Efflorescent Crusts Research Articles

An Assessment of Stratigraphic Completeness in Climate-Sensitive Closed-Basin Lake Sediments: Salar de Atacama, Chile

ABSTRACT Dated subsurface cores, 40 m, 100 m, and 200 m in length, from the Salar de Atacama, Chile, record changes in climate and tectonics over the past 325 ka. These cores were used to assess completeness of the stratigraphic record and how that record was influenced by faulting and climate change. The same basic facies, efflorescent halite crust and cement (subaerial halite), and chevron halite (saline lake) occur in each core. The dominant facies is efflorescent halite formed in a subaerial environment like that in the Atacama basin today. Thick, well-preserved efflorescent crusts in the Salar de Atacama cores illustrate that major sediment aggradation occurred in subaerial environments by growth of efflorescent halite from evaporation of groundwater brines. Similar pedogenically formed evaporites may be pre served in the geologic record. The N-S trending Salar Fault System has downdropped the eastern block of the Salar de Atacama (core 2002) relative to the western block (cores 2005 and 2031). Relative faulting rates over the past 60 ka were determined from stratigraphic offset and uranium-series ages from core 2005. Major faulting along the Salar Fault System occurred between 16.5 and 5.4 ka, with offset of 26.5 m and faulting rates of 2.4 m/kyr. Aggradation of efflorescent halite crusts on the downdropped eastern block served to smooth out topographic variations created by faulting. Although stratigraphic units in core 2002 shifted down > 30 m over the past 60 ka, the thickness and sedimentary features are similar for all but one equivalent stratigraphic unit in all three cores. Faulting along the Salar Fault System, despite significant offset, did not alter the basic stratigraphic record. Temporal completeness of the stratigraphic record, within the limits of the age dates and their errors of greater than ± 3 ka, was examined qualitatively by comparing long-term sedimentation rates (0.6-0.9 m/kyr) and shorter-term sedimentation rates (0.3-3.6 m/kyr). The similarity of sedimentation rates on the 10 kyr scale and the longer 100 to 300 kyr scale suggests temporal completeness of Salar de Atacama sediments, at least at the 10 kyr scale. Although hiatuses, long periods of subaerial exposure, and dissolution of evaporites may have occurred, the main climate fluctuations at Salar de Atacama over the last 100 ka are recorded in all three subsurface cores. Paleoclimate records obtained from such cores are therefore representative of basin-wide climate change and are valid for regional paleoclimate reconstructions.

East African magadi (trona): fluoride concentration and mineralogical composition

Magadi from Lake Magadi, Kenya, Lake Natron, Tanzania, Lake Katwe, Uganda, El-Atrun, Sudan and efflorescent crust from the soil surface (scooped magadi) from northern Tanzania have been analysed chemically to determine fluoride and carbonates concentrations and by X-ray diffraction to determine the mineralogical composition. Magadi from Lake Natron and Lake Magadi are found to be very similar consisting mainly of trona (CO 3 2− + HCO 3 − > 10.4 meq [g magadi] −1) mixed with halite and either kogarkoite or villaumite, respectively, resulting in fluoride concentrations up to 8.7 mg F − [g magadi] −1. The scooped magadi is not as pure with respect to trona as the crystalline magadi, but the fluoride content is of same order of magnitude (0.23–5.1 mg F − [g magadi] −1). The scooped magadi consists of trona (CO 3 2− + HCO 3 = 3.5–9.5 meq [g magadi] −1) with different mixtures of halite, quartz, villiaumite, kogarkoite and thermonatrite. No fluoride containing minerals are identified in magadi from Uganda and Sudan, probably due to the very low fluoride concentrations of 0.02 and < O.24 mg (g magadi) −1, respectively, indicating that these samples are not contaminated with fluoride. The Sudanese magadi is a different mixture of trona, halite and quartz resulting in a variation in the carbonate concentration of 4.6–11.9 meq (g magadi) −1. The magadi from Lake Katwe consists of trona (CO 3 2− + HCO 3 − = 7.0 meq [g magadi] −1) mixed with burkeite and halite.

