BIOSTRATIGRAPHY:Age Dating & Correlation

GEOL 3213

• GEOLOGIC TIME SCALE

• Know the:– periods– eras– eons– Ma:

• 1.6• 65• 251• 410• 544• 2500• 4000• 4650

251201

4650

Simplified evolutionary chart based on wall structure

• 5 major wall types:

– Pseudochitinous

– Arenaceous

– Microgranular

– Porcelaneous

– Hyaline

• Higher taxa have long ranges

• Higher taxa = 1st approximations of ages

FORAMINIFERA

Acmes of Diversity• Late Devonian to

Carboniferous

• Ordovician (especially Medial Ordovician)

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

– Named after the taxon• Acme zone

– Great abundance of a species– Restricted to only a short part of its total duration

• Concurrent-range zone– Overlapping range is the biozone– Named usually after both taxa, e. g., 2 species

• 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

End of File

• Tree rings: