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6 PASS IVE MARGINS: GROUP 3 DIVERGENT CONTINENTAL ~GINS-- POST-RIFTING INTERNAL PROCESSES SUGARY Post-rifting divergent margin basins are the principal low to moderate temperature chemical factories on our planet. It primarily in these basins that erosional debris from the continents accumulates with organic debris from the oceans and slowly "cooks." These chemical reactors produce massive mineralogical changes in the basin sediments, most of the worId's reserves of oil and gas, and mineral deposits. It is important scientifically, economically, and environmentally that we understand how these giant reactors operate. Because fluid flow is fundamentally involved in almost all the reactor processes and products, the single most important scientific objective is to understand the causes and interactions that control fluid flow in post-riftina divergent margins. The processes involved are principally the mechanics of sediment accumulation, faulting, and other kinds of tectonic remobilization (such as diapirism), and burial diagenesis. As with any chemical reactor, the critical problems or principal steps required to understand sedimentary basins on divergent margins are: (1) characterization of the reactor at various instants of time, particularly the fluid and chemical _ _ ~ fluxes within and across the boundaries of the reactor; (2) documentation and interpretation of the cumulative products of the reactor over appropriate intervals of time; and (3) construction of a process model capable of simulating and predicting the operation of the reactor. The studies that are needed are case history investigations carried out with a view toward the refinement and testing process model of margin reactors that joins tectonic, stratigraphic, structural, chemical, and fluid flow processes and accounts for their interactions. 61
The strategy required is the proper coordination of process model development and case history investigations. Process model development and case history investigations should proceed in parallel. Case studies addressing, first, present fluid and chemical fluxes in basins, and then the integrated effects of such fluxes as they have operated over the history of basin development appear to be particularly pertinent. We believe that sedimentary basins on divergent continental margins are especially amenable to effective study at the present time because highly evolved and complimentary land- and sea-based technologies and observations can be merged today in these locations. At least partial joining of land and sea investigations on administrative and funding levels could produce dramatic scientific returns. Post-rift divergent margins are a good place to initiate such coordination because they are geologically simpler than convergent margins, and because land and sea are tectonically contiguous at divergent margins. BACKGROUND The diversity and scale of chemical processes that occur in divergent continental margins are impressive. At the top of a basin, bacteria consume organic matter and produce CO2. As the sediments are buried and move through the normal geothermal gradient,-they are warmed and as a result they slowly "cook." Especially within and below the "oil window" at 60 to 90° C, the organic matter "matures 'I to produce hydrocarbon fluids. These reactions have positive ~ V and consequently are expelled from their host strata in a process called primary migration. Inorganic reactions that expel water also occur at about the same temperatures. The principal inorganic reactions involve the progressive crystallization of clays (e. g., the tranformation of smectite to illite). Thermal expansion of pore fluids and positive TV reactions such as mentioned above cause large volumes of divergent margin basins to become "overpressured" below about 800m depth. Fluid pressures approach lithostatic values in the Gulf of Mexico and the Niger Delta, for example. However, the evolution of fluid pressure and fluid flow within the overpressured zones is not well understood. There is some evidence that specific subzones within a basin behave as independent flow systems (pressure bottles or bladders), but it is not understood how the transition zone between "hard" overpressure and hydrostatic pressure evolves so as to remain at a depth of -80Om, and the implications of this evolution for chemical diagenesis of the entire basin are not clear. Transition zones are important sites of hydrocarbon entrapment in some basins (e.g., the top of geopressure in the Gulf of Mexico) and not in others (e.g., the 62
