Towards a sequence stratigraphic solution set for autogenic processes and allogenic controls: Upper Cretaceous strata, Book Cliffs, Utah, USA

Upper Cretaceous strata exposed in the Book Cliffs of east–central Utah are widely used as an archetype for the sequence stratigraphy of marginal-marine and shallow-marine deposits. Their stratal architectures are classically interpreted in terms of accommodation controls that were external to the sediment routing system (allogenic), and that forced the formation of flooding surfaces, sequence boundaries, and parasequence and parasequence-set stacking patterns. Processes internal to the sediment routing system (autogenic) and allogenic sediment supply controls provide alternatives that can plausibly explain aspects of the stratal architecture, including the following: (1) switching of wave-dominated delta lobes, expressed by the internal architecture of parasequences; (2) river avulsion, expressed by the internal architecture of multistorey fluvial sandbodies and related deposits; (3) avulsion-generated clustering of fluvial sandbodies in delta plain strata; (4) ‘autoretreat’ owing to increasing sediment storage on the delta plain as it lengthened during progradation, expressed by progradational-to-aggradational stacking of parasequences; (5) sediment supply control on the stacking of, and sediment grain-size fractionation within, parasequence sets. The various potential allogenic controls and autogenic processes are combined to form a sequence stratigraphic solution set. This approach avoids anchoring of sequence stratigraphic interpretations on a specific control and acknowledges the non-unique origin of stratal architectures

Analysis of floodplain sedimentation, avulsion style and channelised fluvial sandbody distribution in an upper coastal plain reservoir: Middle Jurassic Ness Formation, Brent Field, UK North Sea

Numerical models and recent outcrop case studies of alluvial-to-coastal-plain strata suggest that autogenic avulsion can control the stacking density and architecture of channelized fluvial sandbodies. The application of these models to subsurface well data was tested by the analysis of upper coastal plain deposits of the late Bajocian Ness Formation in the Brent Field reservoir, UK North Sea. These coastal plain deposits accumulated during the progradation and retrogradation of the wave-dominated ‘Brent Delta’. Sedimentological facies analysis and palaeosol characterization in cores were used to interpret the styles of palaeochannel avulsion. These results were then compared with the dimensions and distributions of channelized fluvial sandbodies that had been quantified by spatial statistical tools (lacunarity, Besag’s L function) applied to interpretative correlation panels between closely spaced wells. The results indicate that the distributions of channelized sandbodies may plausibly have been generated by avulsions and that they influence sandbody connectivity and pressure depletion patterns. Intervals of upper coastal plain strata with relatively wide sandbodies that display some clustering in their stratigraphic architecture are associated with a high proportion of avulsions by incision and annexation in core samples. Such intervals display relatively good vertical pressure communication and relatively slow, uniform pressure depletio

