MICROFOSSILS IN MARINE SEDIMENTS: THE INFLUENCE ON MACRO-SCALE MECHANICAL BEHAVIOR

In this study we analyze the impact of foraminifera in marine sediments on the macro-scale mechanical behavior. We uniformly mix foraminifera with natural marine mudstone in three different microfossil concentrations: 0 wt%, 6.5 wt%, and 12.3 wt%. The mudstone is from Site C0011 in the Nankai Trough, offshore Japan, obtained during Integrated Ocean Drilling Program (IODP) Expedition 322. The foraminifera are extracted by washing and sieving of marine sediments from IODP sites U1337 and U1338 in the Equatorial Pacific near Hawaii. We use resedimentation to prepare the homogeneous microfossil-rich mudstone samples and uniaxially compress them to 100 kPa. Resedimentation simulates the natural processes of sedimentation and burial under controlled conditions. Microfossil-rich sediments are known to initially not consolidate to as low porosities as other marine clays owing to microfossil shells acting as structural components. But they show a delayed compressibility when the yield stress is overcome and microfossil shells collapse resulting in an increase in porosity and compressibility. Here, we investigate the threshold microfossil content at which the microfabric significantly changes during compression and the stress at which foraminifera chambers start to break. For the low stresses considered in this experiment, we find that for increasing foraminifera concentrations from 0 wt% to 12.3 wt%, initial void ratio decreases from 2.65 to 2.27, compression index decreases from 0.69 to 0.53, and the permeability decrease with decreasing porosity remains relatively constant. We also use petrography and scanning electron microscopy to identify the foraminifera shells as well as investigate microstructural changes during compression. This study has large implications for submarine slope failure due to the possible sudden expulsion of water causing a pore pressure increase, which may destabilize continental slopes, and cause a hazard for coastal communities and offshore infrastructure.