Evaporitic Lake Research Articles

Lacustrine and palustrine carbonates in a Brigantian (late Dinantian) intrashelf basin in the Derbyshire carbonate platform

The Brigantian (late Dinantian) Headstone Laminite provides a record of freshwater carbonate deposition in an intrashelf basin when the surrounding Derbyshire carbonate platform was emergent. The laminated grainstone/carbonate mudstone facies contains calcified plant material, peloids, ostracods and lenses of calcite tufa deposited in a lake margin and sublittoral part of a freshwater lake in a humid palaeoclimate. Precipitation of tufa may have taken place along water courses or resurgences fed by meteoric run‐off and seepage from the karsted carbonate platform. The fenestral carbonate mudstone with calcrete and fenestrae represent deposition in well‐drained parts of the lake margin. A shift to more arid conditions is represented by the laminated dolomudstone facies with an allochem assemblage of calcified plants and micropeloidal dolomicrospar grains and occasional alginite in restricted a lake basin setting. Gypsum crystals and desiccation curls are also present in evaporitic lake margin settings. A return to more humid conditions is indicated by deposition of the laminated grainstone/carbonate mudstone facies with organic‐rich mudstone with thin coal seams and carbonized roots deposited in a poorly drained lake margin setting. This may be a reflection of a gradual transgression after emergence. The eastern margin of the intrashelf basin may have been formed by the Edensor Anticline; however, it is not known if a further complex of lacustrine, palustrine and lagoonal facies was present to the east. Deposition of graded beds containing lithoclasts derived from lacustrine facies were associated with slumping generated by reactivation of structures controlling the intrashelf basin margins.

Seismic stratigraphic framework and seismic facies of the Aptian Pre-salt Barra Velha Formation in the Tupi Field, Santos Basin, Brazil

The discovery of giant oil accumulations in the Pre-salt (sag) section of the Santos Basin has attracted huge interest in the lacustrine carbonate deposits of the Aptian Barra Velha Formation. Although for the significant volume of publications on the Pre-salt section, there is still limited understanding of its tectono-stratigraphic evolution, and lack of a coherent published stratigraphic framework for the Tupi Field, the largest and most productive Pre-salt accumulation. This work aims to define a seismic-stratigraphic framework in the Aptian Pre-salt interval of the Tupi Field, using 19 seismic lines, and photomicrographs of thin sections from 13 wells. Four seismic units (SU1, SU2, SU3, and SU4) and two seismic facies (SFA and SFB) were identified. The depositional pattern of the seismic units is of a continuous expansion of the accommodation space, although with significant erosion at the top of SU4. SFA is composed mostly of calcite spherulites and Mg-clays mud, which constitutes the background of lacustrine sedimentation preferentially formed and/or preserved in relatively deeper areas. SFB corresponds to crusts of fascicular calcite shrubs preferentially precipitated in relatively higher areas, and to intraclastic deposits of reworked spherulites and shrubs. The Pre-Alagoas unconformity represents the base of the study interval. The top is characterized by a regional unconformity which represents a temporal gap between the youngest of the units (SU4) and the start of the evaporitic deposits of the Ariri Formation. No additional unconformities were identified within the Barra Velha Formation in the Tupi Field, characterizing a continuously expansive deposition on a flat, tabular and onlapping pattern upon the Outer High, as a product of continuous accommodation creation, under low tectonic activity. The predominant post-depositional configuration, together with the continuity and onlap patterns of seismic units onto a remanent high, support a shallow evaporitic lake model, rather than a deep lake with microbial platforms for the Tupi Field area.

