Structural geology laboratory manual

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STRUCTURAL GEOLOGY LABORATORY MANUAL Fourth Edition by David T. Allison Copyright © 2015 Associate Professor of Geology Department of Earth Sciences University of South AlabamaTABLE OF CONTENTS LABORATORY 1: Attitude Measurements and Fundamental Structures ................ 1-1 Reference system ...................................................... 1-1 Attitude of Planes ...................................................... 1-2 Attitude of Lines ...................................................... 1-5 The Pocket Transit ..................................................... 1-6 Magnetic Declination ................................................... 1-6 Measurement of Planar Attitudes with the Pocket Transit ...................... 1-7 Measurement of Linear Attitudes with the Pocket Transit ...................... 1-7 Locating Points with a Pocket Transit ...................................... 1-8 EXERCISE 1A: Geological Attitudes and 3D Block Diagram Interpretation ...... 1-19 EXERCISE 1B: Geological Attitudes and 3D Block Diagram Interpretation ...... 1-28 LABORATORY 2: Orthographic Projections for Solving True/Apparent Dips and Three-Point Problems ............................................................ 2-1 True and Apparent Dip Calculations ....................................... 2-1 Three Point Problems ................................................... 2-2 EXERCISE 2A: Orthographic Projections. .................................. 2-7 EXERCISE 2B: Orthographic Projections. .................................. 2-9 LABORATORY 3: Basic Stereographic Projections ................................ 3-1 Stereographic Projections ............................................... 3-1 Elements of the Stereonet ............................................... 3-1 Plotting Planes and Lines on the Stereonet .................................. 3-2 Solving Problems with the Stereonet ....................................... 3-2 EXERCISE 3A: Stereographic Projections I ................................. 3-7 EXERCISE 3B: Stereographic Projections I ................................. 3-9 LABORATORY 4: Rotational Problems with the Stereonet. .......................... 4-1 Plotting the Pole to a Plane .............................................. 4-1 Fold Geometry Elements ................................................ 4-1 Finding Paleocurrent Direction from Crossbed Data .......................... 4-2 Rotational fault problems ............................................... 4-10 EXERCISE 4A: Rotations with the Stereonet ............................... 4-19 EXERCISE 4B: Rotations with the Stereonet ............................... 4-21 LABORATORY 5: Contoured Stereographic Diagrams ............................. 5-1 Types of Stereonets .................................................... 5-1 Constructing contoured stereonets ......................................... 5-1 Interpretation of Stereograms ............................................ 5-3 Analysis of Folding with Stereograms ...................................... 5-4 iiProblems Associated with Fold Analysis on the Stereonet ...................... 5-5 EXERCISE 5A: Contoured Stereograms and Interpretation of Folded Data ........ 5-6 EXERCISE 5B: Contoured Stereograms and Interpretation of Folded Data ........ 5-9 LABORATORY 6: Campus Geologic Mapping Project .............................. 6-1 Mesoscopic Structure ................................................... 6-1 Megascopic Structure Symbols ........................................... 6-2 Pace and Compass Traverse.............................................. 6-3 EXERCISE 6: Geologic Map and Structural Analysis General Instructions .............. 6-5 EXERCISE 6A Geologic Map and Stereonet Analysis ......................... 6-6 EXERCISE 6B Geologic Map and Stereonet Analysis ......................... 6-8 LABORATORY 7: Geologic Map & Cross Section Field Project ...................... 7-1 EXERCISE7A: High Fall Branch Geologic Map & Cross-Section ............... 7-3 EXERCISE 7B: Tannehill Historical S.P. and Vicinity Geologic Map & Cross-section .............................................................. 7-6 LABORATORY 8: Thickness and Outcrop Width Problems .......................... 8-1 Thickness of Strata ..................................................... 8-1 Apparent thickness in a drill hole ......................................... 8-4 EXERCISE 8A: Thickness and Outcrop Width Problems ...................... 8-5 EXERCISE 8B: Thickness and Outcrop Width Problems ...................... 8-6 LABORATORY 9: Outcrop Prediction ........................................... 9-1 Outcrop Prediction ..................................................... 