Problems in Dating Sedimentary Rocks (Sidebar 3)

Stratified sedimentary rocks and their included fossils confront us with important concepts relating to the history of the earth. Are sedimentary rocks and fossils millions of years old? Radioisotope dating of sedimentary rocks might be supposed to provide the answer. However, the radioisotope dates that have been obtained by standard geochronological practice have been recognized to be doubtful because:

	1.	these derived rocks are made largely of erosion particles from older rocks
	2.	the sedimentary rocks which form in situ as authigenic minerals do not commonly have enough radiogenic parent for dating and often do not retain the daughter elements. (For example, radiogenic parent 40K is often uncommon in sedimentary rocks, or the radiogenic daughter (40Ar) is lost from open mineral structures.)

Thus, carbonate rocks, evaporates, iron minerals, and phosphorites are usually classified as unsuitable for analysis of age.

Minerals from three types of sedimentary rock could be potentially datable: illite and glauconites from shale and zircon from volcanic ash beds. The clay mineral illite in shales exchanges daughter elements argon and strontium easily within its crystal structure. Glauconite, a clay-like mineral formed in the transformation of sediment to rock, has an open structure allowing argon to escape. Zircon (ZrSiO₄) includes significant amounts of uranium, thorium, and lead in its crystal structure, but these elements are very mobile as well. Hence, radioisotope dating of sedimentary minerals is usually discordant with the conventional ages assigned by the standard geologic column. All told, dating sedimentary rocks from radioisotopes in minerals still remains uncertain and subject to considerable doubt.

References:
Dickin, Alan P., Radiogenic Isotope Geology, Cambridge, UK, Cambridge University Press, 1995, pp. 53-56.

Faure, Gunter, Principles of Isotope Geology, Second Edition, New York, John Wiley and Sons, 1986, pp. 74-78.

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