UTIG RESEARCH PROJECTS ARCHIVE

SEA LEVEL AND CYCLIC SEDIMENTATION
THE CANTERBURY BASIN, NEW ZEALAND

Check out the cruise reports and the data!

THE THICK sedimentary deposits beneath the world's continental shelves respond to one of the Earth's fundamental cyclic processes, the rise and fall of global sea level. Increasing collection of marine seismic reflection profiles by the oil industry during the 1970s revealed that continental margins comprise stacked units, known as sequences, which are separated by gaps in deposition (unconformities). Peter Vail and co-workers at the oil company Exxon proposed that each sequence is the product of a cycle of global sea level; the sequence-bounding unconformities formed by erosion of the shelf during sea-level lows. This work led to the compilation of a global cycle chart purporting to illustrate the history of global sea level for the last 250 million years.

If sequences are caused by a global process, they must be approximately synchronous worldwide, i.e., sedimentary basins around the world must have the same patterns of sequences through time, and must also correlate with proxies for global climate change, such as the oxygen isotope record. However, it is difficult, with existing techniques, to date sequences with sufficient precision to make the necessary global correlations. Furthermore, processes with purely local, as opposed to global, impact can also produce sedimentary cycles. Examples of such local processes are crustal uplift and subsidence, sediment supply rate, and ocean current activity. Therefore, the theory that sequences are predominantly the result of global sea-level change remains one of the central controversies in the earth sciences.

SCIENTIFIC OBJECTIVES
This new research effort, funded by the National Science Foundation, will attempt to distinguish global and local controls on sequence formation in the Canterbury Basin, on the eastern margin of the South Island of New Zealand. This is an exceptional location in which to gain such understanding. Large-scale basin-fill geometry is a response to an 80 million-year rise and fall in relative sea level controlled by regional plate tectonic evolution. The basin is close to a strike-slip plate boundary (the Alpine Fault) between the Pacific and Australasian plates. Uplift and erosion of the Southern Alps at the plate boundary have led to high rates of sediment supply to the offshore basin during the last 20-25 million years. This, in turn, has resulted in the preservation of unusually high-frequency (0.5-1 million-year periodicity), seismically resolvable sequences. Sequences are bounded by unconformities with distinct paleo-shelf edges (clinoform breakpoints) that are well preserved and not planed off by erosion. Absence of faulting and folding offshore, measurements of strain distribution and vertical movement onshore, and tectonic subsidence curves from an offshore exploration well all suggest that the nearby plate boundary has not added undue tectonic complexity during the Neogene development of the offshore Canterbury Basin.

An 18-day cruise is planned on R.V. Maurice Ewing in January 2000 to collect high-resolution, multichannel seismic (MCS) data in order to improve upon exisiting interpretations based on low-frequency, commercial MCS data. The new seismic profiles will have a vertical resolution of approximately 5 m; the comparable figure for the existing MCS data from the basin is approximately 20 m. Experience with similar profiles, already acquired on the New Jersey and northern California margins, indicates that the improved resolution will allow enhanced imaging of features diagnostic of depositional and erosional processes, e.g., fluvial shelf channels, slope canyons and sediment drifts. Interpretation and mapping of the new MCS data, coupled with forward stratigraphic modeling, will allow evaluation of the relative roles of eustasy, tectonics, sediment supply and currents in the stratigraphic development of this relatively young (80 Ma rifting) continental margin for direct comparison with its antipodal counterpart, the New Jersey margin. Based on ties to existing exploration wells, the sequences are of late Miocene age, correlative with those drilled on the New Jersey margin (Ocean Drilling Program (ODP) Legs 150, 150X, 174A and 174X). This is the essence of the global sea-level approach advocated by several ODP planning groups since 1987 and the planned survey will augment an existing ODP proposal (JOIDES Proposal No. 511) to drill this margin.

EDUCATION & TRAINING
This is a three-year project that will include graduate student participation in all phases, from data acquisition at sea, to processing and interpretation. The graduate student will gain experience in MCS data processing, sequence stratigraphic interpretation, and basin analysis and modeling This training is invaluable to students interested in careers in the oil industry and service industries. Participation will also include two K-12 science teachers and involve collaboration with researchers in other institutions, both within the US and New Zealand and a role in planning for ODP drilling.

SIGNIFICANCE
Sea level change is perhaps one of the earth's most societally relevant processes; rises expected in the next century will affect directly hundreds of millions of people. Texas will be one of the regions most affected, because of its vast, low-lying coastal expanse. This project will enhance our ability to decipher the complex record of geological and climatic processes contained within continental-margin sediments. It will also help us to understand the links between depositional and erosional processes and preserved stratigraphic architecture. Coupled with future ODP drilling, it will provide new information on sediment types associated with seismic geometries, results of direct relevance to oil exploration.

Publications related to this study:
Lu, H., Fulthorpe, C., and Mann, P., 2003, Three-dimensional architecture of shelf-building sediment drifts in the offshore Canterbury basin, New Zealand, Marine Geology, v. 193, p. 19-47.

Carter, R. M., C. S. Fulthorpe, and H. Lu, 2004, Canterbury Drifts at Ocean Drilling Program Site 1119, New Zealand: Climatic modulation of southwest Pacific intermediate water flows since 3.9 Ma, Geology, 32, 1005-1008.

Lu, H., and C. Fulthorpe, 2004, Controls on sequence stratigraphy of a middle-Miocene to Holocene, current-swept, passive margin: offshore Canterbury Basin, New Zealand, Geol. Soc. Amer. Bull., 116, 1345-1366.

Lu, H., C. S. Fulthorpe, P. Mann, and M. A. Kominz, 2005, Miocene-Recent tectonic controls on sediment supply and sequence stratigraphy: Canterbury basin New Zealand, Basin Res., 17, 311-328.

Two additional papers/dissertation chapters are underway.