Stratigraphic evolution of an aeolian erg margin system: the Permian Cedar Mesa Sandstone, SE Utah, USA

The Permian Cedar Mesa Sandstone of south‐east Utah is a predominantly aeolian succession that exhibits a complex spatial variation in sedimentary architecture which, in terms of palaeogeographic setting, reflects a transition from a dry erg centre, through a water table‐controlled aeolian‐dominated erg margin, to an outer erg margin subject to periodic fluvial incursion. The erg margin succession represents a wet aeolian system, accumulation of which was controlled by progressive water table rise coupled with ongoing dune migration and associated changes in the supply and availability of sediment for aeolian transport. Variation in the level of the water table relative to the depositional surface determined the nature of interdune sedimentary processes, and a range of dry, damp and wet (flooded) interdune elements is recognized. Variations in the geometry of these units reflect the original morphology and the migratory behaviour of spatially isolated dry interdune hollows in the erg centre, locally interconnected damp and/or wet interdune ponds in the aeolian‐dominated erg margin and fully interconnected, fluvially flooded interdune corridors in the outer erg margin. Relationships between aeolian dune and interdune units indicate that dry, damp and wet interdune sedimentation occurred synchronously with aeolian bedform migration. Temporal variation in the rates of water‐table rise and bedform migration determined the angle of climb of the erg margin succession, such that accumulation rates increased during periods of rapidly rising water table, whereas sediment bypassing (zero angle of climb) occurred in the aftermath of flood events in response to periods of elevated but temporarily static water table. During these periods in the outer erg margin, the expansion of fluvially flooded interdunes in front of non‐climbing but migrating dunes resulted in the amalgamation of laterally adjacent interdunes and the generation of regionally extensive bypass (flood) supersurfaces. A spectrum of genetic depositional models is envisaged that accounts for the complex spatial and temporal evolution of the Cedar Mesa erg margin succession.     

Archean to Recent aeolian sand systems and their sedimentary record: Current understanding and future prospects

The sedimentary record of aeolian sand systems extends from the Archean to the Quaternary, yet current understanding of aeolian sedimentary processes and product remains limited. Most preserved aeolian successions represent inland sand‐sea or dunefield (erg) deposits, whereas coastal systems are primarily known from the Cenozoic. The complexity of aeolian sedimentary processes and facies variability are under‐represented and excessively simplified in current facies models, which are not sufficiently refined to reliably account for the complexity inherent in bedform morphology and migratory behaviour, and therefore cannot be used to consistently account for and predict the nature of the preserved sedimentary record in terms of formative processes. Archean and Neoproterozoic aeolian successions remain poorly constrained. Palaeozoic ergs developed and accumulated in relation to the palaeogeographical location of land masses and desert belts. During the Triassic, widespread desert conditions prevailed across much of Europe. During the Jurassic, extensive ergs developed in North America and gave rise to anomalously thick aeolian successions. Cretaceous aeolian successions are widespread in South America, Africa, Asia, and locally in Europe (Spain) and the USA. Several Eocene to Pliocene successions represent the direct precursors to the present‐day systems. Quaternary systems include major sand seas (ergs) in low‐lattitude and mid‐latitude arid regions, Pleistocene carbonate and Holocene–Modern siliciclastic coastal systems. The sedimentary record of most modern aeolian systems remains largely unknown. The majority of palaeoenvironmental reconstructions of aeolian systems envisage transverse dunes, whereas successions representing linear and star dunes remain under‐recognized. Research questions that remain to be answered include: (i) what factors control the preservation potential of different types of aeolian bedforms and what are the characteristics of the deposits of different bedform types that can be used for effective reconstruction of original bedform morphology; (ii) what specific set of controlling conditions allow for sustained bedform climb versus episodic sequence accumulation and preservation; (iii) can sophisticated four‐dimensional models be developed for complex patterns of spatial and temporal transition between different mechanisms of accumulation and preservation; and (iv) is it reasonable to assume that the deposits of preserved aeolian successions necessarily represent an unbiased record of the conditions that prevailed during episodes of Earth history when large‐scale aeolian systems were active, or has the evidence to support the existence of other major desert basins been lost for many periods throughout Earth history?     

