New and interesting

This page highlights a selection of recently published, interesting scientific articles dealing with evaporite-related topics, along with a link that takes you to the article source.

Salt anomalies in potash beds of the Esterhazy Member, Devonian Prairie Evaporite Formation, Saskatchewan, Canada

The Esterhazy Member of the western Canada Prairie Evaporite has been mined underground for sylvite (KCl) since the early 1960s. Although the geology of the Esterhazy Member ore body is largely considered a regional flat lying continuous series of thin potash hosting beds, there are numerous occurrences where the ore has been either replaced or removed leaving behind uneconomical halite-rich sections. An explanation of the underlying controls on the formation of these salt anomalies has been somewhat elusive although the overwhelming assumption remains that these features developed in lows on a salina. This paper proposes that salt anomalies formed because of two processes, early compaction of carbonate shoals of the Winnipegosis Formation and tectonics that resulted in multiple stages of block movement during the deposition of the upper Prairie Evaporite. Since these two processes can result in a significantly different size to a salt anomaly, encountering one or the other type can have a significant effect on the economics of the ore body. This paper looks at some of the geological methods that might provide geologists with means to predicting salt anomalies.

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A production room was cut over this Winnipegosis shoal after a 3D seismic survey was shot. The room is approximately 20ms above the shoal top. Elevation changes in the room mimic the “W” shape of the Shell Lake Member (blue line) over the shoal, characteristic of the subsidence found over isolated shoals in the area. The profile was hung from the underlying Ashern Formation. TWTT = two way travel time content.

Volcanic and Saline Lithium Inputs to the Salar de Atacama

The Li-rich brine contained within the halite body of the Salar de Atacama is uncommon for two reasons: First, it has an exceptionally high Li concentration, even compared to other closed basins in the Li triangle of South America; and second, it is widespread within the halite nucleus and not restricted to a localized area. This study focusses on the southern half of the salar where Li production occurs and draws comparisons with its northern neighboring basin through which the Loa river flows. Concentration and isotope data for water inflowing to this part of the salar were obtained from surface inflow as well as wells located within the alluvial fans on its eastern margin. Lithium varies between 0.2 and 20 mg/L before reaching the salar where small amounts of the brine and or salts that precipitated from it can increase its concentration up to 400 mg/L or higher. The δ7Li of the inflow water varies between +4.9‰ and +11.2‰ and increases to +12.6‰ within the salar margin, consistent with salar brine based on reported measurements. Boron isotopes indicate that it is unlikely that solutes are derived from sedimentary evaporites or mineral cements, unlike the situation in the adjacent Loa basin. Water that flows through an aquifer laterally confined by a basement block and a line of volcanoes has a notably higher δ7Li than other inflow water, around +9‰, and increasing to +10.5‰. δ7Li values are overall higher than were measured in the adjacent Loa basin, indicating that here the water–rock reactions for Li are more evolved due to longer residence times. Lithium concentrations increased with sodium and chloride, but sedimentary evaporites are shown to be unimportant from δ11B. This is accounted for two ways: evaporated saline inflow leaks from higher elevation basins and inflows are partly derived from or modified by active volcanic systems. Active and dormant volcanoes plus the massive Altiplano–Puna magmatic body are important as heat sources, which enhance water–rock reactions. The large topographic difference between the mean elevation of Altiplano on which these volcanoes sit and the salar surface allows hydrothermal fluids, which would otherwise stay deep below the surface under the modern arc, to uplift at the salar.

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Geology and sampled wells in the Salar de Atacama basin

Evaporite-bearing orogenic belts produce ligand-rich and diverse metamorphic fluids

Detailed petrologic and chemical investigation of mid-amphibolite facies calcareous, scapolite-rich metasedimentary rocks from the Mount Isa region in northern Australia is used to explore changing fluid chemistry with prograde metamorphism. The presence of widespread scapolite with Cl- and variably SO4-rich compositions in upper amphibolite facies rocks makes it unavoidable that the regional metamorphic fluids were locally highly saline and oxidised, and that high salinities persisted throughout metamorphism. Electron microprobe analyses and chemical maps of individual scapolite grains show zoning in Cl and S, likely to reflect buffering of the metamorphic fluid by scapolite during progressive metamorphism. The zoning in Cl and S demonstrates that scapolite has the potential to record changes in fluid chemistry during metamorphism. The variation in scapolite composition between samples, in combination with whole rock geochemistry, shows that different layers within this heterogenous rock package generated fluids of different chemistries. Interaction between scapolite-bearing rocks and externally-derived magmatic or metamorphic fluids that are out of equilibrium drives scapolite breakdown, releasing Cl to the fluid. In the Mount Isa region, metamorphic fluid production was enhanced by periods of magmatism, which promoted development of a regionally extensive and unusually saline fluid system that was active at multiple stages over a 250 million-year period. The highly saline and oxidised fluids formed through interaction with scapolite are well suited to transporting a broad range of metals, and may explain the diverse range of syn-orogenic mineral deposits in the Mount Isa Inlier. Metamorphic belts with large volumes of evaporitic material are ideal for generating a broad spectrum of syn-orogenic hydrothermal ore deposit types - including Fe oxide Cu-Au, Fe sulphide Cu-Au, Mo-Re and U-REE, but lacking the Au-only deposits found in typical orogenic belts. Unlike regions hosting traditional orogenic gold deposits, belts containing evaporitic sequences can preserve Cl-rich minerals such as scapolite in the metamorphosed source region, allowing them to remain active as ore forming systems through relatively high-grade metamorphism and multiple stages of tectonism. Periods of supercontinent breakup, such as the Mesoproterozoic, may have resulted in the formation of large, intracontinental basins well suited to the development of widespread evaporitic sequences. This, in combination with overprinting orogenesis and high temperature magmatism, may have provided the ingredients for widespread ore deposit formation at a global scale.

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Map of the Eastern Fold Belt, after Oliver et al. (2004). Sedimentary rocks are depicted in grey, with the Corella Formation and equivalents depicted as the darkest grey. Sedimentary rocks equivalent to the Corella Formation are widespread in the Eastern Fold Belt. Historical Cu and Au workings, major deposits and currently operating mines (Cu, Au and Zn) are also plotted. Data from https://minesonlinemaps.business.qld.gov.au/Html5Viewer/Tablet.html?viewer=momaps. KLB: Kalkadoon–Leichhardt Belt; MKFB: Mary Kathleen Fold Belt
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Schematic model for the generation of saline fluids in a purely metamorphic system, interlayered metasedimentary rocks containing variable proportions of mudstone, carbonate and evaporite will generate fluids of different chemistries. Calcareous and halite-bearing layers will contain Cl-rich scapolite. Fluids escape along layers or small shear zones, preserving internally buffered metamorphic assemblages. Mixing between different metamorphic fluids may result in small-scale albitisation. In a mixed magmatic-metamorphic system, crystallising magmas are a source of heat to drive metamorphic fluid production and will also introduce fluids of different chemistries. These fluids may result in scapolite breakdown, releasing Cl and increasing fluid salinity where migration of saline, oxidised fluids through the crust may result in phase separation.

