Sediment Input Points Research Articles

The evolution of catchment‐depositional system relationships on the dip slopes of intra‐rift basement highs: An example from the Frøya High, Mid‐Norwegian rifted margin

AbstractBasement highs form one of many potential sediment source areas during the evolution of continental rifts and rifted margins and add to the topographic complexity typical of active rifts. Footwall basement highs acting as a source area to sedimentary systems in the hangingwall of major faults have been documented in many systems worldwide. However, the back‐tilted footwall dip slopes of such highs have received comparatively little attention. Here, we investigate a subsurface case study from the Norwegian continental shelf, where catchments and shallow marine syn‐rift sedimentary systems on a dip slope are preserved due to early transgression of an intra‐rift high. At the onset of Late Jurassic rifting, the Frøya High emerged as a prominent, N‐S trending, 25 km‐wide basement high tilted towards the east in response to several kilometres of displacement along the Klakk Fault Complex, a major normal fault zone at the Frøya High's western edge. Using well‐calibrated 3D seismic reflection data, we observe a series of conspicuous Upper Jurassic wedges along the eastern edge of the Frøya High along the margin of the Froan Basin. Internally, these wedges show sigmoidal geometries marking top and foresets of clinoform packages with a maximum thickness of ca. 200 m with foresets between 30 and 200 m high, dipping ca. 10° towards the east, southeast and northeast. We interpret these wedges to represent a series of eastward prograding deltas positioned along a constructional shoreline, connected to E‐W trending valleys and river catchments up‐dip. The deltas show strong progradation, interpreted to reflect the impact of the continued uplift of their catchments prior to the abrupt termination of sediment supply from drainage capture by footwall scarp drainages. The presence of a connected, largely constructional shoreline has implications for Late Jurassic sediment distribution around the Frøya High, providing primary sedimentary input for longshore‐driven sedimentary systems in the Draugen Ridge to the north. Comparisons with other syn‐rift dip slope systems highlight a broadly similar evolution but shows a distinct lack of the protracted backstepping observed in other dip slope systems. We postulate that different structural configurations of dip slope systems, being footwall uplift or hangingwall subsidence driven, may drive the strongly progradational character of the deltaic systems on the Frøya High. The Frøya High example highlights the need to constrain primary sediment input points to aid the interpretation of volumetrically significant, but short‐lived and subtle depositional systems, especially within complex, tectonically active settings.

Sedimentation on structurally complex slopes: Neogene to recent deep‐water sedimentation patterns across the central Hikurangi subduction margin, New Zealand

AbstractDeep‐water sedimentation on active margins often entails complex sediment transport pathways through slope accommodation. Sedimentation in such settings is commonly differentiated into ‘fill and spill’ vs. ‘tortuous corridor’ models. To investigate the utility of these models in convergent settings 15,344 km2 of 3D seismic data is used to investigate sedimentation and erosion patterns across the Hikurangi subduction margin. A series of thrust‐bound trench‐slope basins, each tens of kilometres long by kilometres wide, have been diachronously forming, filling and deforming through the Neogene until today. Five primary input points delivered sediment to the basins along the studied part of the margin. Channels display both axial and transverse orientations, the run‐out lengths of which vary temporally. At various times, relatively coarse‐grained sediment was trapped in the interior basins, occasionally then to be cannibalised during landsliding or erosion of growing structures. At other times, coarse‐grained sediment was bypassed to distal basins or the trench. Multiple sediment input points and occasionally tortuous sediment dispersal corridors result in the evolution of convoluted depositional systems, often with similar styles of sedimentation occurring contemporaneously in proximal and distal basins, contrary to simple models of basin fill. A hierarchy of controls on sediment distribution can be distinguished. At the highest level, sediment distribution is controlled by external factors, for example, glacio‐eustacy and tectonics. At basin scale, the interaction of sedimentary systems with local relief (e.g. evolving seafloor structures and landslides) dictates the location and style of deposition. At the lowest level, autocyclic factors (e.g. flow response to earlier deposits) influence the spatiotemporal variation in erosion and sedimentation. The complex interplay of these factors dictates whether basins were filling, spilling or some combination at any point in time, whilst basins that were filled and spilled may subsequently resume filling due to changes in the bounding conditions. Hence simple use of ‘fill and spill’ or ‘tortuous corridor’ models to tectonically active margins is not advised. Furthermore, as sedimentation may influence structure growth, constraining the controls on sediment distribution may improve understanding of the broader evolution of convergent margins and their resource distribution.

