Curiosity rover detects elemental sulphur on Mars.

NASA's Curiosity Rover has detected crystals of elemental sulphur on the surface of Mars, the first time sulphur has been detected as a pure element on the planet. The crystals were observed at a location within Gale Crater called Convict Lake on 7 Jun 2024, and form a patch about 12 cm across. The crystals are thought to have been exposed by the rover itself driving over a rock and crushing it several days previously.

A patch of minerals including crystals of pure elemental sulphur on the surface of Mars. The image has been colour enhanced to for the benefit of Human eyes; the rover used an X-ray spectrophotometer to detect the element. NASA/JPL/CalTech/Malin Space Science Systems. 

The presence of sulphur on Mars is hardly surprising not surprising. The element is one of the most common in the universe and has been detected on all planets in the Solar System, as well as meteorites, asteroids, and comets. But most sulphur previously found on Mars has been in the form of sulphate salt evaporites, which formed as lakes and other bodies of water dried out on the surface of the planet long ago.

On Earth, sulphur deposits typically take the form of sulphates (the oxidised form of the mineral) or sulphites (the reduced form) with elemental sulphur forming in sedimentary rocks through the actions of sulphur-reducing micro-organisms in anaerobic (i.e. oxygen free) environments, and in volcanic rocks by the reaction of gaseous hydrogen sulphide and sulphur dioxide. The geology of Gale Crater is dominated by sedimentary deposits, including evaporites, but is generally low in sulphates. 

It is possible that the Convict Lake rock is of volcanic origin, and reached the Gale Crater locality as ejecta. However, the images of the rock resemble the surrounding sedimentary rocks, making it more likely that it is local in origin. This makes it likely that the sulphur has been derived from an original sulphate source in some way, although this does not necessarily imply the presence of sulphur reducing micro-organisms, as in the oxygen-free atmosphere of Mars, abiotic reducing reactions impossible on Earth become far more likely.

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Evidence of recent liquid water at low latitudes on Mars.

The surface of Mars today is a hyperarid desert, yet in many places has features apparently formed by liquid water. It is now generally accepted that liquid water was present on early Mars, when the planet had a very different atmosphere. However, that early atmosphere has subsequently disappeared, and it is now assumed that the atmospheric pressure on Mars is now to low for liquid water to form.

This being the case, it came as a great surprise to scientists when, in March 2009, droplets of liquid were observed on one of the robotic arms of NASA's Phoenix Rover. Studies of the data recovered by Phoenix eventually concluded that the conditions for hypersaline water could exist on Mars at high latitudes during the summer, when the temperature rose above the melting point of such solutions (which is significantly lower than 0°C), resulting in a freeze-thaw cycle that might help to explain some of the features seen in these regions. 

However, the wider presence of apparent water features, such as slope lineae and gullies, is harder to explain. This had led scientists do develop alternative explanations, under which such features could have developed under dry conditions, since it is difficult to understand how sufficient liquid water could be present on the surface of Mars to explain features such as slope flows hundreds of metres in length. This disparity between a theoretical presence of small amounts of water on Mars, and a necessity to invoke dry formation of features we would typically associate with the presence of large amounts of water on Earth, makes it necessary to study smaller (less than one metre) features on Mars's surface in order to understand the planet's hydrological conditions properly. This is particularly true of features at lower (i.e non-polar) latitudes, where milder conditions make a freeze-thaw cycle unlikely, and there is a higher potential for the presence of some form of microbial life.

The China National Space Administration's Zhurong Rover landed on the southern Utopia Planitia on 15 May 2021, and spent nine months exploring the Late Hesperian northern lowlands, including studying the microstructure and chemical composition of the dune features observed there.

In a paper published in the journal Science Advances on 28 April 2023, Xiaoguang Qin of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology andGeophysics of the Chinese Academy of Sciences, Xin Ren of the Key Laboratory of Lunarand Deep Space Exploration at the Chinese National Astronomical Observatories, Xu Wang, also of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Jianjun Liu, also of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Haibin Wu, again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, and of the  College of Earth and Planetary Sciences at the University of Chinese Academy of Science, Yong Sun of the Institute of AtmosphericPhysics of the Chinese Academy of Sciences, Zhaopeng Chen, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Shihao Zhang, also of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Yizhong Zhang, Wangli Chen, Bin Liu, Dawei Liu, and Lin Guo, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, Kangkang Li, again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Xiangzhao Zeng, Hai Huang, Qing Zhang, Songzheng Yu, and Chunlai Li, again of the Key Laboratory of Lunar and Deep Space Exploration at the Chinese National Astronomical Observatories, and Zhengtang Guo, once again of the Key Laboratory of Cenozoic Geology and Environment at the Institute of Geology and Geophysics of the Chinese Academy of Sciences,  present the results of a study of the surficial microstructure, morphology, and chemical compositions of dunes studied by the Zhurong Rover, and the implications of these results for the possibility of liquid water having existed on the surface of Mars at low latitudes in the recent past.

A series of detached barchan (crescent-shaped) dunes with sinuous profiles are present in the Zhurong Rover landing area, each completely detached from its neighbour. The Zhurong Rover encountered four of these as it made a north-to-south transect of the area in the first four months after it landed. 

The rover discovered that the dunes were covered by two very different types of sand, one light and one dark, with the dark sand overlying the lighter dunes, implying a second generation of deposition. The barchan dunes are composed of the lighter sand, and are 15-30 m long and 3-10 m wide. The darker sand matches the surrounding soils, and forms longitudinal dunes and ridges running over the barchans, most commonly at northwest orientated longitudinal dunes crossing the western flank of the barchan. These longitudinal dunes appear more recent, and are likely to have formed under the current modern conditions.

Exploration route of Zhurong Rover and cracks on bright sand dunes. (A) Map of the exploration route of Zhurong from May to September 2021. The HiRIC photo (0.7-m resolution) was taken by the Tianwen-1 orbiter. Dunes 1 to 4, marked by white rectangles, were measured in situ on Sols 45, 64, 92, and 99, respectively. (B) Panorama mosaics acquired by Zhurong Rover's Navigation and Terrain Camera of longitudinal dunes on barchan Dune 2, with white rectangles indicating positions of the cracks. (C) and (D) Cracks developed on the southwestern slope of longitudinal dune on the western wing of Dune 2, with a white arrow pointing to one of the cracks. (E) Panorama mosaics acquired by Navigation and Terrain Camera of barchan Dune 3, with white rectangles indicating positions of the cracks. (F) Cracks on the northern slope of Dune 3. Qin et al. (2023).

Close examination of the surface of these dunes shows more that one form of cementation holding the particles together, with a continuous crust having formed on the light dues and the particles of the darker ridges held together in agglomerated clusters. Examination of the agglomerated particles suggests the presence of hydrated sulphates, hydrated silica (particularly opal), iron oxide minerals, and possibly chlorides. The hydrated sulphates and hydrated silica are most likely to be forming the cements holding these particles together.

