Poverty Shelf, New Zealand from the Holocene to Present: Stratigraphic Development and Event Layer Preservation in Response to Sediment Supply, Tectonics and Climate

• Main Author: Pierce, Lila Eve Rose
• Format: Others
• Language: English
• Published: W&M ScholarWorks 2012
• Subjects: Geochemistry; Pharmacy and Pharmaceutical Sciences; Sedimentology
• Online Access: https://scholarworks.wm.edu/etd/1539616809; https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=2376&context=etd

The small, high sediment yield Waipaoa River is located on the tectonically active, mountainous Poverty Margin on the east coast of the North Island, New Zealand. In contrast to sedimentary sequences on passive margin shelves, active margins, such as the Poverty Margin, can preserve continuous records of changing geological and environmental conditions at the land-sea interface during rapid sea-level rise. Two subsiding mid-shelf basins on the Poverty Shelf contain thick transgressive sequences which provide a record of evolving river, climate, landscape, and oceanographic conditions since the Last Glacial Maximum (LGM). This dissertation investigates the stratigraphic development of Poverty Shelf, including event layer preservation, over the last ∼14ka using geochemical proxies and physical properties analyses of the sedimentary record. The work herein was conducted under the auspices of the MARGINS Source-to-Sink program, a multi-national, interdisciplinary study focused on understanding sediment routing, transformation and fate through the Waipaoa Sedimentary System (WSS) from catchment sources to final sinks on the adjacent Poverty Shelf and Slope. A suit of five giant piston cores and eight box cores were retrieved from Poverty Shelf during two cruises to address long (Holocene) and short (modern) timescales, respectively, of deposition. Geochronological, geochemical and sedimentological profiles from the giant piston cores are used to reconstruct the processes that influenced shelf infilling during this critical period of recent earth history. Accumulation rate analyses from high-resolution radiocarbon profiles and tephrachronology, along with X-radiographic facies analysis, enable tracking of rapidly shifting loci of deposition from the slope to mid-shelf depocenters. Grain size profiles and delta13C values reveal sympathetic changes that track the waxing and waning influences of sediment supply via the Waipaoa River as sea level rose to maximum flooding at ∼7ka, subsequent Poverty Bay shoreline reorganization and changing shelf accommodation, and, in the last ∼1ka, a strong anthropogenic signal. Imprinted upon this record is evidence for temporal and spatial changes in event layer frequency and type throughout the Holocene. Event layers may be produced by extreme and episodic storms, floods, earthquakes and other perturbative events that punctuate background marine sedimentation with large additions of terrestrial sediment. Event layers can be identified by unique textural and geochemical characteristics. An event layer, likely emplaced via hyperpycnal flow, attributed to Cyclone Bola (1988), the most severe modem cyclone on record in this location, serves as a modem benchmark for other storm events in the Poverty Shelf records. Examination of X-radiographs and lithostratigraphy, textural and isotopic variability reveal that the Poverty Shelf stratigraphic record preserves evidence of exceptional event sedimentation in the past and present that can be distinguished from shifts in supply, transport, and accumulation of sediments due to longer-time scale perturbations related to climate, sea level, and tectonics. A period of increased fidelity of the event record in the mid-Holocene is observed associated with increased accommodation within rapidly flooded depocenters.