Abstract:
Three lower Miocene units occur in the Waipatiki/Mangatuna area: Mangapuku Mudstone (Otaian), Westcott Sandstone (Otaian-Altonian), and Tunakore Mudstone (Altonian). They are regarded as members of the Ihungia Formation and crop out mostly in the axis of the Waipatiki Syncline. Major faults near Toi Flat trend west-east and, with an associated zone of minor faults, appear to thicken Westcott Sandstone in this area. Rocks showing gradual deepening from initially shallow water (Westcott Sandstone and Tunakore Mudstone) overlie those of deep-water origin (Mangapulu Mudstone). Mangapuku Mudstone is a dark-grey, calcareous, sandy mudstone with rare macrofaunas and diverse and abundant foraminiferral fauns of abyssal to shelf break origin. It is separated from older units by a deep-water unconformity and was uplifted at rates of up to 1.1 km/m.y. to shallow-water depths, with submarine erosion preceding deposition of Westcott Sandstone. Westcott Sandstone is dominated by grey, calcareous, fossiliferous muddy sandstones to sandy siltstones with intercalated unfossiliferous turbidite sandstones, deposited in a forearc basin subsiding from inner shelf to shelf break depths. Westcott Sandstone has diverse and abundant shallow molluscan faunas along with rich non-molluscan macrofaunas and foraminiferal faunas. Tunakore Mudstone is a dark grey, calcareous, sandy mudstone with minor thin sandy turbidites, deposited at shelf break to mid-bathyal depths, and grades up from Westcott Sandstone from continued subsidence. It contains rare macrofaunas and highly diverse foraminiferal faunas. Sedimentation in Westcott Sandstone and Tunakore Mudstone was ?225 m/m.y in combination with an average basement subsidence rate of 880 m/m.y.