Facies and architectural element analysis of a meandering fluvial succession: The Permian Warchha Sandstone, Salt Range, Pakistan

The 30 to 155 m thick Early Permian (Artinskian) Warchha Sandstone of the Salt Range, Pakistan is a conglomerate, sandstone and claystone succession within which seven lithofacies types (Gt, St, Sp, Sr, Sh, Fl and Fm) occur in a predictable order as repeated fining-upward cycles. Common sedimentary structures in the conglomerates and sandstones include planar and trough cross-bedding, planar lamination, soft sediment-deformed bedding, compound cosets of strata with low-angle inclined bounding surfaces and lags of imbricated pebbles. Structures in the finer-grained facies include desiccation cracks, raindrop imprints, caliche nodules and bioturbation. Groups of associated facies are arranged into nine distinct architectural elements (channels, gravel bars, sandy bedforms, downstream and laterally accreting barforms, sand sheets, crevasse splays, levees, floodplain units and shallow lakes), which is consistent with a fluvial origin for the succession. The types of architectural elements present and their relationship to each other demonstrate that the Warchha Sandstone preserves a record of a meandering river system that drained the northern margin of Gondwanaland. The dominance of fine-grained (floodplain) facies over gravel-grade (channel-base) facies and the widespread occurrence of large-scale lateral accretion elements supports the interpretation of a high-sinuosity, meandering fluvial system in which channel bodies accumulated via the lateral accretion of point bars but in which the active channels covered only a small part of a broad floodplain at any time instant. Although the regional and temporal distribution of these deposits is complex, in broad terms the lower part is dominated by stacked, multistorey channel bodies, whereas single-storey channel elements isolated in abundant fine-grained floodplain deposits dominate the middle and upper parts of the formation.     

Petrography and provenance of the Early Permian Fluvial Warchha Sandstone,Salt Range,Pakistan

The Warchha Sandstone of the Salt Range of Pakistan is a continental succession that accumulated as part of a meandering, fluvial system during Early Permian times. Several fining-upward depositional cycles are developed, each of which is composed of conglomerate, cross-bedded sandstone and, in their upper parts, bioturbated siltstone and claystone units with distinctive desiccation cracks and carbonate concretions. Clast lithologies are mainly of plutonic and low-grade metamorphic origin, with an additional minor sedimentary component. Textural properties of the sandstone are fine- to coarse-grained, poorly to moderately sorted, sub-angular to sub-rounded, and with generally loose packing. Based on modal analyses, the sandstone is dominantly a feldspathoquartzose (arkose to sub-arkose). Detrital constituents are mainly composed of monocrystalline quartz, feldspars (more K-feldspar than plagioclase) and various types of lithic clasts. XRD and SEM studies indicate that kaolinite is the dominant clay mineral and that it occurs as both allogenic and authigenic forms. However, illite, illite-smectite mixed layer, smectite and chlorite are also recognised in both pores and fractures. Much of the kaolinite was likely derived by the severe chemical weathering of previously deposited basement rocks under the influence of a hot and humid climate. Transported residual clays deposited as part of the matrix of the Warchha Sandstone show coherent links with the sandstone petrofacies, thereby indicating the same likely origin. Illite, smectite and chlorite mainly occur as detrital minerals and as alteration products of weathered acidic igneous and metamorphic rocks. Based primarily on fabric relationship, the sequence of cement formation in the Warchha Sandstone is clay (generally kaolinite), iron oxide, calcareous and siliceous material, before iron-rich illite and occasional mixed layer smectite–illite and rare chlorite. Both petrographic analysis and field characteristics of the sandstone indicate that the source areas were characterised by uplift of a moderate to high relief continental block that was weathered under the influence of hot and humid climatic conditions. The rocks weathered from the source areas included primary granites and gneisses, together with metamorphic basement rocks and minor amounts of sedimentary rocks. Regional palaeogeographic reconstructions indicate that much of the Warchha Sandstone detritus was derived from the Aravalli and Malani ranges and surrounding areas of the Indian Craton to the south and southeast, before being transported to and deposited within the Salt Range region under the influence of a semi-arid to arid climatic regime.     

