Erg Margin Research Articles

Loess or lake sediments: Resolving the origin of massive beds in the Lower Jurassic of southern Africa

Throughout Earth's history, dust deposits have been considered sensitive markers of fluctuations in global atmospheric circulation patterns. However, the identification of ancient dust deposits remains problematic, as their massive (structureless) nature impedes an unequivocal classification. In southern Africa, the Lower Jurassic Clarens Formation is dominated by intriguing structureless sandy siltstones that have been interpreted as loess even though classical diagnostic outcrop‐scale loess features are not observed. In this study, we utilized grain characteristics such as size, shape, sorting and fabric to assess the origin of these ubiquitous massive beds, as compared to strata with a known origin in the unit. Based on these grain characteristics, the massive beds can be grouped into six different facies types (Facies 1–6). Facies 1 has very similar grain features to the large‐scale cross‐bedded strata and thus it is interpreted as migrating dune deposits, only apparently massive. The dominant grain size with a large proportion of very fine‐grained sand and the random grain fabric suggests that Facies 2 resulted from sandy dust fall (short‐term suspension), whereas Facies 3 is interpreted as aeolian dust, having features consistent with modern and ancient loess due to its dominant silt grain size and random grain fabric. Features in Facies 4 are undiagnostic and could reflect a range of aeolian processes, although a dominant silt grain size may suggest an overwhelming loess component. Facies 5 and 6 show primary aeolian signatures with the preferred grain orientation being evidence of reworking in an aqueous setting (e.g., lake, stream). These six facies types represent a spectrum of depositional processes that occurred in response to erg migration and climatic changes along this Early Jurassic erg margin. Their facies diversity is evidence for: (1) dry and windy conditions in SW Gondwana during the Sinemurian to Pliensbachian, and (2) reworking of the aeolian sediments in various fluvial and lacustrine settings during the development of one of the largest known sand seas in Earth's history.

Lake dynamics in an Early Jurassic desert: Evidence from the Clarens Formation in southern Africa

ABSTRACTInterdune deposits are sensitive to climatic fluctuations and ancient interdune lacustrine systems can reveal the drivers of erg dynamics through time and space. This southern African study details the dynamics of interdune deposition in a vast ancient desert system that was active over Pangaea in the Middle Mesozoic and formed part of one of the largest known sand seas in Earth's history. Focusing on the Sinemurian to Pliensbachian Clarens Formation in the main Karoo Basin (south‐west Gondwana), three facies associations were identified that formed in meso‐scale and macro‐scale lakes amongst aeolian dunes and sand sheets of the Clarens erg system. The northern facies association is restricted to interdune hollows within an actively climbing dune system. High‐sediment availability and high‐wind speeds limited the space available for these meso‐scale lakes to expand. Here, migrating dunes outpaced the water table rise promoting the formation of discontinuous lenses of lake deposits, as is typical in erg centres. The southern facies associations developed both in spatially isolated meso‐scale lakes, where flooding was controlled by groundwater inundation (water table rise), as well as in macro‐scale lakes, where flooding was related to groundwater inundation and floods (surface runoff) that induced terminal splays. Relative to the north, the southern facies associations, with their more significant thickness and lateral extent, show that, here, the basin was characterized by a higher rate of relative subsidence. Moreover, the southern lake systems were established along the erg's margin, where the interdunes expanded during pluvial phases. Ultimately, the facies associations in the Clarens Formation show that the widespread arid conditions in the Early Jurassic of south‐west Gondwana were subjected to prolonged and repeated pluvial phases in an otherwise harsh environment. Given the prominence of Jurassic aeolianites across Pangea, this study provides a southern African perspective on global palaeo‐erg dynamics and Early Jurassic climatic shifts.