An epeiric glass ramp: Permian low-latitude neritic siliceous sponge colonization and its novel preservation (Phosphoria Rock Complex)

Glass ramps are shallow-marine depositional settings in which siliceous sponge meadows dominate coastal environments. They are increasingly recognized throughout the Phanerozoic and represent a biosiliceous counterpart to neritic carbonate factories. Detailed reexamination of the Permian Tosi Chert in the Bighorn Basin indicates that it records a glass ramp that extended over at least 75,000 km2. Outcrops, cores, and wireline logs are used to discriminate previously unidentified shallow subtidal to peritidal facies in its landward extent. These facies indicate that sponge meadows ranged from variably oxygenated offshore settings through low-energy, well-oxygenated, and saline shallow subtidal settings, with spicules transported into supratidal environments affected by enterolithic evaporite growth. This range of subenvironments is largely unique among glass ramps. This is the result of the Tosi's accumulation in an epicontinental sea where waves impinged offshore but frictional attenuation caused low-energy nearshore environments. As a result, the Tosi shares similarity with epeiric sea carbonate deposition and is referred to herein as an epeiric glass ramp. The low palaeolatitude of the Tosi and hot and arid desert it bordered also contributed to its uniqueness as shallow waters were warmer and more saline than higher-latitude counterparts. As a result, a minor sea-level fall at the termination of biosiliceous deposition was associated with increased lagoonal circulation and refluxing brines that caused evaporite and dolomite precipitation within the upper Tosi. Preservational attributes of the Tosi also add to the range of unique traits that can be used to reconstruct neritic biosiliceous environments. These include three disparate colours of chert (black, grey, and purple) related to the host strata and diagenetic redox conditions, early chertification that preserved sedimentary structures within nodules, and nodule shape related to bioturbation intensity. The Tosi glass ramp thus expands the known extent and context of Permian glass-ramp deposition along the western Laurentian margin and illustrates key properties that will aid future glass ramp identification.

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Depositional model of the Tosi epeiric glass ramp (Time 1) and the end-Tosi minor sea-level fall (Time 2) that led to seepage reflux of hypersaline brine into the Tosi in the shallow subsurface. All models are vertically exaggerated. SB, sequence boundary; TST, transgressive systems tract.
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Tosi chert: A) A simplified representation of the formation of chert nodules from silica remobilized from sponge spicules and subsequent replacive silicification of host rock. B) Summary figure showing the range of replacive chert nodule shapes and colours as they relate to interpreted depositional setting along the glass ramp.

Mass-transport complexes (MTCs) document subsidence patterns in a northern Gulf of Mexico salt minibasin

Mass-transport complexes (MTCs) dominate the stratigraphic record of many salt-influenced sedimentary basins. Commonly in such settings, halokinesis is invoked as a primary trigger for MTC emplacement, although the link between specific phases of salt movement, and related minibasin dynamics, remains unclear. IN this papare the authors use high-quality 3D seismic reflection and well data to constrain the composition, geometry and distribution (in time and space) of six MTCs preserved in a salt-confined, supra-canopy minibasin in the northern Gulf of Mexico, and to assess how their emplacement relate to regional and local controls.

They define three main tectono-sedimentary phases in the development of the minibasin: (a) initial minibasin subsidence and passive diapirism, during which time deposition was dominated by relatively large-volume MTCs (c. 25 km3) derived from the shelf-edge or upper slope; (b) minibasin margin uplift and steepening, during which time small-volume MTCs (c. 20 km3) derived from the shelf-edge or upper slope were emplaced; and (c) active diapirism, during which time very small volume MTCs (c. 1 km3) were emplaced, locally derived from the diapir flanks or roofs. They present a generic model that emphasizes the dynamic nature of minibasin evolution, and how MTC emplacement relates to halokinetic sequence development. Although based on a single data-rich case study, the model may be applicable to other MTC-rich, salt-influenced sedimentary basins.

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(a) W-trending un-interpreted seismic section. (b) Interpreted W-trending seismic section showing the overall salt-tectonic structure of the study area, and the nine key seismic horizons (H0 to seabed) and main MTC-bearing intervals (MTC 1 to MTC 5).

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Conceptual model for extrabasinal MTCs, intrabasinal MTCs, slope channels and background slope sediments.

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Stock market response to potash mine disasters

The authors show that news about a mining accident affects the stock of the competitors of the affected company as well the greenfield potash firms. Moreover, the impact of the accident on the stock of the competitors and greenfield firms strongly depends on the type of mining disaster.

The stock of the affected companies responds the most to information on brine inflow in potash mines. Inflows of water into a potash mine can result in its closure, which can lead to significant losses at the company, as this type of accident is often uninsured. In contrast, man-made accidents result in only a small reaction of the stock of the affected companies. In most cases, such accidents do not have a negative impact on potash production and potential losses related to the event are insured. The stock of competing companies and greenfield firms reacts, however, negatively to information on work accidents in the affected companies.

They also find that the stock of competing companies is not affected by natural disasters and attribute a lack of reponse to the oligopolistic structure of the market controlled by cartels, which have a surplus of capacity. 

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Closed depressions in Kotido crater, Arabia Terra, Mars. Possible evidence of evaporite dissolution-induced subsidence

The identification of karst sinkholes in Mars may provide evidence of dissolution processes caused by liquid water and information on paleoclimatic and paleohydrological conditions. This work presents a comprehensive cartographic inventory of 513 closed depressions developed on evaporite-bearing Equatorial Layered Deposits (ELDs) within Kotido crater, Arabia Terra. Detailed mapping, morphometric analyses and spatial distribution relationships reveal a number of features supporting that the depressions correspond to collapse sinkholes related to evaporite dissolution: (1) suitable topographic and litho-structural conditions for the development of a fracture-controlled epigene evaporite karst; (2) presence of open fissures at the foot of the scarped margins; (3) dimensions and frequency-size distributions comparable with those reported on Earth; (4) spatial association with high-permeability zones (i.e., fractures).

Some characteristics of the depressions indicate that they have been re-shaped and enlarged by wind erosion: (1) dominant orientation consistent with the prevalent one-directional winds; (2) differing morphological characteristics on the downwind- and upwind-sides; and (3) nested depressions associated with the upwind sector. The relatively fresh appearance of the depressions and the lack of impact craters suggest a poorly constrained Amazonian karstification phase in the region.

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HiRISE images (ESP_016776_1810 and ESP_016921_1810) illustrating some of the main features in the study area and the mapped closed depressions. A: Elongated depressions with a broader and steeper SW side and a pointed and a gentler NE edge (star). Sharp-crested NE-oriented ridge attributable to a yardang (arrow). B: Aligned and partially coalesced elongated depressions associated with fractures with a prevalent NNE trend. Note the difference between the gentler NE and steeper SW sides of the depressions and the prominent ridge-like morphology of some edges. C: Composite depressions with scarped edges developed on fractured light-toned resistant layers. The floor of the depressions is largely mantled by aeolian deposits a scattered fallen blocks. D: Elongated scarp-edged depressions some of them with stepped floors and nested basins associated with the SW sector. E and F: Depressions with fissures at the foot of the marginal scarps indicative of ground deformation. Butte capped by a resistant layer in the NE corner of the image. Inset images show enlarged fissures. Inset in Fig. F corresponds to a RGB image.

