Stratigraphy and structure of Cretaceous (Neocomian-Maastrichtian) sedimentary rocks in the Anini-Okaura Stream area, Urewera National Park, New Zealand

Greywacke basement rocks (Urewera Group) in the Anini-Okaura Stream area have yielded well-preserved dinoflagellate assemblages indicating an earliest Cretaceous (Neocomian) age. This represents the fIrst definite record of Neocomian strata in the North Island. Urewera Group includes a thick mudstone-dominated unit previously considered to have a Clarence or Raukumara Series (mid-Late Cretaceous) age. Two structural blocks are recognised. based on the presence of different mid-Late Cretaceous sequences on either side of the NNE-trending Anini Fault. In the western block, Urewera Group rocks are unconformably overlain by Clarence-Mata Series sediments (Karekare Formation, Tahora Formation), but in the eastern block they are overlain directly by Mata Series sediments (Tahora Formation, Whangai Formation). There is a facies change in the Tahora Formation across the Anini Fault. The fault was probably active during the Late Cretaceous and has a post-Haumurian, pre-Miocene vertical displacement in excess of 500 m. Miocene strata unconformably overlie all older units. Petrographic studies indicate Urewem Group sandstones were derived from an igneous source area, whereas more mature mid-Late Cretaceous sediments wereprimarily derived from reworking of older sedimentary rocks. Cretaceous rocks in the area are immature for petroleum generation and have little or no hydrocarbon source potential.