Gypsum Lithofacies Research Articles

Origin and Paleoenvironmental Conditions of the Köprüağzı Evaporites (Eastern Anatolia, Turkey): Sedimentological, Mineralogical and Geochemical Constraints

Köprüağzı evaporites mainly consist of primary (selenite and gypsarenite) and secondary (massive, laminated and satin-spar) gypsum lithofacies, as well as minor anhydrite, and are interbedded with clastic and carbonate units. Sedimentological-mineralogical and geochemical findings reveal that the depositional basin extends from a lagoon to the hinterland. These data confirm that too much detrital input was transported into the basin with multiple salinity and pH value fluctuations, organic matter activity and reducing conditions. Geochemical data point out the shallow environment and the mixing of hydrothermal fluids and fresh waters. The investigated evaporites were exposed to diagenesis and alteration under the influence of a hot—dry and minor humid climate, tectonism and pressure. The 87Sr/86Sr, δ34S and δ18O isotope values of Köprüağzı evaporites reveal that they are of Miocene marine sulfate origin.

Preliminary Mineralogical Characterization of Branching Selenite Gypsum: New Insights for the Paleoenvironmental Reconstruction and Mechanical Characterization

The present study proposes a characterization of textural and mineralogical features of branching selenite gypsum lithofacies from the Monferrato area (NW Italy). This facies is considered to have appeared during the sequence of the primary lower gypsum of the Messinian Salinity Crisis starting from the sixth stratigraphical cycle, providing a useful tool for stratigraphic correlation throughout the Mediterranean. It is often associated with thick and continuous beds that are exploited by open-pit and underground quarries. We provide the results of a characterization of the non-gypsum minerals that represent approximately 10% in weight of the rock. Mineralogical data were collected with energy-dispersive X-ray spectroscopy (EDS) and X-ray powder diffraction (XRPD) techniques. In addition, a detailed characterization of clay families was performed through dedicated XRPD analyses. Results highlight the presence of detrital minerals (quartz, feldspars and clay minerals), calcite, dolomite and epsomite. Clay analysis registered the existence of five different families (illite, chlorite, smectite and two mixed layers). These mineralogical data suggest that the origin of this lithofacies is connected to both depositional processes and in situ precipitation processes under different conditions of saturation of the brine (respectively undersaturated and supersaturated in sulfates). The mineralogical and textural heterogeneity also represents an important element in controlling the mechanical response of the material and the consequent stability of quarry sites.

Sequence Stratigraphy of the Fatha Formation in Shaqlawa Area, Northern Iraq

A surface section of the Fatha Formation (Middle Miocene) was studied in the Shaqlawa area, Erbil, Northern Iraq. It consists of siliciclastic silt, evaporates, and carbonates in a mixed siliciclastic silt composition. The Fatha Formation in the study area can be divided into two members of variable thickness based on rocky differences. Depositional settings ranged from shallow open-marine and restricted-hypersaline to supratidal and continental (sabkha, fluvio-deltaic, and exposure). It is bounded below by a type one sequence boundary above the Eocene Pila Spi Formation and marked by conglomerates. The upper sequence boundary with the Injana Formation is conformable. Thirteen sedimentary facies were distinguished in the Fatha Formation within the Shaqlawa region of northern Iraq and include sandstone to mudstone, wavy bedded sandstone to mudstone, Flaser bedded sandstone to mudstone, Marl, sandstone, cross lamination sandstone, Trough cross bedded sandstone, Planar cross bedded sandstone, marly limestone lithofacies, bioclastic grainstone to packstone microfacies, bioclastic lime mudstone to wackestone microfacies, lime mudstone-wackestone microfacies, and gypsum lithofacies. The depositional environment of the formation was inferred based on the facies association concepts. The succession formation can be divided into several third-order cycles, which reflect fluctuations in the relative sea-level rise. High-frequency cycles of transgressive System Tract and Highstand System tract. Fundamental to the evolution of the sequence, in this case, is the local tectonic component.