Ichnofossils, Cracks or Crystals? A Test for Biogenicity of Stick-Like Structures from Vera Rubin Ridge, Mars

New images from Mars rover Curiosity display millimetric, elongate stick- like structures in the fluvio-lacustrine deposits of Vera Rubin Ridge, the depositional environment of which has been previously acknowledged as habitable. Morphology, size and topology of the structures are yet incompletely known and their biogenicity remains untested. Here we provide the first quantitative description of the Vera Rubin Ridge structures, showing that ichnofossils, i.e., the product of life-substrate interactions, are among their closest morphological analogues. Crystal growth and sedimentary cracking are plausible non-biological genetic processes for the structures, although crystals, desiccation and syneresis cracks do not typically present all the morphological and topological features of the Vera Rubin Ridge structures. Morphological analogy does not necessarily imply biogenicity but, given that none of the available observations falsifies the ichnofossil hypothesis, Vera Rubin Ridge and its sedimentary features are now recognized as a privileged target for astrobiological research.

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Stick-like structures imaged on the surface of Mars. Are they mud cracks, preserved crystal efflorescences, water escape features, or biogenic?

Origin and Evolution of the Halo-Volcanic Complex of Dallol: Proto-Volcanism in Northern Afar Rift (Ethiopia)

Contextual early observations on volcano genesis are valuable but scarce. Resembling a shield volcano, the Dallol dome is a complex 40 m-high geological structure on the Danakil depression, a North-South-elongated salt plain lying 120 m below sea level in the North Afar (Ethiopia). Dallol has become a tourist destination famous for its colorful hydrothermal features and raised scientific interest due to its life-challenging polyextreme conditions. Although some general models for its genesis exist, little is known about the origin and temporal evolution of both, the dome and its geothermal activity resulting in hyperacidic and halite-oversaturated brines. In this study, data obtained from three multidisciplinary field campaigns (January 2016, 2017, and 2019) are combined to refine the geological mapping of the North Danakil and the Dallol dome. The analysis of stratigraphic, geomorphological, geochemical, and hydrogeochemical data as well as satellite, drone and infrared aerial images shed light in its complex temporal evolution. Results suggest that the recorded history of the dome began when at least one deep magmatic basalt intrusion occurred later than 6000 years ago, forcing the uplifting of the lacustrine deposits of that age covering the west side of the dome. The interaction of the magma with the buried salt deposit resulted in a halo-volcanic activity with, likely, several melted-salt effusion events. Substrate accommodation after effusion led to the current collapsing crater on the dome top and the geothermal still-ongoing degassing. An important hydrothermal reactivation took place after a dyke intrusion event in October–November 2004. It triggered the appearance of new fractures on the dome top and the northward migration of the hydrothermal activity, as inferred from the analysis of historical aerial images combined with high-definition visible and infrared images taken from a drone during our field campaigns. Based on theseobservations, an updated hydrogeothermal conceptual model linking deep magmatic activity with halokinetic processes and geothermal fluids is used to explain the origin and evolution of the Dallol halo-volcanic complex. These geothermal manifestations may potentially inform about rarely documented premises of a volcano’s birth.

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Main steps in the evolution of the halo-volcanic Dallol complex.(A) Salt lake precipitates on the west side over an incipient dome in the east side of Dallol. (B) Magmatic intrusion and updoming. (C) Salt flowing and collapsing. (D) Dyking, faulting and recent geothermal activity. Note a lack of at-surface volcanic material.

Deep-burial hydrothermal alteration of the Pre-Salt carbonate reservoirs from northern Campos Basin, offshore Brazil: Evidence from petrography, fluid inclusions, Sr, C and O isotopes

Petrographic, mineralogical, elemental, isotopic and fluid inclusion analyses were integrated to unravel the diagenetic evolution of Brazilian Pre-Salt lacustrine carbonate reservoirs of northern Campos Basin, southeast Brazilian margin. Detailed thin section and cathodoluminescence petrography, scanning electron microscopy and electron microprobe analyses established a paragenetic evolution of diagenetic processes and products, comprising extensive dolomitization, silicification, and dissolution. A paragenesis including saddle dolomite, macrocrystalline calcite, mega-quartz, Sr-barite, celestine, fluorite, dickite, sphalerite, galena, and other metallic sulfides filling fractures and dissolution porosity, and aqueous fluid inclusions with homogenization temperatures of 92–152 °C and salinities between 13 and 26 wt % eq. NaCl characterized a hydrothermal system with some analogy to carbonate-hosted Pb–Zn Mississippi Valley (MVT) and Irish-type deposits. Petroleum inclusions and solid bitumen testify atypical oil generation and migration, associated with the hydrothermal flow. The host Pre-Salt spherulitic and fascicular carbonates present highly radiogenic 87Sr/86Sr ratios, indicating strong interaction with continental crust materials. Hydrothermal phases show δ18O values more negative than syngenetic and diagenetic carbonates. The δ13C values are interpreted as result of interaction between the hydrothermal fluids and the host rocks. The combined data set provides clear evidence of intense hydrothermal alteration of northern Campos Basin Pre-Salt reservoirs at deep-burial conditions (>2 km), possibly related to Late Cretaceous or more probably Paleogene magmatic activity. Mixed-sourced fluids bearing a basinal signature fed the hydrothermal system and promoted dissolution of the host rocks. The hydrothermal alterations had strong impact on the porosity, permeability, and heterogeneity, contributing, together with the associated fracturing, to the excellent production performance of the Pre-Salt reservoirs.

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Schematic representation of the deep-burial hydrothermal system affecting the northern Campos Basin Pre-Salt reservoirs, offshore Brazil. The fault-focused hydrothermal system probably involved mixing of fluids derived from several sources: (B) Pre-Cambrian basement (mainly granitic-gneissic felsic rocks), (C) volcanic rocks from the Cabiúnas Formation (mainly basaltic), (R) rift deposits from the Atafona and Coqueiros formations, and (S) serpentinization of the upper mantle. Note the intrusive magmatic (mafic) rocks (M) in the Coqueiros (rift interval) and Macabu (sag section) formations; hydrothermal alteration (H) comprising extensive dolomitization, silicification, and dissolution, with the paragenesis including saddle dolomite, macrocrystalline calcite, mega-quartz, Sr-barite, celestine, fluorite, dickite, sphalerite, galena, and other metallic sulfides filling fractures and dissolution porosity; and anhydrite thick layer (A) at the top of the sag reservoir affected by the hydrothermal alteration (base of evaporites).