Evolution of syn‐ to early post‐rift facies in rift basins: insights from the Cretaceous–Paleocene of the Great South Basin, New Zealand

AbstractEvolution of rift basin fill and geometry depend on the complex interactions between fault growth, sediment supply, base level changes and pre‐existing basement fabric. This study integrates multiple datasets in the Great South Basin (GSB), southeast New Zealand, and provides key insights into the evolution of depositional environments in rift basins, including the interplay between normal faulting, sediment supply and sediment dispersal patterns. It also examines the control of pre‐existing basement fabric on rift geometry and sediment distribution in the syn‐ and post rift successions. The syn‐rift is up to ~5.5 km thick in the GSB, and is underlain by several different basement terranes. Three syn‐rift stages are recognised; c.105–101, 101–90 and 90–83 Ma. During the initial syn‐rift, isolated northeast‐trending graben developed, with resultant alluvial fan/fan delta, fluvial, coastal and lacustrine sediment fill. The balance between sediment supply and accommodation space exerted considerable control on facies, especially the presence of lacustrine facies. During the later stages of syn‐rift, marine transgression occurred and connectivity between the graben developed, with shelfal, shoreface and marginal‐marine facies deposited. With marine transgression across the hinterland, sediment supply was significantly reduced in the northeast of the basin, leading to underfilling of graben, and preservation of rift topography for up to ~20 Myr after the cessation of faulting. In the west, where sediment supply was higher, rift topography was quickly filled. The NW‐trending basement terrane boundaries controlled accommodation space development during the initial stages of graben formation. Later in the syn‐ and post rift stages, these terrane boundaries formed long‐lived sediment input points into the basin, and controlled the position of repeated large deltaic depositional units.

How is stratigraphic modeling of frontier basins dependent on data: A case study of the Shelburne sub-basin, offshore SE Canada

Frontier basins are difficult to model with forward stratigraphic modeling software, due to the limited amount of geological information available from sparse wells and limited seismic coverage. Five different simulations with varying amounts of geological information are compared to a geologically constrained reference case model using all available data for the Shelburne sub-basin, in order to understand the relative importance of various geologically constrained parameters to modeling frontier basins. The modeling has been performed with DionisosFlow™, a 4D multilithology diffusion-based deterministic forward stratigraphic modeling software. All five less constrained models resemble the major features of the reference case model, with a similar distribution of sand on the shelf and localized progradation of sand across a delta into deep-water. In detail, there are significant deviations in sedimentation patterns. Not all geologically constrained parameters are equally important to modeling; the most influential parameters are paleobathymetry and sediment thickness defined by seismic surfaces. In this case study, the model could not be calibrated unless three sediment input points were chosen, as demonstrated independently from detrital petrology studies. Predefining certain modeling parameters generally reduces computational time. Unconstrained models offer a first overview of the probable architecture of the basin, and are a useful tool which can reduce the overall computation time of the modeling process and give insight into sedimentation patterns.

Determining sediment provenance history in a Gondwanan erg: Botucatu formation, Northern Paraná Basin, Brazil

The Lower Cretaceous Botucatu Formation comprises a dry-aeolian system developed in western Gondwana and preserved within the Paraná Basin of central South America. Multiproxy provenance analysis (detrital zircon UPb dating, heavy mineral, petrographic and granulometric analysis) was undertaken along two 500 km transects of the northern Paraná Basin (NW and NE transects). To assess the provenance variation within the aeolian stratigraphy, samples were taken from the base and top of the Botucatu Formation and from sandstones interbedded with volcanics in the overlying Serra Geral Formation. Granulometric analysis shows a mean fine- to very fine-grained sand. The bulk petrography yields an average composition of Q91F6L2, and heavy mineral analysis gives a mean of 86% of zircon-tourmaline-rutile, demonstrating the overall polycyclic nature of the sediment. The discrete occurrence of garnet, apatite and epidote indicates local sediment input points, providing indirect evidence for point-sourced fluvial input into an overall dry aeolian system. Detrital zircon UPb LA-ICP-MS dating demonstrates a dominant Cambrian to Late Neoproterozoic (520–750 Ma) population with subsidiary Tonian-Stenian (0.9–1.2 Ga) and Orosirian-Rhyacian (1.9–2.3 Ga) contributions. The Botucatu Formation along the northern basin margin displays similar provenance to the southern part of the basin, pointing to a predominance of recycling processes within the basin. Statistical plots illustrate the detrital zircon age variability - recording an upward decrease in the age proportion of Permian and Tonian-Stenian populations and an increase in Cambrian-Late Neoproterozoic dates towards the top. Such variability can be related to the recycling of distinct Paraná Basin strata. The proportion of Cambrian-Late Neoproterozoic dates increases in the NE, indicating a possible input from the Ribeira Belt granitoids. Similar to major-scale dune-fields, Botucatu erg register variable sand sources there are possibly linked with coexisting depositional systems. In summary, the northern Botucatu erg is mostly fed by local sources reworked from underlying Paraná Basin units, with little direct input from basement sources and limited impact of regional wind-pattern variability.