Late Cretaceous plateau deserts in the South China Block, and Quaternary analogues; sedimentology, dune reconstruction and wind-water interactions

Plateau deserts are among those continental systems with the most significant examples of clastic deposits associated with the complex combination of the action of ice, wind and water. The Late Cretaceous rocks of China contain the record of desert systems migration and partial destruction in a palaeoplateau desert. The evolution of the South China Block during Late Cretaceous was characterized by widespread destructive plate margin activity, this favoured orogenic uplift and high-altitude mountain ranges of which a direct consequence was a widespread rain shadow effect, triggering desertification and erg expansion similar to that of Quaternary counterparts in the Tibetan Plateau. This work presents a detailed analysis of the sedimentology, stratigraphic architecture, palaeogeography and palaeocurrent reconstruction of aeolian and associated alluvial depositional systems. The reconstruction of aeolian bedforms based on field-data analysis indicates that, during Late Cretaceous, draas and crescentic dunes accumulated and migrated in extensive intermontane desert systems. Heavy rains led to partial destruction of aeolian dunes and the reworking of aeolian sands in interdraa corridors. The fault-controlled subsidence and an increasing input of windblown sediments favoured the long-lasting accumulation of the erg system. Possible monsoonal rains recharged the groundwater system causing the rise of the regional phreatic level that favoured the ulterior long-lasting preservation of the aeolian system in the Xinjiang Basin. The dimensions and processes inferred from this Cretaceous desert system are similar to those observed today in the linking zone of the Taklamakan-Gobi Deserts, where meltwater flows from alpine glaciers in surrounding mountain ranges recharge the alluvial and fluvial network of the erg margin. The occurrence of striated cobbles in the alluvial facies suggests glacial activity in the catchment areas of the Cretaceous plateau.

Enigmatic clastic pipe swarms and implications for fluidization dynamics in aeolian deposits

AbstractSoft‐sediment deformation of contorted and massive sandstone is common throughout much of the siliciclastic record, but clastic pipes represent a distinctive class of pressurized synsedimentary features. Remarkable centimetre to metre‐scale clastic pipe exposures in the Jurassic Navajo Sandstone of Utah (USA) establish a range of pipe sizes, expressions and relationships to the host rock in an erg margin setting, traditionally thought to be just a dry desert system. In particular, the field and laboratory characterizations of cylindrical pipes show internal concentric, annular rings that imply water fluidization, with alignment of long grain axes due to shear flow along pipe margins. Central interior parts of decimetre‐scale pipes appear massive in plan view, but display weakly developed pseudobedding from post‐pressure release, gravitational settling in the cross‐sectional view. Deformation features of conjugate fractures, ring faults, hypotrochoid patterns (geometric arcs and circles) and breccia in the host material reflect both brittle and ductile behaviour in response to the fluidization and injection of the clastic pipes. The stratigraphic context of individual pipes and the stratabound intervals of pipe features imply dynamic deformation nearly coincident with deposition in this Early Jurassic aeolian system related to multiple factors of groundwater expulsion, timing and local host sediment properties that influenced pipe development. Although the pipe features might be easily overlooked as a smaller scale feature of soft‐sediment deformation in dune deposits, these are valuable environmental indications of disrupted fluid pathways within porous, reservoir quality sands, associated with possible combinations of periodic springs, high water‐table conditions and strong ground‐motion events. These pipe examples may be important analogues where exposures are not so clear, with applications to diverse modern and ancient clastic settings internationally on Earth as well as in planetary explorations such as on Mars.

The Sedimentology of an ERG Margin: The Kayenta–Navajo Transition (Lower Jurassic), Kanab, Utah, U.S.A.

The Sedimentology of an ERG Margin: The Kayenta–Navajo Transition (Lower Jurassic), Kanab, Utah, U.S.A.

