Modern and Holocene carbonate sedimentology of two saline volcanic maar lakes, southern Australia

The Basin Lakes are two adjacent maar lakes located in the centre of the Western Volcanic Plains District of Victoria, Australia. Both lakes are saline and alkaline; West Basin Lake is meromictic whereas East Basin is a warm monomictic lake. The carbonate mineral suite of the modern offshore bottom sediments of these Basins consists mainly of dolomite and calcite, with smaller amounts of hydromagnesite and magnesite in West Basin and monohydrocalcite in East Basin. The dolomite, hydromagnesite, magnesite, and monohydrocalcite are endogenic in origin, being derived by primary inorganic precipitation within the water columns of the lakes or at the sediment-water interface. The calcite is biologically precipitated as ostracod valves. In addition to the carbonates in the modern offshore (deep-water) sediments, the lakes also contain a girdle of nearshore carbonate hardgrounds. Both beachrock and microbialites (algal boundstones) are present. These modern lithified carbonate units exhibit a wide range of depositional and diagenetic fabrics, morphologies and compositions. In West Basin, the hardgrounds are composed mainly of dolomite, hydromagnesite, and magnesite, whereas dolomite and monohydrocalcite dominate the East Basin sediments. Aragonite, high-Mg calcite, kutnahorite, siderite, and protohydromagnesite also occur in these lithified carbonate units. Stratigraphic variations in the carbonate mineralogy of the Holocene sediment record in the lakes were used to help decipher the palaeochemistry and palaeohydrology of the Basins. These changes, in conjunction with fluctuations in organic remains and fossil content, indicate a pattern of lake level histories similar to that deciphered from other maar lakes in western Victoria.     

Depositional environments of Upper Miocene (Messinian) evaporite deposits of the Sicilian Basin*

In Sicily, Messinian evaporitic sedimentary deposits are developed under a wide variety of hypersaline conditions and in environments ranging from continental margin (subaerial), to basin-margin supratidal, to intertidal, to subtidal and out into the hypersaline basin proper. The actual water depth at the time of deposition is indeterminate; however, relative terms such as ‘wave base’ and ‘photic zone’ are utilized. The inter-fingering relationships of specific evaporitic facies having clear and recognizable physical characteristics are presented. These include sub-aerial deposits of nodular calcium sulphate formed displacively within clastic sediments; gypsiferous rudites, arenites and arenitic marls, all of which are reworked sediments and are mixed in varying degrees with other clastic materials (subaerial, supratidal, and intertidal to deep basinal deposits). Laminated calcium sulphate alternating with very thin carbonate interlaminae and having two different aspects; one being even and continuous and the other of a wavy, irregular appearance (subtidal, intertidal, and supratidal deposits). Nodular calcium sulphate beds, usually associated with wavy, irregular laminated beds (supratidal, sabkha deposits); very coarsely crystalline gypsum beds (selenite), associated with more even, laminated beds (subaqueous, intertidal to subtidal deposits); wavy anastomozing gypsum beds, composed of very fine, often broken crystals (subaqueous, current-swept deposits); halite having hopper and chevron structures (supratidal to intertidal); and halite, potash salts, etc. having continuous laminated structure (subaqueous, possibly basinal). Evidence for diagenetic changes is observed in the calcium sulphate deposits which apparently formed by tectonic stress and also by migrating hypersaline waters. These observations suggest that the common, massive form of alabastrine gypsum (or anhydrite, in the subsurface) may not always be ascribed to original depositional features, to syndiagenesis or to early diagenesis but may be the result of late diagenesis.     

Precipitation of dolomite using sulphate-reducing bacteria from the Coorong Region, South Australia: significance and implications