Predicting river mouth location from delta front dip and clinoform dip in modern and ancient wave‐dominated deltas

AbstractWave‐dominated deltas and strandplains make up the majority of the world’s depositional coastlines, provide an important record of sea‐level change and serve as hydrocarbon reservoirs worldwide. Satellite imagery forms a great source of data on the recent depositional history of modern deltaic systems. In the subsurface, three‐dimensional seismic and well data make the three‐dimensional assessment of large‐scale deltaic reservoir bodies possible but struggle to resolve internal heterogeneities away from wells. To bridge this gap in characterizing deltaic sedimentation, this study combines measurements from both the shallow, high‐resolution section of three‐dimensional seismic data of the Eocene Halibut Delta in the Outer Moray Firth, offshore Scotland, with information from Google Earth’s satellite imagery and digital elevation model on south‐east Brazilian river deltas (São Francisco, Jequitinhonha, Doce and Paraíba do Sul) to present a means of predicting the location of fluvial sediment input points with respect to clinoform geometry. The key measurement for this study is the delta front and clinoform dip which has been measured at multiple locations along strike of the coastline of the examined deltas. Dip decreases away from the inferred river mouth for all deltas by 50% within 7.2 km. The river mouth location was inferred from the position of palaeo‐channels visible on the delta top and coarse sediment recorded in grab samples offshore for the south‐east Brazilian deltas, and from imprints of palaeo‐channels on attribute maps for the Eocene Halibut Delta. In summary, this study found that delta front dip is steepest at the location of the river mouth and decreases, along with grain size, away from it. This suggests that high dip values correlate with the proximity to the channel mouth and can be used to predict fluvial channel facies in modern deltaic systems and subsurface reservoirs.

Quantitative analysis of a footwall‐scarp degradation complex and syn‐rift stratigraphic architecture, Exmouth Plateau, NW Shelf, offshore Australia

AbstractInteractions between footwall‐, hangingwall‐ and axial‐derived depositional systems make syn‐rift stratigraphic architecture difficult to predict, and preservation of net‐erosional source landscapes is limited. Distinguishing between deposits derived from fault‐scarp degradation (consequent systems) and those derived from long‐lived catchments beyond the fault block crest (antecedent systems) is also challenging, but important for hydrocarbon reservoir prospecting. We undertake geometric and volumetric analysis of a fault‐scarp degradation complex and adjacent hangingwall‐fill associated with the Thebe‐2 fault block on the Exmouth Plateau, NW Shelf, offshore Australia, using high resolution 3D seismic data. Vertical and headward erosion of the complex and fault throw are measured. Seismic‐stratigraphic and seismic facies mapping allow us to constrain the spatial and architectural variability of depositional systems in the hangingwall. Footwall‐derived systems interacted with hangingwall‐ and axial‐derived systems, through diversion around topography, interfingering or successive onlap. We calculate the volume of footwall‐sourced hangingwall fans (VHW) for nine quadrants along the fault block, and compare this to the volume of material eroded from the immediately up‐dip fault‐scarp (VFW). This analysis highlights areas of sediment bypass (VFW > VHW) and areas fed by sediment sources beyond the degraded fault scarp (VHW > VFW). Exposure of the border fault footwall and adjacent fault terraces produced small catchments located beyond the fault block crest that fed the hangingwall basin. One source persisted throughout the main syn‐rift episode, and its location coincided with: (a) an intra‐basin topographic high; (b) a local fault throw minimum; (c) increased vertical and headward erosion within the fault‐scarp degradation complex; and (d) sustained clinoform development in the immediate hangingwall. Our novel quantitative volumetric approach to identify through‐going sediment input points could be applied to other rift basin‐fills. We highlight implications for hydrocarbon exploration and emphasize the need to incorporate interaction of multiple sediment sources and their resultant architecture in tectono‐stratigraphic models for rift basins.

