Initial sedimentation processes and the early geological evolution of three maar craters, Hindon Maar Complex, Otago
Murphy, C. E. J. (2018). Initial sedimentation processes and the early geological evolution of three maar craters, Hindon Maar Complex, Otago (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/12107
Permanent Research Commons link: https://hdl.handle.net/10289/12107
The Hindon Maar Complex, located near Hindon, 25 km NW of Dunedin, consists of four volcanic craters infilled by lake sediments and mass flow deposits. The maars form part of the Waipiata volcanic group, which was active from 25–9 Ma. Drilling of Maar 1, Maar 3 and a sediment-filled depression between the inferred locations of Maar 2 and Maar 3 revealed up to 10 m of laminated biogenic lake sediments underlain by siliciclastic mass flow deposits. This project describes the siliciclastic deposits from these drill-cores and from limited outcrops of Maar 2. Physical properties, stratigraphic logs, X-ray fluorescence spectrometry, Also XRD, SEM and optical petrography are used to create a facies analysis and infer the processes of initial sedimentation into the maar craters following the maar-forming eruptions. A total of 10 facies are identified within the Hindon Maar complex. Facies 1–6 show the progression of crater wall stabilisation from rockfalls which occurred weeks to years after the initial eruption to the formation of organic rich lake-sediment units which would have been deposited hundreds of years post eruption. Facies 1–6 have a high proportion of mica, quartz and schist grains, indicating extensive incorporation of the country rock. Nonconsolidated mottles are found throughout facies 2–5 and are interpreted as the remnants of weathered pyroclastic materials which were sourced from failures in the upper crater walls or tephra ring. Facies 1 is a laminated carbonaceous lake sediment consisting almost entirely of organic matter. Facies 2 is a nonconsolidated laminated silty clay formed by debris flows. Facies 3 is a consolidated silty fine sand with discrete gravel lenses and 5% mottles formed as a result of mass flows. Facies 4 is a fine gravel breccia comprising up to 35% mottles, which is formed by mass flows originating in the upper crater or tephra ring. Facies 5 is a poorly consolidated fine gravel breccia which formed as a result of turbidity currents. Facies 6 is loose schist and quartz grains of fine gravel to medium pebble size, formed by rockfalls into the early lake. Facies 7 is a silty clay which is geochemically and mineralogically different to all other facies. Facies 7 exhibits convoluted bedding and is an example of a slump deposit occurring locally in Maar 3. Due to its significant differences to all other facies, it is assumed to have had a different parent material. Facies 8–10 are found in the area of a major gravity anomaly associated with Maar 3. These deposits are composed of silt and clay sized particles and have elevated Fe. They have high magnetic susceptibility and density and are interpreted as weathered pyroclastic material of the tephra ring. The infilling of Maar 1 occurred initially as a series of coarse-grained mass flows into the crater (Facies 6 and Facies 5). Once the crater wall began to stabilise and the occurrence of rockfalls decreased, Facies 4 was deposited, resulting from high crater wall collapse. This was followed by furthered, small cater wall collapse of Facies 3. Facies 2 was later deposited once crater wall failures halted, and resulted from the erosion, rather than the collapse, of the crater wall. Once the crater wall was fully vegetated, Facies 1 began to accumulate, forming thick organic rich deposits, indicating the maar lake was stratified and had an anoxic bottom layer. The infilling of Maar 3 followed the same pattern. However, there is a significant lateral offset between some units of the same facies, which may indicate more complex processes, such as faulting. The exact history of Maar 3 cannot be determined with the information acquired.
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