Dolomite was successfully precipitated in culture experiments that simulated microbiogeochemical conditions prevailing during late stages of evaporation in ephemeral, hypersaline dolomitic lakes of the Coorong region, South Australia. Analyses of lake- and pore-water samples document rapid geochemical changes with time and depth in both dolomitic and non-dolomitic lakes. Extremely high sulphate and magnesium ion concentrations in lake waters decline rapidly with depth in pore waters throughout the sulphate-reduction zone, whereas carbonate concentrations in pore waters reach levels up to 100 times those of normal sea water. Ultimately, sulphate is totally consumed and no solid sulphate is recorded in the dolomitic lake sediments. ‘Most probable number’ calculations of lake sediment samples record the presence of large populations of sulphate-reducing bacteria, whereas sulphur-isotope analyses of lake-water samples indicate microbial fractionation in all the lakes studied. Viable populations of microbes from the lake sediments were cultured in anoxic conditions in the laboratory. Samples were then injected into vials containing sterilized clastic or carbonate grains, or glass beads, immersed in a solution that simulated the lake water. Falls in the levels of sulphate and rising pH in positive vials were interpreted as indicating active bacterial sulphate reduction accompanied by increased concentrations of carbonate. Within 2 months, sub-spherical, sub-micron-size crystals of dolomite identical to those of lake sediments were precipitated. It is concluded that bacterial sulphate reduction overcomes kinetic constraints on dolomite formation by removing sulphate and releasing magnesium and calcium ions from neutral ion pairs, and by generating elevated carbonate concentrations, in a hypersaline, strongly electrolytic solution. The results demonstrate that bacterial sulphate reduction controls dolomite precipitation in both the laboratory experiments and lake sediments. It is proposed that dolomite formation, through bacterial sulphate reduction, provides a process analogue applicable to thick platformal dolostones of the past, where benthic microbial communities were the sole or dominant colonizers of shallow marine environments.     

Diagenetic processes in carbonate mound sediments at the south‐west Rockall Trough margin

Cold water coral covered carbonate mounds at the south‐west margin of the Rockall Trough form ridges several kilometres long and up to 380 m high. Piston cores obtained at three mound crests reveal the complex internal structure of the mound build up, with alternating unlithified coral‐dominated intervals and lithified intervals. The most recent lithified interval is covered by corals embedded in a fine‐grained matrix, comprising ca 11 000 years of continuous mound evolution. Before this time ²³⁰Th/U dating shows the presence of several hiatuses in mound build‐up. Aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth. Downcore X‐ray fluorescence scanning, computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration. The upper boundary of the most recent lithified interval shows some erosional features, but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long‐term non‐sedimentation, but to earlier sub‐surface diagenesis. Organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons. Depending on the openness of the system, this can lead to precipitation of a more stable low‐magnesium carbonate. A model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals.     

Microbially mediated carbonate precipitation in a hypersaline lake,Big Pond (Eleuthera,Bahamas)

Microbial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.     

贵州中三叠统上部垄头组的沉积性质——再论其钙结壳的成因

钙结壳是土壤层之下的岩石在出露地表渗流带环境内受大气雨水的溶解,并通过上覆土壤层的淋滤而成的产物。由于雨水的不断的溶解,使原始的岩石或沉积物发生破裂,从而形成各种大小的砾块或砾石。随着雨水不断的溶解,雨水逐渐增加 CaCO_3的含量。这些富含碳酸钙的雨水,当它流到位于潜水面之上的下渗流带时,就在土壤层之下、原来岩石之上沉积了特殊的、层纹状方解石胶结物。它们往往覆盖岩石表面或沿着砾石的外缘、呈包覆状沉积,从而形成典型的层纹状钙结壳。渗滤豆粒的外皮也属于层纹状钙结壳的性质,这些外皮有向下方加厚的趋势,这表明渗流带的含碳酸钙的雨水,受重力作用影响下,具有往下悬垂现象,类似于洞穴内的钟乳石。豆粒之间的相互衔接呈多边形构造,也表明它是受重力作用而形成的构造。     

Formation of modern dolomite in hypersaline pans of the Western Cape,South Africa

Authigenic calcite and dolomite and biogenic aragonite occur in Holocene pan sediments in a Mediterranean‐type climate on the western coastal plain of South Africa. Sediment was analysed from a Late Pleistocene coastal pan at Yzerfontein and four Holocene inland pans ranging from brackish to hypersaline. The pans are between 0·08 and 0·14 km² in size. The δ¹⁸O_PDB values of carbonate minerals in the pan sediments range from ?2·41 to 5·56‰ and indicate precipitation from evaporative waters. Covariance of total organic content and percentage carbonate minerals, and the δ¹³C_PDB values of pan carbonate minerals (?8·85 to ?1·54‰) suggest that organic matter degradation is a significant source of carbonate ions. The precipitation of the carbonate minerals, especially dolomite, appears to be mediated by sulphate‐reducing bacteria in the black sulphidic mud zone found in the brine‐type hypersaline pans. The knobbly, sub‐spherical texture of the carbonate minerals suggests that the precipitation of the carbonate minerals, particularly dolomite, is related to microbial processes. The ⁸⁷Sr/⁸⁶Sr ratios of pan carbonate minerals (0·7108 to 0·7116) are slightly higher than modern sea water and indicate a predominantly sea water (marine aerosol) source for calcium (Ca²⁺) ions with relatively minor amounts of Ca²⁺ derived from the chemical weathering of bedrock.     

