Abstract:
Miocene aged rocks in the Taranaki Basin were deposited into a shallowing marine prograding system. Some of these (The Mount Messenger Formation and the Urenui Formation) ouctop along the western coast to the north of the Taranaki peninsula, and form the focus of this study. The Mount Messenger Formation consists alternating sandstones and mudstones seen as turbidites that were deposited into a basin-slope environment. The Mount Messenger Formation has been studied extensively, though these studies have primarily focused on the sandstone facies. The Urenui overlies the Mount Messenger, and consists of mostly silty mudstones with occasional in-filled channels. This study aims to characterize the mudstones found in these two formations, using several techniques on 62 samples. These techniques include Interpretation of sedimentary logs, laser diffraction particle sizing, bulk and clay Xray diffraction, thin section, scanning electron microscopy, and energy-dispersive x-ray spectroscopy. The sedimentary logs were used to interpret the environment of deposition and laser technique provided grain size distribution curves that were plotted again several factors such as age, location, distance from bed-base. Trends of decreasing mean grainsize, increasing amounts of volcanic minerals, and increasing thickness of mudstone beds were identified from north to south (old to young). The mudstones of the Mount Messenger and Urenui formations are volcaniclastic, poorly sorted, silty mudstones. Their mineralogical composition is fairly consistent, but their grain size distribution varies dramatically. They are anisotropic and this study demonstrates that not only do they vary from bed to bed, but within beds too, sometimes only centimeters apart from each other.
