The emplacement of overlapping submarine deposits of monogenetic Surtseyan-style volcanoes onto a submerged continental shelf that built-up over millions of years in the Waiareka-Deborah Volcanic Field, New Zealand

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Moorhouse, Benjamin Luke

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Since the eruption of Surtsey (1963 - 1965) many studies have been made of the resulting island, but the pre-emergent base remains submarine, un-incised and little studied. The same is true for many recently studied Surtseyan volcanoes, and means the pre-emergent parts of subaqueous eruptions are not well documented. This thesis presents an in depth study of the superb deposits of Surtseyan-style eruptions preserved in the South Island of New Zealand in Otago sea-cliffs along the coastline between Oamaru and Moeraki. These Eocene-Oligocene intraplate basaltic field deposits erupted in Surtseyan-style onto a submerged continental shelf and have since been exposed above sea level. They are inferred to be typical products of submarine processes such as those that built Surtsey to the sea surface. Volcanic fields typically include many small, monogenetic, volcanoes formed by single eruptions fed by short-lived magma plumbing systems that solidify after eruption. However, the stratigraphy of Cape Wanbrow suggests that eruptions produced multiple volcanoes whose edifices overlapped within a small area, but separated by millions of years. The small Cape Wanbrow highland is shown to include the remnants of 6 volcanoes that are distinguished by discordant to locally concordant inter-volcano contacts marked by biogenic accumulations or other slow-formed features. This discovery challenges the traditional view of monogenetic volcanoes and calls for researchers in monogenetic fields to start evaluating both unstudied and previously studied monogenetic volcanoes with this in mind. The 6 volcanoes contain several lithofacies associations: (a) the dominantly pyroclastic E1 comprising well-bedded tuff and lapilli-tuff, emplaced by traction dominated unsteady, turbulent high-density currents; (b) E2, massive to diffusely laminated block-rich tuff deposited by grain-dominant cohesionless debris flows; (c) E3, broadly cross stratified tuff with local lenses of low to high-angle cross-stratification which was deposited by either subaerial pyroclastic currents or subaqueously by unstable antidune and chute-and-pool forming supercritical flows; (d) E4, very-fine- to medium-grained tuff deposited by turbidity currents; (e) E5, bedded bioclast-rich tuff with increasing glaucony content upward, emplaced by debris flows; (f) E6, pillow lava and inter-pillow bioclastic sediment; and (g) E7, hyaloclastite breccia.

These lithofacies associations aid interpretation of the eruptive evolution of each separate volcano, which in turn grew and degraded during build-up of the overall volcanic pile. Sedimentary processes played a prominent role in the evolution of the volcanic pile with both syn- and post-eruptive remobilization of debris from the growing pile of primary pyroclastic deposits of multiple volcanoes separated by time. An increase in bioclastic detritus up-sequence suggests that the stack of deposits from overlapping volcanoes built up into shallow enough waters for colonization to occur. This material was periodically shed from the top of the edifice to form bioclast-rich debris flow deposits of volcanoes 4, 5 and 6. Bedform geometries of volcanogenic sedimentary structures produced in both subaqueous and subaerial environments can be incredibly similar, if not identical, and this has resulted in a long history of difficulties in unambiguously distinguishing primary from reworked deposits, and gas-deposited from water-deposited ones. Sedimentary structures such as dunes and low- and high-angle cross-stratification produced by numerous flow types make interpretation of setting difficult. In particular the architecture of such deposits and often field observations of contextual detail can be extremely difficult to interpret based on their ubiquitous presence in many settings and flow types. This is made increasingly difficult when structures are poorly preserved, exposures are limited and independent palaeoenvironmental indicators are absent or ambiguous. However correct interpretation of such sedimentary structures along with the deposits they are part of can be crucial to understanding the host volcanic sequences. The origins of dunes and associated structures that occur within the pyroclastic deposits at Cape Wanbrow serve as an example, and have long been debated. To determine the depositional setting of dune-bearing deposits, careful analysis of contextual information has been completed along with the examination of well-described examples of; (1) subaerial dry pyroclastic deposits, (2) subaerial moist pyroclastic deposits, (3) deposits of gaseous fluid-gravity flow (e.g. eolian currents), (4) unidirectional fluid-gravity water flow deposits (e.g. rivers, tides) and (5) aqueous sediment-gravity flow deposits split into those comprising pyroclastic material and those with non-pyroclastic material. This includes examination of the physical controls that shape each example and the factors controlling bedform deposition in that environment with the aim of being able to distinguish between major flow types in each environment. For Cape Wanbrow, this analysis showed that ambiguous bedforms were formed subaerially, which thus provides direct evidence for emergence and subaerial growth of one of the the volcanoes represented in the succession. To understand multiple eruptions of monogenetic volcanoes at one site at Cape Wanbrow a study of peridotite xenoliths from Kakanui and Boatmans Harbour revealed that the peridotitic portions of sub-continental lithospheric mantle (SCLM) beneath North Otago contains a degree of compositional variation but is relatively fertile. The origins of the xenoliths indicate that the magmas are from either the base of the spinel facies lithosphere, from within the predicted narrow zone of garnet facies lithosphere or from within the asthenosphere. The xenoliths indicate that the magma source was fairly deep, and therefore the process that led to multiple eruptions over a small geographical area has to be one that affected the asthenosphere or the lower lithosphere.

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xx, 219 pages A4

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2015Moorhouse

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Open Access

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POLYGON ((170.940006954538575 -45.086981031540319,171.00179816729198 -45.089032006114557,171.000225480588284 -45.153310855850748,170.940970196037284 -45.153686530991479,170.940006954538575 -45.086981031540319))

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http://download.otagogeology.org.nz/temp/Abstracts/2015Moorhouse.pdf

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Moorhouse, Benjamin Luke, “The emplacement of overlapping submarine deposits of monogenetic Surtseyan-style volcanoes onto a submerged continental shelf that built-up over millions of years in the Waiareka-Deborah Volcanic Field, New Zealand,” Otago Geology Theses, accessed October 20, 2021, http://theses.otagogeology.org.nz/items/show/609.

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