Drawing Geological Structures - Jörn H. Kruhl - ebook

Drawing Geological Structures ebook

Jörn H. Kruhl

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Despite the modern dominance of computer graphics programs and digital cameras, the ability todraw geological structures manually remains a necessity in academic geology and beyond. Drawings serve for quick and simple documentation in the field or at the microscope. They can be applied as a language of their own as well as be adapted to suit specific requirements. Moreover, geological drawing improves observational ability and contributes to the understanding of geological structures and structure-forming processes. Geological drawing is assisted scientific thinking. Drawing Geological Structures provides undergraduate as well as graduate and practicing geologists with a thorough, step-by-step practical guide to the art of geological drawing. Beginning with the basics, the book covers thin sections, sample sections, samples and geological stereograms. The chapters provide examples of how drawings evolve and are complemented by exercises, allowing the reader to practice their drawing prior to going out into the field or working at the microscope. Users of this unique guide will develop their knowledge and technical vocabulary whilst also improving their drawing skills.

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Table of Contents

Cover

Title Page

Copyright

About the Author

Preface

Chapter 1: Introduction

1.1 Why Do We Need Drawings?

1.2 The Tools

1.3 Sizes of Drawings

1.4 Geological Versus Artistic Drawing

1.5 Drawing With Symbols

1.6 Realistic Drawing

1.7 The Fractal Geometry of Geological Fabrics

1.8 Basic Rules of Geological Drawing

References

Chapter 2: Rock Thin Sections

2.1 Drawing as a Form of Microscopy

2.2 Drawing with Various Tools

2.3 Foundations of Thin-Section Drawing

2.4 Minerals and Their Characteristic Fabrics Under the Microscope

2.5 Sketches for Fast Documentation

2.6 Development of Precise Thin-Section Drawings

2.7 Digital Reworking of Manual Thin-Section Drawings

2.8 Digital Drawing

2.9 Summary

References

Chapter 3: Specimen Sections

3.1 The Geological Message of a Drawing

3.2 Schematic Representation of Minerals

3.3 Schematizing Rocks and their Structures

3.4 Development of Drawings

3.5 Illustration on Different Scales

3.6 Detailed Drawings of Sample Cuts

3.7 Summary

Chapter 4: Drawing Rock Structures in Three Dimensions

4.1 Foundations

4.2 The Development of Schematic 3D Drawings

4.3 Field Drawing

4.4 Digital Processing

4.5 Rules of Labeling

4.6 Summary

References

Chapter 5: Geological Stereograms

5.1 Foundations

5.2 Orientation and Extension

5.3 Additions and Labeling

5.4 Digital Processing

5.5 Summary

References

Chapter 6: Solutions

Index

End User License Agreement

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Guide

Cover

Table of Contents

Preface

Begin Reading

List of Illustrations

Chapter 1: Introduction

Figure 1.1

One of the author's early, but failed, attempts to draw samples and outcrops in the field, and a better version of the same drawing.

(a)

Monoclinic fold in psammopelite and quartzite layers of the Moinian (Grampian Highlands at Loch Leven, Scotland); field drawing; outcrop KR513; field book 6 (Kruhl, 1973). The drawing contains numerous shortcomings; above all, imprecise layout of lines, a sloppy perspective, and an incorrect positioning of foliation planes in the metapsammopelitic layers.

(b)

The same drawing redrawn years later. Cross bedding and S1 foliation planes are more precisely placed; the perspective is correct and, consequently, the 3D appearance of the drawing is better; the carbonate spots are more realistically illustrated; and the labeling is more closely related to the structures. Circles L and K indicate positions of samples. Both drawings ca. A6; black ballpoint pen.

Figure 1.2

Drawing of part of the Grand Canyon, “Vishnu's Temple” (Dutton, 1882, plate XXXIV): a felicitous combination of artistic, geological, and geomorphological representation.

Figure 1.3

(a)

Photo of the “Spitznack” Fold (near the Loreley, Middle-Rhine region, Rhenish Massif, Germany). Centimeter- to decimeter-thick metapsammopelitic layers are bent to an open, monoclinic fold. The boundaries of bedding are represented by strong fractures in the horizontal fold limb and by weak fractures and differences in brightness in the vertical limb. In addition, a schistosity can be recognized. It is represented by narrow-spaced, nearly parallel fractures. The schistosity is pronounced and steep in the horizontal limb, fan-shaped in the fold crest, and is barely visible in the steep limb. Additional fabric details cannot be recognized. Hammer as scale.

(b)

Schematic drawing of the same fold. Based on the small protrusions and the fabrics that can be recognized on them, the planar, 2D view has been supplemented to form a 3D block. Highlighted are (i) the partitioning of the schistosity to two different sets of foliation planes in metapsammitic layers, (ii) the pile shape of the schistosity in metapelitic layers, (iii) the stretching lineation on an obviously bedding-parallel foliation plane, (iv) the compressed and sheared quartz veins in the steep limb, and (v) the slickensides on steep bedding-parallel shear planes. All these structures are not visible in the photo and are only revealed by close observation of the fold. Modified after Zurru and Kruhl (2000, Figure 33); size of original drawing ca. B4. A more comprehensive drawing is shown in Figure 4.27.

Figure 1.4

Six well-known droodles. Together with other ones, they can be easily found in the internet.

(a)

Cat that peers into a mousehole;

(b)

giraffe standing in front of a window;

(c)

bear climbing a tree;

(d)

person with a large sun hat on a bicycle;

(e)

four elephants nosing at a ping-pong ball;

(f)

chessboard for beginners.

Figure 1.5

Geological 2D pictograms ranging from kilometer to decimeter scale, reduced to the essentials.