Water traces on bright sand dunes. (A) Topographic contour map of the environs where the trace is located. The coordinate system is east-north-up local Cartesian coordinate, and the origin is that of the rover coordinate system. The background digital orthophoto map photo was taken by the Navigation and Terrain Camera. (B) Multispectral Camera bird’s-eye-view photo showing a strip-like trace and a likely water-soaked fragmented soil block. (C) Enlarged photo showing polygonal cracks and bright polygonal ridges. (D) Enlarged photo showing circular region with the strip-like trace as a part. (E) Navigation and Terrain Camera three-dimensional image of an interdune depression between two dark longitudinal dunes. (F) A cross section of the dune along the profile of the white dash line in (E). Qin et al. (2023).

Compositionally, both the light and dark sands have high iron and magnesium contents, although silica remains the most abundant material, comprising between about 52% and about 90% of all samples. Oxides make up about 15% by weight of the light sand and about 10% by weight of the dark sand. The instrumentation used is known to be incapable of detecting volatile elements such as sulphur, chlorine, and phosphorus, as well as hydrogen and hydroxide ions.

Images showing features of agglomerates and crust on the bright sand and dark sand surfaces. (a), (d) Panorama mosaics of Dunes 1 and 3 acquired by the Navigation and Terrain Camera, where white crosses denote the target positions of the laser-induced breakdown spectrometer . (b, c) The Multispectral Camera images (band centered at 699.2 nm, with a Full Width Half Maximum of 14.8 nm) of dark sand and bright sand regions denoted by the white boxes in the photo in (a). (e) The Multispectral Camera image located at the white box in the photo in (d). The upper side of the image is the dark sand region, and the lower side is the bright sand region. For (b), (c), (e), the imaging distance are 2.67m-2.81m, 2.21m-2.28m and 3.15m-3.39m, respectively and the maximum resolutions are 0.42mm, 0.34mm and 0.51mm, respectively. Qin et al. (2023).

The second and third dunes encountered are covered with polygonal cracks, although these are seen only on the underlying bright sand dunes, not the darker sand ridges running across them. The polygons formed by the cracks have an average area of 55.2 cm², and an average side length of 4.8 cm, far smaller than cracks previously observed on Mars by remote sensing. Assuming that the cracks have a depth to width ratio of between 1/3 and 1.4, this would equate to a depth of 1.25-1.7 cm. The majority of the polygons are pentagons, though they range from triangular to heptagonal in shape. The average internal angle of the polygons is 120°, and intersections between cracks are typically Y-shaped.

The MI images of bright sand and dark sand. (a), (d) Panorama mosaics of Dunes 2 and 3 acquired by Navigation and Terrain Camera. (b), (c) The MI images before and after ablation by e laser-induced breakdown spectrometer at the marked target (cross) on the dark sand surface of a longitudinal ridge on the western flank of Dune 2. The image size is 1024 pixel × 1024 pixel. (e), (f) The MI images before and after ablation by e laser-induced breakdown spectrometer at the marked target (cross) on the bright sand surface of Dune 3. The yellow dashed ellipse encircles the e laser-induced breakdown spectrometer crater. (g) The quartzite used in the laboratory experiment. (h) The MI image of the rock surface lasered by the laser-induced breakdown spectrometer. (i) The MI image of onboard Nontronite calibration target after probing with the laser-induced breakdown spectrometer obtained on Sol 58. The red arrow points to the crater created by laser-induced breakdown spectrometer laser ablation. Qin et al. (2023).

A light-toned, strip-like trace, over 40 cm long and about 1.5 cm wide was observed within the interdune depression of the second barchan dune. This ran along the lowest part of the trough depression, and separates light and dark bands of sand, with a dark sand slope to the north and a light sand slope to the south, with abundant polygonal cracks. The shape of this trace appears to be exactly what would be expected by pooled water, should this be able to exist here, and the underlying crust be impermeable to water.

Based upon the superposition of the features, Qin et al. conclude that the light-coloured barchan dunes were formed first, then became encrusted with sulphates, and possibly chlorides, during a more humid climatic phase. 

In order to determine the age of these dunes, Qin et al. looked at the density of craters on the land-surface they form part of (the rate at which asteroids randomly impact Mars is considered to be approximately constant, so that parts of the Martian surface can be dated by the density of impact craters), concluding that this surface was between 400 000 and 1.4 million years old. 

The polygonal cracks which have formed on the surface of some of these cracks are believed to have been caused by a loss of moisture, either through drying or desiccation, with the dark, longitudinal dunes forming after this, and finally the sand in the longitudinal dunes becoming agglutinated into clumps. The cementing of the sands requires a liquid or gas which was able to fill the pore spaces between them, then transform into a solid state. Such substances would include carbon dioxide gas turning into dry ice, liquid water freezing into ice, or various salts and other hydrated chemicals precipitating out of solution as the water in which they were dissolved evaporates. The conditions around the Zhurong landing site make the formation of dry ice and/or water ice highly improbable, and both of these would be detectable by the laser-induced breakdown spectrometer on the Zhurong Lander, which has found no evidence of their presence. However, hydrous sulfates, opaline silica, ferric oxides, and probably chlorides, have bee detected, and this mixture would provide a suitable cement for the sand grains.

The formation of a cement from a mixture of salts and hydrated minerals requires the presence of liquid water. This could have originated from rain, snow, or frost, or have upwelled from a subterranean source, although the evaporation of groundwater drawn upwards by capillary action seems unlikely, as there are cracked evaporation surfaces on the raised dunes, but not the surrounding flatlands, which makes the precipitation of water, either as rain or frost/snow which then thawed before evaporation, the most likely explanation.

The saturated vapor pressure (point at which the atmosphere can hold no more evaporated water, and it begins to precipitate out) is unrelated to the atmospheric pressure, although the temperature must be above 0°C for liquid rain to fall. At 0°C on Mars the saturated vapor pressure would be 611 pascals, while the atmospheric pressure observed on Mars by the Zhurong Lander  is between 786 and 834 pascals, meaning that the atmosphere would need to be about 72% water for rain to fall. Since the modern Martian atmosphere is about 95% carbon dioxide, liquid precipitation on Mars is currently impossible.

Several different landers have now taken atmospheric readings on Mars, giving a range of surface temperatures between -105°C and -5°C, a range of atmospheric pressures between 683 and 849 pascals, and a vapor pressure of 0.27 pascals. Under these conditions, the frost point (point at which the temperature drops so low that water absorbed into the atmosphere precipitates out as frost) would be about -74°C, which means frost would be possible at the Zhurong landing area. More widely, it is assumed that frost and snow are relatively common on Mars.

Mixing water ice, from frost or snow, with salts could potentially lead to its melting point being lowered sufficiently for highly saline liquid water to form. Any subsequent raise in temperature could subsequently lead to water evaporation, with seasonal or even daily cycles of frost formation, melting, and evaporation enabling the formation of cements.

The temperature on Mars is thought to rise rapidly between 5.00 and 6.00 am, local true solar time, providing an interval in which frost can sublimate, and potentially also in which it could melt and then evaporate in a hyper-saline environment. This happens seasonally, with steeper rises and higher temperatures achieved in local summer.

