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I Principles II Lithostratigraphic Units III Contacts IV Correlation V Subsurface Techniques

Lithostratigraphy. I Principles II Lithostratigraphic Units III Contacts IV Correlation V Subsurface Techniques. I Principles Hutton - Superposition & Original Horizontality Potential problems - deformed beds Walther’s Law Matching Environments (Lithofacies) vs Time.

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I Principles II Lithostratigraphic Units III Contacts IV Correlation V Subsurface Techniques

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  1. Lithostratigraphy I Principles II Lithostratigraphic Units III Contacts IV Correlation V Subsurface Techniques

  2. I Principles • Hutton - Superposition & Original Horizontality • Potential problems - deformed beds • Walther’s Law • Matching Environments (Lithofacies) vs Time Siccar Point Scotland

  3. How to determine original “stratigraphic-up” orientations? • Geopetals • Cross-strata • Channels • Inclusions • Truncation • Fossils

  4. 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 Larger More Inclusive

  5. 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

  6. 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

  7. 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)

  8. Short-Term Event Beds • Ash • Storm (Tempestites) • Seismites • Floods (Inundites) • Impact ejecta • Longer-Term • Hardgrounds • Transgressive Surfaces • Maximum Flooding Surfaces P&S p. 330

  9. Geology 3550 Sedimentation & Stratigraphy Subsurface Techniques I Rock samples from wells II Well logs (wireline logs) III Seismic stratigraphy

  10. I Rock samples from well • Continuous core • - Expensive • - Possibly limited recovery • Side wall cores • - May fracture rock • Cuttings (Mud logs) • - Position approximate

  11. 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

  12. 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

  13. 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)

  14. SeismicWave velocity (1-8 km/sec) frequency (10-80 Hz) (velocity / wavelength) amplitude (20-600m) wavelength

  15. Character of Seismic Waves I Amplitude (~energy) increases with increasing 1) Fluid content 2) 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) Density 2) External pressure Velocity decreases (seismic “sag”) with increasing 3) Porosity 4) 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?

  16. 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

  17. Biostratigraphy • I Ecostratigraphy (Biofacies) • Biofacies vs lithofacies vs time • Provide evidence for eustatic cycles • II Biochronology (Biozones) • Index fossils • Range zones • Problems

  18. Biochronology - Index Fossils • Characteristics • Short range • - Rapid evolution • or • - Rapid extinction • Widespread (planktonic, • float after death, or • dispersed by wind) • Little ecologic control • Abundant • Examples • Trilobites • Forams • Ammonites • Pollen

  19. Biochronology - Zones • 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

  20. Biochronology - Problems • “Fuzzy boundaries” • - Species identification • - Timing of appearance/disappearance • - Migration over time • - “Lazarus” species • - “Zombie” species • Environmental control

  21. Geology 3550 Sedimentation & Stratigraphy Chronostratigraphy I Time Units - Geochronologic II Time-Rock Units - Chronolithologic III Relative Dating IV Absolute Dating V The Geologic Time Scale

  22. more time less time

  23. 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

  24. 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

  25. V The Geologic Time Scale more time • 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 less time

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