Megaclasts within mass-transport deposits: their origin, characteristics and effect on substrates and succeeding flows

Megaclasts transported within submarine landslides can erode the substrate, can influence the flow which transports them, and, if they form seafloor topography, can influence subsequent flows and their deposits. We document grooves up to 40 km long formed by megaclasts carried in submarine landslides which scoured tens of metres deep into the contemporaneous substrate of the deep water Taranaki Basin, New Zealand. A 1,925 km2 3D seismic reflection survey records six mass transport deposits (MTDs) interbedded with turbidites. Here, we focus on three MTDs, labelled A (oldest), B and C (youngest). MTD-A features megaclasts that internally have coherent parallel strata, and formed striations 4-15 km long and 2-3 km wide, with protruding megaclasts that are onlapped by younger sediments. The seafloor expression of these megaclasts partially obstructed the submarine landslide that created MTD-B. MTD-B contains megaclasts that incised through the rugose topography of the underlying MTD-A, and formed divergent grooves on the basal surface of MTD-B (8-40 km long and 200-250 m wide) which suggest radial flow expansion where flows exited topographic confinement. MTD-C features grooves 2-6 km long and 100-200 m wide that terminate at megaclasts which internally are characterised of highly deformed reflectors surrounded by a chaotic matrix. This study directly links megaclasts to the grooves they form, and demonstrates that markedly different styles of scouring and resultant grooves can occur in closely-related MTDs.