Structural Control Structural Control of Landformsof Landforms

SAND, HOSES, Slickensided Rock, Pencil, Rubber bandGum, Foam sediments, Cardboard fault models, 2 Plastic boxes, Food Coloring ,Paper, wood, Ice

Photo from Drury: Two distinct units.  One dendritic drainage pattern is sparsely vegetated.  Parallel contours suggest it is horizontal.  Other formation banded, with straight wooded ridges, controlled by steep dips. The boundary truncates the ridges. Horizontal unit lies unconformably on the steeply dipping strata (angular unconformity). 

The wide spacing of drainage in the younger unit suggests that it is a massive, coarse clastic rock.  The older unit comprises shales and limestones. From Steve Drury, Image Interpretation in Geology,

adopted for this course

Mostly Chapter 12

Plus a review of folds and faults

Some photos in this PowerPoint made available online, courtesy of Steve Dutch, click here

From our lab workbook Image Interpretation in Geology by Steve Drury

ErodabilityErodability

• Relative Erodability– Layered rocks = wide range

• Sedimentary• Volcanic

– Massive rocks = narrow range• Metamorphic• Intrusive igneous

– Erodability is not absolute but • typically shale > limestone > sandstone ~ gneiss Canadian Shield.

Pale granite and darker metavolcanic rocks, the granite having resisted glaciation best. Drury IIG

ErodabilityErodability

• "… shale, limestone, marble and some types of [mica] schist are less resistant "valley-makers" in humid climates" …

• "whereas [quartz] sandstone, quartzite, [quartz] conglomerate and various igneous rocks [ granite has ~20% quartz H= 7] are resistant "ridge-makers" ….

• Easterbrook (1969) Principles of Geomorphology

[words in brackets added]

Lithology/ClimateLithology/ClimateErodability: shale > limestone > sandstone ~ gneiss

In humid areas, weathering and erosion are faster, slopes are more eroded, gentler after the same duration of exposure to weathering

In arid terrains (a) the intermittent violent erosion develops steep-sided gullies and valleys. Note differential erosion

Horizontally layered rocks – outcrops parallel topographic contours. 

In humid climate the

topography is more muted.

Monadnocks resistant rock ridges Colorado

Undisturbed Sediments Undisturbed Sediments showing differential erodabilityshowing differential erodability

Dry Climate, intermittent strong storms

Review: Stream Vees

Vees are pointing in direction of dip

TablelandsTablelands

In horizontal beds, rock outcropswould follow contours

Tablelands: note horizontal Tablelands: note horizontal layers, differential erosionlayers, differential erosion

• Plateau>mesa>butte>chimney

• Ratio surface area of top to height

Dry Climate, intermittent strong storms

In horizontal beds, rock outcropswould follow contours

Pediment (gentle slope < 5%,erosional concave up surface w thin veneer of gravel etc.)

Inselbergmesa

Butte chimney

Desert Landforms Desert Landforms near Mountainsnear Mountains

Alluvial Fan

(often exposed bare rock with gravel veneer)

Mountains eventually erode away to Inselbergs

Rain-shadow desert in the lee of mountains

Compression, Tension, Compression, Tension, and Shearing Stressand Shearing Stress

Convergent Divergent Transform

Convergent Plate Boundaries Convergent Plate Boundaries and Foldingand Folding

Subduction causes Arc: Under Ocean Lithosphere Japan,Aleutians, Cent. Am.; under continent Andes, Cascades

Continent-Continentcollision formsFold and Thrust Mountains: Alps, Himalayans, Appalachians

Strike and DipStrike and Dip

Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface

Map Symbols: Strike shown as long line, dip as short line. Note the angle of dip shown: 45o

Tilted StrataTilted Strata• Monoclinal folds, or one

side (limb) of a fold • Name = f(dip angle)

– Cuesta (moderate dip)– Hogback (steep dip)– Flatiron remnant of

dissected Hogback w triangular face

Dip Slope vs. Scarp slope

Hogback

Cuesta

Hogback dip slope greater 30° - 40° with near symmetric slope on each face

RidgesRidges• Dip of Cuesta < Hogback

Copyright © J. Michael Daniels 2002http://www.alperry.com/coal/grand_hogback.html http://www.aureo.org/conference/boulderconference.html

Folds are typical of convergenceFolds are typical of convergenceFolded Rock Before ErosionFolded Rock Before Erosion

Folded Rocks, Hwy 23 Folded Rocks, Hwy 23 Newfoundland, New JerseyNewfoundland, New Jersey

Source: Breck P. Kent

Adjacent Anticline and Syncline

Note highest point

Folded Rock After ErosionFolded Rock After Erosion

Eroded Anticline, older rocks in center. Syncline is opposite.

