Mid-tertiary stratigraphy of Silverdale district

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Abstract
Argillaceous limestone and greensands (Mahurangi Limestone) possibly up to 1,000 ft thick form the local basement rocks in this area 20 miles north of Auckland, New Zealand. They were previously considered Bortonian (Mid-Eocene) in age, but from their contained microfossils they are now thought to be Whaingaroan to Duntroonian (Lower Oligocene), They are overlain by at least 100 ft of Waitakian (Upper Oligocene) grey-white calcareous siltstone (Wainui Siltstone), which in turn is unconformably overlain by 2,000 ft or more of Otaian (Lower Miocene) mudstone, siltstone, sandstone, and conglomerate (Waitemata Group). Completing the upward sequence is a chaotic deposit up to 500 ft thick, of sandstone, mudstone, claystone, chocolate shale, serpentinite, and concretionary boulders (Onerahi Formation), containing fossils of Mata, Dannevirke, Arnold, and Landon age. Drillhole data, microfaunal age determinations, and economic notes on ground water, limestone, road metals, and serpentinites are given.

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