Processes of carbonate precipitation in modern microbial mats

Microbial mats are ecosystems that arguably greatly affected the conditions of the biosphere on Earth through geological time. These laminated organosedimentary systems, which date back to > 3.4 Ga bp, are characterized by high metabolic rates, and coupled to this, rapid cycling of major elements on very small (mm-µm) scales. The activity of the mat communities has changed Earth's redox conditions (i.e. oxidation state) through oxygen and hydrogen production. Interpretation of fossil microbial mats and their potential role in alteration of the Earth's geochemical environment is challenging because these mats are generally not well preserved.Preservation of microbial mats in the fossil record can be enhanced through carbonate precipitation, resulting in the formation of lithified mats, or microbialites. Several types of microbially-mediated mineralization can be distinguished, including biologically-induced and biologically influenced mineralization. Biologically-induced mineralization results from the interaction between biological activity and the environment. Biologically-influenced mineralization is defined as passive mineralization of organic matter (biogenic or abiogenic in origin), whose properties influence crystal morphology and composition. We propose to use the term organomineralization sensu lato as an umbrella term encompassing biologically influenced and biologically induced mineralization. Key components of organomineralization sensu lato are the “alkalinity” engine (microbial metabolism and environmental conditions impacting the calcium carbonate saturation index) and an organic matrix comprised of extracellular polymeric substances (EPS), which may provide a template for carbonate nucleation. Here we review the specific role of microbes and the EPS matrix in various mineralization processes and discuss examples of modern aquatic (freshwater, marine and hypersaline) and terrestrial microbialites.     

Mélanges and other types of block-in-matrix formations in the Cantabrian Zone (Variscan Orogen,northwest Spain): origin and significance

Block-in-matrix formations in the Variscan foreland of Spain (Cantabrian Zone) occur in two different geological settings. The major block-in-matrix formations are mélanges, which appear as carpets beneath or ahead of submarine thrust systems. These mélanges may reach up to kilometric thickness and are mostly composed of broken formations (boudinaged sequences) of late Carboniferous age and scattered ‘exotic’ blocks derived from older Palaeozoic formations. Moreover, the mélanges in the Cantabrian Zone also include subordinate debris flow deposits with a chaotic block-in-matrix fabric (olistostromes). The source of the mélange blocks was the front of advancing nappes, chiefly the upper part of the nappe stacks. Therefore, the Cantabrian mélanges are interpreted as originated through submarine sliding and slumping associated with steep slopes at the orogenic front. The different types of rock bodies of these mélanges may be related to the degree of lithification of the sediments or rocks during slumping. So, broken formations are boudinaged sequences where the boudins or blocks resulted from extensional faults developed in lithified or semilithified limestones and sandstones, whereas the unlithified muddy matrix underwent continuous deformation. The scattered ‘exotic’ blocks ranging in age from early Cambrian to early Carboniferous were incorporated into the mélanges as individual blocks from competent well-lithified formations, originally located in the lower part of the nappe stacks. Although the Cantabrian Zone mélanges include olistostromic intervals, most of the olistostromes of this zone occur in a different geological setting. They are usually intercalated in the normal marine deposits of the Variscan foreland basin and, in contrast to the mélanges, they are mostly related to the margins of carbonate platforms, ahead of moving nappes. Finally, other instances of olistostromes are related to slopes generated by limb rotation of growth folds, which developed on submarine wedge-top successions.     