Quantitative characterisation of deltaic and subaqueous clinoforms

AbstractClinoforms are ubiquitous deltaic, shallow-marine and continental-margin depositional morphologies, occurring over a range of spatial scales (1–104m in height). Up to four types of progressively larger-scale clinoforms may prograde synchronously along shoreline-to-abyssal plain transects, albeit at very different rates. Paired subaerial and subaqueous delta clinoforms (or ‘delta-scale compound clinoforms’), in particular, constitute a hitherto overlooked depositional model for ancient shallow-marine sandbodies. The topset-to-foreset rollovers of subaqueous deltas are developed at up to 60m water depths, such that ancient delta-scale clinoforms should not be assumed to record the position of ancient shorelines, even if they are sandstone-rich.This study analyses a large dataset of modern and ancient delta-scale, shelf-prism- and continental-margin-scale clinoforms, in order to characterise diagnostic features of different clinoform systems, and particularly of delta-scale subaqueous clinoforms. Such diagnostic criteria allow different clinoform types and their dominant grain-size characteristics to be interpreted in seismic reflection and/or sedimentological data, and prove that all clinoforms are subject to similar physical laws.The examined dataset demonstrates that progressively larger scale clinoforms are deposited in increasingly deeper waters, over progressively larger time spans. Consequently, depositional flux, sedimentation and progradation rates of continental-margin clinoforms are up to 4–6 orders of magnitude lower than those of deltas. For all clinoform types, due to strong statistical correlations between these parameters, it is now possible to calculate clinoform paleobathymetries once clinoform heights, age spans or progradation rates have been constrained.Muddy and sandy delta-scale subaqueous clinoforms show many different features, but all share four characteristics. (1) They are formed during relative sea-level stillstands (e.g., Late Holocene); (2) their stratigraphic architecture and facies character are dominated by basinal processes, and are quite uniform; (3) their plan-view morphology is shore-parallel and laterally extensive; (4) their sigmoidal cross-sectional geometry contrasts with the oblique profiles of most subaerial deltas. Holocene-age, delta-scale, sand-prone subaqueous clinoforms occur on steep (≥0.26°) and narrow (5–32km) shelves, at typical distances of 0.6–7.2km from the shoreline break. That contrasts with mud-prone subaqueous deltas, which form clinoforms on gently-sloping (0.01–0.38°), wide (23–376km) shelves, at usual distances of 7.5–125km from the shoreline. Delta-scale sand-prone subaqueous clinoforms have diagnostically steep foresets (0.7–23°). Similarly steep gradients were observed in much larger shelf-prism- and continental-margin-scale clinoforms. Gentler foreset gradients are shown by sand-prone subaerial deltas (0.1–2.7°), and mud-prone subaqueous and subaerial deltas (0.03–1.50°). Due to the lack of connections with river mouths, Holocene delta-scale sand-prone subaqueous clinoform deposits have progradation rates (1–5×102km/Myr) and unit-width depositional flux (1–15km2/Myr) that are up to 3–4 and 2–3 orders of magnitude lower, respectively, than age-equivalent input-dominated subaerial deltas and muddy subaqueous deltas. Lower progradation/aggradation ratios are reflected in a larger spread of clinoform trajectory angles (from −0.4° to +3.5°) than the very low values displayed by age-equivalent subaerial and muddy subaqueous deltas.As slowly prograding, steep, sigmoidal clinoforms are strongly suggestive of sand-prone subaqueous deltas, the Sognefjord Formation and Bridport Sand are likely Jurassic examples of this clinoform type, and host hydrocarbon reservoirs. In contrast, the Campanian Blackhawk Formation is an outcrop example of delta-scale compound clinoforms with a muddy subaqueous component

Preserved stratigraphic architecture and evolution of a net-transgressive mixed wave- and tide-influenced coastal system: Cliff House Sandstone, northwestern New Mexico, USA

The Cretaceous Cliff House Sandstone comprises a thick (400 m) net- transgressive succession representing a mixed wave- and tide-influenced shallow-marine system that migrated episodically landwards. This study examines the youngest part (middle Campanian) of the Cliff House Sandstone, exposed in Chaco Cultural Natural Historical Park, northwest New Mexico, U.S. A. Detailed mapping of facies architecture between a three-dimensional network of measured sections has allowed the character, geometry, and distribution of key stratigraphic surfaces and stratal units to be reconstructed. Upward-shallowing facies successions (parasequences) are separated by laterally extensive transgressive erosion (ravinement) surfaces cut by both wave and tide processes. Preservation of facies tracts in each parasequence is controlled by the depth of erosion and migration trajectory of the overlying ravinement surfaces. In most parasequences, there is no preservation of the proximal wave-dominated facies tracts (foreshore, upper-shoreface), resulting in thin (4–7 m) top-truncated packages. Four distinct shallow marine tongues (parasequence sets) have been identified, consisting of ten parasequences with a total stratigraphic thickness of ~ 100 m. Each tongue records an episode of complex shoreline migration history (multiple regressive–transgressive phases) in an overall net-transgressive system. The ravinement surfaces provide a stratigraphic framework in which to understand partitioning of tide- and wave-dominated deposits in a net-transgressive system, and a model is presented to account for the sediment distribution and stratigraphic architecture observed in each parasequence. Despite a complex internal architecture, parasequences exhibit a predictable pattern which can be related to the regressive and transgressive phases of deposition. Preservation of wave-dominated facies tracts is associated with shoreline regression, while tide-dominated facies tracts are interpreted to record sediment accumulation during shoreline transgression that also resulted in significant erosion of the underlying regressive deposits. The interplay between erosion, sediment bypass, and deposition during regression and transgression is shown to ultimately control the preservation and stratigraphic architecture of the larger-scale net-transgressive coastal system. While the Cliff House Sandstone exhibits a facies composition and quantitative stacking patterns (shoreline trajectory) similar to other studied examples, differences in the dip-extent of the wave-dominated sandstone tongue has resulted in a more disconnected architecture between the high-fr equency cycles. Understanding the variety of stratal geometries that ravinement surfaces can generate is therefore crucial to predicting the spatial distribution and facies architecture in transgressive systems

