Techniques of mollusc zonation in Texas Cretaceous

Abstract

The objective of the investigation, the kinds and numbers of fossils available, and the scale of the investigation all influence the type of zonation that will be used in any particular sequence of rocks. In an Edwards limestone study, the purpose being paleoecologic, the zones are based on sessile molluscs and are biosomal, but a single lithosome may be coextensive with more than one zone. No zones are coextensive with more than one lithosome; zonation is a technique with which to obtain data for further interpretation. A study in the Georgetown limestone consists of a detailed correlation of appearances and disappearances of mutually overlapping ammonite teilzones. In this study the zones are those parts of teilzones that are or are not overlapped, arbitrarily depending on whether an appearance or a disappearance is more useful for the investigation. Strata which can be correlated with the Austin chalk at Austin change rapidly from shale to chalk to limestone to black shale to flaggy limestone. The shale and limestone lithosomes contain ammonite sequences which are usually mutually exclusive. Within a single lithosome assemblage zones are useful and practical, but correlations from one lithosome to another is made only by the rare occurrences of single ammonites which seem to have been buried outside of their optimum environment. These single occurrences are neither index fossils nor guide fossils in the most recent interpretations. None of the 4 techniques used above produce units that can be called zones in a chronostratigraphic sense. All of them are zones in the wider interpretation of that word, and the Edwards limestone zones also approach the meaning of the term zone as used by some ecologists.