Multi-proxy palaeoenvironmental record of coastal tectonic uplift and abandonment (ca. 6th c. CE) of Lechaion's inner harbour, ancient Corinth, Greece

Lechaion's inner harbour basin was constructed in the 7th-6th c. BCE and served as Corinth's principal port for over a millennium. The harbour decline and abandonment in the 6th c. CE has been attributed to several causes: natural siltation, co-seismic uplift, coastal subsidence, and damage by tsunami impacts. A multi-proxy palaeoenvironmental study was conducted on seven cores from Lechaion's inner harbour to determine changes in the coastal environments and timing and cause of harbour abandonment. Palaeoenvironments were reconstructed using high-resolution micro-XRF core scanning of sedimentary facies, isotopic (δ18O, δ13C), and micropalaeontological analyses (foraminifera, palynomorphs).The harbour lithostratigraphy consists of an uppermost (∼1 m) sequence of laminated mud and marl overlying interbedded pebbly sand and mud containing abundant marine microfossils and Roman pottery refuse. A thin (<12 cm) calcrete layer at the base of the marl defines a basin-wide paraconformity, marking a transition from a marine-estuarine harbour basin to a restricted, evaporitic lake. Basin restriction is recorded by a sharp decline in terrigenous elements (Si, Ti, K, Fe), increased Sr, δ18O, a decline in foraminifera and marine dinoflagellate cysts, and an increase in freshwater algae. The event is constrained by AMS 14C age modelling to the 6th c. CE and interpreted as rapid, co-seismic uplift of the harbour floor, most likely during destructive earthquakes of 524 and 551/552 CE. These seismic events have been linked to a ∼1.1 m uplift of the nearby Perachora Peninsula and sediment liquefaction structures on-site. No evidence was found for 2nd c. BCE or 6th c. CE tsunami events proposed in previous work. This study represents the most comprehensive geoarchaeological study completed to date in Lechaion's inner harbour and confirms its destruction and abandonment in the 6th c. CE as a result of co-seismic uplift and rapid shoaling of the inner basin.

Reinterpreting the South Atlantic Pre-Salt ‘Microbialite’ reservoirs: petrographic, isotopic and seismic evidence for a shallow evaporitic lake depositional model

The first encounter of the Pre-Salt Aptian ‘Microbialite’ carbonate reservoirs (the Barra Velha Fm) in 2005, in the Parati field, Santos Basin, was followed by additional discoveries such as the multi-billion barrel Lula (Tupi) field. Now, nearly 30 more discoveries have been made in the basin such as Libra and Sapinhoa, with recoverable reserves estimated as >30 BBOE, according to ANP. In addition, discoveries have been made in the adjacent Campos Basin, including the Pao de Acucar field (Viera de Luca et al., 2017), where the same unit is known as the Macabu Fm., and also in the Kwanza Basin, West Africa (Saller et al., 2016). After deposition in a late rift setting, the Barra Velha Fm and its equivalents were buried by more than 1 km of marine origin evaporites of the Ariri Fm and its equivalents, as the Albian Ocean seeped and poured into the basin.

Reconstruction of a saline, lacustrine carbonate system (Priabonian, St-Chaptes Basin, SE France): Depositional models, paleogeographic and paleoclimatic implications

A 220-m thick carbonate-dominated succession has been deposited in shallow-water, saline lake environments during the early to middle Priabonian (MP17A-MP18 mammal zones) in the Saint-Chaptes Basin (south–east France). The palaeoenvironmental, paleoclimatic and palaeogeographic significance of such saline lake carbonates has been deciphered on the basis of a multi-proxy analyses including: 1) depositional and diagenetic features; 2) biological components (molluscs, benthic foraminifera, characean gyrogonites, spores and pollens); 3) carbon and oxygen stable isotopes; 4) trace elements; and 5) clay mineralogy. Five stages of lacustrine system evolution have been identified: 1) fresh-water closed lake under dry climate (unit U1); 2) fresh to brackish water lacustrine deltaic system with a mixed carbonate-siliciclastic sedimentation under relatively wet climatic conditions (unit U2); 3) salt-water lacustrine carbonate system under humid climatic setting (unit U3); 4) evaporitic lake (unit U4); and 5) closed lake with shallow-water carbonate sedimentation under subtropical to Mediterranean climate with dry seasons (unit U5). Upper Eocene aridification is evidenced to have started as early as the earliest Priabonian (unit U1: MP17A mammal zone). A change from humid to dryer climatic conditions is recorded between units U3 and U4. The early to middle Priabonian saline lake is interpreted as an athalassic (inland) lake that have been transiently connected with neighboring salt lakes influenced by seawater and/or fed with sulfates deriving from recycling of evaporites. Maximum of connection with neighboring saline lakes (Mormoiron Basin, Camargue and Central grabens, Hérault Basin) likely occurred during unit U3 and at the base of unit U5. The most likely sources of salts of these adjacent basins are: 1) Triassic evaporites derived from salt-diapirs (Rhône valley) or from paleo-outcrops located east of the Durance fault or offshore in the Gulf of Lion; or 2) marine incursions from the south, through Paleogene grabens in the Gulf of Lion.