(a)

Detachment;

(b)

metamorphic core complex;

(c)

strike-slip fault that displaces two halves of a granite pluton;

(d)

stratovolcano;

(d)

pillow lava;

(f)

cross-bedding.

Figure 1.6

Pictogram of a meteorite impact; redrawn symbol from a sign-board of the geopark “Nördlinger Ries” (Germany).

Figure 1.7

Schematic representation of rock fabrics.

(a)

Granitic (top) and volcanic vein (bottom) in a layered gneiss. Crosses represent feldspar cleavage planes in granite. “v” stands for volcanic rock. The homogeneous distribution of symbols represents the homogeneity of the rock fabric.

(b)

Orthogneiss layer in schist. The “wave” ∼ symbolizes the wavy structure of the gneiss, which originates from the lenticular shapes of deformed feldspars and the distribution of the surrounding biotite flakes. The schematized fabric includes the information that the gneiss was deformed at temperatures high enough for crystal-plastic deformation of feldspar. The parallel lines represent the foliation in the schist. In addition, they contribute to the light-dark contrast that typically exists between orthogneiss and schist.

(c)

Bedded limestone. The cross-strokes symbolize fractures that form perpendicular to bedding during diagenesis and compaction and are rarely present in other rocks in such formation.

(d)

Folded limestone layer with schematized fabric adapted to the form. The cross-fractures were formed prior to folding and, consequently, remain perpendicular to the layering after folding. Watch out! This is an interpretation that needs to be supported by observation. Fractures generated during or subsequently to folding can be oriented differently.

(e)

Folded limestone layer with schematized fabric not adapted to the form. Geometry of the fold and orientation of the schematized fabric communicate different messages. Their contrast may confuse viewers.

(f)

Boudinaged layer. Solid and broken lines represent foliation planes, which are curved as a result of boudinage. The light-dark contrast emphasizes layers of different composition. In general, the impression is generated that a strong gneiss layer is boudinaged in a mantle of weak schist.

(g)

The computer-generated gray-white gradation preserves the “correct” light-dark contrast; however, the foliation geometry cannot be represented.

Figure 1.8

Complex (fractal) geological structures.

(a)

Devonian metapsammopelite with bedding of variable thickness; Hartland Quay (Devon, England).

(b)

Fe-Mn dendrites on a bedding plane of “Plattenkalk” (Solnhofen, Germany). The dendritic pattern is similar on different scales.

(c)

Fracture pattern on a block cut of Malm limestone; sample KR5149B; Unterwilfingen Quarry (Nördlinger Ries, Germany). The fractures are clustered, that is, form few large and many small fragments. The size distribution of these fragments follows the power law.

(d)

Photomicrograph of a sutured quartz grain boundary; sample KR4846B; syntectonically crystallized tonalite (Abbartello, Golfo de Valinco, Corsica, France). The geometrical arrangement of the few large and many small sutures follows the power law.

Figure 1.9

Sketch of a folded rock layer with two different types of schematized lineations:

(a)

variably long and evenly distributed strokes;

(b)

variably long and clustered strokes. Sketch (a) looks artificial, in contrast to sketch (b).

Figure 1.10

Schematic distribution (flow) pattern of feldspar phenocrysts in a porphyritic granitoid.

(a)

Crystals of equal size with equal orientation and spacing.

(b)

Crystals of different size but with equal orientation and nearly equal spacing.

(c)

Crystals of different size (few large, many small crystals) with slightly different orientations and clearly different spacing (clustering).

Chapter 2: Rock Thin Sections

Figure 2.1

Crystal shapes and characteristic sections of some rock-forming minerals. In addition to the 3D form, transverse and longitudinal sections, as well as an arbitrary section, are shown. 3D forms modified after Tröger

et al.

(1979); see text for details.

Figure 2.2

Schematic illustrations of important rock-forming minerals with characteristic shapes and internal fabrics as observed in thin section. For each mineral, the range of refractive indices, n (after Tröger

et al.

, 1979), and the resulting approximate range of line weights are specified. The line weights relate to sketch sizes of ca. A5 to A4. Black bars represent 1 cm in the original sketch. Characteristic internal fabrics should be used specifically in schematic sketches. The illustrated ones represent selected examples and by far do not cover the entire range of potential fabrics.

Figure 2.3

Greenschist-facies orthogneiss from the eastern Tauern Window (Eastern Alps, Austria); sample KR2916A; mineralogical composition: quartz (Qtz), plagioclase (Pl), biotite (Bt), chlorite (Chl) and epidote (Ep); pencil drawing based on a microscope image in plane-polarized light; size of original drawing ca. A4; line weight 0.5 mm and locally lighter, due to obliquely set lead. The drawing develops successively.

(a)

Contouring of the larger mineral grains and the epidote layer with the same light line weight for easier correction. The end faces of the biotite platelets are sketched straight, indicating the magmatic origin of the crystals.

(b)

Intensification of contours and internal fabrics in plagioclase and biotite. Saussuritization (sauss) in plagioclase is schematized for the purpose of saving of time. The grain margin, poor in saussurite, is highlighted, because it reflects a magmatic zoning and, consequently, the magmatic origin of the crystals.

(c)

Finalization of the drawing by labeling, filling the epidote layer, drawing of the quartz subgrain boundaries, and adding the second (incomplete) epidote layer that was missing before. This relatively cursory sketch serves to illustrate the characteristics of rock deformation: (i) an early magmatic to sub-magmatic deformation, which led to foliation (S1: shape orientation of biotite, plagioclase and coarsely sutured quartz) and local recrystallization of plagioclase, and (ii) a subsequent brittle deformation that generated epidote-filled fractures (S2) oblique to S1.

