Biostratigraphic units Geology By Misson Choudhury
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Hey I'm Misson Choudhury ,Pursuing My m.sc in Applied geology at Bangalore university
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Biostratigraphic units Geology By Misson Choudhury
- 1. Biostratigraphic Units A. Purpose of Biostratigraphic Unit • The purpose of biostratigraphic classification is to organize rock strata systematically into named units based on content and distribution of fossils. B. Nature of Biostratigraphic Units • Rock strata are classified biostratigraphically by dividing them into units distinguished by differences in their fossil content. • A biostratigraphic unit may be based simply on the presence of fossils as contrasted with their absence
- 2. • There are many different kinds of biostratigraphic units depending on the paleontological feature considered. • A biostratigraphic unit may be considered to be present only within the observed limit of occurrence of the paleontologic feature on which it is based. Distinctive Nature of Units • Biostratigraphic units are distinct from many other kinds of stratigraphic units. • All parts of all rock strata have lithostratigraphic character, and all parts of all rock strata have chronostratigraphic character, but many parts of the Earth’s stratigraphic sequence lack significant fossil remains and thus lack biostratigaphic character and are not amenable to biostratigraphic classification.
- 3. Continuity of Fossils in Relation to Outer Limits of Units • Fossils usually constitute only a minor, disseminated, fractional part of a rock stratum. • Even within fossiliferous sequences fossils are rarely found in every bed or formation, nor are they found everywehre along a bed or formation.
- 4. Definitions 1. Biostratigraphy • The element of stratigraphy that deals with the remains or evidences of former life in strata and with the organization of strata into units based on their fossil content. 2. Biostratigraphic Classification • The organization of strata into units based on their fossil content. 3. Biostratigraphic Unit • A body of rock strata unified by its fossil content or paleontological character and thus differentiated from adjacent strata. • A biostratigraphic unit is present only within the limits of observed occurrence of the particular biostratigrphic features on which it is based.
- 5. 4. Biostratigraphic Zone (Biozone) • A general term for any kind of biostratigraphic unit • Biozone is a short alternative term for biostratigraphic zone. • Biozones vary greatly in thickness and geographic extend. • They may range from a local bed to a unit thousands of meters thick or extending worldwide • The total time represented by a biozone may be referred to simply as its time or time-value, or its biochron.
- 6. 5. Superzone and subzone • Several biozones with common biostrtigraphic features may be grouped into superzones (superbiozones) • Any kind of biozones may be divided into subzones (subbiozones) to express finer biostratigraphic detail. • A zone need not be completely subdivided into subzones. • Subzones have also been divided into very small units called zonules.
- 7. 6.Barren interzones and intrazones • Intervals lacking in fossils between successive biozones may be called barren interzones. • The barren intervals of substantial thickness within biozones may be referred to as barren intrazones. 7. Biohorizons • Surfaces of biostratigraphic change or of distinctive biostratigraphic character, valuable for correlation, commonly used as a biozone boundary though often recognized as horizons within biozones. • In theory, a biohorizon is strictly a surface or interface • In practice, the term may refer to a thin but biostratigraphically distinctive bed. •
- 8. • Features on which biohorizons are commonly based include “first appeareances”, “ last coccurence”, distinctive occurrences, changes in the character of individuals taxons (Eg. Changes in direction of coiling in foraminifera, or in number of septa in corals). • Biohorizons have been called surfaces, horizons, levels, limits, boundaries, bands, markers, indexes, datums, datum planes, datum levels, key horizons, key beds, marker beds and so on.
- 9. Kinds of Biostratigraphic Units 1. General • Biozone is the general term for any kind of biostratigraphic unit. • There are different kinds of biozones • The following four general types of biozones are in common use a) Assemblage-Zone • A groups of strata characterized by a distinctive natural assemblage of all forms present or of the forms present of a certain kind or kinds.
