Geology 106 Geological Mapping Techniques SUNY University at Buffalo

Transcription

1 Geology 106 Geological Mapping Techniques SUNY University at Buffalo

2 Why are maps important in geology? Geology and Relief of the Conterminous US USGS "Tapestry of Time and Terrain" - The shape of Earth s surface, the spatial arrangement of different rock types, and the relative ages of those rock units allow us to reconstruct Earth history, deduce the workings of plate tectonics, and understand surface processes - Geologic maps are, in a sense, a synthesis of what s known about the geological history of an area

3 Course Objective At the end of this course students should be able to read and interpret maps commonly used in the Earth and environmental sciences to understand two- and three-dimensional spatial relationships in the surface and subsurface. - This course will focus mainly on the reading and interpretation of existing maps (w/ paper and pencil) -We ll aim for a quantitative description of the surface and subsurface - My primary goal for you in this course is to be able to move easily between a 2D representation (a map) and the 3D reality it depicts

4 3D 2D Learning how to position ourselves on the globe, and how to represent that position on a flat piece of paper, has been the focus of cartographers and geodesists for centuries

5 3D 2D The Grand Canyon N The topography provides a window into the Earth

6 A geologic map depicts that window in 2D, providing a tool for interpreting the 3D arrangement of rock units we can t observe directly 3D 2D Geologic Map of the Grand Canyon N

7 Course Materials (Bring every day!) - Graph paper tablet (1/4 inch rule) - Regular pencils and colored pencils (12 color set is fine) - Ruler AND protractor - Calculator (need trig functions) - Tracing paper - Spencer Text (will use some of the maps included with the text)

8 Course Outline* Labs 1-4: Introduction to maps of the Earth s surface: topography and geomorphology Labs 5-9: Introduction to geologic maps: connecting surface and subsurface in 3D Lab 10: Interpreting deformation from geologic maps: faults and folds Lab 11: Making measurements in the field** Lab 12: Applied subsurface mapping in petroleum geology *Subject to change **This lab may happen earlier in the semester

9 GLY 106 Laboratory 1 Reading a Topographic Map - Today we ll practice reading a standard USGS Topographic Quadrangle map of South Buffalo - Before starting, let s have a look at some essential map elements

10 USGS Topographic Quadrangle Map Legend (Notice your map is not square.why?)

11 Map Scale For example: On a 1:24000 quad map, scale factor = 24000, so a distance of 1cm on the map represents a true distance of 24000cm or 240m. Note: Your map has been rescaled to fit on an 11 x 17 sheet of paper, so it s no longer 1:24000! The scale bar, however, is still accurate.

12 Map Projection

13 Map Projection The Datum: A mathematical model of the Earth (Basis for NAD83 and WGS84)

14 Map Projection The Transverse Mercator Projection Tangent Case Secant Case Secant Case Yellow zone bounded by two standard meridians and one central meridian A conformal projection, meaning local angles and shapes are preserved.

15 The Transverse Mercator Projection Each UTM zone = 6 degrees of longitude

16 Universal Transverse Mercator (UTM) Zone Coordinate System (-,+) (+,+) Central Parallel False Origin (-,-) (+,-) Eastings Northings Central Meridian Northings/Eastings are reported in meters

17 Universal Transverse Mercator (UTM) Zone Coordinate System Example: Easting m, Northing m (Note orange UTM grid: 1000mx1000m)

18 True North, Magnetic North, and Grid North

19 True North vs. Magnetic North - Earth exhibits a magentic field roughly analogous to a magnetic dipole - However, the poles of the magnet do not coincide with the geographic poles (i.e., the spin axis) - At any point on the planet, a compass reading (which measures the magnetic field) will show magnetic north, which in most places is offset from true (geographic) north

20 True North vs. (UTM) Grid North Grid North GN

21 Topographic Contours - Contours join lines of equal elevation above a datum ( sea level) - Provide a 2D representation of a 3D surface

22 Topographic Contours Landscape Perspective View (3D) Topographic Contours Topographic Contour Map (2D) Topographic Cross-Sections

23 GLY 106 Laboratory 2 Making a Topographic Profile

24 Topographic Contours - Contours join lines of equal elevation above a datum ( sea level) - Provide a 2D representation of a 3D surface

25 Topographic Contours - Any arbitrary traverse across a topographic map can be converted to a topographic profile (i.e., a plot of elevation)

26 Topographic Contours Elevation Distance Resolution (level of detail) depends on contour interval; scale can be adjusted to give the desired vertical exaggeration

27 Traverse From Map Elevation Profile Elevation (ft) Distance (Scale = Map Scale)

28 Topographic Contours Note: Both plots convey the same information, but the vertical exaggeration affects the appearance of the plot VE=Vertical Scale/Horizontal Scale H 2 Elevation High Vertical Exaggeration H 1 Distance H 2 Elevation Low Vertical Exaggeration H 1 Distance

29 Topographic Contours - For this lab, you ll make three topographic profiles; each parallels a 4km road segment in the SE Buffalo Quadrangle - You are given specific instructions on the scale and vertical exaggeration for the plot - You are given specific instructions for the plot labels and interpretation (and don t forget instructions #6 and #7) - Take your time!

