Stuart J. Jones

Sediment Flux to Basins: Causes, Controls and Consequences

There is an increasing trend in the Earth sciences towards the integration of many subdisciplines. The sedimendatry basin, is a fundamental focal point of many studies, which as a consequence often neglects the complimentary drainage basin or catchment. Sedimentary basins provide a record of Earth history, reflecting the geographical, lithological, oceanographic and ecological development through the rock record. Drainage basins in comparison record ephemeral landscape evolution, where topography is eroded and provides the flux of sediment to the basin. The basin fill reflects the sediment flux from the hinterland and provides evidence of the dynamic geomorphic processes. In context the drainage system and sedimentary basin can be regarded as a ‘production line’ with sedimentary record giving valuable insight into long-term landscape evolution and geomorphological processes illuminating the evolution of sedimentary basins. This volume assesses the current position of understanding sediment supply to basins with the integration of the many sub-disciplines in the Earth sciences. It documents a mix of hinterland and sedimentary basin studies with a gradation from orogenic belts to the deep marine. The authors represent a wide spectrum of Earth scientist, with leaders in the science providing review papers and new-directive papers in their field of specialization.

Braided stream and flood plain architecture: the Rio Vero Formation, Spanish Pyrenees

Early- to middle-Miocene fluvial sandstones of the Rio Vero Formation were studied, in an area around the town of Barbastro, south central Pyrenees Spain. The outstanding quality of outcrops in this area allows a three-dimensional study of architectural elements. Six architectural elements are recognised, described in detail, and interpreted from three key localities. Seven main lithofacies were identified and sub-divided into gravelly, sandy and fine-grained lithofacies. The architectural elements and lithofacies have been combined with a hierarchy of depositional bounding surfaces to fully interpret the evolution of the depositional system at the meso- and macro-scale. Not only the different architectural elements and lithofacies of the complete braided fluvial system, but also the lateral variation of the architectural elements were emphasised in this study. Differential tectonic movements, seasonal climate change, and their effect on vertical and lateral evolution of the area were the main control on basin sedimentation, channel interconnection, palaeocurrent patterns, and consequently the fluvial architecture. The presence of lateral ramp anticlines caused the fluvial system to be laterally restricted, with the main channel-belts being located in the areas of highest subsidence and lowest topography. Intervening topographic highs acted as both flood plains and lateral barriers between the main channel systems. The proposed depositional model comprises broad, low-sinuosity, perennial, but seasonal moderate-energy streams. The sandstone architecture is dominated by channel-fill and sheet sands, and associated simple and more complex bars. Adjacent to the main channel-belts fine-grained sandstones, siltstones and immature paleosols occur. The along-strike relationship between major fluvial systems and their outlets into a foreland basin has important implications for the infill of the basin and the modelling of fluvial systems along mountain belt fronts.

The role of fluid pressure and diagenetic cements for porosity preservation in Triassic fluvial reservoirs of the Central Graben, North Sea

Anomalously high porosities and permeabilities are commonly found in the fluvial channel sandstone facies of the Triassic Skagerrak Formation in the central North Sea at burial depths greater than 3200 m (10,499 ft), from which hydrocarbons are currently being produced. The aim of our study was to improve understanding of sandstone diagenesis in the Skagerrak Formation to help predict whether the facies with high porosity may be found at even greater depths. The Skagerrak sandstones comprise fine to medium-grained arkosic to lithic-arkosic arenites. We have used scanning electron microscopy, petrographic analysis, pressure history modeling, and core analysis to assess the timing of growth and origin of mineral cements, with generation, and the impact of high fluid pressure on reservoir quality. Our interpretation is that the anomalously high porosities in the Skagerrak sandstones were maintained by a history of overpressure generation and maintenance from the Late Triassic onward, in combination with early microquartz cementation and subsequent precipitation of robust chlorite grain coats. Increasing salinity of pore fluids during burial diagenesis led to pore-filling halite cements in sustained phreatic conditions. The halite pore-filling cements removed most of the remaining porosity and limited the precipitation of other diagenetic phases. Fluid flow associated with the migration of hydrocarbons during the Neogene is inferred to have dissolved the halite locally. Dissolution of halite cements in the channel sands has given rise to megapores and porosities of as much as 35% at current production depths.

Landscape evolution : denudation, climate and tectonics over different time and space scales

The morphology of Earth’s surface reflects the interaction of climate, tectonics and denudational processes operating over a wide range of spatial and temporal scales. These processes can be considered catastrophic or continuous; depending on the timescale of observation or interest. Recent research had required integration of historically distinct subjects such as geomorphology, sedimentology, climatology and tectonics. Together, these have provided new insights into absolute and relative rates of denudation, and the factors that control the many dynamic processes involved. Specific subject areas covered are sediment transport processes and the timescales of competing processes, the role of the geological record and landscapes in constraining different processes, the nature of landscape evolution at different spatial scales and in contrasting geological environments.

