Biostratigraphy: time scales from graphic and quantitative methods

Abstract

Semi-quantitative and quantitative biostratigraphy methods are assisting with scaling of stages, as exemplified in the Ordovician—Silurian and Carboniferous—Permian segments of GTS2004. This chapter focuses on some theory and practical considerations. INTRODUCTION The larger part of the Phanerozoic time scale in this book relies on a construction where stages are first scaled “geologically” with biostratigraphic compositing techniques, and than stretched in linear time using key radiometric dates. The advent of versatile and “clever” semi-quantitative and quantitative biostratigraphy methods is assisting with this geological scaling. The methods also add a new dimension to the construction of local or standard biochronologies, and its time scale derivatives. In particular, three methods, each with their own PC-based programs, merit attention when it comes to scaling biostratigraphic data for standard or regional time scales: graphic correlation, constrained optimization ranking and scaling. Each of these three methods aims at a particular segment of time scale building and its application, using complex and/or large microfossil data files. Constrained optimization is directly utilized in building the early Paleozoic segment of GTS2004, and graphic correlation plays a key role in building the biostratigraphic composite for the late Paleozoic. Ranking and scaling has been used in construction of local biochronologies. In this chapter more general examples will be given of the approaches; a summary of the numerical and graphic methods is presented in Table 3.1.