Lower Cambrian facies architecture and sequence stratigraphy, NW France: framework for evaluation of basin-wide processes of sedimentation

AbstractA correlation and interpretation of the sequence stratigraphy of the lower Cambrian strata of NW France is presented and used to characterize basin-wide controls on processes of sedimentation. Deposition occurred in two basins, as two third-order stratigraphic sequences. A northern basin in the Cotentin was dominated by deposition of marine siliciclastic sediments of Sequence 1 (Cambrian Stages 2–3). The primary sediment input was from the NW at La Hague. A middle Normandy basin, SW of Caen, was characterized by deposition of platform carbonates and fine-grained, offshore siliciclastics in Sequence 1 and fluvio-deltaic and marine siliciclastics in Sequence 2 (Cambrian Stages 3–4). Major sediment input points lay to the west in Brittany and to the NE of Caen (in Sequence 2). Lowstand and transgressive systems tract strata dominate both sequences, with local preservation of highstand deposits in basin-centre locations. Fluvial deposits occur at basin margins and display sandstone-dominated facies typical of prevegetation systems. Two styles of fluvially dominated siliciclastic delta are recognized, which show delta-front strata composed of fluvial channels overlying either wave-formed deposits or hyperpycnites. The former formed on shallow shelves, whereas the latter formed where basin bathymetry was steeper and commonly fault controlled. Fossil microbial life forms are conspicuous in Sequence 1, as are intervals of low-diversity bioturbation. The balance of life forms is consistent with the punctuated replacement of algal matgrounds with mixgrounds during early Cambrian time. Super-mature quartz arenites were formed by high-energy wave processes on slowly aggrading basin flanks following marine transgression.

Controls on Early Cretaceous desert sediment provenance in south‐west Gondwana, Botucatu Formation (Brazil and Uruguay)

AbstractThe Lower Cretaceous Botucatu Formation records the development of widespread dry–aeolian desert sedimentation throughout the Paraná Basin in south‐west Gondwana. To reconstruct the provenance of the aeolian sediment, petrography, granulometric analysis, U‐Pb detrital zircon ages have been determined from along the southern basin margin in Rio Grande do Sul state (southern Brazil) and Uruguay (Tacuarembó region). The dataset reveals a mean composition Qt89F8L3, comprising very fine to medium‐grained quartozose and feldspatho‐quartzose framework. Heavy mineral analysis reveals an overall dominance of zircon, tourmaline and rutile grains (mean ZTR0.84) with sporadic garnet, epidote and pyrolusite occurrences. The detrital zircon U‐Pb ages are dominated by Cambrian to Neoproterozoic (515 to 650 Ma), Tonian to Stenian (900 to 1250 Ma) and Orosirian to Rhyacian (1.8 to 2.2 Ga) material. The detrital zircon dataset demonstrates a significant lateral variation in sediment provenance: Cambrian to Neoproterozoic detrital zircons dominate in the east, while Tonian to Stenian and Orosirian to Rhyacian ages predominate in the west of the study area. Sandstones are quartz‐rich with dominantly durable zircon, tourmaline and rutile heavy mineral suite, with subtle but statistically significant along‐strike differences in heavy mineral populations and detrital mineralogy which are thought to record local sediment input points into the aeolian system. The similar age spectra of Botucatu desert with proximal Paraná Basin units, the predominance of quartzose, and zircon, tourmaline and rutile components, suggests that recycling is the mechanism responsible for the erg feeding.

PALEOENVIRONMENTAL IMPLICATIONS OF TIME-AVERAGING AND TAPHONOMIC VARIATION OF SHELL BEDS IN LAKE TANGANYIKA, AFRICA

ABSTRACTThe lake bottom along structural platforms in Lake Tanganyika, Africa, is carpeted with numerous large shell beds, known to be of late Holocene age, but of uncertain assemblage process. The shell beds may be the result of sedimentological (physical) assembly processes, or biological processes, or both. Previous work focused on the distribution of shell-rich facies, and showed time averaging of the surficial shell bioclasts over the last ∼ 1600 calendar years BP. We focus on an extensive shell deposit along a deltaic platform in Kungwe Bay, Tanzania and examine time-averaging and taphonomy of Neothauma tanganyicense shells to constrain sedimentological and biological processes forming concentrations of shells. New radiocarbon dating indicates that Neothauma shells are time-averaged over the last ∼ 3000 calendar years. Younger shells predominate shallow-water and exhibit unimodal age distributions, while shells from deeper-water exhibit a broader age distribution. Taphonomic results indicate that water depth and distance from the delta river mouth influence shell abrasion and encrustation with more encrustation developing away from sediment input points. Shells with black coatings and reddish-orange oxidation patinas suggest local burial and exposure. The age-frequency distributions of the shells suggest production rates of the shells vary over time and with water depth, tracking climatically driven lake-level changes (e.g., Little Ice Age, ∼ 100–650 BP). In addition, age-distributions suggest that (1) mixing of different populations are more prevalent along the steeper deltaic slopes, and (2) recent decreasing production rates may reflect anthropogenic land-use change and attendant sedimentation, which has implications for Neothauma itself, and for organisms that are obligate occupants of the shell beds. These results suggest both climatic and depositional processes play unique roles in the distribution and accumulation of shell beds in Lake Tanganyika, which informs interpretation of similar paleoenvironments in the geologic record.

