Transcript law of original horizontality

Physical Geology Review
the expanse of
material is vast!
1
Internal Structure of the Earth
granitic/basaltic
crust = 0-40 km; mantle = 40-2900 km;
outer core (liquid) = 2900-5200 km; inner core = 5200-6400 km
from: www.usgs.gov
2
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)
3
Basic Principles
• principle of superposition
– younger rocks are deposited on top of older
rocks
4
Basic Principles
• law of original horizontality
– all sediments originally laid down in horizontal
layers
Grand Canyon, Arizona
5
Basic Principles
• law of original horizontality
– non-horizontal bedding implies some sort of
deformation
6
The Alps of Sisteron, France
Interpreting Field Data
• strike and dip measurements
• topography following geologic structure
• geologic cross-sections
7
“Undeformed”, horizontal rock
Cedar Point, UT
8
Horizontal Bedding
in Sedimentary Rocks
younger
older
•
•
•
original horizontality
lateral continuity
superposition
9
Strike and Dip
10
Looking in strike direction,
dip is angle from horizontal
arrow pointing down dip
11
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
12
Vertical Beds
Miocene Monterey Fm., California coast
Laminated bedding with selective dolomitization (yellow beds)
1 meter
13
Orientation of Planes (strike)
• 3d block diagram
Strike is
intersection
between plane
and horizontal
Horizontal is
reference frame
North is
reference
direction
14
Orientation of Planes (dip)
• vertical cross-section view
strike
dip angle
Dip is inclination of plane from
horizontal, measured in vertical
plane perpendicular to strike
15
Folds and Faults
- folds are evidence of
ductile rock deformation
- faults represent brittle
rock deformation
16
Monocline on the San Juan River, Utah
17
Lateral Continuity (or lack thereof!)
18
Faults
• vertical cross-section view
dip angle
Hanging Wall
Footwall
19
Faults
• 3d block diagram
Fault
Hanging Wall
Footwall
20
Dip-Slip (Normal)
Hanging Wall
Footwall
21
Dip-Slip (Normal)
Hanging Wall
Footwall
horizontal stretching = extension
vertical thinning
22
Dip-Slip (Reverse)
Hanging Wall
Footwall
23
Dip-Slip (Reverse)
Hanging Wall
Footwall
horizontal shortening = contraction,
vertical thickening
24
Strike-Slip
Hanging Wall
Footwall
strike-slip faults are typically vertical,
but they can be dipping like this one
25
Strike-Slip (right-lateral)
Hanging Wall
Footwall
as you step across the fault,
the block you are stepping onto moves to the right26
Strike-Slip (left-lateral)
Hanging Wall
Footwall
as you step across the fault,
the block you are stepping onto moves to the left 27
Oblique-Slip
Hanging Wall
Footwall
28
Oblique-Slip
Hanging Wall
Footwall
both strike-slip and dip-slip
29
Normal Fault with Basin Shaped
Fold in Hanging Wall
shear sense = normal
Foot
Wall
Hanging Wall
Slip vector
Fault
Plane
Bedding
30
Fault drag & shale smear
31
Strike-slip fault
and Drag folding
32
Slickensides = shear
33
Reverse Fault in Core
34
Geologic Cross-section
Map
view
B
A
A
Crosssection
view
B
35
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
36
Joint are “just” cracks….
(looking at dip slopes)
orientation changes
from bed to bed
37
Joints
(looking at bed scarps)
38
A Joint filled with minerals = vein
Bristol Channel, UK
Ouachita Mtns., Arkansas
39
A “Joint” filled with magma = dike
40
Mt. Moran, Grand Tetons, WY
Famous Fractures in the Movies
The Middle East?
hardly!
41
from Transformers
How about New
Mexico?
volcanic
source
propagating dike
curving path to line up with earth stresses
Volcanic Neck
and Igneous
Dike:
Shiprock dike
volcanic
42
source
Igneous Rock Features
43
from Press and Siever, Understanding Earth
Other Geomorphic Features
44
Glacial Moraine & U-shaped Valley
45
Glacial Moraine
46
Yellowstone River
Immature River – Steep, Vshaped Canyons
47
Mature, meandering river (Yellowstone River)
48
High energy rivers – heavy sediment load
channel cut & fill,
graded bedding
braided stream
49
Alluvial fan
50
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)
51
Angular
Unconformity
52
Disconformity
53
Nonconformity
Cambrian Flathead ss on top of
Precambrian granite
54
55