Biostratigraphy is the science of correlation of sedimentary units base on the identifiable fossils they contain.Paleontologists examine fossils of all kinds, but micropaleontology (the study of microscopic organisms) is perhaps the most useful method of dating because the remains of tiny organisms tend to be better preserved, more widely distributed, and may provide more precise age determinations than larger shells or bone material.The Sr geochronology method involves extracting these isotopes from fossil shell material (only several milligrams of sample are necessary for X-ray fluorescence spectroscopy).
The basic science behind this method is that calcareous shell material incorporates the two strontium isotopes in the same ratio that occurs in seawater at the time the organism was alive.
At different times in Earth's history, the relative abundance of these two isotopes in seawater gradually changed through time (such as during the Permian, the Late Cretaceous, and parts of the Tertiary).
Relatively young deposits can be sometimes dated using tree rings, varved-lake sediments, coral growth patterns, and other methods.
Paleontology is the study of life in past geologic periods (fossil plants and animals), incorporating knowledge of an organism's phylogeny, relationships to existing organisms, and correlation to an established chronology of Earth History.
Great volcanic eruptions in the Western United States in the geologic past produced airfall deposits that have been recognized as far away as the East Coast.
The USGS maintains a tephrochronology laboratory in Menlo Park, CA.
A geologic map or report typically is only a summary of investigations that frequently involve the collecting and processing of hundreds of rock samples, followed by the evaluation and interpretation of data from a variety of analytical techniques.
A relative age is the age of a fossil organism, rock, or geologic feature or event defined relative to other organisms, rocks, or features or events rather than in terms of years.
Tradition paleontological and biostratigraphic correlation methods are still perhaps the most common relative dating methods used by geologists.
More modern correlation technologies include use of marine stable isotope records, paleomagnetic dating, tephrachronology, geomorphological methods, sedimentation characteristics, and other geochemical and radiometric methods.
A relative age of the original shell can be established by comparing the strontium isotope ratio of the shell material to published data for the time periods where this method is usable.