Figure 2.4

Amphibolite-facies gneiss (Scheelite deposit Mittersill, Tauern Window, Eastern Alps, Austria); sample KR3768B; mineralogical composition: quartz (Qtz), amphibole (Am), biotite (Bt), chlorite (Chl), apatite (Ap) and scheelite (Tu); pencil drawing based on a microscope image in plane-polarized light; line weight 0.5 mm and locally lighter, due to obliquely set lead; original size of a single sketch ca. A4. The drawing develops successively.

(a)

Contouring of quartz areas in order to outline the layering and the grain distribution of other minerals; contours in light line weight for easier correction.

(b)

Intensification of contours and fillings according to the difference in refraction between quartz and other minerals.

(c)

Contouring within quartz areas with light line weight.

(d)

Complete filling of grains with high refraction, specifically of amphibole (with characteristic angle between cleavage planes) and scheelite; completion of grain boundaries in the amphibole layer (based on a microscope image with crossed-polarized light) and labeling. Grain boundaries in the scheelite areas illustrate the partitioning to larger grains with irregular shape and smaller isometric ones and thereby indicate the recrystallization of scheelite.

Figure 2.5

Metasomatically overprinted conglomerate; Mary Kathleen Mine (Mt. Isa Inlier, Australia); sample KR5090D; mineralogical composition: K-feldspar (Kf), plagioclase (Pl), clinopyroxene (Cpx), epidote (Ep), apatite (Ap) and zircon (Zr); three pencil drawings based on microscope images in plane-polarized light; original size of sketch assembly ca. A4; development in two steps.

(a)

Contouring of all grains with the same line weight and labeling.

(b)

Intensification of grain outlines and characteristic internal fabrics: twin lamellae in plagioclase, diffuse gridiron (microcline) twinning in K-feldspar, and cleavage planes in clinopyroxene. These internal fabrics are schematized, that is, represent the real fabrics only roughly but characterize the minerals. The line weight of the boundaries between pyroxene and other minerals of lower refraction is increased, in contrast to the line weight of internal boundaries.

Figure 2.6

Amphibolite-facies andalusite schist (Sesia Zone, Western Alps, Val Loana, Northern Italy); sample KR1481; mineralogical composition: quartz, white mica, biotite, andalusite, staurolite and ore (opaque); pencil drawing based on microscope image in plane-polarized light; original size of drawing ca. A4. Biotite and white mica show outlines with similar line weights and cleavage planes typically pronounced close to the end faces of the thin tabular crystals. The brown, inherent color of biotite is represented by fine dotting, however, without considering the (meaningless) different intensity of color due to pleochroism. In comparison to mica, the sutured quartz grain boundaries, with their straight facets, are drawn with lighter line weight. The few subgrain boundaries are drawn as dotted lines. Based on the relatively high refraction of staurolite, the three small rounded grains (St) are silhouetted against the surrounding grain fabric by thick grain boundary lines and thick internal dotting.

Figure 2.7

Garnet-albite schist from the garnet zone of Barrow metamorphism (Dalradian of the Scottish Highlands; Power Station Ardlin, Loch Lomond, Scotland); sample KR1434; mineralogical composition: quartz (Qtz), albite (Ab), white mica, biotite, garnet, epidote (Ep), apatite (Ap) and ore (opaque); free-hand ballpoint pen drawing of thin-section image in plane-polarized light; variation of line weight through different pressure on the ballpoint pen; size of original drawing ca. A5. Garnet, epidote, and apatite grains serve as initial “support points.” Subsequently, mica and albite grains are drawn and, finally, quartz and the inclusions in albite. In order to save time, the drawing is kept incomplete. Biotite and white mica show the same line weight but can be distinguished by internal dotting of biotite. Apatite (Ap) and epidote (Ep) differ from their albite-white mica surroundings by heavier line weight and from each other by different dotting that mimics the different refraction of the two minerals.

Figure 2.8

Breccia of the Zuccale Fault (Punta di Zuccale, Elba, Italy); sample ZV13a; mineralogical composition: quartz (Qtz) and calcite (Cc); pencil drawing of a thin-section image in plane-polarized light; size of original drawing A4. The drawing develops successively. (a) Contouring of the larger quartz and calcite areas, in order to outline the layering and record proportions and distribution of the crystals. In addition, (i) the position of the structure in thin section, (ii) the orientation of the thin section in relation to the sample and to the lineation (“striation”), and (iii) the orientation of the sample in relation to the fault (“bottom”) are indicated. (b) Increasing of line weight of the palisade-quartz–calcite boundaries and drawing of fillings and small structures. (c) Drawing of (i) fine-grained areas, (ii) fluid inclusions (FI—only locally for saving time), and (iii) an enlarged section (A) for better visualization of fluid inclusions. Labeling and indication of dominant quartz-c orientations and the supposed shear sense.

Figure 2.9

Metagabbro of the Finero Complex (Ivrea Zone, Southern Alps; Valle Cannobina, Northern Italy), deformed under amphibolite-facies conditions; sample KR740; mineralogical composition: garnet (Grt), clinopyroxene (Cpx), amphibole (Am), plagioclase (Pl), apatite (Ap), and ore (opaque); original size of the drawing ca. A4.

(a)

Pencil drawing of a thin-section image in plane-polarized light; all grain outlines and internal fabrics are drawn with equal line weight; minerals are labeled and marked by hatching and internal fabrics; ore grains are outlined in black.

(b)

Drawing finalized with ink pen. The high relief of garnet is represented by line weight 0.5 mm and dotting along the grain boundary; the high chagrin by thick internal dotting (line weight: 0.5 mm). The outlines of pyroxene and amphibole grains are drawn with line weight 0.35 mm. The closely spaced parallel lines in spotted areas represent fine exsolution lamellae. The brown, inherent color of the amphibole is silhouetted against the light green of pyroxene by dense dotting. This contrast clearly visualizes the amphibole-pyroxene intergrowth. Plagioclase is kept blank, in order to increase the contrast to the other minerals.

