geology 106 geological mapping techniques suny … · lab 12: applied subsurface mapping in...

Geology 106 Geological Mapping Techniques

SUNY University at Buffalo

Why are maps important in geology?

Geology and Reliefof the Conterminous US

- The shape of Earth’s surface , the spatial arrangement of different rock types, and therelative 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

Course ObjectiveAt 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

Learning how to position ourselves on the globe, and how to represent thatposition on a flat piece of paper, has been the focus of cartographers and geodesists for centuries

2D3D

The Grand Canyon

N

The topography provides a “window” into the Earth

2D3D

Geologic Map of the Grand Canyon

N

A geologic map depicts that window in 2D, providing a tool for interpreting the3D arrangement of rock units we can’t observe directly

2D3D

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)

Course Outline*Labs 1-4: Introduction to maps of the Earth’s surface: topography and geomorphologyLabs 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

GLY 106Laboratory 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

USGS Topographic Quadrangle Map Legend

(Notice your map is not square….why?)

For example: On a 1:24000 quad map, scale factor = 24000, so a distance of1cm 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.

Map Scale

Map Projection

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

Map Projection

The Transverse Mercator Projection

A conformal projection, meaning local angles and shapes are preserved.

Tangent Case Secant Case

Secant Case

Yellow zone bounded by two standard meridians and onecentral meridian

Map Projection

The Transverse Mercator Projection

Each UTM zone = 6 degrees of longitude

Universal Transverse Mercator (UTM) Zone Coordinate System

CentralMeridian

CentralParallel

(+,+)(-,+)

(-,-) (+,-)

“FalseOrigin”

EastingsN

orthings

Northings/Eastings are reported in meters

Example: Easting 675000m, Northing 4750000m(Note orange UTM grid: 1000mx1000m)

Universal Transverse Mercator (UTM) Zone Coordinate System

True North, Magnetic North, and Grid 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

True North vs. Magnetic North

Grid North

GN

True North vs. (UTM) Grid North

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

Topographic Contours

Landscape Perspective View (3D)

Topographic Contour Map (2D)

Topographic Contours

Topographic Cross-Sections

Topographic Contours

GLY 106Laboratory 2

Making a Topographic Profile

Topographic Contours

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

Topographic Contours

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

Topographic Contours

Distance

Elevation

Resolution (level of detail) depends on contour interval; scalecan be adjusted to give the desired vertical exaggeration

780800820840860

Distance(Scale = Map Scale)

Elev

atio

n (ft

)

Traverse From Map

Elevation Profile

Distance

Elevation

H2

H1

Distance

Elevation

H2

H1

High Vertical Exaggeration

Low Vertical Exaggeration

Note: Both plots convey the same information, but the vertical exaggeration affects the appearance of the plot

Topographic Contours

VE=Vertical Scale/Horizontal Scale

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!

GLY 106Laboratory 3

Surface Hydrology and Topography

Have you noticed a relationship between topography and watercourses (i.e., creeks and rivers)?

Today we’ll explore this in more detail

Drainage Area Above Point P

P

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

- Not fixed over geologic time

- “Nested”

Drainage Basins of

W NY

Example of aregional drainage dividecrossing the Ellicottvillearea

To Cattaraugus Ck…

To Allegheny River…

From the pattern of the streams, we can make out the regional drainage divide

Ellicottville Quad

To Cattaraugus Ck…

To Allegheny River…

From the pattern of the streams and topography, we can map surface drainage in great detail

A

BC

D

Ellicottville Quad

A B

Elev

atio

nEl

evat

ion

Distance

Distance

C D

Stream Stream

Ellicottville Quad

To Cattaraugus Ck…

To Allegheny River…

From the pattern of the streams and topography, we can map surface drainage in great detail

11

1 11

21

2

33

6

Drainage Area Above Point P

P

Stream Networks

- Drainage areas evolve stream networks

-The networks are commonly characterized by their branching patterns

- Stream Order indices are usedto describe the branching pattern in a network

To Cattaraugus Ck…

To Allegheny River…

Example of Stream Ordering

11

2 1

3

1

1 1

2 1

3

?

Ellicottville Quad

Elevation

Distance Downstream

?

Stream Long-Profile

Just a topographic profile that traces the axis of a stream or river

Rainfall rate (L/T)

Area of Bucket Opening (L2)

Discharge (L3/T)

How to think about question #4

Water level (constant in time)

GLY 106Laboratory 4

Contouring Spot Heights (known points of elevation)

Idealized Case

Actual Case

814ft777ft

793ft

What if we wanted contours at 785 and 800?

