Stratigraphic guide

Stratigraphy is one of the most demanding and fascinating of geological disciplines since it is concerned with all attributes of rock sequences (in particular sedimentary successions), including their form, distribution, lithology, fossil content, depositional environment, vertical sequence and age.  It is therefore a synthetic subject concerned with the origin and temporal and spatial distribution of layers of sedimentary and other rocks.  Indeed, there is no doubt that stratigraphy is the foundation of geology, because it is only by demonstrating the order and lateral continuity of rocks that earth history can be established.

Anyone who has been active in the field of geology for the last few decades will be aware of the tremendous advances that have occurred in the physical, chemical, taphonomic and sedimentological fields.  New techniques of absolute and relative dating, palaeo- and environmental magnetism, stable isotopes and automated logging methods, to name but a few, have all added to the palette of techniques, each of which can be applied to the fundamental problem of correlation.  However, with this tremendous diversity goes the problem of being able to assess and apply these techniques in a scientific world where individuals tend to become increasingly specialised.  It is clearly vital that someone remains who can draw together the strands to see the problem as a whole.  Hopefully this whole will be greater than the sum of the parts.  However, to achieve this successfully there is no doubt that the stratigrapher must truly be a person for all seasons!

Stratigraphical problems are often still those uppermost in the minds of Quaternary scientists. Of course, unlike those geologists working on Palaeozoic, Mesozoic or earlier Cenozoic rocks, we are testing stratigraphical principles to their limit and beyond, often by attempting to subdivide impossibly short periods of time across different environments: the classical facies problem.  However a particular problem exists in Quaternary stratigraphy that arises because many practitioners are not geologists.  It is crucial that workers fully understand the implications of stratigraphical analysis and the essential distinctions between the various subdivisions of rock strata if we are effectively to divide, correlate and reconstruct history from rock sequences.

Please note that these pages do not include correlation and timescale charts. These items are available here.

Introduction

This guide is based on that produced by the Stratigraphy Commission of the Geological Society of London (Rawson et al. 2002).  These pages are designed to explain the different stratigraphical methods, to illustrate how they can be applied by the practising Quaternary scientist and, where appropriate, to indicate their limitations.  It is not a formal stratigraphical code; for this the International Stratigraphic Guide (Salvador 1994), the North American Stratigraphic Code (North American Commission on Stratigraphic Nomenclature 1983) or the on-line version on the International Commission on Stratigraphy (ICS) website can be consulted.  The aim is to encourage a common approach to stratigraphical practice by offering clear guidelines on usage of direct relevance to the Quaternary scientist.

This guide is not only based on the 2002 volume but also on previous editions (George et al. 1967, 1969; Harland et al. 1972; Holland et al. 1978; Whittaker et al. 1991), with modifications and additions from other sources relevant to Quaternary stratigraphical issues.  Together, they lead to a holostratigraphical approach which is increasingly providing precise correlation for most Phanerozoic successions, inevitably including those of the Quaternary.

The diverse areas of stratigraphy are essentially concerned with:

  • calibrating and correlating successions with one another and with the chronostratigraphical standard.  In the Quaternary, the main tool remains biostratigraphy – the use of fossils in  correlation.  However, many other methods are also utilised and are becoming increasingly important, see below.
  • establishing a formal (relative) time framework – chronostratigraphy. This is ultimately defined by the placing of fixed reference points, often called ‘golden spikes’, at carefully-chosen levels within sedimentary or ice-core successions.
  • of particular importance to the subdivision of Quaternary successions is the division based on interpreted climatic significance.  This gives rise to climatostratigraphy which is based on the recognition of an inferred widespread climatic episode. Closely allied to this is the scheme of divisions related to establishment of an event stratigraphy.
  • isotope stratigraphy is a method of determining relative ages of sediments based on measurement of isotopic ratios of a particular element.
  • magnetostratigraphy is dependent on variations in the magnetic properties of sediments and rocks as a basis for geological correlation.