North Sea ~ ~ There is some suggestion f ram base metal mineralization in salt domes and in Mississippi Ball By-type lead-z inc deposits at basin margins that large, overpressured regions in sedimentary basins episodically rupture and t' squirt" brines out escape structures on margin edges . If true, such sul f ide accumulations could tell us a great deal about the f luid dynamics of basins. Fluids moving within divergent continental margins often precipitate minerals on preexisting sediment grains or dissolve these grains. This chemical alteration can completely plug or completely remove entire sedimentary units. Mineral overgrowths are referred to as "cements" because they tend to cement the pore space and change loose sediments into competent rock. Careful petrographic, chemical and fluid inclusion studies of cements, particularly in cases where "cement stratigraphy'' or identifiable zoning in the cements al lows a sequence to be deduced, provide a record of mineral precipitation from basin pore fluids and thus a record of fluid movements within the basin. Fluids escaping from divergent sedimentary basins can produce dramatic effects, particularly at sea. If the fluid expulsion is rapid enough, near surface temperatures and heat fluxes may be affected. Gas hydrates accumulate in the upper sediment prism of many continental margins where methane and other small gases combine with water to form a crystalline solid that fills the interstitial pore space. Hydrates form under conditions of low temperature, high pressure and high gas saturation. They naturally break down at depth in the sediment column because of geothermal heating. The base of the hydrate layer thus migrates through the sediment column as sedimentation continues. Pressure changes associated with changes in sea level also cause the hydrate boundary to migrate. Such migration may cause the generation of fluid overpressures and promote strain (faulting) at the base of the hydrate layer. Progressive migration of the hydrate zone through the upper sediment column alters the fabric of the strata in ways that could profoundly change the porosity and permeability structure of whole sections of a margin. Perhaps the most dramatic consequence of fluid venting is the chemosynthetic-communities it supports. The first chemosynthetic communities in the deep sea were discovered at hydrothermal vents at mid-ocean ridges. The communities live off the oxidative contrast between reduced vent waters and oxygenated seawater. In the last few years chemosythetic communities have alto been discovered in continental margin settings. The first site discovered was along a fault zone off San Diego. Later, abundant communities were found around saline seeps at the base of the Florida Escarpment. Since then, communities have been found on accretionary prisms off Oregon and Japan, around oil seeps in the Gulf of Mexico, around slump scars off Nova Scotia, 63
and in. several submarine canyons. Along one careful ly surveyed 2 On long segment of the Florida Escarpment, chemosynthetic communities ~ ine ~10% of the escarpment base. Major box canyons developed in carbonate strata in the surrounding area may have f ormed as the result of rock dissolution associated with pore water discharges at the canyon heads. Sing ar canyons are common along the eastern margin of North America. THE S INGLE MOST IMPORTANT SCIENTIFIC O=ECTIVE The above discussion shows that a rich variety of recent observations bear on f luid movements and chemical processes in post-rift continental margins. The problem of comprehending and describing post-rifting changes in cli~rergent continental margins can be usefully focused by the important parameter of fluid flow, because fit uid flow is directly involved in many of the changes and refit ects nears y al ~ the others. We are really dealing with a giant, natural, fixecl-bed fluid chemical reactor. The single most important scientif ic obey ective for post-rift divergent margins can be stated as understanding the interactions that control fluid flow and its chemical ant] tectonic consequences as a function of time. Processes Involved A large number of processes interact to control f luid f low in post-rift divergent margins (and post-rift sedimentary teas ins in general ~ . These are illustrates} schematically in Figure ~ . Cal KgedImentatio~ Folding Faulting, Fracturing, Jointing Salt Heat Flow Fluid Generation Rock Alteration metamorphism diagenesis Seal Formation gas hydra~ces C~3 Overpressure - Density Differences salinity therma1 Topographic FIGURE 1 Interrelated processes controlling fluid flow in divergent margin teas ins . 64
The movement of pore fluids in a sedimentary teas in is driven by a number of mechanisms, but principal ly by the fluid overpressures produced by compaction, heating, and diagenetic reactions that occur as the sediments are buried. The geometries and grain size of the sediment packages are the first control s on pernT eabil ity and thus on the movements of fluids, but as the margin evolves, faulting, fracturing, j Dinting, and salt and mud do apiri sm impose additional control s. The movements of fluids nemselves cause permeabil ity modi f ications through chemical alteration. All these processes exert first order infix uences on fig uid movement. None can be disregarded!. Critical Probe ems There are two especially critical problems : ( 1~ visual- izing the effects of the operation of such a large-scale thermo-chemical-structural reactor; and (2) developing an integrated process model that can describe the operation of such a reactor. The steps required to address these problems are: (~) definition of the present distribution of fluid flux in divergent margin basins (fluid flux snapshots); (2) definition of the cumulative effects of past fluid fluxes; and (3) development of the interpolation and process models required to interpret and understand the observed fluid fluxes and their products. It is