Cretaceous silicic volcanic deposits in the Shag Valley and at Naseby

Hill, Matthew Galloway.

Introduction There is a curious geological pattern pointing to the fact that silicic volcanism was active during the Late Cretaceous in the South Island of New Zealand. Small localised 110 - 100 M a ago silicic volcanic deposits are found outcropping in Nelson-North Westland, in the Naseby district of North Otago and in the Shag Valley district of NE Otago. This study investigates the Otago silicic volcanic rocks with the purpose of seeing how they compare with each other. Emphasis has also been placed on clarifying the field relations and documenting the lithology of the Shag Valley volcaniclas'1c sediment. 1.1 Geological Setting In both North and East Otago small amounts of Cretaceous silicic tuff are found intercalated within r r the Kyebum and Horse Range formations respectively. These formations consist of thick sequences of breccias and conglomerates which have accumulated in fault depressions that formed under an extensional regime during the Mid to Late Cretaceous. Lower Cretaceous to Early Tertiary marine deposits overWthe Horse Range Formation in East Otago. In North Otago marine transgression reached up into the Maniototo but not as far as the Naseby field locality. A shift to a compressional regime, which has affected the South Island since the early Tertiary, has reactivated Cretaceous normal faults in the reverse sense, producing uplift of basement rocks and the overlying sequences along the NW/SE trending Waihemo Fault Zone (WFZ). The Horse, Kakanui and Ida ranges were formed and these now act as the northern border of the Shag Valley 1 and the M~miototo plains. The NW/SE trending faults mark a change in the Haast schist basement geology where low grade TZ I-ll schists have been uplifted on the northern side above higher grade TZ Ill-IV schist. Figure 1 is a locality map, this shows the location of the two field localities and their relation to the Cretaceous sediments and major faults. The Shag Valley Locality Note that grid references (GR) referring to the Shag Valley locality are from Sheet 143 (NZMS, 1980). The field area is five minutes drive north of Palmerston. It lies within farmland at the foot of the QC Horse Range on the northern side of the lower Shag Valley. A ~trip 1.5 km wide and 2.5 km long, trending NW has been mapped. Nineteen days were spent in the field, the majority of which were spent at two localities. 1.2.1 The Geology On the northern margin of the Shag Valley schists of grade no higher than TZ 11 have been pushed up along faults of the WFZ, forming the Horse Range. Cretaceous sediments of the Horse Range Formation lie north of the WFZ within the Horse Range. These sediments have been uplifted above Horse Range Formation to the south. The latter sediments lie within a fault bounded strip trending NW -SE. These in turn have been uplifted, juxtaposing Cretaceous sediments against Late Tertiary marine sediments. The silicic volcanics outcrop only in this fault bounded strip. As you move outside the field area, within the infaulted strip towards the coast, you move up through the local Tertiary marine sequence. 2 In the first chapter the stratigraphy and structure is briefly presented. It should be noted that lab and field work has not been focussed on these surrounding rocks.

60 leaves : ill. (some col.), maps (some col.) ; 30 cm.

1999Hill

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