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

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.

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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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