南海表层沉积物中的碳酸钙含量分布及其影响因素

通过测定南海213个表层沉积物样品中的碳酸钙含量, 综合分析整个南海海域碳酸钙含量分布特征及其控制因素.结果表明, 不同的区域海洋环境, 控制表层沉积物中碳酸钙含量变化的因素也不尽相同: 大陆架区, 碳酸钙含量主要受陆源非碳酸盐物质的稀释作用而较低; 大陆坡区, 碳酸钙因丰富的物源量、低的陆源物质输入量和弱的碳酸钙溶解作用等因素而呈较高含量; 深海盆区, 碳酸钙含量因强烈的溶解作用而较低.根据碳酸钙含量在南海整个表层沉积物中的分布趋势, 推测南海纬度14°N以北的海域碳酸钙补偿深度(CCD)为3700m左右, 纬度14°N以南的海域CCD为4000m左右.Pearson相关分析表明, 南海表层沉积物中钙质超微化石对碳酸钙的含量分布具有较高的贡献率.      

The role of endolithic cyanobacteria in the formation of lithified laminae in Bahamian stromatolites

The microboring activity of endolithic cyanobacteria plays a major role in the formation of the dominant lithified laminae in modern marine stromatolites in the Exuma Cays, Bahamas. These stromatolites are composed primarily of fine-grained carbonate sand that is trapped and bound by the filamentous cyanobacteria Schizothrix sp.. Periodic introduction of coccoid endolithic cyanobacteria Solentia sp. during hiatuses in stromatolite growth associated with very low rates of sedimentation results in the formation of lithified horizons, 200–1000 μm thick. These layers consist of micritized grains that are welded together at point contacts. The micritization is caused by extensive microboring and carbonate precipitation within boreholes concurrent with endolithic activity. Grain welding occurs when boreholes cross from one grain to another at point contacts. Thus, microboring destroys original grain textures but, at the same time, plays a constructional role in stromatolite growth by forming lithified layers of welded grains. These lateral bands of fused carbonate grains help to stabilize and preserve the stromatolite deposits.     

Reflection of seismic paleoevents in Mesozoic–Cenozoic terrigenous sequences of the northern Caucasus

Numerous traces of paleoseismic events (seismites) were established in Mesozoic–Cenozoic marine sedimentary sequences of the northern Caucasus. These traces are most prominent in the terrigenous Middle Miocene sandy–clayey sediments. Impact of seismic shocks upon the relatively weakly lithified sediments provoked distortion of the primary sedimentary structure, liquefaction of the sandy material, and injections of different morphologies (neptunic dikes and sills). The formation of jointing in sediments fostered their vertical permeability and promoted the migration of diagenetic solutions into the adjacent horizons, which stimulated the formation of subvertical carbonate bodies. The amount and intensity of seismic events varied at different stages of the accumulation of sequences and in different areas of the paleobasin. In the eastern sector of the northern Caucasus, seismic activity similar to the present-day general pattern was likely developed as early as the Middle Miocene: maximum activity in the Dagestan and its westward attenuation. Traces of seismic activity are also recorded in the Maikopian (Oligocene–Lower Miocene) and Lower–Middle Jurassic rocks.     

The Cold Carbonate Connection Between Mono Lake, Californiaand the Bransfield Strait, Antarctica

Ikaite is a rare form of carbonate – calciumcarbonate hexahydrate (CaCO3·6H2O) and isthe precursor to thinolites. Metastable 'ikaite'crystals, discovered in unconsolidated marinesediments in the King George Basin in the BransfieldStrait, Antarctica, are related to diageneticremineralization reactions of organic matter. StableC, O, and H-isotopes track the response of ikaitecrystals, during growth, to changing interstitialfluid conditions as a result of bacterial sulphatereduction and methanogenesis. Ikaites form inpreference over calcite or aragonite at the prevailingsurface sediment conditions of -1.6 °C and 200bar in the King George Basin.The calcareous tufa towers of the terrestrial,hypersaline Mono Lake of northern California areCaCO3-precipitates formed by the influx ofsubmerged springs of calcium-rich freshwaters enteringthe alkaline lake (CO2 = 0.5 m, pH =9.8). Under current climatic conditions the mineralcalcite precipitates, but during the colder Tiogaglacial period of Late Wisconsian age (12,000 to 9,000years BP), and possibly present day during the winterseason, the monoclinic calcium carbonate hexahydrate(ikaite) was the dominant phase formed. Thesepaleo-ikaites have since recrystallized to form thecalcitic pseudomorph 'thinolites'. They are foundelsewhere in recent and ancient sediments of polarregions, e.g., as 'glendonites'. The environmentaloccurrence of ikaites and their pseudomorphs deem themas potential paleoclimatic indicators of coldenvironments. The larger crystals are typicallyrestricted to colder, deeper organic-rich sediments orin moderately evaporitic basins. In these cases, theikaite formation and decomposition may be influencedby additives such as phosphate or amino acids.     

