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

Relative Age - Principles 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 N. Lindsley-Griffin, 1999

Relative Age - Principles 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 N. Lindsley-Griffin, 1999

Relative Age - Principles Cross-cutting relationships: A rock unit is always older than any feature which cuts across or disrupts it. Fractures cutting sandstone layers, Merseyside, UK N. Lindsley-Griffin, 1999

Relative Age - Principles Cross-cutting relationships: A rock unit is always older than any feature which cuts across or disrupts it. Fractures cutting sandstone layers, Merseyside, UK N. Lindsley-Griffin, 1999

Relative Age - Principles 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 N. Lindsley-Griffin, 1999

Relative Age - Principles 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 N. Lindsley-Griffin, 1999

Relative Age - Principles Faunal succession used for correlation: Look for same fossils in same sequence of similar strata. Fig. 3.6, p. 63 N. Lindsley-Griffin, 1999

Absolute Age - Principles 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 © Houghton Mifflin 1998; N. Lindsley-Griffin, 1999. All rights reserved

Absolute Age - Principles 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 N. Lindsley-Griffin, 1999

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 © Houghton Mifflin 1998. All rights reserved

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

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 © Houghton Mifflin 1998. All rights reserved

Relative Age - Principles Unconformities: Gaps in the rock record - recognized by an erosional surface Signifies a major break in deposition Columbia River Gorge, OR-WA N. Lindsley-Griffin, 1999

Relative Age - Principles Original horizontality Superposition Cross-cutting relationships Horizontal sedimentary rocks Youngest sedimentary rock Dike cuts strata - dike is younger Fault cuts dike and strata - fault is youngest Oldest sedimentary rock © Houghton Mifflin 1998. All rights reserved

Dating the Geologic Time Scale Radiometric dating and cross-cutting relationships bracket ages of sedimentary rocks Radiometric age of intrusion Y is 350 m.y. 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. © Houghton Mifflin 1998. All rights reserved

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 - Disconformity N. Lindsley-Griffin, 1998

“Baked zone” - soil formed on older lava flow before being covered and cooked by a new lava flow - Disconformity N. 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 Nonconformity - sedimentary gravels over garnet schist. What is the history of this outcrop? N. Lindsley-Griffin, 1998

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

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

Cross-cutting relationships: dike cutting gneiss is younger N. 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

Grand Canyon Formations Superposition Disconformity Original horizontality Angular unconformity Cross-cutting relationships Nonconformity © Houghton Mifflin 1998. All rights reserved

Grand Canyon Formations 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 7. Erosion of fault blocks, deposition of Tapeats sandstone 6. Faulting © Houghton Mifflin 1998. All rights reserved

Geologic Time - Summary 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 N. Lindsley-Griffin, 1999