Sedimentological and mineralogical-petrographic characteristics of Miocene evaporitic deposits (SW Erzincan)

This study aimed to find the depositional environment and formation conditions of the gypsum-dominated Miocene succession alternated and intercalated with clastics and carbonates in the southwestern part of the Erzincan Basin. As a result of sedimentological, mineralogical-petrographic investigations, it was determined that the gypsum in this sequence was formed as primary and secondary. Primary lithofacies; nodular anhydrite, selenitic, discoidal, gypsum arenitic secondary lithofacies; It is divided into massive, laminated, banded, nodular, brecciated, satin-spar. The facies repetitions and folds, fractures, cracks and faulting in the sequence showed the effect of tectonism with the water level fluctuations in the basin. In addition, these investigations revealed sedimentary structures such as cross-bedding, ripple undulations, chicken-wire, enterolithic and bacterial-algal structures, parallel laminations in gypsum lithofacies. In addition to these, it was understood that gypsum lithofacies were exposed to diagenetic processes under the influence of a hot-humid and semi-arid climate, organic matter activity, meteoric/underground-water and hydrothermal solution factors, and were subjected to high temperature, pressure, salinity and pH conditions.

Gypsum lithofacies and stratigraphic architecture of a Messinian marginal basin (Piedmont Basin, NW Italy)

During the Messinian salinity crisis (5.97–5.33 Ma) the Mediterranean was transformed into the youngest salt giant in Earth history. In marginal Mediterranean subbasins, the first stage of the crisis is recorded by up to 16 lithological cycles made up of shale/gypsum couplets (Primary Lower Gypsum unit, PLG), making a downslope transition to organic-rich shales. Gypsum comprises a wide array of lithofacies, including bottom-grown gypsum, cumulate deposits (i.e., resulting from gypsum nucleation in the water column and subsequent deposition on the seafloor) and the branching selenite. The latter appeared synchronously in many sections of the Mediterranean in the 6th PLG cycle (i.e., at about 5.87 Ma). To investigate the still enigmatic stratigraphic architecture of the lateral lithofacies transition from the shallower basin margin to the depocenter and the paleoenvironmental modifications driving the development of the branching selenite lithofacies, we carried out sedimentological, petrographic and geochemical analyses of the different types of gypsum lithofacies exposed along a margin-to-depocenter transect in the Piedmont Basin (NW Italy), corresponding to the northernmost Mediterranean subbasin. The results of this study suggest that the lateral and vertical distribution of gypsum lithofacies mostly reflects the position of an oxycline with respect to the seafloor. Bottom grown gypsum (cycles PLG1–PLG5) was deposited on a temporarily oxygenated seafloor above the oxycline in the marginal part of the basin. In the case of cumulate deposits (laminar gypsum and gypsiferous mudstones, cycles PLG6–PLG15), gypsum nucleated within the water column at the oxycline and was eventually deposited on an anoxic seafloor. The branching selenite lithofacies corresponds to a syngenetic product formed below the seafloor at the expenses of cumulate gypsum. Deposition of cumulate gypsum and formation of the branching selenite were controlled by the balance between the rate of sulfate reduction in bottom waters and nucleation of gypsum in the water column. The appearance of cumulate gypsum and the branching selenite from the 6th PLG cycle upwards reflects the intensification of water column stratification in response to increased isolation of the basin.

Geochemical characteristics of gypsum lithofacies in northeastern of Muş (Eastern Anatolia-Turkey): an indication of the Neotethys closure

The study investigates the origin, age, paleo-depositional environment and formation conditions of gypsum lithofacies deposited in northeastern Mus in Eastern Anatolia using element analysis and sulfur, oxygen and strontium isotope combinations. The correlation diagrams of major ions and trace elements plots in the gypsum lithofacies indicate that carbonates and clastics carried by terrestrial waters (streams, rivers, etc.) significantly contributed to the evaporitic phases. Furthermore, the effects of hydrothermal solutions, increased salinity of the basin and presence of biological activity were associated with high values for major ions and trace elements. While the δ18O and δ34S isotope contents of some gypsum lithofacies samples show that they are of terrestrial or marine origin, others do not indicate either origin because they are affected by other factors such as water salinity variations, bacterial sulfate reduction, new terrestrial water and detrital inputs carried into the basin. However, 87Sr/86Sr isotope concentrations analyzed in gypsum lithofacies in the study area indicate Rupelian (Lower Oligocene) time, which is compatible with the Cenozoic age curve. Based on all these data, gypsum lithofacies in this basin were deposited in a transition zone (shallow sea-lagoon-inland sabkha-mudflats) under the influence of both marine and terrestrial conditions. Therefore, we claim that this basin formed during the Rupelian (Early Oligocene), at a time when the sea connection between the Indian Ocean and the eastern Mediterranean was restricted; that is, the southern branch of the Neotethys was extremely shallow during this period and almost terminated in the study area.