Rates and geochemical processes of soil and salt crust formation in Salars of the Atacama Desert, Chile

The hyperarid Atacama Desert contains numerous local basins with surficial salt crusts, known as salars, where evaporation of shallow groundwater drives the major soil processes. We examine chemical and isotopic profiles in two soils of differing ages from the Salar Llamara to determine the geochemical processes involved in their formation. Evaporation, which provides salts to the soils through mineral precipitation, decreases with increasing salt crust thickness, and average~0.03mmm−2d−1 over geological time frames. Salt distribution varies predictably with depth and soil age, with the most soluble compounds concentrated nearest to the land surface, indicating the direction of fluid flow. δ34S values of mineral sulfate tend to decrease with decreasing soil depth, following a pattern indicative of Rayleigh-like fractionation as solute-rich waters migrate toward the land surface. δ13C and δ18O values of carbonate suggest that the uppermost halite layers, which contain very small amounts of carbonate, have a strong biological signature. In contrast, carbonate-rich layers deeper in the profiles consist of largely unmodified lacustrine carbonate that formed in highly evaporitic lake conditions. The continuous upward evaporation of water and dissolved solutes creates a rugged and physically dynamic halite crust composed of rounded salt nodules. The crust undergoes deliquescence as atmospheric relative humidity rises from marine air intrusions, and we found that the halite nodules on the surface of the Salar Llamara are nearly always at or above deliquescence relative humidity. The interiors of these nodules are therefore able to buffer the large diurnal changes in atmospheric relative humidity allowing for the survival of halophilic microbial communities in an otherwise very dry environment. Radiocarbon measurements of occluded organic C in the surface crusts indicate that C cycling occurs at differing rates depending on local micrometeorological conditions, and that a given salt crust feature may persist for thousands of years once formed.

The origin of solutes within the groundwaters of a high Andean aquifer

This paper investigates the origin of solutes within the groundwaters of the Monturaqui–Negrillar–Tilopozo (MNT) aquifer system within the high Andes of the Atacama Desert that discharges into the Salar de Atacama. Key questions include the relative significance of volcanic hydrothermal processes and evaporitic brine recycling over solute supply as well as the pathways of solute ingress to the MNT aquifer system. Groundwaters were analysed for elemental (major, minor and trace) and isotopic (δ18O/δ2H; δ13C–DIC; δ34S–SO4; 87Sr/86Sr) constituents to which various hydrochemical and multivariate statistical methods have been applied. Groundwaters are all classified as thermal and show increasing temperatures (27–35°C) and concentrations of HCO3 (4.4–10.4mmolL−1 dissolved inorganic carbon [DIC]) with increasing proximity to Volcano Socompa resulting from an increasing mass flux of steam and magmatic CO2 (pCO2=0.016 to 0.10atm; δ13C–CO2=−9.3 to −3.6‰ (V-PDB)) boiled off a deep hydrothermal reservoir. Superimposed upon this gradational and relatively smooth spatial increase in heat and mass flow is a sharp, structurally controlled, increase in TDS (826–3632mgl−1) and a concomitant change in δ34S–SO4 (+0.79 to 4.9‰ (V-CDT)) and 87Sr/86Sr values (0.707375–0.706859) associated with the inflow of evaporitic solutes. Evaporitic inputs are chemically and isotopically distinct from localised secondary hydrothermally derived solutes with major, minor and trace element data suggesting an origin within a highly oxidising, alkaline, evaporitic lake receiving dilute inflows enriched in volcanic/fumarolic sulfur mineralisation probably from volcanoes Socompa, Salín or Pular that delimit the eastern topographic extent of the aquifer system. The conceptual model presented in this paper proposes that basal leakage of evaporitic brines from active salar(s), within the high altiplano/volcanic arc, are actively entrained by sub-regional groundwater flow and conveyed to the MNT aquifer system where they mix with solutes derived from localised secondary hydrothermal gas–water–rock interaction. This work provides detail on the origin and processes controlling the solute composition of groundwater inflows to the Salar de Atacama within the volcanically active and hyperarid Atacama Desert and may be of significance to conceptual models of evaporitic brine evolution, recycling of evaporitic brines and hydrothermalism in arid regions.

