Physical compaction of vertic palaeosols: implications for burial diagenesis and palaeo-precipitation estimates

Burial compaction is one of several major obstacles to estimating palaeoprecipitation from depth to pedogenic carbonate in favourably preserved palaeosols. Palaeosols must be decompacted and the preburial depth to the pedogenic carbonate obtained. Vertic palaeosols may be particularly good candidates for palaeoprecipitation estimates, because of their increased likelihood of preserving clastic dykes, one of the best features for estimating burial compaction. Compaction estimates from clastic dykes and literature-based depth of burial estimates suggest vertic palaeosols undergo significantly less burial compaction than may be commonly assumed. Late Carboniferous vertic palaeosols, buried to 2·5–3·0 km, compacted to 93% of their original thickness. In contrast, clastic dykes in a nonpedogenic shale directly underlying one of the Late Carboniferous palaeosols records compaction to 70% of original thickness. Similarly obtained burial compaction and burial depth estimates for Early Carboniferous, Ordovician, and Proterozoic vertic palaeosols were used to test a burial compaction curve and equation specific to vertic palaeosols. Results suggest this ‘vertic-calibrated’curve and equation can be used to estimate burial compaction for vertic palaeosols lacking clastic dykes, but additional testing is needed. Naturally high bulk densities may have limited the compactibility of vertic palaeosols. Likewise, high initial bulk density and an abundance of swelling clays may have severely limited the transmissivity of some vertic palaeosols as they passed from pedogenic to burial environments. Upon burial these vertic palaeosols may have behaved as closed systems, which has implications for understanding their diagenetic modification. Additional efforts to understand burial compaction of vertic palaeosols also promises to improve our understanding of aquifer/aquiclude and hydrocarbon reservoir/seal relationships in sedimentary basins containing intercalated palaeosols.     

Role of latitudinal shift and climate change in evolution of red and yellow palaeosols of the Himalayas: Implications for early Oligocene seasonality and Mid-Miocene enhanced precipitation

Cenozoic atmospheric circulation, climatic changes, sedimentation and weathering over the Indian sub-continent were mainly influenced by the northward drift of the Indian Plate, the shrinking Paratethys, India-Asia collision and the rise of the Himalayas. This study is aimed at exploring the fluvial sedimentary record of the north-west part of the Himalayan Foreland Basin to interpret weathering and pedogenesis during early Oligocene to Mid-Miocene time. Palaeopedological investigation of a 3.1 km thick succession from Kangra sub-basin of the Himalayan Foreland Basin shows that the lower 2 km part of the succession is characterized by the red (10R hue) and the upper 1.1 km part of the succession by the yellow (2.5Y hue) palaeosols with varying intensity of weathering and pedogenesis. The association of sedimentary rocks and pedogenic expression in palaeosols indicate four (Type-A to Type-D) pedofacies in the entire Oligocene–Miocene succession. The pedofacies are defined by a decrease in the intensity of palaeopedogenic development from strongly-developed palaeopedofeatures in Type-A, moderately-developed palaeopedofeatures in Type-B, weakly-developed palaeopedofeatures in Type-C and to the only incipient stage of palaeopedogenesis in Type-D pedofacies. The palaeolatitudinal shift during the convergence of the Indian Plate played a major role in weathering and palaeopedogenesis with the inception of seasonality during the early Oligocene, which is demonstrated by the formation of the red palaeosols with pedogenic CaCO₃ and vertic features in tropical conditions. The transition to yellow palaeosols at about 20 Ma is marked by increased humidity, rapid aggradation, pronounced uplift and enhanced erosion of the hinterland. These yellow palaeosols are characterized by the abundance of weakly-developed Bw and Bss horizons, pure clay pedofeatures and absence of any pedogenic CaCO₃ during short pedogenic intervals in subtropical conditions.     

Were Ediacaran siliciclastics of South Australia coastal or deep marine?