Topography may be opposite of Structure Topography may be opposite of Structure

AnticlineAnticline Before/After Erosion Before/After Erosion

Notice center rock oldest

Topography may be opposite of Structure Topography may be opposite of Structure

Syncline Before/After ErosionSyncline Before/After Erosion

Notice center rock youngest

Various FoldsVarious Folds

Various Folds (cont'd)Various Folds (cont'd)

Various Folds (cont'd)Various Folds (cont'd)

Various Folds (cont'd)Various Folds (cont'd)

Axial plane near axis should be close to horizontal

Axis

Plunging Folds and Nose RulesPlunging Folds and Nose Rules

Nose of anticline points direction of plunge, syncline nose in opposite direction

UpEnd Down

End

Demo: Plastic box, water, paper folds

Plunging Folds Plunging Folds

Nose

Nose

Nose

Joints: Fractures – with no movementJoints: Fractures – with no movement

Source: Martin G. Miller/Visuals Unlimited

vs. Faults with relative movement

Sandstone, note no streams here, too many cracks

Dip-Slip Dip-Slip FaultsFaults

Demo: Cardboard Models

Continental Rift into Ocean Basin - Tension => Divergence

Rift Valleys and Oceans are the same thing

Normal Faults

Normal Faults at Divergent Normal Faults at Divergent Margins - IcelandMargins - Iceland

A new graben, down dropped hanging wall block - Normal Fault – divergent zone MOR

Overhanging Block

Footw

all

Fault Line scarp(High-angle

Normal Fault)

Convergent MarginsConvergent MarginsShallow Reverse Fault = Thrust FaultShallow Reverse Fault = Thrust Fault

Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)

Same layer

Lewis Thrust Fault (cont'd)Lewis Thrust Fault (cont'd)

Source: Breck P. Kent

PreCambrian Limestone over Cretaceous Shales

Geologists are frequently called upon to find the ore bodyGeologists are frequently called upon to find the ore body

Younger

Miners pay geologists to find their lost orebodyOne friend earned enough to buy a house

This poor guy is out of luck

What phase of magma fractionation would result in the placement of this ore body?

Which formed first, the ore body or the fault?What common mineral is mostly likely in the ore body?

This guy is rich

Normal

Reverse

Horizontal Movement Along Horizontal Movement Along Strike-Slip FaultStrike-Slip Fault

Landscape Shifting, Wallace CreekLandscape Shifting, Wallace Creek

San Andreas Fault

Normal Fault Quake - NevadaReverse Fault Quake - Japan

Strike Slip Fault Quake - California

HW Down

HW UpConvergent

Divergent

Transform

Fracture Zones and SlickensidesFracture Zones and Slickensideshttp://pangea.stanford.edu/~laurent/english/research/Slickensides.gif

Part 2 Structural Control Part 2 Structural Control of Streams mostly Ch. 12of Streams mostly Ch. 12

• Consequent streams follow slope of the land over which they originally formed.

• Subsequent streams are streams whose course has been determined by erosion along weak strata.

• Resequent streams are streams whose course follows the original relief, but at a lower level than the original slope

• Obsequent streams are streams flowing in the opposite direction of the consequent drainage.

consequent (c follow slope) subsequent (s along weak)

obsequent (o opposite main slope) resequent streams (original slope but lower level)

Insequent (random dendritic)

Insequent Streams= Initial ConsequentInsequent Streams= Initial Consequent• Almost random drainage often forming dendritic

patterns. • Typically tributaries - developed by headward

erosion on a horizontally stratified rocks, or a substrate with ~ constant composition.

• NOT controlled by the original slope of the surface, its structure or the type of rock.

