Lithofacies characterization of fluvial sandstones from outcrop gamma-ray logs (Loranca Basin, Spain): the influence of provenance

Natural gamma spectral (NGS) log motifs and cluster analysis were used to characterize outcropping sandstone bodies formed in braided and highsinuosity streams of the Tertiary Tórtola fluvial system of the Loranca Basin (Spain). Five coarse-grained lithofacies comprise these deposits and determine distinct NGS log motif. Cross-plots and cluster analysis of NGS log data of the lithofacies suggest three distinct clusters. These clusters reflect distinct values for sandstones with small sets of ripple lamination, cross-stratification, and conglomerates and pebbles. Ripple-laminated sandstones show the most variability in NGS signature, whereas conglomeratic sandstones show the most uniform signature. Such cluster analysis may be used to assign NGS log data points of unknown origin to a specific fluvial lithofacies under conditions of equal rock provenance and diagenetic history. A sedimentaclastic (i.e. sedimentary parent rock) origin of sediments appears to be the main control on detrital composition that, in turn, varies with grain size.Depto. de Mineralogía y PetrologíaFac. de Ciencias GeológicasTRUEpu

Back-flow ripples in troughs downstream of unit bars: Formation, preservation and value for interpreting flow conditions

Back-flow ripples are bedforms created within the lee-side eddy of a larger bedform with migration directions opposed or oblique to that of the host bedform. In the flume experiments described in this article, back-flow ripples formed in the trough downstream of a unit bar and changed with mean flow velocity; varying from small incipient back-flow ripples at low velocities, to well-formed back-flow ripples with greater velocity, to rapidly migrating transient back-flow ripples formed at the greatest velocities tested. In these experiments back-flow ripples formed at much lower mean back-flow velocities than predicted from previously published descriptions. This lower threshold mean back-flow velocity is attributed to the pattern of velocity variation within the lee-side eddy of the host bedform. The back-flow velocity variations are attributed to vortex shedding from the separation zone, wake flapping and increases in the size of, and turbulent intensity within, the flow separation eddy controlled by the passage of superimposed bedforms approaching the crest of the bar. Short duration high velocity packets, whatever their cause, may form back-flow ripples if they exceed the minimum bed shear stress for ripple generation for long enough or, if much faster, may wash them out. Variation in back-flow ripple cross-lamination has been observed in the rock record and, by comparison with flume observations, the preserved back-flow ripple morphology may be useful for interpreting formative flow and sediment transport dynamics

Bedforms and sedimentary structures related to supercritical flows in glacigenic settings

Upper-flow-regime bedforms, including upper-stage-plane beds, antidunes, chutes-and-pools and cyclic steps, are ubiquitous in glacigenic depositional environments characterized by abundant meltwater discharge and sediment supply. In this study, the depositional record of Froude near-critical and supercritical flows in glacigenic settings is reviewed, and similarities and differences between different depositional environments are discussed. Upper-flow-regime bedforms may occur in subglacial, subaerial and subaqueous environments, recording deposition by free-surface flows and submerged density flows. Although individual bedform types are generally not indicative of any specific depositional environment, some observed trends are similar to those documented in non-glacigenic settings. Important parameters for bedform evolution that differ between depositional environments include flow confinement, bed slope, aggradation rate and grain size. Cyclic-step deposits are more common in confined settings, like channels or incised valleys, or steep slopes of coarse-grained deltas. Antidune deposits prevail in unconfined settings and on more gentle slopes, like glacifluvial fans, sand-rich delta slopes or subaqueous (ice-contact) fans. At low aggradation rates, only the basal portions of bedforms are preserved, such as scour fills related to the hydraulic-jump zone of cyclic steps or antidune-wave breaking, which are common in glacifluvial systems and during glacial lake-outburst floods and (related) lake-level falls. Higher aggradation rates result in increased preservation potential, possibly leading to the preservation of complete bedforms. Such conditions are met in sediment-laden jökulhlaups and subaqueous proglacial environments characterized by expanding density flows. Coarser-grained sediment leads to steeper bedform profiles and highly scoured facies architectures, while finer-grained deposits display less steep bedform architectures. Such differences are in part related to stronger flows, faster settling of coarse clasts, and more rapid breaking of antidune waves or hydraulic-jump formation over hydraulically rough beds. © 2020 The Authors. Sedimentology published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologist

