Subdivision and petrology of the Mesozoic rocks Of Coromandel (Manaia Hill Group)

Abstract

The original unconformable threefold subdivision of the sedimentary Mesozoic basement rocks of Coromandel is redefined as two conformable formations (Moehau and Tokatea Hill) within Manaia Hill Group on the basis of gradational detrital content and structural continuity; the new sequence is inverted with respect to previous subdivision. Standard sections are described for both formations (Moehau 520–610 m minimum; Tokatea Hill 530 m maximum). The older formation (Moehau) is characterised by a lithic volcanic greywacke and sub-greywacke suite derived from calc-alkaline volcanic and plutonic rocks, and minor sedimentary rocks, whereas the younger formation (Tokatea Hill) is a feldspathic greywacke suite, almost devoid of volcanic detritus except near the base, derived from a more mature calc-alkaline plutonic landmass with minor sedimentary rocks. Moehau Formation and the lower part of the Tokatea Hill Formation are slope-deposited turbidites, whereas the upper Tokatea Hill Formation resulted from stable deposition in deeper water. Moehau Formation locally includes prehnite, analcime, actinolite, and zeolite vein minerals and has a chloritic matrix; Tokatea Hill rocks contain none of these minerals and typically have a sericitic matrix. The structure of the Manaia Hill Group is relatively simple, the fold axes striking north-west to north-north-west with the Tokatea Hill Formation occupying the core of a syncline.