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ABSTRACT: The Northland Allochthon is a structurally displaced rock unit located between Three Kings Island and Mt. Camel in northern North Island, New Zealand. Lithologically, the allochthonous unit is composed of different lithofacies comprising sandstone, siltstone, greensand, siliceous mudstone, argillaceous micritic limestone and rare coal measures along with dispersed organic matter. The objective of this paper is to study modal analysis of different lithofacies of Northland Allochthon to infer relationship between sandstone composition and tectonic setting of displaced allochthonous unit. For this purpose, petrographic technique is used following Point Counting method for classification of grains. Results indicate that facies of Tokerau Clastics and Mangakahia complex of the Northland Allocution rocks plot within the recycled orogenic fields while samples from Omahutta facies of the Motatau Complex fall in the transitional and Craton interior zones. QmPK plot reflects increasing maturing of the detritus with time. The relative concentration of quartz over feldspar indicates that feldspar has either been destroyed by abrasion/chemical weathering or has been diluted by cycle quartz.
Keywords: Northland Allochthon, Modal Analysis, Provenance, Tectonic Setting, Petrography, Ternary & discrimination diagrams.
INTRODUCTION
The Northland Allochthon is present both NE and SW of a structural high located between the Three Kings Island and Mt. Camel (Figure 1) in northern North Island of New Zealand. This is a thick widespread displaced allochthon rock unit contains rocks of Late Cretaceous to Early Miocene age (Kear and Waterhouse, 1977; Balance and Sporli, 1979). The Northland Allochthon is estimated to have a present day volume of 32000km3 in an area of 26000 km3. The volume of allochthon eroded from onshore Northland can only be guessed and the volume from the Three Kings Island-Mt. Camel Terrance is not yet known. Lithologically the allochthonous rocks are mostly composed of sandstone, siltstone, greensand, siliceous mudstone, argillaceous micritic limestone and rare coal measures along with dispersed organic matter. It overlies the autochthonous, paleogene non-marine to marine transgressive sediments (Hayward et al. 1989). It is unconformably overlain by the Waitemata and correlative groups of Upper Oligocene-Lower Miocene age.
Tectonism is the fundamental factor that affects sedimentation and bulk characteristics of sedimentary rocks (Pettijohn, 1957 and Krumbion and Sloss, 1963). It controls the source area, uplift and erosion, transport conditions to the depositional sites, degree...