Example: Linear Interpolation(793-777)/(Map Distance)=Elevation change per unit map distance, or slope

So for the 785 contour:

(8/(793-777)) x Map Distance = Map Distance from the 777 measurement

785

800

793ft

777ft

Map distance

Elev.

814ft777ft

793ft

- 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?

785

800

A? B?

C?

What if we wanted contours at 785 and 800?

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)

GLY 106Laboratory 5

Introduction to Geologic Maps

Geologic Maps- Geologic maps are those that show the spatial distribution of materials exposed at the Earth’ssurface, i.e. rocks or broken up bits of rock (sediment)

- Geologic maps are most commonly overlain on a topographic base, i.e. information aboutsurface 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 (fromwells), 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 deposits2) Bedrock Geologic Maps exclude surficial deposits to show the distribution of bedrock and arethe most common type of geologic map

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 materialthat intrudes and cross-cuts strata are younger than that strata; if strataare broken (faulted) or highly deformed (folded), than the faulting ordeformation 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

Unit AUnit BUnit CUnit DOld

Young

Unit AUnit BUnit CUnit DOld

Young

Old

Young

Simple Superposition

Superposition & Cross-Cutting (Intrusion)

Deformation (Tilting)

Unit AUnit BUnit CUnit DOld

Young

Superposition & Cross-Cutting (Erosion)

Unit E

Angular Unconformity, Superposition

Topography

Unit AUnit B

Unit C

₊ ₊ ₊₊ ₊₊ ₊

₊₊

₊₊ ₊₊

₊₊₊ ₊ ₊₊

₊₊

₊ ₊₊ ₊₊ ₊ ₊₊

₊ ₊₊ ₊₊ ₊₊

Noncomformity, Superposition

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) Topography2) Shape and orientation of the geologic units

Unit AUnit BUnit CUnit D

Unit AContact A:BContact B:CContact C:D

3D Block Diagram 2D Geologic Map

Old

Young

“Outcrop”

3D Block Diagram 2D Geologic Map

Unit A

Unit B Unit C

Unit D U

nit

A

Topography

Un

it A

Un

it B

Un

it C

Un

it D U

nit

C

Un

it B

Old

Young

Young

Young

Contact C:D

Contact B:C Contact A:B

Unit A Unit B Unit C

Old

Unit A

Unit B Unit C

Unit D

Unit A

Contact A:B

Contact B:C

Contact C:D

Old

Un

it E

Contact A:E

The Stratigraphic Column

Unit AUnit BUnit CUnit D

Uni

t A

Topography

Unit A

Unit B

Unit C

Unit DU

nit C

Uni

t B

Old

Young

Unit A

Unit B

Unit CUnit D

Distance

Elev

atio

n

- The 2D map pattern of geological surfaces (contacts, or top and base of a rock unit) isstrongly dependent on the 3D orientation of that surface

- The orientation is generally described by two measures: Strike and Dip

Strike and Dip

dip

N

S

Dip Angle: Maximum inclination ofthe surface from the horizontalDip Direction: Direction towards which the surface is inclinedStrike: A line on the surface 90°from the dip direction

Example shown here:

Strike is 0° (or 180°)Dip is 40°W

Strike and Dip

N

What are the strike and dip shown here?

Contact C:D

Contact B:C Contact A:B

Old

Unit A

Unit B Unit C

Unit D

Contact A:B

Contact B:C

Contact C:D

Old

Unit A

Unit B Unit C

Unit D

Topography

Old

Young

Young

Young

N

N

N

What’s Unit E’s dip direction and strike?

What Units A-D’s dip direction and strike?

What Units A-D’s dip direction and strike?

Today: Some practice interpreting geologic maps

The Grand Canyon National Park(Spencer Map, Ap2-13)

Note the following:

PCbr = Brahma Schist PCr = Rama Schist PCgd = Zoraster GranitePCg = Zoraster GranitePcum = Ultramafic intrusives

Vishnu

IgneousIntrusives(ZorasterComplex)

GLY 106Laboratory 6

Planar Contacts and Topography

N

- Last week, we introduced the “Strike” and “Dip” definition of a planein 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)

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

Old

Young

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 structurecontours

The intersection of structure contours and topographic contours on a map define where a unit ‘outcrops’, or is exposed at the Earth’s surface

