HR: 1340h
AN: OS23C-1332    [Abstracts]
TI: Coupled Passive Margin Stratigraphic Evolution and Fluid Flow
AU: * Wolinsky, M A
EM: maw@duke.edu
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708-0227 United States
AU: Pratson, L F
AF: Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC 27708-0227 United States
AB: Continental margin processes are strongly influenced by patterns of overpressure and fluid flow within basin sediments. These patterns effect consolidation, slope stability, and the transport of heat and solutes. Numerous processes generate overpressure, but in many passive margins rapid deposition is thought to be a dominant mechanism, producing a two-way coupling between sedimentary processes and fluid flow. We study this coupling using a fully 2D margin-scale coupled stratigraphic evolution and fluid flow model. The sedimentary process model describes transport of sand and mud through coastal plain, shoreface, shelf, slope, fan, and rise depositional environments, as well as migration of internal boundaries between environments. The fluid flow model uses the finite element method (FEM) to solve for fluid flow on a dynamic grid. We explore the effects of 2D coupling for various basin geometries, sea-level histories, and sediment influx rates. We find that on a margin-scale, deposition tends to be localized in depocenters, which are initiated by sediment flux divergence across depositional environment boundaries (e.g. shoreline, shelf-break). Over timescales relevant to pore-pressure evolution, these boundaries and their associated clinoforms migrate in response to sedimentation and relative sea level change, resulting in complex dynamic patterns of overpressure. Although in some regions fluid flow is quasi-1D (vertical and forced only by local deposition), in many regions fluid flow is strongly 2D. Complex flow results from heterogeneous stratigraphy, with strong contrasts in sand and mud permeability determining preferential flow paths. On regional scales, confined horizontal sandy units, such as buried turbidite fan complexes, act as fluid flow conduits, inducing flow anisotropy. On smaller scales, interbedded sandy and muddy deposits produce anisotropic effective permeabilities with preferential horizontal flow. The complex dynamic geometries inherent to margin stratigraphic evolution have heretofore prevented effective margin-scale modeling coupled 2D stratigraphic-hydrologic system, but our computational approach introduces two innovations. A dynamic meshing technique allows for rapid meshing of arbitrary deposits while maintaining bounds on FEM conditioning and error. And a hierarchical mesh structure, which aggregates sediment layers into mesh nodes into finite elements, embeds sub-grid dynamics into the FEM model via layer averaged anisotropic permeabilities. Our results show that in regions of homogeneous lithology, quasi-1D numerical or asymptotic approximations are applicable, but that in regions with complex stratigraphy, strong anisotropy effects cannot be ignored. Our meshing and FEM approach is general and easily adapted for use with any sedimentary or backstripping model, and models including more grain sizes or processes have an even greater potential for complex 2D fluid flow. Hence our results demonstrate the viability and necessity of including 2D effects in continental margin fluid flow models.
DE: 4255 Numerical modeling
DE: 3022 Marine sediments--processes and transport
DE: 3230 Numerical solutions
DE: 1815 Erosion and sedimentation
DE: 1829 Groundwater hydrology
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting