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Eruption, erosion and post-volcanic marine faunal colonization, the Red Bluff Tuff Formation, Chatham Islands, New Zealand
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posted on 15.02.2017by Sorrentino-Mariconda, Leonor
This thesis reports for the first time phreatomagmatic deposits and preserved Surtseyan tuff cones from the Chatham Islands, New Zealand. This research located the relicts of at least six, closely-spaced, Paleogene Surtseyan cones and associated bioclastic sediments within the Red Bluff Tuff Formation (RBT). The complete stratigraphic section was studied in detail by means of fieldwork (sampling, logging, sketching and recording main structural and textural features) as well as comprehensive laboratory work (marine faunal taxonomy, grain-size, componentry and vesicularity analyses). It was found that RBT consists of two parts: 1) the lower part represents volcanic aggradational processes that constructed tuff cones in a short period of time (years) and is composed of a bedded interval of explosively fragmented, vesicular glassy basaltic pyroclasts (ash and lapilli sizes) as well as feeder-dykes, pillow-lavas and pillow-sills and 2) the upper part represents the rapid denudation of these cones by shallow marine currents or gravity-flows reflecting the instability of the tephra-pile forming the cones, and a much later complete marine faunal colonization stage (e.g. corals, brachiopods, molluscs). Erosion could have occurred almost immediately after (or even during) the emplacement of the volcanic pile, similar to what has occurred at Surtla vent, a satellite submerged cone of the basaltic island volcano Surtsey, Iceland; the Waiareka-Deborah Volcanics Bridge Point, Aorere Point, and Lookout Bluff Surtseyan cones (Otago, New Zealand); and Marion and Prince Edward islands (Southwest Indian Ocean). By contrast, the entire faunal colonization and stabilization of a diverse marine community could have taken hundreds, or perhaps even thousands of years to reach their acme following the volcanic pulses. The structural, textural and compositional characteristics of RBT support a phreatomagmatic mode of fragmentation similar to that at Surtsey Volcano, Iceland. Although RBT represents an extremely interesting, rare, and novel example to study pre-historic submarine volcanism very little research has been done on this volcanic sequence. Because the RBT sequence characterizes one of the most complete marine bioclastic tuff cones described in the geologic record the present study fills a large gap in our knowledge of the eruption and faunal recovery phases of submarine volcanoes.