Depositional evolution of a progradational to aggradational, mixed-influenced deltaic succession: Jurassic Tofte and Ile formations, southern Halten Terrace, offshore Norway

Predicting the hydrodynamics, morphology and evolution of ancient deltaic successions requires the evaluation of the three-dimensional depositional process regime based on sedimentary facies analysis. This has been applied to a core-based subsurface facies analysis of a mixed-energy, clastic coastal-deltaic succession in the Lower-to-Middle Jurassic of the Halten Terrace, offshore mid-Norway. Three genetically related successions with a total thickness of 100–300 m and a total duration of 12.5 Myr comprising eight facies associations record two initial progradational phases and a final aggradational phase. The progradational phases (I and II) consist of coarsening upward successions that pass from prodelta and offshore mudstones (FA1), through delta front and mouth bar sandstones (FA2) and into erosionally based fluvial- (FA3) and marine-influenced (FA4) channel fills. The two progradational phases are interpreted as fluvial- and wave-dominated, tide-influenced deltas. The aggradational phase (III) consists of distributary channel fills (FA3 and FA4), tide-dominated channels (FA5), intertidal to subtidal heterolithic fine-grained sandstones (FA6) and coals (FA7). The aggradational phase displays more complex facies relationships and a wider range of environments, including (1) mixed tide- and fluvial-dominated, wave-influenced deltas, (2) non-deltaic shorelines (tidal channels, tidal flats and vegetated swamps), and (3) lower shoreface deposits (FA8). The progradational to aggradational evolution of this coastal succession is represented by an overall upward decrease in grain size, decrease in fluvial influence and increase in tidal influence. This evolution is attributed to an allogenic increase in the rate of accommodation space generation relative to sediment supply due to tectonic activity of the rift basin. In addition, during progradation, there was also an autogenic increase in sediment storage on the coastal plain, resulting in a gradual autoretreat of the depositional system. This is manifested in the subsequent aggradation of the system, when coarse-grained sandstones were trapped in proximal locations, while only finer grained sediment reached the coastline, where it was readily reworked by tidal and wave processes

Effective flow properties of heterolithic, cross-bedded tidal sandstones: Part 1. Surface-based modeling

Tidal heterolithic sandstones are commonly characterized by millimeter- to centimeter-scale intercalations of mudstone and sandstone. Consequently, their effective flow properties are poorly predicted by (1) data that do not sample a representative volume or (2) models that fail to capture the complex three-dimensional architecture of sandstone and mudstone layers. We present a modeling approach in which surfaces are used to represent all geologic heterogeneities that control the spatial distribution of reservoir rock properties (surface-based modeling). The workflow uses template surfaces to represent heterogeneities classified by geometry instead of length scale. The topology of the template surfaces is described mathematically by a small number of geometric input parameters, and models are constructed stochastically. The methodology has been applied to generate generic, three-dimensional minimodels (9 m3 volume) of cross-bedded heterolithic sandstones representing trough and tabular cross-bedding with differing proportions of sandstone and mudstone, using conditioning data from two outcrop analogs from a tide-dominated deltaic deposit. The minimodels capture the cross-stratified architectures observed in outcrop and are suitable for flow simulation, allowing computation of effective permeability values for use in larger-scale models. We show that mudstone drapes in cross-bedded heterolithic sandstones significantly reduce effective permeability and also impart permeability anisotropy in the horizontal as well as vertical flow directions. The workflow can be used with subsurface data, supplemented by outcrop analog observations, to generate effective permeability values to be derived for use in larger-scale reservoir models. The methodology could be applied to the characterization and modeling of heterogeneities in other types of sandstone reservoirs

Tectonic controls on the spatial distribution and stratigraphic architecture of a net-transgressive shallow-marine syn-rift succession in a salt-influenced rift basin: Middle-to-Upper Jurassic, Norwegian Central North Sea

