Subaqueous Gypsum Research Articles

Revealing the mechanisms of soil gypsum formation in the Atacama Desert through triple oxygen and hydrogen isotopes of gypsum hydration water

The soils in the hyper-arid core of the Atacama Desert (Chile) harbor substantial quantities of soluble salts, including sulfates and nitrates. Gypsum (CaSO₄∙2H₂O) is a prevalent mineral in the Atacama region. It manifests as ∼10-cm-thick surface crusts exhibiting a spatial pattern resembling polygons, in mixtures with other minerals beneath the surface (<3 m deep) or growing in brine ponds of salars. We examine the triple oxygen and hydrogen stable isotopes (δ17O, δ18O and δD) of structurally-bonded gypsum hydration water (GHW) from Atacama gypsum deposits to investigate their formation mechanisms. We observed considerably positive Δ17O anomalies (Δ17O > 1 ‰) in GHW of samples containing substantial amounts of nitrate (generally soil horizons 0.5–2 m deep). This is interpreted as resulting from oxygen isotope exchange between soil nitrate (Δ17O > 15 ‰) and GHW (Δ17O ∼ 0 ‰), either occurring in-situ within the soils due to bacterial activity, or more likely, during cryogenic extraction of GHW at high temperature and low pH in the laboratory. However, when analyzing pedogenic gypsum in soils with negligible amounts of nitrates, the isotopic composition of its parent solution is preserved. These samples showed that gypsum formed from solutions that did not undergo significant evaporation. These waters are isotopically similar to contemporary unevaporated freshwater sources in the region. This indicates that gypsum formation likely occurred through the hydration of anhydrous or hemihydrate calcium sulfate, either in the atmosphere or in the soil. Subsequently, the gypsum was subject to recrystallization in the soil due to occasional rainfall events. This stands in contrast to subaqueous gypsum crystals growing in brine ponds of salars in the Atacama Desert, which reflect the isotopic composition of highly-evaporated parent fluids.

The Absolute Age and Origin of the Giant Gypsum Geode of Pulpí (Almería, SE Spain)

Subaqueous gypsum (CaSO4·2H2O) crystals are relatively common in epithermal systems where sulfide ore deposits are present. The Giant Geode of Pulpí (Almería, SE Spain) hosts some of the largest (up to 2 m in length) subaqueous gypsum crystals discovered to date. Here, we present the first U-series ages of its crystals and reconstruct the oxygen and hydrogen isotopic composition (δ18O and δ2H) of the Pulpí paleo-aquifer from which the crystals formed by using stable isotopes of gypsum hydration water. We successfully dated the onset of gypsum precipitation in the Geode at 164 ± 15 ka. However, the extremely low U concentration (11 ppb) and relatively high detrital Th content (230Th/232Th 3.2) hinder accurate dating other gypsum samples. The δ18O and δD values of the paleo-aquifer during the growth of the crystals aligned with the local meteoric water line, suggesting that the sulfate-enriched mother solution consisted of meteoric water that recharged the aquifer during that period. The mean isotopic composition of the Pulpí paleo-aquifer (δ18O = −6.5 ± 0.1‰ and δ2H = −42.3 ± 0.5‰) during the formation of the crystals was similar to the current groundwater in this area (δ18O = −6.1 ± 0.8‰, δ2H = −42 ± 6‰). The isotopic differences observed in samples collected from distinct locations and in individual crystals were probably related to changes in the isotopic composition of the aquifer, as a consequence of varying climate that impacted on the isotopic composition of rainwater during thousands of years in this region. Our results indicated that subaqueous selenite crystals may be useful for paleo-hydrological reconstructions. However, improving the current analytical techniques for dating gypsum with low U concentrations will be essential to obtain accurate and reliable records from Quaternary gypsum cave crystals in the future.

Gypsum salina–coral reef relationships during the Last Interglacial (Marine Isotopic Stage 5e) on the Egyptian Red Sea coast: a Quaternary analogue for Neogene marginal evaporites?

