Fossils are used to determine the ages of rock layers. Index fossils are of organisms that lived over a wide area. They lived for a fairly short period of time. An index fossil allows a scientist to determine the age of the rock it is in.

Index fossils are used by both geologists and paleontologists to study the rocks and species of the past. They help to give a relative age for the rock layers and other fossils found in the same layer.

Index fossils are used by geologists and palaeontologists as significant aids to determine the correlation and age of rock sequences [2]. Geologists use both large fossils or ‘macrofossils’ and microscopic fossils or ‘microfossils’ as indices to define and identify geologic periods.

To establish the age of a rock or a fossil, researchers use some type of clock to determine the date it was formed. Geologists commonly use radiometric dating methods, based on the natural radioactive decay of certain elements such as potassium and carbon, as reliable clocks to date ancient events.

Index Fossils – are fossils that must be widely distributed and represent a type of organism that existed only briefly. Index fossils are useful because they tell the relative ages of the rock layers in which they occur.

A useful index fossil must be distinctive or easily recognizable, abundant, and have a wide geographic distribution and a short range through time. Index fossils are the basis for defining boundaries in the geologic time scale and for the correlation of strata.

Biostratigraphy is only possible because species evolve and become extinct at specific times, and the evolution of one species is not repeatable. Diatoms are particularly advantageous for biostratigraphic studies of high-latitude sediments where calcareous microfossils are often poorly preserved or of low diversity.

Different fossils work well for sediments of different ages. Microfossils specially foraminifera, calcareous nannoplanktons, dinoflagellates, spore and pollen are widely employed in biostratigraphy because of their high resolution, better preservation and more abundance for quantitative analysis.

Biostratigraphy is the branch of stratigraphy that uses fossils to establish relative ages of rock and correlate successions of sedimentary rocks within and between depositional basins. A biozone is an interval of geologic strata characterised by certain fossil taxa.

Alcide d’Orbigny

geologist William Smith

Explore lithostratigraphy, which studies rock order; biostratigraphy, which looks at fossils; and chronostratigraphy, which uses absolute and relative dating methods.

The principles of biostratigraphy stem from the fundamental precept that William Smith claimed to be a general law: “The same strata are found always in the same order of superposition and contain the same peculiar fossils.” The subject can be considered under four headings: (1) biostratigraphic correlation; (2) …

Lithostratigraphy is the ‘classification of bodies of rock based on the observable lithological properties of the strata and their relative stratigraphic positions’1. Stratigraphy includes information about processes, geographical distributions, and the palaeo-environment of past glaciers and glaciation.

C (the period of time after which half of a given sample will have decayed) is about 5,730 years, the oldest dates that can be reliably measured by this process date to approximately 50,000 years ago, although special preparation methods occasionally make accurate analysis of older samples possible.

radiometric dating methods

Radiocarbon dating is therefore limited to objects that are younger than 50,000 to 60,000 years or so. (Since humans have only existed in the Americas for approximately 12,000 years, this is not a serious limitation to southwest archaeology.) Radiocarbon dating is also susceptible to contamination.

What can be dated? For radiocarbon dating to be possible, the material must once have been part of a living organism. This means that things like stone, metal and pottery cannot usually be directly dated by this means unless there is some organic material embedded or left as a residue.

There are a number of limitations, however.

• First, the size of the archaeological sample is important.
• Second, great care must be taken in collecting and packing samples to avoid contamination by more recent carbon.
• Third, because the decay rate is logarithmic, radiocarbon dating has significant upper and lower limits.

easy state radioactive decay. Carbon 14 has what is known as “half lives”. Each one lasts about 5,730 years. By the time you get out 60,000 years, the effectiveness is too diminished to be accurate.