Sedimentology and evolution of a foreland desert basin, Middle Eocene Gercus Formation (North and Northeastern Iraq)

The Middle Eocene Gercus Formation was studied at four surface sections (Badi/Zawita, Dohuk Dam, Brifca, and Shaqlawa), in north and northeastern Iraq. It is dominated by distinct red beds of continental clastic sediments consisting of mudstones, sandstones, conglomerates, and minor carbonates and evaporites. The detailed sedimentological study reveals that these rocks accumulated in three main distinct facies associations: aeolian, fluvial and lacustrine. The first consists of deflation lags/desert pavements, aeolian sandsheet, aeolian dune, aeolian bimodal, and interdune deposits. The second comprises pebble-sand and sand bedload stream, mud playa/ephemeral floodplain, gravel bedload stream, debris flow, sheet flood, hyperconcentrated flow, and intra-erg mass-flow deposits. The third consists predominantly of freshwater carbonates and playa gypsum. The overall characters of the facies associations indicate that the Gercus Formation was deposited in diverse suites of sedimentary environments. These are: arid to semiarid (dry) alluvial fans, ephemeral streams “wadis,” aeolian dunes, interdunes, and lakes. The close association of these related environments throughout the studied sections indicates that the Gercus Formation represents the sediments of a former desert environment under arid to semiarid conditions. The distribution of these environments generally displays a coarsening upward sequence in which erg margin deposits occurs at the base of the succession and an alluvial fan system at the top. The erg system seems to reflect an expansion of central erg over the underlying erg margin. This may be attributed to an increase of aeolian supply to the desert over a long haul prior to basin-wide progradation of the overlying alluvial fan deposits. The development and the preservation of Gercus desert systems are associated with syndepositional tectonism in a rapidly subsiding foreland basin.

Spatial variability of multi-controlled aeolian supersurfaces in central-erg and marine-erg-margin systems

During the Albian Iberia was under the influence of the Northern-Hemisphere Hot Arid Belt favouring the development of an extensive sandy desert system with a marine-erg margin where prograding aeolian dunes interacted with Tethyan waters. The interplay of different controls, such as synsedimentary tectonics, compaction of the underlying coal-bearing unit, eustatic sea-level variations, climate modulation, and the autodynamics of the different sedimentary subenvironments determined the character of bounding surfaces, which separate four erg sequences. These bounding surfaces, or supersurfaces, may display a different sedimentary expression in adjacent areas. Bounding surface 1 is a sand-drift surface (SDS) in the central-erg and a transgressive surface (TS) in the marine erg margin. Bounding surface 2 is associated with a basin re-configuration associated to active extension tectonics, followed by deflation. Bounding surface 3 marks the end of erg expansion, the start of its partial destruction and redeposition and reworking in restricted marine environments. Bounding surface 4 marks the return to more arid conditions and draa progradation into Tethyan waters. These bounding surfaces separate four erg sequences. On the basis of the relative role of allocyclic processes, two megasequences are defined. The first comprises erg sequences 1–3, and the second megasequence comprises erg sequence 4. Erg megasequence 1 developed while synsedimentary tectonic activity and substrate (peat) compaction were active. Erg megasequence 2 was mainly modulated by climate (change). A nomenclature for supersurfaces is proposed based on the types of external control.

Controls on marine–erg margin cycle variability: aeolian–marine interaction in the mid‐Cretaceous Iberian Desert System, Spain

