Chapter 4: Biostratigraphy – using fossils to date and correlate rocks

This document provides information pertinent to the interpretation of biostratigraphical data collected from cuttings material obtained when drilling boreholes. Caving of rock material is prevalent in many boreholes (younger/higher rock material and fossils being mixed with in situ older/lower material and fossils) and can be a significant problem. Unrecognized, it can lead to false range extensions of taxon bases (i.e. first/lowest occurrences). Caving is caused by wellbore instability (especially wall breakout) or by certain drilling techniques. The document also discusses reworking, which occurs when fossils, especially easily remobilized microfossils, are eroded from older sediments and incorporated into younger deposits, mixed with younger in situ taxa. Guidance and recommendations are given on how to recognize and mitigate the effects of such phenomena.