The origin and significance of groundwater-seepage gypsum from Bristol Dry Lake, California, USA

Gypsum and anhydrite fabrics observed in trenches and deep (500 m) cores from Bristol Dry Lake, California, USA, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentological evidence indicate that the gypsum formed in Bristol Dry Lake precipitated displacively within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater-seepage gypsum). Evidence for displacive growth of gypsum is: (i) the geometry of the deposit, (ii) stable isotopic data and the water chemistry of the brine, and (iii) inclusions of matrix which follow twin planes and completely surround crystals as they grow. The bulk of the gypsum precipitated in the playa occurs around the edges of the playa in the playamargin facies and completely rings the lake. Sulphate concentrations in the groundwater increase toward the gypsum zone in the playa margin. Basinward of this zone, sulphate concentrations decrease sharply to trace element levels in the basin centre brine. Authigenic gypsum is rare in the centre of the playa. Stable (δ¹⁸O values measured for gypsum waters of crystallization (GWC) are similar to the values calculated for groundwater in the playa margin and alluvial fan sediments (?– 6%0), whereas measured brine δ¹⁸O values range from + 0·5 to + 3·7%0. Deuterium values measured for groundwater are ?– 70%0, GWC are ?– 60 to – 65%0 and brine values are ?– 57%0. The geometry of the deposit and the chemical data suggest that the water precipitating the gypsum is more closely associated with the groundwater than the brine. However, some mixing between groundwater and brine is likely. Within 100 m of the surface, the gypsum dehydrates to anhydrite, although the same vertically aligned fabric is retained through the diagenetic process. The similarity of displacive vertically aligned gypsum and anhydrite fabrics seen in Bristol Dry Lake to subaqueously deposited gypsum in modern brine ponds indicates that the criteria used to define subaqueous fabrics must be better constrained.     

Sedimentology of the Cretaceous Maha Sarakham evaporites in the Khorat Plateau of northeastern Thailand

Evaporites of the Cretaceous to early Tertiary Maha Sarakham Formation on the Khorat Plateau of southeast Asia (Thailand and Laos) are composed of three depositional members that each include evaporitic successions, each overlain by non-marine clastic red beds, and are present in both the Khorat and the Sakon Nakhon sub-basins. These two basins are presently separated by the northwest-trending Phu Phan anticline. The thickness of the formation averages 250 m but is up to 1.1 km thick in some areas. In both basins it thickens towards the basin centre suggesting differential basin subsidence preceding or during sedimentation. The stratigraphy, lithological character and mineralogy of the evaporites and clastics are identical in both basins suggesting that they were probably connected during deposition. Evaporites include thick successions of halite, anhydrite and a considerable accumulation of potassic minerals (sylvite and carnallite) but contain some tachyhydrite, and minor amounts of borates. During the deposition of halite the basin was subjected to repeated inflow of fresher marine water that resulted in the formation of anhydrite marker beds. Sedimentary facies and textures of both halite and anhydrite suggest deposition in a shallow saline-pan environment. Many halite beds, however, contain a curious `sieve-like' fabric marked by skeletal anhydrite outlines of gypsum precursor crystals and are the product of early diagenetic replacement by halite of primary shallow-water gypsum. The δ³⁴S isotopic values obtained from different types of anhydrite interbedded with halite range from 14.3‰ to 17.0‰ (CDT), suggesting a marine origin for this sulphate. Bromine concentration in the halite of the Lower Member begins around 70 ppm and systematically increases upward to 400 ppm below the potash-rich zone, also suggesting evaporation of largely marine waters. In the Middle Member the initial concentration of bromine in halite is 200 ppm, rising to 450 ppm in the upper part of this member. The bromine concentration in the Upper Member exhibits uniform upward increase and ranges from 200 to 300 ppm. The presence of tachyhydrite in association with the potassic salts was probably the result of: (1) the large volumes of halite replacement of gypsum, on a bed by bed basis, releasing calcium back into the restricted waters of the basin; and (2) early hydrothermal input of calcium chloride-rich waters. The borates associated with potash-rich beds likely resulted from erosion and influx of water from surrounding granitic terrains; however, hydrothermal influx is also possible. Interbedded with the evaporites are non-marine red beds that are also evaporative, with displacive anhydrite nodules and beds and considerable amounts of displacive halite. The δ³⁴S isotopic values of this anhydrite have non-marine values, ranging from 6.4‰ to 10.9‰ (CDT). These data indicate that the Khorat and Sakhon Nakhon basins underwent periods of marine influx due to relative world sea-level rise but were sporadically isolated from the world ocean.     

