Verdine and other associated authigenic (glaucony, phosphate) facies from the surficial sediments of the Southwestern continental margin of India

Abstract

Green grains, pale green-brown infillings of foraminifer tests and brown friable aggregates occur in the coarse fraction of the surface sediments from the southwestern continental margin of India, between the Periyar river in the north and Quilon in the south. Study of the nature, distribution, mineralogy and internal structure of these particles resulted in (1) the discovery of a verdine facies and (2) determination of the relationships of phosphate with glaucony and verdine in these sediments. Two distinct verdine facies associated zones, a shelf zone at about 40 m and a slope zone between 100 and 280 m water depth, are distinguished. Verdine grains are abundant (up to 30%) where biogenic debris dominates in the sediments and are rare (<5%) where terrigenous detritals dominate. On the continental shelf the verdine grains are dark green and predominantly occur as angular to subrounded grains, whereas on the slope they are represented by the coexistence of different types of pellets (dark green glossy and green to light green rugose pellets) and infillings. Scanning electron microscopy (SEM) studies indicate that the shelf grains are mostly homogeneous and the slope grains contain several detrital components. The most common nannostructure of the authigenic clays is the association of closely spaced contorted clay blades and globules. X-ray mineralogy suggests that these grains are a mixture of verdine dominated minerals. Phyllite C is the principal verdine mineral in the shelf zone. On the continental slope phyllite V dominates between 100 and 205 m water depth followed by phyllite C at about 280 m. Glaucony and phosphate predominantly occur as pale green to brown infillings of foraminifers and as phosphatic friable aggregates in the sediments of the terrace at 330 m water depth. Glauconitic smectite and carbonate fluorapatite are the respective mineral phases. The terrace sediments are sandy and often associated with non-glauconitized and non-phosphatized mollusc shells. Phosphatized verdine grains occur at 170 m water depth. Scanning electron microscopy studies indicate that phosphate is in the form of globular to rod-shaped bacteria-like structures or microbial filaments, or both. This type of phosphate occurs within the foraminifer tests and as cement in the friable aggregates. Glauconitized foraminifers are enclosed in phosphatic matrix and phosphate occurs within the glaucony matrix. The intergrowth of green clays and phosphatic globules is observed locally in phosphatized glaucony aggregates and also in phosphatized verdine grains. The formation of verdine, glaucony and phosphate in these sediments is relict. The age of their formation is difficult to estimate precisely, but appears to be older than mollusc shells dated at 5710 yrs B.P. Planktonic foraminifers from the terrace indicate a Quaternary age. Verdine formation on the shelf and on the slope is diachronous. The shelf verdine facies most probably formed during the Late Pleistocene high sea level stands or during the Early Holocene. The slope verdine may have formed contemporaneously with glaucony and phosphate on the terrace during low sea level stands. The terrace most probably acted as a sort of hardground facilitating glauconitization and phosphatization of the sediments. Upwelling, oxygen minimum zone and low terrigenous sediment accumulation provided adequate conditions for phosphatization which was mediated by microbial processes. The intergrowth of phosphate with the authigenic clays of verdine and glaucony may be due to the local confinement of the microenvironment, which was probably controlled by microorganisms.