Hypotheses for the origin of fine-grained sedimentary rocks at Santa Maria crater, Meridiani Planum

En route to Endeavour crater, the Mars Exploration Rover Opportunity embarked on a short but significant campaign at Santa Maria crater during sols 2450–2551. Santa Maria crater is a relatively young impact crater, approximately 100m in diameter and 11–17m deep. Opportunity performed detailed analyses on several ejecta blocks and completed an extensive imaging campaign around the crater. Many of the ejecta blocks are composed of sandstone with abundant wind ripple laminations suggestive of eolian deposition. However, other ejecta blocks are massive, fine-grained, and exhibit a nodular texture. These rocks are interpreted to be the first rocks of a grain size smaller than the Microscopic Imager can resolve, and may represent the first mudstones observed by the rover. Several depositional environments are considered for the origin of the fine-grained rocks, and the observations are best fit by a transient evaporitic lake. If the inferred mudstones were deposited in a lacustrine setting, then surface water may have been present in a broader range of surface environments than previously documented at Meridiani Planum.

Chronology of sand ridges and the Late Quaternary evolution of the Etosha Pan, Namibia

Etosha Pan, situated at the southern border of tropical Africa, is a vast endorheic plain in Namibia's semi-arid north. The most recent studies agree that the pan was the floor of a former lake with varying water levels. Here we explored this idea further by investigating the link between lake-level change and records of late Pleistocene and Holocene climate change. The varying lake levels were inferred through sediment analysis and optical dating of sand deposits that form ridges parallel to the current shore along the northern and western margins of the pan. Our results support the view that the sand ridges are shoreline deposits of an evaporitic lake. The ridges result from the interplay between intermittent river discharge and riverine sediment supply from the north, prevailing north-easterly wind and shore-parallel waves. Therefore they are a proxy for former levels of a perennial lake. We infer higher levels during the late Pleistocene and a drastic drop shortly after 10ka. Since around 8ka Etosha Pan was covered by a shallow water body. This lake water-level reconstruction is not in line with the histories of ITCZ migration and strength of Benguela current upwelling. We confirm that the linkages between the evolution of the Etosha Pan and the climate mechanisms driving hydrological changes in subtropical southwest Africa are poorly resolved and need further investigation.

Geochemical profile of a layered outcrop in the Atacama analogue using laser‐induced breakdown spectroscopy: Implications for Curiosity investigations in Gale

AbstractWe performed laboratory laser‐induced breakdown spectroscopy (LIBS) and laser Raman spectroscopy measurements on samples from a layered outcrop from the Atacama Desert, Chile. This outcrop is a terrestrial morphological and possibly mineralogical analogue for similar formations that will likely be investigated by the Curiosity rover at Gale Crater. Our results demonstrate that fast LIBS analysis can generate semiquantitative chemical profiles in subminute times using automated data processing tools. Therefore, the LIBS instrument can be an invaluable tactical tool on the Curiosity rover for remote, rapid geochemical survey of layered outcrops, thus serving daily operational needs. The derived chemical profiles, supported by the range of minerals identified by Raman spectroscopy, is consistent with the products of a continental evaporitic lake. In the framework of future surface exploration on Mars, a combined Raman/LIBS investigation may provide a rapid mineralogical/chemical evaluation of targets that can be useful for selecting samples to be eventually collected for sample return purposes or for selecting sample sites to be drilled in the search for astrobiology‐relevant species.

