Stratigraphy & Chronostratigraphy

Chronostratigraphy Defines  The element of stratigraphy that deals with the relative time relations and ages of rock bodies

Seismic Stratigraphy  Powerful and growing field  Identify horizons and determine depositional environment by shape  Recognize unconformities  Recognize transgressional- regressional history.  Less costly than drilling hole

Methods  Use airgun to create bubble  Hydrophone picks up sound wave  Produces long line  Vertical scale is two way travel time Must know seismic velocities through different rock types and seawaterMust know seismic velocities through different rock types and seawater  2D produced by array of recording points  Ships tow array woodshole.er.usgs.gov D. M. Finlayson

Thumper Method on Land  Developed by Conoco  Array of trucks produce vibrations

 Seismic reflection produced by density contrasts  Reflectors not always correlative with lithologic change  Two beds with same density may not be discernible  Contrastingly, chert layer stong reflector  One wiggle represents s meters  Stratigraphic layers and structures evident Seismic section of Western Sukhothai Depression, Thailand

CR McClain

Gulf Coast Salt Dome images/salt_dome.jpg

Gas Hydrates woodshole.er.usgs.gov

Seismic Sequences  Unconformities identified  Packages of rocks bounded by unconformites = seismic sequence  Determine ages of rocks through biostratigraphy  Determine age of onlap and offlap  Sea level curve constructed

Sea Level Curve  Established through seismic stratigraphy  Recognized unconformities related to rise and drop of sea level  Caution: data made to fit curve Matching unconformities to curves confirms curveMatching unconformities to curves confirms curve Those that don’t match are said to be sloppily dated or local tectonismThose that don’t match are said to be sloppily dated or local tectonism Many unconformities in record that any could match curveMany unconformities in record that any could match curve Curve constructed from passive margin basins with similar tectonics & agesCurve constructed from passive margin basins with similar tectonics & ages Must take into account local tectonismMust take into account local tectonism

Magnetostratigraphy  Correlates rocks through similarities in magnetic signature  Correlations can be world wide  But not used independently  Time scale constructed mostly from lavas

Methods  Rocks contain Fe-oxides Magnetite(Fe3O4)Magnetite(Fe3O4) Hematite (Fe2O3)Hematite (Fe2O3)  Lock in magnetism after cool below Curie point 650o –hematite650o –hematite 578-magnetite w/no titatium578-magnetite w/no titatium  Magnetitzation retained is thermal remanent magnetization (TRM)  Detrital Remanent Magnetization (DRM)- transported, deposited magnetic grains  Chemical Remanent Magnetization (CRM)- precipitated iron minerals. Usually noise or overprinting

Magnetometer  Samples collected and run though magnetometer  Magnetometer records intensity and direction of magnetic vector  Must eliminate overprinting of younger magnetizations through Alternating field demagnetization or thermal demagnetization.  Cryogenic Magnetometer Liquid Helium at 4 degrees above absolute zeroLiquid Helium at 4 degrees above absolute zero Creates superconducting region around sensorsCreates superconducting region around sensors Magnetized specimen sets up current that can be measuredMagnetized specimen sets up current that can be measured

 After AF demagnetization  Declination angle between magnetic north and true north  Inclination angle between horizontal and compass dip: 0 at equator 90o at poles

History  Bruhnes-1906-French Physicist- volcanic rocks oppositely magnetized  Cox,Doell, Dalrymple- 1960s- systematically studied rocks Showed rocks of same age had same magnetic polarityShowed rocks of same age had same magnetic polarity  Magnetic polarity time scale produced

 Need to match local polarity scale with global polarity scale  Correlations can be worldwide Polarity reversals happen worldwidePolarity reversals happen worldwide Polarity reversals happen independent of lithology/faciesPolarity reversals happen independent of lithology/facies Can directly correlate terrestrial and marine recordCan directly correlate terrestrial and marine record  Reversals geologically instantaneous—about 4-5k years Scott and Hall

Limitations  Polarity events common Single polarity sample cannot be usedSingle polarity sample cannot be used Hiatuses or change in sedimentation rates hamper correlationsHiatuses or change in sedimentation rates hamper correlations  Rocks have to be recent or  Need independent method (biostratigraphy/ radiometric dating) Match to scaleMatch to scale  Magnetic record of rocks before Carboniferous poor