Transition from arid to hyper-arid environment in the southern Levant deserts as recorded by early Pleistocene cummulic Aridisols

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ～2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ～1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (10⁵–10⁶ years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ¹⁸O and relatively high δ¹³C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ～1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 10⁶ yr.     

Structure, chemistry and origins of gypsum crusts in southern Tunisia and the central Namib Desert

Gypsum crusts are broadly defined as accumulations at or within about 10 m of the land surface from 0.10m to 5.0 m thick containing more than 15% by weight gypsum (CaSO₄·2H₂O) and at least 5.0% by weight more gypsum than the underlying bedrock. The deposits are often, but not invariably, consolidated owing to cementation by gypsum. The crusts are found in many of the world's deserts where mean monthly potential evaporation exceeds mean monthly precipitation throughout the year. Using structural, fabric and textural criteria, three main types of crust may be distinguished:(1) bedded crusts, found either at or beneath the land surface, which are made up of discrete horizontal strata up to 0.10 m thick, each showing a gradation in gypsum crystal size from less than 50 μm at the top to more than 0.50 mm at the base; (2) subsurface crusts, of which there are two forms, one made up of large, lenticular crystals (up to 0.50 m in diameter)—the desert rose crusts—and the other, a mesocrystalline form, with gypsum crystals up to about 1.0 mm in diameter; and (3) surface crusts, which are subdivided into columnar, powdery and cobble forms, all of which are made up of predominantly alabastrine gypsum (crystallites less than 50 μm in diameter). In southern Tunisia and the central Namib Desert, bedded crusts are found around ephemeral lakes and lagoons. They are characterized by size-graded beds, gypsum contents of 50–80% by weight and comparatively high concentrations of sodium, potassium, magnesium and iron. They are interpreted as shallow-water evaporites which accumulate when saline pools evaporate to dryness. Desert rose crusts or croûtes de nappe generally contain 50–70% by weight gypsum, and have higher sodium concentrations than the second subsurface form. Texturally they are characterized by poikilitic inclusion of clastic material within large lenticular crystals. They are interpreted as hydromorphic accretions, which precipitate in host sediments at near-surface water tables through the evaporation of groundwater. The second form of subsurface crust—the mesocrystalline—often occurs in close association with the various surface forms. Unlike the hydromorphic crusts, they are not restricted to low-lying terrain. They are characterized by gypsum contents reaching 90% by weight, and have a close chemical and textural similarity to columnar surface crusts. This mesocrystalline form represents an illuvial accumulation; the surface forms—excluding the bedded crusts—are exhumed examples at various stages of solutional degradation. Subsurface precipitation of gypsum from meteoric waters containing salts leached from the surface, results in displacive gypsum accumulation in the soil zone. In southern Tunisia, the gypsum is derived from sand and dust deflated from evaporitic basins; in the central Namib, salts dissolved in fog water are the most likely source. Where other salts are present, differential leaching may form two-tiered crusts, calcrete—gypsum or gypsum—halite, if rainfall is sufficient to mobilize the less soluble salt yet insufficient to flush the more soluble. Gypsum crust genesis is restricted to arid environments, and if their susceptibility to post-depositional alteration is acknowledged, they can provide valuable palaeoclimatic indicators.     

Holocene evolution of the Anichab Pan on the south-west coast of Namibia

Coastal sediment-filled depressions (pans) are one of the few areas that contain Quaternary records of sea-level and palaeoenvironmental change along the western margin of southern Africa. Anichab is a 128 km² salt-encrusted pan on the hyper-arid southern coast of Namibia with an emergent, well-preserved and in-place mid-Holocene mollusc assemblage. The molluscs are typical of subtidal sands on the sheltered side of offshore islands but include several warm-water species no longer found living along this coast. The Holocene evolution of the pan was largely influenced by changes in sea level and supply of sand along the coast. Calibrated radiocarbon ages of mollusc shells indicate a maximum Holocene sea level of ca 2 m above mean sea level (msl) from 7·0 to 6·3 ka and a return to near present-day sea level by 5·3 ka. The pan surface is 2 m below msl and has been emergent since 4·9 ka from the build up of sandy beaches and coastal dunes. A thin (1–4 cm) halite crust occurs over much of the pan surface but a layer of halite-cemented sand up to 40 cm thick is restricted to the central pan. Gypsum occurs near the subsurface brine interface and is limited by calcium to the edges of the pan. Nodules of calcite-cemented sand are forming in brackish, relatively high alkalinity subsurface waters in the south-east corner of the pan and nodules of aragonite-cemented sand are forming in brines 1 m below the central pan surface. Although modern dolomite has been reported from coastal lagoons of Brazil and Australia, carbonate cements are a minor feature of Anichab Pan and dolomite was restricted to a single reworked nodule most likely of Late Pleistocene age. Therefore, Anichab Pan does not appear to be a modern analogue to extensive, mixed-water dolomite cements found in Upper Pleistocene sediment-filled depressions on the Namibian shelf.     