Oligocene to Pliocene palaeogeographic evolution of the Çankırı-Çorum Basin, central Anatolia, Turkey

The Çankırı-Çorum Basin is one of the largest basins of Central Anatolia to have developed Oligocene to Quaternary terrestrial environments. The basin contains significant raw materials for industry and energy, but the determination of their distribution patterns requires a good knowledge of the stratigraphic and spatial distribution of deposits. Besides, evolving tectonic regimes in the basin and the sedimentary sequences controlled by them can play key roles in revealing the regional paleogeography. On the basis of a new chronological framework provided by mammalian fossils and sedimentological data, ten sedimentary formations have been identified for the Oligocene–Pleistocene time interval. From the oldest to the youngest these are the İncik, Güvendik, Kızılırmak, Kılçak, Kumartaş, Hançili, Bayındır, Akkaşdağı, Bozkır and Değim formations. During Early Oligocene times, an alluvial fan and braided and meandering rivers deposits developed at the northern and eastern rims of the basin, whereas lacustrine products dominated by evaporitic deposits occurred in vast areas at the center. In Late Oligocene, the lake area contracted and dominant floodplains and braided rivers took place in the central area of the basin. During Early–Middle Miocene, large lacustrine areas dominated by organic/clastic deposits developed in the northern and middle parts of the basin; lake shore facies were formed in northern parts of the basin and on some paleohighs surrounding the lake. In the Orta and Şabanözü regions volcanic activity started during the Middle Miocene and lasted until the beginning of Late Miocene. During Late Miocene, there were different types of lacustrine environments in the northern and southern parts of the basin: a hypersaline lake characterized by evaporitic deposits in the north and carbonate-rich shallow lakes and swamps in the south. A younger evaporitic lake formed in Early Pliocene in the northern part of the basin and covered vast areas. Its deposits are as thick as 400 m. From Late Pliocene, the thick sedimentary sequences deposited during the Oligocene–Early Pliocene interval become fragmented and deformed by a tectonic wedge bounded by a normal fault in the west and a thrust in the east. Sedimentary products of this tectonic regime are represented by thick alluvial fan sequences of the Değim Formation.

Evidence of potash salt formation in the Pliocene Sedom Lagoon (Dead Sea Rift, Israel)

The Dead Sea is a Ca-chloride evaporitic lake close to halite saturation. Carnallite (KMgCl 3·6H 2O) is the next mineral predicted to precipitate and is currently produced in solar ponds on the shores in Israel and Jordan. The particular composition of the Dead Sea has been attributed to the contribution of Pliocene residual brines formed during the deposition of the Sedom formation that currently reappear as hydrothermal flows. The chemical composition of these returning flows suggests that Sedom brines reached the potash-forming (sylvite and/or carnallite) stage. The Sedom formation is a thick (more than 2000 m) evaporite succession made up of bedded salt rock accumulated during the Pliocene when an arm of the Mediterranean sea seeped through the Dead Sea Rift forming the Sedom Lagoon. The present study is focused on the upper part of the Sedom formation (Mearat Sedom member) where detailed petrology, bromine contents in salt, Cryo-SEM-EDS microanalysis of fluid inclusions in halite, and isotopic compositions ( δ 34S and δ 18O in sulfates, and δ 37Cl in salt) were investigated. Although halite is the main evaporite mineral, minor sylvite and carnallite occurrences in the form of early diagenetic nodules, cements and/or authigenic crystals are commonly present. The geochemical modeling of the primary fluid inclusion composition in halite points to the evaporation of seawater or mixtures of seawater and continental water, with a very minor (< 1.5%) proportion of CaCl 2-rich and MgCl 2-rich brines similar to those discharging into the present-day Dead Sea. This suggests that the refluxing of residual brines was already active in Sedom times. A cyclic evolution of the basinal brines can be deduced from the composition of fluid inclusions together with the bromine contents in halite and the sulfate isotopes. Thus, stages of dilution involving fresh marine and/or continental waters alternated with stages of high concentration when the saturation of the lagoon brine with respect to the potash salts was reached. Potash minerals could crystallize on the shores or in the marginal zones of the lagoon and could be dissolved and recycled during the subsequent dilution stages.

