Ch. 10—Key terms

1. Ch. 10—Key terms • Principle of faunal succession • Homotaxis • Zones (or biozones) • Taxon-range zone; interval zone; concurrent range zone • Biostratigraphic correlation • Causes of diachroniety • Index fossils (know examples)

2. Chapter 10—Biostratigraphy • Biostratigraphy = the branch of stratigraphy that is concerned with the spatial and temporal distribution of fossils and fossil-bearing strata • Biostratigraphic unit = body of rock characterized by its (specified) fossil content • Present and recognizable only where its characteristic taxa are present

3. Region 1 Region 2 Region 3 F Region 4 C C E H A B Dev. B Devonian A Mississippian Pennsylvanian G C S i l u r i a n D Miss. Devonian S i l u r i a n Devonian Ordovician Silurian Unexplored Region 5 (1) Determine the total stratigraphic ranges of species A–H on the basis of known occurrences in regions 1–4 (2) What is the age of a sample that contains species H, C and E? (3) How would you interpret a sample that contains species A, B, G and H?

4. Biostratigraphy • William “Strata” Smith—father of biostratigraphy (~1800–1815) • Principle of faunal succession • Homotaxis: Biotas follow one another in an orderly succession through geologic time (as a consequence of evolution) • First documented in “Coal Measures” then in Jurassic rocks in England

5. Biostratigraphic units • Principal biostratigraphic unit is the zone (or biozone) • Taxon-range zone = group of strata containing the total stratigraphic range of a selected taxon • Interval zone = group of strata between two biohorizons • Concurrent range zone = group of strata containing the overlapping ranges of two or more specified taxa

6. Biostratigraphic units • Regardless of the kind of zone employed, it is imperative to define zones in such a way as to make the boundaries unambiguous and easily recognizable

7. Taxon-range zone C E Commonly used; lower and upper limits of zone are defined by lowest and highest occurrence of zonal name-bearer (taxon E) A D B

8. Interval zone Most commonly used; boundaries defined on first or last occurrences of specified taxa C E A D B

9. Concurrent range zone Commonly used: Zonal boundaries defined by the overlapping ranges of two or more specified taxa (D & E) C E A D B

10. Practical recognition of zones • Few, if any, biozones are recognizable worldwide • Most organisms live only in a particular biogeographic province • Within a given biogeographic province, organisms only live where the environment is favorable

11. Shallow marine biogeographic provinces easward deflection of Labrador Current cold easward deflection of Gulf Stream warm

12. Biostratigraphic correlation • Biostratigraphic correlation = procedure by which approximate age equivalency of strata in separate areas is determined on the basis of fossils • Starting point is always determining the local stratigraphic ranges of taxa in a vertical section of rock • Then identify zonal boundaries and compare with same at some distant section of interest

13. Local stratigraphicranges and identificationof zonal boundaries(Pennsylvanianfusulinids)

14. Accuracy of correlation • Biostratigraphic correlations almost never are exact time correlations • Total stratigraphic range of a given taxon is never preserved at any one locality • Lowest and highest stratigraphic occurrences of a given taxon are diachronous (time-transgressive) over large areas • Evolutionary origin followed by dispersal • “Last survivor” of a species’ extinction

15. Accuracy of correlation (cont.) • Despite shortcomings, zonal biostratigraphy is the most accurate, most efficient, and most practical method for correlating sedimentary rocks • Intercontinental resolution can approach ± 0.5 M.y. or better • Intrabasinal resolution even better • Example: 26 conodont zones recognized worldwide in Upper Devonian tropical carbonate biogeographic province

16. Accuracy of correlation (cont.) • Best zonations utilize “guide” or “index” fossils that exhibit: • Rapid rates of evolution • Widespread geographic distribution • Occurrence in variety of sedimentary facies • Abundant occurrences • Typically marine, planktonic or nektonic organisms

17. Index fossils • Graptolites (most useful in Ordovician and Silurian) • Conodonts (useful throughout Paleozoic and Triassic) • Ammonoids (useful in Devonian through Cretaceous) • Planktonic forams (useful in Jurassic through Holocene) • Calcareous nannoplankton (useful in Jurassic through Holocene) • Acritarchs/dinoflagellates (useful throughout Phanerozoic)

18. Biostratigraphy & chronostratigraphy • Chronostratigraphy = the branch of stratigraphy concerned with organizing and classifying rocks into named units that correspond with intervals of geologic time • Objective is to subdivide geologic time into units so that there are no gaps or overlaps • Chronostratigraphic unit = a body of rock that formed during a specified interval of geologic time • Geochronologic unit = an interval of geologic time (during which rocks of the corresponding chronostratigraphic unit formed)

19. Chronostratigraphic & geochronologic units Chronostratigraphic unitGeochronologic unit Eonothem Eon Erathem Era System Period Series Epoch Stage Age

20. Definition of chronostratigraphic units • Chronostratigraphic units (e.g., systems) are defined by lower boundary stratotypes • Physical stratigraphic horizon that defines base of unit • Boundary horizon must coincide with the appearance of an agreed-upon guide fossil in a depositionally continuous rock sequence • Top of a chronostratigraphic unit is defined by the base of the next higher unit

21. Lower boundary stratotypes • Base of Pennsylvanian Subsystem • 82.9m above base of Bird Spring Formation at Arrow Canyon, Nevada • Evolutionary appearance of conodont Declinognathodus noduliferus (Gnathodus girtyi  D. noduliferus chronocline) • Base of Permian System • Base of limestone beds 19.5f at Aidaralash Creek section, northern Kazakhstan • Evolutionary appearance of conodont Streptognathodus isolatus (S. wabaunsensis S. isolatus chronocline)

22. Triassic lowerboundary stratotype Evolutionary appearance of Hindeodus parvus in Bed 27 at Meishan section (South China)