3550 Strat i Graphy Text
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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
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