A rare natural gypsum ooide (Gypsolites) in an evaporitic playa lake of late miocene (?) to pliocene age in central Anatlia, Turkey

Gyposolites are found in the western part of Polatli town, Polatli-Sivrihisar basin, about 90 km from Ankara, Turkey. Gypsolites are observed in a transition zone between a siliciclastic marginal lake and an evaporitic inner lake. The ooide-bearing strata have a small to moderate thickness (10 to 50 cm), are laterally restricted, and partially exhibit a sandy appearance, including various amounts of volcanoclastic grain and mud pellets. They sometimes show micro-gradations, parallel or cross-laminations and rippled surfaces, in which macroscopically there are evidences of biogenic activity such as fossil tracks and burrows, but microbial activity is intensively present within the gypsum cortex, yielding micro-moldic porosity. Gypsolites fabrics suggest either (I) they occur under bacterial activity in local salines or short-lived ponds that occupied the low-energy part of the lake or (ii) weak currents or wind activity exert a significant influence on the gypsolites fabric. Some traces of microbial mats within the cortex and micro-gradations, parallel or cross-laminations indicate that the concomitance of bacterial activity and inorganic processes contributed to the construction of gypsum ooide structures in the Neogene evaporitic lake environment.

The Ninyerola Gypsum unit: an example of cyclic, lacustrine sedimentation (Middle Miocene, E Spain)

The Ninyerola Gypsum unit is a Middle Miocene lacustrine deposit, close to 200 m in thickness, which crops out mainly along an anticline structure trending NNE-SSW near the city of Valencia. This anticline structure is comprised of Keuper (Upper Triassic) diapiric materials in the core and a Miocene succession on its fl anks. The Ninyerola Gypsum unit appears at the base of this succession and is overlain by alluvial deposits. The latter deposits grade upwards to terrigenous, coastal marine sediments presumably of Tortonian age. The anticline fl anks of the Miocene succession differ markedly not only in terms of their depositional and diagenetic facies but also in thickness and cyclicity. These differences suggest that synsedimentary deformation of the Triassic basement exerted control on Miocene sedimentation. On the eastern fl ank, where the succession is thicker (>230 m) and well exposed, we distinguished the following stratigraphic intervals from base to top: nodular gypsum (a), bioturbated gypsum (b), alternation of laminated gypsum and carbonate (c), a calcareous interval (d), red lutites and bioturbated gypsum (e), and a clastic alluvial alternation (f); this assemblage unconformably underlies a clastic, coastal marine interval (g). The Ninyerola Gypsum unit constitutes the lacustrine sediments −intervals (a) to (e)− of the succession. In this unit, numerous carbonate-gypsum cycles are recorded in intervals (a) to (c), whose average thickness approaches 6 m. The lacustrine stages of this succession were interpreted as the result of the gradual development of a saline lake of the sulphatecarbonate type overlying the Triassic materials, which extruded during the Lower Miocene. Interal (c) represents the maximum extension, depth and subsidence of this saline lake. This deep lake stage (c) was preceded by a shallow lake stage (b), which initially experienced anhydritic sabkha conditions (a). Stage (c) was followed by a diluted lake stage (d) and a fi nal evaporitic lake stage (e). Subsequently, an input of alluvial clastic materials (f) and marine transgression (g) brought an end to the evaporative conditions in the area.