Mg- and K-bearing borates and associated evaporites at Eagle Borax Spring, Death Valley, California; a spectroscopic exploration

Efflorescent crusts at the Eagle Borax spring in Death Valley, California, contain an array of rare Mg and K borate minerals, several of which are only known from one or two other localities. The Mg- and/or K-bearing borates include aristarainite, hydroboracite, kaliborite, mcallisterite, pinnoite, rivadavite, and santite. Ulexite and probertite also occur in the area, although their distribution is different from that of the Mg and K borates. Other evaporite minerals in the spring vicinity include halite, thenardite, eugsterite, gypsum-anhydrite, hexahydrite, and bloedite. Whereas the first five of these minerals are found throughout Death Valley, the last two Mg sulfates are more restricted in occurrence and are indicative of Mg-enriched ground water.Mineral associations observed at the Eagle Borax spring, and at many other borate deposits worldwide, can be explained by the chemical fractionation of borate-precipitating waters during the course of evaporative concentration. The Mg sulfate and Mg borate minerals in the Eagle Borax efflorescent crusts point to the fractionation of Ca by the operation of a chemical divide involving Ca carbonate and Na-Ca borate precipitation in the subsurface sediments. At many other borate mining localities, the occurrence of ulexite in both Na borate (borax-kernite) and Ca borate (ulexite-colemanite) deposits similarly reflects ulexite's coprecipitation with Ca carbonate at an early concentration stage. Such ulexite may perhaps be converted to colemanite by later reaction with the coexisting Ca carbonate--the latter providing the additional Ca (super 2+) ions needed for the conversion. Mg and Ca-Mg borates are the expected late-stage concentration products of waters forming ulexite-colemanite deposits and are therefore most likely to occur in the marginal zones or nearby mud facies of ulexite-colemanite orebodies. Under some circumstances, Mg and Ca-Mg borates might provide a useful prospecting guide for ulexite-colemanite deposits, although the high solubility of Mg borate minerals may prevent their formation in lacustrine settings and certainly inhibits their geologic preservation. The occurrence of Mg borates in borax-kernite deposits is also related to fractionation processes and points to the operation of an Mg borate chemical divide, characterized by Mg borate precipitation ahead of Mg carbonate. All of these considerations imply that Mg is a significant chemical component of many borate-depositing ground waters, even though Mg borate minerals may not be strongly evident in borate orebodies.The Eagle Borax spring borates and other evaporite minerals were studied using spectroscopic and X-ray powder diffraction methods, which were found to be highly complementary. Spectral reflectance measurements provide a sensitive means for detecting borates present in mixtures with other evaporites and can be used to screen samples rapidly for X-ray diffraction analysis. The apparently limited occurrence of Mg and K borate minerals compared to Ca and Na borates may stem partly from the inefficiency of X-ray diffraction methods for delineating the mineralogy of large and complex deposits. Spectral reflectance measurements can be made in the laboratory, in the field, on the mine face, and even remotely. Reflectance data should have an important role in studies of existing deposit mineralogy and related chemical fractionation processes, and perhaps in the discovery of new borate mineral resources.

Major-ion geochemistry and mineralogy of the Salt Lake (Tuz Gölü) basin, Turkey

In the Salt Lake basin of Turkey, chemical composition of inflow surface waters defines a continuous trend from CaHCO 3-rich spring waters to NaSO 4Cl-rich brines. Considerable compositional variation exists among the surface waters. Water-rock interaction governs compositional variations in springs, streams and rivers, and is enhanced by evaporation and precipitation of calcite and protodolomite. Solute concentrations of the streams and the rivers are partially controlled by the mineralogy of the playa deposits. The concentration increase from inflow surface waters to a NaCl-type lake surface brine is not accessible to direct observation. The principal cause of the evolution from SO 4- rich brine to CI-rich brine in the lake is interpreted as the recycling of solutes through the differential dissolution of efflorescent crusts. In the lake, major-ion concentrations generally exhibit an evaporation-dependent evolution trend that is further modified by precipitating halite, gypsum, aragonite and calcite. Sediments of the lake are predominantly composed of gypsum, dolomite, huntite, magnesite, and subordinately of polyhalite minerals. Magnesite and huntite are mostly early diagenetic minerals that have been formed by the transformation of dolomite in the presence of pore fluids with a high Mg Ca ratio.

