Geologic Time and Stratigraphic CorrelationGeology 200Geology for Environmental ScientistsGeology for Environmental ScientistsRelative TimeRelative Time•Geologists first worked out the sequence ofGeologists first worked out the sequence of events recorded in the rock record using the principles of relative time:principles of relative time:– original horizontalityii ll l i i–original lateral continuity– superposition– fossil succession–cross-cutting and intrusive relationshipscrosscutting and intrusive relationships– unconformitiesOriginal Horizontality - Sediments usually form flatlying deposits on the earth’s surfaceflat-lying deposits on the earth’s surface.Original Horizontality - Sedimentary rocks are horizontal because the original sediments were horizontalsediments were horizontal.Original Lateral Continuity: a logical extension of original horizontality Individual bedsextension of original horizontality. Individual beds are the same age along an outcrop.Unconformities: They are significant in that they aresignificant in that they are indicators of missing time in the rock record.8_9(a)LayeredsedimentaryrocksNonconformityNonconformityMetamorphicrockIgneousii k(b)intrusive rockYoungersedimentaryrocksAngularunconformityOlder, foldedsedimentary(c)rocksDisconformityBrachiopod(290 million years old)Trilobite (490 million years old)James Hutton, 18thCentury founder of GeologySiccar Point, Scotland, where Hutton discovered the meaning of unconformities.gSiccar Point, Scotland, June 2004Angular Unconformity in the Grand Canyon bet een Protero oic and Cambrian Rocksbetween Proterozoic and Cambrian Rocks. 100s of million years (MY) are missing.Nonconformity between metamorphic and sedimentary rocks in the Grand CanyonyyDisconformity within Pennsylvanian age rocks, West Virginia The disconformity shows an olderVirginia. The disconformity shows an older topography. Maybe only 1000s of years are missing from the rock recordfrom the rock record.Can you interpret the sequence of geologic events using superposition, intrusive relationships, andusing superposition, intrusive relationships, and cross-cutting relationships?aWhat is the sequence of events?Dike and SillWhat is the sequence of events?These various principles wereThese various principles were used to construct the Geologic Time Scale (p. 199 in Text), which was done without anywhich was done without any knowledge of the number of years involved.Fig. 8.8. The standard geologic time scaleTime Units of the GeologicTime Units of the Geologic Time Scale• Time can be separated into “pure” time and “rock” time. Rock time is divided into time stratigraphic units. Pure time gpis divided into geochronologic units.•Time stratigraphic units sometimes•Time stratigraphic units sometimes parallel formation boundaries, but fh f ibdioften they cross formation boundaries.Time Units• Geochronologic•Era• Time Stratigraphic•(Erathem)•Era–Period•(Erathem)–System•EpochAge•SeriesStage–Age• Devonian Period–Stage• Devonian SystemRock Units, not a part of theRock Units, not a part of the Geologic Time Scale• Sedimentary rocks are divided into fiformations.• Formations can be divided into members.F ti b bi d i t•Formations can be combined into groups.Rock Units, cont’d,• Formation name consists of two parts:p–Geographic nameih l i l i–Lithology or simply Formation• Examples:p–Burlington Limestone–Waynesburg Sandstone–Juniata Formation (no dominantJuniata Formation (no dominant lithology)Rock Units, cont’d• Example of rock unit divisions:•Conemaugh Group: 2 formations•Conemaugh Group: 2 formations–Casselman Formation–Glenshaw Fm.: several members•Ames Member•Ames Member•Harlem Coal Mbr.•Pittsburgh Red Shale Mbr.The Grand Canyon of the Colorado River in Arizona Sedimentary rocks are divided intoArizona. Sedimentary rocks are divided into formations, which can be combined into groups.Absolute Time: Gh lGeochronologyRadiometric Dating: the source of the dates on the Geologicof the dates on the Geologic Time ScaleRadiometric Dating• Actually a simple technique.•Only two measurements are needed:•Only two measurements are needed:• 1. The parent:daughter ratio measured with a mass spectrometer.•2Thedecay constantmeasured by a2. The decay constant measured by a scintillometer.Basis of the Technique• Radioactive elements “decay.” Decay occurs as an element changes tooccurs as an element changes to another element, e.g. uranium to lead.hlidiih•The parent element is radioactive, the daughter element is stable.• The decay rate is constant.What is Radioactivity?What is Radioactivity?•Radioactivity occurs when certainRadioactivity occurs when certain elements literally fall apart.Ull d•Usually protons and neutrons are emitted by the nucleus.• Sometimes an electron is emitted by the nucleus which changes a neutronthe nucleus, which changes a neutron to a proton.Si l i d•Sometimes an electron is captured.Electron emissionAlpha particle emissionWhat causes radioactivity?• Carbon-14 is produced by cosmic ray bombardment of Nitrogen-14 in thebombardment of Nitrogen14 in the atmosphere.All h di i l•All other radioactive elements were produced by supernova explosions before our solar system formed. This is called explosive nucleosynthesis.pyCommon Radioactive ElementsCommon Radioactive Elements, Parents and Daughters• Carbon-14, C14Nitrogen-14, N14,g,• Uranium-235, U235Lead-207, Pb2074040• Potassium-40, K40Argon-40, Ar40•Uranium-238, U238Lead-206, Pb206Uranium238, ULead206, Pb• Rubidium-87, Rb87Strontium-87, Sr87Basis of the Technique• As the parent element decays, its amount decreases while the amount ofamount decreases while the amount of the daughter element increases. This gives us a ratio of parent:daughtergives us a ratio of parent:daughter elements.• The decay rate is geometric rather than linear. Unaffected by heat or pressure.UypKey Term• Half-Life: the amount of time for half the atoms of a radioactive element tothe atoms of a radioactive element to decay. Doesn’t matter how many atoms started half will decayatoms started, half will decay.Radioactive Decay80100clide4060rcent NucParentDaughter0200246810Per0246810Half LivesOn a log scale geometric depletion plots as aOn a log scale, geometric depletion plots as a straight line.Decay of Parent Nuclide10010100rcent110Scale, PeParent0110246810Half LivesLog