دا كورس جيولوجية عامه مفيد جدا لطلاب اولى ود الرابط اللى عليه الكورس كامل وفى تحت امثله من الكورس
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II. Fundamental principles of Geology
Most sedimentary rocks occur in the form of layers called beds or strata.
Each layer is the result of the deposition of sediment during some natural event (such as a flood or storm).
A. Steno's Laws Named for Nicholaus Steno, a Danish physician living in Florence, Italy in the 1600's.
Levin, 6th edition, p. 3-4.
1. Principle of Superposition
1. Oldest rocks on the bottom
2. Younger rocks on top
2. Principle of Original Horizontality
1. Sediments are deposited in flat layers
3. Principle of Original Lateral Continuity
1. Sediments are deposited over a large area in a continuous sheet
B. Other basic principles of Geology which we can use for relative dating (or determining which rocks are older or younger)
1. Principle of intrusive relationships
The intrusion is younger than the rocks it cuts.
2. Principle of cross-cutting relationships
See Levin, 6th edition, p. 10
The fault is younger than the rocks it cuts.
3. Principle of components or inclusions
See Levin, 6th edition, p. 10-11
Note the irregular erosional surface. This is an unconformity.
The clasts (in the bed above the unconformity) are derived from the underlying (older) bed.
The gravel clasts are older than the layer which contains them.
The layer containing the gravel must be younger than the layer from which the clasts originate.
The principle of components or inclusions also applies to xenoliths.
A xenolith is a fragment of country rocks which has been broken off during an intrusion, and has become surrounded by magma. The xenolith is older than the igneous rock which contains it.
Compare illustrations in Levin, 6th edition, p. 11
4. Principle of fossil succession
Fossils occur in a consistent vertical order in sedimentary rocks all over the world.
(William"Strata Bill" Smith, late 1700's, England).
This principle is valid and does not depend on any pre-existing ideas of evolution. (In fact, Charles Darwin's ideas on evolution did not appear until 50 years later - 1858).
Geologists interpret fossil succession to be the result of evolution - the natural appearance and disappearance of species through time.
See Levin, 6th edition, p. 7-8.
We can use these 7 principles to establish the relative ages of rocks.
Unconformities
Unconformities are buried surfaces of erosion or non-deposition.
See illustrations in Levin, 6th edition, p. 8, 11, 17, 91 and 92
1. Angular unconformities
Implies tectonic deformation and erosion of underlying strata.
2. Nonconformity
Sedimentary strata overlying igneous or metamorphic rocks (in an erosional - not intrusive- contact)
3. Disconformity
An irregular surface of erosion betwen two units of parallel strata
4. Paraconformity
A planar surface between two parallel units of sedimentary rock, representing a period of non-deposition, but no erosion.
Criteria for recognizing unconformities:
1. Sedimentary criteria
1. Basal conglomerate - Many unconformities are overlain by a layer of conglomerate or gravel. The clasts are commonly fragments eroded from the underlying rock.
2. Buried soil profiles.
3. Layers of phosphatized pebbles, glauconite (greensand), or manganese-rich beds.
2. Paleontological criteria
