Re-interpretation of the age and environment of deposition of Paleogene turbidites in the Andaman and Nicobar Islands, Western Sunda Arc
Highlights
► Palaeogene turbidites of Andaman and Nicobar Islands are examined and reinterpreted. ► Ophiolite and arc-derived volcaniclastic and continent-derived siliciclastic sandstones. ► Deposition in different tectonic and sedimentary environments. ► Revise the tectonic/depositional model of this part of Western Sunda Arc.
Introduction
The Andaman and Nicobar Islands record thick and extensive developments of Paleogene turbidites, described as true turbidites and fluxoturbidites (Karunakaran et al., 1964a, Karunakaran et al., 1964b, Karunakaran et al., 1968a, Karunakaran et al., 1975). These authors described the Bouma division-bearing fine to medium grained and unfossiliferous sequences of greywacke sandstone-shale alternations as true turbidites and the fossiliferous, mostly massive bedded pebbly (grits) and coarse to medium grained lithic sandstones rich in ophiolitic detritus as fluxoturbidites. Karunakaran et al. (1964b) suggested that the fluxoturbidites (Namunagarh Grit of the Mithakhari Group, Table 1) were mostly derived from sea mounts and guyots situated within the ocean basin at its origin, whereas true turbidites (Andaman Flysch, Table 1) were derived from beyond the borders of the ocean and were transported and deposited by the turbidity currents. Field mapping and stratigraphic studies since the early sixties (Karunakaran et al., 1964a, Pawde and Roy, 1964, Chatterjee, 1964, Chatterjee, 1967, Pandey et al., 1992) established an upper Eocene–Oligocene age for the Andaman Flysch and a Late Palaeocene to upper Eocene age for the Mithakhari Group, consistent with the depositional ages constrained for the Paleogene formations by isotopic dating of terrigeneous and volcanogenic minerals (Allen et al., 2008). The Mithakhari Group (=Baratang Group; Pandey et al., 1992) is most abundant in North and Middle Andaman Islands (Fig. 1) whereas the Andaman Flysch is predominant in the South Andaman and Great Nicobar Islands.
Recently Chakraborty and Pal (2001), observing the apparent resemblance between the turbidites exposed on the Kalipur–Shibpur coast, North Andaman and at Corbyn’s Cove, South Andaman Islands, identified the Kalipur turbidites as Oligocene-aged Andaman Flysch. They suggested that the gritty and coarse grained channelized sandstones of the Kalipur turbidites are proximal inner fan, and medium to fine grained unchannelized greywacke sandstones of the Corbyn’s Cove turbidites, distal mid fan facies of an Oligocene forearc fan. Several subsequent papers dealing with the soft sediment deformation of turbidite successions (Chakraborty and Mukhopadhyay, 2003), geodynamic evolution of the Andaman arc (Pal et al., 2003) and the statistical analysis of bed thickness patterns in turbidite successions (Chakraborty et al., 2002, Mukhopadhyay et al., 2003) identified the Kalipur–Shibpur turbidites as Oligocene Andaman Flysch. This obviously necessitates reinvestigation of the turbidites of the Andaman Islands. Curray and Allen (2008) stated that the turbidites in the North and South Andaman Islands are not necessarily the product of the same fan, since the fan facies interpretations and palaeocurrent directions (Chakraborty and Pal, 2001) appear incompatible.
This study aims to provide an improved geological account of the Andaman and Nicobar Islands based on the author’s several years of field study, and to characterize the Paleogene turbidites, based on detailed examination of key sections. Turbidites outcrops on the Kalipur–Shibpur, Ramnagar and on Rampur coasts (Fig. 1), at Corbyn’s Cove and in Great Nicobar Island provide an opportunity to document the different kinds of turbidite. This study establishes that the turbidites on the Kalipur–Shibpur coast and also in adjoining areas (Fig. 1) are part of the Mithakhari Group, based on field, petrographic and palaeontologic evidence, providing new insights into the Paleogene turbidite sedimentation in the Andaman arc and is proved to be useful in recognition, correlation and interpretation of the different types of turbidites incorporated into the subduction complex (Dickinson, 1982, Bailleul et al., 2007).