The central Australian groundwater discharge zone: Evolution of associated calcrete and gypcrete deposits

The groundwater discharge zone of central Australia is marked by a chain of playas extending 500 km from Lake Hopkins in Western Australia through Lake Neale, Lake Amadeus and numerous smaller playas to the Finke River in the Northern Territory. This great valley is devoid of surface drainage, and the playas and their associated landforms and chemical sediments are a result of groundwater transmission, and discharge from a large regional flow system. The valley contains extensive groundwater calcrete deposits, which are commonly silicified. The playas contain gypsum and glauberite deposits resulting from the evaporative concentration of discharging groundwater to 250 g/1total dissolved solids. Thin gypcrete crusts have developed on the playa margins and islands as a result of induration of bordering gypsum dunes. Diagenetic changes in the calcrete and gypcrete are caused by dissolution and reprecipitation through groundwater movement.

Calcrete and gypcrete samples from playas near Curtin Springs, NT, dated by electron spin resonance (ESR) — calibrated by ¹⁴C dates on calcrete and the uranium content of gypcrete — are mostly in the range 8000–16 000 years BP, and are evidence of groundwater discharge conditions similar to those of the present day. Calcrete in the vadose zone, above the present water‐table, gives ESR dates in the range 22 000–27 000 years BP suggesting episodes of high intensity rainfall at that time. Phreatic calcrete, below the water‐table, with ages in the range 34 000–75 000 years BP, provides evidence of older episodes of calcretization.

The ESR and ¹⁴C dates for the chemical sediments suggest that this groundwater flow system has been an important feature in the region for much of the Late Quaternary. Although there are problems with the interpretation of ESR dating for chemical sediments with diagenetic alteration, the technique shows promise as a dating tool.     

Carboniferous calcretes in the Canadian Arctic

Calcrete palaeosols have been found in the Upper Carboniferous Canyon Fiord Formation of southwestern Ellesmere Island, Canadian Arctic. These calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins, in which the tectono-sedimentary environment led to the deposition of five different sedimentary facies: (i) floodbasin sandstones; (ii) alluvial fan sandstones; (iii) alluvial fan conglomerates; (iv) braided fluvial sandstones; and (v) shallow marine limestones. Nodular/massive palaeosol profiles, consisting of cryptic, nodular, massive and laminar horizons, occur within the floodbasin sandstone and alluvial fan sandstone facies. Plugged palaeosol profiles, consisting of cryptic, plugged and laminar horizons, are restricted to the alluvial fan conglomerate facies. Massive/brecciated palaeosol profiles, consisting of cryptic, massive/brecciated and laminar horizons, occur essentially within the shallow marine limestone facies. The relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material: nodular/massive profiles are restricted to silicate-rich sandstone hosts, plugged profiles are restricted to carbonate-rich conglomerate hosts and massive/brecciated profiles are restricted to limestone hosts. Important relationships also exist between the maturity levels of nodular/massive profiles and sedimentary facies: profiles are generally mature in the floodbasin sandstones, invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones. These different maturity levels were probably controlled mainly by exposure time, vegetation and substrate composition.     