Interpreting complex fluvial channel and barform architecture: Carboniferous Central Pennine Province,northern England

The Bashkirian Lower Brimham Grit of North Yorkshire, England, is a fluvio‐deltaic sandstone succession that crops out as a complex series of pinnacles, the three‐dimensional arrangement of which allows high‐resolution architectural analysis of genetically‐related lithofacies assemblages. Combined analysis of sedimentary graphic log profiles, architectural panels and palaeocurrent data have enabled three‐dimensional geometrical relationships to be established for a suite of architectural elements so as to develop a comprehensive depositional model. Small‐scale observations of facies have been related to larger‐scale architectural elements to facilitate interpretation of the palaeoenvironment of deposition to a level of detail that has rarely been attempted previously, thereby allowing interpretation of formative processes. Detailed architectural panels form the basis of a semi‐quantitative technique for recording the variety and complexity of the sedimentary lithofacies present, their association within recognizable architectural elements and, thus, the inferred spatio‐temporal relationship of neighbouring elements. Fluvial channel‐fill elements bounded by erosional surfaces are characterized internally by a hierarchy of sets and cosets with subtly varying compositions, textures and structures. Simple, cross‐bedded sets represent in‐channel migration of isolated mesoforms (dunes); cosets of both trough and planar‐tabular cross‐bedded facies represent lateral‐accreting and downstream‐accreting macroforms (bars) characterized by highly variable, yet predictable, patterns of palaeocurrent indicators. Relationships between sandstone‐dominated strata bounded by third‐order and fifth‐order surfaces, which represent in‐channel bar deposits and incised channel bases, respectively, chronicle the origin of the preserved succession in response to autocyclic barform development and abandonment, major episodes of incision probably influenced by episodic tectonic subsidence, differential tilting and fluvial incision associated with slip on the nearby North Craven Fault system. Overall, the succession represents the preserved product of an upper‐delta plain system that was traversed by a migratory fluvial braid‐belt system comprising a poorly‐confined network of fluvial channels developed between major sandy barforms that evolved via combined lateral‐accretion and downstream‐accretion.     

Controls on the morphology of braided rivers and braid bars: An empirical characterization of numerical models

Braided rivers exhibit highly variable morphologies, morphodynamic behaviours and resulting depositional records. To evaluate relationships between characteristics of braided-river channel belts and river depth, water discharge and streambed gradient, 39 numerical modelling experiments were conducted with the software Delft3D to simulate braided-river evolution under a broad range of boundary conditions. Data from model outputs were integrated with observations from 63 natural braided rivers differing with respect to river depth and streambed gradient. The modelled rivers each underwent similar evolutions, yet each culminated in markedly different final river morphologies, dependent on discharge and riverbed gradient. The rivers underwent evolutionary stages of: (i) formation of transverse unit bars with limited relief from an initially featureless bed; (ii) channel development around bars and in some cases dissecting transverse unit bars; (iii) formation of relatively simpler compound bars; and (iv) amalgamation of these simpler compound bars into more complex compound bars. Quantitative relationships relating to braided-river channel-belt morphology and organization are established, and the following results are noted: (i) bar elongation (length-to-width ratio) is correlated positively with riverbed gradient; (ii) bar height and area are correlated positively with discharge, and negatively with riverbed gradient; (iii) the river depth is the main predictor of mean braid-bar area; and (iv) the degree of braiding is primarily associated with river width-to-depth ratio and riverbed gradient. Results arising from this research improve our understanding of controls on the morphology and architectures of braided fluvial channel belts; they provide a novel empirical characterization that can be applied for predicting channel depth, bar morphology, streambed gradient, and degree of braiding of modern fluvial systems and of the formative rivers of ancient preserved successions.     

