Borradaile Understanding Geology Through Maps

1. Auflage 2014
ISBN: 978-0-12-801093-8
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 196 Seiten

Understanding Geology through Maps guides young professional geologists and students alike in understanding and interpreting the world's dynamic and varying geological landscapes through the liberal use of visual aids including figures, maps, and diagrams. This highly visual reference introduces the skills of interpreting a geological map and relating it to the morphology of the most important types of geological structure. Thoroughly revised, and with more international examples, it is ideal for use by students with a minimum of tutorial supervision. Maps of geological structures provide all of the realism of a survey map without the huge amount of data often present, so readers can develop or hone their skills without becoming overwhelmed or confused. In particular, emphasis is placed throughout on developing the skill of three-dimensional visualization so important to geologists. - Authored by a master geologist with more than 40 years of experience in research and instruction - Features more than 130 figures, diagrams, and illustrations-many in full color-to highlight major themes and aid in the retention of key concepts - Leads to a broad understanding of Earth's geology through the use of real and theoretical map - Exercises conclude each chapter, making it an ideal tool for self-guided and quick study

Dr. Graham Borradaile, BSc, PhD, DSc (University of Liverpool), is a professor of Geology at Lakehead University in Ontario, Canada. Graham's research and instruction experience spans 43 years and his research has been continuously funded by NSERC (Ottawa) since 1979 with occasional funding for specific projects from other research organizations (e.g. NATO, Province of Ontario, and commercial sources). Dr. Borradaile's research focuses on two sub-disciplines: magnetic properties of rocks and structural-tectonic geology. His rock magnetic laboratory occasionally tackles archaeological problems, including the nature of ancient paints and pigments, and the age-determination of stone buildings from their magnetization. The latter has involved studies in Cyprus, Israel and England and has been featured in Discovery Magazine, on Discovery Channel TV and on BBC TV (UK). Graham is also the author of more than 100 journal papers and his more recent book, Statistics of Earth Science Data, was published by Springer in 2003.

Autoren/Hrsg.

Borradaile, Graham

Inhaltsverzeichnis

1;Front Cover;1
2;Understanding Geology Through Maps;4
3;Copyright;5
4;Contents;6
5;Foreword;8
5.1;MINIMUM MATERIALS REQUIRED FOR THIS COURSE ARE;9
5.2;OTHER LITERATURE;9
5.3;FURTHER READING;9
5.4;ADVANCED STRUCTURAL INTERPRETATION OF GEOLOGICAL MAPS;9
6;Acknowledgments;12
7;Chapter 1 - Geological Maps and Some Basic Terminology;14
7.1;UNIFORMITARIANISM (AND ITS LIMITATIONS);17
7.2;STRATIGRAPHIC CORRELATION;19