Diagenetic evolution of Aptian evaporites in the Namibe Basin (south‐west Angola)

The widespread and dissected nature of the Angolan gypsiferous salt residuals offers a uniquely detailed view of the lateral and vertical relations inherent to secondary evaporite textures, which typify exhumed salt masses worldwide. Such secondary textures are sometimes misinterpreted as primary evaporite textures. Thin, metre‐scale and patchy, dome‐like gypsum accumulations are well‐exposed within strongly incised present‐day river valleys along the eastern margin of the Namibe and Benguela basins (south‐west Angola). These sections are time equivalent to the main basinward subsurface evaporites (Aptian Loeme Formation) which mostly consist of halite. The gypsum (here called the Bambata Formation) is interpreted to represent the final residual product of fractional dissolution and recrystallization of the halite mass that occurred during Late Cretaceous margin uplift and continues today. This halite underwent multiple episodes of diagenetic alteration between its deposition and its final exhumation, leading to the formation of various secondary gypsum fabrics and solution‐related karst and breccia textures that typify the current evaporite outcrop. Four different diagenetic gypsum fabrics are defined: thinly bedded alabastrine, nodular alabastrine, displacive selenite rosettes and fibrous satin‐spar gypsum. Current arid conditions are responsible for a thin weathered crust developed at the top of the outcropping gypsum, but the fabrics in the main core of the current at‐surface evaporite unit mostly formed during the telogenetic stage of uplift prior to complete subaerial exposure. Alteration occurred as various dissolving and rehydrating saline minerals encountered shallow aquifers in the active phreatic and vadose zones. Geomorphological and petrographic analyses, mostly based on the cross‐cutting relations and crystallographic patterns in the outcrop, are used to propose a sequence of formation of these different fabrics.     

Late Pliocene to Early Quaternary extensional detachment in the La Paz–El Cabo area (Baja California Sur, Mexico): implications on the opening of the Gulf of California and the mechanics of oblique rifting

We demonstrate that Pliocene to Early Quaternary sedimentary formations in Baja California Sur (Mexico) were deposited syn-tectonically over a major detachment associated with the exhumation of Mesozoic crust. The detachment dips to the ENE and is associated with E–W stretching. This large extensional structure strikes almost parallel to the general trend of the Gulf of California and extension is oblique to the East-Pacific seafloor-spreading direction. Crustal-scale stretching in this area was still active after the beginning of seafloor spreading c.  3.6 Ma ago. The detachment is capped by Late Pleistocene–Holocene alluvial sediments the deposition of which seems to be partly syn-tectonic and controlled by minor stretching subparallel to the present-day North American–Pacific kinematic vector. We discuss the implications of our observations on strain partitioning during opening of the California Gulf as well as on the structure of the Gulf of California margin.     

Miocene to recent history of the western Altiplano in northern Chile revealed by lacustrine sediments of the Lauca basin (18°15′–18°40′ S/69°30′–69°05′W)

The intramontane Lauca Basin at the western margin of the northern Chilean Altiplano lies to the west of and is topographically isolated from the well-known Plio-Pleistocene lake system of fluvio-lacustrine origin that covers the Bolivian Altiplano from Lake Titicaca to the north for more than 800 km to the Salar de Uyuni in the south. The Lauca Basin is filled by a sequence of some 120 m of mainly upper Miocene to Pliocene elastic and volcaniclastic sediments of lacustrine and alluvial origin. Volcanic rocks, partly pyroelastic, provide useful marker horizons. In the first period (6–4 Ma) of its evolution, the Lago Lauca was a shallow ephemeral lake. Evaporites indicate temporarily closed conditions. After 4 Ma the lake changed to a perennial water body surrounded by alluvial plains. In the late Pleistocene and Holocene (2-0 Ma) there was only marginal deposition of alluvial and glacial sediments. The basin formed as a half-graben or by pull-apart between 10 and 15 Ma (tectonic displacement of the basal ignimbrite sequence during the Quechua Phase) and 6.2 Ma (maximum K/Ar ages of biotites of tuff horizons in the deepest part of the basin). Apart from this early basin formation, there has been surprisingly little displacement during the past 6 Ma close to the Western Cordillera of the Altiplano. Also, climate indicators (pollen, evaporites, sedimentary facies) suggest that an arid climate has existed for the past 6 Ma on the Altiplano. Together, these pieces of evidence indicate the absence of large scale block-faulting, tilt and major uplift during the past 5–6 Ma in this area.     