Figure 2.10

Metagabbro of the Finero Complex (Ivrea Zone, Southern Alps; Valle Cannobina, northern Italy) formed under amphibolite-facies conditions; sample KR741-3; mineralogical composition: garnet (Grt), clinopyroxene (Cpx), amphibole (Am), plagioclase (Pl), and ore (opaque); drawing of a thin-section image in plane-polarized light, generated in A4 size with the aid of a drawing tube.

(a)

Pencil drawing of grain outlines and internal fabrics with equal line weight.

(b)

Ink pen drawing on tracing paper, based on (a). The contrasts in color and refraction of the different minerals are illustrated by different line weights and fillings: garnet 0.5 mm line weight, amphibole 0.35 mm, clinopyroxene 0.35 mm. Strong chagrin and high relief of garnet are represented by internal dotting and dotting along the grain boundaries (0.5 mm). Dense dotting (0.35 mm) and thick dotting along grain boundaries imitate the dark green inherent color of amphibole and the relief of the amphibole versus the surrounding plagioclase, respectively. Amphibole and plagioclase form coarse symplectite between garnet, clinopyroxene, and ore grains. Light-green clinopyroxene is optically separated from amphibole by light dotting (0.35 mm) and parallel cleavage planes. Although the twin lamellae of plagioclase are not visible in plane-polarized light, they are drawn as dotted lines in order to visualize the small distortions of crystal parts of plagioclase. The fine symplectite along the clinopyroxene margin, with small ore grains enclosed (black), is represented by approximately parallel lines (line weight: 0.25 mm).

Figure 2.11

Mica schist of the Silbereck succession (eastern Tauern Window, Eastern Alps, Austria); sample KR2901B; mineralogical composition: quartz, white mica, epidote; drawing of a thin-section image generated in A4 size with the aid of a drawing tube.

(a)

Pencil drawing with equal line weight for all minerals.

(b)

Ink pen drawing of (a) on tracing paper; line weight of white mica 0.35 mm, quartz 0.25 mm, and epidote 0.35 mm; dotted roundish area in quartz: hole filled with epoxy resin. During revision, the cleavage planes at the ends of the white mica platelets are intensified in order to increase the contrast between white mica and the surrounding minerals. Already in the initial pencil drawing, the quartz grain boundaries are delineated as straight segments with sharp corners, characteristic of quartz, but not rounded as often shown in quick sketches. Further characteristics of quartz include: (i) accentuated 120° angles at triple junctions, often accomplished by short bends at the triple junction, and (ii) quartz grain boundaries that meet the flat faces of micas at right angles, again accomplished by short bends at the mica face.

Figure 2.12

Dark gabbro from Mailam/Pondicherry (southern India); sample KR5018; mineralogical composition: clinopyroxene, plagioclase, and ore.

(a)

Photomicrograph (plane-polarized light). The up to 1 mm large colorless plagioclase is tabular, free of cleavage planes and shows pigmented dark cores. The calcium-rich crystals (labradorite with An content of ca. 55 to 60%) form a grid with clinopyroxene filling. The clinopyroxene is anhedral. It is intergrown with massive ore (opaque) and segregates fine particles of ore.

(b)

Drawing of the photomicrograph (a); ink pen on tracing paper placed on the photomicrograph; original size ca. A4. The different pigment content of the plagioclase is highlighted by different dotting. Thus, the tabular shape of the crystals and their interlocking are visualized without drawing the grain boundaries. The outlines of the higher refractive clinopyroxenes are represented by dotting closely along the grain boundaries—not by a higher line weight. The visual contrast to plagioclase is increased by additional internal dotting, which also reflects the weak zoning, and by fractures (0.25 mm line weight, like the grain boundaries).

Figure 2.13

Kinzigite of the fossil Variscan continental lower crust (Serre/Calabria, southern Italy); sample KR3279; mineralogical composition: garnet (Grt), quartz (Qtz), biotite (Bt), sillimanite (Si), cordierite (Crd), and ore (opaque).

(a)

Photomicrograph in plane-polarized light.

(b)

Ink pen drawing on tracing paper placed on a paper print of the photomicrograph; original size ca. A4. One purpose of the drawing is to show the distribution and random orientation of the sillimanite needles and the high amount of cordierite. Neither grain boundaries nor internal fabrics of quartz are displayed, and, therefore, quartz forms the white background of the drawing. This conforms to the relatively lowest refraction of quartz and, in addition, increases the total range of light-dark contrast in the drawing.

Figure 2.14

Gabbronorite from the Harz Mountains (Germany); sample M6; thin-section collection of TU Munich Geology; mineralogical composition: plagioclase (Pl), amphibole (Am), clinopyroxene (Cpx), olivine (Ol), apatite (Ap), and ore (opaque); long side of photo equivalent to 5.7 mm; original size of a single drawing ca. A4.

(a)

Photomicrograph in plane-polarized light.

(b)

Pencil drawing of grain boundaries in (a) on tracing paper placed over a print of the photomicrograph. Ore grains are marked by x.

(c)

Based on (a), the ink drawing is successively developed from minerals of high to minerals of low refraction. (i) Blackening of the ore grains; (ii) drawing of outlines of all mineral grains with appropriate line weights, from high to low: olivine 0.7 mm, pyroxene and amphibole 0.5 mm, plagioclase 0.25 mm; (iii) filling of the olivine grains (fractures and dotting along the grain boundaries and internally); (iv) filling of the clinopyroxene grains (cleavage planes, exsolutions [represented by short thick strokes], light internal dotting); (v) dense internal dotting of the amphibole grains, in order to generate the brightness contrast to pyroxene; (vi) characterization of plagioclase by twin lamellae (dotted lines with 0.25 mm weight), although the twins are only visible in crossed-polarized light but not in the photo (a).