7.3;WHAT IS MEANT BY “MAPPING”;19
7.4;INITIAL TERMINOLOGY FOR LITHOLOGY;22
7.5;SEDIMENTARY ROCKS;23
7.6;SUPERFICIAL DEPOSITS (SEDIMENTS AS OPPOSED TO SEDIMENTARY ROCK);24
7.7;MAP REPRESENTATION OF SUPERFICIAL DEPOSITS;25
7.8;IGNEOUS ROCKS;25
7.9;METAMORPHIC ROCKS;26
8;Chapter 2 - Relative Ages;28
8.1;MORE ADVANCED CONSIDERATIONS IN RELATIVE DATING;31
8.2;PALEOMAGNETISM: A SPECIALIZED APPLICATION OF RELATIVE AGES;33
9;Chapter 3 - Absolute Ages;40
9.1;UNITS OF MEASURE;40
9.2;EARLY ESTIMATES OF ABSOLUTE GEOLOGICAL AGES;40
9.3;RADIOACTIVE DECAY AND GEOCHRONOLOGY;41
9.4;DENDROCHRONOLOGY;44
9.5;VARVE CHRONOLOGY;44
9.6;RADIOCARBON AGE DETERMINATION;44
9.7;PERIODIC SECULAR MAGNETIC VARIATION (PSV) AND GEOMAGNETIC REVERSALS (GPTS);45
9.8;GEOCHRONOLOGICAL CONSEQUENCES FOR GEOLOGICAL HISTORY;46
10;Chapter 4 - Age Relationships from Map View;50
11;Chapter 5 - Layered (Stratified) Rocks and Topography;58
11.1;DIP, STRIKE, AND THEIR MAP REPRESENTATION;61
11.2;RECORDING ORIENTATIONS: CONVENTIONS;63
11.3;HOW DOES THE FIELD GEOLOGIST MEASURE THE ORIENTATION OF A PLANE?;64
11.4;THE “WAY UP” OR POLARITY OF STRATA: YOUNGING;66
11.5;TECTONIC PLANAR AND LINEAR STRUCTURES;68
11.6;ORIENTING SPECIMENS RETRIEVED FROM THE FIELD;68
11.7;READING PUBLISHED GEOLOGICAL MAPS;70
12;Chapter 6 - Strata and Plane-Dipping Features;74
12.1;STRUCTURE CONTOURS: CONSTRUCTED, EXTRAPOLATED, INTERPOLATED, AND TOPOGRAPHIC INTERSECTIONS;76
12.2;DETERMINING DIP ANGLES FROM STRUCTURE CONTOURS;76
12.3;EXTRAPOLATING GEOLOGY AND DETERMINING DIPS USING STRUCTURE CONTOURS;77
12.4;COMPLICATIONS IN DRAWING CROSS-SECTIONS: VERTICAL EXAGGERATION AND APPARENT DIP;78
12.5;DIPPING STRATA, UNCONFORMITIES ON PUBLISHED GEOLOGICAL MAPS;82
12.6;SIMPLE CONSTRUCTIONS FOR MAPPING OUT STRATA FROM A FEW OUTCROPS;83
13;Chapter 7 - Dips, Thicknesses Structure Contours and Maps;90
13.1;TRUE THICKNESSES OF DIPPING BEDS;93
13.2;STRUCTURE CONTOURS FROM DIPS;96
14;Chapter 8 - Unconformities;102
14.1;TYPES OF STRATIGRAPHIC DISCORDANCE;104
14.2;STRATIGRAPHIC DISCORDANCE IMPLIED BY DIFFERENCES IN DEGREE OF REGIONAL METAMORPHISM;105
14.3;ONLAP AND OFFLAP WITH ANGULAR UNCONFORMITIES;106
14.4;NONSEDIMENTARY CONTACTS;106
14.5;UNCONFORMITIES AND SUBCROPS;107
14.6;UNCONFORMITIES AND STRUCTURE CONTOURS;107
14.7;PREUNCONFORMITY DIPS;111
14.8;EXERCISES WITH UNCONFORMITIES;111
14.9;THICKNESS VARIATIONS: ISOPACHYTES;113
15;Chapter 9 - Faults;124
15.1;TENSION FRACTURES;130
15.2;SHEAR FRACTURES;131
15.3;SIMPLIFIED MECHANICAL DETAILS;132
15.4;EXPRESSION OF FAULTS AT THE SURFACE;133
15.5;DIP-SLIP AND STRIKE-SLIP COMPONENTS OF MOTION ON A FAULT;134
15.6;TRANSFORM FAULTS AND OTHER GROWTH FAULTS;136
15.7;REVIEW QUESTIONS CONCERNING FAULTING;137
15.8;SIMPLIFIED PROCEDURE FOR UNDERSTANDING A FAULT FROM A MAP;139
16;Chapter 10 - Folds;154
16.1;EXCERPTS SIMPLIFIED FROM PUBLISHED GEOLOGICAL SURVEY MAPS;172
17;Appendix 1;190
18;Appendix 2 - Final Project Possible after Completionof Studying This Book;192
19;Index;194

