Spatial patterns of deformation and paleoslope estimation within the marginal and central portions of a basin-floor mass-transport deposit, Taranaki Basin, New Zealand

Abstract

This study describes the character of submarine mass movement and associated deformation as revealed by an exceptionally well-exposed portion of a seismic-scale masstransport deposit (MTD) within the upper Miocene Mohakatino Formation (Taranaki Basin, New Zealand). The North Awakino MTD is at least 55 m thick and crops out along the northern Taranaki coastline for ~11 km in wave-cut platforms and in cliffs as much as 100 m high. Spectacular softsediment deformation features are developed in remobilized sediment gravity fl ow deposits that initially accumulated within a lowgradient intraslope basin. Sedimentary facies within the North Awakino MTD comprise laterally extensive, thick- to thin-bedded volcaniclastic sandstone and mudstone. Distinct postdepositional deformation styles are associated with bedding type: folds developed in thick-bedded sandstone are larger (fold heights to tens of meters) and more laterally continuous (to 1 km) than those developed in thinner bedded facies. Regional geologic relationships suggest that nearly the full width of the North Awakino MTD is exposed in outcrop, providing a rare opportunity to observe lateral relationships between the marginal and central portions of the MTD. We conducted a rigorous paleoslope analysis of slump fold, fault, and bedding orientations using both existing and newly proposed methodologies. Separate analysis of seven subregions within the North Awakino MTD reveals that the predicted MTD transport direction varies widely along the outcrop extent. Most notably, slump folds and faults within the inferred margins have mean orientations that are suborthogonal to those within the central portions of the MTD. This relationship is hypothesized to be a consequence of edge effects that may be related to lateral compression along the margins of the MTD. Our analysis demonstrates the importance of accounting for spatial hetero geneity in slump structure orientations when determining the paleoslope orientation through kinematic analysis. Our inference of west-directed translation suggests that the North Awakino MTD formed in response to a local change in the bathymetric slope orientation that was likely the result of tectonically induced basin deformation.