Three-dimensional Reduced-Complexity Simulation of Fluvio-Deltaic Clastic Stratigraphy

Abstract A novel reduced-complexity approach to 3D forward modeling of siliciclastic stratigraphy is presented for the simulation of erosion, transport, and sedimentation in continental, transitional, and marine depositional domains. The numerical model is based on defining centerlines that connect sediment input points to the shoreline. For each centerline, erosional and depositional surfaces bound depositional domains, and sand and mud proportions are assigned to each domain. The position of each depositional surface follows a set of geologic rules and ensures mass balance with sediment input. The numerical model is tested by simulating the basin-fill architecture of the XES02 laboratory experiment run at the University of Minnesota, which generates stratigraphy mimicking a passive-margin basin fill. Automatic calibration is used to test multiple combinations of uncertain model input parameters to find those that produce scenarios consistent with the experimental stratigraphy. Calibrated models accurately reproduce shoreline and mass-balance centroid migration, marine sediment proportions, and shoreline trajectories measured in the XES02 experiment. The models also provide reasonable approximations of sand distribution. Of interest, falling shoreline trajectories in the experiment and the calibrated models develop coeval to topset aggradation or topset incision depending on rate of base-level fall. The results reported herein validate the numerical approach for simulating sand distribution in an experimental basin, representing a first step towards the application of the numerical model in fluvio-deltaic settings. For field applications, analogue data can be used to independently constrain the geometry of surfaces bounding depositional domains and their composition. The simplicity of the numerical model then enables multiple realizations by quickly varying uncertain boundary conditions, allowing probabilistic predictions in settings with limited data constraints.

Palaeoenvironmental significance of lacustrine stromatolite forms from the Middle Old Red Sandstone of the Orcadian Basin

AbstractThe form of microbialite accumulations is largely the product of environmental processes and microbial activity. Recent work has largely concentrated on the identification and classification of microbialites with little attention being paid to their environmental significance. This study describes the environmental distribution of the varied stromatolite forms recorded from the Middle Old Red Sandstone sequences of the Orcadian Basin. Comparisons are made with Triassic examples from East Greenland and modern microbialite accumulations. The Middle Old Red Sandstone of Northern Scotland was deposited in a predominantly lacustrine setting. Stromatolites are recorded from both steep basin margin coincident settings and lower gradient settings where the lake margin was distant from the basin margin. In the latter case stromatolite development is largely restricted to transgressive lacustrine sequences, during the deposition of which reduced rates of sedimentation resulted from the migration of sediment input points towards the basin margin. Stromatolite sheets, domal mounds, aligned mounds (and associated runnels), sand-cored stromatolite mounds and reefal stromatolite accumulations have been identified representing the transition from more sheltered to more exposed environments. In basin margin coincident settings stromatolite accumulation is restricted to areas of low sedimentation where microbialites coat boulders and pebbles. A model for the palaeoenvironmental distribution of the stromatolite forms described is proposed and is shown to be applicable to similar examples from the Triassic of East Greenland. It is suggested that this model may be more widely applicable to stromatolitic accumulations in similar lacustrine settings through large portions of the Phanerozoic.

