Long-timescale variation in bulk and clay mineral composition of Indian continental margin sediments in the Bay of Bengal, Arabian Sea, and Andaman Sea
Introduction
Sedimentary environments of the northern Indian Ocean are influenced by dynamic tectonic and climatic conditions that link the uplift of the Tibetan Plateau and Himalayas and development of the Indian monsoon (e.g., Clift et al., 2008, Curray, 1994, Molnar et al., 1993). Himalayan uplift and monsoon variation result in changes in provenance and weathering that are reflected as stratigraphic variation in mineralogy in marine records over tectonic and orbital timescales (e.g., Brass and Raman, 1990, Debrabant et al., 1993, Krissek and Clemens, 1991, Liu et al., 2003). Sedimentary basins along the Indian margin are host to accumulations of gas hydrate (Collett et al., 2008, Jaiswal et al., 2012, Lee and Collett, 2009, Mazumdar et al., 2014, Ramana et al., 2009, Rees et al., 2011). These basins also record the effects of past monsoon variation (Ponton et al., 2012, Phillips et al., 2014, Rashid et al., 2011, Tripathy et al., 2011).
Bulk and clay mineral abundances in marine sediments are useful indicators of detrital provenance and dispersal patterns (e.g., Forsberg et al., 1999, Moros et al., 2004, Underwood and Pickering, 1996, Yuste et al., 2004), as well as paleoclimatic conditions in the source areas (e.g., Singer, 1984, Thiry, 2000). Smectite-group minerals (e.g. montmorillonite, nontronite, saponite) can be detrital or authigenic in origin; they are widely regarded as chemical weathering products of volcanic rocks and pedogenesis in arid-to-semiarid climates, whereas kaolinite is a product of intense leaching in humid tropical soils (Biscaye, 1965, Chamley, 1989, Chamley, 2001). In this paper, we refer to smectite-group minerals simply as smectite. Chlorite and illite are formed via mechanical weathering of plutonic, metamorphic, and sedimentary rocks in continental areas (Chamley, 1989).
The occurrence of marine gas hydrate on continental margins is controlled primarily by pressure, temperature, and methane saturation conditions and additional factors associated with host sediments such as ionic strength of pore water (Sloan and Koh, 2008). Methane saturation within marine sediments is influenced by a variety of factors including the rate of methanogenesis (a function of organic carbon quantity and quality), the rate of advection of methane or higher order hydrocarbons, and porosity and permeability (both lithologic and fracture related). Higher values of porosity in hemipelagic mud deposits, which are influenced by mineralogy, grain-size distribution, microfabric, and compaction, may enhance gas hydrate accumulation by sustaining pathways for fluid migration (e.g., Clennell et al., 1999; Lorenson, 2000, Xu and Ruppel, 1999). The relationship between mineralogy and enhanced gas hydrate accumulation in marine sediments is primarily related to elevated quartz and feldspar abundance in intervals of coarser grain size and moderate to high porosity (e.g. Bahr et al., 2008; Expedition 311 Scientists, 2006) or other related authigenic mineralization, which may help sustain porosity with depth (Rose et al., 2014).
Laboratory and modeling studies indicate that smectite-group clays, specifically montmorillonite, promote the formation of methane hydrate (Cha et al., 1988, Ouar et al., 1992, Park and Sposito, 2003, Riestenberg et al., 2003). The mineral surface of hydrated clay minerals, especially 2:1 layer swelling clays, have been shown to provide reaction surfaces for geochemical reactions (Sposito et al., 1999). Smectite surfaces can thermodynamically promote the formation of structure I methane hydrate by the ordered absorption of water molecules (Cha et al., 1988, Park and Sposito, 2003). This promotion effect has been shown to significantly reduce the pressure required for methane hydrate formation, but this effect can be negated by the presence of dissolved silica (Riestenberg et al., 2003).
