Differential Sedimentary Loading Research Articles

Sedimentary architecture of the rhone turbidite system during the pliocene.

Our study aims to assess the role of salt tectonics on the architecture of the Pliocene sequence of the Rhone deep-sea fan, Gulf of Lions. The study is based on seismic and chronostratigraphic interpretation of multichannel seismic profiling and boreholes. Sediment thickness distribution illustrated by an isopach map indicates how salt deformation has influenced the fan’s sedimentary architecture. Syn-depositional (syn-Pliocene) halokinesis is the main mechanism responsible for the turbidite organization as a series of isolated depocenters. Pos-depositional (syn-Quaternary) mechanism of differential sedimentary loading has also affected locally the present-day sedimentary distribution by post-depositional stretching and compaction of the Pliocene sequence.

Carbonate platforms in wedge-top basins: An example from the Gunung Mulu National Park, Northern Sarawak (Malaysia)

The Melinau carbonate platform initiated during the Mid-Eocene on a rotating slice of the Rajang accretionary prism. The differential sedimentary loading enhanced a rotation of the mobile substratum and created an elongated, asymmetrical wedge-top basin. The extensional southern margin of the basin consists of a 2100–2200-m-thick section of Eocene-to-Oligocene carbonates. These thin laterally towards the northern margin of the basin, where a carbonate factory was active on a postulated underlying thrust. Backstepping and dismemberment of the carbonate system started during the latest Oligocene and deep-marine sedimentation became prevalent over the entire region during the Early Miocene.

Salt tectonics driven by differential sediment loading: stability analysis and finite‐element experiments

AbstractAt many continental margins, differential sediment loading on an underlying salt layer drives salt deformation and has a significant impact on the structural evolution of the basin. We use 2‐D finite‐element modelling to investigate systems in which a linear viscous salt layer underlies a frictional‐plastic overburden of laterally varying thickness. In these systems, differential pressure induces the flow of viscous salt, and the overburden experiences updip deviatoric tension and downdip compression. A thin‐sheet analytical stability criterion for the system is derived and is used to predict conditions under which the sedimentary overburden will be unstable and fail, and to estimate the initial velocities of the system. The analytical predictions are in acceptable agreement with initial velocity patterns of the numerical models.In addition to initial stability analyses, the numerical model is used to investigate the subsequent finite deformation. As the systems evolve, overburden extension and salt diapirism occur in the landward section and contractional structures develop in the seaward section. The system evolution depends on the relative widths of the salt basin and the length scale of the overburden thickness variation. In narrow salt basins, overburden deformation is localised and characterised by high strain rates, which cause the system to reach a gravitational equilibrium and salt movement to cease earlier than for wide salt basins. Sedimentation enhances salt evacuation by maintaining a differential pressure in the salt. Continued sedimentary filling of landward extensional basins suppresses landward salt diapirism. Sediment progradation leads to seaward propagation of the landward extensional structures and depocentres. At slow sediment progradation rates, the viscous flow can be faster than the sediment progradation, leading to efficient salt evacuation and salt weld formation beneath the landward section. Fast sediment progradation suppresses the viscous flow, leaving salt pillows beneath the prograding wedge.

Dynamic restoration of profiles across diapiric salt structures: numerical approach and its applications

The backstripping method that is widely used in basin analysis sometimes fails for salt-bearing basins because the highly mobile and buoyant salt deforms its sedimentary overburden. We present a numerical approach for 2-D dynamic restoration of cross-sections through successive earlier depositional stages. The approach is based on a solution of the inverse problem of the gravitational (Rayleigh–Taylor) instability and combines the Galerkin-spline finite-element method with interface tracking and a backstripping method. Our model interprets basin profiles as multiple layers of viscous fluids with various densities and viscosities. The evolution of salt structures is modelled backward in time by removing successively younger layers and restoring older layers and any diapirs to the stage they were likely to have been. We test the sensitivity of the restoration technique to small variations in density of the layers at different stages in the evolution of diapiric structures. The applicability of the technique was demonstrated by reconstructions of upbuilt and downbuilt diapirs. The technique is used to restore a depth-converted seismic cross-section through the south-eastern part of the Pricaspian salt basin. Mature salt diapirs in the section are shown to have been downbuilt from a salt layer with an initially uniform thickness as a result of differential sedimentary loading until the end of the Triassic before one of the diapirs was buried and actively upbuilt. The numerical approach is well suited for restoration of cross-sections with ductile overburdens, but despite limitations can be developed to 3-D restorations and other rheologies.