Stratigraphic Analysis of Gercus Formation in Dohuk area, Northern Iraq

A surface section of the Gercus Formation (Middle-Late Eocene) was studied in Berafat area, Dohuk Governorate, Northern Iraq. The Gercus Formation consists of a mixed siliciclastic sediments, evaporates and carbonate sequences in the studied region, predominantly in the upper and middle parts. Nevertheless, it usually consists of upward-fining carbonate-rich sandstone cyclothems, marl, conglomerate and siltstone along with a gypsum lens and thin micrite carbonate beds. The Gercus Formation was deposited in delta and delta front of occasionally depositional environment which is represented by red-brown claystone and reddish-brown mudstone lithofacies. Cross bedded pebbly sandstone, trough cross-bedded sandstone and laminated cross-bedded sandstone lithofacies are deposited in braided delta environment. Marl lithofacies and gypsum lithofacies are deposited in intertidal and supratidal environments. The sea level fluctuation caused the river base level to occasionally rise and fall. In addition, the process changed from erosion to deposition, while the grain size also changed at different environments from gravel to sand and clays. The Gercus succession of northern Iraq was developed during the Middle-Late Eocene in an active margin basin, where the last stage closure of the New-Tethys and its collision with the Eurasian plate took place between the northeastern Arabian plate. It caused major episodes of uplifts and subsidence along with base level variations due to eustatic ups and downs. Within Gercus Formation, several fourth order cycles can be recognized, reflecting generally asymmetrical cycles, as well as the difference between sediment supply and accommodation space.

The paleodepositional environment, diagenetic and depositional conditions of the Middle-Late Miocene Koluz gypsum member (NE Van, Eastern Turkey)

This study investigates the depositional setting and diagenetic processes of gypsiferous sediments known as the Koluz gypsum member located in the northeast of Van (Eastern Anatolia-Turkey). The commencement of still-active contractional regime due to the collision of Eurasian and Arabian plates gave way to uplift events on the active Eurasian margin. This uplift initially resulted in termination of marine sedimentation and commencement of continental deposition in the region. The Middle-Late Miocene Kurtdeligi formation is one of the oldest products of the continental setting in the Eastern Anatolia. Sedimentologic and petrographic analyses conducted on the Koluz gypsum member of the formation, the main concern of this study, indicate that the member is represented by secondary gypsum (massive, laminated, nodular, satin spar) and primary gypsum lithofacies (selenite, gypsum arenite, discoidal and radial). The secondary gypsum facies are formed after hydration of anhydrite and anhydritizated primary gypsums. Additionally, mineralogical and geochemical studies suggest that carbonate (dolomite and calcite) and detrital materials found within the member formed during the deposition or late diagenetic processes. The high major and trace element values determined by XRF analyses also indicate intense detrital influx and accordingly terrestrial origin for the basin. Sedimentary textures and lithofacies of the member show a typical ephemeral playa complex, which was likely fed by both evaporation and groundwater discharge. Tectonism, climate, salinity variation, biologic activity and diagenetic and hydrothermal fluids play major roles in the formation and transformation of these sediments to each other.

Mineralogical-petrographical study of evaporites from Mali Kukor, Vranjkovići and Slane Stine quarry (Upper Permian evaporites from Dalmatia, Croatia)

The evaporite deposits examined in this study are located in the central part of middle Dalmatia, Croatia. In this region, Upper Permian evaporite sediments were deposited under favourable conditions onto the Variscan basement around the northern margins of Gondwana. These sediments can be subdivided into three members, a lower evaporite unit (an anhydrite member), a middle evaporite unit (a gypsum member), and an upper unit (a clastic member), and are mainly comprised of secondary gypsum that formed via the hydration of precursor anhydrite rocks. The middle evaporite unit comprises beds of gypsum as well as early diagenetic dolomites that contain gypsum sequences, extending up to 60 m maximum thickness, and overlying clastic sequences that themselves are up to 20 m thick. These Upper Permian evaporite sediments contain horizontal, irregular, gypsum lithofacies that exhibit pronounced enterolithic and boudinage structures. The characteristics of these sediments are indicative of deposition in supratidal and sabkha settings (i.e., early diagenetic dolomites and evaporites) within a shallow epicontinental marine environment with highly varied coastlines, bays, and lagoons. The secondary gypsum seen within this Upper Permian middle evaporite unit displays alabastrine and porphyroblastic secondary textures and includes corroded anhydrite relics; associated minerals include muscovite, chlorite, potassium (K)-feldspar, quartz, and amphibole. The Upper Permian evaporite sediments discussed in this study are composed of irregular, locally brecciated secondary gypsum that probably formed as a result of multiple synsedimentary collapse of pre-existing soluble mineralsand/or synsedimentary and post-sedimentary tectonics.