difficult to collect and synthesize data in a million cubic kilometers of sediments so that the interlinked processes that affected them can be seen. Furthermore, interrogating such a volume of rock and determining the cumulative products of reaction is a daunting prospect. Unusual audacity is required to build ~ model that incorporates and integrates all the diverse processes that influence operation of the reactor. Nonetheless, it is our view that progress in understanding post-rifting processes in divergent margin basins can best be promoted by attempting just such a large-scale, process-oriented synthesis. With regard to process modeling, each of the individual processes circled in Figure 1 are relatively well understood. What is particu~arly-needed at this point is a quantitative understanding of the linkages between individual processes, and the formulation of an integrated model that can describe post-rift internal processes in divergent continental margins as a whole. NEEDED STUDIES The studies required to meet our principal scientif ic goal of understanding the interactions that control f luid f low and 65
its chemical and tectonic consequences as a function of time can best be performed as a series of case studies that: 1. define the present flow rates crossing basin surfaces, 2. define the cumulative products of fluid flow in representative rock volumes (e.g., the total fluxes), 3. develop methods to extend geographically sparse data to geologically reasonable volumes, and 4. integrate components of a numerical model (Figure 1) that will eventually be capable of simulating the history of fluid flow and chemical reactions in a sedimentary basin. Obtaining Snapshots of Fluid and Chemical Far uses in Divergent Margin Basins Understanding the present rates and directions of fat uicI flow In divergent margins is the most direct approach to determining overall flux rates and the evolution of fluid flow through time. We strongly endorse studies that measure or def ine the velocities of fluid flow across any surface within a given basin. Studies that determine average flux rates through 100 to 10,000 square meter areas should be especially encouraged. Several examples of projects that might merit special attention are: Fluid Seeps on Flanks and Toes of Divergent Margins Mapping chemosynthetic community distributions could provide a measure of the present-c3ay fluid efflux from divergent margins and is an example of a basin survey that can only be made at sea. Geopressured Fluid Flow Much could be learned from comprehensive measurements of the present pressure distribution within an active overpressured basin. Heat Flow Surface heat flow surveys on continental margins constrain models of fluid movement. To filter out the effects of seasonal bottom water temperature variations, surveys using shallow boreholes or a deeper insertion of temperature and pressure probes are needed. Heat flow surveys within basins could address the question of whether fluid flaw between the underlying 66
basement and the overlying sediments is an important agent of mass transfer over time, or only in the initial stages of a basints evolution. Heat flow surveys within a basin could reveal the rate and distribution of fluid movement through pressure seals. Determining present fluid fluxes in divergent margins will require substantial resources. As indicated in Tabs e 1, mapping of submarine seeps wil ~ require ship time, and sophisticated instrumentation like deep tow sidescan sonars with photographic calibration. Submersible time will al so be required to obtain reliable samples of subsurface fluids. Much of the data on geopressured areas have been collected by the petroleum industry, and strong academic-industry ties wil ~ be required! to make use of this important database . Better heat f l ow and pore pressure measurements will require modification and development of new technologies to penetrate below the zone of biannual fit uctuations. Determining the Cumulative Effects of Fluid and Chemical Fluxes in Divergent Margin Basins The previously suggested studies would provide a current snapshot of fluid movements and chemical fluxes. Studies of the cumulative products of diagenesis and fluid flow would potentially provide a record of all post-rifting alteration and fluid flow. We encourage any study that can measure the cumulative consequences of sustained fluid flow. Constraints should be sought on the interval of time over which particular phases of alteration takes place, so that chemical reaction rates and chemical fluxes can be deduced and compared with the determinations of the snapshot studies. Establishing chrono~ogic constraints is especially important. Examples of possibly significant integrated studies include: Diagenetic Cements Studies that determine the volumes and spatial distributions of diagenetic products, and the timing of chemical reactions would provide a direct record of movements of basin pore fluids. The main challenges to such studies are: (1) the labor intensiveness of the required petrographic, chemical and fluid inclusion analyses; (2) difficulties in properly extrapolating from local drill hole analyses to obtain estimates of the average alteration over a broader region; and (3) problems dating specific cement "strata." New chemical logging technologies may facilitate both data collection and extrapolation and help solve the first two of these challenges, especially if selected core is available to calibrate the data 67