9-1 Special Cases ......................................................... 9-1 General Solution for Outcrop Prediction .................................... 9-2 EXERCISE 9A: Outcrop Prediction ....................................... 9-7 EXERCISE 9B: Outcrop Prediction ....................................... 9-9 LABORATORY 10: Stereographic Statistical Techniques ........................... 10-1 Least-squares Vector of Ramsay (1968) ................................... 10-2 Least-squares Cylindrical Plane .......................................... 10-2 Least-squares Conical Surface of Ramsay (1968) ............................ 10-3 Goodness of Fit Measures .............................................. 10-7 EXERCISE 10A: Stereograms and Statistical Techniques .................... 10-10 EXERCISE 10B: Stereograms and Statistical Techniques .................... 10-13 LABORATORY 11: Stress Analysis ............................................ 11-1 Stress Field Ellipsoid .................................................. 11-1 Mohr Circle Diagram .................................................. 11-1 Constructing the Mohr Circle Graph ...................................... 11-3 Determining the Attitude of Stress Axes and Fracture Planes................... 11-3 iiiMathematical Basis for Mohr Circle ...................................... 11-4 EXERCISE 11A: Mohr Circle and Stress Calculations ....................... 11-6 EXERCISE 11B: Mohr Circle and Stress Calculations ........................ 11-7 LABORATORY 12: Strain Analysis ............................................ 12-1 Strain Analysis ....................................................... 12-1 Use of the Hyperbolic Net (De Paor's Method) .............................. 12-2 Plotting the Attitude of the Finite Strain Ellipse ............................. 12-3 Solving for the Dimensions of the Finite Strain Ellipse ....................... 12-4 EXERCISE 12A: Strain Analysis ........................................ 12-7 EXERCISE 12B: Strain Analysis ........................................ 12-9 LABORATORY 13: Fault Displacement Vectors ................................. 13-1 Introduction to Fault Translation ........................................ 13-1 Apparent Translation (Separation) ....................................... 13-1 Net Slip ............................................................ 13-2 Rotational Faults ..................................................... 13-2 ................................................................... 13-5 EXERCISE 13: Fault Solutions .......................................... 13-5 ......................................................................... 13-5 LABORATORY 14: Down-plunge Fold Projections ............................... 14-1 Introduction ......................................................... 14-1 Constructing the Down-Plunge Profile Plane ............................... 14-1 EXERCISE 14: Fold Projection ......................................... 14-5 LABORATORY 15: Constructing Geologic Cross-sections from Geologic Maps. ........ 15-1 Exercise 15A: Geologic Cross-Sections ................................... 15-5 ivLIST OF FIGURES Figure 1-1 : Geologic time scale. ............................................... 1-9 Figure 1-2 :Rule of “V”s for contacts. .......................................... 1-10 Figure 1-3 : Steeply dipping strata. ............................................. 1-10 Figure 1-4 : Moderately dipping strata. ......................................... 1-11 Figure 1-5 : Vertical strata. ................................................... 1-11 Figure 1-6 : Overturned strata. ................................................ 1-12 Figure 1-7 : Apparent and true dips in a block diagram. ............................ 1-13 Figure 1-8 : Example anticline/syncline pair. ..................................... 1-13 Figure 1-9 : Example of an unconformity. ....................................... 1-15 Figure 1-10 : example of a geological basin- younger strata in core with circular geometry contacts. ............................................................ 1-15 Figure 1-11 : Example of a plunging anticline/syncline pair. ......................... 1-16 Figure 1-12 : Example of a non-plunging overturned anticline/syncline pair. ............ 1-17 Figure 1-13 : Example of a left-lateral strike slip fault. ............................. 1-17 Figure 1-14 : Example of a reverse dip-slip fault. ................................. 1-18 Figure 1-15 : Example of an oblique slip fault. ................................... 1-18 Figure 1-16 : Diagram for problem 1A-1. ....................................... 1-21 Figure 1-17 : Diagram for problem 1A-2 ........................................ 1-21 Figure 1-18 : Diagram for problem 1A-3.................................... ..... 