Sedimentology, stratigraphy and structural geology of the Burwood sub-basin, Mount Hamilton - Mararoa River area, western Southland, New Zealand
Structural and sedimentological evolution of the Burwood sub-Basin has resulted in the formation of a range of depositional environments in the Mount Hamilton - Mararoa River area in western Southland, New Zealand. The basement rock in the area is Triassic Murihiku Supergroup sediments (siltstone, sandstone, conglomerate and tuft). The basement is unconformably overlain by a series of sedimentary units that represent a range of depositional environments - from braided alluvial plain through to deep marine basin. With the initiation of extension during the Eocene, the Nightcaps Group began to accumulate; the Beaumont Formation was deposited by a braided river system that flowed from the southwest carrying Median Tectonic Zone and Takitimu Group detritus. Subsequent to this the Orauea Mudstone accumulated in a large lake that covered much of western Southland. During the early-mid Oligocene the tectonic regime became transtensional and the basin continued to subside, forming a sequence of marine units (Waiau Group). The early Whaingaroan Spear Peak Formation was deposited first, as a small, debris flow-dominated, proximal submarine ramp or fan. Detritus contained in this unit is derived from the Takitimu Group and Murihiku Supergroup to the southeast. As sediment supply rapidly decreased, the hemi-pelagic mudstone of the Waicoe Formation was deposited, giving way temporarily to a turbidite-dominated, outer submarine fan (Weydon Formation), flowing from the northeast, carrying Caples Terrane material, before another period of Waicoe Formation deposition. The Waitakian Haycocks Formation was then deposited as a thick, turbidite-dominated outer fan. The sediment is derived from the Caples Terrane and was transported by currents flowing from the north. Deposition of the W aiau Group reflects the change from early-mid Oligocene transtension to early Miocene transpression that continued and increased until the late Miocene, resulting in deformation in the area. Following deposition of the Tertiary units, all the units in the area have been folded and faulted, with orientation of minor structures controlled by the regional Moonlight Fault System and Southland Syncline. Faulting in the southern part of the area parallels the north-south axis of the Southland Syncline, while in the west is parallel to the northeast-southwest Moonlight Fault System. Folding of the units is often disharmonic: the Beaumont and Spear Peak Formations are tightly folded, while the overlying units are more broadly folded. Fold axes in the west are orientated 20" from the Moonlight Fault System, suggesting that they have formed as a result of transpressional strain related to movement on this fault system. Structural analysis indicates that 30% shortening and 5km dextral shear across the area is compatible with the relative orientations of folds and faults. Comparison of bedding orientation above and below the unconformity reveals that the western limb of the Southland Syncline was sub-horizontal until the late Miocene, while the east limb dipped approximately 40". The structural and sedimentary record of the Burwood sub-Basin throughout the Tertiary parallels that of the other western Southland Basins, with a number sedimentary units deposited in a subsiding basin that trended north-south and was bounded by the regionally extensive Moonlight Fault System. The structure in the area also shows evidence for distributed late Cenozoic deformation that resulted in the bending of the South Island terranes.
xii, 164 leaves : ill. (some col.), maps (1 folded) ; 30 cm.
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McDonnell. Michael Peter., “Sedimentology, stratigraphy and structural geology of the Burwood sub-basin, Mount Hamilton - Mararoa River area, western Southland, New Zealand,” Otago Geology Theses, accessed November 12, 2019, http://theses.otagogeology.org.nz/items/show/415.