Map showing the number of days (noted on contour lines in terms of sols) during a Martian year and locations where the ground temperature is exceeds 0˚C. Contour intervals are 40 sols. The Zhurong landing site marked with a red star. Qin et al. (2023).

The orbital obliquity of Mars (i.e. the angle at which it is turned to the Sun, which determines the severity of the seasons) is thought to have been equal to or greater than it is now throughout the past 1.4 million years, which would mean that the climate of Mars has been comparable to or more humid than Today throughout this interval. This would imply that the formation of liquid water at low latitudes on Mars has remained at least as possible as it is today over this period.

Such a process of repeatedly forming hypersaline solutions would facilitate the dissolution of silica from sand grains to form opal, as well as attacking other minerals, enabling hydrated sulphates and iron oxides to form.

The polygonal cracks on the surface Mars are also almost certainly the result of either freeze-thaw thermal contraction or desiccation, in response to seasonal or daily changers in temperature, with their general shape suggesting the later is more likely. Meteorite impact effects and carbon dioxide freeze/sublimate cycles have been suggested as an origin for similar cracks elsewhere on Mats, but there are no signs of any meteor impacts large enough to have caused these cracks near the Zhurong landing site, and carbon dioxide is unable to freeze out of the Martian atmosphere this far from the Martian poles.

Qin et al.'s study is the first small-scale study of such cracks at low latitudes on Mars. They believe that these features are almost certainly the result of desiccation, but cannot rule out an alternative hypothesis, in which the cracks are formed by the freezing of hypersaline water, causing cracks to form in the crust under tensile stress. 

All of the features seen in the Zhurong landing area point towards the presence of saline water, providing evidence that liquid water can form at low latitudes on Mars. Qin et al. propose that water accumulated on top of the dunes as frost or snow after the atmospheric temperature dropped below the frost point, then melted due to a combination of rising temperatures and contact with salt within the sands. which would in turn facilitate the formation of hydrated silica (opal) and iron oxides. This water would then evaporate away, at fairly low temperatures due to the low atmospheric pressure on Mars, leaving the salts to precipitate out and form a cement between the sand grains, forming cracks on the dune surface as they dried out. This cycle would likely repeat numerous times.

If this hypothesis is correct, then it suggests that the amount of liquid water available on the surface of Mars in the recent past is considerably higher that previously suspected. It has previously been suggested that transient films of water might have formed on the surface of rocks in the recent past due to acid weathering, and that small amounts of water might have formed duricrusts and rock surface coatings over geological timescales. The situation at the Zhurong landing site appears quite different, with apparently mobile sands unlikely to have become cemented together by any process operating on a geological timescale. Rather this appears to be the result of an evaporative process operating over a relatively short period. less than 1.4 million years ago, and possibly less than 400 000 years ago.

This recent presence of water at 'tropical' latitudes on Mars becomes less unreasonable when it is remembered that Mars is thought to have undergone a significant change in the obliquity of its orbit about 5 million years ago, and only to have reached its current, low-obliquity orbital configuration about 3 million years ago. This may suggest that the thick ice caps present at the current Martian poles are a relatively modern feature, a result of a fairly recent transfer of water from lower latitudes, something which may well have still being occurring 1.4 million years ago. This provides further support for the theory that high-obliquity excursions in the Martian orbit might well have provided enough water for gully formation. Thus the presence of sufficient saline water at low latitudes on Mars for evaporite formation in the recent past is in fact in accord with our current understanding of the planet's recent geological past.

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Two Earthquakes in Moyen-Ogooué Province, Gabon.

The United States Geological Survey recorded two Earthquakes in Moyen-Ogooué Province, Gabon, on Tuesday 27 April 2021. The first, a Magnitude 4.5 Earthquake at a depth of 10 km, occurred roughly 44 km to the southwest of the town of Lambaréné, about 2 minutes before  1.40 am local time (about two minutes before 0.40 am GMT), with the second, a Magnitude 4.7 Earthquake also at a depth of 40 km, which happened about 40 km to the southwest of Lambaréné, happening about two minutes later. There are no reports of any damage or injuries associated with these events, but they are likely to have been felt locally.

 

The approximate locations of the 27 April 2021 Gabon Earthquakes. USGS.

Earthquakes are extremely rare in Gabon, which lies over Precambrian basement rocks which for the most part have not been tectonically active since the rifting which separated Africa from South America as the Atlantic Ocean in the Mesozoic. However, the North Gabon Sub-basin is cross-cut by a series of northwest-southeast trending faults associated with this Mesozoic rifting. Movement on these rift zones is now extremely limited, but the area is overlain by extensive evaporite salt deposits, which are structurally weak, and altering the way in which faults propagate.

 

Fault systems and tectonic units division of the Gabon Coastal Basin. Fz. Fault. Chen et al. (2013).

Because salt deposits are dense and structurally weak, movement on faults below them does not typically propagate upwards though them. Instead, the salt layer will often expand laterally, accommodating the movement of the fault. Eventually, however, this displacement becomes to great for the overburden layer, which leads to the development of new faults in that layer, offset from the faults in the basement. Thus, gradual movements in the basement rock can be translated into sudden, shallow faulting in surface layers, which we experience as Earthquakes.

 

Fault development on a salt layer. Wikipedia.

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Magnitude 5.2 Earthquake in Ngounié Province, Gabon.

The United States Geological Survey recorded a Magnitude 5.2 Earthquake at a depth of 10 km, roughly 45 km to the west of the town of Fougamou in Ngounié Province, Gabon, slightly before 6.10 pm local time (slightly before 5.10 pm GMT) on Saturday 6 March 2021. There are no reports of any damage or injuries associated with this even, but it was felt across the west of Gabon.

 

The approximate location of the 6 March 2021 Gabon Earthquake. USGS.

Earthquakes are extremely rare in Gabon, which lies over Precambrian basement rocks which for the most part have not been tectonically active since the rifting which separated Africa from South America as the Atlantic Ocean in the Mesozoic. However, the North Gabon Sub-basin is cross-cut by a series of northwest-southeast trending faults associated with this Mesozoic rifting. Movement on these rift zones is now extremely limited, but the area is overlain by extensive evaporite salt deposits, which are structurally weak, and altering the way in which faults propagate. 

 

Fault systems and tectonic units division of the Gabon Coastal Basin. Fz. Fault. Chen et al. (2013).

Because salt deposits are dense and structurally weak, movement on faults below them does not typically propagate upwards though them. Instead, the salt layer will often expand laterally, accommodating the movement of the fault. Eventually, however, this displacement becomes to great for the overburden layer, which leads to the development of new faults in that layer, offset from the faults in the basement. Thus, gradual movements in the basement rock can be translated into sudden, shallow faulting in surface layers, which we experience as Earthquakes.

 

Fault development on a salt layer. Wikipedia.

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The end of the Jabłonna Reef system of the Zechstein Sea.