The demise of a ‘salt giant’ driven by uplift and thermal dissolution

The development of giant salt basins and eventual cessation of rapid salt deposition is founded on a delicate balance of salinity and heat fluxes within the water body governed by tectonic, climatic and eustatic change. The onset of salt deposition in such basins is widely accepted to be initiated by basin restriction. However, the processes that lead to the termination of salt deposition are comparatively unclear. Here the authors use an array of 2D and 3D seismic surveys to reveal that the truncation surface at the top of a thick salt sequence in the Eastern Mediterranean is far more extensive than previously thought. Tery show that uplift of the salt driven by deformation and thermal dissolution initiated the demise of the ‘salt giant’, even prior to the final dilution and emplacement of brackish Lago Mare and fluvial deposits. Progressive uplift of the salt through the thermocline and into the under-saturated epilimnion led to dissolution. They argue that dissolved salt was recycled and re-precipitation from the hypolimnion in the deepest sections of the basin contemporaneous with dissolution of halite from the shallower epilimnion. Their findings explain how rapid basinwide salt deposition was brought to an end in the Eastern Mediterranean and present a novel process for sculpting the final architecture of a ‘salt giant’.

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Examples of deformed intra-salt reflections that have been truncated from around the Eastern Mediterranean. Reconstructions of the truncated salt stratigraphy (red) demonstrate the significant amount and variability of vertically truncated stratigraphy around the basin. BES – Bottom Erosional Surface; IMTS – Intra-Messinian Truncation Surface. a: Truncation in the North Levant Basin. b: Truncation in the central Levant Basin. c: Truncation to the east of the Eratosthenes Block. d: Truncation in the Latakia Basin.

Deep Mediterranean's Messinian evaporite giant: How much salt?

Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Aenean commodo ligula eget dolor. Aenean massa. Cum sociis natoque penatibus et magnis dis parturient montes, nascetur ridicEvaporite giants can precipitate out of high salinity waters in semi-enclosed ocean basins when the rate of evaporation outstrips the inflow of new saline and fresh waters. The thick saline series deposited in the deep Mediterranean during the Miocene Epoch's late Messinian age (between 5.97 and 5.3 Ma) is among the youngest of such salt giants in Earth history. During this event, also known as the Messinian Salinity Crisis, voluminous evaporites accumulated in the deep basins when the main inflow of saline water to the Mediterranean through the Gibraltar Straits was severely diminished. The evaporites are interbedded with sediments eroded from the shelves and slopes or mass-transported downslope. Recent advances in geochronology and better understanding of the geodynamic history of various basins have led to insights about the timing of the salinity crisis and the tectonics and role of Gibraltar passage and other western narrows from the Atlantic to the Mediterranean. New estimates based on ocean-wide seismic facies analysis presented here reveal that there is between 821 ± 50 and 927 ± 50 thousand cubic km of late Messinian salt, and a total of up to 1.2 ± 0.1 million cubic km of salt plus associated sediment tied up in the deep Mediterranean basins. This suggests that after the initial restriction the Mediterranean had to be either continuously supplied with brine, or partially to completely refilled several times to produce the total salt edifice. The amount of Atlantic saline water needed to amass this evaporite giant is between 7 and 8 times the modern-day Mediterranean's equivalent of saline water. Our new volumetric estimates have implications for the Mediterranean's Messinian sequestration and desiccation scenarios and should lead to more meaningful geodynamic models and provide constraints for many of the controversies that still surround this major event in geological history. Maps of salt distribution in various basins also have important implications for sub-salt exploration geoscience.ulus mus. Donec quam felis, ultricies nec, pellentesque eu, pretium quis, sem. Nulla consequat massa quis enim.

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New estimates of thicknesses of Messinian salt in the Mediterranean: Isopach map of evaporites and Messinian Salinity Crisis (MSC)-related sediments based on seismic facies analysis from the deep Mediterranean basins (excluding basins on land and intermediate basins such as the Adriatic). These quantitative volumetrics of salt are relevant for the restriction/sequestration and desiccation scenarios of the MSC and geodynamic modeling of the consequences of rapid accumulation of the salt giant at abyssal depths. [Major deep Mediterranean basins where significant quantities of MSC salt accumulated include L-PB: Liguro-Provençal Basin AB: Algerian Basin IB: Ionian Basin HB: Herodotus Basin SB: Sirte BasinLB: Levant Basin, and on the fringes of NC: the Nile Cone]

Chronology with a pinch of salt: Integrated stratigraphy of Messinian evaporites in the deep Eastern Mediterranean reveals long-lasting halite deposition during Atlantic connectivity

The Messinian Salinity Crisis (MSC; 5.97–5.33 Ma) is considered an extreme environmental event driven by changes in climate and tectonics, which affected global ocean salinity and shaped the biogeochemical composition of the Mediterranean Sea. Yet, after more than 50 years of research, MSC stratigraphy remains controversial. Recent studies agree that the transition from the underlying pre-evaporite sediments to thick halite deposits is conformal in the deep Eastern Mediterranean Basin. However, the age of the base and the duration of halite deposition are still unclear. Also disputed is the nature of the intermediate and upper MSC units, which are characterized as periods of increased clastic deposition into the Eastern Mediterranean based on marginal outcrops and seismic data. We provide a multidisciplinary study of sedimentary, geochemical, and geophysical data from industrial offshore wells in the Levant Basin, which recovered a sedimentary record of deep-basin Mediterranean evaporites deposited during the MSC. In combination with previous observations of the MSC throughout the Mediterranean Basin, our results promote the need for a new chronological model. Remarkably, the one-kilometer-thick lower part of the evaporitic unit is composed of essentially pure halite, except for a thin transitional anhydrite layer at its base. The halite is undisturbed and homogeneous, lacking diverse features apparent in more proximal sections, indicating a deep-sea depositional environment. We find that distinct, meters-thick non-evaporitic intervals interbedded with the halite, previously thought to be clastic layers, are diatomites. While XRD analysis confirms an increase in clastic components in these sediments, they are composed primarily of well-preserved marine and freshwater planktonic diatoms. The occurrence of marine planktonic diatoms in these intervals indicates the input of Atlantic waters into the Mediterranean Basin during the deposition of the massive halite unit. Seismic stratigraphy and well-log cyclostratigraphy further support deep basin halite deposition, which started about 300 kyr earlier than widely assumed (~5.97 Ma). We propose that halite deposition in the deep Mediterranean took place during stage 1 of the MSC, rather than being limited to the short 50 kyr MSC acme when sea level was presumably at its lowest. Thus, brine formation, salt precipitation, and faunal extinction occurred at least in part in a deep, non-desiccated basin, with a restricted yet open Mediterranean-Atlantic connection that allowed inflow of oceanic water. We observe an increase in heavy minerals and reworked fauna within the clastic-evaporitic, Interbedded Evaporites of the basinal MSC section, and argue that these settings correspond in the deep basins with a significant sea-level drawdown during stage 2 of the MSC, as observed in the marginal sections. This correlation is corroborated by astrochronology and chemostratigraphic markers, such as the distribution of n-alkanes and biomarker-based thermal maturity indices.