The hydrocarbon prospectivity of the Egyptian North Red Sea basin

Abstract Recent work by a multi-disciplinary team has led to a significantly better understanding of the prospectivity of the North Red Sea. New regional biostratigraphic and environmental analysis from north to south through the Gulf of Suez and into the Red Sea have placed the Nubian sequences into a regional chronostratigraphic framework. The Nubian Upper Cretaceous pre-rift sandstones are observed in the field on both the Egyptian and Saudi Arabian side of the North Red Sea. This regionally extensive sequence was deposited in a continental to shallow marine setting fringing the Mesozoic Tethys Ocean, which lay further north. Extensive onshore fieldwork and mapping of sediment input points, fault orientations and fault linkages have helped to develop an understanding of the expected controls on syn-rift sandstone and carbonate deposition offshore. Thick halite with interbedded evaporite and clastics in the Late Miocene sequences of the Red Sea pose seismic imaging challenges. Recent reprocessing and newly acquired seismic data have produced a step change improvement in imaging of the prospective pre-rift section. Petroleum systems modelling incorporating new information on rift timing and crustal thinning as well as onshore core analysis for source rock properties and temperature variation through time indicates that oil expulsion occurs in the inboard section of North Red Sea – Block 1. This is supported by hydrocarbon shows in the drilled offshore wells which can be typed to pre-rift source rocks from stable isotope and biomarker data. All the key elements of the Gulf of Suez petroleum system exist in the North Red Sea. An integrated exploration approach has enabled prospective areas in the North Red Sea – Block 1 to be high-graded for drilling in early 2011.

Post-rift marine transgression of the southern Browse Basin margin: controls on hydrocarbon reservoir development and exploration potential

The formation of the Precipice Fold Belt (560 Ma) marked the last significant modification of the basement drainage pattern and catchment area supplying sediment to the southern Browse Basin. Seismic mapping of the Proterozoic basement and the results of exploration drilling confirm the extension of both the geological formations and structural features exposed on the Proterozoic Kimberley Plateau into the offshore study area. As with the present-day shelf, sedimentation during the Late Jurassic to Early Cretaceous marine transgression directly onlapped the basement in the study area and showed the influence of basement topography on stratigraphy. A marked increase in net-to-gross and reservoir sand thickness is observed moving northeast from the Leveque to the Yampi shelves. This broad change in stratigraphy can be related to specific basement-controlled morphologies. A >250 m-high escarpment formed by the Proterozoic Kimberley Plateau basement terrane bounds the Yampi Shelf to the southeast of the Palaeozoic zero-edge line. This escarpment fixed the location of shelf sediment input points forming a barrier to marine transgression, which ensured that the Yampi Shelf was relatively immune from sediment starvation during highstand systems tracts. In contrast, the broad low angle Leveque Shelf experienced large lateral shifts in shelf sediment input points during eustatic changes in sea level with sedimentation dominated by transgressive systems tracts. In addition to shelf morphology, the larger basement-controlled drainage area (80,000 km2) supplying sediment to the Yampi Shelf tended to increase sedimentation rates, leading to shelf delta progradation during highstand systems tracts. The quality and thickness of mass flow deposits seen in the Caswell Sub-basin and along the Prudhoe Terrace are the direct result of the concentration of high net-to-gross sand close to the offlap break on the Yampi Shelf. Given the extreme long term stability of the southern Browse Basin margin, recognition of the influence of basement structure on shelf sedimentation has refined the geological model for the post-rift marine transgression. This model could be used to reduce the risk on reservoir and seal distributions in the Late Jurassic to Early Cretaceous study interval. In a broader context, observations on basement-controlled reservoir and seal distribution from the Browse Basin might be usefully applied to other basins where sediment is supplied from long-lived Proterozoic margins.

The Deep-Water Architecture Knowledge Base: towards an objective comparison of deep-marine sedimentary systems

A quantitative method is proposed for the comparison of deep-marine clastic depositional systems and the analysis of their architectural properties. The method comprises a knowledge base of quantitative, literature-derived information from modern and ancient, surface and subsurface deep-water systems, implemented as a relational database management system and referred to as the Deep-Water Architecture Knowledge Base (DWAKB). The types of information contained within the knowledge base include: (1) internal and external parameters controlling system architecture, such as tectonic setting, grain size of available sediment, degree of basin confinement and number and distribution of sediment input points; (2) the dimensions of architectural elements, such as channels, levees, lobes, open slope and basin plain; (3) the spatial organization of the architectural elements; and (4) the bed thickness distribution and proportion of different lithologies within the architectural elements. The potential value of the DWAKB for comparative studies of deep-marine clastic systems is considerably higher than that of classification schemes and system analogue concepts presently available. Thus, in contrast to classification schemes, the knowledge base is not limited in the number of controlling parameters and it does not have a limited time span because of the flexibility to update existing records and add new datasets. Moreover, system analogues can be selected more objectively through the use of statistical methods, and the knowledge base allows unsurpassed integration of large-scale architectural data with bed- and facies-scale data. The expected value of the DWAKB is illustrated with several examples of quantitative data analysis. These include the determination of the frequency of occurrence of levees in systems of different grain size, the calculation of the dimensions of submarine channels as a function of grain size and proximity to the source area, and the construction of idealized models for sand-rich and mixed mud–sand systems based on probabilities of spatial transitions between architectural elements.