Sedimentology of a saline playa in the northern Great Plains, Canada

Ceylon Lake, a small salt playa located in southern Saskatchewan, is typical of many shallow ephemeral lacustrine basins found in the northern Great Plains of western Canada. The present-day brine, dominated by magnesium, sodium, and sulphate ions, shows wide variation in composition and concentration on both a temporal and a spatial basis. The modern sediments overall exhibit relatively simple facies relationships. An outer ring of coarse grained shoreline and colluvial clastics surrounds mixed fine grained clastics and salts and, in the centre of the basin, salt pan evaporites composed mainly of mirabilite, thenardite, and bloedite. Coring of the late Pleistocene and Holocene sedimentary fill shows that the lake has evolved from a relatively dilute, deep water, clastic dominated basin through a shallower, brackish water, carbonate-clastic phase, and finally into the present salt dominated playa. The thick sequence of evaporites preserved in the basin suggests evolution of the brine from a Na-rich solution to a mixed Mg-Na system. The most important post-depositional processes affecting Ceylon Lake sediments are mud diapirism and salt karsting.     

Sedimentology of gravelly Lake Lahontan highstand shoreline deposits,Churchill Butte,Nevada, USA

Gravelly shoreline deposits of the latest Pleistocene highstand of Lake Lahontan occur in pristine depositional morphology, and are exposed in gravel pits along Churchill Butte in west-central Nevada. Four environments differentiated at this site are alluvial fan/colluvium, lakeshore barrier spit, lake lower-shoreface spit platform, and lake bottom. Lakeshore deposits abut, along erosional wave headcuts, either unsorted muddy to bouldery colluvium fringing Churchill Butte bedrock, or matrix-supported, cobbly and pebbly debris-flow deposits of the Silver Springs fan. The lakeshore barrier spit is dominated by granule pebble gravel concentrated by wave erosion of the colluvial and alluvial-fan facies. The lakeward side of the barrier consists of beachface deposits of well-sorted granules or pebbles in broad, planar beds 1–10 cm thick and sloping 10–15°. They interfinger downslope with thicker (10–25 cm) and less steep (5–10°) lakeward-dipping beds of fine to medium pebble gravel of the lake upper shoreface. Interstratified with the latter are 10–40-cm-thick sets of high-angle cross-beds that dip southward, alongshore. Higher-angle (15–20°), landward-dipping foresets of similar texture but poorer sorting comprise the proximal backshore on the landward side of the barrier. They were deposited during storm surges that overtopped the barrier berm. Gastropod-rich sand and mud, also deposited by storm-induced washover, are found landward of the gravel foresets in a 15-m-wide backshore pond. Algal stromatolites, ostracodes, and diatoms accumulated in this pond between storm events. The lake lower shoreface, extending from water depths of 2 to 8 m, consists of a southward-prograding spit platform built by longshore drift. The key component of this platform is large-scale sandy pebble gravel in 16° southward-dipping `Gilbert' foresets that grade at a water depth of about 6–7 m to 4°-dipping sandy toesets. A shift from bioturbated lower-shoreface sand and silt, to flat and laminated lake-bottom silt and mud, occurs between water depths of 10–40 m and over a shore-normal distance of ≥250 m. This lake-bottom mud facies, unlike the others, is areally expansive.     

The enigma of fine-grained alluvial basin fills: the Permo-Triassic (Cumbrian Coastal and Sherwood Sandstone Groups) of the Solway Basin,NW England and SW Scotland)

