The Carboniferous Southern Pennine Basin, UK

Many of the Carboniferous outcrops located in the Derbyshire region of the Peak District National Park, UK, have provided sites for both significant and pioneering research relating to the clastic sedimentology of marine palaeoenvironments, particularly so during the 1960s and 1970s when early models describing the sedimentary architecture of fluvio-deltaic, submarine slope and deep-marine submarine-fan sedimentation were first developed. The area was subject to hydrocarbon exploration from the 1920s to 1950s, which although unsuccessful in economic terms left a legacy of sub-surface data. Despite a long-history of sedimentological research, the deposits exposed at several classic localities in the Pennine Basin continue to broaden and challenge our current understanding of sedimentary processes to this day

The lithofacies organization of fluvial channel deposits: a meta-analysis of modern rivers

Environmental interpretations of subsurface fluvial successions are commonly based on facies observations from core and are often attempted by generalist geologists by reference to classic facies models. However, for fluvial channel deposits, the value of observations on lithofacies proportions for interpretations of depositional environment has yet to be assessed quantitatively. Here, a test is presented that is based on a comparative study of facies data from 77 reaches of 46 modern rivers. The analysis is undertaken on datasets from published case studies stored in a sedimentological database, with consideration of causes for observational bias, and with particular attention paid to sandy lithofacies. The observed variability in the proportion of facies assemblages in the channel deposits of sandy river systems is quantified for classes of environments categorized according to channel pattern (braided, low sinuosity, meandering), climatic setting (arid to perhumid), and discharge regime (ephemeral to perennial). By capturing the variability in facies organization within fluvial systems of certain types, these outputs serve as facies models that provide a measure of uncertainty to sedimentological interpretations. Concurrently, the statistical analysis presented enables a test of the significance of relationships between the relative proportions of channel lithofacies and parameters that either represent controlling factors (e.g., water-discharge characteristics) or covariates (e.g., channel pattern). For classes of river systems grouped by channel pattern, climate, and discharge regime, emerging features of facies organization can be identified. Statistically, it is observed that relationships exist (i) between channel pattern and the frequency of the preserved expression of bedforms, and (ii) between controls on river hydrology (climate, discharge regime and seasonal variability) and the record of upper and lower flow-regime conditions. Thus, the results corroborate existing qualitative facies models in some respects. However, observations of the relative dominance of facies in channel deposits demonstrate limited value for interpretations or predictions in subsurface or outcrop studies, as variability within each type of depositional system is significant. Corehole data of fluvial channel deposits may be commonly overinterpreted

On the geological significance of clastic parasequences

Parasequences recognized in clastic sedimentary successions of shallow-marine origin are considered by some geologists to be the fundamental building blocks of depositional sequences, even though problems in their definition and application have been identified by others, who instead advocate their abandonment as formal sequence stratigraphic units. To elucidate the geological significance of clastic parasequences and inform the debate on their use in stratigraphy, a quantitative characterization of the geometry, facies characteristics and timescale of deposition of 1163 parasequences has been undertaken based on a synthesis of data from outcrop and subsurface studies that are available in the scientific literature. Through a database compilation, the attributes of the studied parasequences are analysed with respect to the interpreted geological origin of the units, and with consideration of sources of bias and uncertainty. Particular emphasis is placed on assessing the following: (i) the importance of heuristics, and of data types and coverage in the recognition of parasequences; (ii) differences in parasequence characteristics observed across deltaic and shoreface depositional systems, and between the Quaternary and the ancient rock record; (iii) possible explanations for the range in timescales of deposition of parasequences; and (iv) the role of autogenic dynamics on the development of deltaic parasequences, partly based on a comparison with the recent evolution of modern deltas. The results demonstrate that parasequence definition and physical correlation suffer from subjectivity, and that significant variability exists in the spatio-temporal and architectural attributes of clastic parasequences. This gives rise to uncertainty that affects the use of parasequences as a framework for comparison of the architecture of packages of strata originating via shoreline regression: this uncertainty must be considered when using analogue data for subsurface predictions or when attempting comparative studies of clastic successions

A classification scheme for fluvial–aeolian system interaction in desert-margin settings