Imaging submarine channels in the western Nile Delta and interpreting their paleohydraulic characteristics from 3-D seismic

Predicting sand distribution and reservoir presence is a major exploration risk in the Nile Delta offshore Egypt. However, integrating state-of-the-art technologies including 3-D seismic reflection surveys, seismic attributes, amplitude extractions, and coherency techniques can reduce this risk through recognition and more accurate mapping of submarine valleys and channel complexes. In addition, identification of sediment input points and pathways may result in more accurate interpretations of sand distribution patterns.

Facies architecture and sequence stratigraphy in glacially influenced basins: basic problems and water-level/glacier input-point controls (with an example from the Quaternary of Ontario, Canada)

Sequence architecture is controlled by two fundamental factors; the water level relative to the depositional surface (which controls accommodation space), and the points or areas of sediment injection (which control how this space is filled in). Relative water level, accommodation space and injection points normally vary together and various genetic terms are used to define systems tracts. But in glacially influenced basins, accommodation space is controlled not only by water level, but also by the glacier. Injection points are similarly dually controlled — by water level and by the terminus of the glacier. During high lake levels the injection point of the glacier may be underwater at the base of the slope. Genetic terms like `highstand' and `lowstand' therefore are meaningless in successions controlled by sediment input points which are independent of water levels and accommodation space. Distinct water-level and glacier input-point systems can occur side by side: the water-level system controlled by lake level or sea level, the glacier input-point system controlled by the position of the subaqueous glacier front. And both can form juxtaposed systems passing laterally into each other. A bewildering and incompatible terminology is needed if the terms of genetic sequence stratigraphy are used for both water-level and glacier input-point systems, because the sedimentary injection points and sequence boundaries of the two systems fluctuate out of phase with one another. Sequences developed by movement of the glacier are completely independent of those controlled by water levels. If genetic sequence stratigraphy is applied to a basin with both lake-level and glacier input-point systems, then each needs separate genetic terminologies to describe them, which leads only to confusion. However, descriptive sequence stratigraphy (allostratigraphy) can be readily applied to successions like the Quaternary of Lake Ontario. We therefore suggest that genetic sequence terms be discarded and simpler allostratigraphic terminology used in glacially influenced basins.

Impact of salt on the structure of the Central North Sea hydrocarbon fairways

The Central North Sea lies within the North Permian evaporite Basin, which was largely flexural but with locally significant basement faults. While the main evaporite basin fill is halite, seismically reflective basin margin facies delineate the salt basin shape. The marginal Zechstein strata record Permian salt tectonics including listric faulting, slumping of kilometre-scale carbonate rafts and halite diapirism, all related to localized pre-Triassic gravity-driven collapse of the Zechstein basin margin. Triassic thickness in the Central North Sea (CNS) was directly influenced by salt, the most significant phase of salt tectonics in the history of the CNS in terms of volume of salt redistributed and dissolved. Lower–Middle Triassic ‘minibasins’ subsided into the salt over much of the CNS. Different initiating mechanisms operated around the basin–differential loading was important near sediment input points, and thin-skinned extension on the platforms balanced a pulse of basement extension in the Central Graben. Minibasin subsidence was accommodated by salt wall growth; salt was eroded during periods of base-level fall and redeposited locally and in the Southern North Sea. The minibasins touched down on basement progressively from the basin margins towards the Central Graben, the timing being a function of salt thickness. The diachronous halting of accommodation space generation combined with the effects of erosion led to a variety of field relationships between the Middle–Upper Triassic Skagerrak sands and the underlying salt–minibasin system. Regionally, the Skagerrak sands are thickest near input points and in the Central Graben, where accommodation space was supplemented by basement faulting. The platforms were subaerially exposed and eroded for much of the period between the late Triassic and late Jurassic. On these platforms, the distribution of the relatively thin Late Jurassic section was controlled by a combination of regional tilt and pre-existing topography created by differential erosion (including dissolution) of the salt walls versus grounded minibasins. The late Jurassic Fulmar shallow marine sands are thickest on the differentially eroded salt highs, which constituted several hundred foot-deep palaeovalleys. Rifting occurred in the Central Graben where tilts of detachments on top of major basement fault blocks caused thin-skinned extension in the overlying strata, either for the first time, or reactivating Triassic minibasin systems. The Fulmar sands are thickest where basement-controlled accommodation space was accentuated by detached fault systems (e.g. fallen diapirs) near sediment input points in the Central Graben. The Early Cretaceous was a time of tectonic stabilization following Late Jurassic basement rifting, although the perched detachment systems in the major basement graben continued to evolve. Accommodation space for Upper Jurassic and Lower Cretaceous turbidites was influenced by up-dip salt high collapse and down-dip ‘rim syncline’ growth. By the end of the Early Cretaceous these systems had stabilized and the surrounding basement highs were sufficiently blanketed to shut off salt dissolution. Chalk isopach variations pick out subtle regional tilts and local basement fault reactivations. Major flexural subsidence in the Tertiary tilted the basin again and precipitated local cover slip, e.g. extensional faults balanced by down-dip folds on the West Central Shelf and diapir rejuvenation on the graben margin. A variety of inversion structures are present in the CNS and are dominantly Cretaceous and Tertiary in age. Several styles of inversion structure occur including thin-skinned and thick-skinned compression, diapir rejuvenation, compaction and dissolution-related features. Salt governed the geometry of many of these structures and in turn the geometry of the Tertiary sand fairways.