Syn-depositional deformation in salt-influenced rift basins is complex, being driven by a combination of normal faulting and the growth of salt structures such as diapirs. Due to a lack of data with which to simultaneously constrain basin structure and syn-rift stratigraphic architecture, we have a poor understanding of how these processes control shallow marine deposition in such settings. To improve our understanding we here use seismic reflection and borehole data from the Norwegian Central North Sea to investigate the role that syn-depositional fault growth and salt movement played in controlling the sub-regional stratigraphic architecture of a net-transgressive shallow-marine syn-rift succession (Middle-to-Late Jurassic). The rift-related structural framework, which is usually dominated by normal fault-bound horst and graben, is strongly modified where an Upper Permian salt layer (Zechstein Supergroup) is sufficiently thick and mobile to act as an intra-stratal detachment, giving rise to decoupled rift-related basement and cover structural styles. Furthermore, cover extension allows the salt to rise diapirically, resulting in the formation of large salt diapirs and supra-salt normal faults formed due to late-stage salt withdrawal and diapir collapse. Rift-related normal faulting and the growth of salt structures had a dual control on the depositional thickness and facies distribution within the net-transgressive, predominantly shallow-marine, Middle-to-Upper Jurassic syn-rift succession. The resulting facies architecture reflects a delicate balance between fault- and salt flow-driven accommodation creation and intra- and extra-basinal sediment supply. Where sediment supply and accumulation rate exceeded accommodation, little or no change in facies is observed across syn-depositional structures. In contrast, where accommodation outpaced sediment supply and accumulation rate, footwall-attached shorelines locally developed adjacent to large, thick-skinned normal faults, with deeper water conditions persisting in the adjacent hanging wall. Flooding of individual structural elements was strongly diachronous and influenced by the underlying rift-related topography, which was characterised by intra-basinal horst and graben. This paper highlights the key role that salt plays in modifying the tectono-stratigraphic evolution of rift basins, suggesting that existing models, based on salt-free structural templates, need to be modified

Source‐to‐sink mass‐balance analysis of an ancient wave‐influenced sediment routing system: Middle Jurassic Brent Delta, Northern North Sea, offshore UK and Norway

Sediment mass-balance analysis provides key constraints on stratigraphic architecture and its controls. We use the data-rich Middle Jurassic Brent Delta sediment routing system in the proto-Viking Graben, Northern North Sea, to estimate sediment budgets and mass-balance between source areas and depositional sinks. Published studies are synthesised to provide an age-constrained sequence stratigraphic framework, consisting of four previously defined genetic sequences (J22, J24, J26, J32). Genetic sequence J32 (3.9 Myr) records transverse progradation of basin-margin deltas, sourced from the Shetland Platform to the west and Norwegian Landmass to the east. Genetic sequences J24 (1.1 Myr) and J26 (0.9 Myr) record the rapid progradation and subsequent aggradation of the Brent Delta along the basin axis, sourced from the uplifted Mid-North Sea High to the south, and the western and eastern source regions. Genetic sequence J32 (2.2 Myr) records the retreat of the Brent Delta. Sediment budgets for the four genetic sequences are estimated using palaeogeographical reconstructions, isopach maps, and sedimentological analysis of core and well-log data. The estimated net-depositional sediment budget for the mapped Brent Delta system is 2.0–2.8 Mt/year. Temporal variations in net-depositional sediment budget were driven by changes in tectonic boundary conditions, such as the onset of uplift before the deposition of genetic sequence J24. Over the same time period, the Shetland Platform, Norwegian Landmass and Mid-North Sea High source regions are estimated to have supplied 2.3–5.6, 5.0–14.1, and 2.8–9.4 Mt/year of sediment, respectively, using the BQART sediment load model and independent geometrical reconstruction of eroded volumes, which are constrained by isostatic uplift estimates based on the geochemistry of syn-depositional volcanic rocks. The net-depositional sediment budget in the sink is an order-of-magnitude smaller than the total sediment budget supplied by the source regions (13.9–23 Mt/year). This discrepancy suggests that along-shore transport by wave-generated currents into the coeval Faroe-Shetland Basin and/or down-dip transport by gravity flows into the coeval western Møre Basin played a key role in redistributing sediments away from the Brent Delta system