Character of the Middle Ordovician Sauk-Tippecanoe sequence boundary in the Ottawa Embayment (eastern Ontario): possible evidence for platform-interior, Taconic tectonism

In eastern Ontario, a Middle Ordovician disconformity that separates predominantly sandstone of the Rockcliffe Formation from underlying dolomite of the Carillon Formation (Beekmantown Group) has, historically, been considered equivalent to the Sauk-Tippecanoe sequence boundary. The character of the formation contact varies considerably, and includes (1) rare, serrated micropinnacles (centimetres in height); (2) smoothed, irregular to undulose erosional bedforms along shallow (<30 cm) incised channels at least ten's of metres in width; (3) a planar boundary with no obvious evidence of erosion; (4) compaction effects across the boundary, as well as within uppermost Carillon and lowermost Rockcliffe strata; and (5) interbedding of Carillon- and Rockcliffe-like lithologies. The latter two observations suggest that, locally, the disconformity represents a relatively minor gap in time. Local, irregular paleotopography is interpreted as some combination of paleokarst, mechanical erosion, and differential sediment loading. The Carillon-to-Rockcliffe transition, from peritidal carbonates to marine or estuarine sandstones, is punctuated by local, anomalous structural and stratigraphic features: ptygmmatically folded fractures, local disconformities, and deposition, then differential erosion of high-energy fluvial-dominated sediments. Differential erosion, fracturing, and paleokarst are local, anomalous attributes of the pre-Carillon paleosurface as well. Combined, these events bracket Carillon deposition, and are interpreted to reflect local platform-interior tectonism coincident with onset of distal Taconic orogenesis. Platform-interior expression of distant, developing changes in plate-boundary dynamics may be linked to incipient basement reactivation along an underlying, shallowly buried Proterozoic fault system, later manifest as the Ottawa-Bonnechere Graben.

Gravity-driven thin-skinned extension above Zechstein Group evaporites in the western central North Sea: an application of computer-aided section restoration techniques

Normal faulting and halokinesis have been important controls on the deposition and subsequent deformation of Mesozoic and Tertiary strata in the North Sea. In addition to the previously documented mechanisms of salt withdrawal, dissolution and differential sedimentary loading, it is recognized that gravity-driven thin-skinned extension above inclined salt layers has played an important part in North Sea basin development. Commercial section restoration software has been used to facilitate depth conversion, restoration and decompaction of seismic sections selected from an interpreted regional database in the western central North Sea, allowing validation of the interpretations, and a graphical and highly quantitative description of salt-assisted extension. Results of this work show that Zechstein Group evaporites were deposited in shallow sag basins during the Permian. Triassic sedimentary pods were generated by localized deposition in synclinal basins and grabens above the evaporites. Bedded salt became folded, while mobile salt flowed to fill anticline cores. Since the early Jurassic, regional tectonic tilting related to post-rift subsidence and increasing sedimentary overburden have caused allochthonous Mesozoic and Tertiary strata to extend by gravity spreading above the mobile salt layer, which detaches the allochthon from the underlying autochthonous Late Palaeozoic rocks. Concave-up listric normal faults sole out in the salt layer, propagate into the overlying cover sewuence, and have been active at different geological times causing stratal thickening and folding within the allochthon. Antithetic and synthetic normal faults have developed, producing complex upward branching fault systems. In map view, the listric faults form curvilinear en echelon arrays, the faults linked by relay ramps. Fault blocks are typically 3–7 km wide, 2–3 km thick and 7–10 km long. Salt movement during the Jurassic-Tertiary has been driven by active extension of the cover, causing salt to fill potential voids created by fault block rotation. Thus salt highs occur beneath sites of extension. The listric faults generally dip in the same direction as the sub-salt surface, although there are also some major counter-regional faults. During extension, regional dips have increased up to about 5, which is sufficient for gravity-driven extension above a salt layer. A total extension of about 6% has occurred. The gravity-driven thin-skinned extension documented in the western central North Sea is a phenomenon which can be recognized elsewhere in the North Sea basin, and can be readily compared with similar phenomena already documented in offshore Angola, Brazil, Nova Scotia and the Gulf of Mexico.

