Wind-ripple Strata Research Articles

Eocene dry eolian system in the Jianchuan Basin, southeastern Tibetan Plateau: Implications for regional wind regime and paleoclimate

East Asia experienced a climatic transition from a zonal pattern (climate zones defined by latitude) to a monsoon-dominated pattern during the Cenozoic. However, hitherto little consideration has been paid to the atmospheric circulation of the planetary wind system that prevailed during the early uplift of the Tibetan Plateau, due to the lack of reliable proxies. Here, we conduct a study on the eolian successions from the Eocene Baoxiangsi Formation in the Jianchuan Basin, southeastern Tibetan Plateau to reconstruct regional atmospheric circulation and paleoclimate. Two facies associations are recognized in the eolian deposits of the Baoxiangsi Formation: a dominant eolian dune association and a subordinate interdune association. Eolian dune deposits are characterized by large-scale cross-stratified sets of strata composed internally of packages of grainflow, wind-ripple strata and pinstripe lamination. These packages of laminae and cross-stratified eolian dune sets are bounded internally by superimposition and reactivation surfaces and interdune surfaces, respectively. Interdune deposits are uncommon and composed dominantly of subcritical climbing translatent strata to form sets of strata and these are themselves bounded by interdune surfaces to form cosets of strata. The association of dominant eolian dune deposits, widely distributed interdune surfaces of great lateral extent (> 450 m) and that climb at a shallow but positive angle (< 1°) is consistent with the climbing dry eolian system. The commonly occurring superimposition surfaces indicate the presence of compound dunes (megadunes, draas). Based on the analysis of 715 dip–azimuth readings of foresets of eolian cross-beds, we reconstruct the prevailing wind directions: the paleowind in the northern basin flowed toward the northeast; in the southern basin it flowed toward the southwest. This wind regime indicates that the Jianchuan Basin was located at the zonal boundary between the westerlies and the trade winds, and the construction of the dry eolian system was controlled by the subtropical high-pressure belt during the middle–late Eocene. Following the accumulation of the dry eolian system, a monsoon climate regime gradually established itself and prevailed in the Yunnan Province; this expanded northward over time. This was closely related to the uplift of the central Tibetan Plateau and the retreat of the Paratethys ocean in the late Eocene.

Interpreting pre-vegetation landscape dynamics: The Cambrian Lower Mount Simon Sandstone, Illinois, U.S.A.

ABSTRACTThe Cambrian Mount Simon Sandstone has been the subject of extensive study and multiple industrial-scale carbon storage demonstrations at Decatur, Illinois, USA. The development of a reliable paleoenvironmental model is critical to successful large-scale carbon dioxide (CO2) storage, but is complicated by the need to interpret pre-vegetation sedimentation processes. The present study presents a paleoenvironmental model of the Lower Mount Simon Sandstone, based on analysis of primary sedimentary structures in two cores and four complete high-resolution resistivity logs (FMI).The Lower Mount Simon Sandstone represents a vertical “drying-up” sequence composed of three associated depositional units: a north–south oriented coastal system at the base, an eastward-directed fluvial unit in the middle, and a westward-directed eolian system at the top that recycled medium- and fine-grained sand in the basin. Quantitative analysis of fluvial cross-strata indicates that the perennial river system was shallow (c. 1 m deep) with relatively narrow channel belts (c. 1 km). Adjacent sandy eolian-floodplain deposits contain abundant thin, crinkly planar laminae that are enriched in fines and are interpreted as cementation surfaces, likely of biological origin. Deflation lags and wind-ripple strata are commonly interbedded with the crinkly strata, suggesting that the recurrence of erosion and deposition that controlled sedimentary preservation on the floodplain were dominated by eolian transport, re-wetting, and (bio-) cementation. Such a prominent role of exposure to the wind, basin-scale sediment recycling, and eolian removal of fine-grained sediment would have ceased to exist for most climates after the development of vegetation on land, yet, may well be key to understanding the environmental context for early life on Earth.

