Glauconite in the Subathu Beds (Eocene) of the Simla Hills of India

Abstract

AUDEN¹ described the geology of the Pre-Tertiary and Tertiary formations of the Simla Hills and mentioned in passing the occurrence of glauconite in the Subathu beds. No detailed study of the mineral has so far been made. A petrographical investigation of the Subathu rocks of the type area (lat. 30° 58′ N., long. 77° E.), which is still in progress, has revealed that there are several glauconitic horizons in the Subathu Series. In the type area, the Subathus have thicknesses of about 2,500 ft. and are lithologically comprised of limestones, including coquinites, calcareous sandstones and shales which are carbonaceous at the base. Glauconite has so far been found to occur in the limestones and calcareous sandstones. These are fine to medium grained rocks, greyish in colour and at places traversed by secondary veins of calcite. Under the microscope they show variable amounts of clastic constituents—quartz, feldspars, rock fragments (limestones and quartzite) and accessories like glauconite, rutile, zircon, tourmaline, pyrite, chlorite, micas and iron oxides embedded in a calcareous matrix. Among the organic constituents, microfossils (tests of foraminifera) are quite abundant.