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Lecture 5: Sedimentation and Fossilization
September 08, 2016
I. Sedimentary Rocks and Sedimentation A. The dynamic evolution of the lithosphere has led to the formation of 3 basic types of rocks that are found at the Earth's surface. 1. Igneous rocks, which are formed by the cooling and crystallization of molten magma originating deep in the crust or in the mantle. a. includes volcanic rocks, which cool and crystallize at the surface of the crust b. includes plutonic rocks which cool and crystallize deep in the crust 2. Sedimentary rocks, which form as a result of the transport and deposition of particles eroded from pre- existing rocks, or from organic activity, or from chemical precipitation of minerals. 3. Metamorphic rocks, which form as pre-existing rocks are transformed by great heat and pressure.
B. Only sedimentary rocks have high potential for preserving fossils. 1. In rare cases, fossils have been recovered from volcanic deposits. 2. Metamorphosis of sedimentary rocks usually destroys useful information about fossils.
C. Most sediments are formed by water and gravity. 1. Wind and chemical erosion also occur, but are minor compared with the effects of water. 2. Mountains and highlands are the ultimate sources of sediment; the oceans the ultimate destination of the water carrying sediment.
D. Sediments and sedimentary rocks can be classified by where they were formed. 1. Marine sediments are deposited in oceans. 2. Non-marine sediments are deposited mostly in river beds and flood plains (FLUVIAL environments), in lakes (LACUSTRINE environments), and in river deltas (DELTAIC environments). 3. Dinosaur fossils can be found in rocks deposited in both environments.
E. Rivers and streams have gradients. 1. Water moves most rapidly down steep gradients; water carries the largest sedimentary particles down steep gradients. 2. Particles carried down steep slopes tend to suffer heavy abrasion and breakage. 3. Areas of steep gradient, like mountains, tend to be areas of net erosion.
F. Areas of low gradient tend to be areas where sediment is deposited. Any deposition of sediments in mountains and highlands tends to be temporary.
G. Tectonic activity can lead to the formation of basins in the earth's crust that catch thick sequences of sediment. Without continued tectonic subsidence, a basin would quickly fill and additional sediments would be carried elsewhere.
H. Tectonic activity and the water cycle drive sedimentation. 1. Tectonically created mountains are the source of sediment and much of the water (melted snow, rain) that moves it. 2. Tectonically formed basins catch the sediment.
II. Fossilization - or how to find your way into the fossil record A. Organism must grow hard parts: 1. Hard parts must be strong enough to withstand the dynamic forces of sedimentation and burial. 2. Necessary strength varies in different environments.
B. Death and burial: 1. Best if organism is not eaten. 2. Nearly all organisms become food for someone else 3. Best if burial is the cause of death or occurs immediately after death.
C. Corpse should be buried to great depth 1. This requires tectonically active burial site 2. Shallow burials are re-excavated sooner or later by routine sedimentary processes. 3. Most, but not all, fossilization processes operate deep in the crust.
D. Types of fossilization: 1. Mineralization (=petrification) 2. Carbonization (mostly affects plants) 3. Trace fossils (e.g. tracks, skin impression) 4. Other
E. Even after burial and fossilization, fossils must avoid chemical dissolution. 1. Most minerals that fossilize bones, as well as the bones themselves, are soluable. 2. Bones often dissolve even after burial and fossilization.
F. Tectonically activity must continue - a little. 1. Without tectonism, burial won't be very deep. 2. Without tectonism, the grave won't be uplifted, eroded, and exposed for discovery. 3. Tectonism often destroys fossils.
G. Fossil must be discovered by a scientist.
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