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

K. S. R. Murthy ¹, T. C. S. Rao ¹

Affiliations
1 National Institute of Oceanography, Regional Centre, Plot No. 52, Kirlampudi Layout, Vishakapatnam 530 023, IN

Abstract

Loss of seismic reflections, termed as acoustic wipeouts, are observed in the high resolution sparker seismic records over the continental shelf and slope of Krishna, Godavari and Mahanadi river mouths, east coast of India. The wipeouts observed in the sub-surface layers beneath the outer shelf off these basins (150 to 250 m water depth) result in a chaotic appearance, probably indicating the presence of gas mixed sediments, while those observed over the continental slope (beyond 500 m water depth) are mostly associated with faulted contacts and uplifts and represent an abrupt weakening of seismic energy or break in the pattern of surface and/or sub-surface reflectors. Though acoustic wipeouts result due to a variety of reasons, their association with vertical faults and location at depth off major river basins suggest that some of these might be due to migration of oil/gas along the faulted contacts from increased depths.

Keywords

Geophysics (Seismology), Marine Geology, Continental Margin, East Coast of India, Krishna Basin, Godavari Basin, Mahanadi Basin.

Full Text

     

A. S. Subrahmanyam ¹, S. Lakshminarayana ¹, D. V. Chandrasekhar ¹, K. S. R. Murthy ¹, T. C. S. Rao ¹

Affiliations
1 National Institute of Oceanography, Regional Centre, Visakhapatnam - 530 017, IN

Abstract

Detailed analysis of magnetic and bathymetric data over shelf and slope regions of Cauvery basin, east coast of India. between Mahabalipuram and Vedaranyam demarcated three major offshore lineaments trending N-S, NE-SW and E-W. The N-S lineation has been interpreted as due to dyke intrusions. NE-SW lineament reflects the offshore extension of a major basement depression, viz., the Pondicherry depression. E-W lineation, south of Porto Novo reveals a basement high suggesting the seaward extension of Kumbakonam ridge within the Pondicherry depression. The present analysis gives mosaic of a deep seated graben block flanked by rift related volcanic intrusives over the shelf region of the basin.

Keywords

Structure, Marine Geophysics, Cauvery Basin.

Full Text

     

K. Mohana Rao ¹, T. C. S. Rao ¹

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

Abstract

The Holocene sea level changes in the shelf areas off Visakhapatnam was studied from sediment distribution pattern and shallow seismic profiling. Morphological features on the shelf indicate a Late Pleistocene regression down to about -130 m below the present sea level. A major stillstand of the transgressive sea was recorded at about -80 m where coarse carbonates and shallow water fauna predominate. During this period algal barrier reefs with reliefs varying from 2 to 8 m were developed between -85 m (10790 ± 170 years B.P.) and -100 m (12530 ± 170 years B.P.). Buried reefs observed at about -60 m give a clue for a possible strandline at this depth. Coarse clastic sand patch having mean size of around 1.0 Phi between -20 to -30 m suggests another strand line. The sea level rise during Holocene transgression seems to be discontinuous with temporary pauses.

Keywords

Sea Levels, Holocene, East Coast of India.

Full Text

     

K. S. R. Murthy ¹, M. M. Malleswara Rao ¹, K. Venkateswarlu ¹, A. S. Subrahmanyam ¹, S. Lakshminarayana ¹, T. C. S. Rao ¹

Affiliations
1 Regional Centre, National Institute of Oceanography, 176, Lawson's Bay Colony, Visakhapatnam-530017, IN

Abstract

Magnetic data of the eastern continental margin of India (ECMI) helped in demarcating the offshore extension of the granulite belts of east coast of India and their possible link to those of the East Antarctica. Short wavelength and high amplitude negative magnetic anomalies (200 to 1600 nT) are observed over the northern part of ECMI between Kakinada and Chilka lake and also off Madras shelf, the coastal regions of which are associated with granulite facies rocks . This part of the shelf is associated with structural lineaments with predominant NE-SW trends . In contrast, the offshore basins off Cauvery, Krishna - Godavari (K-G) and Mahanadi rivers are associated with subdued positive anomalies (less than 200 nT) indicating subsidence off these basins by heavy load of Phanerozoic/younger shelf sediments. Structural lineaments with NW-SE or W-E trends delineated over these basins are related to the offshore extension of these rift related Gondwana grabens. Similarity in the trends of the magnetic anomalies and the granulite facies on the east coast of India and their correlation with the granulite belt of Enderby Land of Antarctica supports the view that the entire region comprised of a single and stable block of granulite rocks prior to the fragmentation of the Gondwanaland. The study also suggests probable continuation of Mahanadi and Godavari grabens into East Antarctica.

Keywords

Marine Geophysics, Magnetic Anomalies, Eastern Ghat Granulite Belt, Enderby Land , Antarctica.

Full Text