The Zechstein Sea was once a shallow marine basin that occupied much of the area covered by northeastern England, the North Sea, Germany and Poland during the Late Permian. It was connected to the Palaeotethys Ocean, and formed a shallow sea dominated by microbial reef systems. Towards the end of the period it repeatedly became cut off from the wider ocean and developed into a hypersaline basin similar to the modern Dead Sea, before vanishing completely. The Zechstein evaporite deposits are today mined for salt, gypsum, and potash in England, Germany and Poland. Carbonates preceding the vast accumulation of evaporites reflect changes in the basin hydrology and the degree of connection to the open sea. The resulting sequence of deposits mirrors a shift from normal marine to evaporitic conditions. In the Zechstein Basin microbial deposits abound in the uppermost part of the first Zechstein cycle carbonate, of middle Wuchiapingian age, both in marginal and central parts of the basin. Traditionally, the Zechstein Group is divided into cycles reflecting progressive evaporation: at the base of a cycle are normal marine sediments; these are followed by sediments indicative of increasing salinity, first sulphates, next chlorides and eventually potash salts. Traditionally, four evaporitic cycles were distinguished. The total stratigraphic thickness of the Zechstein deposits in the basin centre exceeds 1.5 km.

In a paper published in the Journal of Palaeogeography on 3 July 2020, Tadeusz Marek Peryt and Marek Jasionowski of the Polish Geological Institute, Paweł Raczyński of the Institute of Geological Sciences at the University of Wrocław, and Krzysztof Chłódek of the Polish Oil and Gas Company, report and interpret the changes in the middle Wuchiapingian sedimentary environments at the transition from carbonate to sulphate deposition at the Jabłonna Reef area in southwest Poland.

Microbial deposits are an essential component of Zechstein Limestone reefs, and their frequency increases upsection. Such a trend was regarded in the past as the record of increasing seawater salinity that eventually led to the deposition of sulphate evaporites. However, a recent study indicated that Echinoids are common throughout the Zechstein Limestone section except close to its top, suggesting that most of the Zechstein Limestone sedimentation was within the normal range of marine salinity and remained at roughly the same level. However, the evaporite drawdown effect caused significant salinity increase at the top of the Zechstein Limestone deposits. The eventual rise of salinity led to the onset of the evaporite deposition in the basinal facies. The sharp boundary between the Zechstein Limestone and the overlying sulphate deposits (Lower Anhydrite) in the basinal facies is due to the nature of evaporites that start to precipitate immediately when the brines reach saturation.

Lithostratigraphy and sequence stratigraphy of the basal Zechstein strata in southwest Poland. Peryt et al. (2020).

Peryt et al.'s choice of the study area was controlled by two factors. First, the uppermost part of the Zechstein Limestone and the transition Zechstein Limestone-Lower Anhydrite was cored in three boreholes (Jabłonna 1, 2, and 3) of four drilled in this particular reef. Secondly, both the Zechstein Limestone (except its uppermost part) and the Lower Anhydrite were characterized in detail in previous studies. Thus, this study fills a gap in the knowledge of depositional history at the carbonate-sulphate transition in the basinal setting.

Location of the study area. (a) The Zechstein Basin, asterisk shows the location of the Jabłonna Reef; (b) Palaeogeography of the Zechstein Limestone, rectangle shows the Wolsztyn reefs shown in (d); (c) The location of arbitrary line 2 showing the location of boreholes (black dots); (d) Reefs of the Wolsztyn palaeo-High; (e) Interpretation of Zechstein along the cross-section shown in (c). Abbreviations: Ca1, Zechstein Limestone reef; eva, evaporites (anhydrite and halite) of the PZ1 cycle (cyclothem); PZ1, PZ2, PZ3, Polish Zechstein cycles (cyclothems); Z₁𝇃, Z₁, Z₂, Z₃; Zechstein seismic reflectors. Peryt et al. (2020).

The Jabłonna Reef is one of many isolated reefs located on the elevated parts of the Brandenburg-Wolsztyn-Pogorzela High that is a part of the Variscan Externides consisting of strongly folded, faulted and eroded Visean to Namurian flysch deposits, capped by a thick cover of Upper Carboniferous–Lower Permian volcanic rocks. The reefs came into existence shortly after the rapid transgression of the Zechstein sea that flooded, probably catastrophically, this intracontinental depression located well below the contemporaneous sea level, some 257 million years ago. The rapid inundation allowed for almost perfect preservation of the uppermost Rotliegend landscape. The rapid inundation was succeeded by several rises in sea level, and thus the Zechstein Limestone section of the Wolsztyn palaeo-high may comprise only the younger part of the unit elsewhere.

The analysis of three dimensional seismic sections indicated that the Jabłonna Reef is composed of three parts: two small, roughly elliptical, and west northwest-east southeast elongated (penetrated by boreholes Jabłonna 3 and Jabłonna 4) and one large, elongated (penetrated by boreholes Jabłonna 1 and Jabłonna 2). Coeval Zechstein Limestone deposits in the depressions between and outside the reefs are thin (a few metres at most), and they are eventually underlain by the middle Wuchiapingian Kupferschiefer, one of the prime correlation markers in northwest and Central European stratigraphy. This unit records a period of basin-wide euxinic conditions, and can thus be considered an excellent time-marker.

The mineralogical composition of the Zechstein Limestone of the Jabłonna Reef varies, although limestone is the main rock type. Most of the Zechstein Limestone sections of the Jabłonna Reef is composed largely of bioclastic (mostly bryozoan) grainstones, and bryozoan and microbial boundstones that were formed in subtidal environments. The general shallowing-upward nature of deposition in the Jabłonna Reef area resulted in reef-flat conditions with ubiquitous microbial deposits in its central part. Subsequently, because of reef-flat progradation, the entire Jabłonna Reef area became a site of very shallow, subaqueous deposition. The uppermost part of the Zechstein Limestone, 2.8–5.1m thick, shows a breccia texture, and is the subject of Peryt et al.'s study.

The Lower Anhydrite consists of nodular anhydrite occurring at the base, which gradually passes into anhydrite with pseudomorphs after gypsum crystals. It is overlain by the Upper Anhydrite. In total, the thickness of PZ1 (Polish Zechstein 1) anhydrite in the Jabłonna Reef area varies from 59.2 to 66.0m; these are followed by PZ2-PZ4 (Polish Zechstein 2-Polish Zechstein 4) that are several 100m thick, and then by Triassic and Cainozoic deposits.

The reefs related to the Wolsztyn palaeo-high are excellent gas reservoirs, and part of their porosity owes its origin to freshwater flushing after deposition of the major part of the Zechstein Limestone and/or during the deposition. Certainly, the freshwater diagenesis occurred before the Lower Anhydrite deposition, as the subsequent geological history indicates that the reef deposits were continuously affected by marine-derived brines. Thus the geological history of the area rules out freshwater diagenesis after the onset of the PZ1 evaporite deposition on the top of the Jabłonna Reef.

