Determining the ages of sedimentary units is difficult, however, unless they contain useful biostratigraphic markers or interbedded volcanics (e.g. It is also made difficult by the fact that sedimentation can occur over relatively lengthy amounts of geologic time.
For that reason, direct dating of deposition/sedimentation is not always possible, but can be closely constrained through radiometric dating of authigenic and diagenetic sedimentary minerals. It is not clear how much of that variability is a result of depositional processes and how much is a function resetting during the subsequent thermal history of the sedimentary basin (Evernden et al. Both the Rb–Sr and the (K)Ar–Ar systems have been shown to commonly yield erroneously young ages, most likely due to the loss of radiogenic daughter products through diffusion or ion exchange processes (e.g. Ca are relatively small and only measurable in materials that are old (Paleozoic—Precambrian) and/or have high K/Ca values.
The term is seldom used today, but was originally used to describe a lithic-rich sandstone with between 10 and 50% mica, clay, or chlorite matrix.
Rocks with greater than 50% clay matrix are called sandy mudstones, and will be discussed in the lecture on mudrocks.
Absolute dating is necessary for knowing specific time e.g.
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In addition to being too young, the K–Ca ages are also highly variable, ranging in age from Silurian to Permian.
The oldest subset of glauconite ages are in agreement with previously published Rb–Sr ages from the same outcrop and provide further evidence for there having been a post-depositional thermal or recrystallization event that reset both the Rb–Sr and K–Ca systems.
The following response, initially presented in three parts, is not as detailed as I would like to make it.
This stems from the fact that I am currently enmeshed in research and also because I want to get this on Schimmrich's net before it closes down.