This study examines 130 case examples from 60 desert regions to propose a generalised framework to account for the diverse types of interaction known to exist between active aeolian and fluvial depositional systems at modern dune-field margins. Results demonstrate the significance of aeolian and fluvial system type, orientation of aeolian versus fluvial landforms, distribution of open versus closed interdune corridors, and fluvial flow processes in controlling the distance and type of penetration of fluvial systems into aeolian dune fields. Ten distinct types of fluvial–aeolian interaction are recognised: fluvial incursions aligned parallel to trend of linear chains of aeolian dune forms; fluvial incursions oriented perpendicular trend of aeolian dunes; bifurcation of fluvial flow between isolated aeolian dune forms; through-going fluvial channel networks that cross entire aeolian dune fields; flooding of dune fields due to regionally elevated water-table levels associated with fluvial floods; fluvial incursions emanating from a single point source into dune fields; incursions emanating from multiple sheet sources; cessation of the encroachment of entire aeolian dune fields by fluvial systems; termination of fluvial channel networks in aeolian dune fields; long-lived versus short-lived modes of fluvial incursion. Quantitative relationships describing spatial rates of change of desert-margin landforms are presented. The physical boundaries between geomorphic systems are dynamic: assemblages of surface landforms may change gradationally or abruptly over short spatial and temporal scales. Generalised models for the classification of types of interaction have application to the interpretation of ancient preserved successions, especially those known only from the subsurface

Assessment of backwater controls on the architecture of distributary channel fills in a tide-influenced coastal-plain succession: Campanian Neslen Formation, USA

The backwater zone of a river is its distal reach downstream of the point at which the streambed elevation reaches sea level. Backwater hydraulics is believed to exert an important control on fluvio-deltaic morphodynamics, but the expressions with which this may be recorded in the preserved stratigraphic record are not well understood. The seaward reaches of modern rivers can undergo flow acceleration and become erosional at high discharges due to drawdown of the in-channel water surface near the river mouth, in relation to the fixed water surface at the shoreline. As coastal-plain distributary channels approach the shoreline they tend to be subject to a reduction in lateral mobility, which could be related to diminished sediment flux at low flow. Current understanding of channel morphodynamics associated with backwater effects, as based on observations from numerical models and modern sedimentary systems, is here used to make predictions concerning the architecture of coastal distributary channel fills in the rock record. On the basis of existing knowledge, distributary channel fills are predicted to be typically characterized by low width-to-thickness aspect ratios, by a clustering of scour surfaces toward their base, by an aggradational infill style, by a facies organization that bears evidence of drawdown-influenced scour filling, possibly resulting in the overprint of tidal signals toward their base, and by co-genetic sand-prone overbank units of limited occurrence, thickness and sand content. To test these predictions, fieldwork was carried out to examine sedimentological characters of channel bodies from an interval of the Campanian Neslen Formation (eastern Utah, USA), which comprises a succession of sandstone, carbonaceous mudstone, and coal, deposited in a coastal-plain setting, and in which significant evidence of tidal influence is preserved. Three types of channel bodies are recognized in the studied interval, in terms of lithology and formative-channel morphodynamics: sand-prone laterally accreting channel elements, heterolithic laterally accreting channel elements and sand-prone aggradational ribbon channel elements. This study concentrates on the ribbon channel bodies since they possess a geometry compatible with laterally stable distributaries developed in the zone of drawdown. Sedimentological and architectural characteristics of these bodies are analyzed and compared with the proposed model of distributary channel-fill architecture. Although conclusive evidence of the influence of backwater processes in controlling the facies architecture of distributary channel fills is not reached, the studied bodies display an ensemble of internal architecture, lithological organization, nature of bounding surfaces and relationships with other units that conforms to the proposed model to a certain extent. The analyzed ribbon sandbodies are all characterized by erosional cut-banks, very limited proportions of mudstone deposits, a lack of genetically related barform units, clustering of scour fills at their base, and a lack of relationships with co-genetic river-fed overbank sandstones. This work provides a guide to future research, which is required to better understand the role of backwater processes in controlling the architecture of distributary channel bodies, their down-dip variations, and how these are expressed in the stratigraphic evolution of prograding coastal plains

Characterizing flow pathways in a sandstone aquifer: Tectonic vs sedimentary heterogeneities

