BIOSTRATIGRAPHY: Age Dating & Correlation GEOL 3213

GEOLOGIC TIME SCALE Know the: –periods –eras –eons –Ma:

Simplified evolutionary chart based on wall structure 5 major wall types: –Pseudochitinous –Arenaceous –Microgranular –Porcelaneous –Hyaline Higher taxa have long ranges Higher taxa = 1 st approximations of ages FORAMINIFERA

Acmes of Diversity Late Devonian to Carboniferous Ordovician (especially Medial Ordovician) Conodonts

Number of Genera Per Epoch 2 major acmes 4 extinctions: –End Ordovician –End Devonian –End Permian –End Triassic

Biostratigraphic Zonation Some conodont zones represent only about 1 Ma, while others may be 2-3 Ma long. This is good for the Paleozoic Era.

Conodont Index Fossil Genera Many conodont genera have short time ranges!

Ranges of Fossils Indicate time Experience Evolution

INDEX FOSSILS Used for dating –rocks –geologic events Biozones –different kinds –range zones are very common –named for 1 or 2 fossils in zone Excellent index fossil: –1) narrow time range –2) wide geographic distribution –3) abundant –4) easily recognized –5) microscopic for petroleum geologists who use “well cuttings” concurrent- range zone assemblage zone time

Why Microfossils for Correlation? 1.Short geologic range. 2.Widespread distribution. Many microscopic organisms are part of the floating plankton in the oceans. These species are readily distributed over large areas by ocean currents. [Facies independence (not restricted to one particular rock type; present in many rock types). For example, plankton falls into any sediment that is forming on the sea floor. ] 3.Distinctive & easily recognized forms by non-specialists. 4.Abundance: Because microfossils are so tiny, hundreds to thousands of specimens may be present in small sediment samples. [Preservable (fossilizable) hard part fossils should be abundant enough to be collected in sufficient quantity for thorough study. (Petroleum geologists like them because many can survive the rigors of the drill bit.)]

Kinds of Biozones Taxon range zone –Defined by 1 st & last appearance of a single taxon –Named after the taxon Teilzone –Observed range zone in a local section Acme zone (=peak zone) –Great abundance of a species –Restricted to only a short part of its total duration Concurrent-range zone (= 2 taxon) –Overlapping range is the biozone –Named usually after both taxa, e. g., 2 species Interval zones –Successive 1 st or last occurrences of partially overlapping ranges. Lineage zone (=phylozones) –Successive first appearances of evolving species in a lineage Assemblage zone –Makes use of almost every species in the assemblage –Hard to work with because involves many taxa –May be the best kind of biozone because data rich

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Tree rings: