all of would like to acknowledge the support of ARCO,
our colleagues who have expressed interest in and Texaco and Princeton University. The authors
support for production of this volume. Reviews of would also like to gratefully acknowledge the
the updated manuscript for the present course by following journals and organizations for
Richard Allmendinger, Steven Usdansky, Emanuel G. permission to reproduce copyrighted material:
Bombolakis and Peter Verrall have been of great Rocky Mountain Association of Geologists, American
help. We are pleased to acknowledge the generous Association of Petroleum Geologists, Eclogae
contribution of technical assistance by Sohio Geologicae Helvetica, Canadian Journal of Earth
Petroleum, Denver, Colorado and secretarial Sciences, American Journal of Science, Geological
assistance by Deborah Love, Joyce Burke, Melody Society of America, Geological Society of London,
Branch, Jolee Gardner and Denise Stansberry of the Royal Society of London, Canadian Society of
Department of Geological Sciences, University of Petroleum Geology, John Wiley and Sons, and the
Tennessee, Knoxville. Woodward and Boyer both Geological Survey of Canada. We would also like
began their use of balanced sections while at The to acknowledge the authors of all of the figures
Johns Hopkins University, and gratefully remember we have been permitted to use, copyrighted or not.
the guidance of Dr. David Elliott. Dr. Suppe
vii
Balanced Geological
Cross-Sections
Balanced Geological
Cross-Sections
Goals techniques for construction of balanced cross-
sections. The first goal is relatively easy, the
The origins of balanced sections lie in the second much more difficult. A balanced section is
petroleum industry in the 1950's and 19601s,m ost only a more rigidly constrained, standard geologic
obviously in Calgary, Canada. The first published section and drawing any good geologic cross-
balanced sections are those by Bally, Gordy and section takes experience and several tries. All
Stewart 119661 of the Canadian Rockies. The of the information needed for any good geologic
concepts of thrust-terrain structural styles, upon section (accurate surface geology, knowledge of
which they are based, have been developing since local stratigraphy, understanding of structural
Peach and Horne's [I9071 classic work on the Moine forms, etc.) are needed for a balanceable section.
thrust zone in Scotland. To begin, how can we Because we add these additional constraints in
define a "balanced" section? Dahlstrom [I9691 "balancingn a section, presumably we can derive
introduced the concept into the literature and more meaningful conclusions from it after it is
suggested two rules: 1) "A simple test of the finished. A balanced section is Q& necessarily
geometric validity of a cross-section is to truth, it is simply a model which satisfies a
...
measure bed lengths these bed lengths must be larger number of reasonable constraints (than does
consistent unless a discontinuity intervenes ..." an unbalanced section); therefore it is more
and 2) "in a specific geological environment, likely to be correct. Balanced sections provide a
there is only a limited suite of structures which more accurate portrait of deformation thus they
can exist..." (p.743, 744). Elliott's [I9831 aid evaluation of exploration targets.
definition was somewhat more restrictive. ".. . The examples worked through here are relatively
the structures drawn on the section are those that straightforward but the nuances of balanced
can be seen in the area in cliffs, road cuts, sections, regrettably, are like any other
-
mountain sides, etc. The use of these structures subtleties they only sneak up on the unwary at
leads to an admissible cross-section. inopportune moments. In this course. we hope to
Additionally, a restored as well as a deformed- provide a basis for further exploration of the
state cross-section should be constructed at the subject of balancing cross-sections.
same Fime. If a section can be restored to an In most of science we spend much of our time
unstrained state it is a viable cross-section. By looking for recognizable patterns. Hand-in-hand
definition, a balanced cross-section is both with "balancing" of cross-sections, we also seek
viable and admissible. . . (p. 101) . predictable and understandable patterns in thrust
If we really understand how structures form, belts. Computer modeling allows many different
then we should be able to take them apart. Thus types of structures to be studied quickly.
the "balance" occurs when bed-lengths, or cross- Modeling of structures in a forward direction,
section areas, are equal in both the deformed and (ie. from undeformed state to observed deformed
the undeformed state. If they are not equal and state), is much more common than backward modeling
an explanation for the inequality is lacking (ie. when the deformed state cross-section can be
(strain ?) then a section is not balanced. If the taken apart by the computer). Forward modeling
deformed section is presented without a restored produces multiple viable cross-sections by varying
version, it may or may not balance, but the author any of the important input parameters. We are
has not proved it balances until the restoration then left to pick the viable model which is most
is presented. If one follows the dip-spectral admissable based on our field problem. The very
analysis rules of fault-bend folding [Suppe, 19831 elegant balanced computer models may not be
in constructing a cross-section, the sections will entirely admissible, but they almost always
restore exactly and are retrodeformable. suggest solutions to balancing problems in real
An area with fault-truncated folding or earlier cross-sections.
faults can be balanced if the section is We should also be careful to distinguish
constructed so that, when the faulting is removed, between two parts of the balanced section problem,
the truncated folds in the hanging wall (HW) and namely: 1) How do the rocks really work in the
the footwall (FW) match. Extensional terrains can physical world? and 2) How do our geometric
also be balanced, although those methods are less constraints work in our cross-section world? Each
well-developed [Gibbs, 19831. Strain data and geometric assumption needed for a "balanced"
unstraining of areas can be included as additional section must be evaluated for its closeness to
data and can add an additional step in the cross- reality in a local area. As one example, the
section restoration process [Woodward and others, geometric methods are usually based on parallel or
-
1986; Ramsay and Huber, 19871. cylindrical folding but mx~yf olds in nature are
The goals of this course are: 1) To discuss neither. It is important to distinguish problems
the origin and utility of balanced cross-sections with balancing which may be derived from our
and; 2) To provide exercises on and instruction in geometric assumptions from those which are real
2 BALANCED GEOLOGICAL CROSS SECTIONS
physical problems (truncated folds and out-of- eroded, is difficult, if not impossible. Again
sequence thrusts for example, may be real interpretations of poorly exposed areas are
problems, bed length problems may be a result of dependent on our understanding of regional
poor drafting instead). relations and well-exposed areas. Thus we are
Cross-sections are our routine interpretations left with very useful, but as Hossack -[I9791
of map data and are only as good as our initial noted, minimum restorations.
information. Deciding the difference between an In conclusion, we hope this course will be a
imbricate fan and a duplex, at an erosion level beginners guide to why and how balanced sections
deep enough for any possible roof thrust to be work. Because the authors are North American and
TURNER VALLEY ROYALITE DEVON. 1
PROJECTED 3 MILES S.E. T-D. 8.978'
TURNER VALLEY PALEOZOICS
HIGHWOOD UPLIFT TURNER VALLEY
:ig. 1. Seismic lines p~c ovide the best evidence that the major beds (reflectors)
Ire defom~e d in ir restori~ blep attern above an undeformed surface. (a) Seismic
section; (b) stnl ctural iL nterpretation [from Bally and others, 19661.