Sandstone aquifers are commonly assumed to represent porous media characterized by a permeable matrix. However, such aquifers may be heavy fractured when rock properties and timing of deformation favour brittle failure and crack opening. In many aquifer types, fractures associated with faults, bedding planes and stratabound joints represent preferential pathways for fluids and contaminants. In this paper, well test and outcrop-scale studies reveal how strongly lithified siliciclastic rocks may be entirely dominated by fracture flow at shallow depths (≤ 180 m), similar to limestone and crystalline aquifers. However, sedimentary heterogeneities can primarily control fluid flow where fracture apertures are reduced by overburden pressures or mineral infills at greater depths. The Triassic St Bees Sandstone Formation (UK) of the East Irish Sea Basin represents an optimum example for study of the influence of both sedimentary and tectonic aquifer heterogeneities in a strongly lithified sandstone aquifer-type. This fluvial sedimentary succession accumulated in rapidly subsiding basins, which typically favours preservation of complete depositional cycles including fine grained layers (mudstone and silty sandstone) interbedded in sandstone fluvial channels. Additionally, vertical joints in the St Bees Sandstone Formation form a pervasive stratabound system whereby joints terminate at bedding discontinuities. Additionally, normal faults are present through the succession showing particular development of open-fractures. Here, the shallow aquifer (depth ≤ 180 m) was characterized using hydro-geophysics. Fluid temperature, conductivity and flow-velocity logs record inflows and outflows from normal faults, as well as from pervasive bed-parallel fractures. Quantitative flow logging analyses in boreholes that cut fault planes indicates that zones of fault-related open fractures characterize ~ 50% of water flow. The remaining flow component is dominated by bed-parallel fractures. However, such sub-horizontal fissures become the principal flow conduits in wells that penetrate the exterior parts of fault damage zones, as well as in non-faulted areas. The findings of this study have been compared with those of an earlier investigation of the deeper St Bees Sandstone aquifer (180 to 400 m subsurface depth) undertaken as part of an investigation for a proposed nuclear waste repository. The deeper aquifer is characterized by significantly lower transmissivities. High overburden pressure and the presence of mineral infillings, have reduced the relative impact of tectonic heterogeneities on transmissivity here, thereby allowing matrix flow in the deeper part of the aquifer. The St Bees Sandstone aquifer contrasts the hydraulic behaviour of low-mechanically resistant sandstone rock-types. In fact, the UK Triassic Sandstone of the Cheshire Basin is low-mechanically resistant and flow is supported both by matrix and fracture. Additionally, faults in such weak-rocks are dominated by granulation seams representing flow-barriers which strongly compartmentalize the UK Triassic Sandstone in the Cheshire Basin

Response of a Coal-Bearing Coastal Plain Succession to Marine Transgression: Campanian Neslen Formation, Utah, USA

The process regime of low-gradient coastal plains, delta plains and shorelines can change during transgression. In ancient successions, accurate assessment of the nature of marine influence is needed to produce detailed paleogeographic reconstructions, and to better predict lithological heterogeneity in hydrocarbon reservoirs. The Campanian lower Neslen Formation represents a fluvial-dominated and tide- and wave-influenced coastal-plain and delta-plain succession that accumulated along the margins of the Western Interior Seaway, USA. The succession records the interactions of multiple coeval sedimentary environments that accumulated during a period of relative sea-level rise. A high-resolution data set based on closely spaced study sites employs vertical sedimentary graphical logs and stratigraphic panels for the recognition and correlation of a series of stratal packages. Each package represents the deposits of different paleoenvironments and process regimes within the context of an established regional sequence stratigraphic framework. Down-dip variations in the occurrence of architectural elements within each package demonstrate increasing marine influence as part of the fluvial-to-marine- transition zone. Three marine-influenced packages are recognized. These exhibit evidence for an increase in the intensity of marine processes upwards as part of an overall transgression through the lower Neslen Formation. These marine-influenced packages likely correlate down-dip to flooding surfaces within the time-equivalent Îles Formation. The stratigraphic arrangement of these packages is attributed to minor rises in sea level, the effects of which were initially buffered by the presence of raised peat mires. Post-depositional auto-compaction of these mires resulted in marine incursion over broad areas of the coastal plain. Results demonstrate that autogenic processes modified the process response to overall rise in relative sea level through time. Understanding the complicated interplay of processes in low-gradient, coal-bearing, paralic settings requires analysis of high-resolution stratigraphic data to discern the relative role of autogenic and allogenic controls

The palaeoshoreline of Early Miocene formations (Euphrates and Jeribe) at the periphery of the Zagros Foreland Basin, Sulaimani Governorate, Kurdistan Region, NE Iraq

A sedimentological investigation of the Early Miocene deposits at the periphery of the Zagros Foreland Basin, Kurdistan Region, around Qishlagh-Sargrma and Darbandikhan, reveals the presence of the Euphrates and Jeribe Formations in this area. A carbonate-dominated unit, comprising four regressive carbonate cycles, at the base of the Fatha Formation, has been investigated to characterize the depositional environments and stratigraphic context of these Early Miocene deposits. Outcrop and thin-section analyses of the carbonate succession reveal a gently inclined, carbonate-dominated ramp based on the depositional lithofacies and microfacies. Eight carbonate microfacies were identified and interpreted to have been deposited in a shallow marine environment. The Euphrates Formation passes up from deposits of restricted lagoon to shoal depositional environments, while the Jeribe Formation was deposited in a hypersaline lagoon to a restricted lagoon. The Early Miocene index fossil Borelis melo melo was identified in the deposits of the Euphrates Formation. Stratigraphic correlation of the studied sections allows the development of a revised palaeogeography for the Early Miocene deposits in the Kurdistan Region