1-22 Figure 1-19 : Diagram for problem 1A-4.................................... ..... 1-23 Figure 1-20 : Diagram for problem 1A-5 ........................................ 1-23 Figure 1-21 : Diagram for problem 1A-6.................................... ..... 1-24 Figure 1-22 : Diagram for problem 1A-7.................................... ..... 1-24 Figure 1-23 : Diagram for problem 1A-8.................................... ..... 1-25 Figure 1-24 : Diagram for problem 1A-9. ....................................... 1-25 Figure 1-25 : Diagram for problem 1A-10. ....................................... 1-26 Figure 1-26 : Diagram for problem 1A-11. ...................................... 1-26 Figure 1-27 : Diagram for problem 1A-12. ...................................... 1-27 Figure 1-28 : Figure for problem 1B-1. ......................................... 1-30 Figure 1-29 : Diagram for problem 1B-2. ........................................ 1-30 Figure 1-30 : Diagram for problem 1B-3. ........................................ 1-31 Figure 1-31 : Diagram for problem 1B-4. ........................................ 1-31 Figure 1-32 : Diagram for problem 1B-5. ........................................ 1-32 Figure 1-33 : Diagram for problem 1B-6. ........................................ 1-32 Figure 1-34 : Diagram for problem 1B-7. ........................................ 1-33 Figure 1-35 : Diagram for problem 1B-8. ........................................ 1-33 Figure 1-36 : Diagram for problem 1B-9. ........................................ 1-34 Figure 1-37 : Diagram for problem 1B-10 ....................................... 1-34 Figure 1-38 : Diagram for problem 1B-11. ....................................... 1-35 Figure 1-39 : Diagram for problem 1B-12 ....................................... 1-35 Figure 2-1 : Example problem 1 solution in spreadsheet form. ........................ 2-2 vFigure 2-2 : Example problem 2 solution in spreadsheet form. ........................ 2-2 Figure 2-3 : Diagram of a three-point problem solution. ............................. 2-3 Figure 2-4 : 3-point problem example in a spreadsheet. .............................. 2-5 Figure 2-5 : Spreadsheet for intersecting planes problem. ............................ 2-5 Figure 2-6 : Map for problem 5. ................................................ 2-8 Figure 2-7 : Topographic map of the USA campus with 3 contact points A, B, and C...... 2-10 Figure 2-8 : Geologic map of a portion of the Dromedary Quadrangle, Utah. ............ 2-11 Figure 3-1 : Example apparent dip problem worked with NETPROG. .................. 3-4 Figure 3-2 : Example Strike and Dip Problem worked in NETPROG. .................. 3-5 Figure 3-3 : Example intersecting planes problem. ................................. 3-6 Figure 3-4 : Equal-area (Schmidt) stereographic lower-hemisphere projection. .......... 3-11 Figure 4-1 : Example crossbedding paleocurrent problem. ........................... 4-4 Figure 4-2 : Crossbed example 1 rotation with Excel “rotation.xlsm”. .................. 4-5 Figure 4-3 : Crossbed example 2 rotation with Excel “rotation.xlsm”. .................. 4-6 Figure 4-4 : Example unfolding fold problem. ..................................... 4-9 Figure 4-5 : Rotational fault example. .......................................... 4-13 Figure 4-6 : Example rotational fault problem solution using “rotation.xlsm”. ........... 4-14 Figure 4-7 : Alternative manual rotational fault example. ........................... 4-15 Figure 4-8 : Example Drill Core problem. ....................................... 4-16 Figure 4-9 : Example drill core problem stereonet. ................................ 4-17 Figure 5-1 : Map for problem 2B. .............................................. 5-11 Figure 5-2 : Counting net (equal area). .......................................... 5-12 Figure 8-1 : Relationship of outcrop width (w) to stratigraphic thickness (t). ............. 8-1 Figure 8-2 : Relationship between apparent (w’) and true (w) outcrop width. ............ 8-1 Figure 8-3 : Cross-section of thickness with slope problem. .......................... 8-2 Figure 8-4 : Scenario where dip and slope directions are the same for thickness calculation. .................................................................... 8-3 Figure 8-5 : Cross-section of depth problem. ...................................... 8-4 Figure 9-1 : Example of horizontal contacts exposed in a valley. ...................... 9-1 Figure 9-2 : Example of geologic Rule of “V’s”. ................................... 9-2 Figure 9-3 : Initial setup of outcrop prediction example problem. ...................... 9-5 Figure 9-4 : Final solution of example outcrop prediction problem. .................... 9-6 Figure 9-5 : Topographic map for problem 1. .................................... 