Altogether 43.4m of core from three borehole sections: Jabłonna 1, Jabłonna 2 and Jabłonna 3 across the uppermost part of the Zechstein Limestone (15 m) and the Lower Anhydrite (41.3 m) were subjected to a detailed sedimentological analysis. Following detailed core measuring, 15 polished core samples and 40 thin sections were examined for sedimentological aspects of the Zechstein Limestone and to record the changes in the frequency of occurrence and the state of preservation of fossil taxa. Twelve thin sections were studied with cathode luminescence.

The uppermost part of the Zechstein Limestone in the Jabłonna 1 borehole is 4.0m thick (depth 2342.0–2346.0 m) and shows a brecciated nature. Clasts are usually sharp-edged and of very various, often centimetric sizes, and show the inclined arrangement. They are composed of limestone and dolomite showing various microbial textures and more rarely organo-detrital texture. These clasts are embedded in nodular anhydrite(−enriched) matrix, and sometimes are accompanied by fine sharp-edged clasts that commonly occur also in the strata underlying the brecciated top part of the Zechstein Limestone. This part of the section smoothly passes into fine nodular, bedded anhydrite that shows abundant carbonate content; the thickness of beds varies from a few to about 10 cm, and in places, the beds are slightly inclined. This portion is 3.2m thick and it gradually passes (0.6 m) into massive anhydrite with clear centimetric pseudomorphs after upright-growth gypsum crystals (this part of the sequence is 2.0m thick), followed by finenodular anhydrite (10.3 m thick). Then, anhydrite breccia (0.6 m thick) occurs, followed by recrystallised anhydrite of conglomeratic appearance (12.3 m thick) with locally occurring clear pseudomorphs after bottomgrowth gypsum crystals.

Samples of the uppermost Zechstein Limestone of the Jabłonna 1 borehole; the depths in relation to the top of Zechstein Limestone are: (a) 0.1 m, (b) 0.9 m, (c) 1.3 m, (d) 2.15 m, (e) 3.4 m, (f) 3.7 m, (g) 4.8 m. Abbreviations: an, anhydrite; dd, detrital dolomite; md, microbial dolomite. (a)-(c) Clasts of microbial carbonates and peritidal laminites within anhydritic and dolomitic matrix; (d) steeply inclined pisolitic dolomite; (e), (f) large clasts of microbial carbonate in nodular anhydrite; (g) microbial encrustations and cement crusts (arrowed) stabilising detrital deposit consisting of sharp-edge clasts, underlying the brecciated deposits shown in (f). Peryt et al. (2020).

In the Jabłonna 2 borehole, the breccias (2.8 m thick) consist of clasts of limestones (mostly Bryozoan grainstone and Stromatolite) in a dolomite matrix. In some instances, dolomicrite with quartz silt and micas (of aeolian origin?) were recorded. These breccias occur at a depth of 2345.4–2348.2 m. Due to the abundance of anhydrite nodules in the top 1.1 m, the transition to the Lower Anhydrite is, in fact, gradual. Above the conventional boundary, now placed at a depth of 2345.4 m, distinctively bedded nodular anhydrite (5.0 m thick) occurs, and the bedding is disclosed by dolomite laminae and lenses showing carbon¹³ and oxygen¹⁸ levels characteristic of the Zechstein evaporite formations. In the upper part of the section, a 2.4-m-thick interval composed of bedded nodular anhydrite occurs, which shows clear pseudomorphs after upright-growth gypsum crystals.

Aspects of the uppermost part of the Zechstein Limestone in the Jabłonna 1 borehole; the depths in relation to the top of Zechstein Limestone are: (a), (b) 2.8 m, (c) 2.4 m, (d)-(h) 0.1 m; Abbreviations: an, anhydrite; bs, Bivalve shell; fe, Foraminiferal encrustation; ga, Gastropod shell; me, microbial encrustation; os, Ostracod. (a), (b) Clast of recrystallised peloidal deposit showing relics of stromatolitic lamination and encrusting Foraminifers; arrow shows the carbonate crust with common pseudomorphs after lenticular gypsum crystals shown in (b); (c) Bivalve shells with microbial encrustations, Gastropods, Ostracods, encrusting Foraminifers and other small allochems in recrystallised micritic matrix and anhydrite cement (sample taken from a clast); (d) sample shown by X (3) above; (e)-(h) fragments of (d) showing aspects of microbial lamination (e), (f), (h) and encrusting Foraminifers (g), (h). Peryt et al. (2020).

The uppermost part of the Jabłonna 2 section resembles most of the underlying deposits consisting of granular sediments with inclined crusts of possible microbial laminites. However, due to dolomitisation and severe recrystallisation, these primary features are poorly (but still) visible. A complex diagenesis in this part of the section might account for the seemingly brecciated nature. But on the other hand, the occurrence, in the close neighbourhood, of clasts of rocks that originated in various environments indicates their transportation.

Polished section (a) and thin sections (b)-(d), uppermost Zechstein Limestone, Jabłonna 2 borehole (the depths in relation to the top of Zechstein Limestone are: (a) 0.6 m, (b) 1.4 m, (c) 1.7 m, (d) 2.2 m). Abbreviations: an, anhydrite; dl, detrital limestone; ml, microbial limestone. (a) Dolomite breccia composed of sharp-edged clasts of peritidal carbonates and, in the top part of the sample, nodular anhydrite; (b) microbial encrustations at the boundary of a clast composed of Bryozoan grainstone that is enclosed in nodular anhydrite; (c) recrystallised limestone composed of crinkle laminations and fine allochems; arrows show Bryozoan zoaria; (d) micritic dolomite showing laminae (arrow) and faint outlines of allochems, with abundant fine quartz grains (white dots) and rare fine mica flakes. Peryt et al. (2020).

The brecciated portion of the Zechstein Limestone in the Jabłonna 3 borehole is 5.1m thick and occurs at a depth of 2348.9–2354.0 m. The clasts show various sizes – from less than 1 mm to several cm. Clasts are accompanied by microbial laminations that also occur at the Zechstein Limestone-Lower Anhydrite boundary. They are overlain by nodular anhydrite (0.9 m thick) containing abundant dolomite in the matrix; the nodules show a clear upward trend toward the bedding arrangement. Next, there is bedded nodular anhydrite (2.4 m thick), most probably clastic, followed by nodular anhydrite with pseudomorphs after upright-growth gypsum crystals up to 2 cm high. The topmost 1.25 m of the cored interval consists of massive anhydrite with gypsum pseudomorphs up to 25 cm high.

Polished sections (a)-(d) and thin sections (e)-(g) from the uppermost Zechstein Limestone, Jabłonna 3 borehole; the depths in relation to the top of Zechstein Limestone are: (a) 0m, (b) 2.2 m, (c) 0.5 m, (d) 1.5 m, (e) 0.6 m, (f) 2.7 m, (g), (h) 3.8 m. Abbreviations: an, anhydrite; dd, detrital dolomite; ml, microbial limestone. (a) Zechstein Limestone–Lower Anhydrite boundary (dotted): microbial (thrombolitic) dolomite overlain by dolomite-rich anhydrite with anhydrite nodules; (b) clasts and microbial laminations steeply inclined within nodular anhydrite; (c), (d) clasts of various size in the anhydrite matrix; (e) aspect of microbial carbonate (filaments?); (f) sharp-edge fine clasts of microbial carbonate with isopachous cement; (g), (h) Stromatolitic encrustations on and accompanied by detrital deposit; X in (h) indicates the location of (g). Peryt et al. (2020).

