Lithostratigraphy

We can classify the layered rock sequences primarily on the basis of their lithological characters. This scheme of classification is known as Lithostratigraphic Classification.

• The smallest unit of a layered sequence is a lamina denoting a single layer with thickness of less than 1 cm.
• A bed is defined as a layer of homogenous lithology with layer thickness of >1 cm.

“A bed or lamina represents a uniform condition of deposition in a basin.”

• The basic unit of lithostratigraphic classification that is mappable on intermediate scale (1:50,000) of geological mapping is called as formation.
  • A formation is named after it’s type area.
  • Two formations can be separated either by a bedding surface or non depositional time known as hiatus.

A formation may comprise a single predominant rock type or an association of two or more rock types. the thickness may go up to few hundreds meters.

• A part of formation that has distinct lithology can be mapped as member that has relatively smaller thickness and aerial extent as comparable to the formation.
  • A member may have characteristic fossil assemblage that is distinct from that of formation.
• Two or more adjacent formations occurring in a rock succession can be combined to form a group.
• Groups can be combined to form a Supergroup.

———- Terminology ———-

Formal terms	Informal terms
Supergroup Group Formation Member Bed	Sequence Complex Marker bed Marker horizon