Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

V. Purnachandra Rao ¹, Ch. M. Rao ¹, A. Mascarenhas ¹, K. Mohan Rao ², N. P. C. Reddy ², H. C. Das ³

Affiliations
1 National Institute of Oceanography, Dona Paula 403 004, Goa, IN
2 National Institute of Oceanography, Regional Centre, 52, Kirlampudi, Layout, Visakhapatnam 530 023, IN
3 Oil India Limited, Duliajan, Assam 786 602, IN

Abstract

Sedimentological and geochemical investigations of the sediments in a core from the eastern continental margin of India, at a water depth of 1200m, revealed two distinct sediment types. The Late Pleistocene sediments are greyish-black in colour and consist of mud turbidites in the lower part and dolostone breccia in the upper part. They also consist of illite and chlorite-rich clay minerals with high organic carbon and sulfur. Organic matter is of dispersed nature. These are similar to black shales and are apparently formed by rapid burial of terrigenous organic matter by turbidites with intermittent reducing conditions during lowered sealeveIs. Dolostone fragments appear to have been transported by mass movement processes during the terminal pleistocene. The Holocene sediments: are moderate yellowish-brown in colour and with detrital vivianite nodules at the base. These sediments are clayey with mont-morillonite-rich, illite-poor clay minerals, lower organic carbon and sulfur contents and ferruginised pyrite grains, deposited under oxidising conditions. The marked changes in the sedimentary environments are attributed to climate and sealevel changes during Pleistocene and Holocene.

Keywords

Environment, Pleistocene, Holocene, Black Shales, Off Penner River, Continental Margin.

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A. S. Subrahmanyam ¹, G. P. S. Murty ¹, K. V. L. N. S. Sarma ¹, K. Mohana Rao ¹, N. P. C. Reddy ¹, M. M. Malleswara Rao ¹, V. Subrahmanyam ¹, P. Suneetha Rani ¹, A. Anuradha ¹, K. S. R. Murthy ¹

Affiliations
1 National Institute of Oceanography, 176, Lawson's Bay Colony, Visakhapatnam- 530 017, IN

Abstract

Earth tremors of low to moderate intensity (3.0 to 5.5) recurring over last century in and around coastal and shelf regions of Visakhapatnam has prompted us to acquirc and study the geophysical data between coastal regions of Kakinada and Kalingapatnam in western part of Central Bay of Bengal (17°N - 18°30'N) for understanding the recent seismicity of the region. Preliminary analysis of magnetic and shallow seismic data reveals the presence of some prominent land-Ocean tectonic lineaments, reactivation of which is considered to be responsible for tremors in the region. Total field magnetic anomaly map of the study area shows two major anomaly lineations such as: (a) N-S trending medium amplitude (˜250 nT) lineation off Visakhapatnam which is considered to be offshore fragment of converging point of two onshore lineaments. Also the lineament is associated with- five earth tremors of magnitude 3.7 to 4.3 at 40 m water depth. Impact of tremors is also observed in the overlying sedimentary strata in the form of vertical faults. (b) A NE-SW high amplitude anomaly zone (˜820 nT) off Kalingapatnam trending NE-SW, which is in close proximity with the epicentral location of earthtremors of 5.5 magnitude. This lineament can be spatially correlated to onshore NW-SE trending structural lineament indicating the probable reactivation of offshore segment resulting in the earth tremors Off Vizianagaram, reactivation of preexisting structural lineaments associated with the NW-SE trending Kandivalasa river fault may be the source of tremors at 20 m water depth. Magnetic basement contour pattern changes from NE-SW to NW-SE off Visakhapatnam. It may be inferred that the basement is bounded by two fault controlled lineaments passing through epicenters of earthtremors off Visakhapatnam and Vizianagaram. Basement structure shows down faulted basement (2 to 3 km) underlying the epicentral location along the lineaments trending NW-SE in the innershelf of visakhapatnam. Shallow seismic data also indicate steepiy dipping sub bottom fault in the study area of Visakhapatnam. South of Visakhapatnam, shallow seismic records indicate vertical faults and shearing reflectors in the lower sedimentary layers. From the deformed lower sedimentary layers. It may be considered that the region might have undergone seismic activity in the recent past, but presently there are no reported tremors in this region. Even undeforrned top sedimentary layers suggest lack of recent tectonic activity between Visakhapatnam and Kakinada in coastal and shelf regions. It may be summarized from the preliminary analysis of the geophysical data that the earth tremors observed north of Visakhapatnam are caused mainly due to reactivation of tectonic lineaments whereas south of Visakhapatnam it is relatively free from any seismicity. Based on the felt reports, tremors are attributed to shallow focus origin. Detailed investigations from multidisciplinary approach like geological, geomorphological, geophysical and remote sensing observations coupled with seismic information in the study area will throw more light on the micro seismic activity and related recent tectonics of this coastal region of east coast of India.

Keywords

Tectonic Lineaments, Magnetic anomalies, Microseismic activity, East coast, Andhra Pradesh.

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