The Late Neoproterozoic Ediacara Member of the Rawnsley Quartzite in South Australia has been considered aeolian, fluvial, intertidal and deep marine by various authors. Palaeosols would not be expected for the deep marine interpretation, but some palaeosols should be evident for the aeolian–fluvial–intertidal interpretations, and this is the first study to examine the Ediacara Member at a petrographic and geochemical scale appropriate to recognize potential palaeosols. Recognition of palaeosols and floodplain facies in Neoproterozoic rocks is a challenge because such rocks are too ancient for diagnostic non‐marine fossils such as root traces. The varied thickness of Ediacara Member red siltstones and white sandstones is distinct from laterally persistent overlying and underlying grey shales and limestones with acritarchs, stromatolites and other marine fossils. The sandstones are trough cross‐bedded and fill palaeovalleys. The red siltstones have poorly sorted, highly angular, silt‐size grains characteristic of loess. Particular sandy and silty beds were sampled for detailed petrographic and geochemical studies, because they include desiccation cracks, sand crystals, ice cracks, carbonate nodules and soft‐sediment deformation like those of palaeosols. Chemical and grain‐size variations within these beds reveal surficial clay formation and oxidation from feldspar as in soils. Petrographic studies also revealed surficial disruption of these palaeosols by filamentous structures comparable with microbial ropes of biological soil crusts. This array of palaeosol features may be of use for recognizing palaeosols in other Neoproterozoic siliciclastic sequences.     

Integrated diagenesis and sequence stratigraphic study of tidal sandstones: the Adedia Formation (Cambro-Ordovician), Sinai, Egypt

This work examines the different effects meteoric versus marine diagenesis had on Cambro-Ordovician tidal sandstones during episodes of fluctuating sea level. The distribution of diagenetic fabrics was compared to a sequence stratigraphic framework. Initially, a rise in relative sea level (RSL) resulted in deposition of transgressive systems tract sands directly onto crystalline basement. These sandstones display evidence of limited cementation by marine, grain-fringing dogtooth-like and fibrous calcite. A fall in RSL resulted in the progradation of a tidal flat complex and deposition of highstand systems tract (HST) and lowstand systems tract (braided fluvial) sandstones. Contemporaneous meteoric-water flux into sands of all the systems tracts occurred. Sequence boundaries (SB) are marked by fluvial incision of tidal sands and by the development of palaeosols. Meteoric incursion during sea-level lowstands resulted in the dissolution and kaolinitization of feldspars, micas and mud intraclasts in all systems tracts, but is most extensive in HST sandstones below the SB. The effect of meteoric-water flux on the dissolution of marine calcite cements is poorly known. Mesogenetic alterations include intergranular pressure dissolution and formation of variable amounts of syntaxial quartz overgrowths in all systems tracts. Telogenetic alteration (i.e. weathering) in the sandstones includes the formation of goethite and calcite. Thus, the integration of diagenesis with sequence stratigraphy provides a useful tool with which to understand reservoir-quality distribution in sand-dominated, tidal sediments.     

Facies and palaeosol analysis in a progradational distributive fluvial system from the Campanian–Maastrichtian Bauru Group,Brazil

The Upper Cretaceous Bauru Group in south‐east Brazil consists of alluvial strata whose characteristics and distribution indicate a fluvial system developed in a semi‐arid to arid climate. Sections exposed within a 90 000 km² study area in Minas Gerais State (in south‐eastern Brazil) were evaluated using facies and palaeosol analysis to formulate depositional and pedogenic models that may account for geomorphic and climate features. From east to west, the study succession records a gradual decrease in grain size, an increase in the width/thickness ratio in channels, a decrease in the lateral and vertical connectivity of channel deposits, and an increase in overbank deposits. The fluvial architecture indicates a braided channel belt, ephemeral ribbon–channels, and an unconfined fluvial facies from east to west in the study area. The lateral and vertical distribution of facies, stratigraphic architecture and palaeocurrent data suggest proximal, medial and distal portions of a progradational distributive fluvial system. The sedimentary dynamics were marked by the building and abandonment of channels related to processes of aggradation, vegetation growth and palaeosol generation. Macromorphological and micromorphological analyses have identified pedological and mineralogical features that indicate an arid to semi‐arid climate with a provenance from the north‐eastern part of the basin (Alto Paranaiba Uplift). From the proximal to the distal portions of the distributive fluvial system, the palaeosol development is different. In the proximal portion, the palaeosols are absent or poorly developed, allowing a possible general comparison with the present soil order: Inceptisols and Aridisols. In the medial portion of the fluvial system, the palaeosols are well‐developed and characterized by Bt, Btk, C and Ck horizons (Alfisols, Aridisols, Inceptisols and Entisols). Poorly drained to well‐drained palaeosols from the base to the top in the distal plain (Aridisols and Inceptisols) are associated with geomorphic and hydromorphic changes in the fluvial system due to progradational evolution. The genetic relationship between the fluvial architecture and the palaeosols enhances the understanding that the sedimentation and pedogenesis that occurs in different portions of the distributive fluvial system are related to the tectonic and climatic evolution of the basin.     

