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Depositional and diagenetic processes of Qa Khanna playa, North Jordan basaltic plateau, Jordan
Journal of Asian Earth Sciences 39 (2010) 275–284
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Depositional and diagenetic processes of Qa Khanna playa, North Jordan basaltic plateau, Jordan F.M. Howari a,*, K.M. Banat b, Y.A. Abu-Salha b a b
Environmental Science Program, College of Arts and Science, The University of Texas of the Permian Basin, 4901 East University, Odessa, TX 79762, United States Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan
a r t i c l e
i n f o
Article history: Received 8 July 2009 Received in revised form 1 April 2010 Accepted 3 April 2010
Keywords: Playa Sediment Clay Evaporate Jordan Sedimentary structures
a b s t r a c t The present study explored mineral occurrences and sediment characteristics of playas from northern Jordan and explained depositional and diagenetic processes as reflected from bulk chemistry and sedimentary structures. Mudcracks of different sizes and shape patterns, laminations, intersediment vesicles, and bioturbation pipes are the main sedimentary structures. Plagioclase, olivine, orthopyroxene, nepheline and other opaque minerals are all of detrital origin, and are derived from the basaltic bedrocks surrounding the studied playa. Evaporites are very rare; they are represented only by trace amounts of gypsum. The identified clay minerals in the clay fraction of the studied sediments, arranged according to their decreasing abundances are palygorskite, illite, kaolinite, smectite and chlorite. The elemental abundances were tied to clay, CaCO3 and nearby igneous rocks. The type of clay minerals, the high pH values of the studied sediments, and the considerable incorporation of Mg and K in palygorskite and illite respectively, may strongly reflect a high evaporative and alkaline environment under arid to semi-arid conditions in an ephemeral lake of the Qa Khanna. Concentrations and distributions of both major and trace elements are essentially controlled by the clay mineralogy and the calcium carbonate content; Ca is mainly incorporated in the CaCO3, which is either generated authigenically or by aeolian deposition. Fe and K are incorporated and fixed by illite under an evaporative and alkaline environment. Mg is incorporated in palygorskite while Mn is adsorbed on various clay minerals. Sr substitutes for Ca in the aeolian CaCO3 and its presence in the studied sediments is independent of the prevailing conditions during the playa evolution. Rb substitutes for K in illite under the prevailing chemical conditions in the studied playa. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction A playa can be described as dry lakebed, generally the shore of, or a remnant of, an endorheic lake. Playas are also known as alkali flats, sabkhas, dry lakes or mud flats; if the surface is primarily salt then they are called salt pans or salt flats (Cooke et al., 1993; Zaaboub et al., 2007). It usually consists of fine-grained sediments infused with alkali salts (Einsele, 1992; Alsharhan & Kendall, 2003; Sadooni et al., 2005). Playa sediments may contain commercially valuable mineral deposits of clays, evaporites and uranium minerals. Clay minerals are typically montmorillonite, palygorskite, sepiolite, kaolin, hectorite and saponite. Evaporites may also contain a wide range of various minerals, such as gypsum, magnesite, siderite, halite, glauberite, thenardite, natron and trona (Einsele, 1992). All playas exist in areas where annual evaporation is considerably greater than annual precipitation, controlling the period for which the standing water persists in the playa. However, Motts (1970) * Corresponding author. E-mail addresses: [email protected], [email protected] (F.M. Howari). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.04.001
suggested that the term playa should be used only when the surface is flooded for less than 25% of the year. The nature of a playa is determined by its sedimentary and hydrological properties, and is a function of many interrelated variables, the most important of which are groundwater, runoff, surface water, pore water, sediments, salts, aeolian processes, and chemical and biological reactions (Torgersen et al., 1986; Boggs et al., 2006). These variables affect the nature of surface morphology, deposition, and diagenesis. Playas have been studied extensively in different areas in the world, especially in the US, Australia, and Africa. In Jordan however studies of playas are rare. For example, Smettan et al. (1993) studied the soil dynamics of playa system in Wadi Araba in Jordan after winter rain. They found that the effect of rain on soils results in an increase of alkalinity due to leaching of salt in the sandy soil. However, there is a need for more studies of playa systems in Jordan. The area selected for the present study is called Qa Khanna, which is located at the northwestern edge of the basaltic plateau in northern Jordan. The studied area lies about 85 km SE of Irbid, and 55 km NE of Amman, and is bounded by 32°000 –32°180 N latitude and by 36°200 –36°310 E longitude (Fig. 1).
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The Qa Khanna is an elongated flat basin trending NW–SE, with a total length of about 20 km. It is composed of two broad basins lying at each end of the playa, with maximum widths of about
4 km. These two basins are linked together by a relatively narrow central band (Fig. 1). Some marginal parts of the playa were reclaimed by farmers for agricultural activities. The surface of the
Fig. 1. Geology and locations of trenches, pits and dust samples of Qa Khanna playa.
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Qa Khanna playa is mostly flat and, vegetation free, except for a few desert bushes at the margins of the playa. The surface is randomly varied in color between bright creamy-white and light brown. This variation may be attributed to differences in evaporation rates of the standing water due to small topographic irregularities. Poor soil profiles are developed as a result of the downward percolating of water when the surface is flooded (e.g. Cooke et al., 1993; Farpoor et al., 2004; Mee et al., 2004). The near shore margins of the playa are covered by a blanket of cherty and basaltic pebbles, ranging in size from 1 to 12 cm. 2. Geology and climate Qa Khanna is located directly at a northwestern margin of the basaltic plateau in NE-Jordan, which affects the geology, mineralogy and geochemistry of the playa sediments. Furthermore this landscape and associated topographic setting impact the prevailing climatic conditions and drainage pattern. Many geological studies have been carried out on the basaltic plateau. One of the most comprehensive was conducted by Van Den Boom and Sawan (1966), which comprised a regional investigation of the plateau basalts in NE-Jordan. In the vicinity of the study area six basaltic flows of Oligocene and Miocene–Holocene age are intercalated with limestone and arenite units. The lowermost sedimentary unit, a cherty limestone of Middle Eocene age which consists of limestone and marls with intercalated flint layers, is best exposed south of Tap-Line Road and north and south of Al-Safawi. This unit is overlain by a nummulitic limestone unit cropping out ESE, SE and S of Azraq. This unit is composed of white, chalky marl, locally sandbearing limestone and hard limestone with chert layers. This unit is of Upper Eocene age. The area of the basaltic plateau is in one of the desert regions of Jordan, with a hot, arid climate, and particularly small amount of rainfall. The area receives its rainfall mainly in winter months with the mean amount of monthly rainfall during winter ranging from 8.9 mm to 37.1 mm (Jordanian Meteorology Department, 1987). The temperature during summer months ranges from 26.5 °C to 45.8 °C, while in winter, it ranges from 6.4 °C to 20.1 °C, and occasionally drops below freezing. The prevailing winds are Southwesterlies (Jordanian Meteorology Department, 1987). 3. Methodology Sediments, sedimentary structures and other geological phenomena of Qa Khanna were described and measured in the field. Twenty-surface sediment samples were collected in trenches (Fig. 1). Fourteen samples were also collected in 50 cm intervals from three pits dug to a depth of 2.5 m to study vertical variations in mineralogical and sedimentological chrematistics. In order to study the distribution of the elements laterally and vertically, three geochemical profiles were constructed. The first profile represents the variation of elements from the flooding area at the southeast part of the playa, and the last is at the farthest region to the NW, along with the elongation direction of the playa. The second and third profiles were made perpendicularly to the first profile in the southeastern and northwestern parts respectively. Two dust samples were collected from the area surrounding the playa. The dust was mineralogically analyzed by XRD to compare its composition with the mineralogy of the playa sediments. 3.1. Geochemical preparation and analyses All sediment samples have been dried at 80 °C, disaggregated and quartered. The sand fraction was separated by a wet sieving method, whereas, a pipette method was used for the separation
of clay and silt fraction (Folk, 1974). Percentages of silt and clay in a representative samples were measured using computerized SEDIGRAPH for 20 surface sediment samples to investigate the lateral variations in the sediment grain sizes. Non-clay minerals of the silt-size fractions were separated and mounted on Canada balsam glass slides, and identified by using the transmitted polarizing microscope. X-ray diffraction techniques were used for the identification of clay minerals, after the preparation of oriented samples. Samples have been scanned as untreated, glycolated clay and heated to various degrees following Brown’s procedure (1961). Samples were cleaned of carbonates, iron oxides and organic materials using the hydrogen peroxide method of Jackson (1979) and Mehra and Jackson (1960). Oriented, flat-layer specimens were prepared for the X-ray study of the clays by allowing a few drops of clay suspension to dry out slowly on a glass slide (Muller, 1967; Banat, 1980). The oriented samples were examined by using a vertical X-ray diffractometer under the following conditions: Radiation = Stabilized nickle filtered Cu K Voltage = 30 kV, Current = 20 mA, Speed = 29 1 cm/min. The relative abundance of clay minerals was determined by comparing the heights of the major reflections for each clay mineral to the summed heights of the major reflections for all clay minerals (Tables 2 and 3). X-ray diffraction techniques were used to identify the non-clay minerals, as well as the evaporite minerals in the sediments of the Qa Khanna playa and in the dust samples collected from the surrounding area. Sr and Rb trace elements, as well as Ca, Mg, K, Fe and Mn major elements were analyzed for the clay fraction of the sediments of the Qa Khanna by using the Atomic Absorption Spectrophotometry. The samples were completely dissolved and treated according to Hesse (1972). Three subsamples were analyzed. The pH values
Table 1 Mean percentage of sand, silt and clay of studied playa sediments by region. Region of basin
Mean% of sand (>63 lm)
Mean% of silt (2–63 lm)
Mean% of clay (<63 lm)
Southern Central Northwestern
6.1 2.1 0.7
84.6 81.3 73.8
9.3 16.6 25.5
Table 2 The relative abundance of clay minerals in the surface samples. No.
