I Principles

II Lithostratigraphic Units

III Contacts

IV Correlation

V Subsurface Techniques

Lithostratigraphy

I Principles

• Hutton - Superposition & Original Horizontality Potential problems - deformed beds

• Walther’s Law

• Matching Environments (Lithofacies) vs TimeSiccar Point Scotland

How to determine original “stratigraphic-up” orientations?

• Geopetals• Cross-strata• Channels• Inclusions• Truncation• Fossils

II Lithostratigraphic Units - no explicit time connotation

• Supergroup

• Group - e.g. Brigham Group (includes Geertsen Fm.)

• Formation - Mappable, recognizable - e.g. Geertsen, Langston Formations

• Member - e.g. Naomi Peak Limestone and Spence Shale

Members of the Langston Formation

• Bed

Lar

ger

Mor

e In

clus

ive

III Bed Contacts• Vertical

- Conformable Gradational (e.g. Geertsen-Langston in

Cataract Canyon) Intercolated

- Nonconformable Angular unconformity Nonconformity Disconformity

Paraconformity Diastem

• Lateral

- Abrupt- Gradational- Intertounging- Wedge

III Bed Contacts (cont.)

• Vertical

• Lateral

- Abrupt Beware faults!

- Gradational Changing facies with environmental

gradient

- Intertounging May show sea level oscillations

- Wedge Thickness trends may reveal source

areas

IV Correlation - establishing equivalence of lithologic units

• Lithology (e.g. well-rounded, well-sorted, medium grained, quartz arenite)

• Vertical Pattern (e.g. SS-SH-LS)

• Allostratigraphic Units (Synthems) - unconformity-bounded packages (e.g.Third-Order Sequences)

• Key Beds - widespread (Short-term event beds vs longer term)

And Perhaps

• Fossils (Biostratigraphy)• Stable Isotopes (Chemostratigraphy)• Magnetic Reversals (Magnetostratigraphy)

Short-Term Event Beds

• Ash• Storm (Tempestites)• Seismites• Floods (Inundites)• Impact ejecta

Longer-Term

• Hardgrounds• Transgressive Surfaces• Maximum Flooding Surfaces

P&S p. 330

Geology 3550Sedimentation & Stratigraphy

Subsurface Techniques

I Rock samples from wells

II Well logs (wireline logs)

III Seismic stratigraphy

I Rock samples from well

• Continuous core - Expensive

- Possibly limited recovery

• Side wall cores - May fracture rock

• Cuttings (Mud logs) - Position approximate

II Well logs (Wire-line logs)

• Dipmeter - bed orientation

• Microcaliper - bore diameter/rock induration

• Acoustic/Sonic - rock density

• “Radiation” - Gamma - measures natural radioactivity in shale, glauconite, arkose - Neutron - measures neutron absorption by H ions in shale, gypsum, fluids (not gas)

• Electric- Self-potential - fluid type (salty or not)- Resistivity - presence/absence fluids

III Seismic stratigraphy

• Refraction

• Reflection

- General

- Interpretation of Sedimentary Packages (Seismic Facies)

Character of Seismic Waves (amplitude, frequency, velocity)

Boundary Types(onlap, downlap, toplap, truncation)

Geometries

III Seismic stratigraphy (cont.)

Interpretation of Sedimentary Packages (Seismic Facies)

• Character of Seismic Waves (amplitude, frequency, velocity)

• Reflector configuration (continuous, discontinuous, chaotic, reflection free) • Boundary Types

(lower - onlap, downlap upper - toplap, truncation)

• Geometries (mounds, channels)

(20-600m)wavelength

Seismic Wave

ampl

itude

velocity (1-8 km/sec)frequency (10-80 Hz) (velocity / wavelength)

Character of Seismic Waves

I Amplitude (~energy) increases with increasing1) Fluid content2) Density contrast (e.g. unconformities)3) Thin beds (additive effect)

High amplitude reflections cause a “bright spot” (actually dark)

II Velocity (1-8km/sec) increases (seismic “pull-up”) with increasing 1) Density2) External pressure

Velocity decreases (seismic “sag”) with increasing3) Porosity4) Pore pressure (Fluids)

E.g. < 6 km depth Terrigenous Seds: 1-3 km/sec Carbonates: 2.5-6 km/sec

> 6 km depth Terrigenous Seds: 3.5-6 km/sec Carbonates: 5-7 km/sec

why this difference?

Character of Seismic Waves (cont.)

III Frequency (10-80 Hz) = velocity / wavelength Increases with decreasing 1) Bed Thickness 2) Fluid Content

IV Wavelength (20-600m) = velocity / frequency

Wavelength (60m typical) Resolution = ½ wavelength, therefore 30m packages resolved Higher frequency gives better resolution, but less penetration

I Ecostratigraphy (Biofacies)

• Biofacies vs lithofacies vs time• Provide evidence for eustatic cycles

II Biochronology (Biozones)

• Index fossils• Range zones• Problems

Biostratigraphy

Characteristics

•Short range

- Rapid evolution or

- Rapid extinction

•Widespread (planktonic, float after death, or dispersed by wind)

•Little ecologic control

•Abundant

Biochronology - Index Fossils

Examples

• Trilobites

• Forams

• Ammonites

• Pollen

Range Zones (= Chron, time unit)

• Taxon• Concurrent (Oppel)• Acme• Partial• Assemblage• Datum (FAD, LAD)

Biomere - extinction-bounded range zones Stage (= Age, time unit), based on several zones

Biochronology - Zones

• “Fuzzy boundaries”

- Species identification - Timing of appearance/disappearance - Migration over time - “Lazarus” species - “Zombie” species

• Environmental control

Biochronology - Problems

Geology 3550Sedimentation & Stratigraphy

Chronostratigraphy

I Time Units - Geochronologic

II Time-Rock Units - Chronolithologic

III Relative Dating

IV Absolute Dating

V The Geologic Time Scale

I Geochronologic (Time) Units -

Eon

Era

Period

Epoch

Age

Chron

II Chronolithologic (Time-Rock) Units -

Eonothem

Erathem

System

Series

Stage

Chronozone

more time

less time

III Relative Dating

• Fossils

• Superposition, Cross-cutting/Truncation, Inclusions

• Magnetostratigraphy

• Secular Trends in Stable (Nonradiogenic) Isotopes - 18/16O, 12/13C, 87/86Sr

• Secular Trends in Trace Elements - Mg, Sr

IV Absolute Dating

• Isotopic U, Pb, K, Ar

• Radiogenic

- Fission Track- Thermoluminescence (TSL, OSL)- Electron spin resonance (ESR)

• Sideral (counts)

- Varves - Sclerochronology - Dendrochronology

• Amino Acid Racemization

•Eons Hadean 4.6 - 3.9 Ga “firey” Archean 3.9 - 2.5 Ga “ancient” Proterozoic 2.5 - .540 Ga “before life” Phanerozoic .540 Ga – present “abundant

life”•Eras

Paleozoic 540 - 248Ma “ancient life” Mesozoic 248 - 65 Ma “middle life” Cenozoic 65 Ma – present “modern life”

•Periods•Epochs

moretime

lesstime

V The Geologic Time Scale