9-10 Figure 9-6 : Topographic map for problem 2. .................................... 9-11 Figure 9-7 : Topographic map for problems 3 and 4. ............................... 9-12 Figure 9-8 : USA campus topographic map. ...................................... 9-13 Figure 10-1 : Examples of eigenvector axial lengths. ............................... 10-5 Figure 10-2 : Example of data set that is normally distributed about a least-squares cylindrical surface according to the chi-square statistic. ................................ 10-9 Figure 11-1 : Example of the Mohr stress circle with fracture envelop. ................ 11-2 Figure 11-2 : Actual physical test specimen for Mohr circle example. ................. 11-3 Figure 12-1 : Simple shear of initially random ellipsoidal pebbles to form a preferred orientation of strain ellipsoids. .................................................... 12-1 viFigure 12-2 : Plot of strain axes and foliation. .................................... 12-3 Figure 12-3 : Undeformed and deformed strain marker reference used for derivation of formulae. ................................................................... 12-5 Figure 12-4 : Scanned photograph of deformed ooids in limestone. .................. 12-10 Figure 12-5 : Tracing of the deformed ooids in Figure 12-4. Use this to calculate R and Φ. F ............................................................... . . . 12-11 Figure 12-6 : Tracing of deformed pebbles in Cheaha Quartzite. Two parallel faces of the same sample (CA-23) are displayed. ......................................... 12-13 Figure 12-7 : Hyperbolic stereonet. ........................................... 12-14 Figure 12-8 : Photograph of deformed pebbles in a metaconglomerate with the cleavage direction indicated. ................................................... 12-15 Figure 13-1 : Example of traces of rotated dikes A and B. ........................... 13-3 Figure 13-2 : Calculation of rotational axis position. ............................... 13-4 Figure 13-3 : Map for problem 2. .............................................. 13-6 Figure 13-4 : Map for problem 3. .............................................. 13-7 Figure 14-1 : Down-plunge projection construction. ............................... 14-4 Figure 14-2 : Map for problem 1 projection. ................................................................... 14-6 Figure 15-1 : Example of apparent dip calculation for a vertical cross-section. .......... 15-2 Figure 15-2 : Example of the geometry of plunging folds and cross-section. ............ 15-3 Figure 15-3 : Geologic Map of the Wyndale and Holston Valley Quadrangles, VA. ...... 15-6 Figure 15-4 : Geologic cross-sections of the Wyndale and Holston Valley Quadrangles, VA. ................................................................... 15-7 viiLABORATORY 1: Attitude Measurements and Fundamental Structures. I. Reference system (A) Geological structures are represented by one or more lines or planes. (B) A line can be defined in three-dimensional space by its angle with three orthogonal axes. A plane can be represented by its normal, which itself is a line. (C) Maps contain two horizontal references: Latitude and Longitude (N-S, E-W) (D) The third reference axis is a vertical line. (E) Geologists typically orient structures with reference to the horizontal (strike, bearing, trace, trend) and the vertical (dip, plunge, inclination). (F) Specifying the orientation or attitude relative to the horizontal and vertical references will specify completely the three-dimensional orientation of a line or plane. (G) Orientation within the horizontal reference plane (map) is read relative to a compass direction (north, south, east, west) in units of degrees. (H) Orientation relative to the vertical is described simply as the angle measured from the horizontal plane to the plane or line of interest, this measurement being made in a vertical plane. This angle ranges from 0 to 90E. II. Important Geometrical Terms (A) Apparent dip: dip (incline) of a plane in a vertical plane that is not perpendicular to the strike. The apparent dip is always less than the true dip. (B) Attitude: orientation of a geometric element in space. ( C) Azimuth: a compass direction measured in degrees clockwise from north with north=0, east=90, south=180, and west=270. ( D ) Bearing: the compass direction of a line, in quadrant format. ( E) Cross section: representation of a geometry on a plane perpendicular to the earth’s surface. (F) True dip: the inclination of a plane measured in a vertical plane trending perpendicular to strike. (G) Dip direction: trend of the dip line; always perpendicular to strike. 