Late Jurassic-Early Cretaceous Erg Deposits in the Mengyin Basin, Western Shandong Province, China: Inferences about the Wind Regime and Paleogeography

The Late Jurassic-Early Cretaceous Santai Formation, sporadically distributed in western Shandong Province, comprises terrestrial alluvial-eolian successions, which records regional wind patterns and paleogeography in eastern North China. This paper conducts an analysis of eolian stratification, bounding surfaces, facies architecture and paleowind direction of the Santai Formation in the east of the Mengyin Basin, western Shandong Province. Three basic types of eolian stratification are recognized in the Santai Formation, including grainflow strata, wind ripple strata and adhesion strata, and have been grouped into eolian dune and interdune facies associations. The occurrences of reactivation surfaces and superimposition surfaces within eolian dune deposits indicate active compound dunes or draas. The association of adhesion strata with grainflow or windripple strata is the development of a wet eolian system. Cross-strata dip direction indicates different paleowind directions from the lower to the upper part of the Santai paleoerg. The lower part of the paleoerg was characterized by paleowinds blowing from northwest to northeast, whereas the upper part was under the influence of paleowinds consistently towards east to northeast. The identified changes in wind directions possibly suggest wind regime shifts from monsoon circulation to westlies of planetary wind system, which may be related with the collapse of the East China Plateau during the Late Jurassic to Early Cretaceous.

Late Jurassic fluvial–eolian deposits from the Tianchihe Formation, Ningwu–Jingle Basin, Shanxi Province, China

Eolian systems were widespread in China in the Cretaceous. However, the reports of pre-Cretaceous eolian and its related systems are scarce, which prohibits studying the construction, accumulation and preservation of eolian systems as well as its allogenic and autogenic controlling factors. In this paper, we report on fluvial–eolian successions from the Late Jurassic Tianchihe Formation in the Ningwu–Jingle Basin. These successions represent the oldest reported inland fluvial–eolian systems in China. Fourteen lithofacies types are recognized in the Tianchihe Formation and have been grouped into six facies associations, including meandering fluvial channel, levee and overbank in the lower part, and eolian sandsheet, eolian dune, ephemeral fluvial channel and floodplain in the upper part. The fluvial system comprises a series of wetting upward cycles, which indicate humid climate culminating in high precipitation and shallow water table level. Eolian dune deposits composed of grainflow, grainfall, wind ripple strata and pin stripe laminations currently implies that all deposits types are internally bounded by reactivation surface and superimposition surface, suggesting compound dunes or draas. Eolian sandsheet deposits consist dominantly of subcritical climbing translatent strata and pin stripe lamination. The association of eolian dune and eolian sandsheet is development of a dry eolian system. Twelve drying upward cycles mainly composed of eolian sandsheet–eolian dune successions and four wetting upward cycles consisting of eolian sandsheet/dune–ephemeral fluvial channel/floodplain successions occur in the eolian system. These repeated cycles likely reflect an evolving erg system directly controlled by sediment supply, sediment availability and relative water table. The vertical transition from fluvial to eolian systems indicate a local climatic oscillation from relatively humid to arid conditions, which may be related with the uplift of the East China Plateau in the Late Jurassic.

3.2 Ga eolian deposits from the Moodies Group, Barberton Greenstone Belt, South Africa: Implications for the origin of first-cycle quartz sandstones

The 3.2Ga Moodies Group in Barberton Greenstone Belt, South Africa, contains the oldest recognized evidence for eolian processes on Earth. These eolian facies indicate that significant wind regimes existed in the Archean atmosphere. Eolian deposits are developed in the lower Moodies Group in the central part of the belt and consist of stacked inversely graded strata ranging in thickness from 2 to 10mm.Individual strata consist of fine- to medium-grained quartz sandstone at the base that passes vertically into very coarse- to coarse-grained quartz sandstone. Strata make up cross-bed sets up to 3–5m thick. Sets of wind ripple strata are either tabular tangential or in the form of broad troughs. This type of inversely graded stratification is the product of wind-ripple migration. The coarse grain size of the sediment combined with the low-angle of inclination of the strata is best interpreted as the product of sand sheet or dune plinth preservation. Based on the geometry of eolian set boundaries, dunes were probably simple barchan form.The tens-of-meters thick eolian deposits are underlain by braided stream deposits composed of subfeldspathic, pebbly sandstones. The vertical change from braided-stream subfeldspathic sandstones to eolian quartz arenites, highlights the underappreciated capacity of eolian processes to generate first-cycle quartz sandstones on landscapes pre-dating the advent of land vegetation.

