Paleoenvironmental analysis of uplifted coastal lake and wetland sequences in the Wellington Region
Author: Ursula Cochran
Paper number: 3701 (EQC 99/324)
Abstract
Each waterbody is located between two major, active faultlines and all have been inlets of the sea at some time in the past. Sediment cores between 2 and 6 metres long were taken from each site and fossils of single-celled algae (diatoms) were removed from the sediment
and identified. Environmental preferences of different diatom species were used to determine what past environments were present at the study sites. Diatom species change dramatically from the base of the cores to the top and indicate that each site has been isolated from the sea in the last 7000 years. In some cases the suddenness, amount, direction and completeness of the changes suggest that large earthquakes were the cause of change. Evidence for the penultimate earthquake on the Ohariu Fault is recorded at two sites and occurred between 800 and 380 BC. Effects of known past large earthquakes are also recorded at the study sites.
This project extends what is known of the history of earthquakes in the Wellington region and demonstrates the use of fossil diatoms for detecting past earthquakes.
Technical Abstract
Diatom analysis of sedimentary sequences from three coastal waterbodies in the Wellington region, indicates that uplift and/or isolation from the sea has occurred at each site in the last 7000 years. Past depositional environments are determined using high resolution diatom analysis of cores from Okupe Lagoon on Kapiti Island, Taupo Swamp north of Plimmerton, and Lake Kohangapiripiri to the east of Pencarrow Head. Marked changes in fossil diatom assemblages occur in each sequence in response to decreased paleosalinity and/or paleo water depth. The suddenness, amount, direction, and completeness of some of the changes point towards coseismic uplift as the cause of change. Evidence for the penultimate large, surface-rupture earthquake on the Ohariu Fault is recorded at two sites and occurred between 800 and 380 BC. Another seven events represent effects at the study sites of known large paleoearthquakes. This research extends what is known of Wellington's paleoseismic history and illustrates the use of diatom analysis as a paleoseismic tool at sites where geomorphologic or stratigraphic evidence is inadequate.
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