SUBAERIAL CEMENTATION AND SUBSEQUENT DOLOMITIZATION OF LACUSTRINE CARBONATE MUDS AND SANDS FROM PALEOTUZ GÖLÜ ("SALT LAKE"), TURKEY

SUMMARY
Along the eastern coast of the Gez Gölö, a salt lake formerly belonging to the larger "Paleo-Tuz Gölö" in central Anatolia, about 6.5 m of lake sediments of probably Pleistocene age were studied.
The sediment series consists of fine-grained unconsolidated dolomite muds representing the basin sedimentation of the old lake.
Nine lithified beds (oomicrites, intramicrites, intraoomicrites, intrabiomi-crites and dololutites with smaller amounts of allochems) are intercalated. These beds were deposited in very shallow water in the littoral zone of the paleo-lake. There is direct evidence that the beds were occasionally exposed to the air after deposition. This led to desiccation (with mud cracks, breaking up of intraclasts) and subaerial cementation due to the influx of fresh water (rain water, springs).
These now lithified carbonate sedimentary rocks were submerged and subsequently covered with mud. During this sub-aquatic phase the rock beds as well as the unconsolidated muds were dolomitized by an extremely high Mg/Caratio of the lake water which in the present Tuz Gölü amounts to about 150! It cannot be excluded, however, that dolomitization already took place or was initiated during the subaerial phase ("supratidal dolomite").     

Lithified carbonate sediment and zeolitic tuff in basalts, Mid-Atlantic Ridge

Basaltic boulders dredged from the Mid-Atlantic Ridge contain lithified coccolith-foraminiferal ooze in fractures and small pockets. Textural and isotopic studies of this sediment provide no evidence for high temperature metamorphism. The lithified carbonate sediment occurs together with palagonitized basaltic glass and zeolitic tuff, and appears to have been squeezed into cracks and other voids within the congealed margins of partly-cooled basalt pillows which intruded soft carbonate ooze. Thermal metamorphism probably was precluded by rapid heat dissipation in convecting pore waters and by the thermal stability of calcareous microplankton. Low temperature alteration of basaltic glass appears to have provided the chemical milieu for precipitation of calcite cement as well as zeolites in the sediment.     

Dinantian sedimentation and the basement structure of the Derbyshire Dome

The depositional history of the Dinantian on the Derbyshire Dome can be divided into three phases: (1) pre-Holkerian: onlap of an irregular basement surface by evaporite and carbonate sediments, (2) Holkerian to Asbian: sedimentation on a carbonate shelf formed by the merging of early Dinantian depocentres following burial of the basement topography, and (3) Brigantian: formation of intrashelf basins and the development of a carbonate ramp on part of the pre-existing shelf. A model of the basement structure underlying the Derbyshire Dome is presented to explain the location of the Brigantian intrashelf basins and carbonate ramp. The basement consists of two main tilted fault blocks separated by a smaller tilt block. Movement on faults bounding the tilt blocks caused the development of intrashelf basins. The basin margins were controlled by structures which developed in the cover sediments. The carbonate ramp present during the late Brigantian developed in response to an eastward tilting of the basement.     

Carbon,oxygen, and sulfur isotope compositions and model of the Silurian rock formation in northwestern Belarus

The paper describes the results of study of the Silurian clayey–carbonate rocks ranging from the Telychian Stage (Llandovery) to the Gorstian Stage (Ludlow) recovered by the Borehole Davtyuny 3k in northwestern Belarus. Rocks of the Sheinwoodian Stage demonstrate a positive excursion of δ¹³C with amplitude of 4.7‰, marking the Ireviken biotic event recorded in the global chemostratigraphic curve. Values of δ¹⁸O for the carbonate material in the studied section (25.5–29.2‰ SMOW) are close to those for Silurian rocks from the Baltic region, Scandinavia, Ukraine, Poland, and Canada. The whole section contains postsedimentary gypsum as nodules and the infilling of fissures and fenestrae. Values of δ³⁴S in gypsum (21.3–26.7‰ CDT) are close to those for the Silurian rocks on the Phanerozoic isotope plot. The formation of gypsum was related to a partial development of the supralittoral environment over the sublittoral and littoral clayey–carbonate substrate. The seawater accumulated in lowlands of the supralittoral plain after storms was intensely concentrated during arid conditions and accumulated in the clayey–carbonate sediment. The subsequent underground evaporation promoted the formation of gypsum as nodules in the unlithified sediments and the infilling of fissures and fenestrae in the lithified rocks.     

