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Lecture 3

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University of Toronto St. George
Earth Sciences
Christine Burton

GLG205 J 3 May 18 Geologic time and fossils How are fossils formed? - Mineralization: cavities filled with different minerals like silica, calcium carbonate, and iron most commonly o Organism dies innear water and its soft parts are consumed by bacteria o After being buried, water passes through the soil and minerals precipitate out of the water into the cavities o If sediments are fine grained, the fossils are more detailed o As more sediments pile on, there is more pressure and it lithifies into rock o Mineral rich water continuously add or modify the shell o Uplift: the rock that was previously buried is lifted up o Erosion J ice, wind, water - Ammonite fossils o An organic gemstone o Iridescent aragonite (a mineral) with trace elements (e.g. Fe, Mn, Al, Ba) that causes coloration - Trace fossils o Preserved tracks, trails, burrows, borings; not the organism itself o Ichnology: the study of trace fossils o Bioturbation: process of disturbing sediment By organisms that walk past or burrowing in, or roots Fossils as clues to ancient environments - Palaeoecology: the study of ancient organisms and their environments - Clues from: o Fossil types E.g. if coral fossils are found, it used to be a shallow water o Assemblages Groups of organisms that live together o Fossil morphology The shape of their bodies o Trace fossils Shows what kind of organisms were there and how they moved - E.g. St. Cuthbert rings o Parts of an ancient sea lily (Crinoid) that were broken up and fossilized Fossils and stratigraphy - How do we use fossils in stratigraphy? o To establish relative age of rock units, correlate units - What information do we need to do this? o Relative age of rock units o Fossil species present in each unit o Establish time of first and last appearance of each species geologic range Long-living organisms are not useful compared to short-lived organisms - William smith o Was digging in England and found out that the same fossils were found in the same depths no matter where he dug
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