The late Permian to Triassic sediments of the Solway Basin consist of a layer-cake succession of mature, predominantly fine-grained red clastics laid down in semi-arid alluvial plain to arid sabkha and saline marginal marine or lacustrine environments. The Cumbrian Coastal Group consists of Basal Clastics and Eden Shales. The Basal Clastics are thin regolith deposits resting unconformably on all-underlying units and are composed of mixtures of angular local gravel and far-transported fine to very fine-grained sands deposited as basal lag. The Eden Shales are predominantly gypsiferous red silty mudstones, with thin very fine-grained sandstone beds, and with thick marine gypsum beds at the base, deposited at a saline lake margin. The overlying Triassic Sherwood Sandstone Group consists of the Annan and Kirklinton Sandstones. The Annan Sandstones are predominantly thick-bedded, multi-storied, fine-grained mature red quartz sandstones in which coarse sand is practically absent despite channels with clay pebbles up to 30 cm in diameter. The overlying, predominantly aeolian, Kirklinton Sandstones consist of festoon cross-bedded and parallel-laminated fine-grained sandstones, almost identical to the Annan Sandstones except that mica and clay are absent. The Stanwix Shales, located above, consist of interbedded red, blue and green mudstones, siltstones, and thin very fine-grained sandstones, with gypsum layers. Although the entire succession can plausibly be interpreted as deposited in a large desert basin opening into a hypersaline marine or lacustrine embayment to the southwest, the uniformly fine-grained nature of the succession is unusual, as is the absence of paleosols, and body and trace fossils. There is almost no coarse sand even in the river channel units, and it seems likely that the basin was not only extremely arid but supplied predominantly by wind rather than water.     

Evaporites and siliciclastics of the Permian Nippewalla Group of Kansas, USA: a case for non-marine deposition in saline lakes and saline pans

The mid‐Permian Nippewalla Group of Kansas consists of bedded evaporites, red‐bed siliciclastics and grey siliciclastics deposited in a non‐marine environment. Lithologies and sedimentary features indicate lacustrine and aeolian deposition, subaerial exposure and palaeosol formation. Grey siliciclastic mudstones characterized by planar and convolute laminations, ostracods, peloids and plant material represent a freshwater‐brackish perennial lake facies. Bedded anhydrites containing gypsum‐crystal pseudomorphs, clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes. Bedded halites consist of chevron and cumulate crystals, dissolution surfaces and pipes and mudcracked microcrystalline salt crusts, which were deposited in saline pans dominated by flooding, evaporative concentration and desiccation. Chaotic halite, composed of red‐bed mudstone and siltstone with displacive halite crystals, formed in saline mudflats. Red‐bed mudstone and siltstone with little or no displacive halite, but with abundant cracking, root and plant features, suggest deposition in a dry mudflat. Red‐bed sandstone, composed of well‐sorted, well‐rounded quartz grains cemented with halite, indicate aeolian and rare shallow‐water deposition. Most deposition took place in halite‐dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. Evaporation, desiccation, flooding and wind played significant roles in this environment. The Nippewalla Group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non‐marine, acidic saline pan system in the mid‐continent of North America. The problem of distinguishing between ancient marine and non‐marine evaporites, as well as recognizing those evaporites deposited in acid settings, with detailed field, core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed. In addition, interpretations of mid‐Permian palaeoclimate data in the form of short‐term air temperature proxies within longer‐term wet–dry trends have been made. These data provide a new palaeogeographic and palaeoclimatic model for the mid‐Permian of western Pangaea.     

柴达木盆地西部尕斯库勒盐湖280 ka以来沉积特征

以柴达木盆地西部尕斯库勒盐湖干盐滩6个钻孔岩芯为研究对象,从岩性特征、成盐期、沉积类型、沉积结构、沉积幅度等方面探讨尕斯库勒盐湖沉积特征。研究表明,自280 ka以来尕斯库勒盐湖经历相对湿润-干旱的气候波动和气候演化,在距今43.6 ka左右进入最干旱时期,可能属于柴达木盆地第二次成盐期;沉积结构层分异现象不明显;各成盐期平均沉积速率变化不大,平均沉积速率比新疆和内蒙古地区高; 该湖沉积中心在盐湖的西北部。     

美国新墨西哥州钾盐矿床及其开发

新墨西哥州卡尔斯巴德 (Carlsbad)钾盐矿床是美国最早发现的古钾盐矿床 ,该矿床一直是美国钾盐的主要供给地。含钾蒸发岩系产出于得克萨斯州西部、新墨西哥州东南部Delaware盆地上二叠统海相地层中。含钾蒸发岩系可分为 4个建造。由下向上分别为 :①Castile建造 ,由石盐岩和其夹层硬石膏岩或石灰岩构成 ;②Salado建造 ,由钾盐、石盐岩、含泥石盐岩、硬石膏岩、杂卤石岩、白云岩、泥岩构成 ;③Rustler建造。由石盐岩、石膏岩、硬石膏岩、硅质岩、白云岩及石灰岩构成 ;④DeweyLake红层建造 ,由红色泥岩和砂岩构成。其中 ,Salado建造 ,厚 6 70m ,含 12个矿带 ,面积为 492 0km2 ,钾盐矿体主要由钾石盐和无水钾镁矾以及石盐等矿物所组成。次生的钾盐矿物有 :钾盐镁矾和钾镁矾等     