Stratigraphy, mineralogy and depositional environment of the evaporite unit in the Aşkale (Erzurum) sub-basin, Eastern Anatolia (Turkey)

The study area is situated in the Aşkale sub-basin where the Early-Middle Miocene aged Aşkale Formation was deposited in a shallow marine to lagoonal environment, and consists of interstratifications of clastic sediments, carbonates and evaporites. The successions of the Aşkale Formation can be divided into four main members interfingering with one another both vertically and laterally, and composed of the sandstone-mudstone-limestone member, the evaporite member, the gravelstone-sandstone-mudstone intercalations and the limestone member. The evaporite unit comprises of secondary gypsum lithofacies formed by hydration of precursor anhydrite, anhydrite, gypsum-bearing limestone and claystone in the form of wedges and lenses. Massive, nodular, nodular-banded, laminated and laminated-banded gpysum lithofacieses in addition to chicken-wire and rare entrolithic structures were described, indicating a sabhka or a shallow water depositional environment. Alabastrine and porphyblastic textures of gypsum were identified within the all lithofacieses with abundant amount of anhydrite relics. Additionally, saponite and illite/smectite, calcite and dolomite, celestite, epsomite were also observed. Successions of the Aşkale Formation were deposited in stable subtropical climatic conditions within rapidly subsiding sub-basin resulted in conversion of sub-basin to shallow platform and even in lagoon environment.

Coastal salina evaporites of the Triassic-Liassic boundary in the Iberian Peninsula: the Alacón borehole

The evaporite unit (the Lecera Formation), which was formed at the Triassic–Liassic boundary in the Aragonian Branch of the Iberian Chain, was studied at the 01 Alacon borehole (Alacon village, Teruel province), where it is mainly constituted by a thick (>300 m) succession of predominant sulphates (anhydrite, secondary gypsum and carbonate) overlain by brecciated carbonates. In the evaporite succession, a number of lithofacies were recognized, which can be grouped into an “ideal cycle”, from base to top: (C1) massive to banded carbonate mudstone, (C2) alternation of carbonate and anhydrite laminae, (A1) alternation of anhydrite and carbonate bands, (A2) clastic intercalations in the alternation of anhydrite and carbonate bands, (A3) laminated to banded anhydrite, (A4) bedded pseudomorphs, (A5) interstitial pseudomorphs, and (A6) massive to nodular anhydrite. Fine-grained gypsum (anhydrite laminae and bands), bedded selenitic gypsum (bedded pseudomorphs), interstitial selenitic gypsum (interstitial pseudomorphs), and graded-nodular anhydrite (a particular fabric of nodular anhydrite) were the most outstanding depositional lithofacies. The evaporite succession exhibits a marked cyclicity: in the lower part the individual cycles are more similar to the ideal cycle and reflect deeper water settings, whereas in the upper part they correspond to shallower water settings. The evaporite sedimentation mainly occurred in a subsiding coastal basin of the salina or lagoon type. In this setting, the subaqueous precipitation of the carbonate and gypsum lithofacies was followed, in each cycle, by the interstitial growth of anhydrite in exposed conditions. As a whole, the evaporite succession reflects an infilling process. The conversion into anhydrite of the selenitic gypsum -probably also of the rest of depositional gypsum lithofaciesstarted under synsedimentary conditions and followed during shallow to moderate burial diagenesis.

