A stratigraphic, biostratigraphic and structural analysis of the geology at Huatokitoki Stream, Glenburn, Southern Wairarapa, New Zealand

Abstract

The geology at Huatokitoki Stream, Glenburn is presently a complex sequence of highly faulted and folded late Cretaceous to early Tertiary strata. Understanding the structural geology requires detailed analysis of the stratigraphy and biostratigraphy at Huatokitoki Stream to obtain control over ages of the rock units. In addition, a reference section, located at Pukemuri Stream, Tora, provides further control over positions of regional stratigraphic and biostratigraphic unconformities. The stratigraphy at Glenburn was divided into six formations, consisting of Cretaceous aged turbidites (Glenburn Formation), late Cretaceous siliceous sandstone and siltstone (Whangai Formation), two laterally time equivalent but lithologically different strata: the first is comprised of Paleocene aged glauconitic sandstone and calcareous siltstone, marl and a limestone marker bed (Waikekeno Formation). The second lithology is composed of Paleocene aged siliceous siltstone, greensand, marl and a limestone marker bed (Whatipu Formation). Overlying the Waikekeno Formation is a sequence of Paleocene-Eocene smectitic mudstone (Honeycombe Formation), and overlying the Whatipu Formation are Eocene aged glauconitic sandstone and siltstone (Shepherd Formation). The presence of rounded sandstone boulders in late Cretaceous siliceous siltstone, limestone and sandstone boulders in Eocene smectite are interpreted to represent localised erosion events. The structural geology of Huatokitoki Stream consists of five folded thrust sheets which form an antiformal stack. The thrust sheets were stacked during the early Miocene by a complex combination of piggy-back and out-of-sequence thrusting. Restored orientations of axial planes and faults to their early Miocene positions show that they consistenly dip towards the east and are therefore interpreted to represent westward vergence and thrusting to the west respectively. Thrusting of Eocene aged deep water smectite over Eocene shallow water glauconitic sandstones also implies transport to the west. Horizontal displacements of the thrust sheets are in the order of tens of kilometres with stratigraphic throws of approximately 1000m. Horizontal shortening of the thrust sheets is estimated to be 50%. Pliocene-Pleistocene deformation has rotated the strata and faults into their present NNE strike. Analysis of S-C fabrics in a brittle-ductile shear zone are likely to indicate dextral strike slip movement although the amount of offset cannot be determined.