钱塘江下切河谷充填物地质特征及浅层生物气的孔隙水压力封闭机理

近年来,在浙江省北部钱塘江河口湾地区发现并开发了大量的晚第四纪浅层生物气藏。末次盛冰期,全球海平面的下降使河流梯度增加,下切作用增强,导致钱塘江下切河谷的形成。下切河谷内的沉积序列从下到上可划分为4种沉积相类型,分别为河床相、河漫滩-河口湾相、河口湾-浅海相和河口湾砂坝相。 所有的商业浅气田和气藏都分布于太湖下切河谷和钱塘江下切河谷及其支谷的河漫滩-河口湾相砂体中。钱塘江下切河谷的河漫滩-河口湾砂体埋深30～80 m,厚3～7 m,被非渗透的黏土包围,可能代表了下切河谷内分布的潮流沙脊。快速堆积的河口湾-浅海相沉积物为生物气藏的形成提供了充足的源岩和良好的保存条件。 河漫滩-河口湾相的黏土层为研究区浅层生物气藏的直接盖层,主要分布在下切河谷内,其埋深、残留地层厚度和孔隙度范围分别为30～80 m、10～30 m和42.2％～62.6％。河口湾-浅海相的淤泥层为间接盖层,覆盖了整个下切河谷,其埋深、残留地层厚度和孔隙度范围分别为5～35 m、10～20 m和50.6％ ～53.9％。黏土层和淤泥层的孔隙水压力远大于下伏砂体的孔隙水压力,其差值可达0.48 MPa。在储集层和盖层分界面即浅气藏的顶部,孔隙水压力值达到最大。黏土层和淤泥层的孔隙水压力可以超过砂质储集层中气体压力和孔隙水压力之和。黏土和淤泥盖层的高孔隙水压力可能是浅层生物气被完全封闭住的最重要因素。直接盖层的封闭能力比间接盖层要好。黏土层和淤泥层的孔隙水压力消散时间很长,有时候很难达到稳定状态,这表明黏土层和淤泥层的渗透性差、封闭性好。随着埋深的增加,其压实程度和封闭性能增加。与黏土层和淤泥层相比,砂层的孔隙水压力消散较快,很容易达到稳定状态,而且消散时间与埋深无关,表明砂层渗透性好、封闭性差。气体一旦进入砂层,孔隙水就不能有效释放,导致砂层的孔隙水压力消散时间比黏土层和淤泥层的要长,这可能与生物气在孔隙水压力释放后的快速补充有关。     

Sedimentological characteristics and geochemical evolution of Nabq sabkha, Gulf of Aqaba, Sinai, Egypt

Nabq sabkha exists 16 km north of Sharm El Sheikh City occupying the low land topography in the alluvial fan zone along the coastal area, Gulf of Aqaba, Sinai, Egypt. The long axis of the sabkha trends NW–SE receiving water from two different sources: meteoric water drained from the surrounding mountainous area and seawater seepage. Field observations help to divide the area into raised beach, hill slopes, sabkha basin, and coastal area. The sabkha basin can be subdivided from its center outward into (1) basin center hypersaline lake flourished with microbial mat and precipitation of halite as rafts, cumulates, and chevrons, (2) saturated saline sand and/or mud flat zone with the extensive growth of gypsum and halite crystals growing displacively as well as different forms of petee structures, and (3) an elevated marginal dry zone with tepee structures. Mineralogical analysis reveals that quartz, halite, and gypsum are the dominant minerals with subordinate amount of aragonite, anhydrite, thenardite, and/or polyhalite. In addition, clay minerals in the mudflat zone are presented by illite and smectite, indicating derivation of soil from the surrounding basement rocks. Chemical analysis of the collected brine samples reveals alkali character in the saline lake (pH?=?7.6) and high concentrations of Na⁺ (680 meq/l), Cl^? (940 meq/l), Mg²⁺ (208 meq/l), Ca²⁺ (70 meq/l), SO ₄ ²⁺ (30 meq/l), and HCO ₃ ^? (6 meq/l). The high salinity values are due to the aridity of the area, which favors precipitation of halite. Using comparative sedimentological, chemical, and mineralogial methods between such modern and ancient evaporitic environments and by detailed field, petrographic and mineralogical studies of modern evaporite environments help to interpret paleo-depositional environments of ancient evaporites sequences still in debate.     