Sedimentology and paleoenvironmental evolution of Messinian evaporites in the Iskenderun–Hatay basin complex, Southern Turkey

Messinian evaporites, which resulted from the salinity crisis during the final closure of the Mediterranean Sea, are exposed in SE Turkey. These evaporites formed in two isolated sub-basins, Iskenderun–Arsuz (IA) and Hatay–Samandağ (HS), which belong to different depositional configurations and tectonic structures. The Neogene fill of these sub-basins consists of a thick sedimentary succession that started with Early Miocene terrestrial clastics, followed by reefs (Middle Miocene) and shallow water siliciclastics (Tortonian – Early to Late Miocene) and finally Messinian evaporates. These sub-basins accumulated in a diverse range of depositional environments from very shallow to deeper water. Evaporite facies in the IA sub-basin consist of sabkhas, saline lagoons and ponds. They are mainly represented by chemical deposits such as scattered gypsum nodules and balls, nodular bedded gypsum, laminated gypsum (Type-A) and selenites (Type-S1). Evaporites in the HS sub-basin mainly consist of detrital gypsum composed of gypsum laminae (Type-B, C), gypsum arenite–rudites and deeper water selenites (≤ 20 m), and resedimented selenites (Type-S2), which were deposited on a sulfate platform with a slope-basin transitional zone. Secondary gypsum with alabastrine and porphyroblastic textures as well as satin spar veins is commonly associated with the sabkha-type evaporites of the IA sub-basin. Deeper-water clastic evaporites of the HS sub-basin have generally remained as primary gypsum or have only been slightly affected by diagenetic alterations. The isotope values ( 87/86Sr; δ 18O SMOW; and δ 34S CDT) from the different kinds of gypsum lithofacies of the sub-basin are similar to those of the Messinian evaporites in other peri-Mediterranean basins, indicating an origin from marine water without external or basinal contributions. The Messinian evaporites examined in this paper are overlain by Early Pliocene (Zanclean) deposits composed of shallow- and deep-water siliciclastics and carbonates with local intercalations of Lago–Mare-type strata. Throughout the Messinian evaporitic stage, the IA sub-basin was mainly comprised of shallow water evaporites, while the HS sub-basin underwent deepening related to regional tectonics induced by the Dead Sea Fault during the construction of the Hatay Graben.

Plio-Pleistocene lacustrine evaporites of the Baza Basin (Betic Chain, SE Spain)

The Baza Basin (the eastern part of the Guadix–Baza Basin, in SE Spain) underwent a significant evaporitic sedimentation in non-marine settings during the Plio-Pleistocene. The largest of these settings developed in the central part of the basin as a shallow, saline lake system, where abundant gypsum deposits occur (Benamaurel and Galera Gypsum units). This paper studies these deposits from the cartographic, stratigraphic, sedimentologic, petrographic and isotopic points of view. All the gypsum has been preserved as primary enabling a good characterization of the environmental conditions of the original precipitates. Three interconnected subenvironments or zones –inner, intermediate and marginal– are differentiated in this saline lake system. In the inner zone (shallow to relatively deeper central lake), a regular alternation of gypsum and carbonate laminae accumulated suggesting a seasonal regime. Many of the gypsum laminae display size-gradation (both reverse and symmetrical) indicating free precipitation in a stable brine body. In the intermediate zone (mosaic of shallow lakes), carbonate (dolomitic) beds formed in association with dark lutite levels, gypsum beds, and gypsum nodules and micronodules. Moreover, anoxic conditions developed in this zone, which resulted in the formation of native sulphur occurrences of economic value. In the marginal zone, the evaporitic deposits are developed only locally (the Galera Gypsum unit, in particular). The various gypsum lithofacies in these marginal deposits suggest the presence of a number of shallow to exposed settings (saline mudflat, saline marsh and gypsiferous pond). The isotopic composition ( δ 34S, δ 18O) of the gypsum samples is consistent with a Triassic origin of the sulphate as a result of chemical recycling. These data also suggest that a similar recycling mechanism operated in the saline lake system, where the gypsum sediments from the margins were coevally recycled towards the inner part. Both the evaporitic mineralogy and the gypsum and carbonate lithofacies suggest that semiarid conditions prevailed in this basin during the accumulation of the evaporite units.