(d)

Digital processing of the line drawing with four shades of gray.

Figure 2.15

Greenschist-facies metapelite of the Variscan basement (Argentiera, northwest Sardinia, Italy); sample SV180; mineralogical composition: quartz (Qtz), white mica (Wm) and ore (opaque); pencil drawing of a photomicrograph in plane-polarized light; original size of the drawing ca. A5; line weight 0.5 mm; thinner lines generated by obliquely oriented lead; SS = bedding; S1 = foliation of the first deformation event; S2 = foliation of the second foliation event (crenulation cleavage). White mica platelets are represented by strokes. The ore particles and masses are kept black. Quartz grain boundaries are not shown, in order to avoid strong filling and, consequently, illegibility of the drawing. The fine internal fabric of larger ore masses reflects the orientation of the foliation and is represented by the appropriate orientation of strokes (from lower-left to upper right).

Figure 2.16

Garnet-mica schist from the Dalradian of the Scottish Highlands (east of Ben Nevis); sample AH168B; mineralogical composition: garnet, white mica, quartz, epidote, and ore (opaque); ink pen drawing after a pencil drawing of a thin-section image in plane-polarized light, generated with the aid of a drawing tube; size of the original drawing ca. A4. The first foliation is built by white-mica platelets and tabular ore grains and is also preserved as relics of aligned and elongate quartz and ore grains in garnet blasts. The foliation is folded resulting in a weak crenulation cleavage. The high chagrin of garnet is represented by dotting with line weight 0.25 mm and the high relief by grain boundaries of line weight 0.5 mm with dotting along the boundaries (line weight: 0.35 mm).

Figure 2.17

Pegmatite from the fossil Variscan lower continental crust (Ivrea Zone, Alpe Scaredi, Val Loana, Southern Alps, Italy), strongly deformed under retrograde greenschist-facies conditions; sample KR2061. Ink pen drawing of a photomicrograph in plane-polarized light; original size ca. A4; line weight generally 0.35 mm, in plagioclase interior 0.25 mm. First of all, the drawing intends to show the difference between intensive brittle deformation in plagioclase (Pl) and crystal-plastic deformation in quartz (Qtz). The strong fragmentation of the large plagioclase grains predominantly parallel to cleavage planes is indicated by broken lines. Only local deformation twins of plagioclase are marked by thin parallel strokes. Grain boundaries in quartz layers are omitted, in order to increase the visual contrast to plagioclase. Quartz layers are locally lenticular and bent around plagioclase crystals, thus highlighting the strong crystal-plastic deformation of quartz. The fine-grained groundmass composed of quartz, plagioclase and white mica is represented by even dotting and, consequently, appears dark, corresponding to the thin-section image. The absence of grain boundaries in quartz regions intensifies the visual contrast and clarifies the coarse structure of the rock.

Figure 2.18

Jadeite quartzite (Shuanghe, Dabie Shan, China); sample RP01; mineralogical composition: quartz, jadeite, garnet, serpentinite along fractures in jadeite and garnet, albite, and symplectite at jadeite and garnet margins; drawing on tracing paper placed over the print of a thin-section scan in plane-polarized light; original size of the drawing ca. A3; felt-tipped pen with line weights 0.1 to 0.8 mm. Quartz grain boundaries are not shown, thus highlighting the geometry of the phase boundaries between quartz and the other minerals, specifically the cuspate structures along the foliation (ca. parallel to the long side of the drawing). Because pyroxene-typical cleavage planes are absent in the jadeite grains, the distinction from garnet is ensured by a specifically high visual contrast generated by dots and lines of high weight. Strokes of different length within the symplectite rims mark the different orientation of the symplectite columns relative to the thin section surface.

Figure 2.19

Quartz porphyry (Bozen, South Tyrol, Italy); sample M64; thin-section collection of TU Munich Geology; mineralogical composition: (i) phenocrysts: ca. 0.5 to 1 mm large, subhedral to euhedral quartz, partially with resorption embayments; ca. 0.5 to 1 mm large K-feldspar and plagioclase; µm large thin tabular biotite; (ii) fine-grained groundmass: quartz, biotite, and feldspars. Quartz is kept completely blank; feldspars are filled with internal fabrics. For saving time and to increase the contrast to quartz, the groundmass is lightly and schematically dotted. Ink drawing of a microscope image (plane-polarized light) on tracing paper placed over a paper print of two merged photomicrographs; size of original drawing A3; line weight 0.25 mm.

Figure 2.20

Tephrite (Kossal, Bohemia, Czech Republic); sample M45; thin-section collection of TU Munich Geology.

(a)

Ink pen drawing of six merged (ca. 30% overlap) photomicrographs (plane-polarized light) on tracing paper placed over a print of a photomicrograph; original size ca. A2; mineralogical composition: (i) phenocrysts: dark brown amphibole with reaction rims, subhedral light greenish-brownish clinopyroxene, and ore (opaque), (ii) fine-grained groundmass: dominantly thin-tabular plagioclase and ore. In the groundmass only larger plagioclase strips are delineated by short strokes, in order to keep the phenocrysts visible, in contrast to the photomicrograph. Similar to the amphibole and pyroxene phenocrysts, a locally weak “east-west” alignment of crystals is visible in the groundmass. In contrast to the groundmass dominated by plagioclase, amphibole and pyroxene phenocrysts exhibit a higher refraction, which is represented by higher weight of the crystal outlines (5 mm in the A2 original). The strong inherent color of the brown amphibole is illustrated by dense dotting which is absent in pyroxene with its clearly weaker inherent color. In addition, the dotting allows us to represent zoning visible in some of the phenocrysts, for example, in the upper right part of the image. Cleavage planes in amphibole and fractures in pyroxene require lower line weight (0.25 mm in the A2 original).

