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

P. S. N. Murty ¹, G. P. Mohapatra ¹, D. R. S. Reddy ¹, M. Hariprasad ¹

Affiliations
1 Geological Survey of India, Marine Wing, East Coast Division-II, Visakhapatnam-530 023, IN

Abstract

The bathymetry contours show that the morphological features of shelf and slope off Visakhapatnam coast are mainly due to erosion and local deposition, while those of Godavari delta are due to high rate of deposition. Features such as terraces, pinnacles and ridges on the continental shelf at different depths are inferred to be vestiges of palaeostrand lines caused by sea level changes during the Pleistocene/Holocene period. The mass wasting near the shelf break off Godavari River could have led to the levelling up of the rise area with the slope giving rise to a smooth and cancave surface. The dissected topography of the northeastern part of the continental slope is inferred to be the effect of erosion and mass movement due to turbidity currents. The abyssal plain is influenced both by the peninsular rivers draining into the area as also by the Bengal Fan. The bathymetric surveys in abyssal plain delineated the boundary between western sedimentary basin influenced by peninsular river sediment and the eastern sedimentary basin having Bengal Fan sediment. The channels and valleys observed on the eastern side of this boundary are the continuation of Bengal Fan channels.

Keywords

Bathymetry, Morphological Features, Bengal Fan Channels.

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G. Gaitan Vaz ¹, G. P. Mohapatra ¹, M. Hariprasad ¹

Affiliations
1 East Coast Operation II, Marine Wing, Geological Survey of India, 41, Kirlampudi Layout, Visakhapatnam - 530 017, IN

Abstract

Red sediments occur as three shore linear patches between Bavanapadi and Ichchapuram in north Andhra Pradesh coast. The unstructured red sediment lying over an antecedent topography is divisible into lower unit with calcretes and upper unit devoid of calcretes. The field observations, grain size parameters, heavy mineral assemblage and SEM studies of quartz grains signify an aeolian source, possibly formed by sand transport from the exposed continental shelf during the Quaternary regressive cycles. Formation of calcrete and red pigments are considered as post depositional phenomena. SEM studies of ilmenite, XRD and chemical studies of finer fractions reveal the in situ derivation of red haematite and goethite pigments from the alteration of ilmenite and precipitated interstitially to give rise to red colour.

Keywords

Palaeoenvironment, Red Sediments, Quaternary, Aeolian, Ilmenite Alteration, Andhra Pradesh.

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P. S. N. Murty ¹, M. Rama Murty ¹, G. P. Mohapatra ¹

Affiliations
1 Geological Survey of India, Marine Wing, Op East Coast-II, Marripalem, Visakhapatnam - 530 018, IN

Abstract

Lime mud occurs within 100-225 in isobath in the outer shelf along the East Coast of India, Bay of Bengal off Kakinada-Bheemunipatnam coast. A thin capping of Non-Carbonate clay/mud varying from 10-200 cm overlies the lime mud. The capping is thicker in the peripheral zone Lime mud is greyish white to creamy white resembling plaster of Pans and comprises minor ooids, pelloids, mud aggregates, foraminiferal tests and few terrigenous detrials. The maximum thickness of lime mud probed is 2 89 m.

Texturally, lime mud is sandy silt within top one meter subsurface level followed by silt up to 2 83 m subsurface depth. Chemical data show a maximum concentration of CaO -52 08% (CaCO₃ - 92 95%), Sr 0 66% and Li- 500 ppm. The XRD data show that the major constituent mineral is aragonite (85-90%) followed by Mg - calcite/calcite together with or without halite. The aragonite crystals are typically stubby and bladed with irregular outline and pointed terminations under SEM Textural, mineralogical and chemical parameters and coarse fraction studies suggest that lime mud might have been formed in shallow water environment as chemical precipitate during late Pleistocene/early Holocene and subsequently transported to deeper parts of the shelf.

Keywords

Lime Mud, Calcium Carbonate, Outer Shelf, Aragonite, Strontium, Chemical Precipitate, Visakhapatnam Coast, Andhra Pradesh.

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