Regional extension as a geologic trigger for diapirism

Initiation of diapirs is one of the least understood aspects of salt tectonics. Differential sedimentary loading or erosion are both effective but not universal. A survey of 18 major salt-diapir provinces shows that salt upwelling is closely linked in time and space to regional extension. Extended salt basins typically develop salt structures, whereas nonextended basins typically do not. Even in salt basins overprinted by inversion or orogenic contraction, the diapirs were initiated during extension on divergent continental margins or in intracontinental rifts. Regional extension thins brittle overburden by forming grabens and half grabens above flowing salt. These fault structures (1) differentially load the salt by their surface relief and (2) weaken the overburden by fracturing and thinning it. Diapiric walls of pressurized salt rise in reaction to the shitting positions of fault blocks in extending overburdens, regardless of thickness, density, or lithology. If regional extension stops, these reactive diapirs stop rising. Eventually the roof of the reactive diapir can be thinned by extension below a critical thickness. Only then can the diapir break through actively as an independent intrusion. Diapiric alignments have been ascribed to basement faulting, even where such faults were conjectural or had trivial displacements. Physical modeling shows that extension of the basement has only indirect influence on diapirism by creating space for extension of the overburden, which is the direct cause of diapirism, whether extension is thick-skinned or thin-skinned and whether the salt was deposited before, duing, or after rifting.

A systematic technique for the sequential restoration of salt structures

A method is described for the sequential restoration of cross sections in areas of salt tectonics where deformation is confined to the salt and higher layers. The subsurface geometry evolves with time through the interaction of various processes: sedimentation, compaction, isostatic adjustment, thermal subsidence (if present), faulting, and salt withdrawal/ diapirism. The technique systematically calculates and removes the effects of each of these processes during specified time intervals defined by the interpreted horizons. It makes no assumptions about salt kinematics and generally results in the area of the salt layer changing through time. The method is described for restoration of extensional terranes, but it is also suitable for areas of contractional salt tectonics with only minor modifications. After converting an interpreted seismic profile to depth, the top layer is stripped off and the underlying section is decompacted according to standard porosity-depth functions. A deep baseline, unaffected by compaction or deformation, is used to restore any isostatic compensation or thermal subsidence. Isostasy is calculated according to the Airy model, and differential sedimentary loading across a section is shown to be approximately balanced by changes in salt thickness so that the load is evenly distributed. After these processes have been reversed, the resulting geometry and the seismic data are used to create the sea-floor template for structural restoration. Fault offsets are removed and the layers down to the top salt are restored to this template, while the base salt remains fixed. The resulting space between the restored top salt and the fixed base salt defines the restored salt geometry. In addition, the difference between the sea-floor template and a fixed sea level provides a measure of the change in water depth (ignoring eustatic changes in sea level). The technique is applied to an interpreted seismic profile from the eastern Green Canyon/Ewing Bank area of offshore Louisiana. The section is characterized by a variety of salt structures, including salt rollers, a diapiric massif, a remnant salt sheet, and a salt weld, which are shown to have derived from an originally continuous salt sheet which has been modified by sedimentary loading. Early loading created vertical basin growth that was accommodated primarily by salt withdrawal and associated diapiric rise through the process of downbuilding. Once the salt weld formed, continued sedimentation was accommodated by a lateral increase in basin size caused by down-dip extension on listric growth faults.