Abstract In the tectonically stable area of the Egyptian coast of the Red Sea, the uppermost marine deposits of the Last Interglacial (Eemian, 5e=5.5 Marine Isotopic Substage) consist of two contrasting sedimentary units: a subcontinuous fringing coral reef and, on its backward side, shelly and subaqueous gypsum deposited in erosive depressions elongated parallel to the shore. The entrenched geometry and the marine to brackish evolution of the pre-evaporite faunas indicate that during this very short period of time, a brief sea-level drop responsible for the limited erosion separated the succession of reef to marine gypsum and that the coastal area underwent several climatic variations. The marine ingression into drowned valleys (early MIS 5.51) developed khor environments (non-brackish lagoon on the Arabic arid shores) which were quickly replaced by highly restricted conditions leading to gypsum precipitation. Permanence of the salina waters and seasonal lamination interrupting crystal growth suggest an anchialine salina system (littoral haline lakes) fed by permanent seeping of sea-water, balanced by refluxing dense brines. A semi-arid climate is assumed, with seasonal rains allowing limited continental sulphate contribution (from Miocene gypsum outcrops). Both climate and sea-level changes were responsible for the complex evolution of this area. Compiled data enable generalization concerning the climatic oscillations during the Last Interglacial sedimentation. A model derived from these Red Sea Quaternary deposits, as an analogue for the marginal Miocene evaporite deposits in the Mediterranean and Red Sea, is discussed.

Sedimentology of Badenian (middle Miocene) gypsum in eastern Galicia, Podolia and Bukovina (West Ukraine)

ABSTRACTPrimary gypsum is the main evaporite mineral in the middle Miocene (Badenian) of the West Ukraine. The lower part of the gypsum sequence is built of autochthonous gypsum while the upper part is composed of allochthonous gypsum that formed following a major, tectonically induced, change in basin morphology. This change resulted in the destruction of the gypsum deposited on the margins of the basin and formation of redeposition features.Autochthonous gypsum facies were deposited in two main environments: (1) giant gypsum intergrowths precipitated from highly concentrated brines; (2) very shallow subaqueous gypsum deposited in a vast brine pan. The brine pan was characterized by a facies mosaic that reflects an interplay of concentrated brines from the central part of the evaporite basin and diluted brines due to the influx of continental meteoric waters. The facies continuum, microbial gypsum ‐ bedded selenite ‐ massive selenite ‐ sabre gypsum, indicates increasing salinity of the brine with time. This type of facies pattern has been established in recent salinas that are analogous to Badenian gypsum in their lateral facies changes. However, the pattern of facies distribution with respect to the open sea in the Badenian basin is opposite to that found in recent salinas.The pattern of the Badenian gypsum facies in the Ukraine indicates that facies repetition may have been related to climatically controlled salinity changes and not to depth changes, as is commonly used to explain the repetition of sulphate facies in a vertical succession.

Deep‐water resedimentation of anhydrite and gypsum deposits in the Middle Miocene (Belayim Formation) of the Red Sea, Egypt

ABSTRACTThe Middle Miocene evaporites in the Red Sea rift were deposited within a complex system of fault‐bounded basins that were episodically active during sedimentation. Such a tectonic framework is known to be highly favourable to resedimentation processes. An offshore petroleum well in the north‐western Red Sea has cored, below a massive salt unit, an anhydrite‐bearing succession which provides an excellent opportunity to study the processes of gravity induced redeposition of Ca‐sulphates in a deep basin. Anhydrite deposits, interbedded with siliciclastic layers and thin halite layers, are composed of resedimented facies ranging from fine‐grained laminated sediments to coarse‐grained breccias. The components derive from the reworking of shelf sediments deposited initially in shallow water to supratidal settings on the surface and edges of structural highs bordering depressions: proximal siliciclastic deposits with interstitial anhydrite (cement patches, nodules) or gypsum and dolostones with early diagenetic anhydrite facies (nodular, chicken‐wire) formed in sabkha conditions, interstitially grown gypsum crystals and subaqueous gypsum crusts precipitated in hypersaline ponds, and diatom‐rich oozes formed in marine, shallow‐water conditions. The homogeneity of the stable isotope composition and petrography of sulphates argue for the initial crystallization of Ca‐sulphates within brines of the same origin and in closely interconnected sedimentary settings. The unconsolidated sediments redeposited as slope‐foot accumulations were carried both as anhydrite (nodules, soft masses, various fragments, individual grains or crystals released by disintegration of large masses) and gypsum (crystalline aggregates or single crystals) later converted to anhydrite during burial. Layers of chaotic breccia are interpreted as the result of seismic events, whereas the fine‐grained deposits could be related to redistribution by nepheloid layers of suspensions of finer grains released by disintegration of the soft anhydrite masses during downslope transport, or of in situ deposits removed by the turbiditic flows.

