In the West Siang district of Arunachal Pradesh, geologists have found an exposed linear belt of Gondwana sandstones. Their marginal position in the Gondwanic Indian Continent drew the attention of geologists at the Dibrugarh University. When were these sandstones deposited? What were the climatic conditions and tectonic setting when they were deposited?
Ranjan K. Sarmah and Tapos Kumar Goswami from the Department of Applied Geology teamed up with Bashab N Mahanta, from the Geological Survey of India, to conduct a field survey in the region. They took topographic maps and started exploring the area to systematically collect surface sandstone samples from the region.
Image: Bashab N Mahanta
Back in their lab, they prepared thin sections of the samples and studied their mineral compositions under a petrographic microscope. The sandstones seemed to be predominantly composed of quartz, a hard crystalline mineral made of silicon dioxide. The samples contained some other minerals in minor proportions. Feldspar, mica, very fine sized particles and some fragments of rock were all mainly cemented together by binding material rich in iron and clay.
“Based on our field and laboratory evidence, we feel that these sandstones are possibly of marine origin”, says Tapos Kumar Goswami, Dibrugarh University.
Low amounts of feldspar in the sandstones suggested they came from a recycled source. The team analysed the properties of the sandstones using various standard tectonic discrimination diagrams and found that the sandstones cluster in the recycled zone with low grade metamorphic field.
“This suggests that the sandstones had formed at relatively low pressures,” says Ranjan K. Sarmah, Dibrugarh University.
“The Gondwana sandstones in Siang district come from a quartz-rich transitional recycled orogen and were finally deposited in a humid marine condition. In this region, sediments derived from tectonically active highlands, which have undergone low grade metamorphism after rock formation”, says Bashab N. Mahanta, Geological Survey of India.
The humid climate during the deposition of Gondwana sandstones can be explained by the Indian continent’s position at a lower latitude at that time and the overall change in global climate.
Joya Moni Mout
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