Sedimentation patterns during the Neogene in Marlborough, New Zealand

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Abstract
The Neogene of Marlborough, and its immediately preceding period, are characterised by a complex interplay of carbonate and clastic sedimentation at an evolving convergent plate margin. Three major periods of sedimentation and tectonics are recognised: 1) an initial pre‐Neogene phase of regional uplift and karstification during the Late Eocene‐Oligocene, 2) Early Miocene carbonate sedimentation, becoming increasingly clastic‐rich with time, and 3) Late Miocene conglomerate, sandstone and siltstone developed in thrust‐fault bounded, rapidly subsiding sedimentary basins. These phases record an increasing tectonic tempo during the Kaikoura Orogeny from a passive margin (in the Waitakian and earlier), to a convergent plate margin (Waitakian‐Recent). Neogene sedimentation in Marlborough was confined to two depositional areas. In southern Marlborough up to 100 m of Early Miocene (Waitakian) limestone and calcareous sandstone was deposited in western and eastern depocentres. Stratigraphic units include the Spy Glass Formation, Weka Pass Stone, and Whales Back Limestone (new). Deposition was in outer shelf and bathyal settings, with sedimentation rates of up to 20 m/myr. These sediments are little different from the subjacent passive‐margin, fine‐grained carbonate lithologies of the Late Cretaceous to Eocene Amuri Limestone. However, they rest on a major omission surface (the Marshall Unconformity) which in Marlborough represents approximately 14 million years. The carbonate‐rich sediments are overlain disconformably by >400 m of bathyal sandstone and siltstone (Waima Formation) of Otaian to Lillburnian, possibly Waiauan age, that contain interbedded debris flow conglomerate (Great Marlborough Conglomerate). The Waima Formation indicates sedimentation rates of up to 150 m/myr at that time. These rocks were deposited during tectonic uplift of basement greywacke blocks. A small area of Pliocene mudstone with debris flow conglomerate near the Clarence river mouth is the only area in southern Marlborough where late Neogene sediments are recorded. In northern Marlborough a younger (from Late Tongaporutuan) series of tectonically controlled basins developed. The basins are filled with a range of siliciclastic sediments, including conglomerate, sandstone and siltstone deposited in fluvio‐lacustrine and alluvial fan, shoreface, shelf and bathyal settings. Stratigraphic units include the Medway, Upton and Starborough formations. They attain a maximum thickness of >3 km and represent the maximum sedimentation rates (870 m/myr) known from the Neogene in Marlborough.