Strata Formation on the Margins (STRATAFORM) is a program funded by the Office of Naval Research. It is a multiyear, integrated investigation involving modern processes and seismic stratigraphy on the shelves and slopes of northern California and New Jersey.
Introduction
Operating at the boundary between marine and terrestrial environments, continental-margin sedimentation is controlled by processes occurring both in the sea and on land. Because continental-margin deposits have high rates of sediment accumulation and potentials for geological preservation, they provide a high-resolution record of Earth history. Interpreting this record is difficult because of the diversity and variability of processes that concurrently influence margin sedimentation, such as fluctuations in sea level, sediment supply, tectonic activity, and oceanic processes.
Better understanding of continental-margin stratigraphic evolution requires input from a range of geological specialties as well as physical oceanography, benthic biology, and geophysics. The scientific community supports this idea: recent consensus statements call for coordinated oceanographic-geologic investigations of continental shelves and slopes [Nittrouer et al., 1988; Mutter, 1993].
Continental margins are complex environments that are best understood when viewed as a continuum in space and time rather than divided into isolated subenvironments. So studying margins requires innovative approaches such as three-dimensional seismic profiling. Only a coordinated program can factor in the many processes that create continental-margin stratigraphy and can increase our knowledge of Earth history. The Office of Naval Research has begun a program called Strata Formation on Margins (STRATAFORM) to fill this niche. The program is investigating the stratigraphic signature of the shelf and slope portions of continental margins, and its integrated sets of oceanic and geologic data promise to benefit a range of sciences.
The program will conduct field work in two distinctive study areas: the continental margins off northern California and New Jersey. By studying shelf and slope environments together, researchers will be better able to observe and model sediment transport, accumulation, and their imprint on the resulting stratigraphy.
Strategy
Strata are composed of fundamental units that represent various time and space scales and are formed by different processes. On a particular margin, thin laminae may be formed by one process such as tidal currents, while thicker beds may be formed by other processes such as storm events. Facies(which are layers meters to tens of meters thick) are produced by extended sedimentation in distinct depositional environments such as inner-shelf sand or midshelf mud. Predicting stratigraphy requires understanding of how small-scale units combine to form units of the next larger scale, and fully interpreting the history recorded in the stratigraphy requires understanding of the range of processes that form strata at all scales.
Objectives
One goal of the STRATAFORM program is to determine the geological relevance of short-term physical processes that erode transport, and deposit particles and those processes-such as bioturbation and resuspension-that subsequently rework the seabed over timescales ~10^2 y. Combinations of processes with variable intensities create different stratigraphic signatures-layer thickness, shape, spacing, and grain size. The long-term record can be used to identify which signatures are preserved, but only examining the short-term processes can explain how they are preserved.
A second goal is to improve capabilities for identifying the processes that form the strata observed within the upper ~100 m of the seabed commonly representing 10^4-10^6y of sedimentation. Seismic reflection records, coupled with drill hole borings, have been used to develop the concept of sequence stratigraphy, which has unified studies of continental-margin sedimentation. According to this concept, sedimentary deposits can be interpreted within a framework of repetitive successions of facies and associated geometry that are produced by similar processes and are separated from each other by erosional(or nondepositional) surfaces. Sequence-stratigraphic interpretations would be even more powerful if more were known about the processes that produce the sequences and their bounding surfaces.
The third goal is to synthesize this knowledge and bridge the gap between timescales of sedimentary processes and those of sequence stratigraphy. If the stratigraphy of continental margins is composed of fundamental units, then small units observed in process studies can be combined to create larger stratigraphic units, which can be observed by seismic profiling. Determining the relative importance of scale dependence versus independence for geometries of diverse stratigraphic units represents one approach for bridging the gap between different scales. For example, gently dipping strata separating two sections of nearly flat-lying strata are referred to as clinoforms and are typical of prograding sediment on several scales-bedforms, deltas, and continental margins. Can a fundamental understanding of clinoform strata on one scale be used to interpret them on a different scale?
Approach
To observe processes creating strata within a field study, physical events must be frequent and energetic and allow erosion, transport, and deposition of sediment with recurrence intervals less than several years. An active community of benthic organisms also is needed to allow significant postdepositional modification of the seabed on timescales comparable to the erosion frequency. And to preserve resulting strata on time scales of l0^2 y, appreciable sediment supply is necessary. This timescale is relevant to process studies because environmental conditions are relatively well known and several radioisotopes can be used to delineate time within the strata. The range of natural environmental conditions experienced in the field can be supplemented in some cases by laboratory studies. Modeling allows extrapolation of strata production to longer time scales and a broader range of the driving variables such as erosion frequency, bioturbation depth, and accumulation rate.
The principal classification of the sedimentary record by sequence stratigraphy is based on relative sea level. The record is divided into various systems tracts: transgressive, high stand, shelf margin, and low stand. In some places, transgressive systems tracts are now present because of the Holocene sea-level rise. High-stand systems tracts also are found on continental margins, but significant deposits are restricted to areas where appreciable sediment is supplied, filling estuaries and escaping to the shelf. Some modern margins mimic aspects of low-stand systems tracts, where shelves are narrow and sediment is escaping to adjacent continental slopes.
