The Zenne Group is a group

Group (stratigraphy)

A group in stratigraphy is a lithostratigraphic unit, a part of the geologic record or rock column that consists of defined rock strata. Groups are divided into formations and are sometimes themselves grouped into "supergroups"....

 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 central and northwest 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...

. The group consists of three formations, all from the Ypresian

Ypresian

In the geologic timescale the Ypresian is the oldest age or lowest stratigraphic stage of the Eocene. It spans the time between and , is preceded by the Thanetian age and is followed by the Eocene Lutetian age....

 and Lutetian

Lutetian

The Lutetian is, in the geologic timescale, a stage or age in the Eocene. It spans the time between and . The Lutetian is preceded by the Ypresian and is followed by the Bartonian. Together with the Bartonian it is sometimes referred to as the Middle Eocene subepoch...

 ages (early 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...

, 50 to 40 million years old). These formation have their shallow marine facies in common.

The three formations are:

• the Aalter Formation
  Aalter Formation
  The Aalter Formation is a geologic formation in the subsurface of northwest Belgium. The formation consists of marine clay and sand, deposited in the shallow sea thet covered northern and central Belgium in the Eocene epoch....
  
  , greenish 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 and 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, found in East-Flanders and West-Flanders.
• the Brussel Formation
  Brussel Formation
  The Brussel Formation is a geologic formation in the subsurface of central Belgium. The formation is named after the Belgian capital, Brussels. It consists of shallow marine sandstone and calcareous sands, deposited in the sea that covered Belgium 45 million years ago, in the Eocene.The Brussel...
  
  , marl
  Marl
  Marl or marlstone is a calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite. Marl was originally an old term loosely applied to a variety of materials, most of which occur as loose, earthy deposits consisting chiefly of an intimate mixture of clay...
  
  s and sands, found in Flemish
  Flemish Brabant
  Flemish Brabant is a province of Flanders, one of the three regions of Belgium. It borders on the Belgian provinces of Antwerp, Limburg, Liège, Walloon Brabant, Hainaut and East Flanders. Flemish Brabant also completely surrounds the Brussels-Capital Region. Its capital is Leuven...
  
   and Walloon Brabant
  Walloon Brabant
  Walloon Brabant is a province of Wallonia in Belgium. It borders on the province of Flemish Brabant and the provinces of Liège, Namur and Hainaut . Its capital is Wavre...
  
  , Hainaut and Namur
  Namur (province)
  Namur is a province of Wallonia, one of the three regions of Belgium. It borders on the Walloon provinces of Hainaut, Walloon Brabant, Liège and Luxembourg in Belgium, and on France. Its capital is the city of Namur...
  
  .
• the Lede Formation
  Lede Formation
  The Lede Formation is a geologic formation in the subsurface of Belgium. The formation is named after the town of Lede in East Flanders. It consists of shallow-marine limestone and sandstone, deposited in the former sea that covered Belgium during the Eocene....
  
  , calcareous sands, found in almost all central Belgium.

Stratigraphically on top of the Zenne Group are the late Eocene sands and clays 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....

, which is not divided into a group. The Zenne Group is almost everywhere found on top of the early Eocene Ieper Group

Ieper Group

The Ieper Group is a group of rock strata in the subsurface of northwest Belgium. The group is subdivided into three marine formations, all formed during the Ypresian, a single age of the geologic timescale...

.
(eds.); 2001: Paleogene and Neogene lithostratigraphic units (Belgium), Geologica Belgica 4(1-2), p. 135-152.