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Sedimentary Rocks 3 - Principles of

Stratigraphy

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• Nicolas Steno 1667

– Law of Superposition: “in a sequence of layered rocks,any later is older that the layer next above it”

– Principle of Original Horizontality: “layers of sedimentare originally deposited horizontally”

– Principle of Lateral Continuity: “sediments initiallyextend laterally in all directions”

Stratigraphy

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• Stratigraphy is the study ofsuccessions of stratified (layeredrocks) in time and space. In itsclassical days, stratigraphy involved

simply the study of litho-stratigraphy, that is the successionof rock types in stratigraphicsections.

• Strata were grouped according to

lithologic affinity into the followinglitho- stratigraphic hierarchy:

• Supergroup

• Group• Formation

• Member

• Bed

Stratigraphy

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Stratigraphy

• Formations are the basicbuilding block of litho-stratigraphy, in effect theunit that can be mapped inthe field. They are vaguelydefined as any unit that canbe recognized according toits lithologic character. Overshort distances, lithologicformations can commonly becorrelated betweenstratigraphic sections.

• Distinctive units that occurover wide distances, such asisochronous volcanic ashbeds, provide particularlyuseful correlations.

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• Diachronous formation: a formation may have the samplelithological properties but formed at different times in different

places

– Subdivisions of Formations:

• Member - rock unit that have a limited lateral extent andare consistently related to one formation

• Bed - if the bed has particularly distinctive lithology,fossil content or chemistry it may be given a name within

the formation

• Groups – are related formations

• Lithology - field characteristics of a rock

• Sedimentary Facies: Lithology or group of lithologiescharacterizing by a specific set of depositional conditions or

environment.

Key Terms in Stratigraphy

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• Contact - boundary between two differentlithologic units– Conformable - little to no time between deposition of

lower unit and the overlying unit– Unconformable - significant break in sedimentationbetween the two units

• Angular unconformity - angle between beds belowunconformity and beds above

• Disconformity - angle of bedding the same between bedsabove and below disconformity, but erosion occurs making thesurface non-flat

• Paraconformity - angle of bedding the same between beds

above and below paraconformity with the paraconformityparallel to bedding

• Nonconformity - deposition of sediments on nonstratiformrocks (igneous or metamorphic rock)

Vertical Changes in Sedimentary Sequences

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Stratigraphic correlations

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• Stratigraphic correlations can be based on:

– physical changes - lithostratigraphy

– fossil assemblage - biostratigraphy– changes in age - chronostratigraphy

– changes in magnetism - magnetostratigraphy

– changes in position relative to unconformities -allostratigraphy

– relationships to worldwide unconformities - sequencestratigraphy

Stratigraphic correlations

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Stratigraphic Correlation Problems

• Lithologic correlations work well, however, only over

relatively short distances. When attempts are made tocorrelate spatially distant stratigraphic sections, it becomes

apparent that lithologic beds are typically diachronous, andlithologic boundaries do not represent constant time lines.

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Correlation problems

• There are a number of resolutions to the correlationproblem; these include:

• biostratigraphy, which uses fossils to correlate betweensections;

• recognition of isochronous marker horizons such asbentonite (altered volcanic ash) layers.

• In recent years, however, attention has focused onsequence stratigraphy, which uses the cyclic nature ofstratgraphic successions to correlate between sections.

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Sequence Stratigraphy• The recognition of cycles, and cycles within cycles is done largely using

reflection seismic sections, and is becoming increasingly divorced from directconnection with actual rocks. Individual bands in these images do notcorrelate to specific rock types, but rather parasequences , thought torepresent individual shallowing upward sequences.

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Parasequences

• The parasequence is the basic unit ofsequence stratigraphy. A parasequenceis an outcrop scale (meters to 10’s of

meters) conformable succession ofsedimentary rocks that typicallyrepresents a single shallowing upwardcycle, bounded by marine floodingsurfaces.

• A parasequence thus represents asingle episode of sedimentprogradation (the seaward movementof shoreline), typically lasting 10’s to

100’s of thousands of years.

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Parasequences

• Parasequences are terminatedby marine flooding eventspossibly associated with

fluctuations in glaciation drivenby external Milankovichcycles, or reflect tectonicsubsidence.

• Milankovich cycles:

• ~100,000 & 400,000 yrs - cycleof orbital eccentricity

• ~100,000 yrs - cycle of tilt of

orbital plane to the ecliptic• 41,000 yrs - cycle of tilt of

rotation axis

• 21,000 yrs - chandler wobble of

rotation axis

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Sequences

• Sequences are stratigraphic successions bounded by surfaces ofsignificant sub-aerial erosion, representing a major cycle ofsedimentation lasting from ~ 5 to 15 Mys.

• They reflect sea level changes in response to major tectonic activity

such as changes in the volume of oceanic ridges and/or sense of seafloor spreading.

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Vertical Changes in Stratigraphy –

Cross-cutting relationships

• Cross-cutting relationships

– Any unit that has a boundary that cut across other strata must beyounger than the rock it cuts

• Included fragments– Clasts in a clastic rock are older than the rock strata in which they

are found

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ABSOLUTE

GEOLOGICAL

TIME SCALE

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Absolute Geological Time scale –

Radioactive Decay Schemes

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Radioactive IsotopeSystematics

THE BASIC EQUATIONS:

- (dN/dt) = λ N (1)

N = number of atomst = time

λ = decay constant

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The numerical value of λ is characteristicof the particular radionuclide under

consideration; unit of time-1.

Decay constant λ represents probability that atom decays within a stated unit of time.

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Rearranging equation (1) and integrating:

-

dN/N =λ

dt (2)

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This yields:

- ln N =λ

t + C (3)

C = constant of integration

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At t = 0 N = N0

Thus:

C = - ln N0 (4)

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Substituting into equation (3):

- ln N =λ

t - ln N0 (5)

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Which equals:

ln N - ln N0 = - λ t

ln (N / N0) = - λ t

N / N0 = e -λ t

N = N0 e –λ t (6)

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N = N0 e –λ t

Gives the number of radioactive parent atoms (N)

that remain at any time t of the original

number of atoms (N0).

Basic equation describing all radioactive decay

processes.

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Assuming:

The decay of a radioactive parent producesa stable radiogenic daughter.

Number of daughter atoms is zero at t = 0.

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The number of daughter atoms (D*)

produced by the decay of its parent atany time t is:

D* = N0 – N (7)

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Provided that:

No daughter atoms are added to or lost

from the system and

Change in number of parent atoms is due

only to radioactive decay

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Substituting equation (6) into (7) yields:

D* = N0 – N0 e –λ t

D* = N0 (1 – e –λ t ) (8)

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Equation (8) gives the number of stableradiogenic daughter atoms (D*) at any

time t that formed by decay of a

radioactive parent with an initialnumber N0 at t = 0.

Condition is no parent or daughter atomswere added to or lost from the system.

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HALF-LIFE

OF A

RADIOACTIVE ATOM

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Radioactive Decay – Half lives

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HALF-LIFE OF A RADIOACTIVEATOM:

The half-life T1/2 is the time required forone half of a given number of a

radionuclide to decay

when: t = T1/2

then: N= ½ N0

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Substituting these values into (6):

½ N0 = N0 e –λ T1/2

ln (½) = – λ T1/2

ln 2 = λ T1/2

T1/2 = ln 2 / λ = 0.693 / λ (9)

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sedimentary rx 3-02-06 09

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