William Smith (late 1700s) discovered that certain rock units could be identified by the assemblages of fossils they contained. This knowledge led to the Principle of Biologic Succession (or Fossil Succession), which states that fossils occur in a consistent vertical order in sedimentary rocks all over the world.
Fossil species appear and disappear throughout the stratigraphic record. The Geologic Time Scale is based on these appearances and disappearances. Each of the Eras ends with a mass extinction. Period boundaries coincide with smaller extinction events, followed by appearances of new species.
Fossils can be used to recognize the approximate age of a unit and its place in the stratigraphic column.
They can also be used to correlate strata from place to place.
Geologic range = The interval between the first and last occurrence of a fossil species in the geologic record.
The geologic range is determined by recording the occurrence of the fossils in numerous stratigraphic sequences from hundreds of locations.
Cosmopolitan species are found almost everywhere; they are not restricted to a single geographic location in their environment.
Endemic species are restricted to a specific area in the environment in which they live.
Cosmopolitan species are most useful in correlation because they have a geographically widespread distribution. Fossils found only in limited environments (endemic species) are not as useful in matching up rock layers that are far apart.
Appearances and disappearances of fossils may indicate:
Reworked fossils are fossils that were eroded out of the rock, and become incorporated into younger deposits. Finding a reworked fossil might make a geologic range incorrectly extended into younger strata. It is important to look for evidence of reworking if an older fossil is found in a much younger rock.
Index fossils or guide fossils are useful in identifying time-rock units and in correlation.
Characteristics of an index fossil:
Biozone = A body of rock deposited during the time when a particular fossil organism existed.
A biozone is identified only on the basis of the fossils it contains.
Biozones are the basic unit for biostratigraphic classification and correlation (much as the formation is the fundamental unit for lithostratigraphy).
Types of biozones:
September 29, 2005