(b)

Detail from (a). Amphibole phenocryst with reaction rim. The uniform internal dotting only represents the inherent color, in contrast to the smaller pyroxenes without dotting. This blow-up also illustrates the local variation of the flow plane around the phenocrysts.

Figure 2.21

Kyanite-mica schist from the Silbereck succession of metasedimentary rocks (eastern Tauern Window, Eastern Alps, Austria); sample KR2928C.

(a)

Thin-section drawing generated with the aid of a drawing tube (plane-polarized light); pencil on paper, original size A4; in the first step of drawing, kyanite was outlined and ore grains filled black.

(b)

Kyanite and white mica grains are outlined with ink pen, with higher line weight for kyanite. The quartz groundmass is kept blank.

(c)

Digital processing (gray filling of kyanite) of a scan of drawing (b); gray level H,S,V,R,G,B = 0,0,82,210,210,210.

(d)

A darker gray filling of white mica is added to (c); H,S,V,R,G,B = 0,0,56,143,143,143.

Figure 2.22

High-temperature mylonite of a stronalite from the Ivrea Zone (Alpe Lut, east of Colloro, Val d'Ossola, Southern Alps, Italy); sample KR2980B; mineralogical composition: garnet, K-feldspar, and quartz. (a) Ink pen drawing from a thin section in plane-polarized light, based on a pencil drawing generated with the aid of a drawing tube; size of original image ca. A4; only outlines of garnet, and quartz and feldspar layers are presented together with few fractures; line weight for garnet 0.5 mm and for quartz and feldspar 0.25 mm. (b) Elaborated pencil drawing with fracture patterns (line weight: 0.35 mm) and dotting of the interior and along boundaries of garnet crystals, and with dotting of the finely feldspar layers (line weight: 0.25 mm). Relics of coarser grains are accentuated by light dotting. The boundaries between quartz and feldspar layers are delineated as strongly dotted but discontinuous lines corresponding to the diffuse transition visible in the thin-section image. Grain boundaries of recrystallized grains in the quartz layers are omitted, in order to increase the contrast to feldspar layers. Additionally, cross fractures (line weight: 0.25 mm) characterize the quartz layers. Small roundish epidote grains (black) are supplemented, which are linearly arranged in quartz layers. (c) Digital processing of drawing (a) accentuates the garnet distribution, based on gray filling of the blasts. (d) The additional gray filling of K-feldspar layers clarifies the high K-feldspar content of the mylonite.

Figure 2.23

Aegirine-nosean phonolite (Eskişehir, Anatolia/Turkey); sample M102; thin-section collection of TU Munich Geology; mineralogical composition: (i) phenocrysts: tabular plagioclase, green aegirine with zoning, and strongly altered nosean that is only weakly silhouetted against the groundmass as diffuse light spots; (ii) fine-grained groundmass: thin-tabular plagioclase and pigment; size of original photomicrograph ca. A3, generated by two merged A2 images.

(a)

Photomicrograph (plane-polarized light) with white lines retracing (i) outlines and zoning of aegirine (line weight: 4 and 2 pixels), (ii) outlines and fractures of plagioclase (line weight: 4 and 3 pixels), (iii) outlines of nosean (line weight: 2 pixels), and (iv) the orientation of thin plagioclase platelets (short strokes; line weight: 5 pixels) generated by an image processing program.

(b)

Image (a) without photomicrograph as background. White lines are changed to black and gray. The groundmass is evenly displayed in light gray, in order to highlight the phenocrysts and their flow pattern (H,S,V,R,G,B = 0,0,95,243,243,243).

Figure 2.24

Jadeite quartzite (Shuanghe, Dabie Shan, China); sample RP01; digital revision of a thin-section drawing with a standard image processing program; detail from Figure 2.18.

(a)

Boundaries of albite and symplectite versus quartz (1 pixel line weight), outlines of jadeite (4 pixels) and garnet (8 pixels), and fractures in jadeite (2 pixels) and garnet (6 pixels) are retraced.

(b)

Pattern of grain outlines separated from the thin-section scan.

(c)

Grains of the different minerals are filled with different shades of gray.

Chapter 3: Specimen Sections

Figure 3.1

Schematic fillings of rock structures.

(a)

Regular “brick” filling as characterization of limestone, with the joints oblique to the layer boundary; hardly acceptable in a geological map and definitely not in a geological drawing.

(b)

Filling of a limestone layer based on slightly irregular “bricks” with joints oriented parallel to the layer.

(c)

Folded layer of a porphyritic granite with feldspar phenocrysts whose flat faces are oriented obliquely to the layer and to the axial plane of the fold; already unacceptable in a geological map.

(d)

Flat faces of phenocrysts aligned around a folded layer.

(e)

Alignment of phenocrysts to a foliation parallel to the axial-plane of the fold.

Figure 3.2

Alternating sequence of folded quartzite-schist layers, with three possible representations of internal structures. Original drawing 15 × 7 cm; felt-tipped pen with 0.1 mm line weight.

(a)

The two rocks are foliated prior to folding. Accordingly, the schistosity planes are folded. In the schist, the schistosity is formed by short mica films and, in the quartzite, by oriented quartz grains. The schistosity is sufficiently depicted by a few strokes, which also accentuate the contrast in lightness between the two rocks.

(b)

The two rocks were foliated during the early phase of folding. In the relatively stiff quartzite layers, the schistosity developed as fractures in fan position. It forms planes in pile position in the schist.

(c)

The two rocks were foliated prior to folding. In the quartzite, the schistosity is folded. In the schist, new mica is formed after folding by a subsequent metamorphic reaction resulting in a random orientation of the mica platelets.