Physico-chemical environment of pedogenic carbonate formation in Devonian vertic palaeosols, central Appalachians, USA

The morphology and geochemistry of pedogenic carbonate found in vertic claystone palaeosols in the Devonian Catskill Formation in central Pennsylvania preserve a record of the physical and chemical environment of carbonate precipitation. The carbonate is characterized by three distinct petrographic generations. Pedogenic rhizoliths and nodules are the earliest precipitated generation, and typically consist of dull red-brown luminescent micrite. Clear, equant calcite spar cement fills voids in the centres of rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Early spar cements are non-luminescent to dull luminescent, whereas later spar cements exhibit bright yellow-orange luminescence. Late stage pedogenic fractures are always occluded with very bright yellow-orange luminescent spar cements. The incorporation of progressively higher concentrations of Mn (up to 34000 ppm) into successively younger calcite spar cements, without concomitant increases in Fe, suggests carbonate precipitation from an evolving meteoric water in which Mn²⁺ became increasingly mobile over time. The increased mobility is possibly due to decreasing Eh, resulting from oxidation of organic matter after rapid soil burial on the floodplain. The amount of Fe²⁺ available for incorporation into calcite was limited because most iron was immobile, having been earlier oxidized and bound to the palaeosol clay matrix as a poorly crystallized ferric oxide or oxyhydroxide mineral. Carbon isotope compositions of pedogenic carbonate correlate with the inferred depth of carbonate precipitation. Rhizoliths preserved below the lowest stratigraphic occurrences of pedogenic slickensides are consistently depleted in ¹³C relative to nodules, which formed stratigraphically higher, within the zone of active soil shrink and swell processes. Nodular carbonate, precipitated in proximity to deep cracks in the soil, is enriched due to increased gas exchange with isotopically heavy atmospheric CO₂. Accordingly, rhizolith compositions will most accurately estimate palaeoatmospheric levels of CO₂; the use of nodule compositions may result in overestimation of P_CO2 by as much as 30%.     

The influence of salt withdrawal subsidence on palaeosol maturity and cyclic fluvial deposition in the Upper Triassic Chinle Formation: Castle Valley, Utah

Integrated fluvial sequence stratigraphic and palaeosol analysis can be used to better reconstruct depositional systems, but these approaches have not been combined to examine halokinetic minibasins. This study characterizes the temporal and spatial patterns of lithofacies and palaeosols in a sequence stratigraphic framework to reconstruct a model of minibasin evolution and identify halokinetic influences on fluvial deposition. This research documents fluvial cycles and stratigraphic hierarchy, palaeosol maturity and apparent sediment accumulation rates in the Chinle Formation within the Big Bend minibasin. This study also uses palaeosols to help identify fluvial aggradational cycle (FAC) sets. The Chinle is divided into two hectometre‐scale (10² m) fluvial sequences, six decametre‐scale (10¹ m) FAC sets, and variable numbers of metre‐scale FACs depending on proximity to the minibasin. Ten pedotypes representing 225 palaeosol profiles are recognized. The pedotypes include palaeosols similar to modern Entisols, Inceptisols, Aridisols, Vertisols and Alfisols. A maturity index (1–5) is assigned to each pedotype to assess its variability in palaeosol development. Estimated palaeosol development time is used to approximate apparent sediment accumulation rates. Increased subsidence resulted in a greater number and thicker FACs, thicker FAC sets and fluvial sequence sections, and lithofacies associations reflecting more rapid sedimentation along the minibasin axis. Palaeocurrent indicators converge towards the minibasin axis and indicate that it formed and drifted through time. Relative palaeosol maturity is inversely related to stratal thickness, and decreases towards the minibasin where episodic burial by fluvial sediment was more frequent. Metre‐scale FACs are most abundant towards the minibasin axis, and locally have Entisols and Inceptisols developed upon their upper boundaries reflecting increased sediment accumulation rates. Areas outside the minibasin are characterized by fewer FACs that are associated with more mature palaeosols. Palaeosol‐derived apparent sediment accumulation rates are as much as two orders of magnitude greater within the minibasin than in marginal areas. The combined stratigraphic, palaeocurrent and palaeosol evidence is used to develop a model for the evolution of the Big Bend minibasin that illustrates the halokinetic affect on fluvial and landscape processes.     

