Biostratigraphy

1. Biostratigraphy

3. Comparing Rock and Time units

4. 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.

5. 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

6. 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

7. 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

9. 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