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- 1.Geologic Time and Principles of Stratigraphy
- Recall Archbishop James Usher (1581-1665) who made a first attempt to calculate the age of the earth using theBible.
- Date: October 4004 B.C.
- Very young earth!WRONG!
- ITS AS OLD AS DIRT!
- James Hutton and Charles Lyell (1797-1875)
- Estimated soil formation at thousands of years/cm.
- Physical and chemical laws are uniform and have been operating on earth, causing changes for a LONG time.
- The present is the key to the past!
- Charles Lyell publishedPrinciples of Geology1830.
- Small changes in present day processes could, over long periods of time, produce significant changes.
5. Geologic Time
- Two methods to assess geologic time:RELATIVE dating and ABSOLUTE dating.
- Whats the difference?
6. GEOLOGIC TIME Geologic time involves events ranging in time from a few seconds (earthquakes, meteorite impacts) to millions and even billions of years (the movement of tectonic plates, the formation of the earth itself).Geologists use two different methods to determine geologic time and the ages of geologic features and events:1.Relative age datinginvolves establishing the order of features and events, without necessarily determining their actual age in years.2.Absolute age datinginvolves determining the actual age (in thousands, millions, or billions of years) of a rock or other geologic feature. 7. Relative Dating
- Age refers to the sequence in which events took place, not an actual age (in years).
- Method used before radioactivity was discovered.
- Using original horizontality, superposition, cross-cutting relationships and fossil assemblages one can determine which rock is older than the other.
8. Relative Dating Techniques
- Nicolaus Steno (1638-1686)
- Principle of Superposition
- Principle of Original Horizontality
- Principle of Lateral Continuity
- These principles led to the study of stratigraphy (study of layered rocks).
9. Stenos Law of Superposition
- In any
- sequence of undisturbed strata (layers of rock), the oldest is at the bottom and successively higher layers are successively younger.
10. Stenos Principle of Original Horizontality
- Sediments that compose rock are under the influence of gravity.Therefore sediments are deposited horizontally and parallel to the surface they are accumulating onto.
11. Stenos Principle of Original Horizontality
- If the sedimentary rock layers are tilted on an angle, what does this mean?
12. Stenos Principle of Lateral Continuity
- Sediments are deposited layers on top of each other.If you find the layers of rock are the same type of rock and sequence of layers here and over there, we can assume they were once continuous.
- A fault can disrupt once continuous layers of rock.
- Streams can erode a valley making rock layers discontinuous.
13. More Stratigraphy
- Charles Lyell
- In addition to Stenos Principles, Lyell came up with another couple of rules to stratigraphy.
- Principle of Cross Cutting Relationships
- Principle of Inclusions
14. Principle of Cross-Cutting Relationships
- Can determine the relative ages of rocks compared to one another.
- Applies to rocks, faults, unconformities, intrusions, etc.
- For example, If one rock layer cuts across another rock layer then the rock layer doing the cutting into is younger.
15. 16. Principle of Inclusions
- Fragments of one rock type can be included into a totally different rock type (when the two rocks are in contact with one another).
- Whenever two rock types are in contact, the rocks containing the pieces of the other will be younger of the two.
- In order to be included into another rock you must exist first.
17. Relative Age Dating - an example
- Layer #1 is younger than the other layers because it is on top.Layer #5 is older than Layers#1-#4, because it is below them.
- The fault is younger than all the layers, because it cuts through all the layers.
- The blue layer is the same age in both places, because it contains the same fossils.
18. Breaks in the Stratigraphic Record
- Layers of rock are said to beconformablewhen they are found to have been deposited layer after layer without interruption.Although particular sites may exhibit conformable beds representing significant spans of geologic time, there is no place on earth that contains a full set ofconformablestrata.
- Throughout earth history, the deposition of sediment has been interrupted over and over again.All such breaks in the rock record are termedunconformities .An unconformity represents a long period of time during which deposition ceased and/or erosion removed previously formed rocks.
- There are three kinds of unconformities..
- ADisconformityis an irregular surface of erosion between parallel strata.A disconformity implies a cessation of sedimentation, as well as erosion, but no tilting.
- Many disconformities are difficult to identify because the rocks above and below are similar and there is little evidence of erosion.
20. 21. Angular Unconformity
- Angular unconformitiesconsist of tilted or folded sedimentary rocks that are overlain by younger, more flat lying strata.
- Angular unconformities indicate that during the pause in deposition, a period of deformation (folding or tilting) as well as erosion occurred.
22. 23. Nonconformity
- Nonconformitiesare a type of unconformity in which the break separates older metamorphic or intrusive igneous rocks from younger sedimentary strata.
- Intrusive igneous masses and metamorphic rocks originate far below the surface.Therefore, for a nonconformity to develop, there must be a period of uplift and the erosion of overlying rocks.Once exposed at the surface, the igneous or metamorphic rocks are subjected to weathering/erosion prior to sedimentation.
24. The Fossil Record What about life represented in the rock record? William Smiths Principle of Faunal Succession Examined fossils in rocks.Found that you could match them up all over the world based on the assemblages (group of fossils) in rocks.Rocks around the world can be correlated based on their fossil content (formed at the same TIME under the same CONDITIONS just in different locations). Principle of Faunal Succession suggests that throughout geologic time, there has been a regular succession of organisms.Fossils succeed one another in a definite and recognizable order, and therefore any time period can be recognized by its fossil content. 25. Principle of Faunal Succession
- When fossils are arranged according to their age by using the law of superposition on the rocks in which they are found, they do not present a random or haphazard picture.To the contrary, fossils show progressive changes from simple to complex and reveal the advancement of life through time.
- For example, in the fossil record there is represented, in succession, an age of trilobites. an age of fishes, an age of reptiles, and an age of mammals.These ages pertain to groups that were especially plentiful and characteristic during particular time periods.Within each of the ages, there are many subdivisions based on certain species.This same succession of dominant organisms, never out of order, is found on every major landmass.
26. Fossils and Stratigraphy
- Geologic Range
- Because of faunal succession, each fossil species can be given ageologic range the first appearance of the fossil in the rock record to the last appearance of the fossil in the rock record.It is the range that an organism existed and left behind fossil evidence.
- Geologic ranges vary among groups of organisms.
27. Correlation and Faunal Succession Correlation by fossils. Certain index fossils are keys to matching and dating sedimentary strata in widely separated outcrops
- Index Fossils
- Fossils that are good for correlating and dating rocks all over the world.
- Easy to identify.
- Short life span for the species (small geologic range).
- Cosmopolitan Lifestyle
- Geographically Widespread
28. Principle of Faunal Succession
- Fossil assemblagesare generally more useful for dating rocks than a single fossil because the sediment must have been deposited at a time when all the species represented existed.
- As you can see in this image the fossil remains of living things are present in the rock layers at definite intervals, and exist within a discrete period of time. In this instance, using the Law of Superposition, would the age Rock Unit A be older or younger than the age of Rock Unit B?
29. Absolute Dating
- The geologic time scale was based on relative dating.Absolute ages have now been assigned b