Isalo III Formation

The Isalo III Formation is a geological formation in Madagascar, off the eastern coast of Africa. It dates back to the Middle Jurassic.^[1] The use of the term "Isalo III" is somewhat controversial as the two prior units Isalo I and II are Triassic cross-bedded sandstone units that form a continuous depositional sequence, while the "Isalo III" sandstones are not part of the same depositional sequence, and were deposited considerably later. and are perhaps better treated as part of several separate formations.^[2] It is traditionally divided into two subunits the lower, Bajocian aged Isalo IIIa unit also known as the Beronono Formation and the upper, Bathonian aged Isalo IIIb unit also known as the Sakaraha Formation or Sakahara Formation.^[3] The Sakaraha Formation consists of sandstones, marls and carbonates and represents a coastal plain environment, and is laterally equivalent to the predominantly carbonate Bemaraha Formation, which represents a coastal barrier lagoon complex.^[2] The formation is found in the northwest and in the southeast of the country and has provided a variety of fossils.

TypeGeological formation

Sub-unitsIsalo IIIa or Beronono Formation
Isalo IIIb or Sakaraha Formation

OverliesIsalo II Formation (unconformity)

PrimarySandstone

Quick facts Type, Sub-units ...

Isalo III Formation
Stratigraphic range: Bajocian-Bathonian ~168–165 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Type	Geological formation
Sub-units	Isalo IIIa or Beronono Formation Isalo IIIb or Sakaraha Formation
Overlies	Isalo II Formation (unconformity)
Lithology
Primary	Sandstone
Other	Claystone, marl
Location
Coordinates	16.6°S 47.0°E / -16.6; 47.0
Approximate paleocoordinates	23.9°S 25.1°E / -23.9; 25.1
Region	Mahajanga & Fianarantsoa Provinces
Country	Madagascar
Extent	Mahajanga Basin

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