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.

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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.

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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.

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T. V. Ramana Murty ¹, K. Mohana Rao ¹, M. M. Malleswararao ¹, S. Laksrminarayana ¹, K. S. R. Murthy ¹

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

Abstract

Earlier studies on grain-Size analysis of sediment samples collected over the innershelf of Gopalpur coast have revealed two signifiCant depositional environments viz. beach and dune. Sediment samples from water depth of <15 m show the representation of dune type of environment and the samples beyond 15 m in the study area are characterised of beach type of environment suggesting submergence of two physiographic units of beach and dune due to Holocene transgression. Empirical Orthogonal Function (EOP) analysis carried out for the above units on grain-Size distribution of the sediments in the Φ-size class revealed that the first three Φ-dependant eigen function viz. c₁(Φ), c₂ (Φ) and c_{3 (Φ) account for more than 95% of variance predominantly represented by the first - dependent function c1(Φ) alone. This first function c1(Φ) of deeper sediments (15-30 m depth) contribute 76.34 % of the total distribution representing coarser sands, in general, and characterized by predominance of very coarse sand to the extent of 40%. Shallow sediments (0 - 15 m depth) contribute 70% of the distribution-representing medium to fine sand with the dominance of 40% of fine sand. This eigen function c1(Φ) showed significant differences in the concentration of grain size particles across the innershelf reflecting the change in the energy condition that prevailed in the area during the emergence of the innershelf.}

Keywords

Sediment Characteristics, Innershelf, Depositional Environment, EOF Analysis, Gopalpur, Orissa Coast.

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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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K. S. R. Murthy ¹

Affiliations
1 National Institute of Oceanography Regional Centre Visakhapatam- 530 017

Abstract

No Abstract.

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K. S. R. Murthy ¹

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

Abstract

No Abstract.

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K. S. R. Murthy ¹

Affiliations
1 Lawsons Bay Colony, Visakhapatnam - 5300 17, IN

Abstract

No Abstract.

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