Evaporite mineralogy and trace-element content of salt-affected soils in Alberta

A survey of efflorescent crusts and associated surface soils in central and southern Alberta was conducted to determine evaporite mineralogy and elemental composition. X-ray powder diffraction indicates that efflorescence mineralogies are dominated by sodium sulfate minerals, such as thenardite and mirabilite. Sodium magnesium sulfate minerals such as konyaite and bloedite are also frequently present, with eugsterite, halite and thermonatrite among other evaporites identified. The content of selected elements in the salt crusts and surface soils was determined using instrumental neutron activation analysis. Trace-element concentrations from site to site were extremely variable. However, comparisons with elemental abundances previously reported for till and soils indicate that there is generally no mean accumulation of trace elements in salt-affected soils. Exceptions are Br and Cl, which show enrichment in soils infused with soluble salts. Key words: Salinity, trace elements, evaporites, mineralogy

Mapping playa evaporite minerals with AVIRIS data: A first report from death valley, California

Efflorescent salt crusts in Death Valley, California, were mapped by using Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) data and a recently developed least-squares spectral band-fitting algorithm. Eight different saline minerals were remotely identified, including three borates, hydroboracite, pinnoite, and rivadavite, that have not been previously reported from the Death Valley efflorescent crusts. The three borates are locally important phases in the crusts, and at least one of the minerals, rivadavite, appears to be forming directly from brine. Borates and other evaporite minerals provide a basis for making remote chemical measurements of desert hydrologic systems. For example, in the Eagle Borax Spring area, the AVIRIS mineral maps pointed to elevated magnesium and boron levels in the ground waters, and to the action of chemical divides causing subsurface fractionation of calcium. Many other chemical aspects of playa brines should have an expression in the associated evaporite assemblages. Certain anhydrous evaporites, including anhydrite, glauberite, and thenardite, lack absorption bands in the visible and near-infrared wavelength range, and crusts composed of these minerals could not be characterized by using AVIRIS. In these situations, thermal-infrared remote sensing data may complement visible and near-infrared data for mapping evaporites. Another problem occurred in wet areas of Death Valley, where water absorption caused low signal levels in the 2.0–2.5 μm wavelength region that obscured any spectral features of evaporite minerals. Despite these difficulties, the results of this study demonstrate the potential for using AVIRIS and other imaging spectrometer data to study playa chemistry. Such data can be useful for understanding chemical linkages between evaporites and ground waters, and will facilitate studies of how desert ground-water regimes change through time in response to climatic and other variables.

Gypsum ground: a new occurrence of gypsum sediment in playas of central Australia

There are many playas (dry salt lakes) in arid central Australia (regional rainfall about 250 mm/y and pan evaporation around 3000 mm/y). Highly soluble salts, such as halite, only appear as a thin (several centimetres thick), white, ephemeral efflorescent crust on the dry surface. Gypsum is the major evaporite precipitating both at present and preserved in sediment sequences. One type of gypsum deposit forms a distinctive surface feature, which is here termed “gypsum ground”. It consists of a thick (up to 80 cm) gypsum zone which rises from the surrounding smooth white playa surface and is overlain by a heaved brown crust. The gypsum zone, with an average gypsum content above 60%, consists of pure gypsum sublayers and interlayered clastic bands of sandy clay. The gypsum crystals are highly corroded, especially in the direction parallel to the c-axis and on the upper sides where illuviated clay has accumulated in corrosion hollows. Overgrowth parallel to the a- and b-axes is very common, forming highly discoidal habits. These secondary changes (corrosion and overgrowth) are well-developed in the vadose zone and absent from crystals below the long-term watertable (depth around 40 cm). These crystal characteristics indicate a rainwater leaching process. At Lake Amadeus, one of the largest playas (800 km 2) of central Australia, such gypsum ground occupies 16% of the total area. The gypsum ground is interpreted as an alteration of a pre-existing gypsum deposit which probably extended across the whole playa before breaking down, leaving a playa marginal terrace and several terrace islands within the gypsum ground. This pre-existing gypsum deposit, preserved in the residual islands, consists of pure, pale, sand-sized lenticular crystals. It is believed to have been deposited during an episode of high regional watertable, causing active groundwater seepage and more frequent surface brine in the playa. A later fall in watertable, probably resulting from climatic change, caused the degradation of the gypsum deposit by dissolution and leaching processes. The common distribution of the gypsum ground and marginal terraces in the playas of central Australia demonstrates the extent of this hydrologic and climatic history.