TABLE 1 Required Resources for Studies of Fit uid F1 ow in Divergent Margins Seafloor surveying ant} sampling 1. Deep-tow surveys. 2 ~ Submers ibles-sampl ing e f f luents . 3 . Flux measurement devices and experiments e * 4 . Longer heat flow probes (10-20 m ~ ong) to avoid biannual temperature wave on shed yes, instrumented with pore pressure capability. * 5 . Geophys ical surveys to image stratigraphy and alteration. cletailec] 3-D seismics. Fat ux Snapshots Ef fects of Cumulative Fluxes Interpolation and Process Models Downhole devices 1. In-situ pore water samplers. * 2. Pore pressure anti rock stress surveys. 3 . Downhole flowmeters; tracer tests at inj ection wells. 4. Wirel ine re-entry tools. Long-term observatories and/or repeated sampling. * 5. Core acquisition. 6, Logging (routine and special tool s*, detailed cumulative surveys). Other resources 1. Technology to date alteration minerals, strata, anc} f luid inclusions with higher revel ution. ~ 2 . Access to case histories and/or -pi 1 ot tests 0 f sparse data interpolation methods. 3. Access to supercomputer facilities; software e* Models workstations for database/model comparison. 4. Physical and chemical models of basin processes (tectonic, structural, stratigraphic, diagenetic)~* * requires new technology and/or development 68
petrographically and stratigraphically, and augment it by providing paragenetic information. Current methods of determining absolute ages of cement strata are only applicable in very restricted instances, but there is hope that these methods may be broadened. More generally applicable cement dating techniques are urgently needed. Gas Hydrates Hydrates may provide a direct measure of the f flax of methane, carbon dioxide, and other gases from large portions of continental margins. Possible interactions between fluctuating sea levels, gas generation at the base of the hydrate layer, and detachment fault ting promoted by fluid overpressures illustrate the kinds of interactions between fluid overpressure and tectonics that also occur at the larger, whole-basin scale (Figure 1~. Studies of such slumping could provide valuable insights into much larger scale basin phenomena. Hydrocarbon Maturation Hydrocarbon maturation is controlled by the time-temperature history of the source rock. Predicting this thermal history is of critical importance to successful hydrocarbon exploration. The existing indicators of time-temperature history (vitrinite reflectance, thermal alteration index, 40Ar/39Ar, fission tracks, hydrocarbon isomerization and aromatization) need to be extended, carefully calibrated, and broadly debated and reviewed. Spatial and temporal variations in thermal maturation relate to the integrated values of many of the fluxes discussed above. Maturity variations may directly ref lect integrated thermaal ef fects of fluid flow, although conductive heat transfer usuall dominates. The three approaches to measuring integrated effects of fluid flow sketched above illustrate the potential diversity and richness of various measures of integrated flux. They are illustrative only. We anticipate that many approaches will be possible. The approaches listed require the use of conventional tools as well as-the development of new tools and methodologies, as indicated in Table 1. Developing Integrated Models to Simulate Fluid-Chemical Processes in Divergent Margins Snapshot and integrated fluxes measured must be interpolated and simulated by process models. Local interpolation models are needed to convert borehole measurements, chemical logs, or seismic images to measures of alteration over relatively large 69
(~lOOm~ x ~100 m) volumes of sediment. Integrated process moclels are needed to bring data from different basins to bear on common problems of understanding basin reactors. Local Models Sectimentol ogists are enjoying considerable success with ''dif fusion" models that simulate how highlands erode and depressions fit 1. Moclels of sedimentation that take into account the geometry of lakes and seas, ant] the effects of storm wind stress on currents r erosion, and sedimentation are remarkable in their abil ity to s imulate observed sediment patterns . What now needs to be done is to cieter~ine the permeability impl ications of these models so that measured alteration can be extrapolated taking into account the paths of likely fluid movement. Structural geologists have long recognized a scale invariance which they have encapsulated in Pumpelly~s Rule that small-scale structures reflect large-scale structures. Minor and major structures exert a tremendous influence on fluid flow, and it is vital to develop the means to describe=the average effects of structures of all scales. Tracts theory offers the possibility of doing this, and it is an actively developing field. Salt domes and tongues are manifestations of salt tectonics which, in many divergent-margin basins, are the principal agents of deformation. The impermeability of salt and its ability to rotate and fracture surrounding rocks strongly constrains fluid movement. Salt