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Earth in Time The Rock Record and Geologic Time GEOLOGY TODAY Barbara W. Murck Brian J. Skinner Chapter 3 N. Lindsley-Griffin, 1999. Maroon Bells, Colorado.

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Presentation on theme: "Earth in Time The Rock Record and Geologic Time GEOLOGY TODAY Barbara W. Murck Brian J. Skinner Chapter 3 N. Lindsley-Griffin, 1999. Maroon Bells, Colorado."— Presentation transcript:

1 Earth in Time The Rock Record and Geologic Time GEOLOGY TODAY Barbara W. Murck Brian J. Skinner Chapter 3 N. Lindsley-Griffin, Maroon Bells, Colorado

2 Relative 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

3 Relative 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

4 Relative 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

5 Relative 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

6 Relative 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 sequence Coal and sandstone strata, Badlands N.P., SD

7 Relative 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

8 Relative 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

9 Absolute Age - Principles © Houghton Mifflin 1998; N. Lindsley-Griffin, All rights reserved Radioactive Decay - Release of particles from nucleus ALPHA EMISSION: Two protons + two neutrons Mass - 4 Number - 2 BETA EMISSION: Neutron decays to a proton and gives off an electron Mass - no change Number + 1 BETA CAPTURE: Proton captures electron and becomes a neutron Mass - no change Number - 1

10 Absolute 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

11 © Houghton Mifflin All rights reserved Carbon-14 Dating Neutrons in atmosphere change nitrogen-14 to carbon-14 C-14 incorporated into tissue of living organisms Ratio of C-14 to other C-isotopes remains constant in living tissue At death, C-14 not replenished, ratio of C-14 to other C-isotopes decreases Amount of C-14 remaining determines time since death of organism

12 N. Lindsley-Griffin, 1999 Proterozoic Quaternary Cretaceous The Geologic Column [Fig. 3.8, p. 64]

13 © Houghton Mifflin All rights reserved Dating the Geologic Time Scale Sedimentary rocks not easily dated radiometrically Radiometric ages added to geologic time scale by: 1. Cross-cutting relationships 2. Bracketing

14 Relative 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

15 Relative Age - Principles © Houghton Mifflin All rights reserved Horizontal sedimentary rocks Dike cuts strata - dike is younger Fault cuts dike and strata - fault is youngest Original horizontality Superposition Cross-cutting relationships Youngest sedimentary rock Oldest sedimentary rock

16 © Houghton Mifflin All rights reserved 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 relationships bracket ages of sedimentary rocks

17 Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ N. Lindsley-Griffin, 1998

18 Angular unconformity - younger layers over tilted layers - Grand Canyon, AZ N. Lindsley-Griffin, 1998

19 Disconformity Angular Unconformity Time gap without angular relationshipTime gap after folding, faulting N. Lindsley-Griffin, 1998

20 Disconformity Angular Unconformity Time gap without angular relationshipTime gap after folding, faulting N. Lindsley-Griffin, 1998

21 Baked zone - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity N. Lindsley-Griffin, 1998

22 Baked zone - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity N. Lindsley-Griffin, 1998

23 Nonconformity - sedimentary rocks over crystalline (metamorphic or igneous) rocks N. Lindsley-Griffin, 1998

24 Nonconformity - sedimentary rocks over crystalline (metamorphic or igneous) rocks N. Lindsley-Griffin, 1998

25 Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop? N. Lindsley-Griffin, 1998

26 Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop? N. Lindsley-Griffin, 1998

27 Cross-cutting relationships: dike cutting gneiss is younger N. Lindsley-Griffin, 1998

28 Cross-cutting relationships: dike cutting gneiss is younger N. Lindsley-Griffin, 1998

29 Cross cutting relationships: what is the sequence of events in this folded marble with two dikes? N. Lindsley-Griffin, 1998

30 Cross cutting relationships: what is the sequence of events in this folded marble with two dikes? N. Lindsley-Griffin, 1998

31 Superposition Disconformity Original horizontality Angular unconformity Cross-cutting relationships Nonconformity Grand Canyon Formations © Houghton Mifflin All rights reserved

32 Grand Canyon Formations © Houghton Mifflin 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

33 Geologic 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


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