Nathani Basavaiah

Physicochemical analyses of surface sediments from the Lonar Lake, central India - implications for palaeoenvironmental reconstruction

We report the results of our investigations on the catchment area, surface sediments, and hydrology of the monsoonal Lonar Lake, central India. Our results indicate that the lake is currently stratified with an anoxic bottom layer, and there is a spatial heterogeneity in the sensitivity of sediment parameters to different environmental processes. In the shallow (0-5 m) near shore oxic-suboxic environments the lithogenic and terrestrial organic content is high and spatially variable, and the organics show degradation in the oxic part. Due to aerial exposure resulting from lake level changes of at least 3m, the evaporitic carbonates are not completely preserved. In the deep water (>5 m) anoxic environment the lithogenics are uniformly distributed and the delta C-13 is an indicator not only for aquatic vs. terrestrial plants but also of lake pH and salinity. The isotopic composition of the evaporites is dependent not only on the isotopic composition of source water (monsoon rainfall and stream inflow) and evaporation, but is also influenced by proximity to the isotopically depleted stream inflow. We conclude that in the deep water environment lithogenic content, and isotopic composition of organic matter can be used for palaeoenvironmental reconstruction.

Indication for clockwise rotation in the Siang window south of the eastern Himalayan syntaxis and new geochronological constraints for the area

Abstract Palaeomagnetic, rock magnetic and geochronological investigations were carried out on the Abor volcanics of Arunachal Pradesh, NE India. A Late Palaeozoic formation age for part of the Abor volcanics cannot be excluded based on K–Ar whole rock dating. Low-temperature thermochronometers – zircon (U–Th)/He and fission track analyses – yield a maximum burial temperature of c. 150–170 °C during Late Miocene. ZFT thermochronology of the Yinkiong and Miri Fms. indicates a post-Paleocene and post-Jurassic deposition age, respectively. This infers that the volcanic rocks intercalating or intruding them are not part of the Late Palaeozoic sequence but represent one or more, latest Cretaceous to Tertiary event(s). Therefore the Abor volcanics are connected to at least two separate events of volcanism. From palaeomagnetic sites, two characteristic magnetic remanence components were separated: a low-coercivity-component demagnetized below 20 mT and a high-coercivity-component demagnetized between 15 and 100 mT. Fold tests support a secondary origin of both components. Thermochronological and rock magnetic analyses indicate a low-grade overprint event between India–Asia collision and Miocene, which probably represents the time of remanence acquisition. The high-coercivity-component shows a trend of clockwise declinations, which is likely related to vertical-axis rotations of the eastern Himalayas due to eastward extrusion of the Tibetan Plateau.

Solid Earth Geomagnetism

One of the fundamental issues in geosciences relates to formation and evolution of the crust, whose understanding has increased many-fold with advanced geophysical techniques. The geological history spans from Archaean, 3.8 Giga years (1 Giga=109), to the present (Neogene). Geophysical methods and techniques investigate the structure, composition and physical state of the Earth by mapping crustal anomalies associated with mineral deposits, structural features, and hydrocarbons. Since these constituents lie hidden beneath the surface, geophysics has become a preferred tool of exploration compared to geological techniques. The present section deals with crustal anomalies, and deliberates on their form and detection using appropriate geophysical procedures.