Earth in Time The Rock Record and Geologic Time GEOLOGY TODAY Barbara W. Murck Brian J. Skinner Chapter 3 Lindsley-Griffin, 1999. Maroon Bells, Colorado
Mar 30, 2015
Earth in TimeThe Rock Record and
Geologic Time
Earth in TimeThe Rock Record and
Geologic Time
GEOLOGY TODAY
Barbara W. MurckBrian J. Skinner
Chapter 3
N. Lindsley-Griffin, 1999.
Maroon Bells, Colorado
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Original horizontality:
Sediments are deposited on a horizontal surface
Sedimentary rocks form in horizontal layers
Sedimentary rocks not horizontal were disturbed after they formed Folded sedimentary strata, Crete
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Stratigraphic superposition:
Each layer of sedimentary rocks is deposited over the previous layer
Lower layers are always older than upper layers
Sandstone, limestone, and shale strata, Grand Canyon N.P., AZ
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Cross-cutting relationships:
A rock unit is always older than any feature which cuts across or disrupts it.
Fractures cutting sandstone layers, Merseyside, UK
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Cross-cutting relationships:
A rock unit is always older than any feature which cuts across or disrupts it.
Fractures cutting sandstone layers, Merseyside, UK
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Correlation:Lateral continuity --
Strata can be traced from one location to another
Physical similarity --
Same characteristics
= same strata
Similar sequence of strata in two sections
= same sequenceCoal and sandstone strata, Badlands N.P., SD
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Faunal succession:Each formation contains a unique fossil assemblage
Assemblages succeed one another in orderly, predictable sequence
Same everywhere in the world
In general, simpler organisms precede more complex ones in the same group
Dinosaur tracks
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Faunal succession used for correlation:
Look for same fossils in same sequence of similar strata.
Fig. 3.6, p. 63
Absolute Age - PrinciplesAbsolute Age - Principles
© Houghton Mifflin 1998; N. Lindsley-Griffin, 1999. All rights reserved
Radioactive Decay - Release of particles from nucleus
ALPHA EMISSION:Two protons + two neutronsMass - 4 Number - 2
BETA EMISSION:Neutron decays to a protonand gives off an electronMass - no change Number + 1
BETA CAPTURE:Proton captures electron andbecomes a neutronMass - no change Number - 1
Absolute Age - PrinciplesAbsolute Age - Principles
N. Lindsley-Griffin, 1999
Half-life: Time needed for the number of parent atoms to be reduced by one-half
At time zero, 100% P.A.
After one half-life, 50% P.A. and 50% D.A.
After two half-lives, 25% P.A. and 75% D.A.
Figure 3.12, p. 70
© Houghton Mifflin 1998. All rights reserved
Carbon-14 Dating
Carbon-14 Dating
Neutrons in atmosphere change nitrogen-14 to carbon-14C-14 incorporated into tissue ofliving organismsRatio of C-14 to other C-isotopesremains constant in living tissueAt death, C-14 not replenished,ratio of C-14 to other C-isotopes decreasesAmount of C-14 remainingdetermines time since deathof organism
N. Lindsley-Griffin, 1999
Proterozoic
Quaternary
Cretaceous
The Geologic Column[Fig. 3.8, p. 64]
© Houghton Mifflin 1998. All rights reserved
Dating the Geologic Time Scale Dating the Geologic Time Scale
Sedimentary rocksnot easily dated radiometrically
Radiometric ages added to geologic timescale by:1. Cross-cutting relationships2. Bracketing
Relative Age - PrinciplesRelative Age - Principles
N. Lindsley-Griffin, 1999
Unconformities:
Gaps in the rock record - recognized by an erosional surface
Signifies a major break in deposition
Columbia River Gorge, OR-WA
Relative Age - PrinciplesRelative Age - Principles
© Houghton Mifflin 1998. All rights reserved
Horizontal sedimentary rocks
Dike cuts strata - dike is younger
Fault cuts dike and strata - fault is youngest
Original horizontalitySuperpositionCross-cutting relationships Youngest
sedimentary rock
Oldestsedimentary rock
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Dating the Geologic Time Scale Dating the Geologic Time Scale
Radiometric age of intrusion Y is 350 m.y.
Radiometric age of intrusion X is 400 m.y.
Devonian strata (B) must be older than 350 m.y. and younger than 400 m.y.
Mississippian strata are younger than 350 m.y.
Silurian strata are older than both Devonian and 400 m.y.
Radiometric dating and cross-cutting relationshipsbracket ages of sedimentary rocks
Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ
N. Lindsley-Griffin, 1998
Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ
N. Lindsley-Griffin, 1998
Disconformity Angular Unconformity
Time gap without angular relationship Time gap after folding, faulting
N. Lindsley-Griffin, 1998
Disconformity Angular Unconformity
Time gap without angular relationship Time gap after folding, faulting
N. Lindsley-Griffin, 1998
“Baked zone” - soil formed on older lava flow before being covered and cooked by a
new lava flow - DisconformityN. Lindsley-Griffin, 1998
“Baked zone” - soil formed on older lava flow before being covered and cooked by a
new lava flow - DisconformityN. Lindsley-Griffin, 1998
Nonconformity - sedimentary rocks over crystalline (metamorphic or igneous) rocks
N. Lindsley-Griffin, 1998
Nonconformity - sedimentary rocks over crystalline (metamorphic or igneous) rocks
N. Lindsley-Griffin, 1998
Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop?
N. Lindsley-Griffin, 1998
Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop?
N. Lindsley-Griffin, 1998
Cross-cutting relationships: dike cutting gneiss is youngerN. Lindsley-Griffin, 1998
Cross-cutting relationships: dike cutting gneiss is youngerN. Lindsley-Griffin, 1998
Cross cutting relationships: what is the sequence of events in this folded marble with two dikes?
N. Lindsley-Griffin, 1998
Cross cutting relationships: what is the sequence of events in this folded marble with two dikes?
N. Lindsley-Griffin, 1998
Superposition
Disconformity
Original horizontality
Angular unconformity
Cross-cutting relationships
Nonconformity
Grand Canyon FormationsGrand Canyon Formations
© Houghton Mifflin 1998. All rights reserved
Grand Canyon FormationsGrand Canyon Formations
© Houghton Mifflin 1998. All rights reserved
Sequence of events based on geology of the Inner Gorge
1.Deposition of shale in marine environment
2. Metamorphism of shale into Vishnu schist
3. Intrusion by Zoroaster granite
4. Erosion to sea level, unconformity surface
5. Deposition of Grand Canyon Supergroup
6. Faulting 7. Erosion of fault blocks, deposition of Tapeats sandstone
Geologic Time - SummaryGeologic Time - Summary
N. Lindsley-Griffin, 1999
Relative Age -- whether a particular rock or feature is older or younger than another
Absolute Age -- the age of a rock in years (m.y. = millions of years)
Geologic Column -- shows succession of all known strata, in chronological order, based on fossils and other relative ages