河流相层序地层模式与地层等时对比

已有的河流层序地层模式在层序发育的控制因素、体系域的划分等方面存在一定的问题。通过对苏里格气田盒8段河流相准层序对比,发现河谷中存在阶梯状河流阶地。现代河谷地貌的研究表明,这种阶地是构造抬升过程中,次级间歇性构造抬升导致河流下蚀与侧蚀—沉积交替的结果。而构造下降过程所引起的基准面上升和可容空间增速的变化又影响了河道砂体的展布特征。考虑构造运动在河流层序形成过程中的作用以及河流阶地与相邻地层之间的接触关系,修正了Wright等河流相层序地层模式,认为构造运动控制了河流层序的发育,构造上升半旋回形成水退体系域,为一系列相对等时的阶地,构造下降半旋回形成低位体系域、水进体系域和高位体系域,低位体系域为构造抬升早期形成的河谷充填物,水进体系域和高位体系域与Wright等的含义一致。     

Early Mississippian sandy siltstones preserve rare vertebrate fossils in seasonal flooding episodes

Flood‐generated sandy siltstones are under‐recognised deposits that preserve key vertebrate (actinopterygians, rhizodonts, and rarer lungfish, chondrichthyans and tetrapods), invertebrate and plant fossils. Recorded for the first time from the lower Mississippian Ballagan Formation of Scotland, more than 140 beds occur throughout a 490 m thick core succession characterised by fluvial sandstones, palaeosols, siltstones, dolostone ‘cementstones’ and gypsum from a coastal–alluvial plain setting. Sandy siltstones are described as a unique taphofacies of the Ballagan Formation (Scotland, UK); they are matrix‐supported siltstones with millimetre‐sized siltstone and very fine sandstone lithic clasts. Common bioclasts include plants and megaspores, fish, ostracods, eurypterids and bivalves. Fossils have a high degree of articulation compared with those found in other fossil‐bearing deposits, such as conglomerate lags at the base of fluvial channel sandstones. Bed thickness and distribution varies throughout the formation, with no stratigraphic trend. The matrix sediment and clasts are sourced from the reworking of floodplain sediments including desiccated surfaces and palaeosols. Secondary pedogenic modification affects 30% of the sandy siltstone beds and most (71%) overlie palaeosols or desiccation cracks. Sandy siltstones are interpreted as cohesive debris flow deposits that originated by the overbank flooding of rivers and due to localised floodplain sediment transport at times of high rainfall; their association with palaeosols and desiccation cracks indicates seasonally wet to dry cycles throughout the Tournaisian. Tetrapod and fish fossils derived from floodplain lakes and land surfaces are concentrated by local erosion and reworking, and are preserved by deposition into temporary lakes on the floodplain; their distribution indicates a local origin, with sediment transported across the floodplain in seasonal rainfall episodes. These deposits are significant new sites that can be explored for the preservation of rare non‐marine fossil material and provide unique insights into the evolution of early terrestrial ecosystems.     

