Interpretation of some Paleocene fluvial sediments from the Upper Pakawau and Kapuni groups, Pakawau Sub-basin, North-West Nelson (1996)
AuthorsStark, C.J.show all
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 North Cape and Farewell Formations. The downstream coarsening of LAB2 and LAB3 is best interpreted by coarse grained sediment being fed from active (penecontemporaneous) intrabasin antithetic and synthetic faults and/or the Kahurangi fault in the west. The interpretation of the Wakamarama Fault as a growth fault during the deposition of LAB1, LAB2 and LAB3 is supported by the lack of change in grain size of LAB1 up-section and the fine grained texture of LAB3 at Abel Head. The change in position of the North Cape Formation upper contact results in the recognition of a new lithostratigraphic unit, the Wharariki member, for the fluvial deposits upon LAA3. Also LAB1 is deemed sufficiently structurally, texturally and compositionally distinct to suggest it be called the Whanganui Member of the Farewell Formation. The boundary between association A and association B sediments is interpreted as a type 1 sequence stratigraphic boundary.