The Levant deposits indicate that high sea level and partial connectivity with global oceans promoted the deposition of deep-basin deep-water halite, while sea-level drawdown promoted deposition of reworked and transported material from the margins into deep Mediterranean basins. This study modifies the current understanding of the mechanisms governing salt deposition throughout the MSC with implications for other evaporitic events in the geologic record.

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Geophysical data and seismic stratigraphy of the Dolphin and Leviathan-1 wells
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Astronomical age model and regional correlation of the Levant MSC

Origin and recharge model of the Late Cretaceous evaporites in the Khorat Plateau, SE Asia

Evaporites commonly occur in the Mesozoic-Cenozoic Tethyan domain, which characterized by extensive Late Cretaceous potash deposits in the Lanping-Simao Basin (LSB) in southwestern China and in the Khorat Plateau (KP) in Thailand and Laos. The LSB and the KP are located in the eastern Tethyan tectonic belt. The origin of the Late Cretaceous evaporites in these basins is controversial; possibilities include marine, continental, or hydrothermal origins. In addition, the recharge model for the major solutes into these evaporitic basins is inadequate, whether it is from the KP to the LSB or from the Qiangtang to the LSB to the KP. In this study, 34 gypsum, anhydrite, and halite samples from two sediment cores collected from the KP were analyzed to determine their stable B-Sr-S isotopic compositions. This is the first time that δ11B values have been reported for the anhydrite in the study area. The origin and evolutionary relationships of these evaporitic basins were investigated based on the geochemical data, sedimentary features, mineral sequences, and stratigraphic ages of the evaporites in the LSB and KP. The tectonic evolution and stratigraphic comparisons during the Triassic-Cretaceous in the eastern Tethyan domain were systematically reviewed and summarized in order to determine a preferred recharge model for the evaporites in the KP. The following conclusions were reached. (1) The reconstructed δ11B values (+38.20‰ to + 51.23‰) of the parent solution, which were based on those of the anhydrite (+8.20‰ to + 21.23‰), and the isotopic fractionation levels (30.2‰ to 32.7‰), 87Sr/86Sr ratios (0.70743–0.70846), and δ34S values (+14.39‰ to +15.94‰) of the anhydrite and halite in the KP overlap with those of Late Cretaceous seawater, suggesting a marine origin. (2) The similar mineral sequences and B-Sr-S isotopic signatures, and the comparable sedimentary features and inherited ore-forming ages indicate that evaporites in the LSB and KP have similar solute sources and evolutionary relationships. (3) The tectonic evolution and stratigraphy demonstrate that during the Late Cretaceous, paleoseawater from the Shan Boundary Ocean (the eastern segment of the Meso-Tethys Ocean) most likely passed through the southwestern part (Tengchong-Baoshan block) of Sibumasu and preferably recharged marine solutes into the LSB and KP.

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The recharge model for paleoseawater (Shan Boundary Ocean) in the Lanping-Simao Basin and the Khorat Plateau.
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Stratigraphy of the Sibumasu Terrane (sections 1–3), the Indochina Block (sections 6–10), the Sukhothai Arc Terranes (sections 4 and 5), the Lanping-Simao Basin (sections 11 and 12), the Qamdo Basin in the Qiangtang Block (section 15), Biru and Chayu in Lhasa (sections 14 and 16)), and Myitkyina (section 13) in West Burma). The red shaded area is the Mesozoic continental red bed outcrops.

Characterization of Oligo-Miocene evaporite-rich minibasins in the Sivas Basin, Turkey

The Sivas Basin in Turkey displays in its central part an Oligo-Miocene halokinetic province which acts as a major outcrop analogue to study salt-sediment interactions. Based on field geology observations, the present paper focuses on the geometry and sedimentology of several minibasins having the particularity of being mainly filled by gypsiferous deposits. Such type of evaporite-rich minibasins remain difficult to identify and are poorly studied in other halokinetic provinces. In the Sivas Basin, the evaporites were recycled from diapiric salts and precipitated in saline ponds emplaced above deflating diapiric stems. Diapir deflation resulted either from local transtensive strain, cessation of diapir feeding and/or subsurface dissolution of the diapiric salt. Minibasin subsidence was likely enhanced by the fast emplacement rate of the capping evaporites, together with the high density of the depositional sulfates compared to the diapiric halite. The evaporite-rich minibasins stand out from their surrounding siliciclastic counter-parts by their small dimension (lower than 1 km-wide), their encased teardrop shape, and their high internal deformations. The later include well-developed halokinetic sedimentary wedges, aerial mega-slumps or inverted flaps. Such structural features probably resulted from the ductile rheology of the evaporite infill and the complex pattern of downbuilding. Although secondary evaporitic minibasins have never been identified in other ancient halokinetic settings, our study highlights that they could developed in any evaporitic environments, coastal or continental, such as in the Precaspian Basin. The secondary minibasins described here can also act as field analogues of other primary evaporite-rich minibasins already suspected in salt giant basins (e.g. in the Santos Basin, Brazil).

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Synthesis of the depositional evaporites in an evaporite-rich minibasin of the Sivas Basin.

Evaporite occurrence and salt tectonics in the Cretaceous Camamu-Almada Basin, northeastern Brazil content

The Camamu-Almada basin is located at the South Atlantic Margin and, despite the availability of seismic data and the evidence of its hydrocarbon potential, is still considered a new exploratory frontier. In this study, the authors used fifty 2D seismic lines and twelve wells to: a) present a seismic stratigraphic interpretation of the transitional phase (i.e., the strata between the rift and the drift stages; b) investigate the distribution of evaporite deposits at the study area; b) verify the impacts of salt tectonics in the deep-water region, using basins situated in the same geotectonic context as reference. The development of the South Atlantic margin resulted in the formation of several rift basins - including the Camamu-Almada basin – and occurred through four tectonic phases: pre-rift, rift, transitional and drift. The regional deposition of evaporites marks the transitional phase, with occurrences both in Brazil and West Africa that increases in thickness and width southwardly. This work details that in the study area, the salt moved laterally and vertically, forming a highly deformed deposit that marks salt deflation and salt inflation zones, directly associated respectively with diapirs and mini-basins. Features related to salt tectonics that can be directly related to processes of migration and accumulation of hydrocarbons in the study area are also described.

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(A) Chronostratigraphic chart of the Almada Sub-basin. (B) Representative section of a well illustrating the transitional section in the study area; (C) Interpreted seismic section situated in the proximal areal illustrating the main unconformities; (D) Interpreted seismic section situated in the distal areal illustrating the main unconformities and main evaporite deposits.