AbstractThe interaction between aeolian dunes of the Iberian erg and Tethys waters during the mid‐Cretaceous led to a variety of sedimentary facies associations such as subtidal deposits, aeolian dunes, playa lakes, coastal lakes with tidal creeks, and marshes and lagoonal embayments with tide‐influenced delta deposits. Facies associations are organized in several stacked cycles. Every cycle is defined by a sand‐drift surface separating playa lake deposits below from aeolian dune deposits above, followed by a transgressive surface separating aeolian deposits below from shallow marine deposits above. The latter are covered by playa lake deposits and finally topped by the next sand‐drift surface. Similar marine–erg margin cycles have been explained previously in terms of high‐order relative sea‐level variations (normally controlled by glacioeustasy). The studied erg margin developed during the mid‐Cretaceous which is considered to be the archetypal example of a polar ice‐free greenhouse period. This manuscript presents an alternative explanation for the cycles as caused by climate‐induced variations in the aeolian sediment supply. In this scenario, orbitally induced latitudinal shifts of the boundary between climate belts led to alternating periods of increased and decreased precipitation in the highland catchments of the back‐erg system (Variscan Iberian Massif). When climate belts shifted to lower latitudes, back‐erg highlands were under the influence of the Northern Warm Humid Belt. While arid conditions prevailed in the lowland central‐erg, increased precipitation in the highlands and the consequent recharge of ground water led to a rise of the phreatic level in the erg system; that diminished wind erosion and windblown sediment input to the fore‐erg, so that the desert margin contracted. With ongoing basin subsidence this favoured the transgression. The increased desert‐ground water flux favoured permanent coastal lakes in the fore‐erg margin and the local development of vegetation. In places where such lakes (lagoons) were connected to the sea, tidal channels were active, as seen nowadays along the desert coast of Qatar (Persian Gulf). When climate belts moved to higher latitudes, the catchment areas in the back‐erg Iberian Massif were under the influence of the Northern Hot Arid belt and a decrease of precipitation led to a drop of the phreatic level, allowing deflation of dunes and exposed wadi channel floors, and the formation of desert pavements and deflation lags in the back‐erg area; this favoured the import of large volumes of windblown sand and dust, forcing progradation of the erg and consequently a retreat of the coastline. The sand fraction accumulated in climbing aeolian dunes and dust was trapped in extensive playa lake systems. Another long‐term allocyclic control is formed by active extensional tectonics that enhanced the creation of accommodation space, especially in the first cycle. A high ground water table sustained by subsurface water supply from the Variscan Iberian Massif generated coastal fresh/brackish lakes in which tide‐influenced meandering channels and salt marshes developed. Erg margin cycles thin upward in response to a decrease of the rate of basin subsidence, suggesting that transgressions led to deep erosion of the underlying aeolian dune sands.

Advances in Arabian stratigraphy: Allostratigraphic layering related to paleo-water table fluctuations in eolian sandstones of the Permian Unayzah A reservoir, South Haradh, Saudi Arabia

ABSTRACTIn the early stages of development of the Permian Unayzah A gas reservoir at South Haradh, difficulties in intra-reservoir wireline log correlation, poor seismic character, and the recognition of multiple gas-water contacts, necessitated a detailed (core- and image log-based) geological study of the reservoir. This study revealed that the Unayzah A can be divided into two stratal units. The lower unit has poor reservoir quality. It consists of a thin, discontinuous basal eolian dune sandstone that is abruptly overlain by up to 70 ft (21 m) of very fine-grained and silty, irregularly laminated sandstones. These sediments were deposited in a very shallow ephemeral (playa) lake setting. They terminate upwards in a thin but widespread upward-fining interval that represents the Maximum Extent of the Lake (MEL horizon). In the upper unit several facies of varying reservoir quality are recognized, representing deposits that were laid down in a “mixed” eolian depositional system. Although these facies are generally common to all of the studied wells, they vary significantly in their proportions and associations from well to well. Thus a number of depositional settings are identified, which pass southwards across the study area from erg-center, eolian dune cross-bedded sandstones in the north (“dry” eolian system) through erg margin, interbedded dune and interdune deposits to very finegrained sandstones representing ephemeral lake sediments in the south (“wet” eolian system). Depositional cycles in the upper Unayzah A interval are recognized among the facies in each well, and in every case these can be related to fluctuations in the paleo-water table. When the wells are displayed in a stratigraphic section using the MEL horizon as a datum, several of these cycles are seen to be correlatable across the study area. Significantly, these sub-regional correlations are maintained both within and between the various facies tracts. They are allostratigraphic in nature with facies distribution controlled by a number of extrinsic factors including position of the paleo-water table relative to the depositional surface, rates of fluctuation of the water table, sediment availability for eolian transportation, transporting capability of the wind and rates of subsidence. Proximity to local ephemeral stream sedimentation was probably also a factor. It may be that in the long term the pulsed nature of the rising water table in South Haradh reflects the onset of a distant, pulsed rise in sea level, possibly related to the creation of the Neo-Tethys Ocean in the southern and eastern part of the Arabian Plate. Recognition of this allostratigraphic reservoir layering clearly identifies significant stratigraphic compartmentalization within the reservoir. This permits a clearer understanding of the distribution of reservoir bodies within the Unayzah A, and explains the variable gas-water contacts in the area. This conceptual stratigraphic model has been incorporated in object-based geocellular modeling for reservoir simulation at South Haradh and also at other Unayzah A gas fields in the area. Integration with geophysical (impedance) data leads to significantly improved success rates in relation to the strategic location of wells, yielding enhanced results in reservoir development.