Sample no. A.1
Palygorskite
Illite
Kaolinite
Semictite
Chlorite
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
A.1 A.2 B.5 C.7 C.8 C.9 11 14 17 20 22 D.15 E.27 F.30 G.32 G.33 G.34 H.36 I.38 I.40
xxxxx xxxxx xxxxx xxxxx xxxx xxxxx xxxx xxxx xxxx xxxxx xxxx xxxx xxxxx xxxxx xxxx xxxxx xxxx xxxxx xxxxx xxxx
xxxx xxxx xxxx xxxx xxxx xxxxx xxxx xxxx xxx xxx xxx xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxxx xxxx
xxxx xxxx xxxx xxxx xxxxx xxxx xxxxx xxxxx xxxxx xxxx xxxxx xxxxx xxxx xxxx xxxxx xxxx xxxxx xxxx xxxx xxxx
xx xx xxxx xx xx xxxx x xx xx xx xx xx x xx xx xx xx xx xx xx
xx x xx x x xx xx xx x xx xx xx x xx x xx x x x x
xxxxx = major; xxxx = abundant; xxx = common; xx = minor; x = scare; (x) = traces; samples1–9 from southern part; samples 10–14 from central and 15–20 from northwestern.
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Table 3 The relative abundance of clay minerals in the subsurface samples. The samples are arranged from top to bottom for each pit. No. 1 2 4 5 6 7 8 9 10 11 12 13 14
Sample no.
Depth (m)
Palygorskite
Illite
Kaolinite
Smectite
Chlorite
pit pit pit pit pit pit pit pit pit pit pit pit pit pit
0.5 1 1.5 2 2.5 0.5 1 1.5 2 2 0.5 1 1.5 2
xxxxx xxxx xxxx xxxx xxxx xxxxx xxxxx xxxx xxxxx xxxxx xxxxx xxxx xxxxx xxxxx
xxxx xxx xxx xxxxx xxxx xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx
xxxx xxxxx xxxx xxxx xxxxx xxxxx xxx xxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxx
xx xx xxx xxx xx xx x x x x x xx xx x
x x x x x xx x x x x xx x x xx
1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3,
1 2 3 4 5 1 2 3 4 5 1 2 3 4
xxxxx = major; xxxx = abundant; xxx = common; xx = minor; x = scare; (x) = traces.
for the study sediment were measured by using a digital pH meter (PTI 55) for twelve representative samples. The procedure was applied as published by Cottenie (1982). The percentages of calcium carbonate in the fine sediments (clay fraction) from Qa Khanna were estimated by using the Calcium Try (Hesse, 1972) method for 20 surface and subsurface sediment samples from the playa. 4. Results and discussion 4.1. Field description of the sediments The sediments of the Qa Khanna are semi-consolidated and very compacted; even the hand augering process was unsuccessful. Therefore, pits were dug to get the subsurface samples. Many water wells were dug around and within the playa by farmers and by the Jordanian army. The groundwater has no significant effect on the sediments because of its relatively great depth. It ranges between 70 m deep in the northwestern basin and 160 m in the southeastern basin. The playa sediments are composed of finegrained sand with a major amount of mud. The distribution of sediments may be attributed to the annual flooding, which cause reworking of sediments, and produce most of sedimentary structures, especially of the southeastern parts of the playa. The walls of the three pits display different colors. The upper part is a light brown color, and it becomes reddish-brown below at 1.1 m depth. Below this, it is deep dark brown in color. Plant remains and small pipes created by the plant roots are common in the playa sediments. It was not possible to estimate the rate of sedimentation during the time of the field work, which took place during the dry summer. However, the sedimentation in the Qa Khanna should be of relatively high rate based on the observation that garbage and used cans, which were found at a depth of about 1 m, were not completely rusty. 4.2. Sedimentary structures The following sedimentary structures were found in the Qa’ Khanna playa. 4.2.1. Mudcracks Mudcracks are the most common sedimentary structure over the whole surface of the studied playa (Fig. 2). These cracks are non-orthogonal and commonly of six-sided pattern. Irregularshaped cracks also exist. The size of mudcracks ranges from 2.5 cm to more than 30 cm in diameter, with varying crack depths between 1 mm and 2.7 cm, respectively. The variation in sizes is proportional to sediment grain size (Cooke et al., 1993). Thickness
of the material and its dry density are also important (Corte and Higashi, 1964). The profile of desiccated sediment blocks displays flat, convex and concave patterns. The V–shaped cracks are occasionally filled with loose sediments reworked mostly by wind in the northwestern part of the playa, which seems to be the most part affected by the southwesterly winds prevailing in the area. However, in the southeast and central parts of the playa, mud cracks are commonly partly filled by compacted sediments carried and/or reworked by the latest episode of the inflowing water. These filled cracks display a network pattern, and may be modified by small distributaries channels created by the inflowing water. 4.2.2. Laminations In the southeastern part of the playa, which is located directly adjacent to the basaltic bedrock, laminations are entirely obscured, while they are more developed in the northwestern part of the playa (Fig. 3). The changes of lamination morphology in the southeastern sediments may be attributed to the high energy of the inflowing water coming directly from the nearby bedrock causing reworking of sediments. In the northwestern portion of the playa, the inflowing water has lost most of its energy, creating suitably calm conditions for free sedimentation from the suspension, allowing lamination to be formed. 4.2.3. Intersediment vesicles The upper 0.6 m of the playa sediment is characterized by the presence of millimeters-sized ovate to irregular cavities or vesicles that are commonly connected by thin horizontal and vertical cracks formed during desiccation (Fig. 4). Vesicles form in the sediments by trapping air in water-mud slurries during floods. Below 0.5 m, these vesicles were destroyed as a result of compaction of the sediments. 4.2.4. Bioturbation Burrows are the only bioturbation phenomena observed in the playa sediments but they are rarely preserved. They are only present in the northwestern basin, which is expected to be by far the most provisional part from the direct drainage position. These burrows are horizontal cylindrical pipes of a centimeter in diameter, none branched, and 2–3 cm in length (Fig. 5). 4.3. Textural analyses The textures of the sediment were described by using a triangular diagram (Fig. 6). As shown in this figure, silty loam is the most dominant sediment type analyzed from this playa, whereas, silty
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a
Upper part of Qa Khanna, near sample # 20
c
Lower part of Qa Khanna, near sample # 4
b
Upper part of Qa Khanna, near sample # 19
d
Lower part of Qa Khanna, near sample # 6
Fig. 2. Mudcracks which appear non-orthogonal and commonly of six-sided pattern (a, b, and c, samples # 4, 19, 20). Irregular shapes also occur (d, sample # 6).
Southeastern part of Qa Khanna
Lower part of Qa Khanna
10 cm
Fig. 3. Lamination of the studied mud sediments, it appears as obscured, and more developed in the northwestern part of the playa.
clay loam, and silt are less common. Sand is very rare. Based on the variations in the types of sediment in the Qa Khanna, it is possible to differentiate between three different regions; (Table 1): (i) southeastern part, (ii) central part and (iii) northwestern part. Grain size analysis was also used to investigate the vertical variation in sediment types of the Qa Khanna. It is clearly seen from the textural analyses that the vertical variation of grain size is not significant. Grain size analysis of the dust samples collected from the area surrounding Qa Khanna, revealed that the bulk percentage of the dust lies in the silt-size; it is 66% and 92% in the two studied samples. 4.4. Mineralogy 4.4.1. Clay minerals As shown in Tables 2 and 3 the identified clay minerals in the clay fraction of the Qa Khanna playa sediment, arranged according
Fig. 4. Morphology of the intersediment vesicles observed in the studied mud sediments, and characterized by the presence of millimeters-sized ovate to irregular cavities or vesicles.
to their decreasing abundance are palygorskite, illite, kaolinite, smectite and chlorite. Palygorskite is the most abundant clay mineral in the majority of the analyzed samples. Illite and kaolinite are generally equal in quantity in nearly all studied samples. Smectite and chlorite are recorded mainly as minor to scarce minerals except for a very few samples in the southeastern part of the study playa, where they are recorded as major minerals. Generally, all the clay minerals of Qa Khanna are homogenous in distribution. They do not exhibit a significant quantitative variation, either laterally or vertically in the examined samples. Smectite and chlorite show a slight lateral variation in their content, decreasing slightly toward the northwestern part of the Qa Khanna. This may be related to the increase in the distance from area most commonly flooded. However, both smectite and chlorite show similar vertical distributions. In the Qa Khanna, it is assumed that kaolinite is of detrital origin and it is mainly derived from the surrounding basaltic rocks. This
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chemical analyses of the clay fraction of the Qa Khanna sediments revealed the abundance of Ca, Mg and K, and (c) the clay samples were taken from a very shallow depth, and may exclude the diagenetic origin of kaolinite which is typically formed at great depths diagenetically. In the study area volcanic rocks are located around the playa basin, therefore, it is believed that smectite was formed outside of the playa basin, by the alteration of these volcanic rocks, and then transported to the playa as clay.
3 cm
Fig. 5. Morphology of the bioturbation observed in the northwestern part of the studied mud sediments appears as horizontal cylindrical pipes of a centimeter in diameter, none branched, and 2–3 cm in length.
