HR: 09:10h
AN: OS31A-03    [Abstracts]
TI: Stratigraphic Evidence of Sorted Bedform Evolution
AU: * Hume, T M
EM: t.hume@niwa.co.nz
AF: National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, 2001 New Zealand
AU: Coco, G
EM: g.coco@niwa.co.nz
AF: National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, 2001 New Zealand
AU: Green, M O
EM: m.green@niwa.co.nz
AF: National Institute of Water and Atmospheric Research (NIWA), P.O. Box 11-115, Hamilton, 2001 New Zealand
AU: Murray, B
EM: abmurray@duke.edu
AF: Division of Earth and Ocean Sciences, Nicholas School of the Environment and Earth Sciences/ Center for Nonlinear and Complex Systems Duke University, Box 90320, Durham, NC 27708 United States
AU: Trembanis, A
EM: atrembanis@whoi.edu
AF: Woods Hole Oceanographic Institution, Applied Ocean Physics and Engineering Dept., 102 Bigelow, MS 12 Woods Hole, MA 02543 United States
AU: McNinch, J
EM: mcninch@vims.edu
AF: Department of Physical Sciences, Virginia Institute of Marine Science, PO Box 1346, Gloucester Point, VA 23062 United States
AB: An examination of inner shelf sediments off Tairua on the north east coast of New Zealand provides stratigraphic evidence of the manner in which `sorted bedforms' or `rippled scour depressions' evolve and the oceanographic or geological conditions control their growth. Side-scan sonar, seismics and 15 vibracores (6 m long), interpreted in the context of wider field studies including swath mapping, seabed sampling and tripod deployments, provide a picture of the surface features and internal architecture of a Holocene sand body, along with detail of the alternating bands of coarse and fine sedimentary units interpreted as sorted bedforms. The coarse fraction is typically shell hash with some rock fragments. The coarse units are thin and typically the thickness of a bedform or two, while the associated fine sand units can be several meters thick. The boundaries between the units are typically distinct, but can be transitional. The coarse units are massive, show upward fining, or are just abruptly coarser at the base, reflecting differing modes of deposition such as settling from suspension, partial reworking of the coarse layer of lateral movements of large ripple over the bed, and the lateral back-and-forth oscillations of the boundaries of the units that we mapped at the surface. The stratigraphy reveals a picture of continual and progressive sorting and the difference between the coarse sand units represents various stages in bedform evolution frozen in time by burial by fine sand. In contrast the fine sands are massive, or exhibit weak horizontal laminae of shell fragments about 1 sand grain thick. While the coarse units are generally well developed and contain little fine sand, we see some that have much higher sand content and may represent coarse units that failed to develop fully before burial. The occurrence of the coarse units is unrelated to the thickness of the unconsolidated Holocene, and there is no obvious layer of coarse source sediments lying at shallow depth below the coarse and fine sedimentary units. In fact the very old C14 dates returned from shell material in the coarse units of surficial sediments suggest that the coarse layers take a long time to accumulate and shell is reworked from older deposits to do this. We present evidence suggesting that the coarse units develop best in areas of low sediment supply. Numerical modeling may help explain these processes. The Foundation for Research, Science and Technology (contract C01X0401) and the Andrew W. Mellon Foundation supported this work.
DE: 3022 Marine sediments--processes and transport
DE: 4863 Sedimentation
SC: Ocean Sciences [OS]
MN: 2005 Joint Assembly