Clay layers in sedimentary sequences are sometimes the result of alteration of ash deposits, producing bentonites or K-bentonites. Such layers may also result from direct deposition of clay (often lithified into shale), or from deposition of dust from aeolian, extra-terrestrial and/or multiple distant volcanic sources during prolonged cessation of significant “normal” sedimentation. Bentonites are obviously a special case as they are often due to a single short-term event and thus are especially useful in chronostratigraphy. It can be very difficult to determine whether a layer superficially appearing to be a uniform bentonite has a normal detrital component or is the result of multiple volcanic events. In the former case, a detrital component could introduce materials, including phenocrysts used in age determination, which could confound efforts to use the layer in stratigraphy. In the latter case, chemical characterization of the layer may be more difficult from one location to another due to varying amounts of the additive components. A method for determining whether a clay layer is due to a single ash-fall event, without a significant non-ash detrital component could be useful in studies using bentonites.

Chemical analyses of apatites from bentonites and sandstones from the Great Valley Sequence of California provide just such a test. Analyses of trace elements in suites of circa twenty grains from each of several bentonites and sandstones show clearly that apatite populations from bentonites are significantly more uniform that those from sandstones. If a sample has chemically identical grains, the layer is probably due to a single ash-fall event, with no significant detrital contamination, or contributions from more than one eruption. This simple test can be useful in screening suspected clay layers in order to select those likely to be most useful in stratigraphic correlation, and for geochronologic determinations using other phenocrysts (e.g. zircon).

This technique may also be useful in identifying some layers due to multiple volcanic events occurring in a narrow time window. In this case the chemistry of the apatites will define two or more tight clusters, as demonstrated by samples from bentonites taken from the Esopus Formation of New York.