Biostratigraphy

TypeUnitsDefined?True time units? LithoMember Formation Group Rock lithologyNo – time transgressive BioZone Stage FossilsSort of ChronoEon, Era, Period, Epoch, Age TimeYes, but how do you measure? SequenceSystem Tract Sequence Transgression/RegressionNo – occur at different places at different times CyclocyclesAstronomical cyclesYes, but how do you recognize? MagnetoPolarity zonePatterns of magnetic polarity Sort of – if correlated to isotopic dates

Comparing Rock and Time units ChronostratRock-Time (Biostrat)Example EonEonothemPhanerozoic EraErathemMesozoic PeriodSystemCretaceous Epoch Early Middle Late Series Lower Middle Upper Late Cretaceous Upper Cretaceous AgeStageMaestrichtian 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 zoneDefinition Taxon range zone (total)First to last of one species Concurrent range zoneOverlap of taxa, 1 st to last of different species Interval range zoneInterval between two species: 1 st to 1 st, last to last Lineage (consecutive- range) zone 1 st appearance within a lineage (commonly used in forams) Assemblage zoneDefined on 1 st and last of one taxa, characterized by other taxa Acme (abundance) zoneAbundance 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