A chemical model for the evolution of Australian sodium chloride lake brines

We present a chemical model which calculates the composition of waters as they are progressively concentrated from various starting compositions to determine the important processes leading to the development of Na-Cl type brines typical of Australia. The model can be extended to other continental settings which are characterised by low rainfall, low topographic relief and highly weathered regolith. The model extends previous brine evolution models to include clay mineral-solution reactions as a means of controlling aqueous silica concentrations and we suppress K + concentrations via clay mineral interactions. Our model also evaluates the importance of elevated partial pressures of CO 2 which mimics situations encountered in groundwaterfed systems. Several important controls on major ion evolution were observed: 1. (1) Initial Ca/Alkalinity ratios must be greater than 0.5 to generate Cl-rich brines of circum-neutral pH. One-thousand fold concentration of average Australian river water (Ca/Alk = 0.31) generates alkaline, high pH, brines while the same ionic strength brine derived from mean Australian rainwater (Ca/Alk = 1.2) generates a brine which is two orders of magnitude lower in alkalinity. 2. (2) Elevated pCO 2 values which occur in subsurface waterssuppresses precipitation of carbonate minerals. This allows Ca to build up more quickly thereby accelerating the onset of gypsum precipitation. 3. (3) Aqueous silica concentrations are controlled by equilibrium with clay minerals in the “dilute” waters but reach amorphous silica saturation in concentrated brines because of the reduced activity of the neutral SiO 2(aq) species in high ionic-strength brines. 4. (4) Continual dissolution and reprecipitation of salts in the groundwaters or efflorescent crusts are responsible for minor-and trace-element ratios very different from that of the initial starting waters.

Water Table Controlled Syndepositional Alteration Textures and Fabrics in Salt Pan Halite: Modern Analogues and Ancient Examples

At the Devil's Golf Course, Death Valley, CA, vadose zone and phreatic zone alteration of subaqueously accumulated halite produces characteristic textures and fabrics that are recognizable in ancient salt pan halite (Late Permian Salado Formation) exposed in a shaft at the Waste Isolation Pilot Plant. Water table depth and duration control fabric type. The crystal size of surficial halite is reduced by hygroscopic alteration. Efflorescent crusts extend the capillary fringe to the surface along vertical permeability pathways. When the water table is shallow, planar dissolution in the vadose zone removes all or most halite from successive thin depositional sequences and progressively disrupts strata consisting of clay or sulfate into irregular strata, stringers, isolated blebs, and, ultimately, 'blebby' laminae. With a deeper water table, point dissolution occurs along vertically oriented permeability pathways (e.g., polygonal margins) producing characteristic textures. Vertical pits, pipes, and macropores form first. Then the surface becomes slightly hummocky as point solution pathways to the water table widen and coalesce. A complex terrain of spires, hummocks, and columns develops and exhibits characteristic pods and lenses of fine halite surrounded by and containing solution lags of insoluble material. Relief is reduced by solution, and lenses and pods of fine halitemore » become smaller and less common. Ultimately, halite is entirely removed leaving only insoluble material. Halite cements grow in the phreatic zone. Halite passively fills voids in more mechanically competent halite. Displacive halite cements dilate fabrics within less competent bedded halite. These textures can be integrated into an idealized lithologic sequence for ancient salt pan halite.« less

Chemical sedimentation in the semi-arid environment of the Okavango Delta, Botswana