Unit bar architecture in a highly‐variable fluvial discharge regime: Examples from the Burdekin River, Australia

Unit bars are relatively large bedforms that develop in rivers over a wide range of climatic regimes. Unit bars formed within the highly-variable discharge Burdekin River in Queensland, Australia, were examined over three field campaigns between 2015 and 2017. These bars had complex internal structures, dominated by co-sets of cross-stratified and planar-stratified sets. The cross-stratified sets tended to down-climb. The development of complex internal structures was primarily a result of three processes: (i) superimposed bedforms reworking the unit bar avalanche face; (ii) variable discharge triggering reactivation surfaces; and (iii) changes in bar growth direction induced by stage change. Internal structures varied along the length and across the width of unit bars. For the former, down-climbing cross-stratified sets tended to pass into single planar cross-stratified deposits at the downstream end of emergent bars; such variation related to changes in fluvial conditions whilst bars were active. A hierarchy of six categories of fluvial unsteadiness is proposed, with these discussed in relation to their effects on unit bar (and dune) internal structure. Across-deposit variation was caused by changes in superimposed bedform and bar character along bar crests; such changes related to the three-dimensionality of the channel and bar geometry when bars were active. Variation in internal structure is likely to be more pronounced in unit bar deposits than in smaller bedform (for example, dune) deposits formed in the same river. This is because smaller bedforms are more easily washed out or modified by changing discharge conditions and their smaller dimensions restrict the variation in flow conditions that occur over their width. In regimes where unit bar deposits are well-preserved, their architectural variability is a potential aid to their identification. This complex architecture also allows greater resolution in interpreting the conditions before and during bar initiation and development

The sedimentological characterisation of labyrinthian reservoir analogues

Civil Engineering and Geoscience

Holocene sediments of the Rhine-Meuse-Scheldt estuaries as aids to interpret tidal and fluvial-tidal deposits in outcrop and core

Despite enormous technological and statistical advances in the last decade, the building of geologically-based 3D reservoir models will always be demanding. Moreover, it is imperative that uncertainty is handled appropriately, especially as remaining resources are increasingly found in smaller and/or more complicated reservoirs and undrained compartments. One way of reducing information loss is to incorporate complementary data into the interpretation of subsurface core data sets. To achieve this, geologists must continue to study modern and ancient analogues and archive their findings in a systematized ‘library’ (conceptual, qualitative and quantitative). Such a library should capture the natural variability of environments and facies, and include methodologies and guidelines for estimating and reducing subsurface uncertainty. This collective understanding and information can also be used for training purposes. A library of observations and interpretations of modern and ancient outcrop analogues can help to reduce subsurface geological uncertainty and, more generally, aids the development of conceptual depositional models, (sequence) stratigraphic correlation methods, and the construction of quantitative subsurface 3D models suitable for reservoir simulation. As such, it can play a vital role in the reservoir characterization work flow. Such a library will also be a means to mitigate the risks of the overuse of simple structures to try and interpret an entire environment

Depositional environments and sediment partitioning during early stage foreland basin thrusting: The Late Ilerdian-Early Cuisian Roda Sequence, South Pyrenean Foreland Basin (Spain).