干旱地区河流扇三角洲-河流扇演替模式:来自黄羊泉扇的启示

通过地表实地踏勘和浅层探槽解释，结合卫星照片解译，参考第四纪和历史地理研究结果，对准噶尔盆地西北缘黄羊泉扇的沉积特征及其第四纪以来的演变进行了分析.黄羊泉扇是一个发育在内陆干旱地区、以河流沉积为主的河流扇，扇体平面上呈上细下粗的“葫芦型”，可以划分为上扇、中扇和扇缘3个相带，沉积物具有由上扇向扇缘变细的趋势.扇上沉积动力以河流为主，缺乏泥石流沉积，但扇面活动水道不发育，大多数河道为偶尔才有径流通过的暂时性河道，风对扇表面的沉积物有明显的改造作用.黄羊泉扇扇缘发育4种不同的终止方式，一部分进入艾里克湖，一部分终止在沼泽地带，一部分在小型湖泊边缘消失，一部分受地形阻挡终止，与经典冲积扇和扇三角洲都不完全相同.结合前人历史地理研究成果认为第四纪以来黄羊泉扇所在的古玛纳斯湖出现过6次高湖平面，在30 ka以前，黄羊泉扇体一直处于古玛纳斯湖包围中，形成黄羊泉扇三角洲.直到30~10 ka期间才由于湖泊水位下降，扇体逐步出露形成河流扇.研究建立了黄羊泉扇随着湖平面升降从河流扇三角洲不断演化为河流扇的沉积模式，表明黄羊泉扇目前正处于河流扇三角洲沉积体系向河流扇沉积体系演变的过渡阶段.      

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia,Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and west-dipping, N–S to N20°–30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacılar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of left-lateral oblique slip of approximately 4.1 mm·year^–1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period.     

A new facies model for the Upper Gypsum of Sicily (Italy): chronological and palaeoenvironmental constraints for the Messinian salinity crisis in the Mediterranean

The Upper Gypsum unit of the Caltanissetta Basin (Sicily) records the last phase of the Messinian salinity crisis comprising the so‐called ‘Lago Mare’ event. A new facies analysis study recognizes nine to ten depositional cycles consisting of seven rhythmically interbedded primary gypsum bodies, and two to three sandstone bodies separated by marly terrigenous horizons showing laterally persistent vertical organization. A basal thin gypsum bed is overlain by a cluster of five thicker gypsum bodies. A marly interval containing two distinct sandstone horizons separates this cluster from the overlying uppermost (seventh) gypsum body. The terrigenous Arenazzolo Formation, in turn followed by the lower Pliocene Trubi Formation, is considered here to form the uppermost part of the Upper Gypsum unit. The rhythmic alternation in the sandy marls and gypsum/sandstone bodies records the response of sediments from shelfal to deltaic systems to precession‐driven arid‐wet climate fluctuations causing cyclical changes of both base‐level and water concentration. During wet climate phases (at insolation maxima) marl and sandstone were deposited in a hypohaline environment as suggested by: (i) the typical Lago Mare faunal assemblage and (ii) the negative δ¹⁸O values. During arid phases (at insolation minima) the reduced meteoric supply, recorded by higher δ¹⁸O values in the carbonate, caused the development of a negative hydrological budget leading to evaporite precipitation. At a basinal scale the Upper Gypsum unit unconformably overlies a mainly clastic evaporite unit containing carbonate breccia (the so‐called ‘Calcare di Base’) and/or clastic gypsum. Towards the basin centres, where the basal contact becomes conformable, a primary gypsum cumulate horizon is present. This layer is interpreted as a possible lateral equivalent of the Halite unit present only in the deepest depocentres. Based on astronomical calibration of the depositional cycles, the Upper Gypsum unit, including the Arenazzolo Formation, spans the interval between 5·33 and 5·53 Ma. This new age calibration allows the deposition of the Halite unit to be dated between 5·6 Ma (top of the Lower Evaporites) and 5·55 Ma (base of the Upper Evaporites) corresponding to isotopic stages TG12 and/or TG14.     