Figure 3.3

Photo of a rock cut and two schematic drawings. Both drawings highlight different aspects of the rock fabric. Scale always as in Figure (c). Quartz-porphyritic dacite; Sesia Zone, Val Loana (Western Alps, northern Italy).

(a)

Sample KR2261; photo of polished section perpendicular to the foliation. The euhedral, white feldspar phenocrysts clearly stand out from the fine-grained dark ground mass. In contrast, the deformed lenticular and transparent-gray, mostly elongate quartz grains are barely visible.

(b)

Line drawing of feldspar phenocrysts and quartz lenses. This drawing best shows sizes and distribution of quartz and feldspar. Feldspars and groundmass are kept blank. The internal dotting of the quartz lenses mimics the complete, fine-grained recrystallization. The shape contrast between both minerals indicates that deformation took place under conditions of crystal plasticity of quartz and brittle behavior of feldspar.

(c)

Line drawing with emphasis on internal fabrics of feldspar and the groundmass. The quartz lenses are kept blank. Compared to (b), the contrast between quartz and feldspar is generated by blank quartz and filled feldspar. The indistinguishable flow fabric and schistosity in the groundmass are formed by large, magmatic biotite platelets up to 100 μm in size and represented by short strokes. While shape and orientation of the quartz lenses reflect roughly homogeneous planar flattening of the rock, the orientations of biotite in the groundmass indicate a more complex flow and deformation pattern on a smaller scale. Felt-tipped pen with line weights of 0.1 to 0.2 mm; original size of both drawings ca. A4.

Figure 3.4

Schematic sketches of various rocks. Original size of the entire sketch A4; felt-tipped pen with a usual line weight of 0.1 mm, occasionally ranging to 0.5 mm in certain cases; natural size of the rock areas: (a) to (d) ca. 1 m, (e) to (m) ca. 10 cm.

(a)

Sandstone layers of different grain size; from bottom to top: conglomerate, coarse, and fine gravel (only indicated by light dotting); a thin clay layer (represented by short strokes); cross-bedded sandstone (lamellae indicated by dotted lines); top layers: coarse sandstone (indicated by light dotting); solid lines: thin clay layers separating and, consequently, defining the sandstone bedding.

(b)

Two tephra layers of different composition, size and shape of fragments, and grain size of groundmass, separated by a tuff layer with internal layering structure (dotted lines); tephra groundmass represented by dotting.

(c)

Bedded limestone with cross fractures and thin clay interlayers. The fractures are limited to single layers and characterize the limestone.

(d)

Homogeneous granite with a thick pegmatite and a thin aplite vein. The short strokes represent (i) biotite platelets in granite, (ii) biotite and/or crystal faces in aplite, and (iii) crystal faces and feldspar cleavage planes, respectively, in pegmatite. These various types of strokes sufficiently and precisely represent the homogeneity and grain fabric of the rocks and the random orientation of biotite.

(e)

Porphyritic granite with flow fabric. The feldspar phenocrysts are schematized as rectangles and the biotite platelets as short dashes. These dashes also accentuate the isotropy of the groundmass.

(f)

Diorite (top) and gabbro (bottom) characterized exclusively by short, thin and short, thick strokes representing amphibole and pyroxene crystals, respectively. All other minerals are left out for clarity.

(g)

Mica quartzite. Only mica platelets are shown as short strokes. They indicate bedding with various mica content and the schistosity (grain orientation).

(h)

Augengneiss with schistosity represented by the alignment of biotite platelets (short dashes). Deformation under greenschist-facies conditions or lower temperatures is indicated by brittle behavior of feldspars (cross fractures in phenocrysts).

(i)

Augengneiss with conjugate foliation planes which are delineated by aligned biotite (short dashes). The biotite platelets frame lenses of plastically deformed feldspars, thus pointing to deformation temperatures higher than those of greenschist facies.

(k)

Garnet mica schist (top) and chlorite albite schist (bottom). Garnet and albite blasts are optically separated by different line weights and internal fabrics (fractures in garnet and small mineral inclusions in albite). Chlorite occurs in rosettes. The sigmoidal conjugate foliation planes are formed by mica platelets (short dashes).

(l)

Crenulation cleavage of different intensity, represented by solid lines (top) and broken lines (bottom).

(m)

Mica schist with quartz veins that are oriented parallel to the first schistosity and were isoclinally folded during the second deformation event. Typically, the quartz veins are dismembered into lenses and isolated fold crests. The cross fractures are characteristic of such quartz veins and facilitate their identification in the drawing. In addition, only few foliation planes are sketched as longer strokes. For clarity, the groundmass is not illustrated.

Figure 3.5

Alternating sequence of metamorphic cm-thick pelitic and psammitic layers affected by two deformation events, which led to two schistosities, layering-parallel S1 and transverse S2. Lower part of the drawing: cm-thick layers with variable mica and quartz content, which, accordingly, show different intensities of foliation. The gradation of mica content leads to variation in orientation of the foliation planes. Upper part of the drawing: a mica-quartzite layer (qm) with weak layering between schist layers is fractured. In contrast, smooth boudins are developed in a fine-grained quartz-mica layer represented by fine dotting. These structures indicate the relatively higher strength of the mica quartzite.

Figure 3.6

Schematic development of a drawing. Alternating sequence of schist of variable mineralogical composition and a transverse young vein.

(a)

Sketch of the layering and its slightly inclined orientation. The horizontal in the field is equivalent to the “horizontal” in the drawing. Distinctive internal structures are integrated: the main schistosity—as long or short strokes depending on the schistosity intensity; isoclinally folded quartz veins that, in schist, are usually well visible; mineral blasts (garnet and feldspar) with slightly different line weight (higher for garnet, lower for feldspar).