Anatomy and evolution of a Lower Cretaceous alluvial plain: sedimentology and palaeosols in the upper Blairmore Group, south-western Alberta, Canada

The Lower Cretaceous (Albian) upper Blairmore Group is part of a thick clastic wedge that formed adjacent to the rising Cordillera in south-western Alberta. Regional transgressive intervals are superimposed on the overall regressive succession. Alluvial conglomerates, sandstones and mudstones were deposited in east-north-eastward draining fluvial systems, orientated transverse to the basin axis. Five facies associations have been identified: igneous pebble conglomerate, thick sandstone, interbedded lenticular sandstone and mudstone, thick mudstone with thin sandstone interlayers, and fossiliferous sandstone and mudstone. The facies associations are interpreted as gravelly fluvial channels, sandy fluvial channels, sand-dominated floodplains, mud-dominated floodplains, and marine shoreline deposits, respectively. Five types of palaeosols are recognized in the upper Blairmore Group based on lithology, the presence of pedogenic features (clay coatings, root traces, ferruginous nodules, slickensides, carbonate nodules) and degree of horizonization. The regional distribution of the various types of palaeosols enables a refinement of the palaeoenvironmental reconstruction permitting an assessment of the controls on floodplain evolution. In source-proximal areas, palaeosol development was inhibited by high rates of sedimentation. In source-distal locations, poor drainage resulting from high watertables, low topography and lower rates of sedimentation also inhibited palaeosol development. The best-developed palaeosols (containing Bt horizons) occur in intermediate alluvial plain positions (tectonic hinge zone) where the floodplains were most stable due to a balance between sedimentation, erosion and subsidence rates. Extrapolating from the upper Blairmore Group suggests that the tectonic hinge zone of continental foreland basins can be established by palaeosol analysis. At the hinge zone, soil development is controlled primarily by climate and tectonics and their effect on sediment supply, whereas closer to the palaeoshoreline, relative sea level fluctuations, resulting in poor drainage, may have a more significant influence.     

Comparison of modern and ancient barite‐bearing acid‐sulphate soils using micromorphology,geochemistry and field relationships

Although pedogenic barite has been documented in many modern soils and palaeosols, no actualistic studies on its formation have been reported. Because barite is stable over the entire range of pressure and temperature of the Earth's crust, it preserves reliable data about the original environment in which it formed. Pedogenic barite and barite‐bearing soils have been used as indicators of landscape stability, environmental conditions, climate and microbial acti‐vity. This study compares field data, micromorphology and stable isotope geochemistry of a barite‐bearing palaeosol from the Morrison Formation (Jurassic) and a modern analogue soil in south‐central Texas, USA. Morrison barite‐bearing palaeosols are over‐thickened cumulic palaeosols that developed in subaerially exposed lacustrine sediments during an extended lake contraction event. Lateral facies relationships document changes in hydrology and duration of episaturated conditions (perched water table above the Btg horizons) that correspond to differences in barite nodule morphology and abundance. Barite precipitation occurred at a redox boundary higher on the landscape after organic matter was completely oxidized. Sulphur isotope data indicate that the initial source of sulphur was soil organic matter. Meteoric water is the likely source of oxygen for the sulphate. Barium sourced from weathering feldspars and clays. The modern analogue displays similar catenary relationships, redox features and micromorphological characteristics compared to the Morrison palaeosols, suggesting that similar pedogenic processes led to barite precipitation. Synthesized data suggest that conditions favourable to barite‐bearing soil formation are low‐gradient basins that have received feldspar‐rich sediments (i.e. volcanically influenced basins), soils that developed near salt domes, soils that developed in exposed wetland or lacustrine sediments and coastal plain deposits. When studied in a well‐documented palaeogeographic context, barite‐bearing soils are valuable to palaeoclimate, palaeoenvironmental and palaeohydrological studies. Combined with regional interfluve palaeosols, barite‐bearing palaeosols may document temporal changes in drainage, surface stability, and accommodation consistent with sequence boundaries/maximum flooding surfaces and climate changes.     

