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This page includes the outline text from the classroom PowerPoint presentation, together with selected graphics from that presentation. The graphics will appear when the title hotlink is activated.
A copy of the classroom presentation, in .pdf format, can be downloaded from here.
Textbooks referred to are:
McLean and Gribble: Geology for Civil Engineers (MG)
Dennen and Moore: Geology and Engineering (DM)
Hierarchy of divisions named for original observations of rocks, later placed in relative order
- Era - named for general nature of fossils
- Period - named for original locale, or typical feature of rocks
- Epoch - named for nature of fossils
- Formation - local rock type
Ages of Rocks
Most rocks can be given an approximate age, in 4.5 Ba Earth history.
- Actual ages come from radiometric dating of some rocks (MG p105+)
- Relative ages found by inference, from observations of geometry, composition (MG § 4.3)
Stratigraphy
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- Branch of geology concerned with formation and history of stratified (layered) rocks
- Basis of much of knowledge of geological history, and of distribution of rocks
- Generally covers the organisation and interpreation of observations in space and time
- Methods include observation of contacts, and correlation between rock units
- often broadened to cover the (time) relationships of all rocks
- Principle of Uniformitarianism
- Principle of Original Horizontality
- Principle of Superposition
- Principle of Faunal Succession
Unravelling Observations
What is the geological history of the rocks in "Minor Canyon"?
(See Press & Siever: Understanding Earth for original art and discussion.)
- The deepest layers are sediments
- These must have been deposited flat
- These must have been deposited in sequence
- Different composition reflect sdifferent sources of sediment from time to time
- Formation is a term for a group of beds with common features or composition
- Details within beds and formations allow decision about which really are deeper (older)
- Earth movements can and do completely invert sequences
- In the Minor Canyon, tilting may have been the next event
Contacts
- Key to relative ages of rock units found at their contacts
These (and bedding planes) are surfaces which separate units with different origins.
Examples:
- Conformity
- Unconformity
- Disconformity
- Nonconformity
- between the granite and the enclosing rocks. Contact metamorphism would prove that the granite was younger than the sediments.
- Angular unconformity, between the sediments and the Larsonton sediments. The exact time interval between the tilting and the beginning of Larsonton sedimentation (during erosion) is probably unknowable.
- The dike is older than the Forest City Formation, and younger than all the other rocks, because it cuts them. It terminates at the base of the Forest City Fm, showing that that surface is probably an unconformity. The Skinner Gulch Limestone contact is a conformity - there is no evidence of a time gap. However, the stream erosion in the present surface would, if the surface were subsequently buried, be evidence of an unconformity in a future geological section.
- Once unravelled, history is displayed in the map legend as part of a stratigraphic column, an ordered display of the rock formations present
- If fossil evidence is available, or radioactive dates determined, stratigraphic column will include geological age names
- Ages may also be determined by correlation
Example
The Grand Canyon section displays many of the stratigraphic features discussed. The image, an old National Geographic pic of surveying in the Canyon, shows the rocks in general (but note that the target lamp is not on the Great Unconformity, event 2 in the section, but on top of the flat-lying bed above it).
Correlation
- Used to extend relative ages beyond local area
- Generally composition-based - compare rock "materials" from different areas
- In near areas (e.g. boreholes) rock type may suffice
- Over larger areas use fossils - observe that life forms evolve similarly over Earth
- (the Principle of Faunal Succession)
Discrimination between the two possibilities:
- The age of the coarse sandstone is different in each hole in the second solution, but the same in each hole in the first solution
- Fossil evidence could make the decision
- Fossil evidence also shows that different rock types have equivalent ages, if they contain the same fossils.
Cartoon Example of Biostratigraphy
- Fossil entities exist during bounded periods of time
- Groups of fossils mark specific time spans
- Beds B and C in the example each contain similar assemblages of fossils at both sites, so correlate (that is, they have the same age)
- Bed A is missing on the right-hand side, so the B/C contact is a hiatus or unconformity.
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