CLAY 100%
a
Clay
Silty clay
Sandy clay Clay loam Sandy clay loam
Silty clay loam
Loam Loamy sand
Sdany loam
Silt
Silt loam
Sand
SILT 100%
SAND 00% 100%
b
Pit 3
Pit 2
0.0 m
1.0 m
1.0 m
1.0 m
2.0 m
2.0 m
DEP PTH
0.0 m
2.0 m
Silt
0.0 m
Silty loam
Pit 1
Silty clay loam
Fig. 6. Textural classification of (a), and vertical variation (b) of the Qa Khanna playsediments.
assumption is supported by: (a) the chemical alteration of steeply sloped basaltic bedrock surrounding the Qa Khanna which may lead to the formation of clay minerals including kaolinite, (b) the
4.4.2. Mineralogy of the non-clay sediment samples The identified non-clay minerals encountered in the silt fraction of the Qa Khanna sediments as well as in the collected dust samples arranged according to their relative abundance are quartz, calcite, plagioclase, evaporites (gypsum) and dolomite. Microscopic study of the Qa Khanna sediments revealed that the silt-sized sediments of the study playa are mainly composed of quartz grains and calcite. Quartz grains are highly fractured, commonly show large conchoidal fractures with scratched surfaces and sharp edges, but also commonly have rounded to subrounded outlines. Calcite appears in two forms: (a) disseminated anhedral to euhedral crystals, occasionally showing a prominent cleavage traces with a distinctive twinkling optical phenomenon, and (b) secondary growth around or enveloping other mineral grains. Plagioclase exists as a minor to scarce mineral in the studied sediment samples. It is mostly found as anhedral crystals with low alteration, mostly exhibits a well defined lamellar twinning. Evaporites, represented by gypsum only, are very scarce in the sediments of the Qa Khanna. The study of the dust samples under the microscope revealed that quartz and calcite are the major non-clay constituents. Quartz grains of the dust samples show similar optical and morphological characteristics to the quartz grains of the playa sediments, i.e., fractures, sharp edges and scratches. On the other hand, calcite derived by wind commonly occurs as anhedral, smashed crystals, with no distinct optical features. Additional accessory minerals have been identified by the microscopic study, including olivine, orthopyroxene, nepheline and some opaque minerals. Olivine is generally found as anhedral to subhedral, highly fractured and altered crystals. Euhedral crystals are rare. Orthopyroxene was recognized by its high birefringence and parallel extinction. It occurs mostly as euhedral crystals. Nepheline occurs as euhedral to anhedral crystals. It is characterized by its light grey interference color, and it commonly contains mineral inclusions (Fig. 7). The mechanism by which quartz grains are incorporated in the playa sediments is either by direct wind deposition on the playa surface or as a result of flushing of the dust grains deposited on the surrounding basaltic plateau. Quartz may have originated from the Kurnub Sandstone, which is exposed to the southeast of the Qa Khanna via the southwesterlies prevailing winds. Calcite is a common mineral in playas deposits. It can be formed and precipitated whenever there are sufficient concentrations of Ca2+ and CO2 3 ions especially under evaporative conditions (Melvin, 1991). Calcite is widely reported as an authigenic mineral in playas; for example in Amargosa desert playa, Searles Lake (playa) and in many Western Victoria playas (Khoury et al., 1982; Hay et al., 1991; Deckker and William, 1989). Authigenic calcite is recognized through microscopic features, especially the euhedral crystals and the prominent cleavage traces, suggesting in situ growth of the calcite crystals. Einsele (1992) states that calcite is the first mineral to be precipitated chemically or biochemically as a result of increasing salt concentration, regardless of the composition of inflowing water into the closed basin. The microscopic study of calcite from the Qa Khanna sediment samples, as well as dust samples reveal that calcite is characterized by both disseminated smashed anhedral crystals, as well as euhedral crystals with distinct cleavage traces. It has been also observed as overgrowth around other mineral grains. This may indicate that
F.M. Howari et al. / Journal of Asian Earth Sciences 39 (2010) 275–284
Quartz Conchoidal fracture
281
Calcite with secondary growth
Quartz
Subr-ounded Scratched surface And sharp edges
Rounded
Plagioclase anhedral Plagioclase, Laminar twining
Orthopyroxene
Calcite, euhedral Cleavage traces
Dolomite D l it rhombs h b
Olivine, Euhedral crystal
Nepheline
Fig. 7. Photomicrograph of the minerals found in the non-clay size fraction of the studied sediment.
calcite is originated from both authigenic precipitation and aeolian deposition. The quantitative scarcity of evaporites (gypsum) may be attributed to the great depth of groundwater, which exceeds 70 m in the study playa. The precipitation of gypsum is probably due to a displacive crystal growth through infiltration of the standing water in rainy season. A similar conclusion was reached by Melvin (1991) during his work on west Victoria, Australia, playas. Plagioclase, olivine, orthopyroxene, nepheline and opaque minerals are minor to scarce, and are all derived directly from the basaltic rocks surrounding the Qa Khanna by weathering. Transportation of these minerals to the studied playa could take place either during flooding water or by deflation during summer. 4.5. Geochemistry Chemical analyses of the clay fraction from the Qa Khanna sediments including both surface and subsurface samples were carried out in order to determine the concentrations of major elements (Ca, Mg, K and Fe) as well as the trace elements (Mn, Sr and Rb). The results of the chemical analyses shown in Figs. 8 and 9. These indicate there are no significant variations in the distribution of the elements laterally and vertically in the sediments of the Qa Khanna. The concentrations of calcium and magnesium in the analyzed
sediment samples display no significant variations throughout the studied sediments; similarly, the subsurface sample concentrations for both elements show almost identical distribution with the depth. The mean values for the pH in the surface and subsurface samples are 8.3 and 8.2, respectively. The measured concentrations of potassium in both surface and subsurface samples of the playa show a nearly uniform distribution in all four geochemical profiles (Fig. 9). These profiles suggest no evident variations in the playa sediments, either laterally or vertically. The concentrations of iron in the surface sediment samples vary from about 2.0% to 5.7%, with an average content of 4.9%. In the subsurface samples, iron concentrations range between 4.5% and 11.8%, with an average content of 5.9%. The distribution of manganese shows no significant variation in concentration in the four geochemical profiles. Strontium and rubidium display a similar geochemical distribution in nearly the all profiles. The two elements show a slight increase towards the center of Qa Khanna playa, meanwhile their concentrations increase slightly with depth. In the study area, pH values increase slightly from the southeastern part towards the northwestern part in both surface and subsurface samples. In order to assess the possible source of the measured elements, the results of the chemical analyses of the sediments of the Qa Khanna playa are compared with those gi-
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Ca 50000
Mg g
ppm
40000
K 30000
Fe
20000
Mn
10000
Sr Rb
0 0
4
8
12
16
20
24
Sample no Fig. 8. Concentrations (ppm) of the measured elements in the studied surface samples.
ven by Wedepohl (1978), Deer et al. (1985) and Newman (1987). As shown in Table 4, it can be generally concluded that basalt is probably the main source for most of the major elements, which were later incorporated into the different clay species present in the studied area, as well as in CaCO3 (for Ca). However, Mg may be trapped primarily by palygorskite, while Fe, K and Rb may be incorporated in illite. However, the Sr concentration in the studied sediments is comparable with its concentration in Cretaceous car-
a
bonate rocks which may indicate that carbonates are the possible source of the Sr. As shown in Table 4 considerable part of the Ca found in the studied playa sediments originated from CaCO3 that formed authigenically or from the airborne dust. Gosselin et al. (1993) reported that in closed basins precipitation of calcite and Mg-rich clay minerals may be an important source of magnesium and calcium in their sediments. However, calcium may also be derived from Carich clay such as illite, smectite and palygorskite. The relatively weak positive correlation coefficient between Ca and Fe (Table 5) may indicate that basalt is not only the possible source of Ca in the studied sediments; carbonate rocks could have contributed to Ca concentrations as well. Palygorskite, which is present as a major mineral in the Qa Khanna sediments, is probably the main source of Mg concentrations encountered in these sediments; kaolinite, illite, chlorite and smectite form only a minor source of Mg. This conclusion is in agreement with findings of Singer (1984), who has studied the mineralogical and geochemical relations in some Western Australia playas. According to Singer (1984), the Mg-rich sediments and/ or brines associated with saline and alkaline environments lead to the formation of Mg-phyllosilicate minerals, especially palygorskite associated with dolomite and calcareous materials. These materials in turn, reflect an evaporative alkaline environment, which is typical of in playas and sabkhas. These conditions provide a very favorable situation for Mg to incorporate in the authigenic paly-
b
12
12
Fe
Fe
10
Co oncentrattion, log (ppm)
C Concentra ation, log (ppm)
Ca
10
K Mg
8 Mn
6 Sr
4
Rb
2
0
Ca K Mg
8 Mn
6 Sr
4
Rb
2 0.0
2.5
5.0
7.5
40
12.5
15.0
0.0
0.5
1.0
c
d 12
Concentra tion, C t log ( p ppm)
Concentrration, log g (ppm)
Ca K Mg
8 Mn Sr
4
2.5
3.0
Fe
10
6
2.0
12
Fe
10
1.5
Depth (m)
Depth (m)
Rb
Ca K Mg
8
Mn
6
Sr
4
Rb
2
2
0 0.6
0.8
1.0
Depth (m)
1.2
1.4
0.6
0.8
1.0
1.2
1.4
1.6
Depth (m)
Fig. 9. Geochemical profiles showing the lateral and vertical variations of the measured element concentrations in the studied samples: (a) represents the variation of elements in the flooding area at the southeast part of the playa towards the farthest point (NE) from the flooding, correspondingly with the elongation direction of the study area; (c and b) represent the second and third profiles which were made perpendicularly to the first profile in the southeastern and northeastern parts respectively, and (e) represent the fourth profile which was constructed to display the variations of elements with depth.