Depositional models of lacustrine evaporites in the SE margin of the Ebro Basin (Paleogene, NE Spain)

An important evaporitic sedimentation occurred during the Paleogene (Eocene to lower Oligocene) in the Barbera sector of the southeastern margin of the Tertiary Ebro Basin. This sedimentation took place in shallow lacustrine environments and was controlled by a number of factors: 1) the tectonic structuration of the margin; 2) the high calcium sulphate content in the meteoric waters coming from the marginal reliefs; 3) the semiarid climate; and 4) the development of large alluvial fans along the basin margin, which also conditioned the location of the saline lakes. The evaporites are currently composed of secondary gypsum in surface and anhydrite at depth. There are, however, vestiges of the local presence of sodium sulphates. The evaporite units, with individual thicknesses ranging between 50 and 100 m, are intercalated within various lithostratigraphic formations and exhibit a paleogeographical pattern. The units located closer to the basin margin are characterized by a massive gypsum lithofacies (originally, bioturbated gypsum) bearing chert, and also by meganodular gypsum locally (originally, meganodules of anhydrite) in association with red lutites and clastic intercalations (gypsarenites, sandstones and conglomerates). Chert, which is only linked to the thickest gypsum layers, seems to be an early diagenetic, lacustrine product. Cyclicity in these proximal units indicates the progressive development of low-salinity, lacustrine bodies on red mud flats. At the top of some cycles, exposure episodes commonly resulted in dissolution, erosion, and the formation of edaphic features. In contrast, the units located in a more distal position with regard to the basin margin are formed by an alternation of banded-nodular gypsum and laminated gypsum layers in association with grey lutites and few clastic intercalations. These distal units formed in saline lakes with a higher ionic concentration. Exposure episodes in these lakes resulted in the formation of synsedimentary anhydrite and sabkha cycles. In some of these units, however, outer rims characterized by a lithofacies association similar to that of the proximal units occur (nodular gypsum, massive gypsum and chert nodules).

The ‘Evaporiti di Monte Castello’ deposits of the Messinian Southern Apennines foreland basin (Irpinia–Daunia Mountains, Southern Italy): stratigraphic evolution and geological context

Abstract An evaporitic limestone–gypsum succession, belonging to the Evaporiti di Monte Castello Formation, is recognized in the late Messinian (Upper Miocene) of the Irpinia–Daunia Mountains in the Southern Apennines arc (Southern Italy). This unit is formed by diatomaceous marls, massive and laminated evaporitic limestones, and by primary and clastic gypsum. Detailed stratigraphical, sedimentological and strontium geochemistry data has permitted reconstruction of stratigraphic and facies relations of gypsum deposits, depositional environment and basin evolution. Genetically related gypsum lithofacies can be grouped into two facies associations. The autochthonous gypsum facies association consists of shallow water selenitic, acicular and laminated gypsum and is characterized by the absence of high-energy sedimentary structures. The redeposited clastic gypsum facies association consists of shallow- to deeper-water fine-grained laminated gypsum, gypsarenites, pebbly gypsarenites and gypsrudites, showing common features of resedimented deposits. Nodular structures occurring in the laminated gypsum lithofacies seem to be mostly related to late diagenetic processes. The sedimentary evolution during the evaporative phase was characterized by a gradual increase in salinity until gypsum precipitated; then the sedimentary conditions in the basin were characterized by almost homogenous salinity conditions, and influenced by events of gypsum reworking and resedimentation probably related to flooding episodes and local tectonic activity. The gypsum was deposited from mainly marine brines, based on their Sr isotopic compositions. This sedimentary series is an equivalent of the Lower Evaporites of other parts of the Mediterranean. The Messinian Monte Castello Formation evaporites represent an uncommon type of evaporitic succession, probably developed in a extensional setting in a basin located along the Apulian foreland ramp, in contrast with the northern Apennines and Sicilian basins (e.g. Vena del Gesso, and Caltanisetta basins), which are considered to be thrust-top basins of the Apennine–Maghrebian foreland basin system.

