The role of mass transport deposits contributing to fluid escape: Neogene outcrop and seismic examples from north Taranaki, New Zealand

Many sedimentary structures are the manifestation of fluid escape in sedimentary basins. This paper examines outcrop and seismic examples in upper Miocene deep-water sandstones and siltstones of north Taranaki, New Zealand. In outcrop examples of fluid escape features comprise discordant bodies within otherwise uniformly bedded surrounding stratigraphy, features characterized by steep sided, over-hanging, vertical or near-vertical margins, infilled with an assortment of poorly sorted or chaotically arranged sandstone and siltstone. Typically, these features are several metres wide and up to 20 m high in outcrop and always occur stratigraphically below a mass transport deposit (MTD). Examples of similar features from nearby 2D and 3D seismic reflection data consist of localized vertical to sub-vertical zones of disrupted reflectivity and are as much as 300 m in height and 10’s–100’s of metres in width. The structures occur in close association with the basal slide planes of seismic-scale MTDs. The close association of fluid escape structures with MTDs suggests that these features formed by the sudden loading of the sedimentary succession by the emplacement of several metre-thick overlying MTDs. We suggest recurring phases whereby the emplacement of MTDs triggered fluid escape within underlying strata and, in turn, the fluid escape contributed to further instability with potential for mobilization and transport of subsequent MTDs in a dynamic deep-water setting.