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Table 4 Average concentration of major and trace elements (ppm) in the studied sediments, compared with those given for different geological materials and minerals by Wedepohl (1978), Deer et al. (1985) and Newman (1987). Average (ppm)
Ca
Mg
K
Fe
Mn
Sr
Rb
Surface samples Pit 1 Pit 2 Pit 3 Basalt Argillaceous deposit Oxidized muds Limestone Carbonate Clay Palygorskite Illite Kaolinite Smectite Chlorite
32,307 55,544 34,805 29,395 72,926* – – – 290,405* 31,523* 4700n 11424d 1500d 11567d 3400n
7734 29,719 12,230 18,595 38,900* – – 7900* – – 71850n 6300n 850n 20000n 11700n
11,401 14,882 9278 12,225 4400* 17,147* – 2201* – – traces, d 35,700, d 210, d 280, d traces, d
48,882 69,703 54,010 5314 90,360* – – – 3500* – 19,700 36960d 5400d 5800d 115000n
531 654 601 516 1600* – – 500w 1100* – 2800n traces, d traces, d 2530 d 5000d
115 153 111 110 48* – – n 210* – – – – – –
32 37 44 44 30 40* – 45* – – – – – – –
Table 5 Correlation coefficients between the abundance of measured major and trace elements. Elements
Ca
Mg
K
Fe
Mn
Sr
Rb
Ca Mg K Fe Mn Sr Rb
1 0.15 0.07 0.15 0.22 0.91 0.33
0.15 1 0.02 0.19 0.07 0.024 0.12
0.078 0.023 1 0.85 0.69 0.69 0.024
0.15 0.19 0.85 1 0.73 0.07 0.17
0.22 0.07 0.69 0.73 1 0.43 0.086
0.91 0.02 0.02 0.07 0.43 1 0.42
0.33 0.12 0.22 0.17 0.08 0.42 1
gorskite. Spencer et al. (1984) reported that both Mg and K can be readily removed from lake water through exchange and fixation on clays, by diffusion into pore fluids, and by incorporation into diagenetic mineral phases under alkaline and high evaporation conditions. From Table 5 it can be seen that there is highly positive correlation between K and both Fe (0.86) and Mn (0.7). This may indicate that basalt bedrock surrounding the playa basin is an important source of K in the studied playa sediments. According to Hay and Guldman (1987), K fixation by illite or by illitization of smectite (authigenic illite) needs a saline and alkaline environment. They determined that the high concentration of K in the playa-lake complex at Searles Lake, California, is one of the indicative lines of evidence for the playa stages throughout the successive changes in the lake-playa chemistry. Typically, K is readily removed from solution by exchange with/or sorption onto clays. This is ideally achieved in alkaline solutions (Gosselin et al., 1993). The clays have a great ability to adsorb Fe at edges and within the lattice of the clay minerals (Velde, 1992). Hay et al. (1991) reported that the high Fe content incorporated in authigenic illite in the Searles Lake-playa complex favors the oxidizing conditions associated with a high degree of evaporative concentration and presumably of salinity accompanied with a closed playa basin. A high pH is also an important chemical parameter that favors authigenic phyllosilicate formation. In the studied sediments, the possible sources of Fe are (i) illite clay mineral, (ii) Fe2+ bearing minerals derived from basalts, and (iii) Fe-oxides. As shown in Table 5 there is strongly positive correlation between the Fe and Mn, which may indicate that basalt is the main source of Mn in the Qa Khanna playa sediments. It is reported that Mn tends to be adsorbed on the edges of some clay minerals like palygorskite, smectite and chlorite. The adsorption process gets more effective in alkaline evaporative conditions (Weaver and Pollard, 1976). Therefore, in the studied sediments, Mn could be adsorbed on the different clay
species, especially palygorskite and smectite under the alkaline and evaporative conditions characterizing the playa environment. The distribution of Sr in rock-forming minerals is related to Ca and K. Because of the similarity in physical and chemical properties between the three elements, Ca and K can be replaced by Sr in their naturally occurring minerals (Wedepohl, 1978). The negative correlation between Sr and Fe (Table 5) may indicate that basalt has only a minor role as a source of Sr in the studied sediments. In nature, Rb does not form minerals of its own, but it is dispersed especially in K-minerals. Such behavior is caused by the high similarity between Rb and K in their crystal chemistry (Wedepohl, 1978). Rb content in the studied sediment is very comparable with its content in basalt (Table 4). Since the correlation coefficient between Rb and Fe is positive (Table 5), it may be concluded that basalt is the main source of Rb. Stoffers and Kuhn (1974) demonstrated that under high evaporative conditions, Rb substitutes for K in polyhalite in the evaporites of the Red Sea. This enhances the fact that the tendency of Rb for substitution for K increases by increasing salinity and alkalinity. Thus, it is expected that Rb in the studied playa sediments is incorporated in illite after its substitution for K under alkaline and evaporative conditions. 5. Conclusions The observed sedimentary structures in the studied playa include mudcracks of different sizes and shape patterns, laminations, intersediment vesicles and bioturbation pipes. The sediments of the Qa Khanna are compacted to semi-consolidated, and they are dominated by silty loam, whereas silty clay loam and silt are less abundant. On the basis of the variations in grain size, steeply sloped basaltic bedrocks surrounding the southeast of the studied playa, and the preservation of sedimentary structures, especially the bioturbation pipes at the northwestern parts of the playa may indicate that the main direction of flooding is from the southeast towards the northwest. Microscopic study of the silt-sized sediments revealed that quartz and calcite are the major minerals, whereas, detrital minerals of less abundance originated from the basaltic rocks, and include plagioclase, olivine, orthopyroxene, nepheline and opaque minerals. Gypsum is rare in the studied playa sediments, and it is the only evaporitic mineral identified by the X-ray diffraction. The identified clay minerals in the playa sediments arranged according to their decreasing abundance are palygorskite, illite, kaolinite, smectite and chlorite. Palygorskite and illite are of authigenic origin, whereas kaolinite, smectite and chlorite are of detrital origin. The presence of palygorskite clay mineral and the relatively high pH values may indicate an alkaline and evaporative condition under arid to semi-arid climate. Basaltic
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bedrocks surrounding the studied playa represent the main source of most of major and trace elements encountered in the studied sediments. Potassium and iron are essentially fixed by illite under an evaporative and alkaline condition. The high concentration of magnesium is explained by its adsorption and/or absorption by palygorskite enhanced by chemical conditions prevailing in the studied playa. Manganese is adsorbed on the edges of the different clay species in the studied playa sediments. The tendency of Rb to substitute for K increases by increasing salinity and alkalinity. Rb in the studied playa sediments is incorporated in illite after its substitution for K under alkaline and evaporative conditions. The strontium encountered in the clay-fraction is mainly as a result of substitution for Ca in CaCO3 derived from the air born dust. Acknowledgement The author thanks the anonymous reviewer and Dr. Boris Natalin Associate Editor of Journal of Asian Earth Sciences for their useful comments and suggestions. References Alsharhan, A.S., Kendall, C.G.St.C., 2003. Holocene coastal carbonates and evaporites of the southern Arabian Gulf and their ancient analogues. Earth-Science Reviews 61 (3–4), 191–243. Banat, K.M., 1980. Principles of Clay Mineralogy. Baghdad University Press. 138 p (in Arabic). Boggs, D.A., Boggs, G.S., Eliot, I., Knott, B., 2006. Regional patterns of salt lake morphology in the lower Yarra Yarra drainage system of Western Australia. Journal of Arid Environments 64 (1), 97–115. Brown, G., 1961. The X-ray identification and crystal structures of clay minerals. Mineral Society (Clay Min. Group), London. 544 p. Cooke, R., Warren, A., Goudie, A., 1993. Desert Geomorphology 526, 13. Corte, A.E., Higashi, A., 1964. Experimental-research on Desiccation Cracks in Soil. US Army Material Command, Cold Regions Research and Engineering Laboratories Research, Rept. 66, 72 p. Cottenie, A., 1982. Chemical Analysis of Plants and Soils. Gent University, Belgium. p. 63. Deckker, P., William, M.L., 1989. Modem, non-marine dolomite in evaporitic playas of western Victoria, Australia. Sedimentary Geology 64, 223–238. Deer, W.A., Howie, R.A., Zussman, D., 1985. An Introduction to the Rock-Fonning Minerals. Longman, Hong Kong. p. 251. Einsele, G., 1992. Sedimentary Basins. Springer-Verlag, Berlin. p. 628. Farpoor, M.H., Khademi, H., Eghbal, M.K., Krouse, H.R., 2004. Mode of gypsum deposition in southeastern Iranian soils as revealed by isotopic composition of crystallization water. Geoderma 121 (3–4), 233–242. Folk, R., 1974. Petrology of Sedimentary Rocks. Hemphill Publishing Co., Texas. p. 182. Gosselin, D.C., Sibray, S., Ayers, L., 1993. Geochemistry of K-rich Alkaline Lakes, Western Sandhills, Nebraska, USA. Geochimica et Cosmochimica Acta 58 (5), 1403–1418.
Hay, R.L., Guldman, S.G., 1987. Diagenetic alteration of silicic ash in Searles Lake, California. Clays and Clay Minerals 35 (6), 449. Hay, R.L., Guldman, S.G., Matthews, C., Lander, R.H., Duffm, M.E., Kyser, T.K., 1991. Clay mineral diagenesis in Core KM-3 of Searles Lake, California. Clays and Clay Minerals 39 (1), 84–96. Hesse, P.R., 1972. Textbook of Soil Chemical Analysis. Chemical Pub., New York. 520 p. Jackson, M.L., 1979. Soil Chemical Analysis Advanced Course, second ed. Published by the author, Madison, Wisconsin, 895 p. Jordanian Meteorology Department, 1987. Unpublished Disseminate Documents about Climatic Conditions in Jordan. Khoury, H.N., Eberl, D.D., Jones, B.F., 1982. Origin of magnesium clays from the Amargosa Desert, Nevada. Clays and Clay Minerals 30 (5), 327–336. Mee, A.C., Bestland, E.A., Spooner, N.A., 2004. Age and origin of Terra Rossa soils in the Coonawarra area of South Australia. Geomorphology 58 (1–4), 1–25200. Mehra, R.K., Jackson, M.L., 1960. Soil Chemical Analysis. Prentice Hall, Englewood cliffs, New Jersey. Melvin, J.L. (Ed.), 1991. Evaporites, Petroleum and Mineral Resources. Elsevier, Amsterdam. p. 228. Motts, W.S., 1970. Geology and hydrology of selected playas in western United States. Air Force Cambridge Research Laboratories, Office of Aerospace Research, US Air Force. Washington, DC, USA. Final Scientific Report Part II. AFCRL-69-0214. Muller, G., 1967. Methods in sedimentary petrology, E. Schweizerbart Sche Verlags buch handlung (Nagele U. Obermiller), Stuttgurt, 283 p. Newman, A.C.D. (Ed.), 1987. Chemistry of Clays and Clay Minerals. Wiley. pp. 1–128. Sadooni, F., Howari, F., Hamza, W., Abd El-Gawad, E., 2005. Modern Arid Coastal Sabkhas of the UAE: Spatial and Temporal Evolution, Sedimentology and Geochemistry. The Sixth Annual UAE University Research Conference, Research Affairs, vol. 6, SC1-14. Singer, A., 1984. Pedogenic palygorskite in the arid environment. In: Singer, A., Galan, E. (Eds.), Palygorskite-Sepiolite: Occurrence, Genesis and Uses. Developments in Sedimentology, vol. 37. Elsevier, Amsterdam, pp. 169–175. Smettan, U., Jenny, M., Facklam-Moniak, M., 1993. Soil dynamics and plant distribution of a sand dune play a microchore catena after winter rain in the wadi Araba (Jordan). Catena 20, 179–189. Spencer, R.J., Baedecker, K.J., Eugster, H.P., Forester, R.M., Goldhaber, M.B., Jones, B.F., Kelts, K., McKenzie, J., Madsen, D.B., Retting, S.L., Rubin, M., Bowser, C.J., 1984. Great Salf Lake, and precursors, Utah: the last 30,000 years. Contributions to Mineralogy and Petrology 86, 321–334. Stoffers, P., Kuhn, R., 1974. Red sea evaporites: a petrographic and geochemical study. In: Whitmarsh, R.B., Weser, O.E., Ross, D.A. et al., (Eds.), Initial Rep. D.S.D.P, vol. 23, pp. 821–847. Torgersen, T., de Dekker, P., Chivas, A.R., Bowler, M., 1986. Salt Lakes: a discussion of processes influencing paleoenvironmental interpretation and recommendations for future study. Paleogeography, Paleoclimatology, Paleoecology 54, 7–19. Van Den Boom, G., Sawan, O., 1966. Report on Geo-physical and Petrological Studies of the Plateau Basalts in NE Jordan. German Geol. Mission in Jordan, Amman. Velde, B., 1992. Introduction of Clay Minerals: Chemistry, Origin, Uses and Environmental Significance. Chapman and Hall, London, New York. p. 198. Weaver, C.E., Pollard, L.D., 1976. The Chemistry of Clay Minerals. Developments in Sedimentology, vol. 15. Elsevier, Amsterdam. 213 p. Wedepohl, K.K., 1978. Handbook of Geochemistry. Springer Verlag, Berlin. 131 p. Zaaboub, N., Abdeljaouad, S., López-Galindo, A., 2007. Origin of fibrous clays in Tunisian paleogene continental deposits. Journal of African Earth Sciences 43 (5), 491–504.
