Evidence for marine deposition in the late cretaceous pakawau group, northwest Nelson

Abstract

Marine sediments are described from the upper part of the Late Cretaceous (Haumurian) Pakawau Group, exposed in northwest Nelson, New Zealand. The Pakawau Group (onshore) has been previously considered to be entirely nonmarine, but sedimentological and micropaleontological analyses of the North Cape Formation have revealed sedimentary structures and microfossils indicative of marine sedimentation. The North Cape Formation, 300–400 m thick, consists of sandstones, conglomerates, and discontinuous fine‐grained units, but differs from surrounding strata by the absence of coal beds. Sedimentary structures in the North Cape Formation include mud‐draped cross‐bedding, bidirectional ripple cross‐laminations, and some bioturbation. Finegrained units contain late Haumurian marine dinoflagellates, which are commonly fragmented and abraded. The evidence suggests that North Cape Formation was deposited in a tidally influenced marginal‐marine, possibly estuarine, environment. Similar sandstone‐rich marine sediments are likely throughout the Cretaceous of the Taranaki Basin and may represent potential reservoir units. Recent criteria for stratigraphic nomenclature in both onshore and offshore Taranaki Basin exclude marine units from the Pakawau Group. Consequently, the existing Late Cretaceous stratigraphy in the Taranaki Basin requires revision. Pliocene ‐ early Pleistocene. However, during the last 2–3 Ma, regional shortening in North Canterbury has not changed significantly in orientation. © 1992 Taylor & Francis Group, LLC.