In terms of mineralogy, the uppermost portions of the Jabłonna sections are generally dolomites with a variable contribution of anhydrite. They show a more complex mineralogical composition. Besides dolomite and anhydrite, they also contain calcite and a minor admixture of accessory minerals, such as celestite, fluorite and authigenic quartz. 

Medium crystalline nonplanar unimodal dolomite with partly preserved primary fabrics (?boundstone–upper four images, bioclastic grainstone–lower four images). (a), (b), (e), (f) transmitted light microphotographs (plane-polarized light and crossed polars, respectively), (c), (g) cathodoluminescence images, (d), (h) backscattered electron images. Abbreviations: dol, dolomite; anh, anhydrite; fine red dots mark spots of microprobe analyses). The anhedral dolomite crystals are usually few tens of micrometres in size and exhibiting undulatory extinction in crossed polarised light. The dolomite is red with some yellowish patches in cathodoluminescence. Primary porosity is plugged with anhydrite cements. Fine crystalline (dolomicritic) patches in the image (a) are encrusting Foraminifers. Jabłonna 1 borehole, sample located 0.5m below the top of the Zechstein Limestone. Peryt et al. (2020).

Two main varieties of dolomite can be distinguished. The most common one is usually nonplanar mediumcrystalline, unimodal dolomite composed of anhedral crystals, mostly a few tens of micrometres in size, exhibiting undulatory extinction in crossed polarised light. However, planar euhedral dolomite crystals are also visible in places. The dolomite crystals are typically cloudy and are rich in inclusions. They show red cathodoluminescence with yellowish spots or zones in places. The dolomite crystals form massive mosaics or very cavernous masses plugged with coarsely crystalline anhydrite and sometimes coarsely crystalline calcite. In some cases, pores are lined by thin rims dolomite crystals. Dolomitisation was generally fabric-destructive and matrix- and grain-replacive, but remains of original fabrics are still traceable in places. The porosity may be in part both secondary (e.g. after dissolution of some components, e.g. fossils) and primary (e.g. original interparticle porosity); the cavernous portions mimic probably original sedimentary fabrics, such as the grainstone texture.

Coarse crystalline calcite (orange in cathodoluminescence) within medium crystalline dolomite (bright red in cathodoluminescence) with abundant anhydrite cementation. (a), (b), (e), (f) transmitted light microphotographs (plane-polarized light and crossed polars, respectively), (c), (g) cathodoluminescence images, (d), (h) backscattered electron images images. Abbreviations: cal, calcite; dol, dolomite; anh, anhydrite; fine red dots mark spots of microprobe analyses. The calcite crystals are probably burial cements that occlude porosity within dolomite. Jabłonna 3 borehole, sample located 0.6 m below the top of the Zechstein Limestone. Peryt et al. (2020).

The second type of dolomite in the sections studied, volumetrically subordinate, is finely crystalline dolomite (dolomicrite). Some fossils (e.g. sessile Foraminifers) or microbialitic fabrics are mimetically replaced by dolomite.

Calcite is present only in some of the thin sections studied. Petrographically, two calcite varieties can be distinguished: massive calcite mosaics and coarsecrystalline calcite cements distributed within dolomite.

Coarse crystalline calcite mosaic with numerous euhedral fluorite crystals. (a), (b) Transmitted light microphotographs (plane-polarized light and crossed polars, respectively); (c) cathodoluminescence image, (d) backscattered electron image. Abbreviations: cal, calcite; dol, dolomite; fl, fluorite; qtz, quartz; fine red dots mark spots of microprobe analyses. The clearer (inclusion-poor) crystals with distinct internal zonation pattern (orange in cathodoluminescence) are probably cements that occlude porosity within calcite mass composed cloudy (inclusion-rich) dull dark/nonluminescent in cathodoluminescence crystals. Scattered tiny crystals or irregular patches of dolomite (pinkish red in cathodoluminescence) occur within the calcite. Additionally, a euhedral quartz crystal is visible in the backscattered electron image (d). Jabłonna 1 borehole, sample located 0.5m below the top of the Zechstein Limestone. Peryt et al. (2020).

The calcite mosaics are composed of anhedral, medium to coarse crystals that are about 100 μm long; these were encountered only in one thin section derived from the lowermost breccia sequence in the Jabłonna 1 borehole. The calcite crystals usually appear cloudy due to numerous inclusions. Patches or aggregates of small dolomite crystals are chaotically dispersed throughout the calcite mosaics. Additionally, euhedral fluorite crystals are dispersed within the calcitic mosaics.

The second type of calcite is coarse calcite cements that fill the pores after the dissolution of some former crystals/skeletons or just the porosity within crystalline dolomite. They are very clear and translucent (inclusion-poor) in transmitted light and exhibit a faint pale-yellow/orange cathodoluminescence.

Based on petrographic studies, the uppermost portions of the Jabłonna sections studied experienced rather simple diagenetic history. They were affected by only one episode of pervasive dolomitisation that usually obliterated to a significant extent of its original textures. The dolomitisation resulted in one type of dolomite, usually nonplanar medium-crystalline dolomite. Such dolomite texture is thought to originate in a higher-temperature environment. It seems that the dolomitisation took place under shallow-burial conditions and could be a result of the seepage of brines that originated during the deposition of the PZ1 anhydrite, as it is generally constrained only to the uppermost portions of the Jabłonna sections studied. Downward the sections, calcite mineralogy prevails and the Jabłonna Reef deposits are still essentially limestones.

Spatially very limited calcite cementation postdates the dolomitisation. The calcite cements show the characteristics typical of higher-temperature burial diagenesis (large and inclusion-free translucent monocrystals, quite homogeneous in cathodoluminescence). Possibly, their crystallisation could be related to fluids released during gypsum-to-anhydrite transition (dehydration), which were relatively rich in calcium ions. Calcite cementation was followed by pervasive anhydrite cementation (in places preceded by, or undergoing simultaneously with, local celestite crystallization) that reduced the remaining porosity significantly.

The uppermost part of the Zechstein Limestone is, in general, much more altered diagenetically than the other parts of the Zechstein Limestone are. This is interpreted as due to two circumstances. The first is the early spelean-like diagenesis in a carbonate-evaporite salina in which the deposits of the uppermost Zechstein Limestone of the Jabłonna Reef have originated (this is discussed later in this paper). The second is its location in the neighbourhood of the anhydrite deposits being the screen for the ascending fluids. In general, dolomitization of the Wolsztyn reefs was polyphase, and this is particularly characteristic of this part of the profile.