1-1(H) Inclination: angle that the trace of a geometric element (line or plane) makes with the horizontal measured in a vertical plane. The maximum angle is 90 degrees (vertical). The angle of inclination of a plane is termed dip, for a line it is referred to as the plunge. (H) Lineation: general tern for a geological feature that is best represented by a line (mineral lineation, stretched pebbles, fold hinge, etc.) (I) Pitch: the angle between a line and the strike of the plane that contains the line. Pitch is synonymous with rake. (J) Plunge: angle of inclination of a line measured in a vertical plane. (K) Plunge direction: trend of a plunging line. (L) Quadrant: a compass direction measured 0-90 degrees from north or south. An example would be N60W (=300 azimuth) or S30E (= 150 azimuth). (M) Rake: angle measured between a line and the strike of the plane that contains the line. The quadrant of the end of the strike line from which the measurement is made must be included as part of the rake angle unless the rake angle = 90 (i.e. 40NE for a 40 degree rake angle measured from the northeast end of the strike). (N) Strike: the trend (compass direction) of the horizontal line in a geological plane (i.e. bedding, fault, joint, axial plane, etc.). By convention the compass direction of the strike is always assigned to a north quadrant., therefore, the azimuth possibilities are 0-90 and 270-360. Note that 360 azimuth is the same strike as 0. (O) Trace: the line formed by the intersection of two non-parallel surfaces. (P) Trend: azimuth direction of a line in map view. II. Attitude of Planes (A) Bedding, cleavage, foliation, joints, faults, axial plane are some of the geological structures that are represented as a plane. Although some of these features are actually curviplanar (i.e. curved surfaces), over short distances their tangent surfaces can be considered planar. (B) The linear attitude component of a plane that is measured in the horizontal reference plane is termed the strike. The strike of a plane is defined as the compass direction formed by the intersection of that plane with a horizontal reference plane. Another way to define strike is simply as the compass direction of the horizontal line contained in the geological plane of interest. By convention the azimuth direction of a strike line is read to a north quadrant so allowable measures of strike azimuth are in the range “000-090" 1-2and “270-360" for strike azimuth, or (N0E - N90E) and (N0W-N90W) for quadrant format strike line bearing. The only situation where the above definitions are ambiguous would be the special case where the plane of interest is horizontal, in which case there are an infinite number of horizontal lines in the plane. In this special case the strike is “undefined”, and a geologist would describe the plane as “horizontal” or has a “dip = 0". (C) The orientation of the strike line relative to the compass direction can be recorded in one of two ways: 1. Quadrant - N45EE, N15EW, N90EE (always read to a north quadrant) 2. Azimuth- 033E, 280E, 090E (always read to a north quadrant) Note that since there are two possible "ends" to a strike line, by convention strike lines are measured in the northern quadrants. (D) If you are using azimuth convention, be sure to use three digits even if the first one or two digits are "0". This avoids confusion with plunge or dip. (E) The dip of a plane defines its attitude relative to the vertical reference. There are two types of dip values: 1. True dip- all planes have only one unique value for true dip 2. Apparent dip- all planes have many possible apparent dip values that range from zero to less than, but not equal to, the true dip value. (F) The dip angle is the angle measured in a vertical plane from the horizontal down to the plane of interest. The true dip is always measured in the vertical plane that trends perpendicular to the strike of the plane. A dip angle measured in a vertical plane trending in any other map direction will always yield an apparent dip value less than that of the true dip. An apparent dip measured parallel to strike always will yield a dip angle of 0E. (G) Dip values always are in the range 0-90E . A dip angle of 0E defines a horizontal attitude. 90E of dip describes a vertically oriented plane. 