Judicious choices of study areas can allow various components(tracts) to be investigated separately. Then modeling can be used to merge the resulting information to shed light on the formation of complete sequences of continental-margin stratigraphy. This synthetic stratigraphy can be contrasted with seismic observations of real margins to test interpretative skills and to fine tune the modeling.
Study Areas
Northern California and New Jersey were chosen as study sites after much consideration. Northern California is an active collision margin with a coastal mountain range, narrow continental shelf (~20 km), and significant input of muddy fluvial sediment. New Jersey is a passive (trailing-edge) margin with a coastal plain, broad shelf (~150 km), and limited modern sediment input.
Sediment is delivered to the northern California margin by a number of rivers (primarily the Eel and Klamath), with a combined input on the order of 10^7-10^8 tons/yr. This input produces a mud deposit that begins at the seaward boundary of inner-shelf sands and stretches across the shelf onto the continental slope. Sediments on the shelf are frequently eroded and transported by waves and currents associated with winter storms. Between storms, the deposits are mixed by benthic organisms. The resulting shelf strata are interlayered sands and muds modified by bioturbation that have formed a high-stand systems tract 10 m thick.
A sedimentary analog is preserved in the Pleistocene deposits (Rio Dell Formation) of the adjacent coastal range and provides a valuable geological perspective. Although the transfer mechanisms are not well understood, much of the modem shelf sediment is transported to the continental slope. The combination of rapid sedimentation and earthquake activity makes the slope deposits prone to failure. Occurrences of slumps, debris flows, and other types of mass movement have been recognized in this area by their effects on seafloor morphology. The supply of appreciable sediment to the continental slope provides an opportunity to examine sedimentary processes that were present worldwide under different sea level conditions but are not common today.
The New Jersey shelf is characterized by a transgressive sand sheet formed by shoreface retreat during Holocene sea-level rise. The sand sheet is exposed because modem fine-grained sediment input is trapped in estuaries, for example, in the Hudson and Delaware estuaries. The shelf is intensely reworked by storms and has formed an erosional surface molded into ridges. The adjacent continental slope is also sediment starved, although small amounts of fine-grained, organic-rich debris are swept from the shelf. Underlying the New Jersey shelf and slope is <15-km-thick sedimentary section formed when sediment input was greater. Some of the section is Quaternary and Tertiary stratigraphy that preserves a detailed record of the local environment. The history of sea level is well recorded, along with that of subsidence, sediment flux and compaction. Three dimensional profiling with high-frequency seismic techniques has provided even more detailed information about Pleistocene-Holocene sedimentation on the New Jersey margin (Figure 1).
These two study sites located at ~40°N will not answer all the important questions about continental-margin stratigraphy. Tropical and polar margins are notably missing. Important process and substrate categories also are absent, such as areas with very low siliciclastic input where calcium-carbonate sediment dominates the margin and very high siliciclastic input where clay-rich clinoforms are produced. The two study areas were chosen to undertake a major but manageable program, and additional study areas may be introduced in later phases of STRATAFORM.
Research Plans
The STRATAFORM research program will undertake a multiyear, integrated investigation involving modern processes and seismic stratigraphy on the shelves and slopes of the northern California and New Jersey study areas. Although the program is diverse, efforts will be focused on a number of specific scientific studies.
Fine-Scale Shelf Strata. Physical processes produce divergences or convergences in sediment flux that lead to net erosion or net accumulation of sediment. The inner-to-mid-shelf transition from slow accumulation of sand to rapid accumulation of mud is an example of a flux convergence, which ends the journey of many fine-grained particles. This transition commonly is observed on shelves and may be related to changes in bottom slope that influence across-isobath Ekman transport of sediment.
The emplacement of sedimentary structures on shelves typically occurs during energetic transport events (storms). At these times, the surface of the seabed undergoes changes in roughness and sediment size, which in turn affect the flow. This feedback is poorly understood but can be investigated by measuring boundary-layer hydrodynamics coupled with observations of bed development. The frequency of forming different event beds, together with the degree and style of bioturbation, determine the character of preserved strata. Most models of biological stratum modification are one dimensional, although X-radiography clearly demonstrates lateral heterogeneities found in natural environments. More realistic models of bioturbation need to be developed.
Morphologic Changes by Gravitational Processes. The steep gradient of the seabed on continental slopes causes gravitational processes to dominate erosion, transport, and deposition of sediment, and consequently, slope morphology. Failure of slope sediment is triggered by a variety of processes including earthquakes, gravitational instability caused by rapid sedimentation, and venting of fluids. The origins of slope failure and the mechanics of mass movement are poorly understood. Particular attention should be directed toward how slides transform to debris flows and ultimately turbidity currents. An associated factor influencing gravitational processes is the morphology itself. Transport of sediment and the geometry of resulting sedimentary deposits vary along different segments of the slope(such as canyon and noncanyon areas) and these differences need to be understood. One of the most important but least understood factors is fluctuating sea level which modulates the sediment flux to the slope.
