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

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.

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