L4s-GlgTime-24Jan13 - Topics for 24 January Relative...

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Topics for 24 JanuaryRelative Geologic TimeNumerical Geologic Time“The leading idea which is present in all our [geological]researches, and which accompanies every fresh observation, thesound of which to the ear of the student of Nature seems echoedfrom every part of her works, is—Time!—Time!—Time!”— George Julius Poulett Scrope,The Geology and Extinct Volcanoes of Central France, 2nd ed., 1858
The “Spiral of Earth History”
Relative Geologic TimeRelative datingallows determination of thechronological order in whichgeological eventsoccurredIdentify rock formations and place them inchronological orderFormation: a body of rock of considerable thickness that hascharacteristics that set it apart from adjacent rocks.The rocksin a given formation have a common origin.Relative ages: only older or younger,no “numerical” agesSix Basic Principles: (principles in green apply only tosedimentary rocks)Original HorizontalityLateral ContinuitySuperpositionCross-Cutting RelationshipsInclusionFaunal Succession
Principle of Original HorizontalitySedimentary rocks, metamorphosed sedimentaryrocks and some volcanic rocks are originallydeposited ashorizontal layersThus, if layers are now folded or dipping, they musthave been deformed afterdeposition and lithificationHorizontal Layers – Grand CanyonTilted Shale Layers
Principle of Lateral ContinuityWhen deposited, sediment layers extend laterally in alldirections until they thin and “pinch-out” or terminateagainst theedge of the depositional basin
Principle of SuperpositionIn anundisturbedstack of sediments orsedimentary rocks, those on the bottom weredeposited first, followed by those above.
Principle of Crosscutting RelationshipsIf a rock layer or fault cuts across another rocklayer or fault, then the feature that is cut acrossmust beIgneous intrusions (dikes) in country rock.Baked zonesadjacent to a dike.A fault cutting across rock layers.
Principle of InclusionIf a rock contains (includes or encloses) fragments ofanother rock, then the included fragments are olderthan the rockFragments of preexisting rocks in clastic sedimentaryrocks.Fragments ofpreserved “country rock” within an intrudedmass of igneous rock (akaxenoliths).Lithic fragments in conglomerate.Dark xenoliths in an igneous intrusion.
Unconformities = “missing pages”Conformable sedimentary sequence: entire pile ofsedimentary rock layers depositedUnconformity: a surface of discontinuity representingan interruption in sedimentation over a significantamount of geologic time – may be caused byThree basic types of unconformities:Disconformity:missing layers due to erosion ornon-deposition between two parallelAngular Unconformity:tilting and erosion ofsedimentary layers followed byNonconformity:sedimentary rocks deposited on top of
DisconformityDisconformitiesoccur when layersare missing due toerosion or non-deposition between
A LocalDisconformityTop layers = 20,000year old glacialsedimentsBottom layers = 450Ma old tropical seasediments
Angular Unconformity – Tilting & Erosion of OlderLayers Followed by
NonconformityNonconformities occurwhen sedimentary rocksoverlieMetamorphic RocksSedimentary Rocks
Relative Geologic Time “Puzzle”ABCDEFGHIJ

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Term
Spring
Professor
Dong

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