Biostratigraphy

Stratigraphy based on the fossil occurrence

Introduction•  "Stratigraphy based on the paleontological aspects of

rocks".• As early as the 1700’s William Smith realized that fossils

could be used to recognize strata of different ages.• Alcide d’Orbigny, in roughly 1842 came up with the idea of

creating major subdivisions of strata based on a unique assemblage of fossils. D’Orbigny name the fossil assemblage strata groups as STAGES.

• Albert Oppel (1856) conceived the idea of small-scale units defined by the stratigraphic ranges of fossil species. Species stratigraphic range zones are referred to as range zones.  The species which marks a range zone is known as an index fossil.

• A paleontological record is the position of a fossil taxon in a rock sequence

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Types of fossil assemblages

Two types of fossil assemblages– Biocoenosis – Thanatocoenosis

Biocoenosis is an assemblage representing the living assemblage of any particular environment. This is a natural assemblage which is reliable for biostratigraphy.

Thanatocoenosis is an assemblage which undergone transportation or reworked. This assemblage cannot be used for age determination.

Fundamentals of biostratigraphy Biostratigraphy is the branch of stratigraphy that deals with

the distribution of fossils in the stratigraphic record and the classification of bodies of rock or rock material into biostratigraphic units based on their contained fossils.

• Biostratigraphic units are distinct from all other kinds of stratigraphic units because their contained fossils record the unidirectional process of organic evolution.

• The stratigraphic record as a whole contains an unrepeated sequence of fossil taxa that may be used to determine the relative age of their enclosing strata.

• The biostratigraphy is based on the biostratigraphic events which includes the first appearance and disappearance from the geologic record.

Biostratigraphic events• Fossil events are the result of the continuing evolutionary

trends of life on earth; they differ from physical events in that they are unique, non-recurrent, and that their order is irreversible.

• Events that usually utilised in biostratigraphy are– First appearance – Last occurrence– Rapid increase in population– Rapis decrease in population

Boundaries• The boundaries of a biozone are drawn at surfaces that

mark the lowest occurrence, highest occurrence, limit, increase in abundance, or decrease in abundance of one or more components of the fauna or flora.

• Furthermore, the base or top of one kind of biozone may not, or need not, coincide with the base or top of another

kind of biozone.

Index fossils

• Characteristic of Index fossils– Well preserved and easily retreived, particularly

microfossils.– Short stratigraphic range– wide geographic distribution, free from facies. can be

found in many rock types e.g. planktonic forms– abundant– Distinctive & easily recognized forms

Nature of Biostratigraphic Units• A biostratigraphic unit is a body of rock that is defined or

characterized by its fossil content.• Most fossils are contemporaneous with the body of rock

that contains them, including those derived from different but coeval sedimentary environments.

• Fossils not contemporaneous with the enclosing body of rock should not be used to define, characterize, or identify a biostratigraphic unit.

• Biostratigraphic units are based on criteria that differ fundamentally from those used for lithostratigraphic units

• The boundaries of most biostratigraphic units, are both characteristically and conceptually diachronous.

Fundamental Unit

• The biozone is the fundamental unit of biostratigraphic classification.

• Biozone is a condensed expression for ‘‘biostratigraphic zone.’’

Kinds of Biostratigraphic Units• The biozone is the fundamental biostratigraphic unit.

Five specific kinds of biozones are recognized herein: - range biozone, – interval biozone, – lineage biozone, – assemblage biozone, and – abundance biozone

• These five kinds of biozones are not hierarchically interrelated.

• The most common choice of biozone is one in which both the lower boundary and the upper boundary are based on the lowest occurrences of individual taxa; the two taxa may or may not have a direct phylogenetic link.

Range biozone• A range biozone is a body of rock representing the

known stratigraphic and geographic range of occurrence of any selected element or elements of the chosen fossil taxon, or taxa, present in the rock record

• Three types of range biozones– Taxon range biozone– Concurrent range biozone– Partial-range biozone

Taxon range biozone• A taxon-range biozone is a body of rock representing the

known stratigraphic and geographic range of a chosen taxon

• The lower boundary is marked by first appearance datum (FAD) and the upper boundary is marked by last appearance datum (LAD) of the same species.

LAD

FAD

• For example • Globorotalia

pseudomenardii zone

Concurrent-range biozone• A concurrent-range biozone is a body of rock including

the concurrent, coincident, or overlapping part of the ranges of two specified taxa.

• Lower boundary is marked by first occurrence of a taxon and the top is marked by last occurrence of ather taxon

Partial range biozone• Partial range biozone is a body of rock located between

the last appearance of a taxon and the first appearance of another taxon.

• The lower boundary is marked by last appearance and the upper boundary is marked by first appearance.

Interval biozone

• An interval biozone is a body of rock between two specified biostratigraphic biohorizons (surfaces)

• Two types of interval biozones– The boundaries are based on

first appearance– The boudaries based on last

appearance

Interval biozones

• Based on last appearances• eg. • Globorotalia velascoensis

• Based on first appearances• eg. • Globorotalia psuedobulloides

Lineage biozone• A lineage biozone is a body of rock

containing species representing a specific

segment of an evolutionary lineage.

Assemblage biozone• An assemblage biozone is a body of rock characterized

by a unique association of three or more taxa, the association of which distinguishes it in biostratigraphic character from adjacent strata.

• An assemblage biozone may be based on a single taxonomic group, for example, trilobites, or on

more than one group, such as acritarchs and chitinozoans

Abundance (acme) biozone• An abundance biozone is a body of rock in which the

abundance of a particular taxon or specified group of taxa is significantly greater than in adjacent parts of the section.

• Abundance zones may be of limited, local utility because abundances of taxa in the geologic record are largely controlled by paleoecology, taphonomy, and diagenesis

Lower boundary is marked by the increase of population of taxa

The upper boundary is marked by the decrease of population of taxa.e.g. Emiliania huxleyi biozone

Name• The name of a biozone consists of the name of one or

more distinctive taxa or parataxa (for trace fossils) found in the biozone, followed by the word ‘‘Biozone.’’ (e.g., Turborotalia cerrozaulensis Biozone or Cyrtograptus lundgreni-Testograptus testis Biozone).

• The name of the species whose lowest occurrence defines the base of the zone is the most common choice for the biozone name.