Short-range variations in the micromorphology of a palaeosol from Wivenhoe in southeast England

The pedogenic histories of four adjacent profiles of a polygenetic palaeosol developed on a Middle Pleistocene terrace of the proto-Thames from Wivenhoe in southeast England are reconstructed on the basis of superposition of key micromorphological features. Despite a considerable variation in macromorphology, partly resulting from large-scale periglacial features, three of the profiles have similar micromorphological records in that they retain evidence for two phases of clay illuviation separated by a period of periglacial disruption. This reconstruction, however, seems to be incomplete because the fourth profile contains micromorphological evidence for a further illuviation–disruption cycle. The extent of this variation suggests that soil micromorphology should be used only with care to reconstruct pedogenic or pedosedimentary histories of complex polygenetic palaeosols, or to compare such palaeosols on different surfaces of chronosequences spanning periods of major climatic change. The variable and possibly limited resolution of micromorphology, together with the current uncertainty over the exact environmental signifiance of illuvial clay features, means that inferred pedogenic phases should be correlated with specific climatic stages only with considerable caution. © 1998 John Wiley & Sons, Ltd.     

Pliocene pedosedimentary cycles in the southern Pampas, Argentina

Well‐developed Bt horizons of five palaeosols (P1–P5) have been recorded previously within a 20‐m‐thick succession of Pliocene siltstones and clayey siltstones in the southern part of the Buenos Aires Province of Argentina. This paper reports a detailed field and micromorphological (thin section) investigation of a 6‐m portion of the sequence encompassing P2 and P3. Large‐scale faunal burrow infillings occur throughout: other bioturbation features in the form of channel and spongy microstructures are mainly confined to the siltstones. The intervening clayey siltstones (Bt horizons) have been affected more by shrink–swell disruption, as evidenced by slickensides and a range of striated b‐fabrics in thin sections. Clay coatings, indicative of illuvial accumulation of clay translocated in suspension from overlying A or E horizons, occur in both the siltstones and clayey siltstones. The types, microstratigraphic associations and depth functions of features are interpreted in terms of changing interactions, balances and dominances between sedimentary, pedogenic and erosional processes over time, thus providing the basis for the pedosedimentary reconstruction of landscape evolution in the region during part of the Pliocene represented by the whole P1–P5 sequence (4–5 Ma BP). It is envisaged that this period was dominated by aeolian deposition, although fluvial and mass movement processes probably led to reworking and redistribution of some of the materials. Overall rates of subaerial deposition, however, were not substantial: pedogenic processes were active throughout, the balance between sedimentation and pedogenesis varying over time in a cyclical fashion. Phases of reduced deposition and establishment of relatively stable land surfaces were marked by the development of argillic soil profiles with clearly defined eluvial and illuvial horizons. Intervening periods of more rapid accumulation of coarser material were characterized by accretionary soil development and welding of new pedological features on existing soils as the surface accreted, first transforming existing eluvial horizons into BCt/AE horizons (siltstones) and then encouraging the syndepositional upward extension of these complex horizons. The primary basis of the alternating units of siltstones (BCt/AE horizons) and clayey siltstones (Bt horizons) lies in the cyclical change in size of particles deposited, although pedogenic translocation processes enhanced these textural differences. The underlying driving mechanism behind the pedosedimentary cycle can only be speculated upon, although it is tempting to relate the sedimentation pattern to climatic fluctuations linked to glacial advances and retreats in the Patagonian Andes during the Pliocene.     

Contrasting palaeosol development in two different tectonic settings: the Upper Buntsandstein of the Western Iberian Ranges, Central Spain

Palaeosols may offer excellent evidence for the development of sedimentary basins but few studies have used diagenetically altered material: here we show that material of this sort can also reflect the sedimentary environment. The Lower to Middle Triassic red beds of the western Iberian Ranges were deposited in a tectonically active half-graben in which subsidence rates varied along the basin as a response to differential fault movements. During this period the basin was filled by a set of fluvial units with interbedded palaeosols. The palaeosols show typical pedogenic calcrete profiles, although extensive dolomitization has deleted part of their microstructure; however, macrostructure and morphology are preserved. Differences in the maturity stages of the palaeosols are related to the changes in subsidence and sedimentation rates along the basin. Thus, two different scenarios are recognized at: (i) the hanging wall, Riba de Santiuste area, where palaeosols reach stage III as the episodic tilting of the floodplain inhibited the development of more mature soils and (ii) the footwall, Cercadillo area, where palaeosols attain stage V maturity, favoured by prolonged periods of tectonic stability resulting in lower sedimentation rates over the floodplain areas.     