Constraints on Meso- to Neoproterozoic seawater from ancient evaporite deposits

Marine sulfate is intimately connected to the global carbon and oxygen cycles through its important role as an electron acceptor for the microbial respiration of organic carbon. The biogeochemical feedbacks within the sulfur, carbon, and oxygen cycles may have changed through time, reflecting changes in the concentration of sulfate in the oceans. Unfortunately, there is much uncertainty about the size of the marine sulfate reservoir throughout Earth history. In particular, conflicting estimates for marine sulfate exist during the latest Neoproterozoic, an interval of time associated with striking changes in Earth system evolution and oxidation: published interpretations of fluid inclusion chemistry place sulfate greater than 16 mmol/kg, whereas other interpretations of carbonate-associated sulfate data suggest concentrations less than 2 mmol/kg. Calcium isotope ratios in evaporite successions provide an independent method for deriving semi-quantitative constraints on sulfate concentrations, as well as other properties of seawater chemistry. Here, the calcium isotope behavior of bedded sulfate evaporites from ∼1050 Ma (Baffin and Bylot Islands, Nunavut, Canada), ∼830 Ma (Officer Basin, Western Australia), and ∼545 Ma (South Oman Salt Basin, Sultanate of Oman) are examined. In combination with other geological observations, the results suggest relatively low, millimolal-level sulfate in the latest Mesoproterozoic and a more specific range of 6–10 mmol/kg sulfate during the latest Neoproterozoic. These new constraints suggest that previous interpretations of sulfate concentrations and seawater chemistry need to be revised, opening up new possible solution spaces for the major ion composition of seawater.

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Constraints on seawater chemistry during deposition of the Iqqittuq Formation, Browne Formation, and Ara Group, based on evaporite modeling results. Each simulation is represented by a circle, with the color scale showing the degree of calcium isotope distillation for relevant simulations. A: constraint that gypsum saturates before halite with progressive evaporation. B: constraint for sulfate-rich fluid inclusions and magnesium sulfate salts (versus calcium-rich fluid inclusions and potassium chloride salts). C: constraint for a limited range of δ44/40Ca values . D: constraint from fluid inclusion K/SO4 ratios in the Ara Group. Markers T, M, K, P, and S indicate estimated seawater composition of today, Messinian, Cretaceous, Permian, and Silurian time, respectively, which capture the classical “aragonite sea” and “calcite sea” compositions. The pink shaded region is consistent with late Mesoproterozoic and Tonian seawater. The blue shaded region is consistent with latest Ediacaran seawater. 

Dolomitization by hypersaline reflux into dense groundwaters as revealed by vertical trends in strontium and oxygen isotopes: Upper Muschelkalk, Switzerland

The Trigonodus Dolomit is the dolomitized portion of the homoclinal ramp sediments of the Middle Triassic Upper Muschelkalk in the south-east Central European Basin. This study re-evaluates the origin, timing and characteristics of the dolomitizing fluids by examining petrographic and isotopic trends in the Trigonodus Dolomit at 11 boreholes in northern Switzerland. In each borehole the ca 30 m thick unit displays the same vertical trends with increasing depth: crystal size increase, change from anhedral to euhedral textures, ultraviolet-fluorescence decrease, δ18OVPDB decrease from -1·0‰ at the top to -6·7‰ at the base and an 87Sr/86Sr increase from 0·7080 at the top to 0·7117 at the base. Thus, dolomites at the top of the unit record isotopic values similar to Middle Triassic seawater (δ18OVSMOW = 0‰; 87Sr/86Sr = 0·70775) while dolomites at the base record values similar to meteoric groundwaters from the nearby Vindelician High (δ18OVSMOW = -4·0‰; 87Sr/86Sr = >0·712). According to water-rock interaction modelling, a single dolomitizing or recrystallizing fluid cannot have produced the observed isotopic trends. Instead, the combined isotopic, geochemical and petrographic data can be explained by dolomitization via seepage-reflux of hypersaline brines into dense, horizontally-advecting groundwaters that already had negative δ18O and high 87Sr/86Sr values. Evidence for the early groundwaters is found in meteoric calcite cements that preceded dolomitization and in fully recrystallized dolomites with isotopic characteristics identical to the groundwaters following matrix dolomitization. This study demonstrates that early groundwaters can play a decisive role in the formation and recrystallization of massive dolomites and that the isotopic and textural signatures of pre-existing groundwaters can be preserved during seepage-reflux dolomitization in low-angle carbonate ramps.

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Schematic summary of 87Sr/86Sr values in the Trigonodus Dolomit and in surrounding units. Dots: individual samples. Rectangles indicate range of data
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Significant advective groundwater flow mixes with and merges the refluxing brine plumes, resulting in laterally uniform, vertical isotopic gradients. Dolomite proceeds to replace calcitic limestone and acquires the geochemical and isotopic characteristics of the mixed waters. The uppermost dolomites, with the most positive 18O values, are probably eroded by the prograding continental environment.

Depositional model of early Cretaceous lacustrine carbonate reservoirs of the Coqueiros formation - Northern Campos Basin, southeastern Brazil

Recent discoveries of giant hydrocarbon accumulations in coquinas and other lacustrine carbonate rocks of Rift and Post-rift (pre-salt) super-sequences of the Campos and Santos Basins have motivated a series of studies focused on understanding the genesis of these unique reservoirs. An integrated petrologic-sedimentologic-stratigraphic-seismic study of the Coqueiros Formation delineated the depositional model and evolution of reworked bioclastic rocks and other associated rocks deposited on the external high in the northern Campos Basin during the transition between late-rift and sag stages. Bioclastic rudstones and grainstones with moderate to high shell reworking represent 65% of the samples described. The following pieces of evidence point to a predominantly shallow lacustrine depositional environment, dominated by storm currents: (i) presence of high-angle (>14°) longshore cross-bedded rudstones; (ii) limited occurrence of muddy sediments interbedded with thick packages of amalgamated bioclastic rudstones; (iii) great lateral continuity of rudstone deposits devoid of matrix on the structural high, consistent with processes of storm action on large expanses of shallow lakes; (iv) deposits of massive rudstones, with disorganized shells, similar to the modern coastal bioclastic deposits of California and Shark Bay. Three sets of facies association are proposed: (i) high-energy facies, composed of bioclastic rudstones and grainstones devoid of matrix deposited above the fair weather wave base (FWB); (ii) low energy facies, composed of hybrid rocks with muddy or peloidal matrix and other fine grained rocks deposited below the FWB or in protected shallow environments; (iii) an alkaline set, composed of mainly magnesian clay ooids deposited in a more alkaline composition of lake water. The evolution of the facies associations over time demonstrates an overall fall of the relative lake level. High proportion of low energy facies associated with a deeper lacustrine environment occurs at the Coqueiros Fm. base, while at the top predominate high energy facies associated with a shallow lacustrine environment, in addition to magnesian clay-rich facies associated to restricted and alkaline water bodies. Paleocurrents suggest the activity of two main wind direction (northwest and southeast) over the external high that result in clean bioclastic facies with good reservoir potential on both flanks. The knowledge of wind directions during the late Barremian and early Aptian in the northern Campos Basin is extremely important for targeting the best depositional facies for hydrocarbon accumulation in the Coqueiros Formation.