Although microbial deposits often show the inclined (even to subvertical) position, this is probably related to the changing configuration of microbial reef complex in time, as it was previously demonstrated for the Westerstein Reef in the Harz Mountains. The alternative for a part of the inclination, in particular, accompanied by the occurrence of complex coated grains, is that they might have resulted during the development of teepee structures which might be expected in a very shallow subaqueous environment that was subject to quite common episodes of subaerial exposure. In any case, in contrast to the most part of microbial biofacies, the strata characterized in this paper cannot be related to the merely subtidal environments.

Peryt et al. assume that during the evaporative drawdown that resulted first in the deposition of thin microbial carbonate in the basinal sections, the Jabłonna Reef became subaerially exposed. Its top (and possibly slopes) became thus an emersion surface, which led to an irregular, karstified and brecciated relief surface related to a stratigraphic hiatus before the establishment of a salina environment in which the regolith became cemented by precipitated halite. Thus, the deposits composing the topmost part of the Zechstein Limestone actually derive from weathering and erosion of the microbial deposits of the uppermost part of the shallowing-upward sequence of the Zechstein Limestone and from precipitation of gypsum from transgressing brines of the sulphate system developed in the basinal facies.

Diagrammatic presentation of sea/brine level changes at the Zechstein Limestone/Lower Anhydrite boundary along the arbitrary seismic line. (1) progradational deposits; (2) degradational deposits; (3)–(4) progradational to aggradational deposits (3)–(4); (5) aggradational to retrogradational deposits: (1) Final stage of deposition of microbial deposits of the reef flat environment in Jabłonna 1. (2) Final stage of deposition of microbial deposits of the reef flat environment in Jabłonna 2 and 3; subaerial exposure in Jabłonna 1. (3) Stages (3a)-(3c) of sea level fall related to evaporative drawdown – subaerial exposure of the Jabłonna Reef and origin of fresh-water diagenesis and anhydrite cementation (possibly related to longer periods of stabilization of sea level during steps in sea level fall). (4) Deposition of microbial deposits in the top layer of the Zechstein Limestone in the basin. (5) Possibly the (beginning of) deposition of carbonate-enriched strata of the Lower Anhydrite. Peryt et al. (2020).

It was previously shown that the general shallowing-upward nature of deposition in the Jabłonna Reef area resulted in reef-flat conditions with ubiquitous microbial deposits in the central part of the Jabłonna Reef. Then, the reef flat started to prograde, and eventually, the entire Jabłonna Reef area became the site of very shallow, subaqueous deposition. Once the sea-level has dropped slightly, the Jabłonna 1 area became exposed first. At that time, shallow subtidal deposition still continued in the other parts of the Jabłonna Reef. Then, the areas at Jabłonna 2 and Jabłonna 3 became exposed, possibly due to the ongoing fall of sea level. 

The result of the long subaerial exposure of the Jabłonna Reef was the origin of an emersion surface and an irregular, karstified and brecciated relief. The length of the stratigraphic hiatus before the establishment of the salina environment is difficult to ascertain. In fact, there is no accord about the length of individual Zechstein formations and members, and even of the entire Zechstein, but Peryt et al. assume that it possibly took a few 100 000 years.

The duration of this exposure is difficult to specify because of several reasons. First, the depositional duration of the Zechstein and its particular cycles is subject to debate, but the estimate that the Z1 phase was about two million years long seems valid. Second, there are substantial differences in the rate of deposition of carbonates and evaporites. Subaquatic sulphates often have the accumulation rates in the order of 1–40 m per thousand years, and the rate of chloride deposition is 4–5 times greater. The duration of the deposition of the Zechstein Limestone has been estimated at about 400 000 years, based on the average rate of deposition of platform carbonates, but it did not include the time of subsequent exposure of marginal carbonate platform (and the reefs of the Wolsztyn palaeo-High). Considering the scale of freshwater diagenesis, the length of the exposure was presumably similar to the range of Zechstein Limestone deposition.

Microbial carbonates are the primary lithology in the uppermost part of the Zechstein Limestone of the Jabłonna Reef. The increase in the amount of microbial deposits upsection was regarded in the past as the record of increasing seawater salinity that eventually led to the deposition of sulphate evaporites. However, a recent study of basin sections indicated that, for the most part of the Zechstein Limestone sedimentation, the salinity remained at roughly the same level of normal seawater until it increased due to the evaporite drawdown effect at the very end of the Zechstein Limestone deposition. Then, the salinity increase eventually led to the onset of the evaporite deposition in the basinal facies.

Microbial carbonates also abound in the shelf-edge reef of the English Zechstein, where Algal Stromatolites and diverse laminar encrustations form up to 90% of reef-flat rock. Thus, in biofacies terms, this part of the Zechstein Limestone is dominated by Stromatolite biofacies. Microbial carbonates occur in situ, and they compose the majority of clasts. However, also clasts of Bryozoan grainstones occur. These rocks are typical for the biofacies occurring below the Stromatolite biofacies that formed in low-energy (indicated by in situ, or almost complete overthrown, zoaria) and occasional high-energy (indicated by intercalations of coquinas) lagoonal environments. These lagoons could evolve into salinas, possibly when the communication with the basin became cut off. Accordingly, there were many environmental perturbations prior to the evaporative drawdown.

The microbial carbonates that developed in the uppermost Zechstein Limestone throughout the basin are commonly not coeval, though. A thin packet of microbial deposits occurring at the topmost part of the basinal sections of the Zechstein Limestone originated following the sea level fall at the end of the Zechstein Limestone deposition. The deposition of peritidal carbonates in the basinal facies was accompanied by subaerial emergence of the marginal carbonate platform (and the reefs related to the Wolsztyn palaeohigh). Subsequently, as a result of a basin-wide deepening-upward trend recorded in the Lower Anhydrite, the deposition of the Lower Anhydrite began at the reef zone. Such a scenario explains well the gradual change from carbonate to sulphate deposition in the Jabłonna Reef. The change took place in shallow salinas, i.e. in the same environment in which the oldest sediments of the Lower Anhydrite formed close to the Wolsztyn reefs, in the area characterized by condensed sequences. This leads to the conclusion that the uppermost part of the Zechstein Limestone in the reef area postdates the uppermost Zechstein Limestone in the basinal area. As recently commented, 'the dynamic relationships between marine and freshwater systems on carbonate platforms and their responses to sea level rise remain poorly understood. This is surprising given the frequency of platform exposure and flooding events seen in the stratigraphic record.' Considering that the flooding of the Zechstein reefs was executed by saline brines, and that the freshwater system has formed during subaerial exposure of the reefs, a much more complex fluid and diagenetic history can be expected than in the case of marine transgression. During transgression, the displaced freshwater lens created an extensive freshwater and brackish system, a transitional deposystem from marine to non-marine carbonate deposition.

