Investigation Of Evaporate
Deposits On Gavkhoni Playa Lake R.Ajalloeian (B.Sc, M.Sc, Ph.D) Department of Geology, Isfahan University, Isfahan-Iran H.R.Pakzad (B.Sc, M.Sc) Department of Geology, Isfahan University, Isfahan-Iran H.Safaei (B.Sc, M.Sc, Ph.D) Department of Geology, Isfahan University, Isfahan-Iran Keywords: Aeolian, Evaporate, Facies, Playa, Salina mud, Salt pan, Abstract: Gavkhoni playa lake as a depression is located at the south-east of Isfahan in central of Iran. This area is between longitudes of E52°,43' - 52°,53' and latitudes of N32° - 32°,21'.The lake was formed as a result of block faulting (graben) since tertiary period. It has been surrounded by volcanic rocks at the north-east part and sedimentary rocks at the south-west part. Also there is an extensive sand dune at the western part of the playa. Ghavkhoni playa is a permanent lake with a closed drainage basin which mainly feded by a permanent river and two ephemeral rivers. Climatologically, this area is dry and its annual precipitation and evaporation are about 80 mm and 3000 mm respectively. Based on satelite image processing (TM) and field works, this basin can be divided into several subenvironments such as sand flat, mud flat, salina mud flat and salt pan. The most important issue in this area is salt pan which is considerable area and covered more than 70% of the playa lake. According to surface and subsurface investigations, salt pan consist of medium to fine grain terrigenous sediments and evaporates. The thickness of the salt layer ranges between few centimeters to the north and about 1.5m to the south. This paper mostly emphesis on evaporate deposits through the field works and remote sensing studies. 1. Introduction Gavkhoni Playa Lake is located to the southeast of Isfahan in the central of Iran. The surface of the playa is about 1470 meters elevation. The Gavkhoni playa lake similar to many other saline lake basins (e.g.. Death valley, California U.S.A. and Howz-e-Soltan) was formed as a graben as the result of block faulting (Krinsley, 1970). Extensive alluvial fans have covered the northeast, south and east of the area. These alluvial fans, which extend from mountains to the margin of the Playa Lake, derive mainly from alteration and erosion of igneous rocks. The Varzaneh aeolian sand field marks the western boundary of this Playa. The margin of the lake to the north is wetter than other area and is covered with bushes. The Gavkhoni Playa Lake is typical of many permanent lacustrin basins within a closed drainage basin in the central of Iran. Water is delivered to the basin via a permanent river, two ephemeral rivers, numerous diffuse small streams, and direct precipitation on the lake's surface and ground water discharge. The lake receives most of water from northern river (Zayandehrud River). The two temporary rivers enter into the south and the west of the lake and sometimes flood and inundate these parts of the lake. In normal years the ground water contribution and evaporation are the largest components of the hydrologic budget. In this reason the sediment in this playa may be a sensitive indicator of any changes in the hydrological budget. Evaporation exceeds precipitation and inflow. Extensive evaporation begins in May and continuous till September. It is a hypersaline lake and dominated by sodium and chloride ions but shows wide variations in composition and concentration on a spatial basis. Hydrological, cli matological and geological conditions combined for formation of this saline lake. In this paper it will be present the types of environment and facies including alluvial fan, sand flat, mud flat, saline mud flat and salt pan and also brine evolution in the Gavkhoni saline Playa Lake. In order to recognize the various units in playa lake, the landsat TM data was used (path 163 and row 38). Through the optimum index factor, three banding combination formatting false color composite was selected. For better understanding of specific phenomena, almost the interactive processing was used. Different processing include contrast stretching, filtering and formula was considered. Based on field studies and image processing several units has been recognized. Also for investigation of subsurface sediments and chemical analysis 5 trenches and 10 pits with up to 2 m deep were dug. 2. Environments And Facies Several major environmental zones can be easily recognized in the basin. They can be defined on the basis of texture, mineralogy, and the ratio of evaporite to clastic. Although the facies are distinct and can be mapped but boundaries are usually graditional. They include alluvial fan, sand dune, interdune, sand flat, saline sand flat, sand beach, mud flat, saline mud flat and salt pan. 2.1 Alluvial fan facies This facies occurs at the base of the steep slopes of the basin. It grades laterally into the wide sand flat to the west, and mud flat, sand beach and narrow sand flat to the north and east, and narrow sand flat, and mud flat facies to the south of the Gavkhoni playa lake. It consists of a mixture of coarse and fine clastic material that has been derived from the adjacent mountains. They were transported down slope by channalized flow in the upper fan and dominantly by sheet flow on the lower fan. Debris flows also may account for significant downslope movement of sediments. Gypsum crystals are commonly found as cement in the alluvial fans, especially in the southwestern alluvial fan. They are concentrated in surface layer and present mostly in the form of fibrous and twinned crystals. Calcite is another chemical mineral, which is present as fine -grained cement in this facies. 