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doi:10.14379/iodp.sp.355.2014

Stratigraphic framework

The stratigraphic framework around the proposed sites has been developed using regional seismic lines in conjunction with industrial wells on the adjacent shelf (Fig. F3). The seismic stratigraphy of the Indus Fan has received attention in its upper and proximal levels (McHargue, 1991), where multiple fan channels have been identified (Deptuck et al., 2003). Droz and Bellaiche (1991) used a regional, single-channel survey to interpret the uppermost part of the sequence characterized by clear channeling and levee formation (Indus Fan Megasequence) and two lower sedimentary sequences, a “basement cover” and a simple parallel-stratified, topography-filling series, not marked by the channel levee complex of the youngest unit. The latter sequence is separated by a major unconformity from the “basement cover.” Clift et al. (2001) dated the Indus Fan Megasequence as mid-Miocene to Recent. Previous studies have imaged a mid-Miocene unconformity as a key horizon in the shelf region (Chaubey et al., 2002b; Ramaswamy and Rao, 1980; Biswas and Singh, 1988). Further, Krishna et al. (2006) confirmed the regional nature of this unconformity, showing that it runs from the shelf to the outer margin of the fan. Fan sediment is relatively thin in the area selected for drilling, with thicker deposits found locally in Laxmi Basin.

The sedimentary sequence can be divided into different units spanning the Paleocene through Pleistocene (Fig. F4). Thick post-Miocene strata (~1 s two-way traveltime) are observed on the profiles except over volcanic highs. The presence of channel-levee complexes in the upper fan and channels under the slope regions shows that the fan is built up by a series of overlapping lobes sourced from at least three generations of submarine canyons that predate the present bathymetric canyon (Deptuck et al., 2003). The location of the modern shelf has been a clastic delta since at least the early Miocene (Daley and Alam, 2002), although the presence of deep-marine clastic sediments on Owen Ridge and in the Makran suggests that the paleo-Indus River was connected to the deep sea well before that time but that the sediment flux greatly increased shortly after the Oligocene/Miocene boundary when the first major channel-levee complexes developed in the northern Arabian Sea (Clift et al., 2001). The proximal fan has experienced rapid sedimentation, and the stratigraphy is locally disrupted by mud volcanoes below the upper slope (Calvès et al., 2011). Carbonate ooze with a smaller component of siliceous microfossils cover the seafloor around the proposed sites because of postglacial sea level rise allowing sediment sequestration on the Indus Shelf (Clift et al., 2014). Data from nearby cores indicate that sediment in this region contains well-preserved, abundant foraminifers and diatoms (Gupta et al. 2011) giving confidence for good biostratigraphic age control in the recovered sections.

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