The Okavango basin of northern Botswana is an area of active diagenetic carbonate and silica accumulation. The basin is formed by grabens which represent an extension of the East African Rift system. The Okavango Delta, a large, marshy, alluvial fan (22,000 km 2), occupies most of the surface area of the basin. Evapotranspiration exceeds precipitation by a factor of 3 and ∼ 96% of incoming water is lost to the atmosphere. Evaporative concentration of dissolved solids occurs and they precipitate in the capillary fringes of islands. The less soluble components CaCO 3 and SiO 2 precipitate in the soils, which results in a significant volume increase which has important implications for the topography of the area and the character of the sediment fill. The more soluble salts, mainly alkali carbonates, accumulate at the surface as efflorescent crusts but are leached into the soil during the rainy season. This salt accumulation destroys vegetation. Swamp abandonment leads to leaching to greater depth, and, with reflooding, to the removal of these salts, possibly to form a deep saline groundwater. Mass-balance calculations indicate that this form of chemical accumulation is the dominant aggradational process occurring in the delta at present.

Hydrochemical processes in ground water‐discharge playas, Central Australia

AbstractA large groundwater system in the Amadeus Basin, central Australia, discharges to a chain of playa lakes 500 km long. The playas contain highly concentrated brines; these are sodium‐chloride rich waters with appreciable magnesium and sulphate and very low concentrations of calcium and bicarbonate. Gypsum, glauberite, and other evaporite minerals are precipitating in the playas. The groundwaters evolve to brine by concurrent processes of dissolution, evaporative concentration, mineral precipitation, and mineralogical change. Chemical evolution is considered with reference to a concentration factor based on chloride. Ion transfer calculations demonstrate losses of magnesium and bicarbonate throughout, as a result of precipitation. Sodium, potassium, calcium, and sulphate are gained initially as a result of dissolution but lost subsequently as a result of precipitation.Larger playas in the chain, exemplified by Lake Amadeus, have dual shallow and deep groundwater flow paths whereas the smaller playas, exemplified by Spring Lake, have only shallow flow paths. Brines in the larger playas are diluted by deep groundwaters and this is reflected in the degree of saturation attained with respect to particular minerals. Thus, saturation with respect to gypsum and glauberite is attained earlier in Spring Lake than in Lake Amadeus. Saturation with respect to halite is attained in Spring Lake but not in Lake Amadeus. Both playas are undersaturated with respect to hexahydrite and sylvite although these minerals occur in efflorescent crusts in Spring Lake.

Evaporite Mineralogy and Groundwater Chemistry Associated with Saline Soils in Eastern North Dakota

AbstractSoil salinization has resulted from seepage of water from drainage ditches in eastern North Dakota. The present study was undertaken to determine the effect of these processes on the geochemistry of shallow groundwater and on the evaporite mineralogy of the affected soils. In the chloridic system studied, SO2‐4 concentrations of groundwater and saturation extracts were controlled by the solubility of gypsum. Calcium concentrations were controlled by gypsum solubility in the sulfatic system. Shallow groundwater samples collected near the recharge sources were undersaturated with respect to gypsum and oversaturated with respect to calcite. Considerable redistribution of gypsum was observed in soil profiles of strongly salinized transects of the chloridic system. Sulfate enrichment relative to Ca2+ was observed in groundwater and soil extracts collected near drainage ditches of chloridic sites. Mineralogy of efflorescent crusts included gypsum, epsomite, hexahydrite, thenardite, and konyaite in the sulfatic system, and halite, eugsterite, thenardite, konyaite, bloedite, and gypsum in the chloridic system.

Sedimentology of playa lakes of the northern Great Plains

The semi-arid plains of western Canada and northern United States contain a large number of saline and hypersaline lakes, with many of the shallow lakes exhibiting playa characteristics. For over 60 years a number of these playas have provided a source of valuable industrial minerals. The present study deals with the modern sediments and sedimentary processes operating in selected playa basins located throughout the northern Great Plains region.The playas generally occupy small, closed basins commonly having an elongate, riverine shape. Most of the brines are dominated by sodium, magnesium, and sulfate ions; however, examples of lakes rich in chloride and bicarbonate ions are also present. Significant variation in the brine chemistry can exist on a seasonal as well as a diurnal basis. The modern sediments of the playas consist of: (1) very soluble salts of mainly sodium and magnesium sulfates and carbonates; (2) sparingly soluble precipitates, including calcite, protodolomite, gypsum, and mixed layer clays; (3) detrital minerals consisting dominantly of quartz, carbonates, feldspars, and clays; and (4) organic matter.Much of the physical and mineralogical character of the playas is due to processes that are largely of a chemical nature and are associated with either evaporative concentration of the brines or groundwater discharge. The most significant processes include: cyclic flooding and desiccation of the playa surface, formation of salt crusts, efflorescent crusts, spring deposits, and intrasedimentary salts, formation of solution pits and chimneys, wind displacement of brines, and periodic detrital sedimentation by sheet flow and wind. These processes combine to create a very dynamic modern depositional environment. The establishment and prolonged maintenance of delicate physical and chemical equilibria have allowed the deposition of over 40 m of salt in some basins.