贵州省织金矿区晚二叠世晚期潮坪相沉积特征

以贵州省织金矿区四个含煤向斜构造单元勘查资料为基础,对该区晚二迭世晚期沉积特征进行分析,认为区内广泛发育砂泥潮坪相和碳酸盐潮坪相。由于海水影响程度的差异,二者在岩石类型、层理、生物化石、自生矿物、元素地球化学特征等方面特征明显,主要表现在前者更多地具有陆相沉积特征,后者则更多地体现海相沉积的特点。砂泥潮坪相之上常发育较好的可采煤层,煤层硫分也较低,而碳酸盐潮坪相则正好相反,因此在研究区找煤时,建议重点放在砂泥潮坪相发育的区域。     

Recent, arctic deltaic sedimentation: Olivier Islands, Mackenzie Delta, North-west Territories, Canada

Despite a low tidal range and relatively low wave conditions, the Mackenzie Delta is not prograding seaward but rather is undergoing transgressive shoreface erosion and drowning of distributary channel mouths. In the Olivier Islands region of the Mackenzie Delta the resultant morphology consists of a network of primary and secondary channels separated by vegetated islands. New ground is formed through channel infilling and landward-directed bar accretion. This sedimentation is characterized by seven sedimentary facies: (1) hard, cohesive silty clay at the base of primary channels which may be related to earlier, offshore deposition; (2) ripple laminated sand beds, believed to be channel-fill deposits; (3) ripple laminated sand and silt, interpreted as flood-stage subaqueous bar deposits; (4) ripple laminated or wavy bedded sand, silt and clay, representing the abandonment phase of channel-fill deposits and lateral subaqueous bar deposition from suspension settling; (5) a well sorted very fine sand bed, presumed to result from a single storm event; (6) parallel or wavy beds of rooted silt, sand and clay, interpreted as lower energy emergent bar deposits; and (7) parallel or wavy beds of rooted silt and clay, believed to represent present-day subaerial bar aggradation. The distribution of sedimentary facies can be interpreted in terms of the morphological evolution of the study area. Initial bar deposition of facies 3 and channel deposition of facies 2 was followed by lateral and upstream bar sedimentation of facies 3 and 4 which culminated with the deposition of the storm bed of facies 5. Facies 6 and 7 signify bar stabilization and abandonment. Patterned ground formed by thermal contraction and preserved in sediments as small, v-shaped sand wedges provides the most direct sedimentological indicator of the arctic climate. However, winter ice and permafrost also govern the stratigraphic development of interchannel and channel-mouth deposits. Ice cover confines flow at primary channel mouths, promoting the bypassing of sediments across the delta front during peak discharge in the spring. Permafrost minimizes consolidation subsidence and accommodation in the nearshore, further enhancing sediment bypass. Storms limit the seaward extent of bar development and promote a distinctive pattern of upstream and lateral island growth. The effects of these controls are reflected in the vertical distribution of facies in the Olivier Islands. The sedimentary succession differs markedly from that of a low-latitude delta.     

