Lithostratigraphy

Lithostratigraphical units are sedimentary or igneous units that conform to the Law of Superposition.  Lithostratigraphy embraces the description, definition and naming of these units, as well as their correlation.

Description of lithostratigraphical units.

When a new unit is defined, or an existing one formally revised, the following should be described:

  • Lithological characteristics, including petrography, mineralogy, geochemistry, fossil content and lateral variation.
  • Relationship with stratigraphically adjacent units, both vertical and lateral.
  • Nature of boundaries:  whether gradational or sharp,  conformable or unconformable.  In particular, describe how the base can be recognized.
  • Thickness at the type section and its lateral variation.
  • Precise location of the type section, using grid references or, where appropriate, the Global Positioning System (GPS).

Definition of lithostratigraphical units.

Units are defined within a hierarchical framework:

  • Supergroup
  • Group
  • Formation
  • Member
  • Bed

Whatever the scale, the defined unit should have, or originally have had, lateral continuity.

The basic unit is the formation, which is generally defined as the smallest mappable unit.  However, ‘mappability’ is a loose criterion, for it depends on the scale of mapping.  In practice, a formation should be mappable and readily represented on a 1:50 000 map scale: individual members may be mappable at this scale, but are not necessarily so. Formations generally vary in maximum thickness from a few metres to several hundred metres.  A formation has lithological characteristics that distinguish it from adjacent formations. However, it is not necessarily lithologically homogeneous.  Furthermore, there may be some quite distinctive sub-units that are best regarded as members, although informal sub-formations have been used in some cases.

The boundaries of a formation may be sharp, interdigitating or gradational.  In a gradational sequence they may have to be fixed arbitrarily, for example where increasing grain size reaches a particular grade, or at the base of a distinctive marker bed, or where there is a significant change in bed thickness. Interdigitation provides particular problems in nomenclature.

A formation may stand alone or it may be linked with contiguous formations into a group.  Definition of a group may be contentious.  Some authors look for common lithological characteristics,  e.g. the Lias Group is dominantly argillaceous but contains limestone- sandstone- and ironstone-dominated formations. Others focus on the genesis of the group and thus place less emphasis on lithological characteristics and more emphasis on changes in gross depositional and tectonic character.  In the latter case, the bases of successive groups are commonly defined by basin-wide unconformities and their correlative conformities (see also sequence stratigraphy). A supergroup embraces contiguous groups and may also contain formations that have not been assigned to a particular group.

A member is a subdivision of a formation, but formations are not necessarily divided either wholly or partially into members.  Members commonly occur in the marginal areas of a formation, representing, for example, a marginal sandbody prograding into a predominantly argillaceous basin.

The smallest formal unit is a bed. In rare cases of lithologically monotonous sequences, formal bed names for distinctive fossil, marl and flint bands may have considerable reference value.  In more variable sequences it is rarely worth giving a formal name to every individual bed, but there may be distinctive beds which can be traced over long distances that merit naming. If the bed is named after a fossil, the fossil name is not italicised.  If the fossil name subsequently changes, the name of the bed does not.

The term subgroup, though not in the formal hierarchy, has to be used on occasion.

It is recommended that the term division be retained only for informal use as a term without hierarchical connotations.

Naming of new lithostratigraphical units.

Normally a new unit is named after a distinctive geographical feature/area, preferably at the type section.  The same geographical name should not be used for more than one unit. It is recommended that, where possible, a lithological descriptor is added, though this is not mandatory and may be inappropriate where more than one lithology occurs. The name also indicates the hierarchical level of the unit.  All the initial letters of such a formally defined name are capitalised (e.g. Robbins Group, Catfish Creek Formation, Bryany Member, Aala Tephra).  Additional terms such as lower, middle and upper should be used only informally and thus without initial capitals.

Stability and usage of existing lithostratigraphical names.

Long-established stratigraphical names may not fit readily with the principles noted above. Major revision of the stratigraphy of a previously described area may necessitate the introduction of new names.  But it is recommended that for the sake of continuity existing names be retained wherever possible, albeit modified to fit into the modern hierarchical framework.  In practice, this process is already well established; for example, the North Sea Drift is now the North Sea Drift Formation, etc..  Where irregularly-formed names, e.g. ‘Corton Beds’ are assigned a position in the modern hierarchy then ‘Beds’ should be dropped, i.e. one would refer to the Corton Member not the Corton Beds Member.  Other irregularly formed names can be conserved as long as confusion does not arise.

Any formal lithostratigraphical name should normally be written in full. But where the name has three components (geographical, lithological and hierarchical), the lithological epithet can be dropped in abbreviation, following an initial citation of the full name. Where new work demonstrates that the hierarchical part of the name is inappropriate then its rank can be changed;  for example the Brent sands of the northern North Sea were originally described as the Brent Formation but they have been reclassified subsequently as the Brent Group – which is now divided into five formations.

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