THREE-DIMENSIONAL FORWARD STRATIGRAPHIC MODELLING OF EARLY CRETACEOUS SEDIMENTATION ON THE LEVEQUE AND YAMPI SHELVES, BROWSE BASIN

Early Cretaceous clastic sedimentation on the Leveque and Yampi Shelf areas of the Browse Basin has been brought sharply into focus recently as a result of exploration success at Gwydion and Cornea. The changing nature of clastic sedimentation patterns in the shallow-to-deep marine environments during C. delicata to upper S. areolata times is of increasing exploration interest. This study was designed to address the following questions: Could the conceptual geological model discussed by Spry and Ward (1997) generate potential reservoir sands in the region from Leveque-1 to Yampi-1?What would be the geometry and depositional environment of such sands?SEDSIM is a three-dimensional stratigraphic forward modelling program developed at Stanford University. The program models sediment erosion, transport, and deposition, and predicts clastic sediment distribution on a given bathymetric surface. SEDSIM was used to simulate one million years of deposition on the Leveque and Yampi Shelves from approximately 145 Ma. The model illustrates the use of 3D stratigraphic forward modelling in illustrating and testing stratigraphic play fairway concepts and the development of sedimentation patterns. Reprocessed seismic data from Seismic Australia were used together with a depth-converted near Base Cretaceous surface provided by Woodside Offshore Petroleum. Input data include; a high resolution relative sea-level curve, an understanding of wind and wave directions throughout the modelled period, tectonic movements throughout the modelled area, the nature of the underlying strata, sediment input points, and grain-size distributions. The simulation clearly shows the development of linear shelfal sands on the C. delicata flooding surface during the following regression. Further work will extend the study to early M. australis times.

Structural controls on sedimentation in continental rifts

Abstract Structure and climate have each been proposed as the primary control on sedimentation in continental rifts. Numerous Phanerozoic rifts spanning a wide range of inferred palaeoclimates have consistent relationships between structure and sedimentation, and between structural evolution and stratigraphic sequences. Six principal structural features strongly influence sedimentation patterns in rifts. (1) Topographically high accommodation zones segment rifts into 50–150 km long structural half-graben, each one a discrete depocentre. (2) Reactivation of pre-existing structural elements commonly controls the location and orientation of accommodation zones. Drainage systems outside the rift then may be captured and diverted towards the rift axis, establishing some accommodation zones as input points for sediment. (3) Footwall uplift at main border faults directs most drainage away from young rifts. Along the rift flanks, uplift is minimized at accommodation zones, enhancing their role as sediment input points. (4) Gentle roll-over of topography on flexural margins produces drainage systems that are generally small, and that do not introduce large amounts of sediment into the rift. (5) Breakup of the basin floor into elongate, rift-parallel fault blocks greatly enhances along-axis sediment transport. (6) Development of fault relay zones and transfer faults localizes down-dip sediment movement. These structural controls occur in the present-day low latitude rifts of East Africa, where climate ranges from arid to humid. Similar features have been identified in ancient rifts that are interpreted to span a comparable range in climates. Clearly, climate is important in determining the sedimentary history of continental rifts. The present study indicates that climatic effects, however, are superimposed on the fundamental structural geometries that evolve during rifting.