Abstract

A distinctive and widespread belemnite-bearing shale of the Tethyan region is well exposed in the Trans-Indus Salt Range as the Chichali Formation, where it contains two workable ironstone seams. Facies analysis indicates a pronounced deepening in the Upper Oxfordian and Kimmeridgian followed by two regressive cycles marked by a transition from glauconitic–phosphatic shales to glauconitic silts and sands and finally to winnowed and oxidized glauconitic sandy ironstones. The first cycle ends in Berriasian belemnite-rich beds; the second cycle, which has few belemnites, extends into the Valanginian, and is followed by deltaic sediments of the Lumshiwal Formation.

Comparison with other areas indicates that the source area for the clastic components was the Indian Shield to the east and south. The increased clastic supply in the Upper Jurassic–Lower Cretaceous is thought to result from tectonic uplift in this source area rather than from climatic change.

The ironstone beds are distinctive in being pelletal glauconites compared with the oolitic chamosite and goethite so typical of the Jurassic elsewhere. Enrichment of the glauconite to commercial grade was by winnowing and oxidation on the seafloor. The absence of oolitic chamosite and goethite could be related to generally deeper water and an overall Eh in the environment too high to allow iron to migrate in appreciable quantities as Fe2+.

First Page Preview

First page PDF preview
You do not currently have access to this article.