(b)

A vein detected during this stage of drawing is added to the rightward extended drawing. The schistosity, also detected late, and the weak folding between foliation planes are superposed on the main schistosity in certain parts. The main schistosity is locally complemented, in order to visualize grading in one of the layers, and is intensified along the isoclinal folds of quartz veins to increase the optical contrast.

(c)

Garnet blasts are differentiated from the feldspars by their internal fracture pattern. The fine grain fabric of the vein is represented by light dotting. Labeling with typical abbreviations improves the readability of the drawing. Last but not least, the enlarged details of the internal fabrics of feldspars serve for better presentation of the deformation processes. The scale only roughly indicates the thickness of layers and the vein. It is obvious that the scale is not valid for the other structures, such as the sizes of mineral blasts or the spacing of foliation planes.

Figure 3.7

Schematic representation of rocks on three different scales. The boxes mark the previous sketch. Size of the original drawing ca. 16 × 21 cm; felt-tipped pen with variable line weight.

(a)

Meter thick alternating layers of strictly banded amphibolite (dark) and clearly deformed augengneiss (light).

(b)

Meter thick layers of gabbro (dark, dotted) and granulite (striped).

(c)

Meter thick layers of basalt (black) and schist (striped).

(d)

Detail of (a) with centimeter to decimeter thick layers in amphibolite and gneiss. In comparison to these layers, amphibole (thick, short strokes) and biotite representations (thin, short strokes) are not to scale.

(e)

Detail of (b) with schematized and not-to-scale pyroxene (open rectangles) and biotite (short, thick strokes) in gabbro, and with mafic portions in granulite (strokes of different length and thickness).

(f)

Detail of (c). Dot- and stroke-based schematization of basalt fabric and half-schematic representation of S-C fabric in schist. On this scale, the dark-light contrast between these rocks is inverted compared to (c).

(g)

Detail of (d). Amphibole in amphibolite and feldspar in gneiss are schematized based on line weight, grain shape, and internal fabric, following

Figure 2.1

and

2.2

.

(h)

Detail of (e). Pyroxene in gabbro, and garnet and sillimanite in granulite are schematized based on

Figure 2.1

and

2.2

. The isotropic “groundmass” in gabbro and in the light layers of granulite are represented by short strokes.

(i)

Detail of (f). Phenocrysts in basalt (e.g., olivine, pyroxene) are sketched as open or closed polygons. The isotropic groundmass is represented by short strokes. Short strokes divide the distinct foliation planes of the S-C fabric in schist into their biotite and white mica components. (

k

,

l

,

m

) Schematized rock representations of (g), (h), and (i), with more intense omissions, that is, with generally fewer strokes.

Figure 3.8

Low-metamorphic metapelite, foliated and folded during the Variscan orogeny, with mm- to cm-wide quartz veins (Middle Rhine Region near Loreley, Rhenish Massif, Germany).

(a)

Scan of a sample section perpendicular to the fold axis; long side of sample ∼ 15 cm.

(b)

Drawing of the section with only locally indicated structures; drawing on tracing paper placed on a scan print; felt-tipped pen with line weights of 0.1 to 0.2 mm; quartz veins are represented as slightly dotted, bedding and first schistosity as dotted lines, and the second schistosity as continuous lines. Locally, the quartz veins diverge from the bedding planes (circle). The second schistosity, transverse to bedding, is intensified in shear zones (rectangle).

Figure 3.9

Folded biotite-feldspar gneiss from the basement of the eastern Tauern Window (Eastern Alps, Austria); section perpendicular to the fold axis; short side of section ∼ 10 cm; light = quartz-feldspar layers with mm-grain size; dark = biotite-rich quartz-feldspar layers; different graphical representations of the scanned sample section; drawings on tracing paper placed on a scan print; felt-tipped pen with line weights of 0.1 to 0.5 mm.

(a)

Scan of the sanded, but not polished, section. The light feldspar layers are partly sharply confined by dark biotite layers. The changes from feldspar-rich to biotite-rich layers are partly gradual.

(b)

Drawing of the boundaries between light and dark areas.

(c)

Drawing with more strongly schematized boundaries. Size and orientation of biotite platelets are indicated by short strokes. Thus, the folded first foliation and the locally developed second foliation are illustrated (rectangle and circle, respectively).

(d)

Drawing without boundary lines. The layers are represented and separated from each other solely by their different content of biotite. Thereby, even diffuse transitions can also be made visible.

Figure 3.10

Weakly foliated and folded augengneiss from the Sesia Zone (Val d'Ossola, Western Alps, Italy).

(a)

Scan of a polished sample section; mm-cm-sized feldspars stick in a μm-mm-grained groundmass of quartz, feldspar, and biotite; short side of sample ∼ 10 cm.

(b)

Drawing of the scan only representing the approximate size, orientation, and accumulation of biotite by short strokes; drawing on tracing paper placed on a scan print; felt-tipped pen with 0.1 to 0.2 mm line weight.

(c)

Drawing only representing the outlines of larger feldspars with few local biotite platelets.

Chapter 4: Drawing Rock Structures in Three Dimensions

Figure 4.1

Rectangular block of gneiss with one surface frontally facing the viewer and two side surfaces perspectively extending backwards (broken lines). Mica platelets are schematized as shorter and longer strokes. The mica orientation defines a schistosity perpendicular to the front and top face, and parallel to the lateral face. The lineation, developed on the foliation plane, is marked by the long axes of elongate and aligned micas, and runs parallel to one edge of the block. The horizontal and vertical edges of the front face are parallel to the margins of the field book or canvas. Thus, these edges are defined, or perceived respectively, as horizontal and vertical in nature.

Figure 4.2

Rectangular blocks as basic elements of a 3D drawing.

(a)

Blocks with a weak perspective distortion toward the “right back” and “left back.” The prolongations of the edges (broken lines) meet at remote vanishing points. The edges follow the sketched rectangular coordinate system.

(b)