0-20E: Shallow 20-50E: Moderate 50-90E: Steep (H) Specification of the strike orientation and dip angular value does not indicate the three-dimensional orientation of a plane; the direction of the dip inclination must also be known: 1-3Possible Strike/Dip quadrant combinations. Northeast Strike (0-090 Northwest Strike (270-360 azimuth) azimuth) True dip trends east SE NE True dip trends west NW SW (I) Note that it is unnecessary to measure the exact compass direction of the dip direction since it is by definition 90E from the strike. A full strike and dip might be recorded as: N45EE, 30ESE (quadrant strike first, then dip and dip direction) 045E, 30ESE (Strike azimuth first, then dip and dip direction) (J) Several different map symbols have been agreed upon by geologists to represent specific planar structures on geologic maps. All of the symbols have these characteristics in common: 1. The long dimension of the symbol is parallel to the strike line. 2. A tic mark or arrow oriented perpendicular to strike will point in the dip direction. A number next to this part of the symbol is the value of the true dip. 3. Special symbols exist for horizontal and vertical attitudes. (K) Because a geologic map must sometimes show multiple generations of planar structures, geologists must often "invent" symbols for a specific map. One should always explain the meaning of all symbols used within the map legend. (L) Besides strike and dip several alternative methods have been used to define a 3D planar attitude: 1. Right-hand rule: the azimuth direction of the strike is recorded such that the true dip is inclined to the right of the observer. In this case the strike azimuth could be to any quadrant. For example, the traditional strike and dip of 320, 55SW would be recorded as 140, 55. 2. Dip azimuth and Dip angle: this method relies on the implicit 90E angle between the true dip azimuth and the strike. The observer measures the dip azimuth and then the true dip angle. For example, a traditional strike and dip of 320, 55SW would instead be 050, 55. III. Attitude of Lines (A) Many geological structures such as fold hinges, mineral lineation, igneous flow 1-4lineation, intersection lineation, fault striations, flute casts, etc., possess a linear geometry in three-dimensional space. (B) Strike and dip cannot be used to measure the attitude of a line. Bearing and Plunge are the two components of linear attitude. (C) The plunge of a line is the angle that the line makes with the horizontal reference measured in a vertical plane. The plunge angle ranges from 0-90E. (D) The projection of the linear feature directly to the horizontal reference plane forms a line that is the bearing of the linear element. The bearing, like the strike, is measured relative to the compass direction. In this course we will normally use azimuth rather than compass quadrants to indicate bearing direction. (E) Although the bearing is measured in the same horizontal reference plane as the strike, its trend may be to any quadrant of the compass. This is because the bearing of the linear feature describes the compass direction of the plunge inclination, which could be in any direction. (F) To clearly distinguish it from a strike and dip, a linear attitude may be written as a plunge and bearing with the plunge angle first: 55E, 145E (plunge angle first, then the bearing azimuth) Although this convention is not universally followed. (G) A plunge angle value of 0E describes a horizontal line. A plunge angle of 90E denotes a vertical line, in which case the bearing is undefined since it has no component parallel to the horizontal reference. (H) Another term may be used to describe the attitude of a line if the line lies within a plane of known strike and dip. This value is the rake or pitch angle, and it is defined as the angle made by the line with the strike line of the plane in which it is contained. The direction end of the strike line from which the angle is measured must be noted to fix the attitude of the line. (I) Linear elements are displayed on a geologic map with a variety of features. The long dimension of these symbols describes the trend with an arrow pointing in the plunge compass direction. The numeric value next to the arrowhead is the plunge angle value in degrees. (J) Since many lineations are intimately related to certain planar features, such as a metamorphic mineral lineation contained within a planar foliation, these two structural elements may be combined into a composite map symbol on geologic maps. 1-5IV. The Pocket Transit (Brunton Compass) (A) The traditional survey instrument of the geologist has been the Brunton Compass or pocket transit, although the alidade and plane table is used in studies where more accuracy is needed. (B) The Brunton contains a magnetic needle that always seeks true magnetic north. On most, but not all, Bruntons, the white end of the needle points to magnetic north. (C) The perimeter of the compass is divided into degrees based on one of two formats: 1. Quadrant- four quadrants (NE, SE, NW, SW) of 90E each. 2. Azimuth - 0 to 360E. (D) A foldout metal pointer, termed the “sighting arm”, defines the long axis of the instrument. This is used as a sighting alignment for measuring a strike line or bearing. (E) Inside the compass is a bull's eye level and a clinometer level. The round bull's eye levels the body of the compass within the horizontal plane. The clinometer can be used to measure angles within a vertical plane. With