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Depositional and diagenetic processes of Qa Khanna playa, North Jordan basaltic plateau, Jordan F.M. Howari a,*, K.M. Banat b, Y.A. Abu-Salha b a b
Environmental Science Program, College of Arts and Science, The University of Texas of the Permian Basin, 4901 East University, Odessa, TX 79762, United States Department of Earth and Environmental Sciences, Yarmouk University, Irbid, Jordan
a r t i c l e
i n f o
Article history: Received 8 July 2009 Received in revised form 1 April 2010 Accepted 3 April 2010
Keywords: Playa Sediment Clay Evaporate Jordan Sedimentary structures
a b s t r a c t The present study explored mineral occurrences and sediment characteristics of playas from northern Jordan and explained depositional and diagenetic processes as reflected from bulk chemistry and sedimentary structures. Mudcracks of different sizes and shape patterns, laminations, intersediment vesicles, and bioturbation pipes are the main sedimentary structures. Plagioclase, olivine, orthopyroxene, nepheline and other opaque minerals are all of detrital origin, and are derived from the basaltic bedrocks surrounding the studied playa. Evaporites are very rare; they are represented only by trace amounts of gypsum. The identified clay minerals in the clay fraction of the studied sediments, arranged according to their decreasing abundances are palygorskite, illite, kaolinite, smectite and chlorite. The elemental abundances were tied to clay, CaCO3 and nearby igneous rocks. The type of clay minerals, the high pH values of the studied sediments, and the considerable incorporation of Mg and K in palygorskite and illite respectively, may strongly reflect a high evaporative and alkaline environment under arid to semi-arid conditions in an ephemeral lake of the Qa Khanna. Concentrations and distributions of both major and trace elements are essentially controlled by the clay mineralogy and the calcium carbonate content; Ca is mainly incorporated in the CaCO3, which is either generated authigenically or by aeolian deposition. Fe and K are incorporated and fixed by illite under an evaporative and alkaline environment. Mg is incorporated in palygorskite while Mn is adsorbed on various clay minerals. Sr substitutes for Ca in the aeolian CaCO3 and its presence in the studied sediments is independent of the prevailing conditions during the playa evolution. Rb substitutes for K in illite under the prevailing chemical conditions in the studied playa. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction A playa can be described as dry lakebed, generally the shore of, or a remnant of, an endorheic lake. Playas are also known as alkali flats, sabkhas, dry lakes or mud flats; if the surface is primarily salt then they are called salt pans or salt flats (Cooke et al., 1993; Zaaboub et al., 2007). It usually consists of fine-grained sediments infused with alkali salts (Einsele, 1992; Alsharhan & Kendall, 2003; Sadooni et al., 2005). Playa sediments may contain commercially valuable mineral deposits of clays, evaporites and uranium minerals. Clay minerals are typically montmorillonite, palygorskite, sepiolite, kaolin, hectorite and saponite. Evaporites may also contain a wide range of various minerals, such as gypsum, magnesite, siderite, halite, glauberite, thenardite, natron and trona (Einsele, 1992). All playas exist in areas where annual evaporation is considerably greater than annual precipitation, controlling the period for which the standing water persists in the playa. However, Motts (1970) * Corresponding author. E-mail addresses: [email protected], [email protected] (F.M. Howari). 1367-9120/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jseaes.2010.04.001
suggested that the term playa should be used only when the surface is flooded for less than 25% of the year. The nature of a playa is determined by its sedimentary and hydrological properties, and is a function of many interrelated variables, the most important of which are groundwater, runoff, surface water, pore water, sediments, salts, aeolian processes, and chemical and biological reactions (Torgersen et al., 1986; Boggs et al., 2006). These variables affect the nature of surface morphology, deposition, and diagenesis. Playas have been studied extensively in different areas in the world, especially in the US, Australia, and Africa. In Jordan however studies of playas are rare. For example, Smettan et al. (1993) studied the soil dynamics of playa system in Wadi Araba in Jordan after winter rain. They found that the effect of rain on soils results in an increase of alkalinity due to leaching of salt in the sandy soil. However, there is a need for more studies of playa systems in Jordan. The area selected for the present study is called Qa Khanna, which is located at the northwestern edge of the basaltic plateau in northern Jordan. The studied area lies about 85 km SE of Irbid, and 55 km NE of Amman, and is bounded by 32°000 –32°180 N latitude and by 36°200 –36°310 E longitude (Fig. 1).
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The Qa Khanna is an elongated flat basin trending NW–SE, with a total length of about 20 km. It is composed of two broad basins lying at each end of the playa, with maximum widths of about
4 km. These two basins are linked together by a relatively narrow central band (Fig. 1). Some marginal parts of the playa were reclaimed by farmers for agricultural activities. The surface of the
Fig. 1. Geology and locations of trenches, pits and dust samples of Qa Khanna playa.
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Qa Khanna playa is mostly flat and, vegetation free, except for a few desert bushes at the margins of the playa. The surface is randomly varied in color between bright creamy-white and light brown. This variation may be attributed to differences in evaporation rates of the standing water due to small topographic irregularities. Poor soil profiles are developed as a result of the downward percolating of water when the surface is flooded (e.g. Cooke et al., 1993; Farpoor et al., 2004; Mee et al., 2004). The near shore margins of the playa are covered by a blanket of cherty and basaltic pebbles, ranging in size from 1 to 12 cm. 2. Geology and climate Qa Khanna is located directly at a northwestern margin of the basaltic plateau in NE-Jordan, which affects the geology, mineralogy and geochemistry of the playa sediments. Furthermore this landscape and associated topographic setting impact the prevailing climatic conditions and drainage pattern. Many geological studies have been carried out on the basaltic plateau. One of the most comprehensive was conducted by Van Den Boom and Sawan (1966), which comprised a regional investigation of the plateau basalts in NE-Jordan. In the vicinity of the study area six basaltic flows of Oligocene and Miocene–Holocene age are intercalated with limestone and arenite units. The lowermost sedimentary unit, a cherty limestone of Middle Eocene age which consists of limestone and marls with intercalated flint layers, is best exposed south of Tap-Line Road and north and south of Al-Safawi. This unit is overlain by a nummulitic limestone unit cropping out ESE, SE and S of Azraq. This unit is composed of white, chalky marl, locally sandbearing limestone and hard limestone with chert layers. This unit is of Upper Eocene age. The area of the basaltic plateau is in one of the desert regions of Jordan, with a hot, arid climate, and particularly small amount of rainfall. The area receives its rainfall mainly in winter months with the mean amount of monthly rainfall during winter ranging from 8.9 mm to 37.1 mm (Jordanian Meteorology Department, 1987). The temperature during summer months ranges from 26.5 °C to 45.8 °C, while in winter, it ranges from 6.4 °C to 20.1 °C, and occasionally drops below freezing. The prevailing winds are Southwesterlies (Jordanian Meteorology Department, 1987). 3. Methodology Sediments, sedimentary structures and other geological phenomena of Qa Khanna were described and measured in the field. Twenty-surface sediment samples were collected in trenches (Fig. 1). Fourteen samples were also collected in 50 cm intervals from three pits dug to a depth of 2.5 m to study vertical variations in mineralogical and sedimentological chrematistics. In order to study the distribution of the elements laterally and vertically, three geochemical profiles were constructed. The first profile represents the variation of elements from the flooding area at the southeast part of the playa, and the last is at the farthest region to the NW, along with the elongation direction of the playa. The second and third profiles were made perpendicularly to the first profile in the southeastern and northwestern parts respectively. Two dust samples were collected from the area surrounding the playa. The dust was mineralogically analyzed by XRD to compare its composition with the mineralogy of the playa sediments. 3.1. Geochemical preparation and analyses All sediment samples have been dried at 80 °C, disaggregated and quartered. The sand fraction was separated by a wet sieving method, whereas, a pipette method was used for the separation
of clay and silt fraction (Folk, 1974). Percentages of silt and clay in a representative samples were measured using computerized SEDIGRAPH for 20 surface sediment samples to investigate the lateral variations in the sediment grain sizes. Non-clay minerals of the silt-size fractions were separated and mounted on Canada balsam glass slides, and identified by using the transmitted polarizing microscope. X-ray diffraction techniques were used for the identification of clay minerals, after the preparation of oriented samples. Samples have been scanned as untreated, glycolated clay and heated to various degrees following Brown’s procedure (1961). Samples were cleaned of carbonates, iron oxides and organic materials using the hydrogen peroxide method of Jackson (1979) and Mehra and Jackson (1960). Oriented, flat-layer specimens were prepared for the X-ray study of the clays by allowing a few drops of clay suspension to dry out slowly on a glass slide (Muller, 1967; Banat, 1980). The oriented samples were examined by using a vertical X-ray diffractometer under the following conditions: Radiation = Stabilized nickle filtered Cu K Voltage = 30 kV, Current = 20 mA, Speed = 29 1 cm/min. The relative abundance of clay minerals was determined by comparing the heights of the major reflections for each clay mineral to the summed heights of the major reflections for all clay minerals (Tables 2 and 3). X-ray diffraction techniques were used to identify the non-clay minerals, as well as the evaporite minerals in the sediments of the Qa Khanna playa and in the dust samples collected from the surrounding area. Sr and Rb trace elements, as well as Ca, Mg, K, Fe and Mn major elements were analyzed for the clay fraction of the sediments of the Qa Khanna by using the Atomic Absorption Spectrophotometry. The samples were completely dissolved and treated according to Hesse (1972). Three subsamples were analyzed. The pH values
Table 1 Mean percentage of sand, silt and clay of studied playa sediments by region. Region of basin
Mean% of sand (>63 lm)
Mean% of silt (2–63 lm)
Mean% of clay (<63 lm)
Southern Central Northwestern
6.1 2.1 0.7
84.6 81.3 73.8
9.3 16.6 25.5
Table 2 The relative abundance of clay minerals in the surface samples. No.