Quantitative analysis of the stratigraphic architecture of incised-valley fills: a global comparison of Quaternary systems

Facies models of the internal fills of incised valleys developed in shelf and coastal settings during cycles of relative sea-level change are largely conceptual, descriptive and qualitative in form; moreover, they are commonly bespoke to individual examples. Here, a database-driven quantitative statistical analysis of 87 late-Quaternary incised-valley fills (IVFs) has been undertaken to assess the general validity and predictive value of classical facies models for IVFs, and to investigate the relative importance of possible controls on their stratigraphic organization. Based on datasets from the published literature stored in a sedimentological database, the geometry and proportion of systems tracts, and of architectural elements of different hierarchies within IVFs are quantified. These variables were analysed to assess how they vary in relation to parameters that represent potential controlling factors: relative sea-level stage, continental-margin type, drainage-basin area, valley geometry, basin physiography and shoreline hydrodynamics. The stratigraphic organization of the studied coastal-plain IVFs is generally consistent with that represented in facies models, the primary control being the rate and magnitude of relative sea-level change. However, results from this study demonstrate significant variability in the stratigraphic architectures of IVFs, which is not accounted for by existing models. Variations in the facies architecture of coastal-plain and cross-shelf valley fills can be attributed to controls other than sea level, and expressed in relationships with continental-margin type, basin physiography, catchment area, river-system size and shoreline hydrodynamics. The following primary findings arise from this research. (i) Compared to their counterparts on passive margins, coastal-plain IVFs hosted on active margins contain, on average, a higher proportion of fluvial deposits and a lower proportion of central-basin estuarine deposits; estuarine deposits tend however to be thicker. This suggests a control on IVF stratigraphic architecture exerted by distinct characteristics of the tectonic setting of the host continental margins, notably basin physiography, rates and mode of sediment supply, and nature of sediment load. (ii) The thickness and proportion of lowstand systems tract are positively correlated with coastal-plain IVF dimensions, likely reflecting the role of drainage-basin area in dictating the scale of the fluvial systems that carved and infilled the valleys. (iii) Positive correlations are observed between the thickness of fluvial deposits, bayhead-delta deposits and central-basin estuarine deposits, versus coastal-plain IVF dimensions and valley catchment area. This suggests a control exerted by the river-system scale on sediment-supply rates and on the accommodation determined by valley size. (iv) Positive correlations between the thickness and proportion of barrier-complex deposits within cross-shelf IVFs versus mean shelf gradient indicate that the geometry of the shelf might control the establishment and preservation of barrier-island environments in incised valleys located on the shelf. (v) Correlations between the width of coastal-plain IVFs and present-day mean tidal range at the shoreline indicate that tidal dynamics may contribute to the widening of the incised valleys. Positive correlation is observed between the proportion of tide-dominated elements in highstand IVF deposits and IVF width, suggesting possible effects of interplays between hydrodynamic conditions and the geometry of incised valleys on their infills. This study highlights the complexity of the internal fills of incised valleys, which must be considered when attempting the application of facies models of IVFs to rock-record interpretations or as predictive tools in subsurface studies

The geometry of fluvial channel bodies: Empirical characterization and implications for object-based models of the subsurface

The distribution of channel deposits in fluvial reservoirs is commonly modeled with object-based techniques, constrained on quantities describing the geometries of channel bodies. To ensure plausible simulations, it is common to define inputs to these models by referring to geologic analogs. Given their ability to reproduce complex geometries and to draw upon the analog experience, object-based models are considered inherently realistic. Yet this perceived realism has not hitherto been tested by assessing the outputs of these techniques against sedimentary architectures in the stratigraphic record. This work presents a synthesis of data on the geometry of channel bodies, derived from a sedimentologic database, with the following aims: (1) to provide tools for constraining stochastic models of fluvial reservoirs in data-poor situations, and (2) to test the intrinsic realism of object-based modeling algorithms by comparing characteristics of the modeled architectures against analogs. An empirical characterization of the geometry of fluvial channel bodies is undertaken that describes distributions in (and relationships among) channel-body thickness, cross-stream width, and planform wavelength and amplitude. Object-based models are then built running simulations conditioned on six alternative, analog-informed parameter sets, using four algorithms according to nine different approaches. Closeness of match between analogs and models is then determined on a statistical basis. Results indicate which modeling approaches return architectures that more closely resemble the organization of fluvial depositional systems known from nature and in what respect. None of the tested algorithms fully reproduce characteristics seen in natural systems, demonstrating the need for subsurface modeling methods to better incorporate geologic knowledge