Headward Erosion

Drainage Patterns with and without structural control

None Joints fold limbs

Volcano, exposed pluton, diapir

Dendritic PatternsDendritic Patterns• Underlying bedrock has no structural control

over where the water flows. • Characteristic acute angles

• No repeating pattern.

Trellis PatternsTrellis Patterns

• Form where underlying bedrock has repeating weaker and stronger types of rock.

• Streams cut down deeper into the weaker bedrock

• Nearly parallel streams

• Branch at higher angles.

Rectangular patternsRectangular patterns

• Branching of tributaries at nearly right angles

• Form in jointed igneous rocks or horizontal sedimentary beds with well-developed jointing or intersecting faults.

Parallel ErosionParallel Erosion

• Form on unidirectional regional slope or parallel landform features. Small areas.

Radial ErosionRadial Erosion• Flow of water outward from a high point

• Down a volcano cone

• or an intrusive dome, or

• down an alluvial fan.

Annular patternsAnnular patterns

• form on domes of alternating weak and hard bedrocks.

• The pattern formed is similar to that of a bull's-eye when viewed from above

• weaker bedrocks are eroded and the harder are left in place.

Centripetal patternsCentripetal patterns

• Form where water flows into a central location, such as a round bowl-shaped watershed, or a karst limestone terrain where disappearing streams flow down into a sinkhole and then underground.

Structural Control of DrainageStructural Control of Drainage

• Contorted

Folded Rocks

Stream CaptureStream Capture

Headward Erosion

Stream Capture vs. Structural ControlStream Capture vs. Structural Control

Subsequent Susquehanna does not reach Beaverdam Creek flowing through water gap

Susquehanna captures headwaters of Beaverdam Creek, diverting upper Beaverdam trunk to Susquehanna channel.

Dry Valley

Godfrey RidgeBro

dhead Cre

ek

Elbow of Capture

Stream Capture

Headward erosion from Water Headward erosion from Water Gap area cut through Godfrey Gap area cut through Godfrey Ridge and captured Brodhead Ridge and captured Brodhead Creek which was flowing east Creek which was flowing east behind Godfrey Ridgebehind Godfrey Ridge

1. Old river meanders across floodplain2. Base level drops (how?), or region uplifts. Area now much higher above sea level than before. Potential energy increases, water flows faster, better erosion, stream straightens and cut down to base level, less floodplain width and cut lower.3.Terrace forms from previous floodplain. Further incision cuts another terrace

Terraces 1

Next time Terraces 2 and 3: Isostatic Rebound and high water shorelines as glaciers melt Potential gh to Kinetic Energy 1/2mV2

A flight of river terraces

• Antecedent Streams and Superimposed Streams• Meanders in steep, narrow valleys

– Caused by a drop in base level or uplift of region

Delaware Water Gap

• River is older than upliftRiver is older than uplift

Incised (entrenched) meanders

"In this panorama in southwestern Colorado, a stream flows from the right across an uplift (anticline) in the rocks. As soon as the stream enters the uplift, its canyon becomes deep. Note the entrenched [incised] meanders, a couple of which were cut through and abandoned when the canyon was about half its present depth. As soon as the river exits the uplift, the canyon once again becomes shallow. Clearly, the river was there first and the rocks arched upward across its course." Steve Dutch

Some photos in this PowerPoint made available online, courtesy of Steve Dutch, click here

PedimentsPediments and Alluvial Fans and Alluvial Fans

Alluvial fans typically develop at the exits of intermittent streams draining arid mountainous regions. 

And on Mars …And on Mars …

Link courtesy Melissa Hansen

An example of a v-shaped stream, with fairly constant slope and cross section

Conservation of Energy with frictional lossesConservation of Energy with frictional losses

• A stream channel has been uplifted to 300 meters above base level. It’s cross sectional area, slope, and water depth is close to constant. The stream is full of large boulders. At 300 meters it flows out of an alpine lake, where it has an average velocity of 0.01 meters/sec, that is, it has mostly potential energy. At base level it has a velocity of 15 meters per second (so all kinetic energy, plus frictional losses on the way down. Estimate the percent energy lost to friction.

An example for the homework calc.