Tectonically-controlled oolite shoals from the Najwah formation, upper Jurassic, Iraq

Abstract EN

Oolite shoals were developed during the Middle-Upper Jurassic in Iraq.

These sediments were assigned to the Najmah Formation which represents a potential petroleum reservoir in the area.

It is suggested that these shoals were developed in parallel to the shoreline, on the eastern margin of the hinge line separating the Stable Shelf of the African-Arabian Platform from the Unstable Mesopotamian Foredeep.

This line is a NW-SE trending zone with distinctive physiorgraphic and geophysical characters.

It seems that the tectonic and topographic configuration of this zone created favorable conditions necessary for the formation of oolites.

Channels were probably flowing from the relatively higher Platform toward the lower land in die east.

In dry seasons, such channels became the sites of favorable hydrodynamic and chemical conditions that facilitated oolite formation.

As a result, oolite shoals were initiated at the mouths of these channels.

Currents and waves transported and redistributed these sediments along the hinge line forming narrow, longitudinal, shoals.

These shoals were made of ooliths showing extensive variations in shape and sorting.

They include normal (Alpha type).

Beta, Spherulites and Grapestone.

Some of them showed well-developed micritic elongation and micritization.

These features may suggest that ooliths were transported to a quiet environment after their formation.