Microbial signatures in sabkha evaporite deposits of Chott el Gharsa (Tunisia) and their astrobiological implications

This study investigates the geomicrobiological potential of Upper Pleistocene evaporite deposits of the Chott el Gharsa, a wide continental sabkha in southern Tunisia. Organic and inorganic-derived biosignatures are mostly contained in microcrystalline, laminated gypsum lithofacies consisting of light/dark alternations of concordant laminae, which have precipitated from high salt concentrated waters. These biosignatures include mineralized microbial-interpreted morphologies, such as mucilage, rods, and microfibers, and dumbbell morphologies in the hollow cores of dolomite crystals that are associated with sulfates. Mineral products that are induced by microbial activity and their organic compounds lead to the formation of lenticular-shaped gypsum crystals, with a high length/width ratio, dolomite precipitation and formation of pyrite framboids. Morphological and structural aspects of these biosignatures, and their composition, in laminated, dolomite-rich sulfate deposits could be detected through microscopic investigations and micro-analyses performed by the instrumentation that is planned for ongoing Mars sample return missions.

Lithofacies and water-body record of Messinian evaporites in Nijar Basin, SE Spain

The depositional lithofacies and fabrics of Messinian evaporites have been studied to evaluate the palaeodepth of water body in Nijar Basin, SE Spain. The Nijar Messinian strata consist of three formations: Abad marls, Yesares evaporites, and Feos rocks. Gypsum deposits dominate the Yesares Formation and the basal Feos Formation. Six gypsum lithofacies have been identified including coarse twinned selenite, grass-like selenite, hemiradial to radial selenite, turbidite selenite in the Yesares Formation; and gypsum conglomerate, laminated gypsum, and hemiradial to radial selenite in the Feos Formation. The size of Yesares selenites decreases upward while carbonate content and trace-element abundance increase in each selenite bed and the whole formation, suggesting a shallowing-up sequence. These gypsum lithofacies were deposited in subaqueous environments commonly at 10–30 m and occasionally up to 100 m or deeper for the turbiditic selenites based on the interpretations of gypsum fabrics, marl intervals, basin configuration, and geochemistry. The sea level of Nijar Basin experienced two major drops with a magnitude of ∼ 40 m before and after the deposition of Yesares evaporites. However, the basin was never desiccated during the evaporite deposition and the change of sea level for the alternative deposition of gypsum beds and marl intervals was commonly around 50–80 m, probably resulting from the glacio-eustacy fluctuations. Feos gypsum reflects another evaporative event and is correlated to the “Upper Evaporite” while the Yesares selenite is correlated to the “Lower Evaporite” based on the interpretations of lithofacies, geochemistry, and sealevel change. As seawater in Nijar Basin was continuously supplied by the central Mediterranean basins and remained in subaqueous conditions during the Messinian salinity crisis, therefore, the desiccation theory needs to be reexamined.

Evaporite deposition and coeval volcanism in the Fortuna Basin (Neogene, Murcia, Spain)

The Neogene basins of the Eastern Betic Mountains were placed in a marginal position with respect to the deep Mediterranean basin during the Tortonian to the Messinian. They contain thick gypsum–anhydrite formations resulting from the Upper Miocene salinity crisis. The evaporitic sedimentation reached its maximum extension and stratigraphic complexity in the Fortuna basin, which belongs to this geological setting. Five stratigraphic units are distinguished in the Fortuna basin: (1) Lower Gypsum, (2) Chicamo Gypsum, (3) Chicamo diatomitic–evaporitic cycles, (4) Ribera Gypsum and (5) Rambla Salada Gypsum units. Some of these units occur locally, whereas others can be laterally correlated with those of the Archena-Mula, Guadalentín corridor and Lorca basins. Most of the gypsum units consist of nodular-laminated secondary gypsum, however, primary selenitic, laminated gypsarenites and fine-grained microlenticular burrowed gypsum lithofacies are also present. The evaporite units can also be characterized by the geochemical composition of the gypsum (the sulphur and oxygen of the sulphate and the strontium isotopes), which display a wide range of values. The extreme values indicate precipitation from Tertiary marine brines, or from meteoric waters that recycled Triassic sulphate anion and additional non-marine contributions from country rocks and hydrothermal solutions. The intermediate values are attributed to mixing between seawater and non-marine parent brines. The Fortuna basin contains several outcrops of volcanic to subvolcanic igneous lamproites (Cabezos Negros, Derramadores, El Tale). These lamproites, the so-called fortunites, crop out as individual domic volcanoes with lava flows (Cabezos Negros) or as intrusive subvertical dikes (Derramadores, El Tale). There is good evidence of several magma–sediment interactions that produced macro and microglobular peperitic lithofacies in the Fortuna basin. A peperite is produced when two liquids (a hot magma and a fluidized water-rich sediment) interact and no mixing is possible due to the sharp contrast of viscosity between them. This implies that the eruption was nearly coeval with the deposition of the host-sediment, that was water-rich. The latest evaporitic event (Rambla Salada Gypsum Unit) in the Fortuna basin took place coevally to the emplacement of the Cabezos Negros dome and related lavic-structures, and the intrusion of the Tale and Derramadores dikes. This paper illustrates the presence of peperitic lithofacies in the fortunites and discusses their implications in assigning an age to these evaporitic sediments, which in turn allows us to date the evaporitic units of the Fortuna–Lorca basins.