Soil-landscape and climatic relationships in the middle Miocene of the Madrid Basin

The Miocene alluvial-lacustrine sequences of the Madrid Basin, Spain, formed in highly varied landscapes. The presence of various types of palaeosols allows assessment of the effects of local and external factors on sedimentation, pedogenesis and geomorphological development. In the northern, more arid, tectonically active area, soils were weakly developed in aggrading alluvial fans, dominated by mass flows, reflecting high sedimentation rates. In more distal parts of the fans and in playa lakes calcretes and dolocretes developed; the former were associated with Mg-poor fan sediments while the latter formed on Mg-rich lake clays exposed during minor lake lowstands. The north-east part of the basin had a less arid climate. Alluvial fans in this area were dominated by stream flood deposits, sourced by carbonate terrains. Floodplain and freshwater lake deposits formed in distal areas. The high local supply of calcium carbonate may have contributed to the preferential development of calcretes on the fans. Both the fan and floodplain palaeosols exhibit pedofacies relationships and more mature soils developed in settings more distant from the sediment sources. Palaeosols also developed on pond and lake margin carbonates, and led to the formation of palustrine limestones. The spatial distributions and stratigraphies of palaeosols in the Madrid Basin alluvial fans suggest that soil formation was controlled by local factors. These palaeosols differ from those seen in Quaternary fans, which are characterized by climatically induced periods of stability and instability.     

Neotectonic and volcanic characteristics of the Karasu fault zone (Anatolia,Turkey): The transition zone between the Dead Sea transform and the East Anatolian fault zone

Abstract

The Karasu Rift (Antakya province, SE Turkey) has developed between east-dipping, NNE-striking faults of the Karasu fault zone, which define the western margin of the rift and westdipping, N-S to N20°-30°E-striking faults of Dead Sea Transform fault zone (DST) in the central part and eastern margin of the rift. The strand of the Karasu fault zone that bounds the basin from west forms a linkage zone between the DST and the East Anatolian fault zone (EAFZ). The greater vertical offset on the western margin faults relative to the eastern ones indicates asymmetrical evolution of the rift as implied by the higher escarpments and accumulation of extensive, thick alluvial fans on the western margins of the rift. The thickness of the Quaternary sedimentary fill is more than 465 m, with clastic sediments intercalated with basaltic lavas. The Quaternary alkali basaltic volcanism accompanied fluvial to lacustrine sedimentation between 1.57 ± 0.08 and 0.05 ± 0.03 Ma. The faults are left-lateral oblique-slip faults as indicated by left-stepping faulting patterns, slip-lineation data and left-laterally offset lava flows and stream channels along the Karasu fault zone. At Hacilar village, an offset lava flow, dated to 0.08 ± 0.06 Ma, indicates a rate of leftlateral oblique slip of approximately 4.1 mm?year^?1. Overall, the Karasu Rift is an asymmetrical transtensional basin, which has developed between seismically active splays of the left-lateral DST and the left-lateral oblique-slip Karasu fault zone during the neotectonic period. © 2001 Éditions scientifiques et médicales Elsevier SAS     

Quaternary tectonics in a passive margin: Marajó Island,northern Brazil

Marajó Island is located in a passive continental margin that evolved from rifting associated with the opening of the Equatorial South Atlantic Ocean in the Late Jurassic/Early Cretaceous period. This study, based on remote sensing integrated with sedimentology, as well as subsurface and seismographic data available from the literature, allows discussion of the significance of tectonics during the Quaternary history of marginal basins. Results show that eastern Marajó Island contains channels with evidence of tectonic control. Mapping of straight channels defined four main groups of lineaments (i.e. NNE–SSW, NE–SW, NW–SE and E–W) that parallel main normal and strike‐slip fault zones recorded for the Amazon region. Additionally, sedimentological studies of late Quaternary and Holocene deposits indicate numerous ductile and brittle structures within stratigraphic horizons bounded by undeformed strata, related to seismogenic deformation during or shortly after sediment deposition. This conclusion is consistent with subsurface Bouguer mapping suggestive of eastern Marajó Island being still part of the Marajó graben system, where important fault reactivation is recorded up to the Quaternary. Together with the recognition of several phases of fault reactivation, these data suggest that faults developed in association with rift basins might remain active in passive margins, imposing important control on development of depositional systems. Copyright © 2007 John Wiley & Sons, Ltd.     