2.2 Sand dune facies The sand dunes enclose the Playa Lake to west. They grade laterally into the sand flat and salina sand flat to the east and alluvial fan to east. Mineralogical composition of the sand dune grains comprise sedimentary (mainly carbonate), igneous and metamorphic lithics and quartz grains, respectively. Feldspar, heavy mineral grains and shells are found as subordinate. Halite and gypsum are present as trace. 2.3 Interdune facie Some small and large interdunes are located between the sand dunes. They are low relief areas, where vegetation and sand dunes are absent. Its surface layer is a very thin sand layer weakly cemented by fine salt crystals usually covers the interdunes. It is usually dry, soft, polygonal. The detrital sand-sized grains of unit (1) are as a result of wind activity in dry conditions. Sand- sized grains could be transported to this basin by wind from the sand dunes. They are deposited over the interdunes from the sand dunes. The presence of gypsum scattered between sediments is a result of reworking, derived from neighborhood saline mud flat.The dispersal halite crystals is probably the result of evaporation on brine pore water in a dry period (Lowenstein & Haride, 1985). 3. Sand Flat Facies This facies extends as a wide area to the west of the Gavkhoni Playa Lake and as narrow bands to the east and south of the lake. It will be explained only about the western sand flat mostly regarding to evaporitic sediments in this paper. The western sand flat is covered with gypsiferous marls. A porous carbonate debris blanket and tuffa covers sediments in some locations. A Soft, porous, puffy surface encrusted with a flaky, thin efflorescence of salt comprises most of the sand flat surface. The two types of gypsum form are known in this facies; rosette, and twin. Rosette gypsum is usually present as wavy layers, interbedded with aeolian sand layers along the western side of the aeolian sand dunes in the sand flat. They are soft and friable, not more than 2 cm thick. Twinned gypsum crystals are found as dispersal in the marl sediments deposited over the sand flat in some places. They have destroyed laminations of the sediments. The sand flat facies grades laterally to saline sand flat and salt pan in the western part of the lake and to mud flat in the other sides of the lakes. 3.1 Sand beach facies stinct and continuos beach ridges occur at the edge of the salt pan to the east and between the saline mud flat to the north of the Playa Lake as a long narrow zone. The grain size of the sand beach mostly ranges from coarse sand to fine sand. In order to study of sedimentary cycle two pits dug in the marginal sand beach up to 1 m. According to the two pit two facies was observed , gravelly sand and sand facies. 4. Mud Flat & Saline Mud Flat This zone includes the mud flat and saline mud flat. This facies encircle the lake, except in the western part of the lake. The northern clay flat is wide, where the Zayandehrud river reaches into the playa. In the northern mud flat, some meandering channels drain waters and transport sediment to the playa and form Zayandehrud delta. Mud flat and saline mud flats are chiefly composed of very fine sediments (silt and clay), gypsum and halite crystals. During high water levels in the lake, the mud flats are sites of clastic sedimentation. Ground water table measured during about 3 years in this zone indicates that fluctuations are not considerable and it is approximately fixed. The mud flat is no saturated by brine and is commonly desiccated. It is characterized by polygonal mud cracks. Polygonal desiccation is fairly common and cracked up to a few cm across. 5. Salt Flat (Salt Pan) The property of the sediment deposited in the salt pan varies laterally from dominantly clastic facies to dominantly evaporite facies. The salt pan covers the center of the Gavkhoni Playa Lake as an efflorescent crust. It occupies more than % 75 of the playa surface and is the most characteristic feature of this playa lake. It takes place the lowest area of the Gavkhoni closed basin. The flat, salt-encrusted pan is surrounded by a saline-soaked mudflat and sand flat in the east and west permeated with evaporite minerals that grew within the sediments. The saline mud flat in turn grades outward into a dry mudflat and sand flat. The common features of this zone are polygonal halite crusts, efflorescent halite ridges and popcorns (cauliflower). Brine -saturated sediment underlies the surface. Brine level slightly fluctuates during dry and wet season and it does not fall more than 20 cm below the salt surface. This surface is normally moist. Below the surface the voids in salt and sediment layers are filled with halite/saturated brines. Principal salt found in the salt pan is halite but carnalite, tachyhidrate, and calcium chloride hexahydrate are also present as minor. Halite is in hopper cubic and massive form. The size of halite crystal reaches is up to 20mm in diameter. The salt pan is a result of