Sites drilled during Indian National Gas Hydrate Program Expedition 01 (NGHP01) provide an opportunity to document mineralogical changes over sediment depths of 180–690 m below seafloor (mbsf) and on kyr-to-Myr timescales. The cores were recovered from previously un-drilled regions of the east and west margins of peninsular India, as well as the Andaman accretionary wedge (Fig. 1), at water depths ranging from 895 to 2663 m (Table 1), all within the modern gas hydrate stability zone. NGHP01 presented an opportunity to examine how depositional history and sediment composition have influenced gas hydrate distribution on the Indian continental margins. In this paper we describe variations in bulk mineralogy (relative abundances of total clay minerals, quartz, feldspar, calcite) and clay mineralogy (relative abundances of illite, smectite, chlorite, kaolinite) in sediment cores recovered during NGHP01. We measured sediment samples recovered from the offshore Krishna-Godavari (Holes NGHP-01-03B, NGHP-01-05C, NGHP-01-07BD, NGHP-01-10BD, NGHP-01-14A, NGHP-01-15A, NGHP-01-16A, NGHP-01-20AB) and Mahanadi Basin (Holes NGHP-01-18A, NGHP-01-19A), the Andaman Sea (Hole NGHP-01-17A), and the Kerala-Konkan Basin (Hole NGHP-01-01A). These records allow for interpretations of sediment provenance through time across a broad swath of Indian Ocean environments, as well as the fundamental relation between mineral content and gas hydrate occurrence.
Section snippets
Previous studies
The global-scale distribution of clay minerals and quartz in seafloor sediments has been long-established as an indicator of latitudinal climate zonation (Biscaye, 1965, Griffin et al., 1968, Leinen et al., 1986, Petschick et al., 1996, Rateev et al., 1969, Windom, 1975, Windom, 1976). Dispersal of fine-grained suspended sediment is affected by prevailing winds, surface currents, bottom currents, and density currents, so there are many exceptions to the simple latitudinal zonation, including
Krishna–Godavari Basin
The Krishna-Godavari Basin is a pericratonic basin formed during the Jurassic rifting of the Indian and Australia–Antarctica plates and the basin structure is characterized by NE–SW trending horsts and grabens (Bastia, 2006, Gupta, 2006, Rao, 2001). The Krishna and Godavari Rivers drain the Deccan Trap basalts and Precambrian metamorphic rocks (Rao and Kessarkar, 2001) and deliver suspended sediment loads rich in smectite (montmorillonite) with minor feldspar, quartz, kaolinite, and illite (
Sample preparation
Samples for analysis by X-ray diffraction (XRD) were extracted from split cores at a routine spacing of one per core (approximately every 9.5 m). Sampling focused on the dominant lithology rather than minor or unusual intervals. Splits of the bulk sediment samples were freeze-dried, hand crushed by mortar and pestle, and powdered for 5 min using a mixer mill. The bulk powders were then packed gently into round aluminum sample holders to retain random particle orientation.
The splits used for
Krishna–Godavari Basin
Sediments from the Krishna–Godavari Basin are enriched in total clay minerals with site averages ranging from 73 to 82 wt. %. Quartz and feldspar comprise a minor but substantial abundance with averages of 8–12 wt. % and 9–14 wt.% respectively. Quartz and feldspar contents are highest (12 and 14 wt.% respectively) at Holes NGHP-01-07B and –D, and NGHP-01-16A in the northeast of the Krishna–Godavari Basin (Figure 3, Figure 4) compared to Holes NGHP-01-03B, -05C, -10B, -10D, -14A, -15A, -20A, and
Spatial distribution and potential source areas
The range of sedimentary environments and tectonic settings sampled by NGHP01 are reflected by large differences in both the bulk and clay-size XRD results. Calcite abundances increase from the Krishna-Godavari Basin to the Mahanadi Basin to the Andaman Sea and Kerala-Konkan Basin (Fig. 15). The average calcite abundance increases with water depth and distance from land, and decreases from dilution by lithogenic minerals. The high total clay abundance relative to quartz and feldspar at all
Conclusions
XRD analyses of fine-grained sediments from NGHP01 sites in the Krishna–Godavari Basin, Mahanadi Basin, Andaman Sea, and Kerala-Konkan Basin capture a wide range in spatial and temporal variations. Our results reinforce conclusions from several previous studies in the region, but our expanded stratigraphic coverage adds more detail to the temporal perspective. Calcite is most abundant at pelagic sites of the Andaman Sea and Kerala-Konkan Basin, whereas total clay minerals are dominant at the
Acknowledgments
The authors wish to thank those that contributed to the success of the National Gas Hydrate Program Expedition 01 (NGHP01). NGHP01 was planned and managed through collaboration between the Directorate General of Hydrocarbons (DGH) under the Ministry of Petroleum and Natural Gas (India), the U.S. Geological Survey (USGS), and the Consortium for Scientific Methane Hydrate Investigations (CSMHI) led by Overseas Drilling Limited (ODL) and FUGRO McClelland Marine Geosciences (FUGRO). The platform
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