Tertiary geology across an allochthon—autochthon contact, Whangaroa County, Northland, New Zealand

Abstract

Abstract Near Whangaroa, New Zealand, Waipapa Group cherts and metasediments are unconformably overlain by an autochthonous, north-dipping sequence of Bortonian-Kaiatan (Ab-Ak; Mid-Late Eocene) age, correlated with Ruatangata Sandstone. The sequence comprises (base to top): 0–40 m+ of massive, sparsely fossiliferous greensand (Waiare lithofacies) containing low-diversity agglutinated foraminiferal faunas suggesting a shallow, brackish environment; 50 m of massive, grey to red, variably siliceous siltstone (Ketetawa lithofacies); and 120 m+ of massive, bluish grey, brown-weathering finegrained calcareous muddy sandstone (Inumia lithofacies) containing moderately rich benthic and planktic foraminiferal faunas that indicate deposition at mid-shelf depths. This autochthonous sequence is discordantly overlain by allochthonous, sheared and deformed, multicoloured claystone of Teurian (Paleocene) age, containing low-diversity siliceous and agglutinated benthic foraminiferal faunas indicative of deposition at lower bathyal or deeper depths; and containing, near the base, a raft of underlying strata incorporated during emplacement. Both the allochthon and the underlying autochthon were differentially offset by high-angle faults active before, after, and probably during, emplacement of the allochthon. These faults do not offset overlying Miocene volcanogenic andesitic breccia (Wairakau Andesite) or argillaceous and tuffaceous lacustrine sediments associated with it Erosion of the breccia was followed by eruption of basalt, prior to incision of the present drainage system, along the channels of which minor soda water and gas seepages are found.