The Sedimentology and Origins of a Giant Mass Transport Complex: The Nataraja Slide, Arabian Sea

A giant mass transport complex (MTC) was recently discovered in the eastern Arabian Sea exceeding in volume all but one other known complex on passive margins worldwide. The complex, named the Nataraja Slide, was drilled by IODP Expedition 355 in two locations where it is ~300 m (Site U1456) and ~200 m thick (Site U1457). The top is defined by the presence of both reworked microfossil assemblages and deformation structures, such as folding and faulting. The deposit consists of two main phases of mass wasting, each which consists of smaller pulses, with generally fining upward cycles, all emplaced just prior to 10.8 Ma. The base of the deposit at each site is composed largely of matrix-supported carbonate breccia that is interpreted as the product of debris flows. In the first phase, these breccias alternate with very well-sorted calcarenites deposited from a high energy current, coherent limestone blocks that are derived directly from the Indian continental margin, and few clastic mudstone beds. At the top of the deposit, in the second phase, muddy turbidite deposits dominate and are increasingly more siliciclastic in composition. At Site U1456, where both phases are seen, a 20 m section of hemipelagic mudstone is present, that is overlain by a ~40 m section of calcarenite and slumped interbedded mud and siltstone. Bulk sedimentary geochemistry, clay mineralogy, and isotope analysis constrain the provenance of the clastic, muddy material to reworked Indus-derived material, with some input from smaller western Indian rivers (e.g., Narmada and Tapti Rivers), and some material from the Deccan Traps. The carbonate material is a shallow-water limestone from the outer western Indian continental shelf that was likely oversteepened from enhanced clastic sediment delivery during the mid-Miocene. The final emplacement of the material was likely related to seismicity as there is evidence for intraplate earthquakes immediately opposite of the scar. Although I hypothesize this area is at low risk of future mass wasting events, it should be noted that other continental margins around the world could be at risk for mass failure as large as the Nataraja Slide.