Sedimentology and geochemistry of saline lakes of the Great Plains

Southern Saskatchewan and portions of adjacent Alberta, North Dakota and Montana are occupied by hundreds of saline and hypersaline lakes ranging in size from small prairie potholes (less than 1 km2) to relatively large bodies of water (greater than 300 km2). From a sedimentological perspective, distinction must be made between two basic types of saline lakes: playas and perennial lakes. Calcium, sodium and magnesium sulfates, carbonates and bicarbonates form as chemical precipitates in lakes with more concentrated brines. In addition, experimental data suggests mixed layer smectites may form authigenically in some lakes. Clastic sediments in the salt lakes consist mainly of silt and clay-sized quartz, feldspars, carbonates and clay minerals. The dominant physical and chemical processes which are responsible for and act upon the sediment vary widely, mainly in response to basin morphology and brine chemistry. Evaporative concentration and significant groundwater contributions affect all the saline lakes. However, other processes are different in the two basic types of basins. The playa lakes are influenced by: evaporitic pumping and the formation of efflorescent crusts and intrasedimentary crystals, cyclic wetting and drying, precipitation of highly soluble salt layers, and influx of clastic debris by sheetflood and wind. In contrast, in the permanent lakes, precipitation of sparingly soluble salts occurs due to the interaction of biological activity, seasonal temperature fluctuations and brine mixing. In addition, many of the permanent lakes undergo freeze-out precipitation of very soluble salts under a winter ice cover. Detrital sediments are distributed within the basins by normal lacustrine processes, including shoreline deposition and erosion, turbidity flow and pelagic fallout.

Sedimentologic Framework of Green River Formation, Wyoming: ABSTRACT

During deposition of the Green River Formation, ancient Lake Gosiute began as a freshwater lake, evolved to a saline, alkaline lake, and ended as a freshwater lake. This evolution reflects the change from a closed-basin to an open-basin hydrologic regime; as a result, sedimentation in the Lake Gosiute system was strongly influenced by the relation between evaporation and inflow of water into the basin. In the Green River Formation, stratification sequences, sedimentary structures, and mineralogy of lithofacies provide important insights into the evolution of the system and the competing factors that determined the type of sediment accumualated in the lake and fringing environments. Carbonate sedimentation was strongly influenced by lacustrine transgressions and regression across a low topographic gradient. Terrigenous rocks reflect progradation of beach and deltaic shorelines during wetter climatic intervals when detritus that was produced and stored in upland areas during preceding drier intervals was transported to the lake. Hydrochemistry of lake Gosiute during the deposition of the Wilkins Peak member was controlled by ground water discharge; during the deposition of the Tipton and Laney Members it was controlled largely by surface water. Calcite precipitated as a result of mixing calcium-rich inflow and saline-alkaline lake waters. Dolomite formed as a result of periodic flooding and drying of the playa fringe (carbonate mud flat), where carbonate muds were saturated with saline-alkaline lake waters and underwent evaporitive pumping. Some surface waters were preconcentrated by dissolution of efflorescent crusts in alluvial plain and mud-flat sediments. Trona and halite precipitated from brine pools as the lake shrank during periods of intense aridity. During himid periods the lake expanded and oil shal was deposited. End_of_Article - Last_Page 1000------------

Lacustrine sedimentation during the culminating phase of Eocene Lake Gosiute, Wyoming (Green River Formation)