tectonics--have been simulated-realistically by centrifuge experiments and finite element modeling. ., . . . .. ~ Local models and~modeling~approaches--can-be expected to be- diverse. Numerical, analog, and statistical methods are all needed. In addition, they need to be tested and calibrated in a variety of case studies. -Material properties contributed by local models and studies-will feed-directly into an integrated process model. - - . .. . . The Integrated Process Model The final litmus test of the understanding of any reactor is the demonstration that an integrated process model can simulate its operation. We cannot rest until we have developed such a model for the economically most significant geological reactors on our planet--sedimentary basins. The components of the integrated mode] are shown in Figure 1, and they have been discussed previously. Models of virtually all individual components have already been developed. What 70
remains is to fuse these components together into a model of the entire reactor* This can be done using supercomputers, especially machines that allow parallel computation of large three-dimensional models such as sedimentation, structural deformation, and fluid, chemical, and thermal flux. We envision, again, that this will be done in stages, with the first step combining and testing pairs of models, such as structure and fluid flow. Later models will stitch together and test model pairs* Many pilot or case history analyses will be required. To a considerable degree, the greater the number of independent efforts that can be mounted, the faster and surer the progress that can be expected. It is important to recognize that as models are combined, each constrains the others. For example, if sedimentological models predict premeabilities and temperature distributions that, when combined with realistic fluid-drive mechanisms and chemical alteration models, predict cement distributions similar to those observed, we will be encouraged that we are on the right track. STRATEGY REQUIRED TO ACHIEVE GOALS The most appropriate strategy for achieving the scientific goal of understanding the interactions that control fluid flow and its chemical and tectonic consequences as a function of time is to fund a series of case studies that are loosely coordinated around the objective of developing an overall process model. Observational case studies will undoubtedly and very properly occur on a variety of divergent basins, as exposures, data availability, and investigator interest dictate. Case studies should be connected by the common goal of defining, through observations or models, the operation of post-rift divergent margin basins as giant thermo-tectonic-structural- chemical fluid reactors. At the same time, it will be important to fund theoretical or modeling efforts that provide a basis for interpolation of observations and that work toward a fully integrated mode] of basin processes. ~ Perhaps the most important need is for funds in support of the interdisciplinary collaboration that is essential to developing and testing both local and integrated models. Individuals must be encouraged to address problems that lie largely outside their specialties. BROADER IMPLICATIONS Study of the movement of overpressured fluids in divergent margin basins, where resources of land and sea technologies can 71
be brought to bear in an unusually effective fashion, can contribute to the solution of a diverse set of important geological problems. For example, overpressured volatiles in magmas at convergent marg ins in f luence the so ructura ~ evo lut ion o f intrus ions and have prodll~::ed signif icant ore deposits ~ e . g ., porphyry copper , and molybdenum and gold deposits) . overpressure al so allows hydrocarbons to escape f rom the very impermeable, organic rich beds in which they are generated. This primary migration is followed by secondary migration into structural or stratigraphic traps where economic quantities of hydrocarbons may accumulate. Overpressure along faults may control earthquakes in ways not currently understood. Finally, fluid overpressures are thought to fundamentally control the shape and tectonics of accretionary prisms. Progress in detecting, modeling, and predicting overpressured fluid flow in divergent margins will contribute directly to similar research efforts in convergent continental margins where the processes involved are more complex. SELECTED BIBLIOG~PHY Abbott , D . H ., ~ . W . Embley I and M ~ A e Hobart . 19 8 S . Correlation of shear strength, hydraul ic conductivity, and thermal gradients with sediment disturbance: South Pass region, Mississippi Delta. Geo-Marine Letters 5: 113-119 . Bethke , C . M., W. J . Harrison, C. Epson , and S . P. Altaner . 1988. Supercomputer analysis of sedimentary basins. Science 239: 261-267. Cathies, L. M., anc] A. T. Smith. ~ 983 ~ Thermal constraints on the formation of Mississippi Valley-type lead-zinc deposits and their implications for episodic basin dewatering and deposit genesis. Econ. Geoff ~ 78: 983-1002 e Paull , C. J., and A. C. Neumann. 1987 . Continental margin brine seeps: their geological consequences. Geology 15 : 545-548 . Talbot, C. J ., and M. P. A. Jackson. 1987 . Salt tectonics , Scientific American 2 SS ~ 8 ~ : 70-79 72
