Burial diagenesis of Northland allochthon rocks, Northland basin, North Island, New Zealand

Abstract

This thesis provides the petrographic, geochemical (whole rock major and trace element chemistry), clay mineralogy, vitrinite reflectance and stable isotope studies of different litho-facies of the Northland Allochthon rocks of North Island, New Zealand and interprets them in terms of provenance and burial history. Northland Allochthon, composed of bathyal depth sediments, is an originally fining-upwards passive margin wedge which accumulated adjacent to northeastern New Zealand between mid-Cretaceous and latest Oligocene. The wedge was obducted on to northern and north eastern New Zealand at the onset of compressional tectonics about 25 Ma. In places nappes have been shown to be in re-ordered stratigraphic sequence. They are all overlain by a final nappe of Cretaceous ophiolite. The Allochthon is overlain by Miocene arc and arc-basin deposits. The clay mineral sequence observed from younger to older sedimentary rocks is from smectite to illite through illite/smectite interstratified mixed-layered clays. Mixed-layer illite/smectite (I/S) clay from sandstone and shale shows a progressive increase in ordering and number of illite layers with increasing age. Illite crystallinity index (IC) of interstratified illite/smectite mixed-layer clays ranges from 0.70-°2q to 0.30-°q.Most of the values are consistent with advanced diagenesis and anchimetamorphism. Vitrinite reflectance of dispersed organic matter is used to determine the diagenetic trends and low grade metamorphism of the allochthon rocks. Reflectance values range from 0.30 to 2.00% Rm and correspond to an estimate of paleotemperature fo 60° to 180°C. The oldest sedimentary complex may have been affected by heating to 300°C from intrusion of keratophyre dykes. Mean vitrinite reflectance (%Rm), illite crystallinity (IC) and percentage of illite layers in the illite/smectite interstratified clays from different litho-facies show that the rocks have been exposed to a temperature range of approximately 120°C to 200°C. A significant statistical correlation exists between %Rm and 1C and the temperature estimated by clay mineralogy is in accordance with the values obtained by vitrinite reflectance measurements. These results show that the diagenetic sequence seems to be that of original burial. It has not been significantly modified during subsequent re-location, re-ordering and re-burial. A preliminary study of O and C isotopes from 21 samples of calcite cement and vein calcite from units within the Allochthon and the lower part of the overlying Miocene Waitemata Group is also undertaken to establish the source of C and O in the sediments. Veins in Cretaceous claystone close beneath the Miocene rocks have δ18OPDB of -4.6 to -5.95, and δ13CPDB of +2.9 to +14.95. Veins in Oligocene Mahurangi limestone have δ18OPDB of -1.1 to -10.15 and δ13CPDB of +1 to +1.83. Veins in basal Waitemata Group have δ18OPDB of -9.2 to -10.61, and δ13CPDB of -10.4 to -17.14. The depositing solutions were all probably less saline than seawater, but the wide variation in C values suggests different sources of carbon in the three samples. Calcite cement in the concretions of Cretaceous age sandstones has δ18OPDB of -7.6 to -10.6, and δ13CPDB of -20 to -23.6. The O values are more negative than the veins in Cretaceous rocks, suggesting formation at a different time when pore solution were even further from marine composition. The strongly negative C values indicate origin from oxidation of organic matter, and the big difference from the veins in the Cretaceous rocks supports the O evidence of different source waters. Calcite cement in basal Miocene sandstone concretions has δ18OPDB of -4.4 to -3.4, and δ18CPDB of -7.0 to +1.4. Both O and C values indicate a slightly modified seawater source, which support an early diagenetic origin, and a phase of concretion formation separate from the one in Cretaceous-age rocks.