Maxwell Introduction to Geological Maps and Structures

Pergamon International Library of Science, Technology, Engineering and Social Studies
1. Auflage 1982
ISBN: 978-1-4832-2307-0
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 340 Seiten

Introduction to Geological Maps and Structures deals with the preparation of geological maps using topographic contours such as hills, valleys, rock outcrop patterns, faults, veins, rivers, lakes, cliffs, and coasts. A geological formation is a three-dimensional body with a particular shape. Two factors determine the accuracy of boundaries on a geological map: 1) boundaries can only be drawn where there is a sharp contact between adjacent formations; and 2) the ability to follow geological boundaries in the field depends on the degree of exposure, from which the solid rocks tend to be hidden under a cover of soil and superficial deposits. If economic interests are involved, geological maps are very detailed: subsurface information obtained from bore holes and mine workings can be added to surface mapping. The book also describes the construction of a tectonic map, usually drawn on a larger scale, which shows the outcrop of lithostratigraphic units also in very large scales. The book notes that no systematic methodology has yet been developed for the construction of tectonic maps. The book is suitable for geologists, students, or scientists involved in hydrology, meteorology and with general earth sciences.

Autoren/Hrsg.

Maxwell, Robert

Inhaltsverzeichnis

1;Front Cover;1
2;Introduction to Geological Maps and Structures;4
3;Copyright Page;5
4;Table of Contents;8
5;CHAPTER 1. Introduction;10
5.1;Nature of Geological Maps;10
5.2;Solid and Drift Maps;12
5.3;Geological Boundaries;12
5.4;Sections and Symbols;15
6;CHAPTER 2. Sedimentary Rocks and the Outcrop Pattern;16
6.1;Bedding of Sedimentary Rocks;16
6.2;Stratigraphy of Sedimentary Rocks;19
6.3;Nature of the Stratigraphic Record;26
6.4;Use of Structural Observations;29
6.5;Width of Outcrop;34
6.6;Topographic Expression of Geological Structure;36
6.7;Structure Contours and Their Use;37
6.8;True and Apparent Dip;41
6.9;Exercises Using Structure Contours;44
7;CHAPTER 3. Folds and Folding;51
7.1;Geometry of Folded Surfaces;51
7.2;Description and Classification of Folds;55
7.3;Morphology of Folded Layers;68
7.4;Mechanisms of Folding;74
8;CHAPTER 4. Folded Rocks and the Outcrop Pattern;89
8.1;Interpretation of the Outcrop Pattern;89
8.2;Cylindrical and Cylindroidal Folds;99
8.3;Non-Cylindroidal Folds;103
8.4;Horizontal Folds;105
8.5;Anticlines and Synclines;107
8.6;Construction of Fold Profiles;108
8.7;Parallel Folds in Cross-Section;115
9;CHAPTER 5. Joints, Veins and Faults;132
9.1;Introduction;132
9.2;Joints and Their Pattern;133
9.3;Fissures and Mineral Veins;137
9.4;Faults and Fault-Planes;140
9.5;Displacement Across Fault Planes;144
9.6;Classification of Faults;150
9.7;Effect of Faulting on Sedimentary Rocks;154
9.8;Normal Faults;165
9.9;Upthrusts or High-angle Reverse Faults;170
9.10;Low-angle Faults;172
9.11;Strike-slip Faults;178
9.12;Fault Rocks;183
10;CHAPTER 6. Igneous Rocks and their Structure;188
10.1;Introduction;188
10.2;Lava Flows;190
10.3;Pyroclastic Deposits;193
10.4;Lava Plateaux and Central Volcanoes;196
10.5;Statigraphic Record of Volcanic Rocks;197
10.6;Mapping of Volcanic Rocks;198
10.7;Igneous Intrusions and their Form;201
10.8;Types of Igneous Intrusions;206
10.9;Minor Intrusions;206
10.10;Major Intrusions;217
11;Chapter 7. Unconformities and the Geological Record;226
11.1;Breaks in Stratigraphic Sequence;226
11.2;Overstep;230
11.3;Overlap;234
11.4;Unconformities and the Stratigraphic Column;240
11.5;Contemporaneous and Posthumous Uplifts;242
11.6;Use of Unconformities in Stratigraphic Dating;246
11.7;Rejuvenation of Folds and Faults;252
11.8;Palaeogeological and Subcrop Maps;263
11.9;Structural Cross-sections through Unconformities;267
11.10;Synoptic Cross-sections and their Palinspastic Reconstruction;272
11.11;Nature of Earth Movements;276
12;Chapter 8. Cratons and Orogenic Belts;280
12.1;Nature of Tectonic Maps;280
12.2;Structural Features of the Craton;281
12.3;Salt Domes;283
12.4;Block Faulting and Folding;285
12.5;Marginal Zones of Orogenic Belts;287
12.6;Slate Belts;290
12.7;Belts of Polyphase Deformation;303
12.8;Migmatite Complexes;316
12.9;Tectonic Slides;319
12.10;Reworking of Basement Rocks;321
12.11;Relative Dating of Orogenic Belts;325
13;Selected References on Structural Geology and Tectonics;329
14;Appendix: List of Geological Survey Maps;331
15;Index;334