Deformation patterns of upper Quaternary strata and their relation to active tectonics,Po Basin,Italy

Despite increased application of subsurface datasets below the limits of seismic resolution, reconstructing near‐surface deformation of shallow key stratigraphic markers beneath modern alluvial and coastal plains through sediment core analysis has received little attention. Highly resolved stratigraphy of Upper Pleistocene to Holocene (Marine Isotope Stage 5e to Marine Isotope Stage 1) alluvial, deltaic and coastal depositional systems across the southern Po Plain, down to 150 m depth, provides an unambiguous documentation on the deformation of previously flat‐lying strata that goes back in time beyond the limits of morphological, historical and palaeoseismic records. Five prominent key horizons, accurately selected on the basis of their sedimentological characteristics and typified for their fossil content, were used as highly effective stratigraphic markers (M1 to M5) that can be tracked for tens of kilometres across the basin. A facies‐controlled approach tied to a robust chronology (102 radiocarbon dates) reveals considerable deformation of laterally extensive nearshore (M1), continental (M2 and M3) and lagoon (M4 and M5) marker beds originally deposited in a horizontal position (M1, M4 and M5). The areas where antiformal geometries are best observed are remarkably coincident with the axes of buried ramp anticlines, across which new seismic images reveal substantially warped stratal geometries of Lower Pleistocene strata. The striking spatial coincidence of fold crests with the epicentres of historic and instrumental seismicity suggests that deformation of marker beds M1 to M5 might reflect, in part at least, syntectonically generated relief and, thus, active tectonism. Precise identification and lateral tracing of chronologically constrained stratigraphic markers in the ¹⁴C time window through combined sedimentological and palaeoecological data may delineate late Quaternary subsurface stratigraphic architecture at an unprecedented level of detail, outlining cryptic stratal geometries at the sub‐seismic scale. This approach is highly reproducible in tectonically active Quaternary depositional systems and can help to assess patterns of active deformation in the subsurface of modern alluvial and coastal plains worldwide.     

东营凹陷沙四下亚段沉积环境特征及沉积充填模式

沉积盆地中沉积环境特征控制了砂体沉积成因类型及盆地沉积充填模式。通过对东营凹陷丰深2井、官112井及梁120井沙四下亚段泥质沉积物系统取样,进行全岩X衍射分析和常量及微量地球化学元素分析,在此基础上对东营凹陷沙四下亚段古气候和古盐度指标进行分析。结果表明,古气候指标Rb/Sr比值与石英+长石含量、Cr元素含量及P元素含量呈正比,含量增加反映了气候相对潮湿,与古盐度指标碳酸盐+硫酸盐含量呈反比,其含量增加反映了气候相对干旱。东营凹陷沙四下亚段沉积时期气候干湿交替频繁,气候潮湿时期相对湖平面上升,湖水盐度降低;气候干旱时期相对湖平面下降,湖水盐度升高,使得此时期湖泊呈现为高频振荡性盐湖特征。气候潮湿时期以沉积碎屑沉积物为主,包括冲积扇、浅水型三角洲、滨湖滩坝及近岸水下扇沉积,由于湖水盐度较低,洼陷带主要沉积泥岩、灰质泥岩、含膏泥岩和少量泥质膏岩;气候干旱时期相对湖平面下降,湖泊水体盐度升高,洼陷带以沉积膏盐岩为特征,垂向上具有含膏泥岩、泥质膏岩、膏岩和盐岩的演化序列,平面上表现为由湖泊边缘向湖泊中心发育灰质泥岩、含膏泥岩、泥质膏岩、膏岩和盐岩的环带状结构特征。     

Natural and human-induced sinkholes in gypsum terrain and associated environmental problems in NE Spain

The central Ebro Basin comprises thick evaporite materials whose high solubility produces typically karstic landforms. The sinkhole morphology developed in the overlying alluvium has been studied using gravimetry and ground-penetrating radar (GPR) on stream terraces, as well as analyzing the evolution of sinkhole morphologies observed in aerial photographs taken in 1928, 1957, and 1985. The sinkhole morphologies give some idea of possible subsurface processes as well as an indication of the final mechanisms involve in sinkhole development. On stream terraces and cover pediments the most commonly encountered dolines are bowl-shaped in their morphology with both diffuse and scarped edges. In contrast, dolines developed in the gypsiferous silt infilled valleys have a funnel and well-shaped morphology. The diffuse-edged bowl-shaped dolines are developed through the progressive subsidence of the alluvial cover, due to washing down of alluvial particles through small voids and cracks into deeper subsurface caves, resulting in a decrease alluvial density. Future compaction of the alluvial cover will produce surface subsidences. This type of dolines are associated with negative gravity anomalies. In contrast, the scarped-edge dolines are formed by the sudden collapse of a cavity roof. The cavities and cracks formed in the gypsum karst may migrate to the surface through the alluvial deposits by piping, and they may subsequently collapse. In this instance, the cavities can be detected by both gravity and GPR anomalies where the voids are not deeper than 4–5 m from the surface. These processes forming sinkholes can be enhanced by man-induced changes in the groundwater hydrologic regime by both inflows, due to irrigation, ditch losses, or pipe leakages, and by outflows from pumping activities.     

