Interpretation of some Paleocene fluvial sediments from the Upper Pakawau and Kapuni groups, Pakawau Sub-basin, North-West Nelson

• Main Author: Stark, C.J.
• Format: Other/Unknown Material
• Language: English
• Published: University of Canterbury 1996
• Subjects: North Cape
• Online Access: http://hdl.handle.net/10092/13430; https://doi.org/10.26021/6518

Fifteen key measured sections form the basis for detailed facies analysis of Paleocene upper North Cape Formation (Pakawau Group) and Farewell Formation (Kapuni Group) sediments, northwest Nelson. Based on structural, textural and compositional variations, the sediments were divided into eight lithofacies associations (LAA3, LAA4, LAA5, LAA6, LAA7, LAB1, LAB2 and LAB3). Associations A3 to A7, North Cape Formation, represent a progradational sequence of floodplain (LAA3), lacustrine (LAA4), low energy meander (LAA5 and LAA6), and braided river deposits (LAA7). LAB1, LAB2 and LAB3 are interpreted as meander, gravel dominated and sand dominated braided systems. The conformable boundary between LAA3 and LAA5 on the northwestern side (Moki Point) of the Whanganui Inlet means the transition from an axial system to system flowing perpendicular to the axis of the Pakawau Sub-basin does not represent the contact between the Kapuni Group and Pakawau Group. Progradation of a fluvial system passing through the space between en echelon faults in the west would explain the paleocurrent change from parallel to perpendicular to the main axis of the Pakawau Sub-basin. A higher rate of subsidence toward the north along the Wakamarama Fault is inferred from a lateral northerly-coarsening trend between LAA6 and LAA7. Erosional contact between LAA6 and LAB2 on the northern side of the Whanganui Inlet is interpreted as the upper contact of the North Cape Formation. The absence of fluvial association LAA5-LAA7 on the southern side of the Whanganui Inlet Entrance suggests a period of uplift and subsequent erosion of prior to the deposition of associations LAB1-LAB3 deposits. The preferred explanation for the localized uplift and erosion of fluvial association A deposits is by activation of intrabasin antithetic and synthetic faults. LAB1 is inferred to represent a 'feeder' system associated with LAB2 and LAB3. The basal contact of LAB1 on the southern side of the Whanganui Inlet is thus inferred to represent the boundary between the ...