The Tongeren Group is a lithostratigraphic

Lithostratigraphy

Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology...

 unit (a sequence of rock strata

Stratum

In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...

) in the subsurface

Subsurface

Subsurface is the seventh studio album by British progressive metal band Threshold. The album was released in August 2004, and received an Album of the Month award in several European music magazines....

 of Belgium

Belgium

Belgium , officially the Kingdom of Belgium, is a federal state in Western Europe. It is a founding member of the European Union and hosts the EU's headquarters, and those of several other major international organisations such as NATO.Belgium is also a member of, or affiliated to, many...

. It consists of shallow marine, epicontinental and/or continental

Continent

A continent is one of several very large landmasses on Earth. They are generally identified by convention rather than any strict criteria, with seven regions commonly regarded as continents—they are : Asia, Africa, North America, South America, Antarctica, Europe, and Australia.Plate tectonics is...

 clay

Clay

Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

s and sand

Sand

Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...

s from the late Eocene

Eocene

The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...

 to early Oligocene

Oligocene

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...

 epoch

Epoch (geology)

An epoch is a subdivision of the geologic timescale based on rock layering. In order, the higher subdivisions are periods, eras and eons. We are currently living in the Holocene epoch...

s (between 37 and 30 million years old).

The Belgian Tongeren Group correlates with the Dutch

Netherlands

The Netherlands is a constituent country of the Kingdom of the Netherlands, located mainly in North-West Europe and with several islands in the Caribbean. Mainland Netherlands borders the North Sea to the north and west, Belgium to the south, and Germany to the east, and shares maritime borders...

 Tongeren Formation

Tongeren Formation

The Tongeren Formation is a geologic formation in the subsurface of the Netherlands. The formation consists of shallow marine, epicontinental and continental sediments, predominantlyy clay and sand from the late Eocene and early Oligocene epochs...

. The name was introduced by André Hubert Dumont in 1849.

Subdivision

The Tongeren Group is subdivided into three formations. From top to bottom these are:

• the Borgloon Formation, lagunal
  Lagoon
  A lagoon is a body of shallow sea water or brackish water separated from the sea by some form of barrier. The EU's habitat directive defines lagoons as "expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle,...
  
   or fluvial
  Fluvial
  Fluvial is used in geography and Earth science to refer to the processes associated with rivers and streams and the deposits and landforms created by them...
  
   shell rich sand and clay;
• the Sint Huibrechts-Hern Formation, marine shell bearing sand and clay;
• the Zelzate Formation, shallow marine glauconiferous
  Glauconite
  Glauconite is an iron potassium phyllosilicate mineral of characteristic green color with very low weathering resistance and very friable.It crystallizes with a monoclinic geometry...
  
   sand with thin clay layers.

Stratigraphic relations

In northwest Belgium the Tongeren Group (represented in that area mostly by the Zelzate Formation) is stratigraphically lying on top of the Maldegem Formation

Maldegem Formation

The Maldegem Formation is a geologic formation in the Belgian subsurface. The formation consists of alternating marine clay and sand strata, deposited during the late Eocene....

 (middle Eocene clay and sand). This formation is lacking in the east of the country, where the Tongeren Group is found directly on top of rocks with older ages, like the Landen Group

Landen Group

The Landen Group is a lithostratigraphic unit in the Belgian subsurface. The group consists of two formations of Thanetian to Ypresian age...

.

The Tongeren Group is usually overlain by deposits of the late Oligocene Rupel Group

Rupel Group

The Rupel Group is a stratigraphic group of rock strata in the subsurface of Belgium. It is subdivided into three formations that are all marine deposits of Oligocene age.-Name:...

.; 1849: Rapport sur la Carte géologique du Royaume, Bulletin de l'Académie royale de Belgique 16 (2^ième partie), p. 370. (eds.); 2001: Paleogene and Neogene lithostratigraphic units (Belgium), Geologica Belgica 4(1-2), p. 135-152.