Diastrophism 2

8/14/2019 Diastrophism 2

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Harry Williams, Geomorphology(21) 1

Landforms Developed on Folded/Tilted Strata

Underlying geologic structure influences surface landforms in two ways:

1. determines outcrop pattern.

2. The relative elevation and/or slope of outcrops will vary, because

different rocks erode at different rates (depends on RELATIVE hardnessof rock, cementation strength (if sedimentary rock) and joint abundance).

This is called DIFFERENTIAL EROSION.

Strike valley

Resistant rock layers

cuesta


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Resistant rock is left standing higher than less resistant

rock, forming various types of cliffs ("scarps), ridges

or uplands. The actual feature formed depends on the

dip of the strata. Mesas are wide flat-topped hillscapped by a resistant horizontal rock layer (a

caprock); buttes are similar, narrower hills; a cuesta

is the combination of a dipslope and escarpment (or

scarp); a hogback is a fairly sharp ridge; a razorback

is a very sharp ridge (dip of the rock layer determines

which will form). Softer rock layers are eroded lower

forming STRIKE VALLEYS - valleys parallel to strike

direction.


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MARBLE DEFORMATION

fracture flowOutcrop Patterns and

Landforms On Geologic Maps.

Subsurface rock layers are

usually exposed by

downcutting streams and

rivers.

Horizontal Strata1. Outcrop pattern parallels

valleys

2. Contacts between rock units

parallel contours because

strata are flat.


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3. Uplifted horizontal strata form plateaus like the Colorado Plateau.

4. Vigorous downcutting during/after uplift forms canyons like the Grand

Canyon.

5. Resistant strata form steep cliffs; softer rock erodes to gentler slopes.


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mountainshills

cliffs ridges

6. Prolonged erosion isolates blocks of rock, forming mesas, buttes and pinnacles.

Lava flowcaprock on

a mesa in

west Texas


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Tilted Strata

1. Outcrops are roughly

parallel bands.

2. Law of V's - low dipangle -> larger V.

3. Older beds dip

towards younger beds

(except in overturned

folds, which areuncommon).

4. Differential erosion

forms ridges, dipslopes,

escarpments, cuestas,

strike valleys.


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4. Cuestas, hogbacks or razorbacks are formed by resistant beds; strike

valleys formed by softer beds.


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Domes

1. Outcrops are

concentric belts.

2. Oldest rocks are in thecenter.

3. Law of V's -> beds dip

away from center.

4. Resistant beds form

inward-facing scarps.


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5. Core of dome determines relief; if resistant -> central upland; if soft ->

central lowland.


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Basins

1. Outcrops are

concentric belts.

2. Youngest rocks are inthe center.

3. Law of V's -> beds dip

towards center.

4. Resistant beds form

outward-facing scarps.


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Folds

1. Erosion of plunging

folds (most plunge) ->

zigzag pattern.

2. Anticlines V down

plunge; oldest beds at

center. Law of V's -> beds

dip away from center;

resistant rocks -> inward-

facing scarps.3. Synclines V up plunge;

youngest beds at center;

Law of V's

-> beds dip towards

center; resistant rocks ->outward-facing scarps.

ANTICLINE

SYNCLINE

ANTICLINE


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4. Since most folds form at depth, fold outcrop patterns are typical in

eroded ancient mountain belts such as the Appalachians.

compression


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Landforms Developed On Faulted Strata

1. Form sharp linear boundaries of uplands or valleys


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2. Displacement -> fault scarps (300 m high in this example from Utah).


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3. Recognized on geologic maps by abrupt displacement of structures or rocks.


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4. Characteristic of young mountain belts -> fault block mountains

UPLIFT


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5. Basin and Range Province

Caused by crustal stretching due to heat flow from subduction. Tilting of

normal faults resulted in fault-angle depressions (tilted fault blocks).


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Basin and range in

Texas

Death Valley


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Surficial Deposits

1. Unconsolidated

recent sediments e.g.

river sands, glacialdeposits, landslide

deposits.

2. Usually form a thin

cover on underlying

bedrock.

3. Usually associated

with process e.g. river

deposits in present

river valleys; landslide

debris at base of cliff

etc.

bedrock


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Influence of Geologic Structure On Drainage Patterns

1. Streams that develop on "original surface" (e.g. emergent coastal plain)

follow the regional slope = CONSEQUENT STREAMS

2. Continued erosion -> strike valleys occupied by SUBSEQUENT

STREAMS3. Tributaries to subsequent streams that follow regional slope =

RESEQUENT STREAMS (REnewed conSEQUENT); tributary streams

that flow in the opposite direction to consequent streams = OBSEQUENT

STREAMS (Oppsite to conSEQUENT).

Consequent stream, following regional slope


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Subsequent stream

Resequent stream Obsequent stream

Consequent stream

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Diastrophism 2