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P. C. Bandopadhyay ¹, Utpal Chakrabarti ¹, N. R. Mohapatra ¹, Abhinaba Roy ¹

Affiliations
1 Geological Survey of India, Eastern Region, DK-6, Salt Lake City, Kolkata - 700 091, IN

Abstract

This report documents ground uplift of a part of the eastern coast of North Andaman Island during the December 2004 earthquake. Emergence of a part of fringing coral reef above the high tide level has been recorded from the Kahpur beach. The newly emergent part of the coral reef is -75m long, -25m wide and trends ESE paralleling the trend of the shore line. The study records appearance of a -600m long and 200m wide tombolo joining the Smith Island to the Ross Island off the eastern coast of North Andaman Island. Before the December 2004 earthquake it was a sand bar that emerged only during the low tides. Vertical movement of 0.4m is calculated for the Kahpur beach while that between the Smith and Ross Islands is 0.7-0.8m.

Keywords

Coral Reef, Tombolo, December 2004 Earthquake, Kahpur, Smith Island, Ross Island, North Andaman Island.

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Tapan Pal ¹, P. C. Bandopadhyay ¹, S. K. Mitra ¹, S. Raghav ¹

Affiliations
1 Geological Survey of India, Salt Lake, Kolkata - 700 091, IN

Abstract

The 2005 eruption of Barren Volcano was explosive and pulsative Strombolian type (15-30 seconds interval). A thick column of grey smoke along with red-hot pyroclasts was found ejecting out with rumbling sound from the crater. The eruption column of ash particles attained a height of more than 300 m, whereas larger pyroclasts were ejecting out to about 100 m above the crater. The assorted mixture of air fall tephra in the form of blocks, cinders, lapilli and spatter along with ash formed two cinder cones. Pyroclasts are of basaltic composition and show vitrophyric texture with phenocrysts of plagioclase, olivine and diopside set in a glassy groundmass. EPMA of the mineral phases shows that plagioclase phenocrysts are represented by anorthite (An 94.5) and bytownite (An8 1.6-89.0),whereas in the groundmass, feldspar microlites are labradorite (An57.3). Compositionally olivines are forsteritic (Fo72-79), and the clinopyroxenes as phenocrysts are diopside and clinopyroxenes in groundmass as pigeonite. Major oxide analyses of the bulk pyroclastics show that the basalts are high alumina basalt of Island Arc Tholeiitic (IAT) affinity Mineralogical as well as chemical similarities of these basalts with lavas of earlier eruptions from Barren Volcano suggests no major change in magmatic evolution with time in the inner arc setting of Andaman subduction complex.

Keywords

Pyroclast, Ash cloud, Strombolian type, Cinder cone, High alumina basalt, Vitrophyric, Island arc tholeiite, Barren Island, Andamans.

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P. C. Bandopadhyay ¹, Utpal Chakrabarti ¹, Abhinaba Roy ¹

Affiliations
1 Geological Survey of India, Eastern Region, Salt Lake City, DK-6, Kolkata - 700 091, IN

Abstract

The Namunagarh grit of Eocene age in South Andaman island is a submarine fan deposit where sedimentation occurred in deep marine environment mainly by turbidity currents. The mudstone and fine-grained sandstone of the turbidite sequence yielded three ichnogenera viz Thalassinoides, Teichichnus and Lorenzinia.

Typically shallow marine Thalassinoides burrows co-occur with deep water burrows represented by Teichichnus and Lorenzinia, indicating a dominant control of substrate, availability of food and rate of sedimentation against bathymetry on distribution of trace fossils. Trace fossils are particularly abundant in the mudstones pointing to a strong lithofacies control. A well-oxygenated muddy bottom with a low rate of sedimentation is envisaged.

An order in the succession of traces could be deciphered. Simply constructed burrows of Thalassinoides appeared initially followed by complex structures of Teichichnus and ornate Lorenzinia.

Keywords

Trace Fossils, Eocene, Namunagarh Grit, South Andaman Island.

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P. C. Bandopadhyay ¹, M. Ghosh ¹

Affiliations
1 Petrology Division, Eastern Region Geological Survey of India, Calcutta, IN

Abstract

The Tertiary sedimentary rocks around Port Blair, South Andaman are classified into several facies and constraints on their formational environments are established.

The Oceanic Pelagic Sediments ( OPS ) are made up of bedded chert and variegated shale facies. Radiolarian tests and microcrystalline quartz are major constituents of the chert. The variable mixture of silt-size quartz, feldspar, clay minerals, pyroclacstic fragments and outosized terrigenous clasts constitute the variegated shale. The OPS were formed in continental-margin orogenic basin through suspension settling and fine grained sandy debris flow in deep water.

Paraconglomerate, tuff and mudstone constitute three major facies of the Mithakhari Group. Pebble size subrounded to rounded, disorganised and polylithologic clasts, in matrix support characterize the conglomerate facies. Pyroclastic facies are coarse grained and unstratified tuff and fine grained and well stratified tuff. They are crystal-rich tuffs of basic to intermediate composition. Massive mudstones interbedded with conglomerate and tuff are geochemically similar to sedimentary mudrocks. Conglomerates represent debris flow deposit formed in submarine channel. The tuffs are interpreted as pyroclastic debris flow deposit and primary air-fall ash turbidites. Mudstones indicate deposition of pelagic mud from basinal water.

The Andaman Flysch comprises quartzwacke sandstone and dark shale facies. Bouma sequences are conspicuous in sandstone-shale rhythmites. Angular and assorted detrital grains that constitute the framework components and also contributed to the formation of matrix, exhibit diffuse and corroded grain outlines and other textures indicative of burial diagenesis. The Andaman Aysch is interpreted as proximal turbidites formed in mid-fan environment by a combination of sandy debris flow deposition and pelagic sedimentation adjacent to an uplifted source terrane.

Keywords

Petrology, Sedimentary Rocks, Facies, Port Blair, Andaman.

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