Facies and Fracture Architecture of the Tensleep Sandstone, Bighorn Basin, Wyoming: Preiiminary Result of an Outcrop and Subsurface Study: ABSTRACT

The Middle Pennsylvanian to Lower Permian Tensleep Sandstone has been the most prolific producer in the state of Wyoming. Now in its advanced stages of production. it is critical to accurately characterize the Tensleep facies and fracture architecture. Two outcrop locations have been selected: Alkali Creek has cross-strata dip parallel exposures at close spacings for eolian facies architecture Ziesman Dome is an NW-SE trending asymmetric anticlinal closure with the SE axis having a marked dextral dislocation, and is selected for analysis of fracture architecture. Byron Field is also an anticlinal closure with NW-SE trend which is selected for location of subsurface study. Methods of study for outcrop includes high- precision electronic surveying, photomosaics, geologic mapping, and fracture spacing and orientation measurements. The subsurface study includes conventional wireline logs, cores, and porosity and permeability data. In addition, a microresistivity log was evaluated from the Lindsay 3H horizontal well. Tensleep facies architecture is defined by a hierarchy of elements. Parasequences are made up of marine to marginal marine dolomitic sandstones and sandy dolostones (<10[sup -2] to 10[sup 2] md) in the lower 20-50%, and eolian dune, interdune and large-scale deformed sandstones in the upper part. Eolian dune sandstones, the primary reservoir (2more » to 3x10[sup 2] md), am characterized by very large-scale (up to 40 ft thick) tabular-planar cross strata with lower permeability wind ripple strata (1 to 2x10[sup 2] md, strongly anisotropic) concentrated above 1st- and 2nd-order bounding surfaces. Medium-scale trough cross stratified intrasets are common; however, their spatial dimensions and distribution pattern are uncertain at this time.« less

Fluvial-aeolian interactions in a Proterozoic alluvial plain: example from the Mancheral Quartzite, Sullavai Group, Pranhita-Godavari Valley, India

Abstract The coarse-grained alluvial succession of the Proterozoic Mancheral Quartzite encloses within it a few 2–6 m-thick, typically salmon red, fine-grained, very well-sorted sandstone units. Some 25–400f the fine-grained sandstone units is aeolian — mostly adhesion laminae with subordinate adhesion cross-laminae and wind-ripple strata. The remainder of the sequence is aqueous; either massive or with faintly-developed trough cross-bedding. Based on their grain size, sorting and roundness, the bulk of the aqueous deposits appear to be reworked aeolian deposits. Thick adhesion laminated units within them show a number of superimposed drying-upwards sequences, represented by an upward decrease in laminae spacing within each of the sequences. Individual drying-upwards sequences, 5–15 cm thick, are in places bounded by disconformity surfaces marked by iron crusts. Stratigraphic relationships with associated fluvial facies indicate that these aeolian sandstones formed in the distal part of the floodplain. An arid climate, vegetation-free landscape, quickly avulsing channel behaviour and rapid basin subsidence favoured development and preservation of these rather thick alluvial plain aeolian deposits. Fluvial dynamics, in contrast to the erg dynamics as interpreted from other interlayered aeolian-fluvial deposits, was the primary control on the depositional features and their internal organization within these aeolian sandstone units.

Effects of Stratigraphic Heterogeneity on Permeability in Eolian Sandstone Sequence, Page Sandstone, Northern Arizona

Outcrop and core data from the eolian Middle Jurassic Page Sandstone of northern Arizona show that realistic reservoir characterization can be based on stratigraphic architecture. Outcrop displays of differential weathering, coloration, cementation, and seepage reflect stratigraphic control of fluid flow within the sandstone. Measurements of permeability using a field permeameter and conventional analysis yield a five-order-of-magnitude range of values between distinct populations. Extra-erg and/or interdune deposits are least permeable (0.67-1,800 md), followed by wind-ripple strata (900-5,200 md), with grain-flow strata being the most permeable (3,700-12,000 md). Extra-erg and interdune deposits and some extensive bounding surfaces form significant flow barriers and ten to compartmentalize the reservoir. Reservoir characteristics of the dune cross-strata are a function of the types, distributions, and orientations of the internal stratification. Directional-flow properties within the strata are most pronounced within wind-ripple laminae. Recognition of the levels of reservoir heterogeneity is sampling-scale dependent. General models for flow within single sets of cross-strata as well as within eolian sequences can be generated based upon relative permeability between stratigraphic units.

