GEOL 101 1nd Edition Lecture 17 Outline of Last Lecture I Nonfoliated Texture II Metamorphic Environment Outline of Current Lecture I Principles of Dating II Unconformities III Fossils Current Lecture Principles of Dating Relative age dates placing rocks and events in their proper formation Before the discovery of radioactive elements geologists in the late 1700s through the 1900s were able to construct the geologic time scale by rock characteristics Absolute age dates specifying the actual number of years that have passed since the event occurred After the discovery of radioactivity geologists were able to assign number ages to the geologic time scale Law of superposition in a sequence of sedimentary rock layers the oldest rocks are on the bottom and they get progressively younger as you go towards the top Principle of fossil succession fossil organisms that are preserved in the rocks succeed one another in a definite and determinable order so that any time period can be recognized by its fossil content Unconformities Principle of original horizontality Layers of sediment are generally deposited in a horizontal position Rock layers that are flat have not been disturbed Principle of cross cutting relationships These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Younger features cut across older features These include features such as igneous intrusions large or small and faults Inclusions An inclusion is a piece of rock that is enclosed within another rock Rock containing the inclusion is younger Unconformity An unconformity is a break in the rock record produced by exposure and erosion and or no deposition of rock units Three types of unconformities Disconformity An erosional surface separating two units of layered sedimentary rocks sandstone over sandstone Noncomformity An erosional surface separating igneous or metamorphic rocks below and sedimentary rocks above sandstone granite Angular unconformity An erosional surface separating tilted or folded sedimentary rocks below and flat lying sedimentary rocks above shale sandstone Rocks are deposited horizontally Rocks are folded or tilted Folded rocks are eroded New rocks are deposited horizontally on top of folded rocks Fossils Fossil remains or traces of prehistoric life Types of fossils The remains of relatively recent organisms teeth bones shells etc Entire animals flesh included may be preserved but are very rare o Frozen mammoth in Alaska and Siberia o Even human fossil Otzi Given enough time remains may be petrified literally turned into stone o Original bone sell teeth wood that gets replaced w minerals from the surrounding rock usually sliced In 1991 a 5300 year old corpse was found in a melting glacier in the Tyrolean Alps along with a bronze ax and other artifacts The corpse and artifacts are beautifully preserved providing insight into the European Bronze Age culture As they learned more about this person that was he has been given the name tzi after the specific locality where he was found Watch this video http www youtube com watch v WA3AiNup7fY Molds and cast the shape of the organism is preserved in the rock Carbonization thin residue of carbon is left behind Tracks and Trails animal footprints preserved in soft sediments Burrows and resting places traces of animal activity in soft sediments Coprolites fossil poo Gives is information about food habits or organisms Gastroliths Dinosaurs ingested rocks to help digest food The dinosaurs stomachs essentially acted as rock tumblers rounding and polishing the rocks The gastroliths were either passed out of the animal s system or were left behind after the animal died and decayed Conditions favoring preservation Rapid Burial o Scavengers and decomposers cannot destroy the remains Possession of hard parts o Shells bones and teeth are more likely to be preserved than fleshy material Special and uncommon cases for preservation Amber fossilized tree resin insects spiders and even lizards Tarpits Mammoths sloths saber toothed cats dire wolfs Ice Wooly mammoths mastodons tzi Recall the Principle of Fossil Succession Fossil organisms succeed each other in a definite and determinable order Any period can be recognized by its fossil content Certain fossils are much better indicators than others Some fossils span large time periods while others span very short time periods Some fossils are localized in small geographic areas while others are found worldwide The most useful fossils show time periods and are geographically widespread These are called index fossils Examples of index fossils Inoceramus bivalve clam genus found world wide that flourished in the cretaceous modern bivalves still exist Mucrospirifen brachiopod genus that flourished world wide in the Devonian modern brachiopods still exist though rare Paradoxides trilobite genus found world wide and flourished in the middle Cambrian extinct Correlation of rock layers Index fossils can be used to match rocks of similar ages across different geographic regions This process has been used to correlate rock units across continents supporting the theory of plate tectonics Absolute Age Dating Establishing numerical ages for the geologic past Dates obtained using radioactive isotopes Review of basic atomic structure o Atomic structure Protons Positive charged Neutrons Neutral charged Electrons Negative charged o Atomic number An element s identifying number Equal to the number of protons in the atom s nucleus o Mass number Sum of the number of protons and neutrons in an atom s nucleus o Isotope Variant of an atom Differs in number of neutrons Results in a different mass number than the parent atom Parent isotope unstable radioactive isotope Daughter product isotopes resulting from decay of a parent