Microbial biomineralization processes forming modern Ca:Mg carbonate stromatolites

Modern Ca:Mg carbonate stromatolites form in association with the microbial mat in the hypersaline coastal lagoon, Lagoa Vermelha (Brazil). The stromatolites, although showing diversified fabrics characterized by thin or crude lamination and/or thrombolitic clotting, exhibit a pervasive peloidal microfabric. The peloidal texture consists of dark, micritic aggregates of very high‐Mg calcite and/or Ca dolomite formed by an iso‐oriented assemblage of sub‐micron trigonal polyhedrons and organic matter. Limpid acicular crystals of aragonite arranged in spherulites surround these aggregates. Unlike the aragonite crystals, organic matter is present consistently in the dark, micritic carbonate comprising the peloids. This organic matter is observed as sub‐micron flat and filamentous mucus‐like structures inside the interspaces of the high‐Mg calcite and Ca dolomite crystals and is interpreted as the remains of degraded extracellular polymeric substances. Moreover, many fossilized bacterial cells are associated strictly with both carbonate phases. These cells consist mainly of 0·2 to 4 μm in diameter, sub‐spherical, rod‐like and filamentous forms, isolated or in colony‐like clusters. The co‐existence of fossil extracellular polymeric substances and bacterial bodies, associated with the polyhedrons of Ca:Mg carbonate, implies that the organic matter and microbial metabolism played a fundamental role in the precipitation of the minerals that form the peloids. By contrast, the lack of extracellular polymeric substances in the aragonitic phase indicates an additional precipitation mechanism. The complex processes that induce mineral precipitation in the modern Lagoa Vermelha microbial mat appear to be recorded in the studied lithified stromatolites. Sub‐micron polyhedral crystal formation of high‐Mg calcite and/or Ca dolomite results from the coalescence of carbonate nanoglobules around degraded organic matter nuclei. Sub‐micron polyhedral crystals aggregate to form larger ovoidal crystals that constitute peloids. Subsequent precipitation of aragonitic spherulites around peloids occurs as micro‐environmental water conditions around the peloids change.     

Geolimnology of Freefight Lake: an unusual hypersaline lake in the northern Great Plains of western Canada*

Freefight Lake, Canada's deepest salt lake, is a meromictic, hypersaline lake located in the most arid part of the northern Great Plains. The lake has a distinctive basin morphology, with a large expanse of seasonally flooded mud flats and sand flats adjacent to a deep, flat bottomed perennial waterbody. The mixolimnion, dominated by magnesium, sodium and sulphate ions, has an average salinity of 110 ppt and overlies a monimolimnion of 180 ppt total dissolved salts. The entire water column is strongly supersaturated with respect to a variety of calcium and magnesium carbonate minerals; the lower water mass is also saturated or supersaturated with respect to a number of very soluble sodium, magnesium and sodium + magnesium salts. The modern sedimentary processes operating in Freefight Lake give rise to six main sedimentary facies: (i) colluvium, (ii) mud flats and sand flats, (iii) algal flats, (iv) delta, (v) slope and debris apron, and (vi) deep basin. The colluvium, mud flats and sand flats, and delta facies are dominated by physical processes and consist mainly of detrital siliciclastic sediment. The algal flats, slope and debris apron, and deep basin facies are dominated by endogenic and authigenic sediments derived mainly by physicochemical and biologically mediated carbonate and evaporite mineral precipitation. As one of very few deep water lakes in the world in which soluble evaporite minerals are forming and being preserved, Freefight Lake occupies an important position within the realm of lacustrine sedimentology. Although many of the sedimentological and geochemical processes taking place in the basin today are unusual, the delineation and evaluation of these processes is essential in order to decipher properly the stratigraphic records of Quaternary lake sediments in this large area of North America, as well as lacustrine sequences from other arid and semi-arid regions of the world.