Leseproben

Chapter 2 Relative Ages
Abstract
Almost every map and field location reveals the relative ages of rocks with respect to one another. Five simple criteria provide observations that yield relative ages. Cross sections permit the application of the techniques so that stratigraphic columns can be established. Keywords
Baked contact test; Caledonides; Conglomerate test; Cross-cutting relations; Faunal succession; Fold test; Magnetostratigraphy; Masonry test; Mid-ocean ridge (MOR); Paleomagnetism; Relative ages; Reversal test; Storage test; Structural applications; Superposition; Tectonic applications; Tilt test Chapter Outline More Advanced Considerations in Relative Dating?18 Structural and Tectonic Applications?18 Paleomagnetism: A Specialized Application of Relative Ages?20 Storage and Masonry Test?21 Conglomerate Test?21 Fold Test (“Tilt Test”)?22 Baked Contact Test?23 Reversal Test?23 In this book, our attention is almost exclusively focused on relative ages; is this fault older than that granite intrusion or is this sandstone older than that conglomerate? From the map alone, we never know how much older or how much younger. That would introduce the question of absolute ages, discussed in the next chapter. Relative ages are very important to the geologist, enabling us to place the rocks and the structures in a sequence that permits us to draw two important types of conclusion from a map view alone: 1. The sequence of geological events (geological history), from a map view alone. 2. The three-dimensional relations, i.e., positions and shapes of the rock formations and structures beneath the surface, from a map view alone. The latter is of great practical importance; we may estimate useable volumes of economic materials, depths to important features, and the underground extent (subcrop) of rock types (lithologies) of interest. Surprisingly, after training and practice, many of these techniques may be applied largely by visual inspection of the geological map. The principles are very elementary and illustrated on the next page, since diagrams best to explain them (Figure 2.1). However, in brief, the principle techniques of relative age determination include the following. 1. Superposition Younger strata succeed older strata. This is the normal situation during the formation of rocks (deposition of sediment, extrusion of lava, and settling of crystals in a magma chamber). However, the tectonic deformation of rocks may fold or thrust rocks so that the order of the original layers is inverted. Special sedimentary features, “way up” indicators (e.g., cross-bedding, ripples, load casts, burrows, and faunal sequence) may indicate the original top of the bed. Thus, one may confirm that the strata are right way up or upside down. We shall not discuss the exception of inverted strata very much and you may ignore it until perhaps the last chapter of the book. In any case, a good geological map should use a special symbol to warn you where strata are inverted. 2. Crosscutting relationships Logically, a younger feature should be superimposed upon an older one and therefore cut across it. For example, older strata may be cut by a volcanic pipe, an igneous intrusion, an intrusion of salt, or an igneous dike. The crosscutting relationship should be obvious in most views, in cross-sections or in a view of a cliff but also looking down on the map. The crosscutting relationship is the fundamental characteristic of most depositional structures that are used to show which way beds young (i.e., which is the top and which is the bottom of the bed). For example, cross-bedding, channeling, ripple marks, worm tubes, grazing trails, mud cracks, syneresis cracks, and sand dikes all rely on the crosscutting principle. These structures are observed in the field by the geologist to determine the stratigraphic order while he is making the geological map. Note that some care is needed when interpreting relative ages involving minor igneous intrusions such as dikes and sills. Sills and laccoliths (sills with a domed swelling) intrude parallel to the beds or layers of preexisting rocks, exploiting the weakness provided by the contacts between the layers. Although they may appear superficially to conform as layers in sequence, they must in fact be younger than the adjacent strata. For example, the horizontally bedded Proterozoic shale around Thunder Bay, Ontario is about 1900 Ma old. Within this sequence are found thick diabase sills that conform perfectly to bedding but the sills are only 1009 Ma old. Note that a sill is defined as a concordant sheetlike igneous intrusion, whereas a dyke is a discordant (crosscutting) sheetlike intrusion. Most sills dip gently and most dikes dip steeply but that is coincidental and not an essential part of the definition. (There are good mechanical reasons why sills and dikes tend to prefer horizontal and vertical orientations, a subject discussed later.)
FIGURE 2.1 Block diagram showing relative age criteria. Determine the order of formation of the rock types and features and tabulate it in a list with the oldest at the base. One notable exception to the rule of crosscutting relationships occurs with fractures, such as faults and joints, and almost universally with tension joints. A fracture represents a free surface across which it is difficult to transmit stress. Thus after one fracture has formed, it is difficult for a subsequent stress regime to cause a fracture that will cut across the earlier fracture. Consequently, later fractures tend to terminate abruptly against older fractures. This is not universal true, especially for faults with large movements or large extents. 3. Inclusion principle Under certain circumstances, a rock may include some fragments of older rock. For example, fluvial (river) sand may include pebbles derived from older rock. After lithification, the sand and pebbles together form a conglomerate. The pebbles may be very much older, and of sedimentary, igneous, or metamorphic character, whereas the sandstone matrix portion of the rock constitutes a newly constituted sedimentary rock. The pebbles are inclusions; their age represents the age of their provenance, not the age of the conglomerate. Some inclusions may be almost the same age as the rock that includes them. For example, in some energetic depositional environments, fast stream currents or turbidity currents on continental shelves may rip up contemporaneous sediment. These fragments may show evidence of their soft unconsolidated character at the time of inclusion, so that is a clue to their young age. However, if the inclusion is lithified (hard rock), it is not possible to determine how much older it is without specialized geochronological work. The degree to which an inclusion has been rounded and shaped during transport is a poor indicator of its greater age. Terrestrial and submarine landslides commonly pick up other rocks during their movement. Where fragments are abundant these may be described as slump breccias. (Breccia is somewhat like a conglomerate except that the older fragments are obviously not waterworn and they may be angular.) Igneous rocks commonly contain inclusions. They are termed xenoliths (“foreign rocks”); they may be sedimentary or other pieces of country rock that is picked up at the edge of the magma chamber as the magma intrudes. Volcanic pipes may be choked with xenoliths, debris formed as the volcanic vent forces its way through the crust. Kimberlite pipes may bring diamond-bearing xenoliths from depths >700 km, and many basalt lava flows carry inclusions of the upper mantle to the surface of the Earth. Xenoliths may be large enough to appear on some maps, for example, roof pendants of country rock cover several square kilometers in the Peruvian granite batholiths. Certain igneous intrusions that contain many fragments of previously lithified rock are termed agglomerates. Not to be confused with conglomerate, agglomerate has an igneous matrix and usually most of the fragments are of igneous rock, which are mostly angular. 4. Faunal succession Long before Charles Darwin’s comprehension of natural selection (1859) guiding the progressive evolution of organisms, humble miners were aware that certain fossils characterized certain sedimentary strata. William Smith recognized that the order in which different fossils appeared in successive strata was the same in different areas. Moreover, whereas the details of the sedimentary rocks changed from one area to another, a certain bed being sandier here, muddier there, the bed would be characterized by the same fossils. Smith grasped the concept that fossils had a chronostratigraphic value that was far more general than lithostratigraphic correlation based solely on the type of sediment. For example, depositional environments may change from one area to another, at the same time. Although the bed deposited at that time may defy lithostratigraphic correlation because its composition changes from sandy at one location to mud elsewhere, its fauna would be the same. Thus, biostratigraphy is a very powerful way of correlating strata of the same age over great differences, in many cases even globally (Figure 2.3). The change in fauna with time is not...