Modelling sedimentation in ocean trenches: the Nankai Trough from 1 Ma to the present

Abstract Sediment accumulation within ocean trenches located at actively accreting convergent margins is determined by an interplay between sediment supply, sediment subduction/accretion at the toe of the overriding accretionary complex and the rate of subduction. Modelling trench sedimentation provides insight into the principal controlling factors, and a means of deriving, from the pattern of sedimentation, how factors, such as the rates of sediment supply and subduction, have varied over the period of accumulation of the trench sediments. Two DSDP-ODP drill sites within the Nankai Trough reveal a coarsening-upward megasequence, indicating a progressive facies transition from abyssal muds to outer-trench silts to inner-trench sands. The changing geometry of the trench-wedge over the past 1 Myr has been determined by modelling variations in net sediment flux for two trench-perpendicular profiles. The models were constrained to fit the stratigraphy at the drill sites, and the simulated present-day geometries of the trench were matched with those shown on seismic reflection profiles by successive adjustment of the model. Results from both sites confirm a ‘slow’ subduction rate of <20 km Myr^-1. At the south-western site (582), the width of the trench-wedge has ranged from 13 to 21 km over the past 1 Myr. To the north-east, at Site 808, the width has ranged from 7 to 13 km over the past 0.5 Myr. These changes in trench-wedge width are primarily the result of large changes in sediment supply rate. The subduction of the Shikoku Ridge, a fossil spreading centre adjacent to Site 808, has had a major influence on the style of sedimentation within the trench. The style of accretion from the trench to the toe of the accretionary complex has important implications for geometrical adjustment of the trench-wedge. Thrust displacement lifts the protothrust region out of the trench, resulting in a decreased width. This is followed by a phase of increasing width as the trench-wedge adjusts towards a new equilibrium. The cyclical, episodic accretion process results in a periodic second-order variation in trench-fill size that is superimposed on primary trends determined by variations in sediment supply rates and subduction rates over time.     

Stratigraphic evolution and preservation of aeolian dune and damp/wet interdune strata: an example from the Triassic Helsby Sandstone Formation, Cheshire Basin, UK

Abstract New and previously published models of wet aeolian system evolution form a spectrum of types that may be explained in terms of aeolian dune dynamics, rate of water table rise and/or periodicity of interdune flooding. This is illustrated with an example from the Mid‐Triassic (Anisian) Helsby Sandstone Formation, Cheshire, UK. Lenses of damp and wet interdune strata exhibit an intertonguing, transitional relationship with the toe‐sets of overlying aeolian dune units. This signifies dune migration that was contemporaneous with water table‐controlled accumulation in adjacent interdunes. Downwind changes in the geometry and facies of the interdune units indicate periodic expansion and contraction of the interdunes in response to changes in the elevation of the groundwater table and episodic flooding, during which accumulation of dune strata continued relatively uninterrupted. This contrasts with other models for accumulation in wet aeolian systems where interdune flooding is associated with a cessation in aeolian bedform climbing and the formation of a bypass or erosional supersurface. Architectural panels document the detailed stratigraphy in orientations both parallel and perpendicular to aeolian transport direction, enabling a quantitative three‐dimensional reconstruction of genetically related aeolian dune and interdune elements. Sets of aeolian dune strata are composed of grainflow and translatent wind‐ripple strata and are divided by a hierarchy of bounding surfaces originating from oblique migration of superimposed dunes over slipfaceless, sinuous‐crested parent bedforms, together with lee‐slope reactivation under non‐equilibrium flow conditions. Silty‐mudstone and sandstone interdune units are characterized by wind ripple‐, wavy‐ and subaqueous wave ripple‐laminae, desiccation cracks, mud flakes, raindrop imprints, load casts, flutes, intraformational rip‐up clasts and vertebrate and invertebrate footprint impressions and trackways. These units result from accumulation on a substrate that varied from dry‐ through damp‐ to wet‐surface conditions. Interdune ponds were flooded by either fluvial incursions or rises in groundwater table and were periodically subject to gradual desiccation and reflooding. Red silty‐mudstone beds of subaqueous origin pass laterally into horizontally laminated wind‐ripple beds indicating a progressive transition from wet‐ through damp‐ to dry‐surface conditions within a single interdune.     