During deposition of the Laney Member of the Green River Formation, Eocene Lake Gosiute evolved from a saline, alkaline lake to a freshwater lake. This evolution reflects the change from a closed-basin hydrologic regime to an open-basin hydrologic regime; sedimentation in the Lake Gosiute system was strongly influenced by the relationship between evaporation and inflow of waters into the basin and, during deposition of the upper Laney lacustrine beds, to the outflow of lake water into the Piceance Creek basin. Stratification sequences, sedimentary structures, and mineralogy of lithofacies in the Laney Member provide insights into the evolution of the lake and into the competing factors that determined the type of sediment accumulated in the lake and fringing environments. Carbonate sedimentation was strongly influenced by lacustrine transgressions and regressions across a very low topographic gradient. Terrigenous rocks mark progradation of beach and deltaic shorelines during times of greater precipitation when more terrigenous detritus was transported to the lake. Hydrochemistry of Lake Gosiute during the deposition of the Laney Member was controlled largely by surface and spring water. Calcite was precipitated in the lake as a result of mixing of calcium-rich inflow and saline-alkaline lake waters. Dolomite formed as a result of periodic flooding and drying of the fringing carbonate mud flat, where carbonate muds were saturated with saline-alkaline lake waters and underwent evaporative pumping. Some surface waters reaching the lake contained high concentrations of salts dissolved from efflorescent crusts generated by capillary draw of connate waters in alluvial and mud-flat sediments.

Hydrochemistry of the Lake Magadi basin, Kenya

New and more complete compositional data are presented for a large number of water samples from the Lake Magadi area, Kenya. These water samples range from dilute inflow (<0.1 g/kg dissolved solids) to very concentrated brines (>300 g/kg dissolved solids). Five distinct hydrologic stages can be recognized in the evolution of the water compositions: dilute streamflow, dilute ground water, saline ground water (or hot spring reservoir), saturated brines, and residual brines. Based on the assumption that chloride is conserved in the waters during evaporative concentration, these stages are related to each other by the concentration factors of about 1:28:870:7600:16,800. Dilute streamflow is represented by perennial streams entering the Rift Valley from the west. All but one (Ewaso Ngiro) of these streams disappear in the alluvium and do not reach the valley floor. Dilute ground water was collected from shallow pits and wells dug into lake sediments and alluvial channels. Saline ground water is roughly equivalent to the hot springs reservoir postulated by Eugster (1970) and is represented by the hottest of the major springs. Saturated brines represent surficial lake brines just at the point of saturation with respect to trona (Na 2CO 3.NaHCO 3.2H 2O), while residual brines are essentially interstitial to the evaporite deposit and have been subjected to a complex history of precipitation and re-solution. The new data confirm the basic hydrologic model presented by Eugster (1970) which has now been refined, particularly with respect to the early stages of evaporative concentration. Budget calculations show that only bromide is conserved as completely as chloride. Sodium follows chloride closely until trona precipitation, whereas silica and sulfate are largely lost during the very first concentration' step (dilute streamflow-dilute ground water). A large fraction of potassium and all calcium plus magnesium are removed during the first two concentration steps (dilute streamflow-dilute ground water-saline ground water). Carbonate and bicarbonate are the dominant anions, and mechanisms by which they are extracted from the solution include precipitation of alkali and alkaline-earth carbonates, and degassing, as well as precipitation and re-solution of efflorescent crusts. Much sulfate is apparently lost from solution by sorption as well as subsurface reduction. Seasonal runoff, principally from the valley floor north of Lake Magadi, is considered to be the principal recharge to the Magadi ground water system. Evaporative concentration is the overall process responsible for the chemical evolution of the brines. This includes not only simple evaporation, but also mineral precipitation as films and cements in the unsaturated zone, re-solution, and reprecipitation of efflorescent crusts, with consequent recycling of salts. In fact, a large fraction of the solutes are acquired through dissolution of efflorescent crusts. Data were obtained for borehole brines from as deep as 297 m. They show the existence of two distinct brine bodies below the present lake, one shallow, coexistent with bedded salts, and highly concentrated (260 g/kg average dissolved solids), and the other deeper in lacustrine sediments or fractured lavas, and only half as concentrated.