Geomechanics of the South Furious field. An integrated approach towards solving complex structural geological problems, including analogue and finite-element modelling

The South Furious oil field, offshore Sabah (Malaisia), is an asymmetric anticline bounded on the steep southern flank by a series of steep E-W-striking reverse faults and affected by intense E-W crestal normal faulting immediately next to and contemporaneous with the reverse faults. In spite of the very good sand development and a high sand percentage, the reservoir performance is very poor. A study was undertaken to describe and understand the development and the mechanics of this type of structure and to explain their presence in almost undisturbed sediments around them. The regional tectonic development was studied to provide a basis for a geological model of the structure. A structural geological model was necessary to be able to predict the type of faults and fractures and their location in the structure. The proposed structural development was modelled with analogue sandbox experiments and numerically simulated with a finite-element program (GEOFEP). The results indicate that basement faulting related to the closing of the South China Sea, induced differential sediment loading, triggering asymmetric clay diapirism in the core of the anticline. The asymmetry resulted in contemporaneous formation of steep reverse faults at the leading edge of the migrating anticline and normal faults in a crestal collapse system. The reverse faults and associated shear fractures (as seen in cores) have a sealing cataclastic fabric. This explains the severe compartmentalisation of the reservoir with consequently poor production behaviour, in particular in the reverse faulted part of the field.

Physical modelling of North Sea salt diapirism

Scaled analogue models provide a powerful tool for investigating progressive deformation. By varying experimental parameters it is possible to produce a wide variety of diapir morphologies and overburden responses. This paper describes the results of one of a series of experiments which investigates the geometry of overburden sediments around diapirs. The roles of basement faulting and differential sediment loading in the growth of diapirs are assessed. The template for this experiment is provided by analysis of a regional seismic line across the Central Graben of the North Sea. The model produced two diapir types which formed in different ways. Type 1 diapirs are initiated by a large fault smearing the salt along the fault plane which creates a path for later upward migration of salt to produce small diapiric walls. Type 2 diapirs can be triggered by either tectonic or sedimentary differential loading. They are predicted to be longer lived and more voluminous than Type 1 diapirs and will tend to form isolated circular structures.

Basin Architecture, Salt Tectonics, and Upper Jurassic Structural Styles, Desoto Canyon Salt Basin, Northeastern Gulf of Mexico

Despite awareness of the importance of continental extension during rifting, there are few quantitative studies that show the influence of crustal extension on basin architecture, the distribution of salt, and Late Jurassic sedimentation in the DeSoto Canyon Salt basin, northeastern Gulf of Mexico. Application of simplified isostatic principles using a lithospheric buoyancy model allow quantification of total tectonic subsidence, crust thickness, crustal extension, and crust type. Interpretation of over 4800 km of migrated multifold seismic reflection profiles and well data, integrated with computed isostatic relations, provide the basis to characterize Middle Jurassic (Callovian-age) salt halokinetic processes and to describe the structural development of overlying Upper Jurassic strata. An average crustal thickness of 25 km and crustal extension s values between 1.4 and 1.8 suggest the sedimentary succession is underlain by moderately stretched and attenuated continental crust. The widespread distribution and geometry of dipping subsalt reflectors, particularly in the shelfal areas, provide evidence for a Late Triassic-Early Jurassic phase of rifting prior to deposition of Middle Jurassic salt. The distribution of autochthonous salt and the overlying Upper Jurassic sediments reflect the presalt structural imprint and suggest that the basic architecture of the basin was established by the Middle Jurassic following significant attenuation of the crust. Although deposition occurred in a slowly subsiding, stable marginal setting, salt movement and associated growth faulting are the most significant tectonic elements affecting the stratigraphic and structural development of the overlying strata. Faults related to growth of salt structures root at the base of salt in what appears to be a common detachment or decollement for salt movement. The original distribution of salt is widespread and sheetlike with an estimated minimum thickness of 760 m. Progressive basinward subsidence of the margin and differential sediment loading by the overburden are the primary mechanisms initiating mobilization of salt and subsequent deformation of sediments during the Late Jurassic and from the Late Cretaceous through the early Cenozoic.