The celestite problem in gypsum Sr geochemistry: An evaluation of purifying methods of gypsiferous samples

Celestite (SrSO 4) is a very common mineral in gypsiferous rocks. In order to evaluate the possible influence of celestite occurrence in gypsum strontium analysis (Sr content), different fine-grained lithofacies with a high content of enclosed minerals (ranging from 5% to 35% of the bulk rock) were studied and a series of experiments with artificial reagents were carried out. The results showed that even very low concentrations (< 0.5%) of celestite can release enough Sr to produce significantly altered results. The recommended methodology to prevent celestite contamination in Sr content analysis of fine-grained gypsum lithofacies involves three basic steps: 1) celestite identification, 2) celestite elimination, and 3) geochemical analyses. We found that back-scattered electron microscopy is the best method to identify celestite over a wide range of celestite concentrations. In the case of celestite occurrence, microprobe analysis provided reliable results as the obtained Sr values corresponded to the gypsum lattice. If a bulk sample analysis is performed, celestite must be eliminated or partially separated from the pure gypsiferous sample in order to reduce contamination. For this, different methods were modified and tested, including a heavy liquid (dibromomethane) separation procedure. However, because none of these methods were completely satisfactory, a new method based on a HCl 2N leaching was developed and tested. Nevertheless, although the celestite content diminished considerably, it was not totally eliminated. In conclusion, the presence of celestite must always be taken into account when gypsiferous samples are analyzed for their Sr content, or their 87Sr / 86Sr ratio, in order to avoid seriously biased results and non-reliable geochemical and paleoenvironmental interpretations. In celestite-rich samples it is convenient to separate the celestite before carrying out the analysis.

Deep to shallow lacustrine evaporites in the Libros Gypsum (southern Teruel Basin, Miocene, NE Spain): an occurrence of pelletal gypsum rhythmites

ABSTRACT A number of non‐marine evaporite units composed of primary gypsum were deposited in saline lakes that developed in the southern Teruel Basin (NE Spain) during the Miocene. In the basin depocentre, a continuum of lacustrine evaporite lithofacies influenced by the activity of organisms is displayed. The Libros Gypsum was deposited in a deep lake, in which water stratification became unstable with progressive shoaling. Rhythmites, composed of laminae of pelletal gypsum and laminae of very fine lenticular gypsum crystals mixed with siliceous microorganisms, formed in addition to gypsum turbidites, intraformational gypsum breccias and slump structures. The pelletal laminae originated from the faecal activity of animals (crustaceans?) ingesting gypsum crystallites in the lake water during episodes of maximum evaporation, whereas the laminae of very fine lenticular gypsum mixed with microorganisms accumulated during episodes of relative dilution. In the wide marginal zones of the basin, the Libros Gypsum unit consists of massive to thin‐bedded bioturbated gypsum and thin‐bedded clotted gypsum, which formed in intermediate to very shallow (palustrine) water depths. The bioturbated gypsum lithofacies were produced by the action of diverse organisms, presumably worms and coleopterans, and chironomid larvae to a lesser extent; the massive lithofacies precipitated in very shallow water; and the thin‐bedded lithofacies formed in shallow to deeper settings. The thin‐bedded clotted gypsum is a relatively deep facies that may have diverse origins (e.g. bioturbation, compaction, disruption of soft sediments and early diagenesis). There is a well‐developed metre‐scale cyclicity in the marginal lake sequences, which is not observed in the inner lake deposits. This suggests a depth control in the various lacustrine subenvironments to record cyclic evaporitic processes. The isotopic composition of the gypsum indicates early sulphate‐reducing bacterial activity in the bottom of the lake and suggests that the sulphate was derived from the chemical recycling of Triassic evaporites of the country rocks.