Abstract:
An approximately 1200m thick Plio-Pleistocene (upper Mangapanian-upper Nukumaruan) sedimentary sequence from the Lake Tutira-Waikoau area of northern central Hawke's Bay is described. Formal and informal stratigraphic subdivision is based on lithology. The sequence is subdivided into fifteen formations, the youngest eleven comprising the Petane Limestone Group. The stratigraphic and facies relationships of the formations are discussed. In order of decreasing age the formations are: Kotemaori Sandstone, Waikoau Sandstone, Esk Mudstone, Darky's Spur Sandstone and, within the Petane Limestone Group, Tararere Sandstone, Whirinaki Mudstone, Tangoio Limestone, Te Ngaru Mudstone, Waipatiki Limestone, Kaare Mudstone, Muka Limestone. Devil's Elbow Mudstone, Kaiwaka Limestone, Tareha Siltstone and Taihoa Limestone.
Five informal formational names are new. These are Kotemaori Sandstone, Darky's Spur Sandstone, Tararere Sandstone, Whirinaki Mudstone and Taihoa Limestone. Two members are recognised within the inner shelf-nearshore Kotemaori Sandstone. These are the mid-outer shelf Erinore Mudstone and the littoral marine Pohokura Conglomerate. Three members are recognised within the nearshore Darky's Spur Sandstone. These are the littoral marine Ridgemount Conglomerate, the distinctive pink-brown Hikuroa Ash and mid shelf Aropaoanui Mudstone. The nearshore Tararere Sandstone is considered to be a lateral equivalent of Park Island Limestone. Petane Mudstone is renamed Whirinaki Mudstone to avoid confusion with the Petane Limestone Group and is believed to be mid-shelf facies. A previously unnamed detrital limestone capping the sequence is named Taihoa Limestone.
Deposition is inferred to have been controlled by climatic fluctuations and associated glacio-eustatic sea level changes. Superimposed are secular vertical tectonic movements resulting from compression at the convergent plate boundary along the Hikurangi-Kermadec Trench system.
Numerous slope failures in sandstones and mudstones are a natural phenomenon which have been accelerated in the past several hundred years by the forest destruction by man. Bedrock and soil failures are prevalent in the relatively impermeable mudstone units and in the soils overlying them. High intensity rainstorms frequently initiate soil failures and additional movement on active bedrock failures. Soil failures are prevalent on north facing slopes particularly those oversteepened by the limestones of the Petane Group. Lake Tutira is inferred to have formed prior to 3,400 B.P. The lake was formed by damming of Papakiri Stream by two separate landslides, both involving the collapse of the limestone capped cliff on the eastern side of the valley.
Bedrock and soil failure susceptibility maps are presented.