New Interpretation of Vertically Aligned Gypsum Fabrics: Implications for Gypsum Depositional Environments and Diagenesis: ABSTRACT

Gypsum and anhydrite fabrics in trenches and deep 500-m cores from Bristol Dry Lake, California, exhibit a vertical alignment of crystals similar to the fabric seen in bottom-nucleated brine pond gypsum. However, geochemical and sedimentologic evidence indicates that the gypsum formed in Bristol Dry Lake precipitated as a diagenetic displacive mineral within the sediment where groundwater saturated with respect to gypsum recharges around the playa margin (groundwater seepage gypsum). Evidence for displacive growth of gypsum comes from (1) the geometry of the deposit, (2) stable isotopic data and the water chemistry of the brine, and (3) inclusions of matrix that follow twin planes and completely surround crystals as they grow. Because the fabrics and textures of this diagenetic gypsum formed by groundwater in playa settings are similar to those of primary gypsum formed in a brine pond, it is necessary to refine the criteria for the distinction between subaerial, groundwater, and subaqueous gypsum. When compared to the features observed in a Holocene subaqueous gypsum deposit (Marion Lake, Australia) and a Holocene subaerial deposit (Abu Dhabi sabkha), the following criteria can be used to distinguish between the three different types of gypsum deposits: (1) amount of matrix, (2) geometry of themore » deposit, (3) gypsum crystal size and orientation, (4) distribution of matrix and fluid inclusions within individual crystals, and (5) fabric of the matrix around the gypsum. Distinguishing between subaqueous and groundwater gypsum may be difficult. However, sufficient mesoscale differences exist such that, in most cases, the different gypsum types can be resolved from core slabs and thin sections. These criteria may be applied in ancient examples even after gypsum has been converted to anhydrite.« less

Dynamic stratigraphy of an evaporite-to-red bed sequence, Gipskeuper (Triassic), southwest German Basin

The dynamics of evaporite accumulation are studied in the Gipskeuper (Triassic) of the southwest German epicontinental basin based on a hierarchical, three-level stratigraphic analysis:(1)Among the various stratification types, the following are most important: (a) thin carbonate intervals with marine faunas; (b) massive sulfates with selenite, indicating very shallow subaqueous gypsum precipitation; (c) thin-bedded gypsarenites with a variety of sedimentary structures suggesting clastic evaporite accumulation during high-energy events (“gypsum tempestites”); (d) laminated gypsum with spectacular tepee structures, testifying inter- to supratidal settings; (e) green-grey and reddish claystones with nodular gypsum indicating evaporitic playa mudflats.(2)Facies sequences occur in several varieties and commonly form 1–4 m thick transgressive/regressive cycles with shallowing-upwards trends. The long recognized carbonate “marker beds” represent short-term trans- or ingressions into the almost flat Keuper Basin. Highly disturbed tepee horizons form widely correlatable regressive peaks. These sequences form evaporite equivalents to the ubiquitous shallowing-upward cycles known from carbonate systems.(3)The basin-fill consists of a small-scale cycles being vertically stacked upon each other to form the basic building block of the layer-cake architecture of the Gipskeuper succession. The stacking of small-scale cycles is superimposed onto a larger-scale, regressive cycle, which records a general shift from restricted marginal marine to hypersaline marine to the more continental red bed deposition of the overlying Keuper. This hierarchy of cycles probably reflects various orders of eustatic sealevel fluctuations.The Gipskeuper thus exemplifies dynamics of evaporite accumulation and allows us to understand some of the processes that produce a “layer-cake” stratigraphy. Its cycle hierarchy is a common motif in the entire southwest German Basin fill, and appears typical for epicontinental basins.

S-isotope studies of shallow water, laminated gypsum and associated evaporites, Upper Permian, East Greenland

Three types of primary subaqueous gypsum occur in the Karstryggen Formation, Upper Permian of East Greenland: (1) planar laminated gypsum, having a narrow δ 34S range of 10.2 to 10.4ℵ CDT, (2) algal laminated gypsum (9.2 to 12.0ℵ), and (3) bottom nucleated gypsum crystals (11.6 to 11.7ℵ). Type 3 was deposited from a residual brine in the basin while types 1 and 2 consist of seawater or mixed fresh water-seawater-derived sulphate with varying contribution of reoxidized sulphur originally deposited as sulphide or organic sulphur. Diagenetic nodular gypsum (7.3 to 10.8ℵ) was formed by redistribution of marine or mixed fresh water-seawater-derived brines which precipitated the laminated gypsum with contribution of sulphate originally precipitated as sulphide or organic sulphur in the organic-rich sediments.

