grav

ity

deposited first - oldest

deposited last - youngest

Principle of SuperpositionPrinciple of SuperpositionIn an undisturbed sedimentary sequence, the oldest rocks are In an undisturbed sedimentary sequence, the oldest rocks are

on the bottom of the stackon the bottom of the stack

Principle of Original HorizontalityPrinciple of Original HorizontalitySedimentary rocks were deposited in primarily horizontal beds

grav

ity

Principle of SuperpositionPrinciple of SuperpositionIn an undisturbed sedimentary sequence, the oldest

rocks are on the bottom of the stack

Oldest

Youngest

Principle of InclusionPrinciple of InclusionWhen clasts of one rock are found in another, the rock from which

the clasts were derived is the older rock, since it must have already existed in order to be included in the new rock

InclusionsInclusions - pieces of older rock incorporated into younger rock

igneous intrusion

InclusionsInclusions - pieces of older rock (clasts) incorporated into younger rock

Principle of InclusionPrinciple of InclusionWhen clasts of one rock are found in another, the rock from which

the clasts were derived is the older rock, since it must have already existed in order to be included in the new rock

Principle of Cross-Cutting RelationshipsOlder features are cut or crossed by younger features.

Bright Angel ShaleBright Angel Shale

Redwall LimestoneRedwall LimestoneMuav LimestoneMuav Limestone

Tapeats SandstoneTapeats Sandstone

Supai GroupSupai Group

Hermit ShaleHermit Shale

FormationFormation – bodies of rock with recognizable characteristic that are thick enough to map

ContactContact – surface separating two formations

UnconformitiesUnconformities represent missing time in the geological sequence, either due to no rock being formed or rock being removed.

There are three kinds of unconformities:

disconformitydisconformity – unconformity between parallel strata. Represents a time of non-deposition or erosion without deformation of strata.

angular unconformityangular unconformity – unconformity between non-parallel strata. Strata were deformed as well as eroded (not necessarily at the same time)

nonconformitynonconformity – unconformity representing erosion of a non-sedimentary rock

Reconstructing Geologic History

Complex Subsurface Geology

Sedimentary Deposition

Marine Sequence

Intrusion

Tilting & Erosion

Subsidence and New Marine Deposition

Missing Formation

Dike Event

Fluvial Deposition

Complex Subsurface Geology

http://www.cotf.edu/ete/modules/msese/earthsysflr/geotime.html

Radiometric DatingRadiometric DatingAbsolute dating using radioactive decay data

Alpha Particle

emits a particle from its nucleus called an alpha particle (2 protons + 2 neutrons).   

Uranium - Thorium Decay

U He + Th238

92

4

2

234

90

spontaneous

decay

“parent” “daughter product”

alpha particle =2 protons + 2 neutrons = positively charged ion of Helium

Thorium:90 protons + 144 neutrons

Beta Particle Emission

But, Th is also unstable, and it emits a beta particle …

234

90

Alpha Alpha EmissionEmission

Beta Beta EmissionEmission

Electron Electron CaptureCapture

http://www.asa3.org/ASA/resources/Wiens.html

Decay sequence for Uranium-238Decay sequence for Uranium-238

Half lifeHalf life – the time it takes for half of the original element to decay to the new element.

Half Life = 10,000 yrsDecays to DaughteriumDaughterium

Starting with 1,000,0000 nuclei of ParentiumParentium

Hypothetical Element ParentiumParentium

Time Parentium Daughterium

1,000,0001,000,000 000

500,000500,000 500,000500,000+10,000

250,000250,000 750,000750,000+20,000

1/2 1 - ½ = ½

½ * ½ = ¼ 1-¼= ¾

100%

50%

25%

Time Parentium Daughterium

125,000 825,000

62,500 937,500

+30,000

31,250 968,750

+40,000

+50,000½ * ½ * ½ * ½ * ½ = 1/32 1-1/32= 31/32

½ * ½ * ½ * ½ = 1/16 1-1/16= 15/16

½ * ½ * ½ = 1/8 1- 1/8 = 7/8

12.5%

6.25%

3.125%

0 10 20 30 40 50

1,000,000

250,000

750,000

0

Time (thousands of years)

Nuc

lei

ParentiumParentium

DaughteriumDaughterium

500,000Will Parentium reach 0%?