Profile of a piggyback basin: Early Miocene Otaua Group and Waipoua Subgroup, western Northland, New Zealand

Abstract

Abstract The Otaua Group of west Northland is interpreted as comprising three phases of sedimentation: (1) a locally preserved, bathyal, upward‐fining sequence of grit, sandstone, and mudstone of late Waitakian age (Harnetts Formation, new); (2) a deltaic and regressive sequence of sandstone and mudstone (Waitiiti Formation), sandstone and grit (Otueka Formation, new), and conglomerate (Waiwhatawhata Conglomerate, including the new Smoothy Member), deposited at upper bathyal to shelf depth in the late Waitakian ‐ Otaian (first cycle); and (3) a second regressive sequence of sandstone and mudstone (Pokas Formation, new) and conglomerate (Omapere Conglomerate) deposited in upper bathyal to terrestrial conditions in the Otaian (second cycle). The Otaua Group is overlain conformably and unconformably by the late Otaian‐Altonian Waipoua Subgroup of the Waitakere Group, comprising basaltic volcanics (Waipoua Basalt), which contain near‐basal lacustrine sediment in the east (Rhine Member, new) and which are overlain by conglomerate (Pukorukoru Formation) in the northwest. Assessment of stratigraphic relationships suggests that the boundary between the Otaua Group and the Waipoua Subgroup should be redefined as the base of the Waipoua Basalt. Following its deposition upon the Northland Allochthon, Harnetts Formation was tectonically incorporated as a large block into the substrate during late‐stage movement. During deposition of the first sedimentary cycle, deformation about northeast‐striking trends was accompanied by southward‐directed thrusting: these thrusts cut the early sediments (Waitiiti Formation) and are truncated by the Waiwhatawhata Conglomerate. After deposition of the first sedimentary cycle, regional folding about both northeast‐trending and NW‐WNW‐trending axes took place, and the sequence was stacked along observed and inferred southwest‐dipping thrusts. Sediments of the second cycle were then deposited to the west of the first, overstepping the earlier sediments and structures. Subsequent local folding of the second‐cycle sediments (about northwest‐trending axes only) was accompanied by emergence and followed by eruption of the Waipoua Basalt. The Waipoua Basalt and second‐cycle sediments were in turn monoclinally folded about NW‐WNW‐trending axes during the final phase of uplift. Deformation is inferred to have ceased during Waipoua volcanism in the latest Otaian and early Altonian. The sediments and structures are interpreted as recording a progressive change, from extensional deformation within the Northland Allochthon during its southward emplacement at bathyal depth in the Waitakian, to north‐south and then northeast‐southwest compression and regional uplift during the Otaian. These structures are inferred to be related to the formation of an early Miocene convergent margin in the vicinity of Northland. Westward regression of the loci of deposition and of deformation, and the southwestward dip of major thrust structures, suggests northeastward‐directed compression (from the southwest); however, interpretation of offshore seismic data shows no evidence of a convergent margin off Northland's west coast. The structures may instead be related to regional shortening in the basement of east Northland but the evidence to date is inconclusive.