Map View

Structure Contours

Topographic Contours

StructureContours on Top of Unit X

Top of Unit X Outcrop

Unit X

Map View

Structure Contours

Topographic Contours

StructureContours on Top of Unit X

Top of Unit X Outcrop

Unit X

Unit XUnit X

Cross-Section

Depth

The intersection of structure contours and topographic contours on a map define where a unit ‘outcrops’, or is exposed at the Earth’s surface

Map View

Structure Contours

Topographic Contours

StructureContours on Top of Unit X

Top of Unit X Outcrop

Unit X

Formation Depth

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

dip

GLY 106Laboratory 7

Structure Contours and Outcrop

Old

Young

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 structurecontours

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

Structure Contours

Even though the river has eroded them away, we can infer the extension of units andcontacts across the canyon

The Grand Canyon

The Grand Canyon North

Structure contours

Even though the river has eroded them away, we can infer the extension of units andcontacts across the canyon

True vs. Apparent Thickness

Old

Young

Unit AUnit BUnit CUnit DOld

Young

Truethickness

Truethickness vsApparent thicknessdip°

True thickness = Apparent thickness x cos (dip°)

depth

depth

Old

Young Truethickness vsApparent thicknessdip°

True thickness = Apparent thickness x cos (dip°)

depth

True vs. Apparent Thickness

top top top top topbase basebase base base If a unit dips, its thicknessmeasured 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

True vs. Apparent Dip

True dipvs

Apparent dip

True dip refers to the maximum dip, or slope, of a surface, as measured perpendicularto 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

True vs. Apparent Dip

A single 2D exposure will most likely yield only an apparent dip

dip°

NOTE

When reporting strike and dip, the compact and conventional format is:

Strike/Dip Magnitude & Quadrant Direction

So if a unit has a strike of 45° and a dip of 30° towards the NW, we’d write:45°/30°NW

N

WE

Today’s Lab:

More practice relating structure contours, topography, and outcrop

Practice obtaining true vs. apparent dips and thicknesses

GLY 106Laboratory 8

Structure Contours, Outcrop, and Cross-sections I

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

The intersection of structure contours and topographic contours on a map define where a unit ‘outcrops’, or is exposed at the Earth’s surface

Map View

Structure Contours

Topographic Contours

StructureContours on Top of Unit X

Top of Unit X Outcrop

Unit X

Map View

Structure Contours

Topographic Contours

StructureContours on Top of Unit X

Top of Unit X Outcrop

Unit X

Unit XUnit X

Cross-Section

Depth

A

B

A

B

600450

300150

Dept

h

Sample Cross-Section

Vertical Datum

GLY 106Laboratory 9

Cross-Sections II

A

B

600450

300150

Dept

h

Sample Cross-Section

Vertical Datum

Today’s Map: Salem (KY) quad, Spencer Ap-26

- Line of cross-section shown

- Your section will display the unitsabove the base of the Bethel S.S. (Mcb), the shape of which is shownwith the red structure contours

-To construct your cross-section, you should plot points where yourline of section crosses the structurecontours (=28pts)

- I give you the topographic elevations at these points

3D Block Diagram 2D Geologic Map

Unit A

Unit B Unit C

Unit D U

nit

A

Topography

Un

it A

Un

it B

Un

it C

Un

it D U

nit

C

Un

it B

Old

Young

Young

Young

Contact C:D

Contact B:C Contact A:B

Unit A Unit B Unit C

Old

Unit A

Unit B Unit C

Unit D

Unit A

Contact A:B

Contact B:C

Contact C:D

Old

Un

it E

Contact A:E

Unit A

Unit B

Unit CUnit D

A Quick Primer on Folds and Outcrop Patterns

Unit C

Unit B

Unit A

GLY 106Laboratory 10

Mapping in GoogleEarth

Map View

Perspective View

Perspective View

Perspective View

Perspective View

Perspective View

Perspective View

GLY 106Laboratory 12

Field Lab II and Exam Review

Today

1. Finish plotting points collected during last week’s lab (see lab tutorial from last week).

2. Exam overview

3. Open review

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

Part 1: Topographic Maps

Outline of Topics

Outline of Topics

The basics: What does a geologic map depict, why do geologic mapslook 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 onthree points of known elevation

Part 2: Geologic Maps

Outline of Topics

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

Part 3: Geologic Map Interpretation

Outline of Topics

The Brunton compass: Magnetic declination, reading bearings, triangulation, measuring dips using the clinometer, measuring strike

Field Lab

Text Readings

Spencer: Chapters 1,2,3,6, 7, 8, 9