the ability to measure both compass direction from magnetic north and vertical angles with the clinometer, the pocket transit can determine strike and dip or plunge and bearing of any geological structure. (F) Examination of either format compass reveals that the compass directions run in counterclockwise rather than clockwise fashion. This is done so that the north end of the needle reads the correct quadrant or azimuth value if one is sighting along the extended metal pointer arm. V. Magnetic Declination (A) Since magnetic north and geographic north do not coincide, geologic maps and survey instruments must correct for the angular difference in these values. In the United Sates, for example, the magnetic declination ranges from 0 to over 20E. The declination angle is measured as east or west depending on its orientation relative to geographic north. (B) All United State Geological Survey (USGS) topographic maps have the magnetic declination indicated in the margin information. 7.5' USGS topographic maps are the standard mapping tools for geological mapping. GPS receivers typically provide an up- to-date measurement of the magnetic declination. USGS maps published more than several decades ago will have inaccurate declination value. (C) To correct for magnetic declination, the pocket transit can be adjusted by turning the screw located on the side of the compass case. Turning this screw rotates the compass 1-6direction scale. Therefore, the compass can be adjusted for magnetic declination by ensuring that the long axis of the Brunton (sighting arm) points to geographic north when the north end of the needle indicates the 0E position. All USGS maps have the magnetic declination value for the map area printed on the bottom center margin of the map. VI. Measurement of Planar Attitudes with the Pocket Transit (A) Direct measurement of strike. (B) Direct measurement of dip. (C) Use of notebook or compass plate to simulate attitude of plane. (D) Shooting a strike and dip from a distance with peep sight. (E) Dips less than 12E cannot be measured because of the clinometer ring protector. 1. Water will run directly down the true dip direction id dripped on a smooth planar surface. 2. Visually estimate the true dip direction. Measure the dip angle in several directions sub-parallel to this direction. The steepest dip is the true dip direction. The strike is, of course, perpendicular to the true dip direction determined from the above methods. (F) When measuring dip angles remember that the clinometer bubble must be up while the pocket transit is held against the planar structure. VII. Measurement of Linear Attitudes with the Pocket Transit (A) The first component of a linear structure that is measured is usually the bearing. To measure the bearing one must line up the long axis of the compass parallel to the projection of the line to the horizontal. There are several methods that accomplish this: 1. Line the feature with the metal pointer while leveling the compass. 2. Align a clipboard or compass plate with vertical and parallel to the linear structure. Hold the compass against the plate while leveling. 3. Sight to a distant landmark that lies along the lineation using the peep hole sight. 4. Hold the compass against or close to the lineation. Level while keeping the 1-7edge of the compass parallel to the lineation. The azimuth read will be parallel to the structure. 5. Lineations observed on an overhanging surface can be almost impossible to measure directly. First measure the strike and dip of the surface that contains the lineation. With a protractor or compass measure the angle that the lineation makes with the strike line of the surface, carefully noting from which end of the strike line that the angle was measured. This is the rake angle of the lineation in the plane. In this case the strike and dip may not correspond to any geological structure– it is simply a reference plane. The plunge and bearing of the lineation may be calculated later with stereographic office methods (see Laboratory 2). (B) After determining the bearing you must measure the plunge angle. To determine the plunge, arrange the compass edge parallel to the lineation while measuring the plunge angle with the clinometer. It may be necessary for a partner to hold a pencil parallel to the lineation for reference while you measure the plunge on that object. (C) If the lineation lies within a planar structure whose attitude has already been recorded one may simply measure the rake angle of the lineation or bearing of the lineation (see section A-5 above). Either of these can later be converted to a bearing and plunge for plotting on a geologic map. The conversion can be done in the field with a stereonet. (D) If the lineation has a steep plunge, it may be difficult to visualize the correct