Publication Name: Eastern Australian Basins Symposium 2001
Authors: N. Aadil, P. Black and P.F. Ballance
Date Published: November 2001
Number of Pages: 33
Reference Type: Magazine Article
Abstract:Northland Allochthon is a structurally displaced rock unit comprising an originally fining-upwards passive margin wedge which accumulated adjacent to northeastern New Zealand between the Middle Cretaceous
and Late Oligocene. The wedge was obducted onto northern and northeastern New Zealand at the onset of compressional tectonics about 25 Ma. It was thrust over autochthonous, non-marine to shallow marine
sediments (Eocene-Oligocene) and is unconformably overlain by Early Miocene arc and arc basin deposits. In places, nappes have been shown to be in re-ordered stratigraphic sequence.
The clay mineral sequence observed in the study area is from smectite to illite through illite/smectite (I/S) interstratified mixed-layered clays. Mixed-layer I/S clays from sandstone and shale show a progressive
increase in ordering and number of illite layers with increase in geological age. Illite crystallinity index (IC) of interstratified I/S mixed-layer clays ranges from 0.701'>2?9 to 0.301'>2?9. Most of the values are consistent with advanced diagenesis and anchimetamorphism. A significant statistical correlation exists between values of IC index and mean vitrinite reflectance.
Mean vitrinite reflectance (%Rm), IC and percentage of illite layers in the I/S interstratified clays from different lithofacies show that the rocks have been exposed to a temperature range of approximately 120?C to
250?C. This temperature range does not correlate with that deduced from oxygen isotopic composition of calcite cements and veins formed during burial. The oldest sedimentary succession may have been affected by
localised heating to 300?C from the intrusion of keratophyre dykes.
The temperature estimated by clay mineralogy is in accordance with the values obtained by vitrinite reflectance measurements. These results show that the diagenetic sequence of the Northland Basin seems to be that of original burial and has not been significantly modified by obduction or by Miocene burial