The sharp boundary between the Zechstein Limestone and the overlying Lower Anhydrite in the basinal facies represents the nature of evaporites that start to precipitate immediately when the brines reach saturation. The increase in seawater salinity, which eventually led to evaporite precipitation, occurred during the deposition of the uppermost (about 10 cm thick) unit of the Zechstein Limestone in basinal facies, during the sedimentation of heterogeneous deposits composed mainly of ill-sorted oncoids and peloids with Stromatolites, above the last occurrence of Echinoids. This increase in seawater salinity was accompanied by sea level fall (evaporite drawdown). The coeval deposits of the reef (shoal) facies experienced some effect of this general increase in salinity, but it was controlled largely by local conditions in the environment of reef flat where considerable fluctuations in salinity might be expected. In general terms, this environment can be compared to Lake MacLeod and other Australian salinas

Lake MacLeod has a unique karst system, made of coastal limestone, which passes water from the Indian Ocean, 18 km underground using a hydrostatic pressure system, to bring salty water through sinkholes and vents into the lake basin. The lake surface is below sea level and water in the sinkholes and vents may be several metres deep. The sinkholes are connected by channels to a system of permanent mudflats, lagoons, and marshes, which can be up to a metre deep. As water travels through sinkholes, it evaporates creating a hypersaline environment when it reaches the terminal lagoon. Because of the variable environmental conditions, pond areas can vary substantially. The Northern Ponds area of Lake MacLeod include intermittently inundated flats, with water matching the ocean’s salinity coming through seepage vents. Freshwater enters though inflow from the Lyndon and Minilya Rivers. This mixture of the two sources makes Lake MacLeod as a brackish water environment. Gnaraloo Wildlife Foundation.

The uppermost part of the Zechstein Limestone in the Jabłonna Reef abounds in nodular anhydrite that forms the matrix in which carbonate clasts are embedded. However, in places the matrix is predominantly or entirely dolomitic. The lowermost Lower Anhydrite is also nodular, which otherwise is common for the entire Zechstein Basin. But we assume that this is a diagenetic fabric, and the original sulphate mineral was gypsum. In Jabłonna 1, some clasts have been encrusted by microbial mats containing pseudomorphs after lenticular gypsum crystals that have originated in a shallow subaqueous environment, most probably in relation of the transgressive Lower Anhydrite.

In the lower part of the Lower Anhydrite in the basinal facies, there is a unit rich in carbonate composing streaks and discontinuous laminae. The increased carbonate content in this unit can be related to either the dissolution phase of the Jabłonna Reef or the onset of the deposition on the Jabłonna Reef top after the evaporative drawdown. In the Ruchocice 4 section, located east of the Jabłonna Reef, thin microbial deposits (25 cm) of the Zechstein Limestone are followed by nodular anhydrite of the Lower Anhydrite, which contains intercalations of microbial dolomite in its lowermost part. These may correspond to unit B of the Lower Anhydrite in more basinal locations. In any case, the transgressive nature of the Lower Anhydrite is indubitable. In addition, the deposition of chloride deposits contemporaneous with sulphate deposits occurred quite early in the PZ1 history.

In some locations, microbial deposits were lacking in the upper part of the Zechstein Limestone. This was interpreted as due to lowering of the tectonic blocks on which the buildups were located, which could have resulted in the cessation of intensive carbonate deposition characteristic of reefs. 

The subaerial exposure of the reefs and the marginal carbonate platforms in the basin centre is a logical consequence of sea level fall at the end of the Zechstein Limestone deposition, which was related to evaporative drawdown. This major sea level fall could be preceded by earlier sea level falls that have been concluded by several authors based on sedimentary and diagenetic premises. However, there is no doubt that the most important factor, in terms of duration and impact of poroperm properties, was the sea level fall related to the change from a marine carbonate to an evaporite basin. Large parts of the Hessian Basin became subaerially exposed for a long period of time, as is indicated by common karstification. Widely developed shallowing-upward peri-littoral, sabkha and salina successions in the Hessian Basin have been interpreted as an indication of a renewed rise of brine level (a transgressive systems tract) due to inflow of preconcentrated brines from the Southern Zechstein Basin to the north. This inflow was preceded by the development of a karstic, subaerial exposure surface, interpreted as a record of type-1 sequence boundary that formed during a distinct brinelevel fall.

In turn, Stromatolitic facies of the transition interval are contained between carbonate platforms or isolated carbonate buildups. The overlying evaporites showed no evidence of subaerial exposure and formed during a relative sea level rise as transgressive systems tract or early highstand systems tract deposits. They commented, however, that it is highly likely that the thick evaporites in the basin centre formed during local or global sea level lowstands. Peryt et al.'s data indicate that a subaerial exposure episode existed, in the study area, after the deposition of transitional Stromatolitic facies in the uppermost part of the Zechstein Limestone, and the regolith can be related to falling stage systems tract deposits, that can be correlated with the lowest Anhydrite unit in the salt basin located to the northeast of the Jabłonna Reef.

A previous studyindicated a good lateral correlation of anhydritized zones in the reefs, which was regarded as an evidence in favour of their syndepositional origin, namely during sea level falls that have been recorded in the marginal Zechstein Limestone carbonate platform. Another possible mechanism is that the anhydrite zones record the brine-level stands during the abrupt lowering of relative sea level at the end of Zechstein Limestone deposition, or they represent a longer stabilization of brine level during the transgression of the Lower Anhydrite.

Peryt et al. conclude that the thin (2.8–5.1 m) unit of brecciated limestones and subordinate dolomites at the top part of the Zechstein Limestone (Wuchiapingian) in the Jabłonna Reef in western Poland recorded a sudden sea level fall that resulted in a long subaerial exposure of the reef, followed by a slow brine-level rise. This unit, regarded as a regolith, originated during the sea level fall related to evaporative drawdown. Eventually, it was locally reworked during the Lower Anhydrite transgression. Therefore, it can be regarded as a transgressive lag deposit.

The highstand systems tract deposits of the Zechstein Limestone are followed by transgressive systems tract deposits of the Lower Anhydrite in the Jabłonna Reef. The regolith can be related to falling stage systems tract deposits that can be correlated with the lowest anhydrite unit in the salt basin adjacent to the Jabłonna Reef.

The dolomite composing the unit studied originated through the seepage of brines in shallow-burial conditions during the deposition of the PZ1 anhydrite.

The nature of primary sulphate mineral in the lowermost Lower Anhydrite is enigmatic, but it is probable that cyclic gypsum upright-growth deposition occurred in salinas developed during deposition of microbial flats at the final stage of deposition of the Zechstein Limestone, and, consequently, sulphate deposition in the reef area could predate the sulphate accumulation in the basin area.

The complex hydrological setting of the reef controlled its early diagenesis. During the sea level fall, the Jabłonna Reef became exposed and subjected to freshwater diagenesis that improved poroperm characteristics of reef reservoirs. During the sea level fall or/and during subsequent transgression of the Lower Anhydrite, the reefs were subjected to intense anhydrite cementation, although its overall impact on the porosity was quite limited.

The studied case implies that important environmental perturbations related to sea/brinelevel fluctuations existed at the transition from carbonate to evaporite deposition in other giant evaporite basins.

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