Eolian dune types preserved in the Tensleep Sandstone (Pennsylvanian-Permian), north-central Wyoming

The Tensleep Sandstone is one component of an erg system that prograded southward out of north-central Wyoming from Middle Pennsylvanian to Early Permian time. Each erg advance was temporarily interrupted by regional marine transgression. Interpretation of dune types deposited in these ergs is developed from an analysis of the relative proportions of eolian stratification types comprising foreset strata, geometry and relationships of bounding surfaces, and paleodispersal patterns. Two basic morphologic dune types are inferred: simple dunes and compound crescentic dunes. The simple dunes are dominant and are subdivided into 1–2 km wavelength and 1.0 km saddle-spacing slightly crescentic, and 0.1–0.2 km wavelength straight-crested subtypes based on their first-order bounding surface geometries. Foresets are composed of grainfall and wind-ripple strata; avalanche strata are rare. The compound crescentic dunes had wavelengths of 0.5–1 km and saddle spacings of 0.8 km. Foresets are dominated by avalanche and wind-ripple strata. A morphodynamic classification of the dune types is inferred from considerations deduced from paleodispersal patterns and comparison with paleocirculation models. The simple dunes were demonstrably oblique to some elements of the wind field and less oblique to others. The compound crescentic dunes had a predominantly transverse configuration.

Practical Characterization of Eolian Reservoirs for Development: Nugget Sandstone, Utah-Wyoming Thrust Belt: ABSTRACT

The Jurassic-Triassic Nugget Sandstone of the Utah-Wyoming thrust belt is a texturally heterogeneous eolian reservoir with anisotropic directional properties that have been inherited primarily from the depositional environment, but also somewhat modified by diagenesis and overprinted by tectonism. Porosity ranges from several percent to the mid-twenties, and maximum horizontal permeability covers five orders of magnitude from a darcy to hundredths of millidarcys. Where productive, the formation ranges from approximately 800 to 1,050 ft (244 to 320 m) thick, at depths of about 7,500 to 15,000 ft (2,286 to 4,572 m) below the surface. Because some reservoirs are fully charged with hydrocarbons, an understanding of their internal complexity is important to monitor production erformance and maximize hydrocarbon recovery. Depositional processes determine the three-dimensional distribution of facies-related bedding types, which can have unique grain size and sorting textures, primary and diagenetic mineral compositions, and tectonic deformational fabrics, resulting in characteristic porosity, permeability, permeability directionality, and pore geometry attributes. Such characteristics can be quantified or calculated from core analysis, capillary pressure, nuclear magnetic resonance, conventional log, dipmeter, and production data to generate a practical geologic model on whatever scale of detail is necessary. Nugget dune deposits (good reservoir facies) consist of cross-bedded grain-flow and grain-fall strata and generally lower angle, wind-ripple-generated laminae. Interdune, sand-sheet, and other nondune deposits (poor reservoir facies) are characterized by wind ripples and more irregular bedding types, some of which are commonly associated with the presence of damp or wet conditions (e.g., bioturbation, wavy bedding, and soft sediment deformation). Dune grain-flow laminae exhibit the best reservoir quality and the least heterogeneity in bedding texture, followed respectively by grain-fall and wind-ripple strata. Anisotropy in permeability directionality resulting from bedding can range over several orders of magnitude. Irregular bedding types, especially those associated with wetter con itions, commonly contain an abundance of the finest grained material available in the depositional environment and generally have the poorest reservoir properties. Postdepositional modification to the reservoir include diagenesis and tectonism. Although compaction, cementation, clay mineralization, and dissolution events have occurred, diagenesis has not ultimately altered the relative hierarchy of bedding-controlled reservoir quality determined by depositional texture. Tectonic deformation has primarily created networks of low-permeability gouge-filled fractures and microfaults, although intermittently open fractures with somewhat more effective permeability also exist locally. Depositional models incorporating dune morphologies, facies distribution, permeability directionality, and theoretical concepts regarding dune migration through time are useful in defining the correlative intervals that are most likely to have continuity and potential communication of reservoir properties. Well-devised stratigraphic models can be adapted for engineering reservoir simulation studies and can also be used in solving structural resolution problems if a correlatable vertical sequence or a relatively consistent cross-bed orientation are known. End_of_Article - Last_Page 2045------------