The Late Ilerdian-Early Cuisian Roda Sequence spans approximately 3 Ma and formed in the South Pyrenean Foreland Basin (Spain). It is mapped in detail across the present-day Tremp-Graus, Ager and Ainsa sub-basins, and across the Boltaña Anticline into the Jaca sub-basin. The South Pyrenean Foreland Basin basin formed during the early stage of oblique collision of the Iberian Plate with the Eurasian Plate, and was bounded to the north by the Pyrenean chain and to the south by the Sierras Marginales. Within the basin, a time-sequenced series of curved thrusts developed (controlled by the location of inverted Mesozoic extensional faults) of which the Montsec thrust was decisive for the development of the Roda Sequence. This thrust sheet was displaced in a mostly southward direction and has an E-W orientation; it carried the Ilerdian-Cuisian sediments of the present-day Tremp-Graus and Ainsa sub-basins. It is well-expressed at the surface as well as partly buried by younger sediments. The Montsec thrust sheet is bounded east and west by the corresponding lateral ramps oriented SW-NW (Segre line), and SE-NW (La Fueba thrust system). Beyond the SE-NW oriented lateral ramp, the present-day Ainsa and Jaca sub-basins are located. The thrust had a significant influence on sediment dispersal patterns and the nature and location of depositional environments during the development of the Roda Sequence.Applied Geolog

Holocene sediments of the Rhine-Meuse-Scheldt estuaries as aids to interpret tidal and fluvial-tidal deposits in outcrop and core

Despite enormous technological and statistical advances in the last decade, the building of geologically-based 3D reservoir models will always be demanding. Moreover, it is imperative that uncertainty is handled appropriately, especially as remaining resources are increasingly found in smaller and/or more complicated reservoirs and undrained compartments. One way of reducing information loss is to incorporate complementary data into the interpretation of subsurface core data sets. To achieve this, geologists must continue to study modern and ancient analogues and archive their findings in a systematized ‘library’ (conceptual, qualitative and quantitative). Such a library should capture the natural variability of environments and facies, and include methodologies and guidelines for estimating and reducing subsurface uncertainty. This collective understanding and information can also be used for training purposes. A library of observations and interpretations of modern and ancient outcrop analogues can help to reduce subsurface geological uncertainty and, more generally, aids the development of conceptual depositional models, (sequence) stratigraphic correlation methods, and the construction of quantitative subsurface 3D models suitable for reservoir simulation. As such, it can play a vital role in the reservoir characterization work flow. Such a library will also be a means to mitigate the risks of the overuse of simple structures to try and interpret an entire environment

Depositional and Diagenetic Heterogeneity Control on Aquifer Quality: a Case Study of the Lower Triassic Sandstones in the Southeastern Part of Netherlands

A combined study of depositional facies and diagenesis variation was carried out to understand the main controls on aquifer quality of the Middle Buntsandstein in the southeastern part of the Netherlands. Heterogeneities in continental sandstone bodies occur at different spatial scales, ranging from micrometers to hundreds of meters. Commonly, such heterogeneities result from the interaction of depositional processes at various spatial and time scales. These processes partially also influence subsequent diagenetic evolution, hence present-day aquifer properties. Understanding the role of the resulting architectural heterogeneities in controlling the dynamic reservoir behavior is key in determining aquifer properties and improving pre-drilling prediction. The sandstones of the Main Buntsandstein subgroup in the southeastern part of the Netherlands provide an excellent example where different detrital compositions, internal sedimentary architectures, and diverse burial histories have resulted in a wide range of present-day aquifer properties. In the study area, the aquifers are composed of stacked heterogenous alluvial sandstones bodies intercalated with mud-prone intervals deposited in arid to semi-arid conditions. Differences in sediment sources, transport mechanisms, and intrabasinal conditions resulted in a wide distribution of composition and texture. Additionally, the effect of post-depositional burial diagenesis in a basin with complex tectonic history created diverse burial histories across the basin. The study aims to investigate the variation of present-day aquifer hydraulic parameters about changes in aquifer facies and architecture, detrital composition, as well as compaction and cementation during burial. Core sample analysis unfolded a diverse spectrum of sedimentary facies and lithic fragments, which differ between formations. Thin section analysis provides insights about mechanical compaction, cementations, and authigenic phases. By combining these results with petrophysical data on permeability and porosity of core samples, the major controls on present-day aquifer quality can be assessed