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Tracks, Outrunner Blocks, and Barrier Scours: 3-D Seismic Interpretation of a Mass Transport Deposit in the Deepwater Taranaki Basin of New Zealand


A series of Plio-Pleistocene mass transport deposits (MTD) have been identified in the deepwater Taranaki Basin, in New Zealand, using the Romney 3D seismic survey. One of these MTDs has been chosen for description and interpretation based on high confidence mapping of its boundary surfaces. The deposit exhibits an array of interesting features similar to those documented by researchers elsewhere plus a unique basal feature unlike those previously observed. The basal shear surface exhibits erosional features such as grooves, “monkey fingers”, and glide tracks. We have been able to image outrunner blocks at the end of the glide tracks in distal areas of the deposit. Internally, the MTD is typically characterized by low impedance, chaotic, semi-transparent reflectors surrounding isolated coherent packages of seismic facies interpreted as intact blocks rafted within the mass transport complex. These transported blocks scale up to 1 km wide and 200 m high, and commonly protrude above the upper surface of the flow. This yields a very irregular paleo-bathymetric surface on the top of this and other MTDs with local relief attributed these protrusions ranging from 10 m to >100 m . The complexity of this upper surface had local impact on subsequent flows. The term “shield block” refers to those large protruding obstacles on the paleo-seafloor that acted as barriers to subsequent flows as they advanced downslope. Obstacles such as mud volcanos have been documented to act as such barriers resulting in elongate, downflow erosional remnants as positive features. The opposite is the case for shield blocks, which disrupt flow and result in elongate, downflow erosional troughs that are negative features. These local erosional features are then infilled similar to mega flutes and are preserved as elongate isochron thicks on the downflow end of the underlying shield block. Kinematic evidence provided by various structures suggests that the MTD flow direction was SE-NW toward bathyal depths. The features presented and the absence of extensional headwall structures, such as local arcuate glide planes and rotated slide blocks, suggests that this part of the deposit belongs to the translational to distal domain of the MTD, and its source area is expected to be somewhere toward the SE in a paleo continental slope.