- 10. b) Range-Zone • A group of strata representing the stratigraphic range of some selected element of the total assemblage of fossil forms present. c) Acme-Zone • A group of strata based on the abundance or development of certain forms, regardless of either association or range. d) Interval-Zone (Interbiohorizon Zone) • The stratigraphic interval between two biohorizons.
- 11. Assemblage-Zone (Cenozones) Definition and significance • A biostratigraphic assemblage-zone is a body of strata whose content of fossils or of fossils of a certain kind constitutes a natural assemblage. • An assemblage-zone may be based on all kinds of fossil forms present or it may be restricted to forms of only certain kinds. • We may have an assemblage-zone based only on fossil fauna or one based only on fossil flora (an assemblage-zone of corals or of foraminifers, or of mollusks or of dasyclad algae, an assemblage-zone of planktic forms or an assemblage-zone of benthic forms and so on.
- 12. • Whatever its basis, an assemblage-zone is characterized by an assemblage or association of fossil forms that are assumed to have lived together or to have died together or to have accumulated together, or in any case , to have been entombed together. • The assemblage-zone is particularly significant as an indicator of environment. • It is also a general indicator of geologic age • These biostratigraphic units have high value of local correlation.
- 13. Range-Zones • A biostratigraphic range-zone is the body of strata representing the total range of occurrences of any selected elements of the total assemblage of fossil forms in a stratigraphic sequence. • The word “range” implies extent in both horizontal and vertical directions. • A biostratigraphic range-zone may represent the stratigraphic range of some one taxonomic unit or of a grouping of taxons or of a lineage or segment of a lineage or of any of particular paeontological feature
- 14. • There are many kinds of range-zones. Some of the principal kinds are a) Taxon-Range-Zone 1.Definition and significance • A taxon-range-zone is the body of strata representing the total range of occurrence of specimens of particular taxon (species, genus, family etc.,). • The range-zone of a taxon means its maximum geographic and stratigraphic extent. • The terms genus-zone and species-zone have occasionally been used in the same sense as taxon- range-zone.
- 15. • The taxon-range-zone is particularly valuable as an indicator or geologic age because of the limited time range (life spans). • The range-zone of a taxon may also be significant of environment. b) Concurrent-Range-Zone Definition and significance • A concurrent –range-zone is defined as the concurrent or coincident parts of the range-zones of two or more specified taxons selected among the total forms contained in a sequence of strata. • The principles of the concurrent-range-zone has long been used in time-correlation.
- 16. c) Oppel-Zone Definition and significance • The Oppel-zone, named after the German stratigrapher, Albert Oppel (1831-1865),. • The term Oppel-zone has not been widely used. • The Oppel-zone may be defined as a zone characterized by an association or aggregation of selected taxons of restricted and largely concurrent range. • The lower part of the zone is commonly marked largely by first appearance and its upper part by last appearance of certain taxons
- 17. d. Lineage-Zone (Phylozone) Definition and significance • A lineage-zone is a type of range-zone consisting of the body of strata containing specimens representing a segment of an evolutionary or developmental line or trend, defined above and below by changes in features of the line or trend. • This type of zone has also been termed as evolutionary zone, a morphogenetic zone, a phylogenetic zone or a phylozone. • Best for time-correlation • Identification of morphologic differentiation • Variations in rates of evolution
- 18. 4. Acme-Zones • An acme-zone is a body of strata representing the acme or maximum development- usually maximum abundance or frequency of occurrence – of some species, genus, or other taxon, but not its total range. • The acme-zone takes its name from the taxon whose zone of maximum development it delimits, for eg. Didymograptus Acme-zone.
- 19. 5. Interval –Zone • A biostratigraphic interval-zone is an interval between two distinctive biostratigraphic horizons. • The of such a zone might be marked by the horizon of first appearance of taxon-a or the last appearance of taxon-b, and the top might be marked by the horizon of first appearance of taxon-c or the last appearnce of taxon-d • The interal-zone is in common use for correlation purposes.