Origin of depositional cycles in a Permian "saline giant": The Salado (McNutt zone) evaporites of New Mexico and Texas

Two types of metre-scale depositional cycles have been recognized within the McNutt Potash Zone of the Permian Salado evaporites. The cycles record progressive drawdown and concentration of brine in a shallow, marginal marine basin. cycles, made of basal carbonate-siliciclastic mudstone, then anhydrite-polyhalite pseudomorphous after primary gypsum, overlain by halite and capped by muddy halite, are interpreted as marine sea water dominated.They formed by massive spillover of sea water through a connection with the Permian Ocean at high relative sea-level stands (lagoonal mudstone and anhydrite-polyhalite after subaqueous gypsum), followed by relative lowering of sea level and gradual basin restriction (shallow saline-lake and salt-pan halite) and ending with a desiccated basin, isolated from the Permian Ocean (salt-pan halite and saline mudflat muddy halite cycle cap). cycles occur between the Type I cycles and contain halite overlain transitionally by muddy halite. They record a temporal evolution of environments from a shallow saline lake to an ephemeral salt-pan-saline mudflat complex and are interpreted as continental-dominated sequences sourced by meteoric inflow from surrounding land areas that mixed with variable amounts of sea water, either residual or introduced into the Salado basin by seepage. The vertical stacking of Type I cycles is best explained by periodic invasions of sea water into the Salado basin coincident with eustatic sea-level rises. The continental-dominated Type II cycles formed during intervening periods of eustatic sea-level fall and low stands. The maximum time interval between major marine incursions is an average of 10 5 yr.

Development and Fabric Relationships of Anhydrite in Detroit River Group Sour Zone,"Ogemaw County, Michigan: ABSTRACT"

The study of diagenetic events within the Devonian Detroit River Group sour zone carbonates reveals several stages of anhydrite development: (1) massive, bedded anhydrite; (2) nodular anhydrite; (3) pore-filling anhydrite; and (4) postcompactional laths. The massive, bedded anhydrite was deposited as subaqueous gypsum. The nodular anhydrite developed in early stages of shallow burial diagenesis. Abundance of nodules decreases vertically, away from the subaqueous, massive anhydrite deposits. These anhydrite nodules, also originally gypsum, grew displacively within sabkha carbonate muds. Nodular anhydrite is visible in hand samples as individuals or as a network of reticulate ovate patches. Within pelletal micrites, dolomicrites, and oolitic packstones, abundant solution vugs have been greatly or entirely reduced by middle to late diagenetic precipitation of a meshwork of individual crystals or singular crystals of primary anhydrite. Euhedral, randomly oriented anhydrite laths are also present but are not precipitated within macropore spaces. These laths, which reveal a postcompactional origin, are often abundant and form a decussate pattern easily seen in hand sample.

Unconformity-Associated Replacement Limestones After Anhydrite in Mississippian of Williston Basin: ABSTRACT

Locally in southeastern Saskatchewan, Mississippian nodular anhydrites (after subaqueous gypsum) beneath an unconformity have been altered to limestone--limestones that are commonly porous and oil-bearing. Such carbonates are commonly intergrown with pyrite and celestite, and thus are difficult to interpret from logs. At localities with calcitized anhydrite, the unconformity is overlain by Jurassic red beds in which pigments have been reduced to green hues. In the region, carbonates beneath the unconformity are normally overlain by red beds and have been completely dolomitized and plugged with anhydrite to form an impermeable caprock. Mississippian anhydrites subcrop at the unconformity surface and reveal little evidence of alteration--even to gypsum. Textures in replaced anhydrites indicate that calcitization involved both creation of porosity and in-situ (small-scale) replacement leading to retention of anhydrite (and later gypsum) fabrics. Celestite formed as strontium was released from anhydrite during replacement by gypsum and calcite. Sulfur in associated pyrite is isotopically lighter than the anhydrite, suggesting anhydrite-alteration involved the activities of sulfate-reducing bacteria. Evidently, H2S liberated during the reaction migrated across the unconformity to reduce overlying red beds. Limestones of this type do not appear to have been reported previously. Stratigraphic and petrographic evidence indicates replacement, although spacially related to the unconformity, was not a weathering phenomenon. It occurred after the unconformity was buried. Unexpectedly heavy ^dgr13C and ^dgr18O values (+ 1.22 to 1.54, and -1.0 to -3.7) obtained from the replacement limestones seem to preclude the utilization of organic carbon in the reaction. The End_Page 494------------------------------ source of carbonate and of the energy required for sulfate-reducing bacterial activity is therefore problematic. End_of_Article - Last_Page 495------------