High-resolution stratigraphic architecture and lithological heterogeneity within marginal aeolian reservoir analogues

Marginal aeolian successions contain different lithological units with variable geometries, dimensions and spatial distributions. Such variations may result in considerable heterogeneity within hydrocarbon reservoirs developed in successions of this type, which poses a high risk to their efficient development. Here, such heterogeneity is described and characterized at inter-well (<1 km) scales using two well-exposed outcrop analogues of ‘end member’ marginal aeolian deposits from the Permian Cedar Mesa Sandstone and Jurassic Page Sandstone of south-central Utah, USA. The sedimentology and stratigraphic architecture of the Cedar Mesa Sandstone was studied in a 1·2 km2 area in the Indian Creek region of southern Utah, where the interval consists of interbedded fluvial and aeolian deposits representative of a fluvial-dominated erg margin. The Page Sandstone was studied in a 4·3 km2 area near Escalante, close to the Utah-Arizona border, where it consists of interbedded sabkha and aeolian deposits representative of a transitional-marine erg margin. The three-dimensional stratigraphic architectures of both reservoir analogues have been characterized, in order to establish the dimensions, geometries and connectivity of high-permeability aeolian sandstones. Facies architecture of the aeolian-sabkha deposits is characterized by laterally continuous aeolian sandstone layers of relatively uniform thickness that alternate with layers of heterolithic sabkha deposits. Aeolian sandstones are thus likely to form vertically unconnected but laterally widespread flow units in analogous reservoirs. Facies architecture in the aeolian-fluvial deposits is more complex, because it contains alternating intervals of aeolian sandstone and fluvial heterolithic strata, both of which may be laterally discontinuous at the studied length-scales. Aeolian sandstones encased by fluvial heterolithic strata may form small, isolated flow units in analogous reservoirs, although the limited continuity of fluvial heterolithic strata results in vertical connectivity between successive aeolian sandstones in other locations. These architectural templates may be used to condition zonation schemes in models of marginal aeolian reservoirs.

The Moenave Formation: Sedimentologic and stratigraphic context of the Triassic–Jurassic boundary in the Four Corners area, southwestern U.S.A.

The Moenave Formation was deposited during latest Triassic to earliest Jurassic time in a mosaic of fluvial, lacustrine, and eolian subenvironments. Ephemeral streams that flowed north-northwest (relative to modern geographic position) deposited single- and multi-storeyed trough cross-bedded sands on an open floodplain. Sheet flow deposited mainly silt across broad interchannel flats. Perennial lakes, in which mud, silt and carbonate were deposited, formed on the terminal floodplain; these deposits experienced episodic desiccation. Winds that blew dominantly east to south-southeast formed migrating dunes and sand sheets that were covered by low-amplitude ripples. The facies distribution varies greatly across the outcrop belt. The lacustrine facies of the terminal floodplain are limited to the northern part of the study area. In a southward direction along the outcrop belt (along the Echo Cliffs and Ward Terrace in Arizona), dominantly fluvial–lacustrine and subordinate eolian facies grade mainly to eolian dune and interdune facies. This transition records the encroachment of the Wingate erg. Moenave outcrops expose a north–south lithofacies gradient from distal, (erg margin) to proximal (erg interior). The presence of ephemeral stream and lake deposits, abundant burrowing and vegetative activity, and the general lack of strongly developed aridisols or evaporites suggest a climate that was seasonally arid both before and during deposition of the Moenave and the laterally equivalent Wingate Sandstone. We interpret growth of the erg as a consequence of marine regression during the latest Triassic through earliest Jurassic that exposed sediments on the coast of the back-arc sea to eolian reworking. Tectonic processes that created accommodation space enhanced preservation of the erg.