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Limited thermochemical sulfate reduction in hot, anhydritic, sour gas carbonate reservoirs: The Upper Jurassic Arab Formation, United Arab Emirates

Limited thermochemical sulfate reduction (TSR) in hot (130–160°C) and anhydrite-rich sour gas reservoir carbonates of the Arab Formation (Upper Jurassic) is manifested by rare calcitization of anhydrite with slightly lower δ13CVPDB values (−3.2 to −0.1‰) than calcite precipitated in equilibrium with Late Jurassic seawater. Fluid inclusion microthermometry of calcite that has replaced anhydrite indicates that TSR occurred between 130°C and 160°C. The lack of evidence for extensive TSR, despite the suitable current temperatures and abundant sulfates in the gas reservoir, coupled with the relatively more common TSR-related calcite in the flanks (water zone) than crest (gas zone), indicate that: (1) gas emplacement while the reservoir was buried at shallower depth slowed down or inhibited TSR in the crest even when it subsequently reached depths where extensive TSR would occur, and (2) H2S (up to 38 vol%) has migrated from the underlying Permo-Triassic and/or Jurassic sulfate-carbonate deposits. This study demonstrates that constraining the timing of hydrocarbon emplacement within the context of burial-thermal history is crucial for a better understanding of the origin and distribution of H2S in hot, anhydrite-rich, sour gas reservoirs.

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Schematic conceptual model of the extent of TSR inthe Arab Formation across the anticline of the field. (A)Formation of the anticline and gas migration occurred in LateCretaceous during the obduction of the Oman ophiolites. TSRstarted in the flanks at around 130 °C in Early Eocene duringgas migration when the reservoir temperatures were too lowto cause extensive TSR in the crest. (B) TSR continued in theflanks and commenced in the lower parts (i.e. outer ramp) ofthe crest but not in the upper parts (i.e. supratidal and lagoon)because of early gas emplacement in Early to LateEocene. (C) TSR continued in the water-saturated flanks,whereas stopped or slowed down significantly in the crestbecause of gas emplacement during the Late Eocene.

Hydrodynamics of salt flat basins: The Salar de Atacama example

The Salar de Atacama is one of the most well-known saline endorheic basins in the world. It accumulates the world main lithium reserves and contains very sensitive ecosystems. This paper characterize the hydrodynamics of the Salar de Atacama, and quantifies its complex water balance prior to the intense brine extraction. The methodology and results can be extrapolated to the groundwater flow and recharge of other salt flats. A three-dimensional groundwater flow model using low computational effort was calibrated against hundreds of hydraulic head measurements.

The water infiltrated from the mountains ascends as a vertical flux through the saline interface (mixing zone) produced by the density contrast between the recharged freshwater and the evaporated brine of the salt flat nucleus. This water discharges and is largely evaporated from lakes or directly from the shallow water table. On the other hand, the very low hydraulic gradients, coupled with the presence of the mixing zone that operates as barrier, leads the salt flat nucleus to act as a hydrodynamically quasi-isolated area. The computed water table shows the lowest hydraulic head in the salt flat nucleus near the discharge at the mixing zone. The groundwater balance of the Salar de Atacama in its natural regime was quantified resulting in an inflow/outflow of 14.9 m3·s−1. This balance considers the basin as an endorheic system. The very low infiltration values that are generally assumed for hyperarid basins are not consistent with the hydrogeology of the Salar de Atacama. Indeed, very high infiltration rates (up to 85% of rainfall) occur because of the high degree of fracturing of rocks and the scarce vegetation. This high infiltration is consistent with the light isotopic composition of the water from the recharge area (Altiplano). Therefore, the existence of additional inflows outside the basin is unlikely.

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Deposition, diagenetic and hydrothermal processes in the Aptian Pre-Salt lacustrine carbonate reservoirs of the northern Campos Basin, offshore Brazil

The discovery of large oil accumulations in the rift and sag Pre-Salt sections of the Campos and Santos Basins has revived interest in the exploration of the lacustrine carbonate reservoirs in the Brazilian and African marginal basins. More than half of Brazilian oil production originates from the Pre-Salt reservoirs of these offshore basins. A study integrating systematic petrography, cathodoluminescence, scanning electron microscopy, microprobe and X-ray diffraction was performed on seven wells in the northern Campos Basin. This study highlights the major primary, diagenetic and hydrothermal features of the Pre-Salt succession, with the aim to improve our understanding of the factors that influence the porosity and permeability distribution in these important lacustrine carbonate reservoirs. The Pre-Salt deposits correspond to bioclastic grainstones and rudstones, syngenetic crusts of fascicular calcite, and intraclastic grainstones and rudstones of reworked crust fragments and calcite spherulites. Magnesian silicates are frequently associated with carbonate deposits. In the sag phase, stevensitic laminations constitute the substrate for the precipitation of calcite crusts and spherulites, which displace and replace the syngenetic magnesian clay deposits. In the rift section, stevensitic ooids are mixed with bioclasts or form ooidal arenites. Pre-Salt carbonate reservoirs have undergone a complex and heterogeneous diagenetic evolution. Eodiagenetic processes include the dissolution, neomorphism and cementation of bivalve bioclasts in the rift, as well as the dissolution of magnesian silicates and their replacement by calcite spherulites, silica and dolomite in the sag section. Burial alterations are commonly associated with hydrothermal fluids carried through faults and fracture systems. These fluids promote dolomitization, silicification, and dissolution at varying degrees and intensities. Eodiagenetic precipitation and dissolution owing to variations in the lake water chemistry and the episodic flow of hydrothermal fluids under burial conditions control the creation, redistribution, and obliteration of porosity in the Pre-Salt reservoirs.

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Schematic representation of the genesis of typical Aptian Pre-salt deposits. A) Laminated deposits of syngenetic magnesian clays,with dispersed clay peloids and siliciclastic grains; B) Partial replacement and deformation of the Mg-clay matrix by calcite spherulites. Asymmetrical spherulites formed closer to the water-sediment interface (WSI); C) Non-coalesced fascicular aggregates of calcite precipitated on WSI with inter-aggregate growth-framework porosity. Clay peloids and siliciclastic grains included in some fascicular aggregates; D) Characteristic “cycle” showing the syngenetic crust of coalesced fascicular calcite aggregates at the top, and syngenetic Mg-clay matrix partially replaced and displaced by calcite spherulites in the middle and preserved at the base.

