Stratigraphy, structure, and inferred environments of deposition of the Early Cretaceous sequence, eastern Wairarapa, New Zealand

Abstract

Abstract In eastern Wairarapa indurated sedimentary “basement” forms an almost continuous belt some 140 km long and 3–15 km wide, generally overlain unconformably by late Cenozoic strata or faulted against Late Cretaceous‐Early Tertiary rocks. These “basement” rocks, previously considered to be Late Jurassic‐Early Cretaceous (Torlesse Supergroup) or Early Cretaceous (Taitai‐Clarence Series) are mostly Motuan (Albian; Clarence Series), but locally probably as old as Urutawan (Albian). No base to the sequence is known. Two major Early Cretaceous units are present. An older Pahaoa Group (new name; Urutawan‐Motuan) consisting of highly deformed, indurated, poorly fossiliferous alternating sandstone and siltstone ( Mangapokia Formation , new name) and minor matrix supported breccia‐conglomerate ( Stronvar Member , new unit) enclosing at different stratigaphic levels units of mostly featureless sandstone (Taipo Formation), and intruded by penecontemporaneous basic igneous rocks. The younger Mangapurupuru Group (emended; Motuan and younger) is less deformed, contains common calcareous concretions and fossils, and commences with a thick breccia‐conglomerate (“pebbly mudstone”; Gentle Annie Formation , emended) that grades laterally and vertically into predominantly siltstone and alternating sandstone and siltstone ( Springhill Formation , emended). The groups are separated by a regional unconformity repesenting a short span of time within the Motuan. Structural complexity in Pahaoa Group is considered to be largely the result of major pre‐lithification deformation associated with a significant intra‐Motuan tectonic event. Later faulting, shearing, and open folding are superposed upon the earlier structures. Fan‐delta deposition, probably involving mostly mass‐transport and turbidity currents, along the western fault‐controlled margin of a basin, best explains the lithofacies of Pahaoa Group, their facies relationships, and, in part, the paucity of fossils. Faulting during the regional intra‐Motuan tectonic event caused the deposition of an extensive breccia‐conglomerate (Gentle Annie Formation), and changes in the configuration of the basin, formation of an unconformity, and the initiation of open shelf paleo‐environments of Mangapurupuru Group.