30 GLY 106 Laboratory 3 Surface Hydrology and Topography

31 Have you noticed a relationship between topography and watercourses (i.e., creeks and rivers)? Today we ll explore this in more detail

32 Drainage Basins (or Catchment Areas ) - The collection area for water that falls on the Earth s surface - Defined above any point on a stream - Separated by drainage divides Drainage Area Above Point P - Not fixed over geologic time - Nested P

33 Drainage Basins of W NY Example of a regional drainage divide crossing the Ellicottville area

34 Ellicottville Quad To Cattaraugus Ck To Allegheny River From the pattern of the streams, we can make out the regional drainage divide

35 Ellicottville Quad To Cattaraugus Ck C B A D To Allegheny River From the pattern of the streams and topography, we can map surface drainage in great detail

36 A Distance B Elevation C Distance D Elevation Stream Stream

37 Ellicottville Quad To Cattaraugus Ck To Allegheny River From the pattern of the streams and topography, we can map surface drainage in great detail

38 Stream Networks - Drainage areas evolve stream networks -The networks are commonly characterized by their branching patterns Drainage Area Above Point P - Stream Order indices are used to describe the branching pattern in a network P

39 Ellicottville Quad To Cattaraugus Ck ? To Allegheny River Example of Stream Ordering

40 Stream Long-Profile Just a topographic profile that traces the axis of a stream or river? Elevation Distance Downstream

41 Rainfall rate (L/T) Area of Bucket Opening (L 2 ) Water level (constant in time) Discharge (L 3 /T) How to think about question #4

42 GLY 106 Laboratory 4 Contouring Spot Heights (known points of elevation)

43 Idealized Case

44 Actual Case

45 What if we wanted contours at 785 and 800? 793ft ft Elev. 777ft 785 Map distance 777ft 814ft Example: Linear Interpolation ( )/(Map Distance)=Elevation change per unit map distance, or slope So for the 785 contour: (8/( )) x Map Distance = Map Distance from the 777 measurement

46 What if we wanted contours at 785 and 800? A? 793ft B? C? 777ft 814ft - By using linear interpolation between known points, you can incrementally constrain the shape of the desired contours -BUT keep in mind: Some interpretation is still required - Would you choose A, B, or C for the 785 contour?

47 Today: You ll construct the contours specified in the lab tutorial A few suggestions - Work from the outside in - Maintain accuracy and recognize redundancy - Remember, there will always be some interpretation - Keep your contouring objective in mind (see tutorial)

48 GLY 106 Laboratory 5 Introduction to Geologic Maps

49 Geologic Maps - Geologic maps are those that show the spatial distribution of materials exposed at the Earth s surface, i.e. rocks or broken up bits of rock (sediment) - Geologic maps are most commonly overlain on a topographic base, i.e. information about surface geology is combined with information about topography - Geologic maps subdivide the materials at the Earth s surface into units based on lithology (rock or sediment composition), age, or other criterion (e.g. depositional environment for sedimentary rocks, metamorphic grade for metamorphic rocks) - KEY Point: Although some geologic maps contain explicit information on the subsurface (from wells), most are only an expression of what can be observed at Earth s surface.yet, much can be inferred about the subsurface from a geologic map on a topographic base Geologic maps come in two main types: 1) Surficial Geologic Maps show the distribution of loose (uncosolidated, unlithified) material, and are most commonly used where glacial deposits are widespread or to emphasize river courses and deposits 2) Bedrock Geologic Maps exclude surficial deposits to show the distribution of bedrock and are the most common type of geologic map

50 Remember Some Fundamental Principles (1) Superposition: Rocks formed from deposition at the Earth s surface tend to be layered (strata) and widespread, with older rocks beneath younger rocks (2) Cross-cutting and Deformation: Rocks formed from igneous material that intrudes and cross-cuts strata are younger than that strata; if strata are broken (faulted) or highly deformed (folded), than the faulting or deformation postdates the formation of the strata (3) Unconformities: Represent significant breaks in deposition, often accompanied by erosion, and represent a geologic time gap (hiatus).and Some Conventions (1) Sedimentary rocks are classified and grouped hierarchically (by scale): Group -> Formation -> Member -> Beds (2) The first letter defining a map unit typically refers to the geologic period in which the rocks formed

