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Physical Geology Review

the expanse of material is vast!

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Internal Structure of the Earthgranitic/basaltic

crust = 0-40 km; mantle = 40-2900 km; outer core (liquid) = 2900-5200 km; inner core = 5200-6400 km

from: www.usgs.gov

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Guidelines for Structural Analysis

• reference frame– law of original horizontality (rocks deposited

on nominally horizontal surfaces)– law of superposition (deposition of new layers

are on top of old layers)– stratigraphic continuity (deposits tend to be

continuous laterally, or discontinuous strata that lie in the same plane are equivalent in time)

• nature of contacts– stratigraphic (bedding or unconformities)– intrusive (igneous or sedimentary)– tectonic (fractures/faults)

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Basic Principles

• principle of superposition– younger rocks are deposited on top of older

rocks

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Basic Principles

• law of original horizontality– all sediments originally laid down in horizontal

layers

Grand Canyon, Arizona

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Basic Principles

• law of original horizontality– non-horizontal bedding implies some sort of

deformation

The Alps of Sisteron, France

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Interpreting Field Data

• strike and dip measurements• topography following geologic structure• geologic cross-sections

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“Undeformed”, horizontal rock

Cedar Point, UT

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Horizontal Bedding in Sedimentary Rocks

• original horizontality• lateral continuity• superposition

younger

older

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Strike and Dip

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Looking in strike direction,dip is angle from horizontal

arrow pointing down dip

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Interpreting Field Data

• strike and dip = attitude of planes (beds, fractures, ...)– strike is intersection of plane with horizontal– dip is angle between plane and horizontal

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Vertical BedsMiocene Monterey Fm., California coast

Laminated bedding with selective dolomitization (yellow beds)

1 meter

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Orientation of Planes (strike)

• 3d block diagram Horizontal is reference frame

Strike is intersection

between plane and horizontal

North is reference direction

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Orientation of Planes (dip)• vertical cross-section view

dip angle

Dip is inclination of plane from horizontal, measured in vertical plane perpendicular to strike

strike

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Folds and Faults

- folds are evidence of ductile rock deformation

- faults represent brittle rock deformation

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Monocline on the San Juan River, Utah

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Lateral Continuity (or lack thereof!)

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Faults

• vertical cross-section view

Hanging Wall

Footwall

dip angle

Fault

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Faults

• 3d block diagram

Footwall

Fault

Hanging Wall

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Footwall

Dip-Slip (Normal)

Hanging Wall

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Footwall

Dip-Slip (Normal)

Hanging Wall

horizontal stretching = extensionvertical thinning

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Footwall

Dip-Slip (Reverse)

Hanging Wall

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Footwall

Dip-Slip (Reverse)

Hanging Wall

horizontal shortening = contraction,vertical thickening

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Footwall

Strike-Slip

Hanging Wall

strike-slip faults are typically vertical, but they can be dipping like this one

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Footwall

Strike-Slip (right-lateral)

Hanging Wall

as you step across the fault, the block you are stepping onto moves to the right

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Footwall

Strike-Slip (left-lateral)

Hanging Wall

as you step across the fault, the block you are stepping onto moves to the left

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Footwall

Oblique-Slip

Hanging Wall

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Footwall

Oblique-Slip

Hanging Wall

both strike-slip and dip-slip

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Normal Fault with Basin Shaped Fold in Hanging Wall

Hanging Wall

FootWall

FaultPlane

Bedding

Slip vector

shear sense = normal

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Fault drag & shale smear

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Strike-slip fault and Drag folding

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Slickensides = shear

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Reverse Fault in Core

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A

B

Geologic Cross-section

A B

Cross-section view

Map view

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Fracture Types

• shear failure – faults– relative displacement is parallel to fracture face

= shearing mode

• tensile failure – joints, veins, dikes– a fracture with relative displacement

perpendicular to fracture face = opening mode

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Joint are “just” cracks….(looking at dip slopes)

orientation changes from bed to bed

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Joints(looking at bed scarps)

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A Joint filled with minerals = vein

Bristol Channel, UK Ouachita Mtns., Arkansas

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A “Joint” filled with magma = dike

Mt. Moran, Grand Tetons, WY

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Famous Fractures in the Movies

The Middle East?

from Transformers

hardly!

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How about New Mexico?

curving path to line up with earth stresses

volcanic source

volcanicsource

propagating dike

Volcanic Neck and Igneous

Dike:Shiprock dike

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Igneous Rock Features

from Press and Siever, Understanding Earth

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Other Geomorphic Features

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Glacial Moraine & U-shaped Valley

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Glacial Moraine

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Immature River – Steep, V-shaped Canyons

Yellowstone River

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Mature, meandering river (Yellowstone River)

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High energy rivers – heavy sediment load

braided stream

channel cut & fill,

graded bedding

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Alluvial fan

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Unconformities

• represent non-deposition or erosion• often indicate tectonic events or global sea level

change, laterally very continuous• types

– disconformity (parallel sed layers above and below, missing some time)

– nonconformity (sed rocks on top of igneous/metamorphic basement)

– angular unconformity (rocks above and below have different orientation)

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Angular Unconformity

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Disconformity

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Nonconformity

Cambrian Flathead ss on top of Precambrian granite

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