Combining stratigraphy and tectonics to understand intermittent deposition on the outer rise offshore Sumatra, Indonesia

• Main Author: Tan, Li Ching
• Other Authors: Judith Hubbard
• Format: Final Year Project
• Language: English
• Published: Nanyang Technological University 2020
• Subjects: Science::Geology
• Online Access: https://hdl.handle.net/10356/141608
• Institution: Nanyang Technological University

We present a robust method of estimating the ages of deep sea sediments at active margins by relating tectonic deformation of the sediments to a known deformation rate. Stratigraphic and tectonic patterns were observed from depth-converted high resolution seismic reflection profiles in front of the Sunda Trench, west of Mentawai Island, Sumatra, Indonesia. We identified normal faults and channels, defined 8 sedimentary packages and interpreted deposition to be intermittent. The ages and sources of sediments were investigated using two methodologies. We first tracked the rotation and tilting of the sediments as they were transported from the forearc bulge towards the overriding plate by relating the dips of the sedimentary packages to the horizontal displacement since deposition. The age estimates were derived by diving distance travelled since deposition over the convergence rate of the plate. We analysed sedimentation patterns against climate-induced relative sea level fluctuations and the estimated ages of our sedimentary packages corresponds with the five glacial cycles in the past 500,000 years during the Pleistocene. We observed that 5 out of 6 sediment pulses were deposited during sea level middle to low stands, when majority of shelf was exposed. Relative sea level fluctuations exerted a general and large scale control on sedimentation by determining the subaerial exposure of continental slopes. A secondary method of identifying horizontal displacement of sediments involves tracking normal faults from the its current site to the location of formation, which hypothetically occurs at a band of seismicity, which we identified. This robust method of estimating deep sea sediments could be applied to other subduction margins to investigate the interplay of climate and sedimentology. Such findings are rarely documented by cores worldwide since the materials are situated offshore below 4 km water depth, where physical samples are inaccessible. This novel method of estimating ages through detailed analysis of high resolution seismic profiles will provide a new record of sedimentation patterns, which can be examined against climatic records and eustatic sea level changes.