Hybrid event beds are texturally and compositionally‐diverse deposits preserved within deepwater settings. They are deposited by flows exhibiting ‘mixed behaviour’, forming complex successions of ...sandstone and mudstone, which are often challenging to predict. Hybrid event beds are documented in deep‐marine settings, where they have been thoroughly characterized, and are well‐known as effective fluid transmissibility barriers and baffles in reservoirs. By comparison, there are far‐fewer studies of hybrid event beds from deep‐lacustrine settings, where their character and distribution remains relatively under‐explored. In order to provide insights into these deposits, this study presents the detailed analysis of three‐dimensional seismic data, wireline logs and core from a series of ancient deep‐lacustrine fan systems in the North Falkland Basin. Results confirm that deep‐lacustrine hybrid event beds comprise the same idealized sequence of the ‘H1–H5’ divisions. However, in this study H3 ‘debrite’ units can be sub‐divided into ‘H3a–H3c’, based on: sharp or erosional intra‐H3 contacts, bulk lithology, mud‐content and discrete sedimentary textures. This study interprets the H3a–H3c sub‐units as the products of multiple flow components formed through significant rearward longitudinal flow transformation processes, during the emplacement of a single hybrid event bed. Hybrid event beds are observed within lobe fringes, where flow types, energies and transport mechanisms diversify as a result of flow transformation. The temporal context of hybrid event bed occurrences is considered in relation to stages of fan evolution, including: the Initiation; Growth (I); Growth (II); By‐pass; Abandonment; and Termination phases. Hybrid event beds are mainly found in either the initiation phase where flow interaction and erosion of initial substrates promoted mixed flow behaviour, or in the abandonment phase as facies belts retreated landward. The results of this study have important implications in terms of flow processes of hybrid event bed emplacement, in particular sub‐division of the H3 unit, as well as the prediction of hybrid event bed occurrence and character within ancient deep‐lacustrine fan settings, in general.
Fault relay ramps are important sediment delivery points along rift margins and often provide persistent flow pathways in deepwater sedimentary basins. They form as tilted rock volumes between ...en‐echelon fault segments, which become modified through progressive deformation, and may develop through‐going faults that ‘breach’ the relay ramp. It is well established that hinterland drainage (fluvial/alluvial systems) is greatly affected by the presence of relay ramps at basin margins. However, the impact on deepwater (deep‐marine/lacustrine) subaqueous sediment gravity flow processes, particularly by breached relay ramps, is less well documented. To better evaluate the complex geology of breached relay settings, this study examines a suite of high‐quality subsurface data from the Early Cretaceous deep‐lacustrine North Falkland Basin (NFB). The Isobel Embayment breached relay‐ramp, an ideal example, formed during the syn‐rift and was later covered by a thick transitional and early post‐rift succession. Major transitional and early post‐rift fan systems are observed to have consistently entered the basin at the breached relay location, directed through a significant palaeo‐bathymetric low associated with the lower, abandoned ramp of the structure. More minor systems also entered the basin across the structure‐bounding fault to the north. Reactivation of basin‐bounding faults is shown by the introduction of new point sources along its extent. This study shows the prolonged influence of margin‐located relay ramps on sedimentary systems from syn‐rift, transitional and into the early post‐rift phase. It suggests that these structures can become reactivated during post‐rift times, providing continued control on deposition and sourcing of overlying sedimentary systems. Importantly, breached relays exert control on fan distribution, characterised by laterally extensive lobes sourced by widespread feeder systems, and hanging walls settings by small‐scale lobes, with small, often line‐sourced feeders. Further characterising the likely sandstone distribution in these structurally complex settings is important as these systems often form attractive hydrocarbon reservoirs.
Early post‐rift deposition associated with the breached relay structure can be divided into two domains: (1) systems which entered the basin at the breached relay structure via the lower abandoned ramp; (2) systems which entered directly across the hangingwall of structure bounding faults. Into the early post‐rift, systems continue to enter at the same locations, however, fan geometries and sources change in response to overall basin subsidence becoming the main accommodation driver.
