SEDIMENTATION PATTERNS DURING THE NEOGENE IN MARLBOROUGH, NEW-ZEALAND

The Neogene of Marlborough, and its immediately preceding period, are characterised by a complex interplay of carbonate and elastic sediment ation at an evolving convergent plate margin. Three major periods of s edimentation and tectonics are recognised: 1) an initial pre-Neogene p hase of regional uplift and karstification during the Late Eocene-Olig ocene, 2) Early Miocene carbonate sedimentation, becoming increasingly elastic-rich with time, and 3) Late Miocene conglomerate, sandstone a nd siltstone developed in thrust-fault bounded, rapidly subsiding sedi mentary basins. These phases record an increasing tectonic tempo durin g the Kaikoura Orogeny from a passive margin (in the Waitakian and ear lier), to a convergent plate margin (Waitakian-Recent). Neogene sedime ntation in Marlborough was confined to two depositional areas. In sout hern Marlborough up to 100 m of Early Miocene (Waitakian) limestone an d calcareous sandstone was deposited in western and eastern depocentre s. Stratigraphic units include the Spy Glass Formation, Weka Pass Ston e, and Whales Back Limestone (new). Deposition was in outer shelf and bathyal settings, with sedimentation rates of up to 20 m/myr. These se diments are little different from the subjacent passive-margin, fine-g rained carbonate lithologies of the Late Cretaceous to Eocene Amuri Li mestone. 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 >4 00 m of bathyal sandstone and siltstone (Waima Formation) of Otaian to Lillburnian, possibly Waiauan age, that contain interbedded debris fl ow conglomerate (Great Marlborough Conglomerate). The Waima Formation indicates sedimentation rates of up to 150 m/myr at that time. These r ocks were deposited during tectonic uplift of basement greywacke block s. A small area of Pliocene mudstone with debris flow conglomerate nea r the Clarence river mouth is the only area in southern Marlborough wh ere late Neogene sediments are recorded. In northern Marlborough a you nger (from Late Tongaporutuan) series of tectonically controlled basin s developed. The basins are filled with a range of siliciclastic sedim ents, including conglomerate, sandstone and siltstone deposited in flu vio-lacustrine and alluvial fan, shoreface, shelf and bathyal settings . Stratigraphic units include the Medway, Upton and Starborough format ions. They attain a maximum thickness of >3 km and represent the maxim um sedimentation rates (870 m/myr) known from the Neogene in Marlborou gh.