Morphostratigraphy

The unique nature of geological sequences in the Quaternary has meant that in many situations geomorphological approaches have been included in the subdivision of sediment and erosional sequences.  The identification and mapping of land surfaces, either developed upon sediment bodies or in some cases on bedrock, has been used as a means of interpreting relative chronologies in many regions. Typical examples include glacial moraines and associated landforms, dunes, fossil shorelines and river terraces (but see also Allostratigraphy).

In each of these cases the so-called morphostratigraphical unit is used to denote a body of sediment that is identified primarily from the surface form it displays (Frye & Willman 1962).  Central to the recognition of such units is that they include both landform and lithology in their definition (Bowen 1978).  Clearly these units are not directly comparable to standard lithostratigraphical units, where vertical and lateral changes, as well as relationships to other units, can generally be observed unambiguously.  Morphostratigraphical units should, therefore, only be given informal status (Richmond 1959).  However, in some Quaternary sequences, particularly in regions of recent glacial recession, moraine ridges mapped over considerable distances are often afforded virtually formal status, e.g. the Salpausselkä Moraines in southern Finland. Similarly, shorelines, either raised or submerged, have been used in a comparable sense in some regions.

Nevertheless, the apparently simple external morphology of some landforms, such as river terrace surfaces (see also Allostratigraphy), dunes or ice-marginal formations, commonly masks internal complexities of sediment sequences preserved beneath or within them.  For this reason, whilst morphostratigraphy might prove to be very useful in some regions, it should never be regarded as a substitute for, or a short-hand way of referring to, other more precise types of stratigraphical unit, such as outlined under lithostratigraphical units.

 

 

*This guide is based on that produced by Rawson et al. (2002) for the Stratigraphy Commission of the Geological Society of London.