Chapter 10: Geologic Time

Biostratigraphy

(the branch of stratigraphy which focuses on correlating and assigning relative ages of rock strata by using the fossil assemblages contained within them) The correlation of rock units on the basis of their fossil content; No regard to lithology. Biozones are independent of formations or other rock units. Biozones may subdivide or span rock units. Geographic Time Scale is based on biostratigraphy.

Formation

A distinctive series of strata that originated through the same formative processes -must be mappable -consists of lumps of strata

Not in order of Age, Just memorization exercise!!!!

A) Conglomerate B) Shale C) Batholith D) Sill E) Fault A F) Sandstone G) Dike A H) Dike B I) Fault B

How Are The Absolute Ages of Rocks Determined?

Absolute dating is based on determining how the ratio between parent and daughter isotopes change with time. Mass spectrometers are used for isotopic analysis. Absolute dating works best for igneous and metamorphic rocks.

Phanerzoic

Contains three eras Cenozoic ("recent life") Mesozoic ("middle life") Paleozoic ("ancient life")

How Are The Absolute Ages of Rocks Determined?

Elements consist of different isotopes - atoms with the same number of protons, but different numbers of neutrons. Some isotopes are radioactive and naturally decay. These decays produce an isotope of a different element. The original, radioactive isotope is called the parent, and the new isotope is called the daughter. Determined?

Precambrian

Everything before evolution of Shells. Archean and Proterozoic. Makes up 88% of Earth's history. Not divided into smaller time units because the events of Precambrian history are not know in great enough detail.

Zone

Fine subdivision of stratigraphic intervals based upon the occurrence of species without regard to lithology. Defined by German paleontologist Alber Oppel.

Principle of Faunal Succession

Fossils of different organisms first appear at different times in the rock record. Fossils of related organisms exhibit regular changes in progressively younger rocks everywhere they are found. When they become extinct fossil organisms disappear from the rock record everywhere at the same time and do not reappear in younger rocks.

Index Fossil (MARINE ANIMALS ARE BEST- i.e. PLANKTON!)

MUST BE: 1.Fast Evolving (better time constraint) 2.Wide Spread 3. Common (easily preservable & identifiable)

Relative Dating

Observe rocks in the field and determine the order of events that produced them

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Sedimentary rocks are not easily dated, but igneous rocks are.

Sequence Stratigraphy

Stratigraphy based upon global sea level changes through time. -help in correlating rocks between continents. -Developed by the petroleum industry

Absolute Dating

To actually know how long ago an event occurred, or when a rock formed in the past, requires laboratory analysis using naturally occurring radioactive elements in rocks and mass spectrometers to measure them.

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Understanding Steps to Explanation of Diagram

Explanation of Diagram

Use of Principles : 1) Layers of sedimentary rock are deposited, with the oldest at the bottom. 2) Fault A cuts across these rocks. 3) Erosion occurs. 4) Renewed deposition of sedimentary rocks, starting with conglomerate. 5) Fault B cuts across all of the sedimentary rock layers. 6) Magma forms an igneous intrusion which cuts across sedimentary rocks and fault B. 7) Dike B intrudes, cutting across the igneous intrusion, fault B, and forming a sill. 8) Dike A intrudes, cutting across sedimentary rocks and the sill. This may have fed volcanoes at the surface. 9) Erosion produced the current landscape.

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We must measure the isotopic abundances. A radioactive parent isotope decays to a stable daughter isotope. If we know the rate of decay, we can use the ratio of the two to calculate the age of the rock or mineral they are contained in. Half-life - the amount of time it takes for ½ of the parent isotopes to decay to the daughter isotope.

Stages

Zones lumped together

Why does biostatigraphy work?

because animals evolve over time

Relative Time Scale (geologic)

created based on fossils

Relative Ages

establish a sequence of events without knowing exactly how long ago they occurred. Events are put in order: what happened first, what happened next, and what happened most recently. This is all early geologists had.

Absolute Ages

establish when an event took place in the past. Absolute ages are numerical, quantitative, ages of geologic events, and have analytical uncertainties. This is the realm of mass spectrometry measurements in the past ~50-60 years.

Unconformities

gaps in the rock record when erosion occurred rather than deposition, 3 types

Principle of Cross-Cutting Relationships

geologic features such as dikes and faults that cut across rock must be younger than the rock they cut through. ex. Basalt is younger than granite

Disconformity

is a gap between two sedimentary layers that are parallel. Erosion, but no tilting.

Absolute Time Scale (now used)

isotopic dating

Principle of Inclusions

objects enclosed in a rock must be older than the rock itself.

Principle of Lateral Continuity

rock layers are continuous until encountering an obstruction

Principle of Original Horizontality

sedimentary rock layers are deposited horizontally when they form. Flat layers of sedimentary rocks that are no longer horizontal. Some tectonic event, which occurred after they were deposited, has tilted these up to the angle they are now at.

Principle of Superposition

states that sedimentary (or volcanic) rocks are created in succession, with the oldest rocks at the bottom, and progressively younger rocks above. ex. D= Youngest A= Oldest

Era

subdivision of Eon

Period

subdivision of Era

Epochs

subdivision of Period

Geochronology

the determination of geologic time is a quantitative science

Eons

the largest division

Correlation

the process of matching up the ages of rocks found in different places, i.e. finding rocks of equivalent age. (index fossils exist only for a brief period of time)

Nonconformity

where sedimentary or volcanic rocks lie directly on igneous or metamorphic rocks.

angular unconformity

where two layers of rock meet that are inclined at different angles to one another.