Chemical compaction of mudrocks in the presence of overpressure

In sedimentary basins, compaction disequilibrium generates overpressure during rapid burial of fine-grained sediments in the mechanical compaction regime, at temperatures below ~70°C. Mudstones behave differently at greater depths in the chemical compaction regime, at temperatures above ~100°C, where evidence suggests that porosity reduction with increasing depth and temperature continues independently of effective stress up to high values of overpressure. We offer an explanation for this behaviour. The horizontal alignment of clay mineral grains is enhanced during clay diagenesis, creating sub-horizontal, flat pores. Because of their flexibility, the flat pores tend to close even under low values of normal effective stress acting across them. Thus, chemical compaction can proceed unless the net expulsion of pore water from the mudstones is inhibited sufficiently for the flat pores to be held open, which necessarily requires the pore pressure to approach the lithostatic stress. In the Lower Kutai Basin, density log reversals are encountered in mudstones in the chemical compaction regime at depths of 3–4 km, where the pore pressure is close to the lithostatic stress. We attribute these reversals to the inhibition of dewatering during clay diagenesis at shallower depths, when the pore pressure was already close to lithostatic stress. Porosity was preserved by the very high pore pressure holding the flat pores open while the mudstone matrix was being cemented by the products of clay diagenesis. We coin the term ‘chemical undercompaction’ for this process.

Impact of periodicity on sediment flux in alluvial systems ; grain to basin scale.

Abstract Periodicity is a common component of many sedimentological processes, but seldom is it considered across all scales of fluvial processes in order to understand the complete impact on sediment supply to basins. Temporal changes in sediment supply within drainage systems and sedimentary basins are a consequence of the inherent instability in transport processes. The causes of fluctuations are of 2 main types: (i) changes in factors endemic to the supply of sediment but which are at least partly independent of erosive forces and (ii) changes in the magnitude of forces available to transport sediment. Fluctuations at spatial scales from grain — through reach — to basin — scales and at temporal scales from minutes to millennia are discussed and evaluated. All fluctuations are reflected in sedimentary deposits in some way. For example, irregular patterns of bed break-up during erosion can generate bedforms that are recorded in deposits, the passage of waves of sediment can cause cycles of incision and aggradation in a reach; large flood events will flush sediment into coastal regions and will be recorded as an identifiable ‘package’ in the deposits. Many models of basin processes and products assume a consistent supply of sediment which is far from the case in nature. One of the challenges in the coming decade is to move away from using long-term averages of sediment supply and to link models directly into geomorphic processes.

Enhanced porosity preservation by pore fluid overpressure and chlorite grain coatings in the Triassic Skagerrak, Central Graben, North Sea, UK

Abstract Current understanding of porosity preservation in deeply buried sandstone reservoirs tends to be focused on how diagenetic grain coatings of clay minerals and microquartz can inhibit macroquartz cementation. However, the importance of overpressure developed during initial (shallow) burial in maintaining high primary porosity during subsequent burial has generally not been appreciated. Where pore fluid pressures are high, and the vertical effective stress is low, the shallow arrest of compaction can allow preservation of high porosity and permeability at depths normally considered uneconomic. The deeply buried fluvial sandstone reservoirs of the Triassic Skagerrak Formation in the Central Graben, North Sea, show anomalously high porosities at depths greater than 3500 metres below sea floor (mbsf ). Pore pressures can exceed 80 MPa in the upper part of the Skagerrak Formation at depths of 4000–5000 mbsf, where temperatures are above 140°C. The Skagerrak reservoirs commonly have high primary porosities of up to 35%, little macroquartz cement and variable amounts of diagenetic chlorite grain coats. This research sheds light on the complex controls on reservoir quality in the fluvial sandstones of the Skagerrak Formation by identifying the role of shallow overpressure in arresting mechanical compaction and the importance of chlorite detrital grain coatings in inhibiting macroquartz cement overgrowth as temperature increases during progressive burial.

Transverse rivers draining the Spanish Pyrenees: large scale patterns of sediment erosion and deposition

Abstract Data collected from six transverse rivers draining the Spanish Pyrenees highlight how the rate of change in river gradients downstream controls long-term deposition and erosion patterns acroos the mountain belt. The rivers draining the Spanish Pyrenees provide evidence for an important relationship between gradient (S) and distance from the drainage divide (x), such that S is proportional to xΩ. This can be expressed quantitatively as a power function relation S ∞ L−R. In areas dominated by erosion Ω has a value > − 5, while in areas of deposition Ω has a value < −0.4. The contrast between areas of erosion and deposition is also reflected in downstream increases and decreases, respectively, in stream power and bed shear stress along the whole of the mountain front. It is proposed, based upon previous empirical studies of loose to semi-armoured channels, that the downstream variations of stream power and bed shear stress result in variations of bed-load transport rates, which in turn create the erosional and depositional patterns associated with variations of channel gradient. These results highlight the complex response of the fluvial system, emphasizing the need for regional approaches for examining long-term changes in rivers draining orogenic belts and the importance for incorporation of hydraulic data into geomorphological models to assist interpretations of the mass distribution of sediment.