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

AbstractThe 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.

Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)

The Skeleton Coast erg forms a prominent NNW trending dune belt, 6– 22 km wide, comprising dunes up to 50 m high, sub-parallel to the South Atlantic margin of Namibia. To a variable degree along its strike, the dune belt dams west-southwestward flowing ephemeral river systems on their route towards the Atlantic Ocean. The southern rivers, Koigab, Uniab and Hunkab, affecting the erg, are characterized by short, infrequent ephemeral flows from restricted catchments, whereas northern rivers, Hoanib and Hoarusib, are fed by large catchment areas favouring higher discharge and more frequent flows as the intertropical convergence zone and greater likelihood of monsoonal rainfall influence is approached. The potential for river damming increases in the same downwind direction through the erg because of increasing dune belt width and height and the change from low barchanoid dune forms in the south to large composite transverse dunes in the north. Whereas cross-sections of the southern part of the erg show considerable asymmetry with low barchans and sinuous crested dune forms eastwards but a high dune wall at the western edge, cross-sections over the northern erg are more uniformly symmetrical. As a consequence, the northern rivers are effectively dammed at the eastern erg margin, causing flood reservoir basins there. In contrast, southern rivers are dammed within the erg as the most pronounced and commonly encountered barrier is provided by the dunewall at its western border.

Marginal Erg Facies: A Trial Approach toward a Descriptive Classification

During the late 1970s and early 1980s, sedimentologists began recognizing the margins of eolian sand seas as separate, components which differed from interior sand seas in geometry, extent, and facies. Stratigraphers have now observed these differences in eolian rocks. Erg margins may be grouped in five ways: (1) by associations with extradunal environments-coastal plain, lacustrine, periglacial, marine (tidal flat, coastal sabkha, beach, and lagoon), and arid alluvial (alluvial fan, fluvial, playa, inland sabkha); (2) by allocyclic controls-eustasy, plate tectonism, and climate; (3) by autocyclic controls-local tectonism, topography, vegetation, hydrology, structure, sediment source and supply, and wind regime; (4) by geographic position-upwind, downwind, and along-wind margins; and (5) by sedimentary facies-texture and architecture. In contrast with erg interiors, erg margins are characterized by smaller, less complex dune-forms related to thinner sand accumulation; elementary dune architecture; more vegetation and bioturbation; high occurrence of sand sheet, zibar, and serir facies; expansive, low-relief interdunes with widely distributed dunes; and a greater proportion of interbedded extradunal deposits. Some of the published studies on ancient eolian systems have identified erg margin facies that have been influences by marine and arid alluvial processes. Few reports have described lacustrine-eolian and periglacial-eolian interactions. This study is an attempt tomore » organize known features of modern and ancient erg margins into a scheme based on erg margin controls.« less

Erg-margin deposits in the Lower Jurassic Moenave Formation and Wingate Sandstone, southern Utah

