Tectonic Modelling, East Coast Basin, New Zealand.
Honours Degree, 1997
University of South Australia
The purpose of this study was to construct a balanced cross section across the study area and constrain the distribution of the Paleocene Waipawa Black Shale in the East Coast Basin. The Waipawa Black Shale is an excellent potential source rock.
The East Coast Basin lies on the eastern coast of North Island, New Zealand. The basin has undergone three main phases of development: (1) Early to Late Upper Cretaceous active margin, (2) Paleocene passive margin and (3) Early Miocene to Recent oblique subduction. The Pacific plate is being subducted west beneath the Indian plate. The subduction zone lies off the east coast of the North Island, New Zealand. The East Coast Basin is a fore-arc basin which exhibits fault-generated folds and thrust faults related to the oblique subduction.
Data available included a long seismic line, which although not straight, is almost perpendicular to the regional strike. Two short seismic tie lines, two drill holes and surface mapping.
The complex structure has resulted in rapid vertical and lateral facies changes; hence the stratigraphy is poorly understood. Structure is dominated by north-north-east trending, north plunging, fault-generated folds and imbricate thrusts.
The Cretaceous to Late Oligocene section is thrusted and thins to the east. There is a major unconformity between this section and the overlying, mildly deformed Pliocene to Recent rocks. Another time break occurs between the Oligocene and Miocene, when subduction was initiated.
The balanced section exhibits imbricate thrusting overlain by relatively undeformed Pliocene to Recent rocks. These rocks show evidence of reactivation of most of the faults between the Pliocene and present day. The Cretaceous to Miocene thrusted section thins to the west and plunges to the north. Older rocks are exposed in the cores of the fault related anticlines to the south.
By comparing the thicknesses in the thrust slices to the thickness of the section on the map, an idea of the distribution of the Waipawa Black Shale was obtained. The information collected indicates that the Waipawa Black Shale lies under most of the study area. The depth of the oil window has been calculated at 3-4.5 kilmetres. On the section, most of the Waipawa Black Shale lies between 3 and 4 kilometres and so should be mature in this area.