Multiple frequency, overlapping geometry, or "nested," seismic images offer an unprecedented opportunity to study how geological processes create and preserve shelf/slope stratigraphy. Multifrequency seismic coverage, including multibeam backscatter/swath bathymetry (Figure 1), has been collected at two continental margin "natural laboratories," New Jersey and northern California, which are each characterized by distinctive suites of depositional processes.
Before we can decipher past sea level fluctuations and determine the rates of long-term climate change, we need a clear understanding of how the sedimentary products of modern processes are preserved, because continental terrace stratigraphy is the long-term, "frozen" record of Earth history. Investigators are using remote sensing data, sampling, and modeling to link short-term sedimentation processes—those acting over hours to centuries—with sequence stratigraphic geometries and facies characteristic of the upper ~100 m of the rock record and the last ~106 years. Preliminary results suggest that geologic events, like large storms, leave recognizable deposits on continental shelves, cross-shelf sediment transport occurs via channelized drainage at all seismically recognizable scales, and two- and three-dimensional models tied to process inputs effectively reproduce observed seismic geometries.
The New Jersey margin (Figure 1), although now relatively quiescent, contains a well-developed sequence stratigraphy in which late Paleogene-Neogene sea level variations are recorded. In addition to an extensive seismic database, holes have been drilled and logged both onshore and offshore. Ocean Drilling Program (ODP) Leg 174A will drill on the New Jersey shelf in mid-1997 to continue a sea level transect begun by ODP Legs 150 and 150X. In contrast to New Jersey, northern California is undergoing rapid sedimentation. There, voluminous, fluvial sedimentation is punctuated by floods and modulated by storms. Investigators in California recently completed detailed, high-resolution multichannel seismic (mcs) profiling offshore to augment lower frequency mcs profiles collected by industry and single-channel profiles collected by the U.S. Geological Survey.
Fig. 1. Gridded multibeam backscatter/swath bathymetric coverage from mid-1996 reconnaissance mapping of the New Jersey margin. Yellow indicates high backscatter intensity, and blue indicates low backscatter intensity. Bathymetry is artificially illuminated from the north and contoured at 25 m (white lines). The modern shelf break is roughly defined by the 150-m contour. This composite image reveals a more complex seafloor morphology for the mid-outer shelf than was recognized from earlier compilations: east-northeast-oriented sand ridges, erosional pits, shore-perpendicular megaripples, shore-parallel furrows, and, to the northeast on the Hudson Apron, iceberg scours. The presence of scours on this margin suggests that reworking below the sea level lowstand (approximately -130 m) of the last glacial episode has been minor. High-resolution multichannel seismic reflection PROFILES are indicated by dashed lines; detailed multichannel seismic grids represent areas that have been evaluated by the Ocean Drilling Program for hydrocarbon hazards.
Modeling ties the complex seismic images of continental margin stratigraphy to the physical processes that create them. The challenge is to merge event-scale processes—that is, storms and floods—with the seismic "pictures" of strata preserved over much longer timescales. The goal is to quantify how environmental parameters contribute to the stratigraphic record. Only then can that record be understood in terms of its implications for sea level history and climate change. The suite of models now being generated considers the occurrence and stratigraphic effect of individual events, the role events play in the accumulation of bodies of sediment, and the lateral shifting and stacking of sediment sequences that form the stratigraphic record.
The modeled processes must be merged and calibrated at scales appropriate to the observational data. The development of facies in response to events and stratigraphic sequence models must also be formulated in terms of the dynamics of the sediment and underlying lithosphere that control the observed patterns; predictions are ultimately tested against field examples, in both two dimensions and (eventually) three dimensions. Figure 2 shows one example of modeling at the sequence scale (1 million years). The model is interactive. Long-term sediment input can be combined with geophysically constrained alterations to the space available for deposition, then compared with seismic data at various frequencies. Physically based, process-oriented algorithms are now being developed for individual model components.
Fig. 2. Simulation of passive margin stratigraphy showing progradation of several "typical" clinoform sequences across the New Jersey continental shelf. These seismic geometries are characteristic of Neogene continental shelves worldwide, suggesting globally synchronous mechanisms for their formation. The model is driven by falling sea level, with a 30-m amplitude and ~1 million year cyclicity. Time lines are drawn every 100,000 years, colored as follows: nonmarine (green); shoreface (yellow); marine shelf (maroon); clinoform (gray) foresets. As observational databases on selected margins grow, these models can become more sophisticated in their predictions of margin stratigraphy.
Acknowledgments: The work discussed is being performed by STRATAFORM (Strata Formation on Margins), which is supported by the Office of Naval Research. Research updates are available on the SUNY-Stony Brook home page; contact C. Nittrouer by e-mail at cnittrouer@ccmail.sunysb.edu for more details. The STRATAFORM Stratigraphy Project members are M. Field, C. Fulthorpe, J. Goff, L. Mayer, G. Mountain, A. Niederoda, D. Orange, M. Steckler, and D. Swift.—J. Austin Jr., University of Texas, Austin, Institute for Geophysics
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