Internal stratigraphic relationships in the Etendeka group in the Huab Basin, NW Namibia: understanding the onset of flood volcanism

The Etendeka Igneous Province in NW Namibia forms the eastern most extent of the Paraná–Etendeka Flood Basalt Province and, despite only covering about 5% of the Paraná–Etendeka, has been the focus of much interest, due to its extremely well exposed nature. The Huab Basin in NW Namibia forms the focus of this study, and formed a connected basin with the Paraná throughout Karoo times (late Palaeozoic) into the Lower Cretaceous. It contains a condensed section of the Karoo deposits, which indicate early periods of extension, and Lower Cretaceous aeolian and volcanic Etendeka deposits, which have their correlatives in the Paraná. In the Huab Basin, the volcanic rocks of the Etendeka Group consists of the Awahab and Tafelberg Formations, which are separated by a disconformity. Detailed examination of the Awahab Formation reveals an additional disconformity, which separates olivine-phyric basalts (Tafelkop-type) from basalt/basaltic andesites (Tafelberg-type) marking out a shield volcanic feature which is concentrated in an area to the SE of the Huab River near to the Doros igneous centre. Early volcanism consisted of pahoehoe style flows of limited lateral extent, which spilled out onto aeolian sands of an active aeolian sand sea 133 million years ago. This sand sea is equivalent to the sands making up the Botucatu Formation in the Paraná basin. The early expression of flood volcanism was that of laterally discontinuous, limited volume, pahoehoe flows of Tafelkop-type geochemistry, which interleaved with the aeolian sands forming the Tafelkop–Interdune Member basalts. These basalts are on-lapped by more voluminous, laterally extensive, basalt/basaltic andesite flows indicating a step-up in the volume and rate of flood volcanism, leading to the preservation of the shield volcanic feature. These geochemically distinct basalts/basaltic andesites form the Tsuhasis Member, which are interbeded with the Goboboseb and Sprinkbok quartz latite flows higher in the section. The Tsuhasis Member basalts, which form the upper parts of the Awahab Formation, are of Tafelberg-type geochemistry, but are stratigraphically distinct from the Tafelberg lavas, which are found in the Tafelberg Formation above. Thus, the internal stratigraphy of the flood basalt province contains palaeo-volcanic features, such as shield volcanoes, and other disconformities and is not that of a simple layer-cake model. This complex internal architecture indicates that flood volcanism started sporadically, with low volume pahoehoe flows of limited lateral extent, before establishing the more common large volume flows typical of the main lava pile.     

Anatomy and facies distribution of terminal lobes in ephemeral fluvial successions: Jurassic Tordillo Formation,Neuquén Basin,Argentina

In terminal fluvial-fan systems, characteristic proximal to distal variations in sedimentary architectures are recognized to arise from progressive downstream loss of water discharge related to both infiltration and evaporation. This work aims to elucidate downstream trends in facies and architecture across the medial and distal zones of terminal-fan systems, which record transitions from deposits of channel elements to lobe-like and sheet-like elements. This is achieved via a detailed characterization of ancient ephemeral fluvial deposits of the well-exposed Kimmeridgian Tordillo Formation (Neuquén Basin, Argentina). The fine sand-prone and silt-prone succession associated with the medial to distal sectors of the system has been studied to understand relationships between depositional processes and resulting architectures. Facies and architectural-element analyses, and quantification of resulting sedimentological data at multiple scales, have been undertaken to characterize sedimentary facies, facies transitions, bed types, architectural elements and larger-scale architectural styles. Eight bed types with distinct internal facies transitions are defined and interpreted in terms of different types of flood events. Channelized and non-channelized architectural elements are defined based on their constituent bed types and their external geometry. The most common elements are terminal lobes, which are composite bodies within which largely unconfined sandy deposits are stacked in a compensational manner; a hierarchical arrangement of internal components is recognized. Proximal feeder-channel avulsion events likely controlled the evolution of terminal-lobe elements and their spatiotemporal shifts. Stratigraphic relations between architectural elements record system-wide trends, whereby a proximal sector dominated by channel elements passes downstream via a gradational transition to a medial sector dominated by sandy terminal-lobe elements, which in turn passes further downstream to a distal sector dominated by silty terminal lobe-margin and fringing deposits. This work enhances current understanding of the stratigraphic record of terminal fluvial systems at multiple scales, and provides insight that can be applied to predict the facies and architectural complexity of terminal fluvial successions.