Biostratigraphy Type Units Defined True time units Litho
Biostratigraphy
Type Units Defined? True time units? Litho Member Formation Group Rock lithology No – time transgressive Bio Zone Stage Fossils Sort of Chrono Eon, Era, Period, Epoch, Age Time Yes, but how do you measure? Sequence System Tract Sequence Transgression/Regression No – occur at different places at different times Cyclo cycles Astronomical cycles Yes, but how do you recognize? Magneto Polarity zone Patterns of magnetic polarity Sort of – if correlated to isotopic dates
Comparing Rock and Time units Chronostrat Rock-Time (Biostrat) Example Eonothem Phanerozoic Erathem Mesozoic Period System Cretaceous Epoch Early Middle Late Series Lower Middle Upper Late Cretaceous Upper Cretaceous 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 timetransgressive • 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, 1 st to last of different species Interval range zone Interval between two species: 1 st to 1 st, last to last Lineage (consecutiverange) zone 1 st appearance within a lineage (commonly used in forams) Assemblage zone Defined on 1 st 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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