The Moenave Formation (Dinosaur Canyon Member) and the closely associated Wingate Sandstone in southern Utah represent the southwestern erg margin of the Lower Jurassic Wingate erg. The erg-margin deposits, which extend across a 50- to 100-km-wide region, are composed of dune, eolian sand-sheet, mud-flanked eolian, sabkha, ephemeral-stream and lacustrine facies. The succession is divided into a basal erg-margin association (8-27 m) dominated by eolian facies and an overlying erg-margin association (as much as 90 m thick) composed mainly of fluvial and lacustrine facies. The sediments of the basal association are composed of three drying-upward or locally drying-wetting-upward sequences that can be traced from the erg margin 100 km slightly upwind (that is, toward the west-northwest) into the outer erg-margin zone. Sequences of the inner erg-margin zone contain dune facies in their upper parts, sequences in the intermediate erg-margin zone contain sand-sheet facies, and those in the outer erg-margin zone contain sabkha facies. The basal association thins markedly toward the west-northwest. Sediments of the overlying association contain up to six drying-upward or drying-wetting-upward sequences but are apparently only fully preserved in the outer erg-margin zone. Individual sequences show much lateral variation but characteristically contain lower, muddy, ephemeral-stream deposits and upper, sandy, ephemeral-stream deposits; sand-sheet deposits; or incipientdune deposits. The general time trend is interpreted as a gradual change toward a more humid climate. The stacked drying-upward or drying-wetting-upward sequences suggest that this long-term climatic change was characterized by a number of fluctuations between relatively arid and humid conditions. These climatic fluctuations apparently controlled both sediment yield to the basin and the depositional conditions in the erg-margin region. During arid intervals, ephemeral braided streams transported sandy bed-load into the erg-margin area, where eolian reworking took place. During humid intervals, sheet-floods and high-sinuosity rivers transported a more mud-rich sediment load into the basin, where eolian activity was minimal.

Erg deposits in the Lower Jurassic Wingate Sandstone, northeastern Arizona: oblique dune sedimentation

ABSTRACTThe Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3‐D outcrops allow a detailed study of structures and sequenses in the erg body.The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well‐developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low‐angle dipping toesets, and upper steeply dipping foresets.The foresets reveal significant across‐crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross‐bedding that documents strong along‐crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along‐crest transport of sand.The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along‐crest transport of sand.The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.

Erg margin of the Permian White Rim Sandstone, SE Utah

ABSTRACTThe Permian White Rim Sandstone of the Canyonlands National Park, Utah, contains a wide variety of sedimentary structures and features that largely result from stages in erg migration and marine influence on an erg margin. Three spatially distinct lithological and depositional facies are recognized and can be distinguished as informal units within the formation. The aeolian dune facies is composed predominantly of fine‐grained cross‐stratified sandstone of the White Rim erg. This facies is the most widespread and comprises the bulk of the formation. Within the aeolian dune facies are small subfacies that represent interdune deposits. A sheet sand facies, composed of parallel‐bedded sandstone, makes up a significant part of the lowest part of the White Rim Formation. This facies appears to have been the precursor or leading (progradational) edge to the main erg system. The final facies is a reworked or veneer facies of rippled to disturbed sandstone that is localized in its extent. It is restricted to the upper few metres of the formation and is transitional in some places to the Triassic Moenkopi Formation. This veneer facies contains many structures which indicate marine reworking as well as periods of desiccation or subaerial exposure.Some previous interpretations of the White Rim Sandstone have tended to classify the whole formation as one depositional setting. It is clear that at the margin of a sand sea, as shown in the White Rim Sandstone, there are transitional facies due to the interactions with other environments. Additionally, variation in the stratigraphic relationships of facies can be related to stages of erg migration. Erg margin deposits preceded central erg development. Erg initiation occurred during a probable relative sea level low. Sea level influence is recorded at the top of the formation because erg termination accompanied a relative sea level high with cut‐off of sand supply. Transgression of the Permian Kaibab Sea over the White Rim erg was probably the main process in preservation of original dune topographic relief. Sea level fluctuations also may have affected distribution of facies and the complexities of structures at the erg margin. Subsequent fluvial reworking of the veneer facies may have obliterated Late Permian features during lowest Triassic Moenkopi deposition.