Origin of the Neoproterozoic rim dolomite as lateral carbonate caprock, Patawarta salt sheet, Flinders Ranges, South Australia

The “rim dolomite” of South Australia?s Central Flinders Ranges is a prominent ridge-forming, layered dolomitic and siliceous unit. The rim dolomite is interpreted to be a lateral caprock found exclusively at the salt?sediment interface between the Patawarta diapir and the Ediacaran-aged Bunyeroo Formation. Lateral dolomite caprock is defined by the following field relationships: (1) the rugose dolomicrite base that parallels the contact of the diapiric matrix and the bedding in the overlying stratigraphy, (2) the exclusive presence of dolomite at the salt?sediment interface, (3) the lack of sedimentary structures or fossils (cyanobacterial laminites and stromatolites), (4) the lack of interbedded Bunyeroo lithofacies, and (5) the inability to trace the rim dolomite capstone away from the diapir margin into the outboard stratigraphy. In addition to the field relationships, the rim dolomite displays the following capstone fabrics: (1) massive?microcrystalline dolomite, (2) porphyritic?two distinct crystal sizes, one forming microcrystalline dolomite groundmass and the other forming rosettes of silica, (3) banded?microcrystalline dolomite forming pressure-dissolution layers of silica and authigenic hematite, and (4) brecciated?mosaic to disorganised, forming a microcrystalline dolomite groundmass, which locally contains remnant clasts of Callanna non-evaporite lithologies, such as quartz arenite to arkosic sandstones and basalts, surrounded by an anastomosing cement-filled vein network. All capstone fabrics contain various amounts of anhydrite, quartz, feldspar and non-evaporite grains that represent the insoluble residue during halite dissolution and caprock accretion. Three different genetic models for the lateral caprock are described and tested, and that of these, only the halokinetically rotated caprock model fits the data. The field relationships and capstone fabrics of the rim dolomite match other lateral caprocks in salt basins such as the Paradox Basin and Gulf Coast, USA.

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Rim dolomite at the salt–sediment interface at the Patawarta diapir.
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Models of lateral caprock formation, halokinetically rotated caprock; (1) Lower BunyerooFormation is deposited and covers the Patawarta diapir; (2) rim dolomite develops as a crestal caprock; (3) diapir inflation causes extension and fracturing ofroof panel and rim dolomite; (4) continued diapir inflation causes erosion of the roof and rim dolomite, clasts of which are subsequently deposited in the overlyingBunyeroo Formation; (5) the rim dolomite is subsequently buried by the Wonoka Formation forming a halokinetic sequence boundary. With further halokineticrotation , the rim dolomite becomes vertical to overturned in a lateral position.

Stylolite-controlled diagenesis of a mudstone carbonate reservoir: A case study from the Zechstein_2_Carbonate (Central European Basin, NW Germany)

Stylolites are rough dissolution surfaces that form due to intergranular pressure-solution resulting from burial compaction or tectonic stress. Despite being ubiquitous in most carbonate rocks, their potential impact on structural diagenesis and fluid flow remains unclear. The Zechstein 2 Carbonate (Ca2) is a diagenetically complex reservoir in the Southern Permian Basin and represents one of the most prolific gas reservoirs in NW Germany. This investigation focuses on evaluating the relationship between stylolites, fractures/veins and their subsequent influence on the spatial variations in reservoir quality. We utilise drill core samples to carry out a combined analysis of cross-cutting relationships between different structures and diagenetic products. We therefore use a combination of petrography and statistical analyses on stylolite networks, focusing on their occurrence, morphology and sealing capacity. In the study area, the Ca2 carbonate mudstone was deposited in a slope environment and dolomitised under shallow burial conditions, followed by bedding-parallel stylolitisation during burial. Results indicate that calcium-rich fluids percolated from neighbouring evaporite units causing widespread calcitisation within the more distal environments of deposition. Some stylolites locally acted as barriers to affect the migration of the calcitising fluids, resulting in a macroscopic diagenetic stratification of relatively porous dolomite and areas of calcitised dolomite with lower porosity. However, pressure-solution continued during burial and bedding-parallel stylolites also appear postdating calcitisation. During inversion, horizontal stylolites were reopened to act as conduits to enable fluid migration that precipitated metal sulphides. This indicates that stylolites acted as both barriers and conduits for fluid flow depending on variations of the overburden pressure and regional stress regime. Stylolites present a range of sealing capacities between 63 and 89%, depending on their morphology, and can result in partial leakage and subsequent invasive calcitisation in their vicinity. This study highlights the importance of understanding the impact of stylolites on structural diagenesis and spatial variations in petrophysical rock properties that determine reservoir quality.

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Block diagrams indicating the changing role of stylolites observed inthe core. A. Stylolites are created in a dolomite matrix (dark grey) prior to calcitisation.B. Stylolites act as barriers to partially inhibit the vertical migration of calcitising fluid to create horizontal calcitised dolomite layers and patchy calcitised dolomite around stylolites (light grey). C. Stylolites are reopened during inversion, postdating calcitisation therefore not acting as a conduit for calcitised dolomite. D. Reopened stylolites act as conduits for iron-rich fluids which precipitate metal sulphides such as pyrite and galena (gold).

Antecedent aeolian dune topographic control on carbonate and evaporite facies: Middle Jurassic Todilto Member, Wanakah Formation, Ghost Ranch, New Mexico, USA

The Middle Jurassic Todilto Member of the Wanakah Formation is a carbonate and gypsum unit inset into the underlying aeolian Entrada Sandstone in the San Juan Basin. Field and thin section study of the uppermost Entrada and Todilto at Ghost Ranch, New Mexico, identified Todilto facies and their relationship to remnant Entrada dune topography. Results support the previous interpretation that the Entrada dunes, housed in a basin below sea level, were rapidly flooded by marine waters. Mass wasting of the dunes gave rise to sediment-gravity flows that mostly buried remnant dune topography, leaving ca 12 m of relief that defined the antecedent condition for Todilto deposition.

Previously interpreted as seasonal varves deposited in a stratified water body, the Todilto is reinterpreted as a microbial biolaminite. Most diagnostic are organic-rich laminae with structures characteristic of filamentous microbes and containing trapped aeolian silt, and clotted-texture laminae with a fabric associated with calcification of extracellular polymeric substances. The dune palaeotopography controls the spatial arrangement of Todilto facies. A continuous basal laminated mudstone thickens over the dune crest, reflecting the optimum conditions for microbial mat development, and is interpreted to have been deposited when marine waters submerged the topography.

Subsequent drying caused the emergence of the crestal area, the formation of tepee structures and a dissolution breccia. Gypsiferous mudflats and periodic ponds occupied the dune flanks and interdune area, with gypsum concentrated within the interdune area. Entrada sands remained unstable during Todilto deposition with common injection structures into the Todilto, and a remnant slope caused the downslope movement and folding of Todilto strata on the upper lee face. Although some expansion of the gypsum occurred in the subsurface, facies architecture fostered the development of a dissolution front adjacent to the interdune gypsum body with section collapse of gypsiferous limestone on the dune flanks.

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A microbial community developed on the submerged, remnant palaeotopography (facies T1, T2 and T5) during Stage 3. Falling water level gave rise to brines and initial precipitation of gypsum in the interdune depression (facies T2). By Stage 4 the crestal area was emergent and tepees formed (facies T1). Continued exposure of the crest gave rise to development of a dissolution breccia (facies T5). Downslope of the emergent crestal area, microbial mats formed on sabkha mudflats with gypsum nodules (facies T3) and gypsum (facies T6) was deposited in the ponded interdune area. During Stage 5 ponding events below the emergent crest occurred, with a hypothesized repetitive deposition of microbial mat, carbonate and gypsum laminations (facies T4). Gypsum (facies T7) was again concentrated within the interdune area. Stage 6 depicts subsurface collapse of facies T3 and, to a much greater extent, facies T4 along a dissolution front developed adjacent to the gypsum body.
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