Topics for 24 January•Relative Geologic Time•Numerical 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 Time•Relative datingallows determination of thechronological order in whichgeological eventsoccurred•Identify rock formations and place them inchronological order–Formation: 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” ages•Six Basic Principles: (principles in green apply only tosedimentary rocks)–Original Horizontality–Lateral Continuity–Superposition–Cross-Cutting Relationships–Inclusion–Faunal Succession
Principle of Original Horizontality•Sedimentary rocks, metamorphosed sedimentaryrocks and some volcanic rocks are originallydeposited ashorizontal layers–Thus, if layers are now folded or dipping, they musthave been deformed afterdeposition and lithificationHorizontal Layers – Grand CanyonTilted Shale Layers
Principle of Lateral Continuity•When deposited, sediment layers extend laterally in alldirections until they thin and “pinch-out” or terminateagainst theedge of the depositional basin
Principle of Superposition•In anundisturbedstack of sediments orsedimentary rocks, those on the bottom weredeposited first, followed by those above.
Principle of Crosscutting Relationships•If 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 Inclusion•If a rock contains (includes or encloses) fragments ofanother rock, then the included fragments are olderthan the rock–Fragments 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 deposited•Unconformity: a surface of discontinuity representingan interruption in sedimentation over a significantamount of geologic time – may be caused by•Three basic types of unconformities:–Disconformity:missing layers due to erosion ornon-deposition between two parallel–Angular Unconformity:tilting and erosion ofsedimentary layers followed by–Nonconformity: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