Sample no. A.1
Palygorskite
Illite
Kaolinite
Semictite
Chlorite
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
A.1 A.2 B.5 C.7 C.8 C.9 11 14 17 20 22 D.15 E.27 F.30 G.32 G.33 G.34 H.36 I.38 I.40
xxxxx xxxxx xxxxx xxxxx xxxx xxxxx xxxx xxxx xxxx xxxxx xxxx xxxx xxxxx xxxxx xxxx xxxxx xxxx xxxxx xxxxx xxxx
xxxx xxxx xxxx xxxx xxxx xxxxx xxxx xxxx xxx xxx xxx xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxxx xxxx
xxxx xxxx xxxx xxxx xxxxx xxxx xxxxx xxxxx xxxxx xxxx xxxxx xxxxx xxxx xxxx xxxxx xxxx xxxxx xxxx xxxx xxxx
xx xx xxxx xx xx xxxx x xx xx xx xx xx x xx xx xx xx xx xx xx
xx x xx x x xx xx xx x xx xx xx x xx x xx x x x x
xxxxx = major; xxxx = abundant; xxx = common; xx = minor; x = scare; (x) = traces; samples1–9 from southern part; samples 10–14 from central and 15–20 from northwestern.
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Table 3 The relative abundance of clay minerals in the subsurface samples. The samples are arranged from top to bottom for each pit. No. 1 2 4 5 6 7 8 9 10 11 12 13 14
Sample no.
Depth (m)
Palygorskite
Illite
Kaolinite
Smectite
Chlorite
pit pit pit pit pit pit pit pit pit pit pit pit pit pit
0.5 1 1.5 2 2.5 0.5 1 1.5 2 2 0.5 1 1.5 2
xxxxx xxxx xxxx xxxx xxxx xxxxx xxxxx xxxx xxxxx xxxxx xxxxx xxxx xxxxx xxxxx
xxxx xxx xxx xxxxx xxxx xxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx xxxx
xxxx xxxxx xxxx xxxx xxxxx xxxxx xxx xxxx xxxxx xxxxx xxxxx xxxxx xxxxx xxxx
xx xx xxx xxx xx xx x x x x x xx xx x
x x x x x xx x x x x xx x x xx
1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3,
1 2 3 4 5 1 2 3 4 5 1 2 3 4
xxxxx = major; xxxx = abundant; xxx = common; xx = minor; x = scare; (x) = traces.
for the study sediment were measured by using a digital pH meter (PTI 55) for twelve representative samples. The procedure was applied as published by Cottenie (1982). The percentages of calcium carbonate in the fine sediments (clay fraction) from Qa Khanna were estimated by using the Calcium Try (Hesse, 1972) method for 20 surface and subsurface sediment samples from the playa. 4. Results and discussion 4.1. Field description of the sediments The sediments of the Qa Khanna are semi-consolidated and very compacted; even the hand augering process was unsuccessful. Therefore, pits were dug to get the subsurface samples. Many water wells were dug around and within the playa by farmers and by the Jordanian army. The groundwater has no significant effect on the sediments because of its relatively great depth. It ranges between 70 m deep in the northwestern basin and 160 m in the southeastern basin. The playa sediments are composed of finegrained sand with a major amount of mud. The distribution of sediments may be attributed to the annual flooding, which cause reworking of sediments, and produce most of sedimentary structures, especially of the southeastern parts of the playa. The walls of the three pits display different colors. The upper part is a light brown color, and it becomes reddish-brown below at 1.1 m depth. Below this, it is deep dark brown in color. Plant remains and small pipes created by the plant roots are common in the playa sediments. It was not possible to estimate the rate of sedimentation during the time of the field work, which took place during the dry summer. However, the sedimentation in the Qa Khanna should be of relatively high rate based on the observation that garbage and used cans, which were found at a depth of about 1 m, were not completely rusty. 4.2. Sedimentary structures The following sedimentary structures were found in the Qa’ Khanna playa. 4.2.1. Mudcracks Mudcracks are the most common sedimentary structure over the whole surface of the studied playa (Fig. 2). These cracks are non-orthogonal and commonly of six-sided pattern. Irregularshaped cracks also exist. The size of mudcracks ranges from 2.5 cm to more than 30 cm in diameter, with varying crack depths between 1 mm and 2.7 cm, respectively. The variation in sizes is proportional to sediment grain size (Cooke et al., 1993). Thickness
of the material and its dry density are also important (Corte and Higashi, 1964). The profile of desiccated sediment blocks displays flat, convex and concave patterns. The V–shaped cracks are occasionally filled with loose sediments reworked mostly by wind in the northwestern part of the playa, which seems to be the most part affected by the southwesterly winds prevailing in the area. However, in the southeast and central parts of the playa, mud cracks are commonly partly filled by compacted sediments carried and/or reworked by the latest episode of the inflowing water. These filled cracks display a network pattern, and may be modified by small distributaries channels created by the inflowing water. 4.2.2. Laminations In the southeastern part of the playa, which is located directly adjacent to the basaltic bedrock, laminations are entirely obscured, while they are more developed in the northwestern part of the playa (Fig. 3). The changes of lamination morphology in the southeastern sediments may be attributed to the high energy of the inflowing water coming directly from the nearby bedrock causing reworking of sediments. In the northwestern portion of the playa, the inflowing water has lost most of its energy, creating suitably calm conditions for free sedimentation from the suspension, allowing lamination to be formed. 4.2.3. Intersediment vesicles The upper 0.6 m of the playa sediment is characterized by the presence of millimeters-sized ovate to irregular cavities or vesicles that are commonly connected by thin horizontal and vertical cracks formed during desiccation (Fig. 4). Vesicles form in the sediments by trapping air in water-mud slurries during floods. Below 0.5 m, these vesicles were destroyed as a result of compaction of the sediments. 4.2.4. Bioturbation Burrows are the only bioturbation phenomena observed in the playa sediments but they are rarely preserved. They are only present in the northwestern basin, which is expected to be by far the most provisional part from the direct drainage position. These burrows are horizontal cylindrical pipes of a centimeter in diameter, none branched, and 2–3 cm in length (Fig. 5). 4.3. Textural analyses The textures of the sediment were described by using a triangular diagram (Fig. 6). As shown in this figure, silty loam is the most dominant sediment type analyzed from this playa, whereas, silty
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a
Upper part of Qa Khanna, near sample # 20
c
Lower part of Qa Khanna, near sample # 4
b
Upper part of Qa Khanna, near sample # 19
d
Lower part of Qa Khanna, near sample # 6
Fig. 2. Mudcracks which appear non-orthogonal and commonly of six-sided pattern (a, b, and c, samples # 4, 19, 20). Irregular shapes also occur (d, sample # 6).
Southeastern part of Qa Khanna
Lower part of Qa Khanna
10 cm
Fig. 3. Lamination of the studied mud sediments, it appears as obscured, and more developed in the northwestern part of the playa.
clay loam, and silt are less common. Sand is very rare. Based on the variations in the types of sediment in the Qa Khanna, it is possible to differentiate between three different regions; (Table 1): (i) southeastern part, (ii) central part and (iii) northwestern part. Grain size analysis was also used to investigate the vertical variation in sediment types of the Qa Khanna. It is clearly seen from the textural analyses that the vertical variation of grain size is not significant. Grain size analysis of the dust samples collected from the area surrounding Qa Khanna, revealed that the bulk percentage of the dust lies in the silt-size; it is 66% and 92% in the two studied samples. 4.4. Mineralogy 4.4.1. Clay minerals As shown in Tables 2 and 3 the identified clay minerals in the clay fraction of the Qa Khanna playa sediment, arranged according
Fig. 4. Morphology of the intersediment vesicles observed in the studied mud sediments, and characterized by the presence of millimeters-sized ovate to irregular cavities or vesicles.
to their decreasing abundance are palygorskite, illite, kaolinite, smectite and chlorite. Palygorskite is the most abundant clay mineral in the majority of the analyzed samples. Illite and kaolinite are generally equal in quantity in nearly all studied samples. Smectite and chlorite are recorded mainly as minor to scarce minerals except for a very few samples in the southeastern part of the study playa, where they are recorded as major minerals. Generally, all the clay minerals of Qa Khanna are homogenous in distribution. They do not exhibit a significant quantitative variation, either laterally or vertically in the examined samples. Smectite and chlorite show a slight lateral variation in their content, decreasing slightly toward the northwestern part of the Qa Khanna. This may be related to the increase in the distance from area most commonly flooded. However, both smectite and chlorite show similar vertical distributions. In the Qa Khanna, it is assumed that kaolinite is of detrital origin and it is mainly derived from the surrounding basaltic rocks. This
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chemical analyses of the clay fraction of the Qa Khanna sediments revealed the abundance of Ca, Mg and K, and (c) the clay samples were taken from a very shallow depth, and may exclude the diagenetic origin of kaolinite which is typically formed at great depths diagenetically. In the study area volcanic rocks are located around the playa basin, therefore, it is believed that smectite was formed outside of the playa basin, by the alteration of these volcanic rocks, and then transported to the playa as clay.
