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Chapter 13
Seismic, Sequence and Magnetic Stratigraphy
Seismic stratigraphy: the study of seismic data for the purpose of extracting stratigraphic information.
Sequence stratigraphy: the study of a sedimentary sequences and an outgrowth of seismic stratigraphy, but not limited to seismic stratigraphy.
Sedimentary sequence: a stratigraphic unit composed of a relatively conformable succession of genetically related strata that is bounded at its top and base by unconformities or their correlative conformities.
Sequence: one cycle of deposition bounded by nonmarine erosion, deposited by one significant cycle of rise and fall of base level. (Think sea level change.)
Magnetostratigraphy: uses records of changes in polarity of the geomagnetic field preserved in sedimentary sequences to correlate between wells and to date the sediment. Individual normal and reverse polarity intervals ("Chrons") typically range from ~10 thousand to 10 million years in duration.
Principles of Reflection Seismic Methods
Seismic Waves: (P waves—Compressional & S waves—Shear)
P waves: fastest seismic wave given name of primary wave
S waves: transverse seismic wave; secondary wave.
Reflection Refraction
Reflection: a wave that meets a boundary at a specific angle will bounce off the boundary at the same angle as the incident angle.
Refraction: the deflection of a propagating wave at the boundary between two mediums with different refractive indices.
Seismic sources: generate controlled seismic energy that is used in both reflection and refraction seismic surveys. A seismic source can be simple, such as dynamite or even a hammer; or it can use more sophisticated technology, such as a specialized air gun.
1940s Seismic exploration on land
Marine Seismic Surveying
Seismic record using the variable-density method of printing
(light intensity is varied to display wave amplitude differences)
Reflection configuration: the gross stratification patterns identified on seismic records.
Reflection continuity: depends upon the continuity of the density-velocity contrast along bedding surfaces or unconformities. It is closely associated with continuity of strata, and it provides information about depositional processes and environment.
Sequence stratigraphy is based on the premise that sedimentary successions can be divided into unconformity-bounded units that form during a single, major cycle of sea-level change.
Sequence stratigraphy attempts to place stratal units into a predictable, chronostratigraphic framework by demonstrating how their generation is related to accommodation space.
Table 13.2 Hierarchy of Sequence-Stratigraphic Units Depositional Sequence – genetically related strata bounded by surfaces of erosion or nondeposition or their correlative conformities. Stratal units within sequences include: Depositional System – a three-dimensional assemblage of lithofacies, genetically linked by active (modern) or inferred (ancient) processes and environments (e.g., fluvial, deltaic, barrier-island). System tract – a subdivision of a depositional system. Four main kinds are recognized: highstand (sediment deposited during high sea level) , falling-stage (sediment deposited as sea falls from high to low), lowstand (sediment deposited during low sea level and early rising sea level), and transgressive (sediment deposited during rising sea level). Parasequence Set - a succession of genetically related parasequences that form a distinctive stacking pattern that is bounded, in many cases, by major marine-flooding surfaces and their correlative surfaces. Parasequence – a relatively conformable succession of genetically related beds or bedsets (within a parasequence set) bounded by marine flooding surfaces or their correlative surfaces.
Marine flooding surface – a surface that separates younger from older strata, across which there is evidence of an abrupt increase in water depth.
Progradational parasequence:a parasequence set in which successively younger parasequences are deposited farther basinward; overall the rate of deposition is greater than the rate of accommodation.
Retrogradational parasequence: parasequences that advance in a landward direction.
Aggradational parasequence: parasequences that build vertically.
Highstand system tracts: consists of an aggradational to progradational set of parasequences that overlies the maximum flooding surface and that is overlain by the next sequence boundary. During the highstand systems tract, the rate of relative sea level rise begins to slow and relative sea level eventually begins to fall prior to the next sequence boundary.
Falling-stage system tracts: form as sea level falls from a highstand position during a forced regression.