Sedimentology of the Miocene evaporitic succession in the north of Çankiri-Çorum basin, central Anatolia, Turkey

The upper Miocene non-marine sediments of the Cankiri-Corum basin in central Anatolia, have both evaporitic and non-evaporitic successions. These sediments were deposited in an evaporitic lake which had temporary episodes of palustrine conditions in response to seasonal or climatic changes. The successions show different facies such as sulfates, carbonates and siliciclastics. The sulfates comprise primary, reworked and diagenetic gypsum. The primary deposits are predominantly laminated gypsum, bedded gypsum and selenite. The reworked (detrital) gypsum comprises gypsite, gypsarenite, gypsrudite and breccias. The diagenetic type comprises micro- and macrogypsum nodules. The carbonates mainly include clayey limestone, oolitic limestone and dolomite. The siliciclastics comprise red beds and both channel and non-channel, conglomerates and mudstones. Laminated gypsum, composed of alternating gypsum and dolomite, was a result of environmental schizohalinity. Bedded gypsum was precipitated in the deeper part of the lake during high evaporation periods. Chevron-type selenite crystals formed on saline mud flats during the times of aridity, whereas the discoidal-type seen in the organic-rich mudstones occurred in the gypsiferous marshes during the times of humidity. Reworked (detrital) gypsum dominates the lake margin. These formed during periodic wet episodes that caused reworking of primary gypsum. Gypsum nodules occurred as both early and late diagenetic products. Carbonates and siliciclastics were deposited during the freshening periods of the lake. Climatic or seasonal changes were the main causes of the depositional styles of the upper Miocene evaporitic and non-evaporitic lacustrine deposits in Cankiri-Corum basin. Additionally, the transition upward from alluvial to lake environment implies an important change in drainage patterns that likely occurred as a result of marginal fault activity.

Nodular anhydrite growth controlled by pedogenic structures in evaporite lake formations

Several nodular gypsum beds exhibiting a remarkable vertical arrangement of individual nodules are present in the Miocene continental evaporite formations of the Calatayud Basin, northeastern Spain. The growth of the gypsum nodules, initially anhydrite, took place within magnesitic carbonate deposits which display incipient pedogenic features such as rhizoliths and clotted to peloidal textures. Simultaneous and after the pedogenic modification of the carbonate substratum, displacing nodules started to grow, their arrangement being closely conditioned by the development of vertical fissuring related to root penetration. The transformation of anhydrite to gypsum was probably realized early after little burial of the sulphate–carbonate deposits. The occurrence of vertically oriented gypsum nodules can be seen as a diagnostic feature for palustrine conditions developed in evaporitic lake settings, this peculiar morphology of the sulphate nodules having been strongly controlled by the internal structure of the palustrine palaeosols.

Fresh-water carbonates from the Lower Freshwater Molasse (Oligocene, western Switzerland): sedimentology and stable isotopes

The Lower Freshwater Molasse of western Switzerland comprises a thick series of continental deposits of Late Oligocene-Early Miocene age. Fresh-water carbonates occur at two main horizons within this succession. Lower Chattian “palustrine” carbonates (Calcaires Inférieurs) show a shallow fresh-water fauna/flora of ostracods and charophytes, but display clear evidence of subaerial exposure in the form of glaebule/pisoid formation, circumgranular cracking, rhizoliths and microkarstic cavities. Repeated desiccation led to locally intense modification by pedogenesis. These horizons record the development of shallow, ephemeral lakes at the northern margin of the Swiss Molasse Basin. Stable isotope analyses were carried out on 46 calcite samples. Data from matrix and intraclasts reveal typical fresh-water compositions ( δ 13C = −5 to −7.5‰; δ 18O = −5.5 to −9‰). A weak co-variant trend suggests deposition in hydrologically closed lakes, although the range of δ 13C values recorded is also consistent with pedogenetic overprinting associated with subaerial exposure. Spar cements and vadose crystal silts display δ 13C compositions from −6 to −7‰, but show light δ 18O values (from −10 to −11‰), recording formation from meteoric waters. Upper Chattian carbonates (Calcaires d'eau douce et dolomie) include algal-ostracodal laminites, interpreted as profundal lake deposits, as well as bioturbated shallow lake charophyte-ostracod mudstones and wackestones, sandy wackestones and white dolomite. Stable isotope analyses (20 calcites, 2 dolomites) show δ 13C values from −2 to −6‰, and δ 18O from −6 to -11‰. Data for laminites and biomicrites plot in different fields; variations in carbon isotopic composition may be attributable to higher organic productivity during high lake stands (the laminites show heavier δ 13C values) or to areal differences in the proportions of biogenic, authigenic and detrital carbonate ( δ 13C values are generally higher in areas of greater detrital carbonate supply). The heavier δ 18O values shown by the dolomites ( δ 13 C = −3‰ = δ 18 O = 0 to −1‰) suggest formation from evolved, evaporitic lake waters. These carbonates record deposition in an extensive, possibly deeper lake complex subject to clastic input from the Alps in the south.