Leseproben

CHAPTER 1

Introduction

Publisher Summary

Geological maps are prepared in the field using a topographic map as a base. Such a base map shows the form or topography of the land surface by means of topographic contours. These are drawn as lines of equal height above a reference plane, which is known as the “ordnance datum.” Topographic contours are drawn at a particular interval, so that the contour height is a multiple of the contour interval. The topography of the sea floor can be represented by submarine contours. Particular features can be recognized from the contour pattern, which allows a mental picture of the topography to be constructed. The topographic map used as a base in geological mapping has a particular scale. Geological mapping is commonly undertaken at one scale, to be published at a different scale.

Nature of Geological Maps

GEOLOGICAL maps are generally prepared in the field using a topographic map as a base. Such a base map, produced in Great Britain by the Ordnance Survey, shows the form or topography of the land surface by means of topographic contours. These are drawn as lines of equal height above a reference plane, such as mean sea-level, which is known as the Ordnance Datum. Topographic contours are usually drawn at a particular interval, so that the contour height is a multiple of the contour interval. The topography of the sea floor can likewise be represented by submarine contours.

It is important to be able to visualise the form of the earth’s surface by studying the contour pattern drawn on a topographic map (see Fig. 1.1). Particular features can be recognised from the contour pattern, thus allowing a mental picture of the topography to be constructed. Studying the natural features such as rivers, lakes, marshes, screes, cliffs and coasts, which are also shown in topographic maps, helps to identify the hills and valleys which together form the topography of an area.

FIG. 1.1 Topographic map showing the form of the land surface by means of topographic contours drawn at an interval of 50 metres.

The topographic map used as a base in geological mapping has a particular scale. This can be given in writing (1 inch equals 1 mile), as a fractional scale (1:63,360), or as a line printed on the map and divided into segments corresponding to certain distances on the ground.

Geological mapping is commonly undertaken at one scale, to be published at a different scale. For example, the Institute of Geological Sciences in Great Britain now undertakes mapping in the field at a scale of 10 centimetres to 1 kilometre (or 1:10,000), which is the metric equivalent of the old 6 inches to 1 mile scale, while it publishes maps on the following scales:

It is the maps on a metric scale, as marked by asterisks, which are gradually replacing the older maps on a similar scale, which were based on imperial measurements.

Although these maps become less detailed as the scale decreases, they are all based on the same information, as obtained from field mapping. It is the geological maps now being published on a scale of 1:50,000 (previously 1:63,360) which form a series of reasonably detailed maps covering the whole country. On a larger scale, the 1:25,000 maps depict areas of particular geological interest, while some 1:10,560 maps are published for certain areas, mainly coalfields. On a smaller scale, the 1:250,000 maps are intended to provide a complete coverage for the country without showing very much detail, while the geological maps published on the scale of 1:625,000 or 1:1,584,000 are simplified maps covering the whole country as two sheets (1:625,000) or a single sheet (1:1,584,000). The Geologic Quadrangle Maps produced by the United States Geological Survey on a scale of 1:24,000 or 1:62,500 are equivalent to the 1:50,000 maps for Great Britain, while geological maps are produced on a scale of 1:2,500,000 as four sheets covering the whole country. Geological maps on various other scales are also published for paticular purposes by the United States Geological Survey and by the corresponding organisations in the individual States.

