Stratigraphy & Stratigraphy & Chronostratigraphy Chronostratigraphy
Jan 04, 2016
Stratigraphy & Stratigraphy & ChronostratigraphyChronostratigraphy
Chronostratigraphy DefinesChronostratigraphy Defines
The element of stratigraphy that The element of stratigraphy that deals with the relative time relations deals with the relative time relations and ages of rock bodiesand ages of rock bodies
Seismic StratigraphySeismic Stratigraphy
Powerful and growing fieldPowerful and growing field Identify horizons and determine Identify horizons and determine
depositional environment by shapedepositional environment by shape Recognize unconformitiesRecognize unconformities Recognize transgressional-Recognize transgressional-
regressional history.regressional history. Less costly than drilling holeLess costly than drilling hole
MethodsMethods Use airgun to create Use airgun to create
bubblebubble Hydrophone picks up Hydrophone picks up
sound wavesound wave Produces long lineProduces long line Vertical scale is two Vertical scale is two
way travel timeway travel time• Must know seismic Must know seismic
velocities through velocities through different rock types and different rock types and seawaterseawater
2D produced by array 2D produced by array of recording pointsof recording points
Ships tow arrayShips tow array
woodshole.er.usgs.gov
D. M. Finlayson
Thumper Method on LandThumper Method on Land
Developed Developed by Conocoby Conoco
Array of Array of trucks trucks produce produce vibrationsvibrations
Seismic reflection Seismic reflection produced by density produced by density contrastscontrasts
Reflectors not always Reflectors not always correlative with correlative with lithologic changelithologic change
Two beds with same Two beds with same density may not be density may not be discerniblediscernible
Contrastingly, chert Contrastingly, chert layer stong reflectorlayer stong reflector
One wiggle represents One wiggle represents 10-1000s meters10-1000s meters
Stratigraphic layers Stratigraphic layers and structures evidentand structures evident Seismic section of Western Sukhothai
Depression, Thailand
CR McClain
Gulf Coast Salt DomeGulf Coast Salt Dome
www.tsunamidevelopment.com/ images/salt_dome.jpg
Gas HydratesGas Hydrates
woodshole.er.usgs.gov
Seismic SequencesSeismic Sequences
Unconformities Unconformities identifiedidentified
Packages of rocks Packages of rocks bounded by bounded by unconformites = unconformites = seismic sequenceseismic sequence
Determine ages of rocks Determine ages of rocks through biostratigraphythrough biostratigraphy
Determine age of onlap Determine age of onlap and offlapand offlap
Sea level curve Sea level curve constructedconstructed
Sea Level CurveSea Level Curve Established through seismic Established through seismic
stratigraphystratigraphy Recognized unconformities Recognized unconformities
related to rise and drop of sea related to rise and drop of sea levellevel
Caution: data made to fit Caution: data made to fit curvecurve• Matching unconformities to Matching unconformities to
curves confirms curvecurves confirms curve• Those that don’t match are Those that don’t match are
said to be sloppily dated or said to be sloppily dated or local tectonismlocal tectonism
• Many unconformities in record Many unconformities in record that any could match curvethat any could match curve
• Curve constructed from Curve constructed from passive margin basins with passive margin basins with similar tectonics & agessimilar tectonics & ages
• Must take into account local Must take into account local tectonismtectonism
MagnetostratigraphyMagnetostratigraphy
Correlates rocks Correlates rocks through similarities through similarities in magnetic in magnetic signaturesignature
Correlations can be Correlations can be world wideworld wide
But not used But not used independentlyindependently
Time scale Time scale constructed mostly constructed mostly from lavasfrom lavas
MethodsMethods Rocks contain Fe-oxidesRocks contain Fe-oxides
• Magnetite(Fe3O4)Magnetite(Fe3O4)• Hematite (Fe2O3)Hematite (Fe2O3)
Lock in magnetism after cool below Curie pointLock in magnetism after cool below Curie point• 650o –hematite650o –hematite• 578-magnetite w/no titatium578-magnetite w/no titatium
Magnetitzation retained is thermal remanent Magnetitzation retained is thermal remanent magnetization (TRM)magnetization (TRM)
Detrital Remanent Magnetization (DRM)-Detrital Remanent Magnetization (DRM)-transported, deposited magnetic grainstransported, deposited magnetic grains
Chemical Remanent Magnetization (CRM)-Chemical Remanent Magnetization (CRM)-precipitated iron minerals. Usually noise or precipitated iron minerals. Usually noise or overprintingoverprinting
MagnetometerMagnetometer Samples collected and run Samples collected and run
though magnetometerthough magnetometer Magnetometer records Magnetometer records
intensity and direction of intensity and direction of magnetic vectormagnetic vector
Must eliminate overprinting of Must eliminate overprinting of younger magnetizations younger magnetizations through Alternating field through Alternating field demagnetization or thermal demagnetization or thermal demagnetization.demagnetization.
Cryogenic MagnetometerCryogenic Magnetometer• Liquid Helium at 4 degrees Liquid Helium at 4 degrees
above absolute zeroabove absolute zero• Creates superconducting Creates superconducting
region around sensorsregion around sensors• Magnetized specimen sets up Magnetized specimen sets up
current that can be measuredcurrent that can be measured
After AF After AF demagnetizationdemagnetization
Declination angle Declination angle between magnetic between magnetic north and true northnorth and true north
Inclination angle Inclination angle between horizontal between horizontal and compass dip: 0 and compass dip: 0 at equator 90o at at equator 90o at polespoles
HistoryHistory Bruhnes-1906-French Bruhnes-1906-French
Physicist- volcanic Physicist- volcanic rocks oppositely rocks oppositely magnetizedmagnetized
Cox,Doell, Dalrymple-Cox,Doell, Dalrymple-1960s- systematically 1960s- systematically studied rocksstudied rocks• Showed rocks of Showed rocks of
same age had same same age had same magnetic polaritymagnetic polarity
Magnetic polarity time Magnetic polarity time scale producedscale produced
Need to match local Need to match local polarity scale with global polarity scale with global polarity scalepolarity scale
Correlations can be Correlations can be worldwideworldwide• Polarity reversals happen Polarity reversals happen
worldwideworldwide• Polarity reversals happen Polarity reversals happen
independent of independent of lithology/facieslithology/facies
• Can directly correlate Can directly correlate terrestrial and marine terrestrial and marine recordrecord
Reversals geologically Reversals geologically instantaneous—about 4-5k instantaneous—about 4-5k yearsyears Scott and Hall
LimitationsLimitations
Polarity events commonPolarity events common• Single polarity sample cannot be usedSingle polarity sample cannot be used• Hiatuses or change in sedimentation rates Hiatuses or change in sedimentation rates
hamper correlationshamper correlations Rocks have to be recent orRocks have to be recent or Need independent method Need independent method
(biostratigraphy/ radiometric dating)(biostratigraphy/ radiometric dating)• Match to scaleMatch to scale
Magnetic record of rocks before Magnetic record of rocks before Carboniferous poorCarboniferous poor