The geological setting of tourmalinite at Rum Jungle, N.T., Australia ? genetic and economic implications

Tourmalinite is a common rock type associated with Proterozoic strata-bound mineral deposits. Although common, it is often difficult to recognise in the field, leading to misidentification. It occurs as a conformable banded quartz-tourmaline lithological unit comprising at least 15% and as much as 50% of the rock. At Rum Jungle, tourmalinite occurs within the oldest sediments (arenites and magnesites) as distinct lenses, as facies equivalents of quartz-magnetite units and mafic schists (tuffs?) and distal equivalents of polymetallic sulfides. Distinct layering, slump folding, rip-up clasts and the association with diagenetic pyrite suggest a sedimentary environment. Enechelon fracturing of the fine-grained, light green tourmaline crystals spectacularly supports pre-deformation formation. The crystals are optically and chemically zoned parallel to the c axis, with irregular growth lamellae width — which supports a pre-regional metamorphic origin. Analyses show the tourmaline to be the Mg-rich variety “dravite”. Most tourmalinites are interpreted as subaqueous marine deposits. It is more likely that they form in lacustrine, shallow water, evaporitic environments, particularly continental rifts. Suitable B-bearing fluids can be generated by hotspring activity and mobilized by CO2-rich fluids. Association with chemical sediments suggests tourmalinites also have a chemical sediment precursor. Ample evidence at Rum Jungle supports the notion of a continental rift environment, which was the site of deposition of fluvial arenites and alkaline, evaporitic lake sediments. Localised hot-spring activity contributed B-bearing fluids which precipitated chemical sediments according to the pertaining pH, temperature etc. Diagenetic alteration produced the tourmalinite now present. These tourmalinites are comparable to those of similar age elsewhere e.g. Sullivan, Broken Hill. They can be genetically modelled upon Recent borate concentrations, all of which occur in continental rift environments.

A Terra Rossa‐like paleosol complex from the Upper Jurassic of Portugal

ABSTRACTThe basal Upper Jurassic unconformity in the Lusitanian Basin of Portugal commonly exhibits an irregular palaeokarstic surface developed in underlying Middle Jurassic carbonates. In the Serra da Arrabida area south of Lisbon, wire cut quarry faces expose a thick Upper Jurassic paleosol complex overlying fissured and brecciated limestone veneered with a calcrete crust and associated with colluvial deposits. The paleosol complex is a red mudstone with calcrete stringers which superficially resembles present‐day red Mediterranean Terra Rossa soils. A detailed micromorphological study indicates the absence of any clay illuviation in the paleosol unit, which suggests that it is not comparable to the true Terra Rossa Alfisols, but more closely resembles present‐day Aridisols. This difference from true Terra Rossa soils probably reflects formation under a drier climate, which is confirmed by the occurrence elsewhere in Portugal of evaporitic lake deposits of the same age. The paper stresses the role of soil petrography (micromorphology) in interpreting pedogenesis in paleosols.