principles of stratigraphy - indiana university bloomingtong302/time.pdf · comparative records of...

Comparative Records of Time• Nature of the rock record

• principles of stratigraphy:• deposition, succession, continuity and correlation

• Stratigraphic tools

• biological succession of life: biostratigraphy

• magnetic reversals: magnetostratigraphy

• progression of sedimentary rocks fromchanges in sea level: sequence stratigraphy

• temporal excursions and sequential changes inchemical characteristics: chemostratigraphy

G302 Development of the Global Environment

Relative Age DatingRelative Age Dating

Depositional Succession• Sedimentary rocks

• deposited as beds orhorizons in rock units

• record and preservedepositional events

• beds often discontinuous

• beds can be eroded or lost• result: a gap in the

temporal record known asan unconformity or hiatus


Principles of StratigraphyPrinciples of Stratigraphy

continuous

deposition

erosion for

50 years

renewed

deposition

100 years

of sediment

150 year gap

resultant

rock record

unconformity

150 years =100 years lost+ 50 years of

erosion


Unconformities: Grand CanyonUnconformities: Grand Canyon

Characteristic Features• Distinct change in rock type,

age, orientation

or structure“great”

unconformity

Laws Governing Stratigraphic Relationships• Superposition

• relative ages from sequence of rock deposition

• Initial Horizonality

• orientation of beds when deposited

• Lateral Continuity

• spatial correlation of individual

horizons and rock units

• Cross-Cutting Relationships

• sequence of events record in

rock relationships


Principles of StratigraphyPrinciples of Stratigraphy

Nicolaus Steno


Law of SuperpositionLaw of Superposition

Order of Layered Units• Definition:

• A rock unit is younger than the one below

and older than the

one above• Stratigraphic Column

• temporal succession of rockunits

• deposition not necessarily

continuous, but sequential

youngestlayer

oldestlayer


Contraints on Original Orientation• Sediments are deposited as horizontal beds

• Principle applies to sedimentary rocks formed in

an aqueous environment

Law of Initial HorizontalityLaw of Initial Horizontality

Grand Canyon: horizontal strata


Evidence of Deformation• Non-horizontal sedimentary rocks

• Modified by post-depositional

events, e.g. foldingfolded

rock units

Law of Initial HorizontalityLaw of Initial Horizontality


Law of Lateral ContinuityLaw of Lateral Continuity

Spatial Relationships• Sediments form as

continuous layers

• Individual horizons

or layers thin or end

only when the

environment of

deposition changes

• Enables correlation

of beds with specific

characteristics Correlation ofrock units


younger intrusive rocks

Cross-Cutting RelationshipsCross-Cutting Relationships

eroded surface(C; unconformity)

fossiliferoussedimentaryrocks (D;horizontal)

igneousintrusion

(B)

deformedfossiliferoussedimentaryrocks (A)

Recorded Sequence of Events:

1. Deposition of fossiliferous sediments (A)2. Folding (deformation) of A3. Intrusion of igneous rock (B)4. Erosion to create surface (C)5. Deposition of fossiliferous sediments (D)

Event Sequence• Younger units

cross-cut older

units

• erosion surfaces• intrusions

• unconformities

Columns• Sequential order of

deposition determined

by correlation of

separate, related

stratigraphic records

• Unconformities may

be recognized,

uncertainties may

persist


Stratigraphic CorrelationStratigraphic Correlation

A

B

CC’C”

D

E

E F

F

G

H?

G’/H’?

hiatus

Site ASite B


Stratigraphic MethodsStratigraphic Methods

Biostratigraphy• Relative ages determined from fossil assemblages

• Biotic changes are a function of extinctions and

evolutionary processes

• Datums record timing of biotic changes

• first appearances and last appearances

• age-dependent

characteristics• boundaries calibrated

by absolute ages


BiostratigraphyBiostratigraphy

TemporalRecords ofLife• Recognition

of species

unique to

particular

time

intervals

• Index

fossils

youngestlayer

oldestlayer


Reversed PolarityNormal Polarity

S

N

magnetic

equator

magneticaxis

axis of rotation

N

S


Magnetostratigraphy• Approach based on intermittent, irregular reversal

of the polarity of Earth’s magnetic field

• Rocks record field at time of formation (cooling)



normal

reversed

Magnetostratigraphy• Magnetic signals preserved in stratigraphic

sections show alternating sequence of polarity

• Series of polarity

shifts:

• normal (modern)

• reversed (opposite)• Polarity intervals are:

• independent of lithology

• of varying duration



Magnetostratigraphy• Sequence of polarity

reversals recognized

• major intervals• minor intervals

• require excellent

stratigraphic resolution

• Globally uniform series of

time-dependent reversals

• Ages determined by

absolute dating

normal

reversed

minor



Magnetostratigraphy• Record compiled

from multiple,

overlapping

sequences

• Correlations to

stages often

based on

biostratigraphy

• palynology -

plant remains

Cretaceous Magnetic Records

Controls on the Sedimentation Process• Sediment production and accumulation is

controlled by:

• sea level – water depth, accommodation space• tectonic subsidence - accommodation space

• climate - weathering rates, grain production• Cycles in these variables operate over different

time scales

• Result from multiple studies of these cycles:

• a temporal record of changes in sea level


Sequence Stratigraphy: PrinciplesSequence Stratigraphy: Principles

Parasequences• Shallowing upward sequences

produce

defined

patterns

of

sediments

• Rock

record

indicates

shallowing



Spatial Arrangement of Stratigraphic Units• Lateral and vertical relationships in parasequences

• Predicable, recognizable sequences develop



landwardoceanward

Stacking of Parasequences• Vertical sequences record sea level change:

• Progradational: shallowing-upward

• Aggradational:

constant, static

• Retrogradational:

deepening



Depositional Sequence• Sequential order of characteristic elements:

• sequence boundary, lowstand systems tract, transgressive

surface, transgressive systems tract, maximum flooding

surface,

highstand

systems tract,

sequence

boundary

• Relevance:• defines sea

level changes



Correlation ofStratigraphicRecords• Magneto-

stratigraphy

• Biostratigraphy

• Sequence

stratigraphy

• Cenozoic

• 2- 65Ma


Stratigraphic ComparisonsStratigraphic Comparisons

Chemostratigraphy or Isotope Stratigraphy• Stratigraphic variations in specific chemical or

isotopic characteristics

• stable isotopes: C (!13C), O (!18O), S (!34S)

• isotopes: (87Sr/ 86Sr)

• molecules• organic

matter


Stratigraphic MethodsStratigraphic Stratigraphic MethodsMethods

Excursion in !13C in

black shales across

Cenomanian/Turonian

boundary

Eastbourne,

SussexPueblo, CO

Tarfaya,

Morocco

Strontium Seawater Evolution• Changes in 87Sr/86Sr related to sources of Sr

• increasing trend for past 150Ma


Chemostratigraphy: ExamplesChemostratigraphy: Examples

87Sr/86Sr ratios

of ocean change

with Sr sources:

new crust (low)

vs. weathering

(high). Ratios can

be age diagnostic

for much of

Cretaceous (K)

and Tertiary (T)

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