51 Simple Superposition Superposition & Cross-Cutting (Erosion) Young Old Unit A Unit B Unit C Unit D Superposition & Cross-Cutting (Intrusion) Young Unit A Unit B Unit C Old Unit D Topography Noncomformity, Superposition Young Old Unit A Unit B Unit C Unit D Unit A Unit B + Unit C Deformation (Tilting) Angular Unconformity, Superposition Young Unit E Old

52 So Why Are Geologic Maps So Varied? - Patterns on surficial geologic maps arise due to surface processes: mainly sediment deposition by glaciers and rivers -Patterns on bedrock geologic maps are a function of two main factors: 1) Topography 2) Shape and orientation of the geologic units 3D Block Diagram 2D Geologic Map oung Old Unit A Unit B Unit C Unit D Contact A:B Contact B:C Contact C:D Unit A Outcrop

53 3D Block Diagram Contact A:E 2D Geologic Map oung Old Unit A Unit B Unit C Unit D Contact A:B Contact B:C Contact C:D Unit A Unit E Contact A:B Contact B:C oung Contact C:D Unit A Unit B Unit C Old oung Old Unit A Unit B Unit C Unit D Topography Unit A Unit B Unit C Unit B Unit C Unit D Unit A

54 The Stratigraphic Column oung Old Unit A Unit B Unit C Unit D Topography Unit A Unit B Unit C Unit B Unit C Unit D Unit A Unit A Unit D Elevation Unit B Unit C Distance

55 Strike and Dip - The 2D map pattern of geological surfaces (contacts, or top and base of a rock unit) is strongly dependent on the 3D orientation of that surface - The orientation is generally described by two measures: Strike and Dip N dip S Dip Angle: Maximum inclination of the surface from the horizontal Dip Direction: Direction towards which the surface is inclined Strike: A line on the surface 90 from the dip direction Example shown here: Strike is 0 (or 180 ) Dip is 40 W

56 Strike and Dip N What are the strike and dip shown here?

57 N Young Old Unit A Unit B Unit C Unit D Contact A:B Contact B:C Contact C:D What s Unit E s dip direction and strike? N Contact A:B Contact B:C Young Contact C:D What Units A-D s dip direction and strike? Old N Young Old Unit A Unit B Unit C Unit D What Units A-D s dip direction and strike? Topography

58 Today: Some practice interpreting geologic maps The Grand Canyon National Park (Spencer Map, Ap2-13)

59 Note the following: Vishnu Igneous Intrusives (Zoraster Complex) PCbr = Brahma Schist PCr = Rama Schist PCgd = Zoraster Granite PCg = Zoraster Granite Pcum = Ultramafic intrusives

60 GLY 106 Laboratory 6 Planar Contacts and Topography

61 N - Last week, we introduced the Strike and Dip definition of a plane in 3D space -Contacts between geologic units are called planar or homoclinal if they have a uniform strike & dip - Many units are more or less planar at large scale (over small areas)

62 The Three-Point Problem If a contact is planar (or can be approximated as such), its strike and dip can be defined from three points of known elevation 450ft 600ft 300ft Map View 3D Perspective View

63 Structure Contours We can represent the elevation of any surface on a map using contours When that surface is a geologic contact and extends below the Earth s topographic surface, we call the contours defining its elevation structure contours Young Old

64 Structure Contours The intersection of structure contours and topographic contours on a map define where a unit outcrops, or is exposed at the Earth s surface Topographic Contours Structure Contours on Top of Unit X Top of Unit X Outcrop Unit X Map View

65 Structure Contours Topographic Contours Structure Contours on Top of Unit X Top of Unit X Outcrop Unit X Map View Cross-Section Depth Unit X Unit X

66 Structure Contours The intersection of structure contours and topographic contours on a map define where a unit outcrops, or is exposed at the Earth s surface Topographic Contours Structure Contours on Top of Unit X Top of Unit X Outcrop Unit X Map View

67 Formation Depth dip The last question today asks for the depth of a contact at a particular location; the depth is the vertical distance beneath the ground surface

68 GLY 106 Laboratory 7 Structure Contours and Outcrop

69 Structure Contours We can represent the elevation of any surface on a map using contours When that surface is a geologic contact and extends below the Earth s topographic surface, we call the contours defining its elevation structure contours Young Old

70 Structure Contours Structure contours are often extended (interpolated) across regions where they no longer exist because the contact is eroded

71 The Grand Canyon Even though the river has eroded them away, we can infer the extension of units and contacts across the canyon

72 Even though the river has eroded them away, we can infer the extension of units and contacts across the canyon The Grand Canyon North Structure contours