Forced regression: accommodation space is reduced as the shoreline moves in a seaward direction and also moves lower down the depositional profile. As a result the coastal plain is bypassed and sediment is deposited in a more seaward position.
Lowstand system tract: begin to form after relative sea level has fallen to its minimum and begun to rise, creating a small amount of accommodation space.
Transgressive system tract: retrogradational sequences where the site of deposition shifts in a landward direction. The rate at which accommodation space is created is greater than the rate of sediment supply. Transgressive surfaces are marked by marine sediments overlying nonmarine sediments.
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When the sea level approaches its maximum, the rate of sedimentation eventually exceeds the rate of sea-level rise and aggradation to strong progradation generates a new highstand system tract and the cycle begins again.
System tract review
Magnetostratigraphy: uses records of changes in polarity of the geomagnetic field preserved in sedimentary sequences to correlate between wells and to date the sediment.
Curie point: the critical temperature of about 500°C - 600°C (for magnetite) where iron-bearing minerals become magnetized in alignment with the Earth’s magnetic field. When cooled further, the magnetite molecules with retain this orientation unless subsequently heated past Curie point. This semipermanent alignment is referred to as thermal remanent magnetism.
Detrital remanent magnetism: When the preferred orientation of magnetic minerals in sedimentary rocks imparts bulk magnetic properties to the rocks.
How?
During deposition of sediments, small magnetic mineral grains are able to rotate in the loose unconsolidated sediment of the depositional surface and thus align themselves mechanically with Earth’s magnetic field.
Geomagnetic polarity time scale for last 5 million years.
Global magnetic reversal patterns from Spreading ridges.
13.26
13.27
13.28
Paleomagnetic correlations of cores from the Arctic, Pacific, Indian, and Atlantic oceans. Cores have different lithologies and fossil assemblages.
Chapter 14: Biostratigraphy
Biostratigraphy: the characterization and correlation of rock units on the basis of their fossil content.
Stratigraphic paleontology: the study of fossils and their distributions in various geologic formations.
Principle of Faunal Succession:
Sedimentary rock strata are observed to contain fossilized flora and fauna, and that these fossil forms succeed each other in a specific, reliable order that can be identified over wide distances.
Stage: (Defined by d’Orbigny) groups of strata containing the same major fossil assemblages. The boundaries of the stage are defined at intervals marked by the last appearance, or disappearance, of distinctive assemblages of life forms and their replacement in the rock record by other assemblages.
Zone: (Defined by Oppel after Quenstedt criticized d’Orbigny) small-scale units defined by the stratigraphic ranges of fossil species irrespective of lithology of the fossil-bearing beds. Each zone was characterized by the joint occurrence of species not found together above or below the defined zone. (a.k.a. biozone).
Index fossil (species): ubiquitous forms of life which existed during limited periods of geologic time and thus are used as guides to the age of the rocks in which they are preserved.
Biogeographic province: an area within a zone defined by geographic limitations beyond which the zone cannot be traced. (think local versus global).
Index Fossils
Overlapping ranges within an Oppel zone
Principle types of biozones
A. Taxon-range biozone
B. Concurrent-range biozone
C. Interval biozone (lowest occurrences of taxa)
D. Interval biozone (highest occurrences of taxa)
E. Lineage biozone
F. Assemblage biozone
G. Abundance biozone
Two hypothetical assemblage biozones
Abundance zones of three hypothetical fossil species.
**Caution:
Correlation by abundance zones may not yield true time correlation as the same species may achieve maximum abundance at different times in different localities.
Taxonomic classification (Linnaean system)
Models and rates of Evolution
Mass Extinctions: dramatic reductions in numbers and diversity at particular times in the geologic record.
The big five…
Proposed causes for the main Phanerozoic extinction events
Biocorrelation: the correlation of lithostratigraphic units on the basis of their fossil contents.
Correlation on the basis of taxon-range and interval biozones
Globorotalia truncatulinoides
Cool period Warm period