Solid and Drift Maps

Geological maps show the distribution of the superficial deposits and the underlying solid rocks at the earth’s surface. The superficial deposits include glacial sands and gravels, boulder clay, alluvial sands and gravels, lacustrine silts and clays, screes and landslip deposits, beach sands and conglomerates, and peat. Such deposits form a thin and discontinuous mantle of material, covering the underlying rocks of the earth’s crust. This material has been formed during the present cycle of erosion, and the deposits are mostly Pleistocene or Recent in age. This means that they have been formed during the last million years or so.

The geological maps published by the Institute of Geological Sciences for the various parts of Great Britain are produced in two editions. The Solid and Drift Edition shows the distribution of the superficial deposits by means of colouring. The solid rocks are only coloured on these maps where they are not overlain by superficial deposits. The older maps of this type were termed drift maps, simply because most of the superficial deposits in Great Britain are formed by glacial drift. The Solid Edition of the same maps does not show the superficial deposits by means of colouring. Instead, the solid rocks are coloured even where they are overlain by superficial deposits. Symbols printed on the map are used to show the nature of the superficial deposits. However, such maps often show the more widespread areas of river alluvium and wind-blown sand by means of colouring.

The Geologic Quadrangle Maps published by the United States Geological Survey mostly correspond to the Solid and Drift Edition of the geological maps produced by the Institute of Geological Sciences for Great Britain, since the superficial deposits are shown on these maps by means of colouring. However, some maps produced for areas covered by glacial deposits are published in a form similar to the Solid Edition.

Geological Boundaries

The surface distribution of the solid rocks in a particular region is shown by means of geological boundaries, drawn as lines on the geological map (see Fig. 1.2). These boundaries define the outcrops of geological formations which have been mapped as separate entities in the field. A geological formation is simply defined as any body of rock which is sufficiently distinct that its boundaries can be mapped in this way.

FIG. 1.2 Geological map depicting the outcrop of the geological formations recognised within the area of Figure 1.1. The ornament assigned to each geological formation is shown by the stratigraphic column of Figure 1.3.

The outcrop of a geological formation corresponds to the area which is directly underlain by rocks belonging to the formation. The outcrops of all the formations recognised in a particular region combine to form the outcrop pattern shown by a geological map. Exposures are only found where the underlying rocks are not hidden from view by soil or superficial deposits, so that they can be seen at the earth’s surface.

Although the outcrop of a geological formation is mapped at the earth’s surface, the formation is itself a three-dimensional body of rock. It has a particular shape, extending to some depth below the earth’s surface, as defined by its contacts with the adjacent formations. It is very important to realise that such a formation, now outcropping at the earth’s surface, originally extended to some height above the present level of this surface, before it was removed by erosion.

The accuracy of the boundaries shown on a geological map depends on two factors. Firstly, boundaries can only be drawn where there is a sharp contact between adjacent formations. Such boundaries become easier to distinguish as the rocks on either side of the contact become increasingly distinct. A boundary cannot be drawn with any degree of accuracy if it represents a transitional contact across which the rocks change only gradually in character. Secondly, the ability to follow geological boundaries in the field depends on the degree of exposure, according to which the solid rocks tend to be hidden under a cover of soil and superficial deposits. It is common practice to show geological boundaries as solid or dashed lines, depending on whether or not these boundaries have been accurately located in the field.

The detail shown by a geological map depends primarily on the diversity of the rocks which have been mapped, since it is only variations in the nature of the rocks which allow different formations to be mapped. However, it also depends on the scale of the map and the degree of exposure. Geological maps showing areas of economic interest are often very...