冀中坳陷饶阳凹陷留西地区古近系沙河街组沙三上亚段沉积相与隐蔽油藏

通过岩心观察和单井相分析,结合沉积背景资料,认为留西地区古近系沙河街组沙三上亚段发育辫状河三角洲相和湖泊相,以辫状河三角洲前缘亚相和滨浅湖亚相为主,主要发育辫状分流河道、越岸沉积、水下分流河道、河口坝、席状砂等微相。根据层序地层学基本原理,结合前人研究成果,认为本区沙三上亚段为一完整的三级层序,可划分为低位、湖侵和高位3个体系域,分别对应于沙三上亚段沉积时期的早期、中期和晚期,绘制了每个沉积阶段的沉积相图,在此基础上研究了沉积体系的平面展布特征。沙三上亚段沉积时期形成的北高南低的构造背景与辫状河三角洲近东西向展布的砂体形态相互配置,加之良好的油源供给,为后期成藏创造了条件。沙三上亚段沉积早期发育的辫状河三角洲前缘水下分流河道与河口坝砂体是主要的储集体,与沉积中期发育的烃源岩形成了十分有利的生储盖组合,具有优越的隐蔽油藏发育条件。总结了该区隐蔽油藏成藏的4种模式,确定留西地区中南部沙三上亚段沉积早期发育的辫状河三角洲前缘砂体是下一步隐蔽油藏勘探的有利目标。     

Role of permeability barriers in alluvial hydromorphic palaeosols: The Eocene Pondaung Formation,Myanmar

This study examines the lateral distribution of hydromorphy in the fine‐grained alluvial deposits of the Eocene Pondaung Formation, central Myanmar. Through detailed outcrop analysis and using a combined sedimentological and pedological approach, this study proposes a reconstruction of Pondaung overbank floodplain palaeoenvironments. The variations of hydromorphic features in the different overbank sub‐environments are then discussed and used to build a model of hydromorphic variability in alluvial deposits. Two main architectural associations with distinctive lithofacies and pedogenic features were identified, corresponding to different sub‐environments: heterolithic deposits and extensive mudstone successions. The heterolithic deposits display variegated fine‐grained lithofacies and contain poorly developed palaeosols with gley and vertic features, which are interpreted to reflect a seasonal wetlands landscape, developed in actively aggrading avulsion belts. Extensive mudstone successions with Vertisols that locally exhibit mukkara‐style pseudogley features are interpreted to represent a distal open‐forested environment. The palaeosols of both sub‐environments display dense local hydromorphic variations they are also characterized by a gradual shift from gley‐dominated to pseudogley‐dominated features with increasing distance from the avulsion belt. The clay‐dominated lithology of the floodplain parent material, which forms numerous subsurface permeability barriers, is shown to have acted as a fundamental control in limiting water‐table dynamics in coarse‐grained parts of the succession, thereby favouring hydromorphic variability. Palaeosol sequences of the Pondaung Formation contrast with the soil‐landscape associations described in other studies and provide an alternative model with which to account for the hydromorphic variability in poorly drained, alluvial soils. The model proposed as an outcome of this study demonstrates that hydromorphic variations can be dramatic in floodplains where permeability barriers are numerous. Further, the model stresses the importance of undertaking detailed lateral palaeosol analyses prior to making interpretations regarding hydromorphic variability.     