Basin tectonics and erg response

The alignment and distribution of facies in the major erg (eolian sand sea) deposits in the Permian and Jurassic systems on the Colorado Plateau show remarkable coincidence with modern tectonic elements. This indicates that subtle tectonic elements, precursors to the present ones, were coeval with ancient eolian deposition and controlled erg geometry and facies distribution. No other known controls or processes were capable of producing the observed patterns. Facies and architectural analysis of the Permian and Jurassic erg deposits define several major trends across the Colorado Plateau. The High Plateaus trend, parallel to and slightly east of the Wasatch line, is defined by a zone of thinning and facies change of several erg deposits. The Zuni trend lies along the Zuni lineament and is coincident with facies changes or erg margins of several units. The Colorado trend lies along the course of the modern Colorado River and a number of erg margins and several facies changes coincide with it. Most erg centers, as defined by facies and thickness, lay within paleobasins that include the Circle Cliffs trough, Holbrook, Black Mesa, and Blanding basins, and southern Cordilleran Miogeocline. Most erg deposits thin or pinch out across paleoarches, which include the Sedona, Monument, Emery and Defiance arches. Although the exact relation between the paleostructures and facies distribution is uncertain, it appears likely that some of the structural elements defined areas of greater and less sand accumulation. Areas of greater sand accumulation would be expected to be dominated by facies typical of erg centers and areas of lesser sand accumulation would be expected to be dominated by facies typical of the erg margin and eolian sand sheets. The structures may have also formed straight margins in aqueous environments such as rivers and marine coasts; straight, vertically stacked facies changes are commonly associated with erg margins in the stratigraphic record. Early post-depositional uplift of some arches may have defined straight, thin edges of some eolian sequences. The knowledge that subtle intrabasinal tectonic elements had an effect on eolian deposition can be used to reconstruct basin history; therefore, the mapping of eolian geometry and facies patterns can provide powerful information concerning the tectonic evolution of a sedimentary basin.

Sedimentology of an ancient erg margin: the Lower Jurassic Aztec Sandstone, southern Nevada and southern California

ABSTRACTThe Lower Jurassic Aztec Sandstone is an aeolian‐deposited quartzose sandstone that represents the western margin of the southerly‐migrating Navajo‐Nugget sand sea (or erg). Vertical and lateral facies relations suggest that the erg margin encroached upon volcanic highlands, alluvial fan, wadi and sabkha environments.In southern Nevada, 700 m thick facies successions record the arrival of the Aztec sand sea. Initial erg sedimentation in the Valley of Fire consists of lenticular or tongue‐shaped aeolian sand bodies interstratified with fluvially‐deposited coarse sandstone and mudstone. Above, evaporite‐rich fine sandstone and mudstone are overlain by thick, cross‐stratified aeolian sandstone that shows an upsection increase in set thickness. The lithofacies succession represents aeolian sand sheets and small dunes that migrated over a siliciclastic sabkha traversed by ephemeral wadis. These deposits were ultimately buried by large dunes and draas of the erg. In the Spring Mountains, a similar facies succession also contains thin, lenticular volcaniclastic conglomerate and sandstone. These sediments represent the distal margin of an alluvial fan complex sourced from the west.Thin aeolian sequences are interbedded with volcanic flow rocks, ash‐flow tuffs, debris flows, and fluvial deposits in the Mojave Desert of southern California. These aeolian strata represent erg migration up the eastern flanks of a magmatic arc. The westward diminution of aeolian‐deposited units may reflect incomplete erg migration, thin accumulation of aeolian sediment succeptible to erosion, and stratigraphic dilution by arc‐derived sediment.A two‐part division of the Aztec erg is suggested by lithofacies associations, the size and geometry of aeolian cross‐strata, and sediment dispersal data. The leading or downwind margin of the erg, here termed the fore‐erg, is represented by a 10–100 m thick succession of isolated pods, lenses, and tongues of aeolian‐deposited sediment encased in fluvial and sabkha deposits. Continued sand‐sea migration brought large dunes and draas of the erg interior into the study area; these 150–500 m thick central‐erg sediments buried the fore‐erg deposits. The trailing, upwind margin of the erg is represented by back‐erg deposits in northern Utah and Wyoming.