Stratigraphic Sequences. The sedimentary record is interrupted by surfaces of discontinuity that bound packages of strata on many scales. Seismic images also reveal bounding reflections separating relatively conformable packages (sequences). Detailed geometric relationships between and within reflecting surfaces and stratal packages are indicative of formative processes. Previous seismic stratigraphic studies, however, have not rigorously constrained interpretations of the formative processes. The greater the observed detail of the seismic architecture, the greater the information that can be gleaned from the seismic stratigraphy. The detail can be enhanced by nested images of various resolutions — that is, different seismic frequencies and through three dimensional seismic surveys. Direct sampling of the seabed can provide ground truth to resolve ambiguities inherent to seismic observations.
Shelf-Slope Exchange. Shelf and slope sedimentation are intimately linked because particles reaching the slope must escape the convergence of sediment flux that leads to shelf accumulation. Shelf edge processes heavily influence the nature and supply of sediment to the slope. To understand this exchange, estimates of sediment delivery are needed as well as details of the responsible processes. The shelf edge is a large boundary and estimates of total flux to the adjacent continental slope require sediment budgets that construct time- and space-averaged inventories of sediment accumulation. However, the actual mechanisms of off-shelf transport are driven by processes such as shelf storms or breaking internal waves, which operate over short time scales. The processes of off-shelf transport also are sensitive to margin morphology(e.g., the presence of a submarine canyon or a nonincised shelf break).
Long-Term Strata Development. STRATAFORM provides a unique opportunity to test models of strata development on continental margins. Comparing model computations with field measurements is one aspect of model testing for mathematical representations of shelf-slope processes. Also important in testing is the evaluation of stratigraphic-model extrapolations. As the timescale of interest increases, information about physical and biological forcing becomes less reliable and indirect seismic measurements must be used to characterize stratigraphy. Model testing, therefore, becomes more challenging. Most models of margin stratigraphy are simple two-dimensional representations designed to demonstrate sedimentation over millions of years. They are not valid for representing much shorter term sedimentation (that is, <10^4 y) and for characterizing the spatial geometric variability inherent to natural systems. Three-dimensional models are needed that combine the constraints imposed by direct observations of processes, morphology, and strata.
Publications
Alexander, C.R., 1996. Slope sedimentation on the Eel River continental margin. EOS, Transactions, American Geophysical Union, v. 76, p. OS 10.
Borgeld, J.C., 1996. Preservation potential of strata deposited on the Eel River shelf during 1986 and 1989 flooding of the Eel River. EOS, Transactions, American Geophysical Union, v. 76, p. OS 10.
Leithold, E.L. and R.S. Born, 1996. The fate of flood layers on the Eel River shelf — following the organic carbon signature. EOS, Transactions, American Geophysical Union, v. 76, p. OS 10.
Narwold, C.F. and J.A. Curtis, 1996. Particle size and water content analysis for the 1995 flood deposits on the Eel River shelf. EOS, Transactions, American Geophysical Union, v. 76, p. OS 10.
Sommerfield, C.K. and C.A. Nittrouer, 1996. The radioisotope record of flood sedimentation on the Eel River shelf, during 1995. EOS, Transactions, American Geophysical Union, v. 76, p. OS 10.
Sternberg, R.W., A. Ogston, and R. Johnson, 1996. A video system for in-situ measurement of size and settling velocity of suspended particulates. Journal of Sea Research, 36.
Syvitski, J.P.M., M. Nicholson, and K. Skene, 1995. Application of hydrologic model RIVER4.1 to Eel River basin, California, a flood-dominated basin. EOS, Transactions, American Geophysical Union, v. 76, p. F240.
Wheatcroft, R.A., 1996. The evolving geometry and internal character of the 1995 flood deposits on the Eel River shelf. EOS, Transactions, American Geophysical Union, v. 76, p. OS 10.
Yun, J., D.L. Orange, and J.C. Moore, 1996. Changes in seafloor morphology due to gas migration in the Eel River basin, CA: Observations using multichannel seismic images. Abstracts with Programs, Geological Society of America, v. 28, p. 128.
Wheatcroft, R.A., C.K. Sommerfield, C.A. Nittrouer, and J.C. Borgeld (1995) The formation and preliminary modification of a flood deposit on the Eel River Shelf, Transactions, SEPM Congress on Sedimentary Geology.
Niedoroda, A.W., C.W. Reed, D.J.P. Swift, H. Arato, and K. Hoyanagi (1995) Modeling shore-normal large-scale coastal evolution, Marine Geology 126, 181-199.
Austin, J.A., C.S. Fulthorpe, and T.A. Davies (1995) Unraveling the stratigraphic complexities of the last deglaciation: Ultra-high resolution 3D seismic images of the New Jersey continental shelf, EOS, Transactions, American Geophysical Union, 76, F308.
Fulthorpe, C.S. and J.A. Austin (1995) Sequence stratigraphy geometries and neogene evolution of the New Jersey continental margin, EOS, Transactions, American Geophysical Union, 76, F308.
