Mass Transport Complexes from a Late Miocene Deepwater Succession, Taranaki Basin, New Zealand: Scales, Styles, and Significance in Relation to Tectonic, Eustatic, and Autocyclic Drivers

Greg H. Browne¹, Lorna J. Strachan², Peter R. King¹, and Malcolm J. Arnot^1
1 GNS Science, Lower Hutt, New Zealand
² The University of Manchester, Manchester, United Kingdom

Late Miocene (Tortonian), bathyal submarine fan and coeval slope sediments are spectacularly exposed for 50 km along the north Taranaki coast, New Zealand (Mount Messenger and Urenui Formations). The succession of sandstone, siltstone, and rare conglomerate and tuffaceous gravity flow deposits is 2 kilometers thick, and a comparable succession is imaged in adjacent offshore seismic lines. Mass transport complexes (MTC's) make up a significant part of the succession.

Three types of MTC are recognised in outcrop, and examples will also be illustrated from seismic profiles. Each has a distinctive style and significance within the deep-water depositional setting, and are differentiated on the basis of scale. Type 1 seismic-scale MTC's consist of intervals >10 m thick with complex fold and brittle shear structures. Type 2 outcrop-scale MTC's are typically up to 10 m thick. Fault orientations from both seismic and outcrop-scale examples are generally consistent with the regional NE-SW fault trend. Both types have fold axes with variable orientations and generally low-angle plunges. Axial plane data is consistent with their transport toward the NW, down the regional paleoslope, though the data from seismic scale examples may have a more southward directed transport direction. Type 3 MTC's are bed-scale and only a few meters thick, and involve single beds with plastic, low shear strength deformation features.

The scale of Type 1 MTC's is thought to relate to regional tectonic drivers, most likely contemporaneous seismic triggers from nearby andesitic volcanic centres or adjustments on the deep-seated basin bounding Taranaki Fault. Type 2 MTC's occur immediately below 4th- possibly 5th-order sequence boundaries and were likely instigated by tectonic and/or eustatic base level fluctuations. Localised autocyclic factors such as slumping from paleo sea-floor bathymetric highs (eg., from channel or lobe margins), or in situ loading, are the likely triggers for Type 3 MTC's to develop.