Stratigraphy ( Biostratigraphy )

II. Biostratigraphy:

 Two most important things.

A. Biostratigraphy is the single most potent way we have of telling geologic time. It allows us to figure out which rocks are the same age as which other rocks, and thus allows us to piece together the rocks that assemble a given depositional environment. Without fossils, we could only rely on the laws of superpositioning and cross-cutting relations to work out the regional geologic history of a given area. These generally do not offer enough constraining observations to go beyond is generally a very broad impressions of the geologic past of a given area.

B. The main problems in biostratigraphy are as follows:

1.       Not all fossil groups are equally useful. Correlations based on groups that slowly evolve or whose presence depends on the nature of the substrate (for example, clams, snails, brachiopods) may not be nearly as precise and accurate as freely swimming groups such as coccoliths, foraminifera, and the ammonites.

2.       Good fossils can be hard to find. Some sequences yield only a few fossils, and these few relatively poor time constraints may allow several equally valid correlations that dramatically affect how you interpret the paleoenvironment. It is important to understand the limitations of the data so you recognize those cases in which more data are critical.