Ooid fabric in the Jurassic of the Indus Basin, Pakistan: control on the original mineralogy

The present study provides a petrographical and geo-chemical analysis of the coated carbonate grains from the early-middle Jurassic strata of the Indus Basin, Pakistan. The emphasis is on the depositional fabric, original carbonate mineralogy and seawater chemistry of the recovered ooids. These coated grains are believed to have formed in a high-energy depositional setting in grain shoal to peritidal settings on a platform interior. The original mineralogy is preserved, with an overprinted mineralogy observed in a few samples. Three different microfabric types are reported, including (1) finely laminated tangential concentric ooids showing original mineralogy, (2) brickwork and moldic ooids delineating the intermediate stage and (3) micritized and leached ooids representing the overprinted mineralogy. Changes in Sr, Mn, Fe and Mg contents might have occurred during the formation of ooids, which asserts that the ooids are mono- to bi-mineralic in chemical composition. Alongside ooids, oncoids had also widely developed around the skeletal and non-skeletal allochems, which served as their nuclei.