Section snippets
Tectonic setting
The north–south trending arcuate chain of Andaman and Nicobar Islands in the Northeast Indian Ocean provides an important physiographic/tectonic link between the Indo-Burma Ranges to the north and the outer arc–forearc Islands of Simule, Nias, Mentawai, offshore of Sumatra to the south, forming the Western Sunda Arc (Fig. 1) (Curray and Allen, 2008). An inner volcanic arc and the Andaman Sea back arc basin with a complex history of evolution (Curray, 2005) lies to the east of the Andaman and
Geology, stratigraphy and age
The geology of the Andaman and Nicobar Islands comprises deformed and thrust units of pre-ophiolitic rocks, Late Cretaceous ophiolite, Paleogene turbidites and relatively less deformed Neogene shallow marine mixed siliciclastic-carbonate sediments with reworked tuffs (Table 1). The Paleogene turbidites consist of siliciclastic and volcaniclastic sandstones and are contained within the two contrasting geologic domains referred to here as chaotic and coherent terranes. A major thrust known as the
Turbidite deposits
The volcaniclastic turbidites are exposed all along the east coast of the North and Middle Andaman Islands and occur as; (1) north–south striking and easterly dipping coherent units, (2) chaotically deformed sandstone–mudstone and, (3) interbedded/intercalated sandstone–fossiliferous limestones. Siliciclastic turbidites are best exposed in and around Corbyn’s Cove, South Andaman Islands (Fig. 1). All the gravity flow deposits described in this study are not uniquely turbidites but belong to the
Kalipur–Shibpur, North Andaman
The framework compositions of the volcaniclastic turbidites range from predominantly feldspathic volcanolithic to lithic-poor quartzofeldspathic arkosic sandstones that characterize the sandstone compositions of subduction complexes (Dickinson, 1982, Underwood and Bachman, 1986, Critelli and Ingersoll, 1995). The vitric, microlitic and lathwork grains (Critelli and Ingersoll, 1995) of basalt and andesite of neovolcanic origin, a few rounded felsitic grains of paleovolcanic origin and the whole
Re-interpretation of the Andaman Flysch, North Andaman Island
The melange terrane in the North and Middle Andaman Islands indicates an accretionary setting for the turbidite deposits contained therein. The attributes of the coherent Kalipur turbidites, previously described, suggest deposition in a proximal fan fed by transverse channels within shallow basins perched on the accretionary slope. The mud-rich sandstone-shale turbidites, showing accretion-related deformation described in Section 4.2, represent offscraped trench-fill deposits formed at the toe
Broader implications
The Paleogene sedimentation history of the Andaman and Nicobar Islands reveals that the Oligocene-aged siliciclastic turbidites were deposited as part of submarine fan on an open ocean floor, while volcaniclastic turbidites of the Late Palaeocene–Eocene age were deposited in slope basins on an accretionary complex, before being accreted into the forearc ridge. Deposition of siliciclastic turbidites (Andaman Flysch) in the fore arc basin (Chakraborty and Pal, 2001, Pal et al., 2003) is
Conclusion
The Kalipur–Shibpur sandstone turbidites in North Andaman Island are not Oligocene-aged Andaman Flysch, instead they represent Eocene-aged Namunagarh Grit of the Mithakhari Group and that the turbidites of the Kalipur and Corbyn’s Cove sections were not produced by the same gravity flows neither deposited in the same fan system. The Andaman subduction complex incorporates Paleogene turbidites of different ages, provenances, tectonic environments and paleogeographic settings. The
Acknowledgements
I gratefully acknowledge the excellent reviews by J. R. Curray, who has offered several constructive comments, suggested pertinent changes and provided interesting discussion points that immensely help improving the manuscript. I also value the excellence and generosity (of time and knowledge imparted) implicit within reviews by A. J. Barber (3rd Reviewer) that no doubt was seminal for improving the paper. The detailed English language corrections in the revised text were also kindly undertaken
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