Phosphorite from the Continental Margin off Madras, Bay of Bengal

Abstract

A recent survey reveals that phosphorites occur over an area of 800 km2 between 100 and 500 m depths on the outer continental shelf and upper slope ESE off Madras. Appreciable subsurface continuity of phosphorites is not indicated. They mostly occur over two terraces recorded between the 100 and 200, and 350 and 400m isobaths. Sampled phosphorites may be classified under four types: (1) dense conglomeratic phosphorites in the size range 2 5 cm, (2) dark gray phosphatic nodules in the size range 1 3 cm, (3) creamy white calcareous algal nodules with phosphatic laminations in the size range 0.5 3 cm, and (4) recent gastropod shells with phosphatic studs. Conglomeratic phosphorites confined mostly to the upper terrace were recov ered along with calcareous pelecypod and gastropod fossils belonging to Cretaceous to Oligocene age. Gray phosphatic nodules occur over upper as well as lower terraces. The algal nodules are mostly confined to the upper terrace and are less abundant. Acoustic profiles (3.5 kHz) and the recovery of fossils indicate that the upper terrace is a karstic surface of a carbonate platform, probably of Cretaceous to Oligocene age. Petrographic study of the conglomeratic phosphorite shows skeletal matter set in phosphatic matrix and what appears to be the phosphatized limestone. Selective phosphatization of foraminiferal tests is common. Dark gray phosphatic nodules show brownish-gray laminations containing phosphatized skeletal fragments set in phosphatic matrix. Creamy white calcareous algal nodules exhibit alternate gray phosphatic and white calcitic laminations. All the phosphorite types are composed of carbonate fluorapatite and calcite as major minerals, followed by quartz and aragonite. Phosphorous pentoxide content of conglomeratic type varies from 15% to 20%, dark gray nodules from 16% to 19.98%, and calcareous algal nodules from 8.10% to 17.54%. The significant enrichment of Mo and Pb in phosphorites is attributed to their fixation in organic matter under reduced condition. SEM studies indicate a microbial origin for the gray phosphatic and algal nodules. Studies suggest four episodes of phosphatization with intervening reworking and redoposition, probably commencing from the Eocene period. Phosphatization during the late Pleistocene and early Holocene period also can be ruled out.