3 cm
Fig. 5. Morphology of the bioturbation observed in the northwestern part of the studied mud sediments appears as horizontal cylindrical pipes of a centimeter in diameter, none branched, and 2–3 cm in length.
CLAY 100%
a
Clay
Silty clay
Sandy clay Clay loam Sandy clay loam
Silty clay loam
Loam Loamy sand
Sdany loam
Silt
Silt loam
Sand
SILT 100%
SAND 00% 100%
b
Pit 3
Pit 2
0.0 m
1.0 m
1.0 m
1.0 m
2.0 m
2.0 m
DEP PTH
0.0 m
2.0 m
Silt
0.0 m
Silty loam
Pit 1
Silty clay loam
Fig. 6. Textural classification of (a), and vertical variation (b) of the Qa Khanna playsediments.
assumption is supported by: (a) the chemical alteration of steeply sloped basaltic bedrock surrounding the Qa Khanna which may lead to the formation of clay minerals including kaolinite, (b) the
4.4.2. Mineralogy of the non-clay sediment samples The identified non-clay minerals encountered in the silt fraction of the Qa Khanna sediments as well as in the collected dust samples arranged according to their relative abundance are quartz, calcite, plagioclase, evaporites (gypsum) and dolomite. Microscopic study of the Qa Khanna sediments revealed that the silt-sized sediments of the study playa are mainly composed of quartz grains and calcite. Quartz grains are highly fractured, commonly show large conchoidal fractures with scratched surfaces and sharp edges, but also commonly have rounded to subrounded outlines. Calcite appears in two forms: (a) disseminated anhedral to euhedral crystals, occasionally showing a prominent cleavage traces with a distinctive twinkling optical phenomenon, and (b) secondary growth around or enveloping other mineral grains. Plagioclase exists as a minor to scarce mineral in the studied sediment samples. It is mostly found as anhedral crystals with low alteration, mostly exhibits a well defined lamellar twinning. Evaporites, represented by gypsum only, are very scarce in the sediments of the Qa Khanna. The study of the dust samples under the microscope revealed that quartz and calcite are the major non-clay constituents. Quartz grains of the dust samples show similar optical and morphological characteristics to the quartz grains of the playa sediments, i.e., fractures, sharp edges and scratches. On the other hand, calcite derived by wind commonly occurs as anhedral, smashed crystals, with no distinct optical features. Additional accessory minerals have been identified by the microscopic study, including olivine, orthopyroxene, nepheline and some opaque minerals. Olivine is generally found as anhedral to subhedral, highly fractured and altered crystals. Euhedral crystals are rare. Orthopyroxene was recognized by its high birefringence and parallel extinction. It occurs mostly as euhedral crystals. Nepheline occurs as euhedral to anhedral crystals. It is characterized by its light grey interference color, and it commonly contains mineral inclusions (Fig. 7). The mechanism by which quartz grains are incorporated in the playa sediments is either by direct wind deposition on the playa surface or as a result of flushing of the dust grains deposited on the surrounding basaltic plateau. Quartz may have originated from the Kurnub Sandstone, which is exposed to the southeast of the Qa Khanna via the southwesterlies prevailing winds. Calcite is a common mineral in playas deposits. It can be formed and precipitated whenever there are sufficient concentrations of Ca2+ and CO2 3 ions especially under evaporative conditions (Melvin, 1991). Calcite is widely reported as an authigenic mineral in playas; for example in Amargosa desert playa, Searles Lake (playa) and in many Western Victoria playas (Khoury et al., 1982; Hay et al., 1991; Deckker and William, 1989). Authigenic calcite is recognized through microscopic features, especially the euhedral crystals and the prominent cleavage traces, suggesting in situ growth of the calcite crystals. Einsele (1992) states that calcite is the first mineral to be precipitated chemically or biochemically as a result of increasing salt concentration, regardless of the composition of inflowing water into the closed basin. The microscopic study of calcite from the Qa Khanna sediment samples, as well as dust samples reveal that calcite is characterized by both disseminated smashed anhedral crystals, as well as euhedral crystals with distinct cleavage traces. It has been also observed as overgrowth around other mineral grains. This may indicate that
F.M. Howari et al. / Journal of Asian Earth Sciences 39 (2010) 275–284
Quartz Conchoidal fracture
281
Calcite with secondary growth
Quartz
Subr-ounded Scratched surface And sharp edges
Rounded
Plagioclase anhedral Plagioclase, Laminar twining
Orthopyroxene
Calcite, euhedral Cleavage traces
Dolomite D l it rhombs h b
Olivine, Euhedral crystal
Nepheline
Fig. 7. Photomicrograph of the minerals found in the non-clay size fraction of the studied sediment.
calcite is originated from both authigenic precipitation and aeolian deposition. The quantitative scarcity of evaporites (gypsum) may be attributed to the great depth of groundwater, which exceeds 70 m in the study playa. The precipitation of gypsum is probably due to a displacive crystal growth through infiltration of the standing water in rainy season. A similar conclusion was reached by Melvin (1991) during his work on west Victoria, Australia, playas. Plagioclase, olivine, orthopyroxene, nepheline and opaque minerals are minor to scarce, and are all derived directly from the basaltic rocks surrounding the Qa Khanna by weathering. Transportation of these minerals to the studied playa could take place either during flooding water or by deflation during summer. 4.5. Geochemistry Chemical analyses of the clay fraction from the Qa Khanna sediments including both surface and subsurface samples were carried out in order to determine the concentrations of major elements (Ca, Mg, K and Fe) as well as the trace elements (Mn, Sr and Rb). The results of the chemical analyses shown in Figs. 8 and 9. These indicate there are no significant variations in the distribution of the elements laterally and vertically in the sediments of the Qa Khanna. The concentrations of calcium and magnesium in the analyzed
sediment samples display no significant variations throughout the studied sediments; similarly, the subsurface sample concentrations for both elements show almost identical distribution with the depth. The mean values for the pH in the surface and subsurface samples are 8.3 and 8.2, respectively. The measured concentrations of potassium in both surface and subsurface samples of the playa show a nearly uniform distribution in all four geochemical profiles (Fig. 9). These profiles suggest no evident variations in the playa sediments, either laterally or vertically. The concentrations of iron in the surface sediment samples vary from about 2.0% to 5.7%, with an average content of 4.9%. In the subsurface samples, iron concentrations range between 4.5% and 11.8%, with an average content of 5.9%. The distribution of manganese shows no significant variation in concentration in the four geochemical profiles. Strontium and rubidium display a similar geochemical distribution in nearly the all profiles. The two elements show a slight increase towards the center of Qa Khanna playa, meanwhile their concentrations increase slightly with depth. In the study area, pH values increase slightly from the southeastern part towards the northwestern part in both surface and subsurface samples. In order to assess the possible source of the measured elements, the results of the chemical analyses of the sediments of the Qa Khanna playa are compared with those gi-
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F.M. Howari et al. / Journal of Asian Earth Sciences 39 (2010) 275–284 60000
Ca 50000
Mg g
ppm
40000
K 30000
Fe
20000
Mn
10000
Sr Rb
0 0
4
8
12
16
20
24
Sample no Fig. 8. Concentrations (ppm) of the measured elements in the studied surface samples.
ven by Wedepohl (1978), Deer et al. (1985) and Newman (1987). As shown in Table 4, it can be generally concluded that basalt is probably the main source for most of the major elements, which were later incorporated into the different clay species present in the studied area, as well as in CaCO3 (for Ca). However, Mg may be trapped primarily by palygorskite, while Fe, K and Rb may be incorporated in illite. However, the Sr concentration in the studied sediments is comparable with its concentration in Cretaceous car-
a
bonate rocks which may indicate that carbonates are the possible source of the Sr. As shown in Table 4 considerable part of the Ca found in the studied playa sediments originated from CaCO3 that formed authigenically or from the airborne dust. Gosselin et al. (1993) reported that in closed basins precipitation of calcite and Mg-rich clay minerals may be an important source of magnesium and calcium in their sediments. However, calcium may also be derived from Carich clay such as illite, smectite and palygorskite. The relatively weak positive correlation coefficient between Ca and Fe (Table 5) may indicate that basalt is not only the possible source of Ca in the studied sediments; carbonate rocks could have contributed to Ca concentrations as well. Palygorskite, which is present as a major mineral in the Qa Khanna sediments, is probably the main source of Mg concentrations encountered in these sediments; kaolinite, illite, chlorite and smectite form only a minor source of Mg. This conclusion is in agreement with findings of Singer (1984), who has studied the mineralogical and geochemical relations in some Western Australia playas. According to Singer (1984), the Mg-rich sediments and/ or brines associated with saline and alkaline environments lead to the formation of Mg-phyllosilicate minerals, especially palygorskite associated with dolomite and calcareous materials. These materials in turn, reflect an evaporative alkaline environment, which is typical of in playas and sabkhas. These conditions provide a very favorable situation for Mg to incorporate in the authigenic paly-
b
12
12
Fe
Fe
10
Co oncentrattion, log (ppm)
C Concentra ation, log (ppm)
Ca
10
K Mg
8 Mn
6 Sr
4
Rb
2
0
Ca K Mg
8 Mn
6 Sr
4
Rb
2 0.0
2.5
5.0
7.5
40
12.5
15.0
0.0
0.5
1.0
c
d 12
Concentra tion, C t log ( p ppm)
Concentrration, log g (ppm)
Ca K Mg
8 Mn Sr
4
2.5
3.0
Fe
10
6
2.0
12
Fe
10
1.5
Depth (m)
Depth (m)
Rb
Ca K Mg
8
Mn
6
Sr
4
Rb
2
2
0 0.6
0.8
1.0
Depth (m)
1.2
1.4
0.6
0.8
1.0
1.2
1.4
1.6
Depth (m)
Fig. 9. Geochemical profiles showing the lateral and vertical variations of the measured element concentrations in the studied samples: (a) represents the variation of elements in the flooding area at the southeast part of the playa towards the farthest point (NE) from the flooding, correspondingly with the elongation direction of the study area; (c and b) represent the second and third profiles which were made perpendicularly to the first profile in the southeastern and northeastern parts respectively, and (e) represent the fourth profile which was constructed to display the variations of elements with depth.