Inferring bedload transport from stratigraphic successions: examples from Cenozoic and Pleistocene rivers, south central Pyrenees, Spain

Abstract Geologists and geomorphologists have long been concerned with rates of sediment transfer as bedload in gravel-bed rivers, especially as rates of sediment transfer are important factors controlling river aggradation and incision. Bedload transport equations, originally derived for Holocene streams, have been used widely in modern gravel-bed river systems. However, palaeohydraulic reconstructions have received less attention and are generally dismissed as inaccurate since most are estimated to be at least an order of magnitude out. This study focuses on deriving stream power, bedload transport rates and efficiency estimates for Oligo–Miocene and Plio–Pleistocene gravel-bed river deposits from the south central Pyrenees, Spain. The basic data used in the palaeohydraulic calculations are estimates of palaeoslope, palaeovelocity, palaeodepth and the volume of sediment accreted in yearly flood events on gravel bars. Analyses of data from these ancient river systems yield more accurate estimates of relative stream power, bedload transport rates and efficiency parameters. This study illustrates the need for understanding the palaeohydraulics of river systems in order to characterize ancient rivers. Gravel-bed rivers with low sediment supply and high bedload transport rates incise. Conversely, when sediment supply is abundant, bedload transport rates and efficiency are low and the river system aggrades.

The Earth's dynamic surface: an overview

Debate about the relative roles of catastrophic v. continuous processes of landform evolution is as old as the discipline of Earth Science itself. Over the last 10 years or so, research in the Earth Sciences has focussed strongly on the Earth’s surface and particularly in terms of quantifying rates of processes. This research parallels developments in geomorphology and sedimentology in the quantification of surface processes since the 1950s and 1960s. These surface processes are the manifestation of the large-scale interaction of climate and tectonics operating over a wide range of spatial and temporal scales. Thus, recent research had required integration of the historically distinct subjects of geomorphology, sedimentology, climatology and tectonics. Partly as a cause and partly as a consequence of this integration, there have been many recent developments in quantitative modelling and both laboratory and field-based analytical tools. Together, these have provided new insights into absolute and relative rates of denudation, and the factors that control the many dynamic processes involved. One of the outstanding issues concerns the balance between tectonics, climate and denudation, and in particular the limiting effects of one on the others and the nature of dynamic feedback mechanisms. The fact that processes can be considered catastrophic or continuous, depending on the timescale of observation or interest, can hinder the predictability of models, depending on how they are formulated. Certain conditions may lead to a steady-state situation in which denudation balances tectonic uplift, leading to a more or less constant topography. Steady-state topography means that detailed study of present day landforms can provide important insights into the nature of surface processes back in time. Such assumptions underpin debates in geomorphology relating to the process-form linkage and the understanding of characteristic forms in the landscape. Alternatively, the recognition of non-steady-state situations and a clearer understanding of why these situations occur provide the key for resolving the climate– tectonics–landscape evolution feedback loop. The transition between the two states will reflect the process response time, and therefore the transitory state may provide a clearer picture of the time lag of topographic response to changes in the rates of climate change and tectonic forcing. However, the response time is not necessarily constant and may have changed considerably at key points in the past, such as the evolution of plants on land in the Palaeozoic and the acceleration of human activity within the landscape in the Holocene. In terms of denudation (physical erosion and chemical and mechanical weathering), there are clearly catastrophic processes, such as landsliding, which operate discretely and on short timescales and more continuous processes, such as chemical weathering, which can be considerably more protracted. The distinction between discrete catastrophe and continuous modification depends also on the time and spatial scales of interest. These considerations also impact directly on the questions of if and how steady-state topography can be achieved, how the processes controlling this state can be quantified and resolved, what causes departures from a steady-state condition and how topography reflects the coupling between denudation, climate and tectonics. Some of the key current research areas in the world are tectonically active regions, such as New Zealand (southern Alps), Taiwan and Olympic Mountains (USA). However, the link between tectonics and denudation is complicated in these convergent zones (e.g. Willett et al. 2001), as there is a significant horizontal component to the deformation and, additionally, climatic variations often produce marked asymmetry in denudation, which itself then feedbacks into the isostatic component of vertical motion. In practice, this research field necessarily involves a broad range of disciplines including field geologists, geomorphologists, structural geologists, geochemists, climatologists and geophysical modellers. These researchers address the observational constraints on