Depositional theme of a marginal marine evaporite

We have reconstructed the depositional environment of the gypsum-carbonate-shale sequence that comprises the Upper Permian Bellerophon Formation of the southeastern Alps in northern Italy. This formation, which reaches a maximum thickness of 600 m, is roughly divided into two facies: (a) a lower dolomite-gypsum facies, and (2) an upper micritic-skeletal limestone facies. It directly overlies, with transitional contact, a thick red-bed sequence (alluvial fanglomerates, fluviatile sandstones and flood-plain siltstones) and is sharply overlain by Lower Triassic calcarenites (oolites, grapestones, pellets, flat-pebble conglomerates). The lower evaporite facies rocks are found in well-defined cycles, each of which, from bottom to top, consists of (A) thin-bedded, worm-burrowed, vuggy ‘earthy’ micritic dolomite, (B) massive to poorly laminated dark grey to black sandy dolomite carrying isolated gypsum nodules, (C) layered (thin-bedded) nodular gypsum (commonly with ‘enterolithic’ folds) with fragmented partings of dolomite, and (D) massive ‘chicken-wire’ nodular gypsum. At Passo di Valles, just east of Predazzo, and 50 km from the basin margin, we measured forty-six consecutive complete cycles, with an average thickness of 3 m per cycle. We interpret the cyclic sequence as having been deposited in a prograding shallow lagoon—sabkha complex. The worm-burrowed ‘earthy’ dolomite mud accumulated in a shallow hypersaline subtidal lagoon. The black sandy dolomite was an ‘intertidal’ sand-flat devoid of algal mats and constantly churned by burrowers (likely crustaceans). As the shoreline prograded lagoonward evaporative concentration of the groundwater induced diagenetic growth of anhydrite nodules (now gypsum) within the porous sandy dolomite. The layered nodular and ‘chicken-wire’ gypsum of the cycle cap is an extreme product of such displacive intra-sediment growth of anhydrite (now gypsum) above the water table of a completely exposed sabkha, such as is found in the Persian Gulf today. We have observed the same cyclically arranged lithologies in two other evaporite sequences in Italy: the Triassic Raibl Formation of the Southern Alps and the Upper Triassic Burano Formation of the central Apennines. We suggest that this mode of deposition is likely a very common one for at least the early stages of marine evaporite accumulation.     

Sediments in the estuarine environment of the tigris/euphrates delta; Iraq; Arabian Gulf

Studies on surface water characters of the estuarine environment of Iraq, northwestern Arabian Gulf, define three subdivisions of salinity: oligohaline, polyhaline, and euhaline. Textural analysis reveals that surface sediments covering the bottom are composed of six distinct classes: silty clay, clayey silt, sand-silt-clays, clayey sand, silty sand and sand. Five physiographic subdivisions are identified and described fluvial-estuarine, tidal mud flats (subtidal flat, lower intertidal flat, upper intertidal flat, supratidal flat), sand bars, Abdallah-Shetana channel and submerged estuarine distributary channel and bar systems. The content of organic matter in the sediments ranges from 0.24-3.69 per cent by weight. High values were recorded from the Abdallah-Shetana channel while lower values are confined to sand bars and submerged estuarine distributary channel and bar systems. Carbonates, quartz, feldspar, and halite are the main mineral constituents of the non-clay grade sediments; carbonates are present mainly as low-magnesium calcite followed by dolomite and high-magnesium calcite. Aragonite is present only in trace amounts. Both detrital and biogenic sources are suggested for low-magnesium calcite. A detrital source for dolomite and a biogenic source for high-magnesium calcite and aragonite are proposed.     

Aeolian landscapes in central Australia: gypsiferous and quartz dune environments from Lake Amadeus

There are two different dune systems in central Australia; regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes. These two systems, especially gypsiferous dunes at Lake Amadeus, the largest playa in central Australia, provide a sedimentary, geomorphological and environmental history of the region during the late Quaternary. The gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below, a mixture of gypsum sand and quartz sand. No clay pellets have been found in the dune sequence, in significant contrast to the gypsiferous clay dunes in other parts of Australia. Three possible models of the environmental controls of gypsiferous dune formation are discussed. The most plausible one suggests simultaneous gypsum precipitation and deflation. Sandsized gypsum was precipitated in a groundwater-seepage zone around the playa margin during seasonally high water-tables and these crystals were deflated onto land during dry intervals, forming the marginal gypsiferous dunes. These processes require conditions of high regional water-table, strong climatic seasonality and probably a windier and overall wetter climate. At least two separate gypsiferous-duneforming episodes can be recognized. The age of formation of the younger one has been dated by thermoluminescence at 44–54 ka. The gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals, showing evidence of leaching, dissolution and recrystallization. It is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune. After the construction of the younger gypsiferous dune, there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa. An equivalent aeolian sand layer was deposited within the playa. Soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation. Activation of the regional dunefields suggests a major period of dry climate, which, although not dated, may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in Australia.     