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Table 4 Average concentration of major and trace elements (ppm) in the studied sediments, compared with those given for different geological materials and minerals by Wedepohl (1978), Deer et al. (1985) and Newman (1987). Average (ppm)
Ca
Mg
K
Fe
Mn
Sr
Rb
Surface samples Pit 1 Pit 2 Pit 3 Basalt Argillaceous deposit Oxidized muds Limestone Carbonate Clay Palygorskite Illite Kaolinite Smectite Chlorite
32,307 55,544 34,805 29,395 72,926* – – – 290,405* 31,523* 4700n 11424d 1500d 11567d 3400n
7734 29,719 12,230 18,595 38,900* – – 7900* – – 71850n 6300n 850n 20000n 11700n
11,401 14,882 9278 12,225 4400* 17,147* – 2201* – – traces, d 35,700, d 210, d 280, d traces, d
48,882 69,703 54,010 5314 90,360* – – – 3500* – 19,700 36960d 5400d 5800d 115000n
531 654 601 516 1600* – – 500w 1100* – 2800n traces, d traces, d 2530 d 5000d
115 153 111 110 48* – – n 210* – – – – – –
32 37 44 44 30 40* – 45* – – – – – – –
Table 5 Correlation coefficients between the abundance of measured major and trace elements. Elements
Ca
Mg
K
Fe
Mn
Sr
Rb
Ca Mg K Fe Mn Sr Rb
1 0.15 0.07 0.15 0.22 0.91 0.33
0.15 1 0.02 0.19 0.07 0.024 0.12
0.078 0.023 1 0.85 0.69 0.69 0.024
0.15 0.19 0.85 1 0.73 0.07 0.17
0.22 0.07 0.69 0.73 1 0.43 0.086
0.91 0.02 0.02 0.07 0.43 1 0.42
0.33 0.12 0.22 0.17 0.08 0.42 1
gorskite. Spencer et al. (1984) reported that both Mg and K can be readily removed from lake water through exchange and fixation on clays, by diffusion into pore fluids, and by incorporation into diagenetic mineral phases under alkaline and high evaporation conditions. From Table 5 it can be seen that there is highly positive correlation between K and both Fe (0.86) and Mn (0.7). This may indicate that basalt bedrock surrounding the playa basin is an important source of K in the studied playa sediments. According to Hay and Guldman (1987), K fixation by illite or by illitization of smectite (authigenic illite) needs a saline and alkaline environment. They determined that the high concentration of K in the playa-lake complex at Searles Lake, California, is one of the indicative lines of evidence for the playa stages throughout the successive changes in the lake-playa chemistry. Typically, K is readily removed from solution by exchange with/or sorption onto clays. This is ideally achieved in alkaline solutions (Gosselin et al., 1993). The clays have a great ability to adsorb Fe at edges and within the lattice of the clay minerals (Velde, 1992). Hay et al. (1991) reported that the high Fe content incorporated in authigenic illite in the Searles Lake-playa complex favors the oxidizing conditions associated with a high degree of evaporative concentration and presumably of salinity accompanied with a closed playa basin. A high pH is also an important chemical parameter that favors authigenic phyllosilicate formation. In the studied sediments, the possible sources of Fe are (i) illite clay mineral, (ii) Fe2+ bearing minerals derived from basalts, and (iii) Fe-oxides. As shown in Table 5 there is strongly positive correlation between the Fe and Mn, which may indicate that basalt is the main source of Mn in the Qa Khanna playa sediments. It is reported that Mn tends to be adsorbed on the edges of some clay minerals like palygorskite, smectite and chlorite. The adsorption process gets more effective in alkaline evaporative conditions (Weaver and Pollard, 1976). Therefore, in the studied sediments, Mn could be adsorbed on the different clay
species, especially palygorskite and smectite under the alkaline and evaporative conditions characterizing the playa environment. The distribution of Sr in rock-forming minerals is related to Ca and K. Because of the similarity in physical and chemical properties between the three elements, Ca and K can be replaced by Sr in their naturally occurring minerals (Wedepohl, 1978). The negative correlation between Sr and Fe (Table 5) may indicate that basalt has only a minor role as a source of Sr in the studied sediments. In nature, Rb does not form minerals of its own, but it is dispersed especially in K-minerals. Such behavior is caused by the high similarity between Rb and K in their crystal chemistry (Wedepohl, 1978). Rb content in the studied sediment is very comparable with its content in basalt (Table 4). Since the correlation coefficient between Rb and Fe is positive (Table 5), it may be concluded that basalt is the main source of Rb. Stoffers and Kuhn (1974) demonstrated that under high evaporative conditions, Rb substitutes for K in polyhalite in the evaporites of the Red Sea. This enhances the fact that the tendency of Rb for substitution for K increases by increasing salinity and alkalinity. Thus, it is expected that Rb in the studied playa sediments is incorporated in illite after its substitution for K under alkaline and evaporative conditions. 5. Conclusions The observed sedimentary structures in the studied playa include mudcracks of different sizes and shape patterns, laminations, intersediment vesicles and bioturbation pipes. The sediments of the Qa Khanna are compacted to semi-consolidated, and they are dominated by silty loam, whereas silty clay loam and silt are less abundant. On the basis of the variations in grain size, steeply sloped basaltic bedrocks surrounding the southeast of the studied playa, and the preservation of sedimentary structures, especially the bioturbation pipes at the northwestern parts of the playa may indicate that the main direction of flooding is from the southeast towards the northwest. Microscopic study of the silt-sized sediments revealed that quartz and calcite are the major minerals, whereas, detrital minerals of less abundance originated from the basaltic rocks, and include plagioclase, olivine, orthopyroxene, nepheline and opaque minerals. Gypsum is rare in the studied playa sediments, and it is the only evaporitic mineral identified by the X-ray diffraction. The identified clay minerals in the playa sediments arranged according to their decreasing abundance are palygorskite, illite, kaolinite, smectite and chlorite. Palygorskite and illite are of authigenic origin, whereas kaolinite, smectite and chlorite are of detrital origin. The presence of palygorskite clay mineral and the relatively high pH values may indicate an alkaline and evaporative condition under arid to semi-arid climate. Basaltic
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bedrocks surrounding the studied playa represent the main source of most of major and trace elements encountered in the studied sediments. Potassium and iron are essentially fixed by illite under an evaporative and alkaline condition. The high concentration of magnesium is explained by its adsorption and/or absorption by palygorskite enhanced by chemical conditions prevailing in the studied playa. Manganese is adsorbed on the edges of the different clay species in the studied playa sediments. The tendency of Rb to substitute for K increases by increasing salinity and alkalinity. Rb in the studied playa sediments is incorporated in illite after its substitution for K under alkaline and evaporative conditions. The strontium encountered in the clay-fraction is mainly as a result of substitution for Ca in CaCO3 derived from the air born dust. Acknowledgement The author thanks the anonymous reviewer and Dr. Boris Natalin Associate Editor of Journal of Asian Earth Sciences for their useful comments and suggestions. References Alsharhan, A.S., Kendall, C.G.St.C., 2003. Holocene coastal carbonates and evaporites of the southern Arabian Gulf and their ancient analogues. Earth-Science Reviews 61 (3–4), 191–243. Banat, K.M., 1980. Principles of Clay Mineralogy. Baghdad University Press. 138 p (in Arabic). Boggs, D.A., Boggs, G.S., Eliot, I., Knott, B., 2006. Regional patterns of salt lake morphology in the lower Yarra Yarra drainage system of Western Australia. Journal of Arid Environments 64 (1), 97–115. Brown, G., 1961. The X-ray identification and crystal structures of clay minerals. Mineral Society (Clay Min. Group), London. 544 p. Cooke, R., Warren, A., Goudie, A., 1993. Desert Geomorphology 526, 13. Corte, A.E., Higashi, A., 1964. Experimental-research on Desiccation Cracks in Soil. US Army Material Command, Cold Regions Research and Engineering Laboratories Research, Rept. 66, 72 p. Cottenie, A., 1982. Chemical Analysis of Plants and Soils. Gent University, Belgium. p. 63. Deckker, P., William, M.L., 1989. Modem, non-marine dolomite in evaporitic playas of western Victoria, Australia. Sedimentary Geology 64, 223–238. Deer, W.A., Howie, R.A., Zussman, D., 1985. An Introduction to the Rock-Fonning Minerals. Longman, Hong Kong. p. 251. Einsele, G., 1992. Sedimentary Basins. Springer-Verlag, Berlin. p. 628. Farpoor, M.H., Khademi, H., Eghbal, M.K., Krouse, H.R., 2004. Mode of gypsum deposition in southeastern Iranian soils as revealed by isotopic composition of crystallization water. Geoderma 121 (3–4), 233–242. Folk, R., 1974. Petrology of Sedimentary Rocks. Hemphill Publishing Co., Texas. p. 182. Gosselin, D.C., Sibray, S., Ayers, L., 1993. Geochemistry of K-rich Alkaline Lakes, Western Sandhills, Nebraska, USA. Geochimica et Cosmochimica Acta 58 (5), 1403–1418.
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