73 True vs. Apparent Thickness Young Old Unit A Unit B Unit C Unit D depth True thickness Young depth dip True thickness vs Apparent thickness Old True thickness = Apparent thickness x cos (dip )

74 True vs. Apparent Thickness Young depth dip True thickness vs Apparent thickness Old True thickness = Apparent thickness x cos (dip ) top base top base top base top base top base If a unit dips, its thickness measured by differencing overlying structure contours on its top and base will be an Apparent thickness. The difference is negligible for small dips, but appreciable for large dips

75 True vs. Apparent Dip True dip vs Apparent dip True dip refers to the maximum dip, or slope, of a surface, as measured perpendicular to the strike of the surface (and thus perpendicular to its structure contours) Apparent dip refers to a dip less than the true dip, measured oblique (or parallel) to the strike of the surface tan (True dip )= tan (Apparent dip )/cos (β ) β = Angle between Apparent and True dip direction

76 True vs. Apparent Dip dip A single 2D exposure will most likely yield only an apparent dip

77 NOTE When reporting strike and dip, the compact and conventional format is: Strike/Dip Magnitude & Quadrant Direction o if a unit has a strike of 45 and a dip of 30 towards the NW, we d write: 45 /30 NW E W N

78 Today s Lab: More practice relating structure contours, topography, and outcrop Practice obtaining true vs. apparent dips and thicknesses

79 GLY 106 Laboratory 8 Structure Contours, Outcrop, and Cross-sections I

80 Recall Geologic maps typically display two of three types of key information: (1) Topography, (2) Outcropping Contacts, and (3) Structure Contours In general, if we have two of the three structure contours, outcrop pattern, and topographic contours we can obtain the third Today we ll use all three to interpret a map and draw a geologic cross-section

81 Structure Contours The intersection of structure contours and topographic contours on a map define where a unit outcrops, or is exposed at the Earth s surface Topographic Contours Structure Contours on Top of Unit X Top of Unit X Outcrop Unit X Map View

82 Structure Contours A Topographic Contours Structure Contours on Top of Unit X Top of Unit X Outcrop Unit X B Map View Cross-Section Depth Unit X Unit X

83 Sample Cross-Section Depth A B Vertical Datum

84 GLY 106 Laboratory 9 Cross-Sections II

85 Sample Cross-Section Depth A B Vertical Datum

86 Today s Map: Salem (KY) quad, Spencer Ap-26 - Line of cross-section shown - Your section will display the units above the base of the Bethel S.S. (Mcb), the shape of which is shown with the red structure contours -To construct your cross-section, you should plot points where your line of section crosses the structure contours (=28pts) - I give you the topographic elevations at these points Map from Spencer, E.W Geologic Maps: A Practical Guide to the Preparation and Interpretation of Geologic Maps (2nd Ed). Prentice Hall.

87 3D Block Diagram Contact A:E 2D Geologic Map oung Old Unit A Unit B Unit C Unit D Contact A:B Contact B:C Contact C:D Unit A Unit E Contact A:B Contact B:C oung Contact C:D Unit A Unit B Unit C Old oung Old Unit A Unit B Unit C Unit D Topography Unit A Unit B Unit C Unit B Unit C Unit D Unit A

88 A Quick Primer on Folds and Outcrop Patterns Unit D Unit C Unit B Unit A Unit C Unit B Unit A

89 GLY 106 Laboratory 10 Mapping in GoogleEarth

90 Map View

91 Perspective View

92 Perspective View

93 Perspective View

94 Perspective View

95 Perspective View

96 Perspective View

97 GLY 106 Laboratory 12 Field Lab II and Exam Review

98 Today 1. Finish plotting points collected during last week s lab (see lab tutorial from last week). 2. Exam overview 3. Open review

99 Outline of Topics Part 1: Topographic Maps Understanding the USGS topographic quad (map scale, map projection, true/magnetic/grid north, datum) Topographic contours: how they re defined, the contour interval, reading and interpolating elevations at specified locations Using topographic contours to interpret geomorphology and construct topographic profiles

100 Outline of Topics Part 2: Geologic Maps The basics: What does a geologic map depict, why do geologic maps look the way they do? Strike and dip of a planar surface: how they are defined and measured Structure contours on geologic contacts and how they differ from topographic contours The three-point problem: Defining a planar geologic contact based on three points of known elevation

101 Outline of Topics Part 3: Geologic Map Interpretation The relationship between structure contours, topographic contours, and unit contacts on a geologic map Distinguishing true thickness from apparent thickness and true dip from apparent dip Constructing geologic cross-sections from a geologic map on a topographic base

102 Outline of Topics Field Lab The Brunton compass: Magnetic declination, reading bearings, triangulation, measuring dips using the clinometer, measuring strike Text Readings Spencer: Chapters 1,2,3,6, 7, 8, 9