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