bearing. In this case, if the lineation lies within a plane it is more accurate to measure its rake angle with a protractor, after first measuring the strike and dip of the plane containing the linear element. VIII. Locating Points with a Pocket Transit (A) The accuracy of a geologic map is totally dependent upon the accuracy of your field stations. The first job of the geologist is to accurately locate his or her position on the map. The compass can aid you in several ways. (B) Pace and Compass: in areas where no suitable topographic map exists, or where traverses do not follow existing roads or trails on the map, it is necessary to keep track of position with a pace and compass traverse. The traverse is done by estimating distance from point to point with pace counts, while bearings are shot from point to point and recorded. The traverse is later plotted on the map with a protractor and scale. The traverse must start from a known reference point. (C) Triangulation can locate a position by the determination of the bearing to two or more known landmarks that occur on a map. Plotting the reverse azimuth of the landmarks will intersect at the current position on the map. 1-8IX. Geologic Time (A) The geologic time scale must be understood for proper evaluation of geologic structures. Stratigraphic codes utilize the time scale to indicate the relative ages of strata. (Figure 1-1) Geologic Time Pe riod Symbol PeriodSymbol Young Young Mississippian M Quaternary Q Tertiary T Devonian D Cretaceous K Silurian S Ordovician O Jurassic J Triassic Tr Cambrian ‐C Permian P Precambrian p‐C Old Pennsylvanian P Old Figure 1-1 : Geologic time scale. (B) Stratigraphic codes such as “Oc” on a geologic map may mark the exposed area of the Ordovician age Chickamauga Formation. The geologic period abbreviation is always uppercase, whereas the formation name abbreviation is lowercase. Note the special symbols for Precambrian, Cambrian, Pennsylvanian, and Triassic. X. Rule of “V” for Geologic Contacts (A) When inclined beds cross a stream valley the contact will form a “V” pointing in the dip direction. (Figure 1-2) (B) The “V” shape is more pronounced with shallow dips, less so as the dip angle increases. (C) If the contact is vertical (i.e. dip angle =90) the contact will not form a “V” when crossing a valley. Instead there is no offset therefore the contact remains straight in map view. 1-9Rule of “V’s” for Geologic Contacts  XI. Bedding Strike and Dip Crossing Stream Valleys Symbols (A) Figures 1-3 through 1-6 illustrate various strike and dip symbols required for different attitudes of bedding. (B) Note that in the case of 20 50 90 overturned bedding the strata would have to be rotated more than 90 degrees to return the stratigraphic section to its original horizontal Figure 1-2 :Rule of “V”s for contacts. position. ‐Cs Oc Sr Dc 75 75 75 75 Dc 75 Dc Sr ‐Cs Oc p‐Ca Figure 1-3 : Steeply dipping strata. 1-10Dc ‐Cs Oc Sr Dc Sr 35 35 35 35 Oc 35 Dc ‐Cs Oc Sr p‐Ca Figure 1-4 : Moderately dipping strata. ‐Cs Oc Sr Dc Dc 90 Sr Dc ‐Cs Oc Figure 1-5 : Vertical strata. 1-11p‐Ca Sr Oc ‐Cs p‐Ca 35 ‐Cs 35 35 35 Oc 35 p‐Ca ‐Cs Oc Sr Dc Figure 1-6 : Overturned strata. XII. Apparent Dips and Block Diagrams (A) If inclined stratigraphic contacts intersect the vertical sides of a 3D block diagram such that the trend of the vertical side is not perpendicular to the strike of the contact the inclined angle of the strata on the side view will be an apparent dip rather than the true dip. The apparent dip is always less than the angle of the true dip. (B) In block diagrams apparent dips on the side view can constrain the true dip value used for the strike and dip symbol, as in Figure 1-7. 1-12Oc Oc Sr Sr 35 Dc Dc 35 Mt 35 Mt 35 Mt Dc Sr Oc Figure 1-7 : Apparent and true dips in a block diagram. XIII. Non-Plunging Folds and 3D Block Diagrams (A) Fold structures are most effectively displayed on 3D Block diagrams because they display the interpretation of the structure in the subsurface. (Figure 1-8) (B) Remember to use as much information from the map as possible - for example even though a contact does not intersect a vertical face on the block, it is possible that a contact may project to the face in the subsurface. © Anticlinal and synclinal closures should be used whenever possible. If a fold hinge projects above the diagram “ in the air” it 60 60 should be dashed. 40 40 40 40 Dc 50 50 50 (D) Use stratigraphic info- if 60 Sr the map surface displays a 40 thickness for a unit make Oc Sr Dc Mf Dc Sr Oc ‐ Ca Oc Sr Dc sure that is displayed on the 40 50 60 vertical faces of the Dc diagram. Sr Oc (E) The anticline axial trace ?p‐ Ca ‐ Ca symbol indicates that both limbs of the fold dip away Figure 1-8 : Example anticline/syncline pair. 1-13

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