Giant calcite-cemented concretions, Dakota Formation, central Kansas, USA

Giant spheroidal concretions (cannonball concretions; some nearly 6 m in diameter) in fluvial channel‐fill sandstones at two localities of the Dakota Sandstone formed by import of cement constituents at a burial depth of <1 km. During cannonball concretion growth a self‐organizational process restricted concretions to a relatively few but widely spaced, and locally, evenly spaced, sites. Other forms of calcite cements at these localities are cement patches in the form of intergrown grape‐size concretions (grapestone), and, locally, pervasive cement. An early episode of invasion by thermogenically generated H₂S, which reacted with iron oxides on detrital grains, generated scattered pyrite crystals and decimetre‐scale spheroidal pyrite concretions. Intergranular volumes (IGV) in the concretions range from 36% to 27%. The absence of a trend in IGV and of carbon and oxygen‐isotope ratios from cannonball centres to margins indicates that these concretions did not cement progressively outwards from the centre. Rather, the modern spheres represent the spatial extent of nucleation sites that were not otherwise organized within that volume. Carbon and oxygen‐isotope values for concretion calcites plot along a swath between depleted values of δ18C of ?36‰ and δ18O of ?13‰ and enriched values of ?4‰ and ?6‰, respectively. Four groups of calcites are evident on the basis of trace‐element content and suggest that the calcite precipitated across a range of oxidation conditions that do not correlate strongly with the isotopic compositions. Although fluvial overbank sandstones have some pedogenic calcite, the channel sandstones have at most a trace of pedogenic calcite and carbonate rock fragments, so that the bulk of cement components were imported to the sandstones. Carbon and calcium sources for calcite cement include marine limestone, carbonate shells, and anhydrite in addition to HCO derived from oxidized methane, most likely derived from beds underlying or laterally in communication with Dakota sandstones. HCO in ascending formation waters, released during compaction, mixed with meteoric water whose temperature and composition varied with time, to generate the 7‰ range in δ18O_calcite values measured.     

Soil/landform relationships surrounding the Harappa archaeological site,Pakistan

A soil survey around the archaeological site of Harappa, Pakistan revealed alluvial deposits of five distinct ages based on relative position in the landscape and degree of soil profile development. the youngest deposit (age 1) is in the lowest landscape position and has received flood waters as recently as 1988. Soils there are in an incipient stage of development: only organic carbon and soluble salts have accumulated at the surface of the profile. the age 2 deposit has not undergone significant pedogenic change, but is in a slightly higher landscape position than the youngest deposit. Elevated concentrations of P, and the presence of sand-sized pottery and brick fragments, indicate that this deposit was derived at least partially from archaeological material. the presence of small, soft calcite nodules (Stage II) and some soluble salt translocation are the primary pedogenic changes observed in the age 3 deposit. the age 4 deposit shows evidence of both carbonate and gypsum accumulation. Presence of large gypsum nodules in deep By horizons suggests that a high groundwater table has altered these soils. the oldest deposit, age 5, forms a late Pleistocene stream terrace of the Ravi River. the soil formed in this deposit exhibits considerable carbonate accumulation, with large, dense nodules (Stage II + ) and an argillic horizon. A ¹⁴C date from pedogenic calcite gives an age of 7080 ± 90 years B.P., indicating a minimum age of early Holocene. the soil survey suggests that the ancient city of Harappa was built on an age 5 stream terrace remnant, surrounded by Holocene floodplains and a meandering channel of the Ravi River.     

鄂尔多斯盆地北部苏里格庙含油气区二叠系山西组及下石盒子组盒八段岩相古地理

以测井资料和岩心观察为基础，对鄂尔多斯盆地北部苏里格庙含油气区的二叠系山西组及下石盒子组盒八段识别出了辫状河、曲流河、三角洲和湖泊等4种沉积相类型。利用层序地层学原理，将山西组和下石盒子组盒八段划分5个三级沉积层序，建立了苏里格庙含油气区层序地层格架。编制了以体系域为单位的山西组和下石盒子组盒八段沉积期岩相古地理图，探讨了层序地层格架内的山西组和下石盒子组盒八段沉积期岩相古地理特征及演化规律。山西期古地理格局呈南北向相分异的曲流河-三角洲沉积；盒八段沉积期冲积体系大范围向研究区南部迁移，形成了以辫状河沉积为主的河流-三角洲沉积。河道砂体是苏里格庙含油气区有利的砂质储集体类型，主要发育在低位体系域。实践表明，以体系域为单位编制的岩相古地理图，可以清楚地展示不同沉积时期的沉积微相展布与迁移变化，是有利储层预测的一种有效方法。