One design concept for the long-term management of the UK's intermediate level radioactive wastes (ILW) is disposal to a cementitious geological disposal facility (GDF). Under the alkaline (10.013.0) ...anoxic conditions expected within a GDF, cellulosic wastes will undergo chemical hydrolysis. The resulting cellulose degradation products (CDP) are dominated by α- and β-isosaccharinic acids (ISA), which present an organic carbon source that may enable subsequent microbial colonisation of a GDF. Microcosms established from neutral, near-surface sediments demonstrated complete ISA degradation under methanogenic conditions up to pH 10.0. Degradation decreased as pH increased, with β-ISA fermentation more heavily influenced than α-ISA. This reduction in degradation rate was accompanied by a shift in microbial population away from organisms related to Clostridium sporosphaeroides to a more diverse Clostridial community. The increase in pH to 10.0 saw an increase in detection of Alcaligenes aquatilis and a dominance of hydrogenotrophic methanogens within the Archaeal population. Methane was generated up to pH 10.0 with acetate accumulation at higher pH values reflecting a reduced detection of acetoclastic methanogens. An increase in pH to 11.0 resulted in the accumulation of ISA, the absence of methanogenesis and the loss of biomass from the system. This study is the first to demonstrate methanogenesis from ISA by near surface microbial communities not previously exposed to these compounds up to and including pH 10.0.
The separation and characterisation of different deformation events in superimposed basins can be challenging due to the effects of overprinting and/or fault reactivation, combined with a lack of ...detailed geological or geophysical data. This paper shows how an onshore study can be enhanced using a targeted interpretation of contiguous structures offshore imaged by seismic reflection data. Two deformation events, including evidence of fault reactivation, are recognised and associated with the onshore part of the Lossiemouth Fault Zone (LFZ), southern‐central Inner Moray Firth Basin. The basin is thought to record a history of Permian to Cenozoic deformation, but it is difficult to conclusively define the age of faulting and fault reactivation. However, structures in onshore outcrops of Permo–Triassic strata show no evidence of fault growth, and new interpretation of seismic reflection profiles in the offshore area reveals that Permo–Triassic fills are widely characterised by subsidence and passive infill of post‐Variscan palaeotopography. We propose that sequences of reactivated faulting observed onshore and offshore can be correlated and can be shown in the latter domain to be Early Jurassic–Late Cretaceous, followed by localised Cenozoic reactivation. The workflow used here can be applied to characterise deformation events in other superimposed rift basins with contiguous onshore (surface)—offshore (subsurface) expressions.
Summary of basin development using a representative 2D seismic profile.
Geological strain analysis of sedimentary rocks is commonly carried out using clast-based techniques, which rely on assumptions regarding clast shapes and orientations as well as distances between ...clast centres. To evaluate the accuracy of these methods in low strain regimes we have compared strain analysis estimates to the anisotropy of magnetic susceptibility (AMS) in sandstone samples from the Wyoming salient. The Cordilleran Mountain Belt of North America is one of the world's classic foreland fold and thrust belts. The Sevier Belt represents the thin-skinned front of this orogenic event characterized by thrust faults and folds that shortened and translated sequences of Devonian to Cretaceous strata eastward. Deformation increases westward providing an ideal laboratory and geological setting to explore the potential correlation of anisotropy of magnetic susceptibility (AMS) to clast-based strain analyses. Studies attempting to define the relationship between AMS and finite strain have been in vogue since the link between layer parallel shortening and AMS was first established. This relationship, despite proven strong correlations between the AMS tensors and tectonic directions, is generally complicated by competing sub-fabrics as well as the various magnetic anisotropy properties of the minerals contributing to the AMS fabric. Even with these complications, we conclude that AMS is much more sensitive to incipient tectonic fabrics than clast based methods.
•The capabilities of clast-based strain analysis techniques were compared to AMS.•We found that at low strains, errors in these techniques caused considerable inaccuracies.•AMS was found to be more effective at identifying incipient tectonic fabrics.
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