GEOL 1425 1st Edition Lecture 14 Outline of Last Lecture I. IntroductionII. Tectonics of North America Outline of Current Lecture I. Principles in stratigraphyII. Reconstructing geologic historyIII. Geologic timeIV. Measuring absolute timeCurrent Lecture—Clocks in Rocks- Timing the Geologic RecordI. Principles of stratigraphya. Original horizontality**—sedimentary strata is laid down horizontallyb. Superposition—c. Faunal succession—the lower fauna in a sediment is the older, higher is youngerd. Stratigraphy and fossils:i. Original horizontality and superposition—older rocks are the underlying strata. Sedimentation in the sea is one example, lake sediments, and wind in the desert. ii. Faunal succession—fauna and flora will be the same in outcrops of the same age, but naturally fauna and flora that did not live in the same time period cannot be found in the same outcrop/sediment layer.iii. Disconformity—different rocks probably laid down under different conditionsiv. Angular conformity—sediments are laid down but other things are also going on- compression of rocks leads to folding, also there is uplift. Some parts of the sequences are gone from erosion leaving for a more even plane or an irregular landscape. The great unconformity in the Grand Canyon!**II. Reconstructing geologic history from the stratigraphic recordThese 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.a. Cross-cutting relationships**-- identify and review which sediment layers come before/after one anotheri. Faultsii. IntrusionsIII. Geologic time: relative ages.a. Divisions of geologic time include eras, periods, and epochsb. Fossils and sedimentary records help us to define timec. Precambrian period: not an age scale. Earliest time we don’t know much about. There aren’t many rocks but there are some in Greenland, Australia, but no fossils and such so early on. Periods defined by rock types and not the fossil record. d. No detailed stratigraphic record until ~500 million years ago!IV. Measuring absolute time with isotopic clocks: a. Isotopic dating theory—all about unstable elements that decay into daughter elements from something lost in the nucleus. Has a half-life, and may be stable for a very long time or a very short time. b. Isotopic dating methods: serve different purposes b/c they can be used over different time periods. Require different materials to take measurements from. c. Know how to calculate half-lifes**** means it may show up on
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