Biostratigraphy - PowerPoint by Nk0jAc3Y

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									Biostratigraphy
Type       Units               Defined?                   True time units?
Litho      Member              Rock lithology             No – time
           Formation                                      transgressive
           Group
Bio        Zone                Fossils                    Sort of
           Stage

Chrono     Eon, Era, Period,   Time                       Yes, but how do you
           Epoch, Age                                     measure?

Sequence   System Tract        Transgression/Regression   No – occur at
           Sequence                                       different places at
                                                          different times
Cyclo      cycles              Astronomical cycles        Yes, but how do you
                                                          recognize?

Magneto    Polarity zone       Patterns of magnetic       Sort of – if
                               polarity                   correlated to
                                                          isotopic dates
      Comparing Rock and Time units
Chronostrat    Rock-Time (Biostrat)   Example
Eon            Eonothem               Phanerozoic
Era            Erathem                Mesozoic
Period         System                 Cretaceous
Epoch          Series                 Late Cretaceous
Early          Lower                  Upper Cretaceous
Middle         Middle
Late           Upper
Age            Stage                  Maestrichtian
               Zone (regional)        Baculites rex
• Larger units are built from smaller ones
  – Eg, stages are defined by the zones in them.
• We define bottoms only
  – If you define bottoms AND tops, one boundary
    has two definitions that may not coincide.
 Why aren’t biostrat correlations true
         time correlations?
• Are you looking at last appearance or
  unconformity?
• Facies dependence: facies are time-
  transgressive
• Regional speciation & extinction
• Shifting climate zones/biogeographic
  provinces
              Other challenges
• Preservation problems
  – Poorly preserved organisms and less abundant
    organisms are unlikely to be found
  – Signor-Lipps effect: poorly preserved and less
    abundant species appear to go extinct earlier than
    they actually do.
• Lazarus species – apparently come back from the
  dead because they weren’t preserved in between
  two occurrences
• Zombie species - appear above their extinction
  because they were exposed by erosion and
  reworked, then deposited in younger sediment
      What makes a good index fossil?
•   Abundant
•   Facies independent (planktonic, nektonic)
•   Easily preserved and collected
•   Widely distributed (global if possible)
•   Short species life (rapidly evolving)
•   Easily identified

• Best organisms: forams, rads, ammonites, graptolites,
  pollen, nannofossils
• But zones are defined for less-than-ideal organisms,
  e.g., dinosaurs, clams, conodonts, trilobies
Kind of zone               Definition

Taxon range zone (total)   First to last of one species


Concurrent range zone      Overlap of taxa, 1st to last of different species


Interval range zone        Interval between two species: 1st to 1st, last to last


Lineage (consecutive-      1st appearance within a lineage (commonly used in
range) zone                forams)

Assemblage zone            Defined on 1st and last of one taxa, characterized by
                           other taxa
Acme (abundance) zone      Abundance peak of one taxa
     Quantitative Biostratigraphy
• Uses a wider range of data than
  appearance/disappearance:
  – Abundance peaks
  – Ratios of species
• Based in sophisticated statistics
  – Correlation analysis (matches patterns of peaks)
  – Cluster analysis – makes groups for assemblage
    zones

								
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