The type Magwa Formation is divisible into two units, a lower argillaceous limestone with intercalating shaly and marly bands, and an overlying reefal limestone. These units are equated with the types Rumaila and Mishrif, respectively, which suggests their inclusion as two members within the Magwa Formation. Hence the validity of the term "Magwa Formation" is substantiated, and previous calls for its abandonment are rejected.

A study of the Rumaila Limestone Member of the type Magwa Formation in thin section has led to the recognition of three distinctive microfacies associations: a lower pyritic, argillaceous limestone with abundant arenaceous Foraminifera and sparse planktonic forms; an intermediate association of mixed character; and an upper association of skeletal, micritic limestone with pyritic shale partings, abundant plankton, and sparse benthonic Foraminifera. This assemblage prompts the postulate of a quiet to slightly agitated, infralittoral to circumlittoral marine environment for the deposition of that part of the type Magwa, increasing gradually in depth toward its upper part. Intermittent partial restrictions from the open sea led to temporary anoxic anaerobic conditions. Middle to early la e Cenomanian age of the Magwa Formation is substantiated.