Salt Tectonism in the Destin Dome Region, Northeastern Gulf of Mexico

The continental margin of the northeastern Gulf of Mexico consists of a thick sequence of Mesozoic and Cenozoic sediments that have accumulated almost continuously since the Middle Jurassic in a relatively stable, slowly subsiding, tectonic environment. The distribution of Middle Jurassic (Callovian-age ) salt is widespread with an estimated minimum accumulation thickness of 760 m. Salt movement and associated listric normal growth faulting are the most significant tectonic elements affecting the structural development of the overlying sediments. Structural and stratigraphic relationships indicated by seismic reflection data suggest two phases of salt tectonism in the Destin dome region. The earliest record of movement of Middle Jurassic salt is in the Oxfordian-age Smackover interval sediments on the Mississippi-Alabama shelf. The development of expanded Smackover interval sedimentary sections is directly related to the regional basinward dip of the presalt surface and to the growth of salt pillows, or rollers, in response to sediment loading. Syndepositional listric normal growth faults occur on the seaward flanks of these salt structures. Growth of the Destin dome anticline on the Florida shelf reflects a significant late phase of salt movement from Late Cretaceous through early Cenozoic time. Differential sediment loading by thick salt updip onto the Floridamore » shelf, is the interpreted mechanism for growth of the Destin dome anticline. The presalt basement surface appears to be a common detachment surface, or decollement, for salt movement.« less

Dynamic changes and processes in the Mississippi River delta

Research in the modern delta of the Mississippi River has revealed short-term changes and processes that are of significant magnitude. Deltaic lobes, each lobe covering an area of 30,000 sq km and having an average thickness of 35 km, switch sites of deposition on an average of every 1,500 yr. Through short periods of geologic time, this process results in a relatively thick accumulation of stacked deltaic cycles covering extremely large areas. Within a single delta lobe, and operating on an even higher frequency, are bay fills and overbank splays. Bay fills, having areas of 250 sq km and thickness of 15 m, require only 150 yr to accumulate. Four major events have taken place in the modern Balize delta since 1838. Overbank splays are much smaller, covering areas of less than 2 sq km and having thicknesses of 3 m, but are associated with high floods on the river. At the river mouth, continued progradation of the distributary channel can form distributary mouth sand bodies that have dimensions of 17 km long, 8 km wide, and a thickness of 80 m in a period of only 200 yr. Differential sedimentary loading at the river mouth results in formation of diapirs that display vertical movements in excess of 100 m in a period of 20 yr. On the subaqueous delta platform, sediment instabilities operate nearly continuously, mass-moving large quantities of shallow-water deposits to deeper-water environments via arcuate rotational slides and mudflow gullies and depositional lobes. All of these changes and processes operate at differing spatial and temporal scales, but all result in deposition of large volumes of sediment over extremely short periods of time.

Significance of Anoxic Slope Basins to Occurrence of Hydrocarbons Along Flexure Trend, Gulf of Mexico: A Reappraisal: ABSTRACT