Gypsum as a monitor of the paleo-limnological–hydrological conditions in Lake Lisan and the Dead Sea

The isotopic composition and mass balances of sources and sinks of sulfur are used to constrain the limnological–hydrological evolution of the last glacial Lake Lisan (70–14 ka BP) and the Holocene Dead Sea. Lake Lisan deposited large amounts of primary gypsum during discrete episodes of lake level decline. This gypsum, which appears in massive or laminated forms, displays δ³⁴S values in the range of 14–28‰. In addition, Lake Lisan’s deposits (the Lisan Formation) contain thinly laminated and disseminated gypsum as well as native sulfur which display significantly lower δ³⁴S values (−26 to 1‰ and −20 to −10‰, respectively). The calculated bulk isotopic compositions of sulfur in the sources and sinks of Lake Lisan lacustrine system are similar (δ³⁴S ≈ 10‰), indicating that freshwater sulfate was the main source of sulfur to the lake. The large range in δ³⁴S found within the Lisan Formation (−26 to +28‰) is the result of bacterial sulfate reduction (BSR) within the anoxic lower water body (the monimolimnion) and bottom sediments of the lake.

Precipitation of primary gypsum from the Ca-chloride solution of Lake Lisan is limited by sulfate concentration, which could not exceed 3000 mg/l. The Upper Gypsum Unit, deposited before ca. 17–15 ka, is the thickest gypsum unit in the section and displays the highest δ³⁴S values (25–28‰). Yet, our calculations indicate that no more than a third of this Unit could have precipitated directly from the water column. This implies that during the lake level decline that instigated the precipitation of the Upper Gypsum Unit, significant amounts of dissolved sulfate had to reach the lake from external sources. We propose a mechanism that operated during cycles of high-low stands of the lakes that occupied the Dead Sea basin during the late Pleistocene. During high-stand intervals (i.e., Marine Isotopic Stages 2 and 4), lake brine underwent BSR and infiltrated the lake’s margins and adjacent strata. As lake level dropped, these brines, carrying ³⁴S-enriched sulfate, were flushed back to the shrinking lake and replenished the water column with sulfate, thereby promoting massive gypsum precipitation.

The Holocene Dead Sea precipitated relatively small amounts of primary gypsum, mainly in the form of thin laminae. δ³⁴S values of these laminae and disseminated gypsum are relatively constant (15 ± 0.7‰) and are close to present-day lake composition. This reflects the lower supply of freshwater to the lake and the limited BSR activity during the arid Holocene time and possibly during former arid interglacials in the Levant.     

Soil genesis and clay mineralogy on Aliabbas River Alluvial Fan, Kerman Province

Alluvial fan is among the important geomorphic positions in arid and semi-arid environments, and the history of settlement in central parts of Iran plateau has significantly been related to this landform. Soil properties are also related to the stability and the position of geomorphic surfaces. To study and compare clay mineralogy and soil physicochemical responses to geomorphic positions, 11 representative pedons on stable and unstable alluvial fan surfaces and plain geomorphic position in a transect from Sarcheshmeh Mountains toward Rafsanjan Plain were studied. Soil moisture and temperature regimes of the area were aridic and mesic, respectively. Smectite, kaolinite, chlorite, illite, and palygorskite clay minerals were found in almost all the studied soils. More salinity was investigated in stable alluvial fan surfaces than unstable positions. Evidence of a more humid paleoclimate shown by buried Bt horizons in plain surface was observed. Transformation of palygorskite to smectite in surface horizon of unstable alluvial fan position and preservation of palygorskite associated by gypsum crystals in By horizon of stable fan surfaces were among the important findings of the research. There was a significant and mutual relationship between geomorphology and physicochemical and mineralogical properties of soils under study.     

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.