塔里木盆地塔中地区志留系沉积相模式探讨

研究表明塔中志留系为陆表海碎屑潮坪沉积。从其沉积特点及古地理背景出发，结合古陆表海环境的水动力特点，建立起塔中地区志留系的陆表海碎屑潮坪沉积模式，认为它可分为三个微相:泥坪、砂泥坪和砂坪。部分泥坪和大部分砂泥坪发育在水下，砂坪的沉积水动力与现代的潮坪相比相对较弱。障壁岛沉积不发育，潮汐流和风暴流对碎屑搬运和改造起主要作用，砂坪和砂泥坪中潮汐水道沉积和风暴沉积发育。水进时垂向上发育向上变粗的沉积序列，砂体中广泛发育交错层理；水退时可形成大面积连续性好的砂体及分布面积广而稳定的泥坪。砂坪微相的一些水下高地存在可能成为有利储层的纯净的石英砂岩。塔中１１井实例说明此相模式在塔中志留系的研究中是适用的。     

Holocene overbank deposition in the drainage basin of Lake Khanka

Overbank deposits in the Komissarovka River valley consist of alternating silt, clay silt, sand, and soils produced by lacustrine, alluvial, and aeolian deposition and by soil formation. Silt and sand layers in the lower part of the section correlate with the events of Early Holocene transgression and Middle Holocene regression of Lake Khanka. Deposition in the lowermost reaches of the Komissarovka River provides a faithful record of local Holocene landscapes controlled by level changes in Lake Khanka.     

The anatomy of a sulphate platform and adjacent basin system in the Leba sub-basin of the Lower Werra Anhydrite (Zechstein, Upper Permian), northern Poland

The Lower Werra Anhydrite (Zechstein, Upper Permian) deposits of the teba area originated in a deep basin setting, in shallow to deep water conditions. Facies changes occur within small distances and suggest fluctuating boundaries between well defined basins and platforms. This pattern of local platforms and adjacent basins developed during deposition. In basinal areas, the sequence is clearly transgressive, whereas on platforms accumulation kept pace with subsidence after an initial transgression. Nodular anhydrite represents a polygenetic deposit which formed at different times with respect to deposition. Massive anhydrite with pseudomorphs after upright-growth gypsum crystals suggest rapid precipitation in a subaqueous environment and/or fluctuating, but generally high, salinity conditions. Massive clastic sulphate originated due to periodic high energy events and resedimentation, or due to brecciation possibly connected with salinity fluctuations and the dissolution of halite. Massive, textureless anhydrite is locally porous and passes upward into breccia, indicating a strongly saline environment. Bedded anhydrite is considered to form in shallow water environments and laminated anhydrite in deep water. Bedded anhydrites contain portions which are graded. Intercalations of sulphate turbidites and upright-growth gypsum suggest fluctuating water depths, with comparatively deep water during turbidite deposition, but shallower conditions during upright-growth gypsum deposition. The sequence observed in slope zones at platform-basin margins, detrital (parautochthonous) sulphate sand to graded beds to basinal laminites, indicates that redistribution processes were important. At the onset of the Lower Werra Anhydrite deposition bathymetric relief existed between the central part of the basin and its margins, where carbonate platforms remained subaerially exposed. Formation of local platforms and adjacent basins required a relatively high subsidence rate, as pre-existing relief cannot account for the total accumulated thickness of the Lower Werra Anhydrite deposits. One implication of this is that the main argument against ‘the shallow water - shallow basin’ evaporite basin model, i.e.,a very fast rate of subsidence, may not be valid for the Łeba Lower Werra Anhydrite basin.     

Sedimentological and geochemical evidence to elucidate an evaporitic–carbonate paleoenvironment, Middle Miocene Red Sea coast, Egypt

Lithologic succession, microscopic examination as well as X-ray diffraction and chemical data revealed that the surface Middle Miocene evaporites of Wadi Quei are composed of anhydrite beds intercalated with carbonate and green shale, whilst the subsurface evaporites of Gemsa locality are composed of gypsum, anhydrite, carbonates and celestite with a rare amount of halite. The anhydrite is found to be formed diagenetically after gypsum. The carbonate is interpreted as having been of biogenic origin. The strong smell of H₂S and golden crystals of pyrite at Wadi Quei beds are indications of the biogenic action of sulphate-reducing bacteria in the presence of organic matter. It is suggested that the evaporite sequence which was deposited in a supratidal sabkha environment is characterized by alkaline-reducing conditions. The presence of nodular gypsum at Gemsa locality is probably deposited in a supratidal environment with oscillation of sea level.