Recently, Tertiary anoxic slope basins have been proposed as the sources for much of the oil occurring along the Flexure Trend in the Gulf of Mexico. The intraslope basins are thought to have been formed in response to salt diapirism and concomitant salt withdrawal resulting from differential sediment loading between the basins and the diapirs, as well as due to associated faulting. Of the modern intraslope basins, the black, organic-rich muds accumulating in the Orca basin have especially attracted and are suggested to be modern analogs to late Tertiary source rocks accumulated and buried across the continental slope. Although the organic carbon content of the anoxic sediments in the Orca basin is generally high (2 to 3%), the concentration of preserved oil-generative organic matter in these sediments is low. Rock-Eval P2 yields are usually in the range of 340 to 1620 ppm, and hydrogen indices are generally less than 100. Pyrolysis-GC and 13C-NMR data show that up to 30 + % of the organic carbon is contained in carboxyl and other oxygenated groups, which are lost during diagenesis and early catagenesis of the sediments, and that much of the remainder is aromatized and degraded. The degradation was probably by oxidationmore » during settling through the oxic water column. The geochemical data indicate, therefore, that the bulk of the organic carbon in the Orca basin is not capable of forming oil during catagenesis. Published regional cross sections across the Texas-Louisiana continental margin commonly show a thick (0.5-4 km), continuous salt sequence, sourcing salt diapirs and ridges, to underlie the Oligocene(.)/Miocene to Pleistocene sedimentary section of the outer continental shelf and slope.« less

Depositional sequences in clastic continental slope deposits, Gulf of Mexico

Tertiary and Quaternary sediments, overlying diapiric older Tertiary shales and Louann Salt on the continental slope in the western Gulf of Mexico, show cyclicity based on seismic-reflection patterns. A set of indistinct parallel reflections or an acoustically semi-transparent zone, normally onlapping onto diapir flanks, alternates with a set of distinct parallel reflections that drape the sea bottom. The indistinct reflections represent deposits employed by bottom transport during a lowering of sea level. Sea level rise and hight stand are characterized by hemipelagic sediments that form blanket-type deposits. Differential sediment loading causes diapiric activity that may reach maximum upward velocities when sea level rises.

Origin of Intraformational Folds in Jurassic Todilto Limestone, Ambrosia Lake Uranium Mining District, McKinley County, New Mexico: ABSTRACT

The Todilto Limestone of Middle Jurassic age in the Ambrosia Lake uranium mining district of McKinley County, New Mexico, is the host formation for numerous small to medium-size uranium deposits in joints, shear zones, and fractures within small to large-scale intraformational folds. These folds probably were formed as a result of differential sediment loading when eolain sand dunes of the overlying Summerville Formation of Middle Jurassic age migrated over soft, chemically precipitated, lime muds of the Todilto shortly after their deposition in a regressive, mixed fresh and saline lacustrine or marginal-marine environment of deposition. Encroachment of Summerville eolian dunes was apparently restricted to relatively narrow beltlike zones trending radially across the Todilto coastline toward the receding Todilto body of water. Intraformational folding is believed to be confined to the pathways of individual eolian dunes or clusters of dunes within the dune belts. During the process of sediment loading by the migrating dunes, layers of Todilto lime mud were differentially compacted, contorted, and dewatered, producing both small, and large-scale plastic deformation structures including convolute laminations, mounds, rolls, folds, and small anticlines and synclines. During the processes of compaction and dewatering, the mud, in localized areas, reached a point of saturation at which sediment plasticity was lost, causing shearing, fracturing, and jointing of the contorted limestone beds. These areas or zones within the limestone became the preferred sites of uranium mineralization because of the induced transmissivity created by sediment rupture during prolonged sediment loading. Along the Todilto coastline adjacent to the eolian dune belts, both interdune and coastal sabkha environments dominated the Summerville on the margins of the Todilto body of water. Sediment in these areas consists mainly of claystone, siltstone, sandy siltstone, and very fine grained sandstone which was apparently derived from the winnowing of the finer grained fraction of sediment from adjacent eolian dune fields during eolian activity. Most of the sabkha sediments were probably carried in airborne suspension to the low-lying, ground-water saturated, coastal areas where they were deposited as relatively uniform blanketlike layers. Deposition of sabkha deposits was apparently slow and uniform over most of the Todilto coastal and interdune areas, and did not cause the formation of othe than small-scale deformation features in underlying Todilto rocks. Large-scale deformational features as well as uranium deposits are notably absent in the Todilto where it is overlain by finer textured sabkha deposits in the Summerville. End_of_Article - Last_Page 560------------