flooding, evaporation and desiccation. After flooding when the shallow ephemeral lake becomes concentrated by evaporation, the formation stage of salt pan starts. In NaCl rich system of this lake continuous evaporation concentrates the brines until they get saturation with halite. Crystallization starts at the brine surface as small plates and hopper crystals, which sink to the bottom. The individual floating crystals are cemented together where they touch to form rafts. When surface tension is disturbed, the crystals fall to the bottom, forming an accumulation of individual halite crystals and broken rafts on the brine pool floor. These serve as nuclei for further growth and widespread syntaxial overgrowth that takes place on the lake floor, ultimately resulting in the development salt crystals (Lowenstein & Haride, 1985). When the salt surface was exposed, the halite layers were buckled, broke into polygonal crust, and teepe structures are formed. The buckling was caused by a net volume increase due to thermal expansion. The continued growth of halite immediately beneath the dry surface of the pan causes lateral expansive growth of the surface crust, and leads to disruption of the crust into large polygons rimmed by pressure ridges that override each other like tectonic thrust. Preferential evaporites pumping of subsurface brine take place along the cracks between the polygons and leads to precipitation of a spongy efflorescent halite (Lugli, Schreiber and Triberti, 1999). Next inflowing dilute floodwaters originated from meteoric waters partially dissolve the old surface saline crust before reaching supersaturation. With evaporation halite precipitates over the salt pan. In most cases, rapid evaporation does not allow halite to form as a cubic crystal at the surface of salt crust, although, in other environments hopper- shapped crystals generally record rapid growth rate (Fayazi, 1991). The subsurface sediments and stratigraphy of the salt pan are known from few pits drilled holes. Deep cores drilled near the south of the playa by Geology Survey of Iran the reveal presence of a cyclic stragraphic record of non-evaporites facies. Beds exposed in cores and pits range from a few centimeters to over 12 m thick. The sedimentology facies found in the saline pans through cores and pits consist of layers of crystalline salt and detrital siliciclastic (mud and sand). They range from salt, black mud, loose sand., sandy mud and brown clay facies. In this paper it is described only the first three facies in detail, because there is no sufficient data about other facies. Salt (unit 1): The thickness of salt crust ranges from a few centimeters to the north and up to150 centimeters to the south. They are usually clear and white in color, but black, pink and green colors are also is marked due to impurities. The surface layer comprises some sand- sized grains. The volume percentage of these sediments is in different places. There are thin layers of dark mud interbedded with crystalline halite layers. The largest number of mud layers separating crystalline halite is observed on the southern margin of the salt pan in close proximity to one of detrital sediment sources. In contrast, the layer of crystalline halite without intermittent mud layer is observed in the central portion of the salt pan where only major floods succeed to reach mud sediments. Black mud (Unit 2): The black mud layer underlies the salt layer .The thickness of this unit reaches to 10 cm in the lake center and. It consists of gray to black clay and silt. The ratio of clay to silt is high (about 80% clay-sized). Clay minerals include illite, chlorite, kaolinite and smectite. The organic content is approximately high in this layer. Halite as hopper cubic and gypsum as prismatic form are scattered through the gray mud. The size of halite and gypsum crystals is up to 20 mm in diameter. A large number of gastropode and ostracode shells are present in the sediments of this unit. Sand (Unit 3): This unit is the lowermost layer in one location. It is also exposed in one drilled core below the salt layer and repeated between brown clay layers. It is mostly composed of sand grains. They are well sorted and mostly range between fine sand to medium sand. Mineralogical composition of detrital sediments is composed of mostly sedimentary, igneous lithics and quartz grains. Feldspars and heavy minerals are as subordinate. It consists of gastropoda and ostracoda shells similar to unit 2. Gpsiferous marl (Unit 4): This unit directly overlies unit 5 in one drilled core. The thickness of this facies is about 2m. It is greenish to dark in color. Brown mud (Unit 5): This facies is recognizable in one drilled core. It includes two parts, upper and lower part. The upper one underlies immediately gypsiferous marl facies. The lower one is the lowermost layer in this core. This unit ranges from 4m to more than 12m thick. During spring, minor floodwaters originated from meteoric waters (rainstorm runoff and snow meltwater), cover the saline pan and form a temporary shallow brackish lake. The depth of this lake is usually no more than a few tens of centimeters. Minor flooding is much more common occurrence on saline pans than major flooding, and can create a temporarily undersaturated saline lake without deposition of a detrital mud layer. Repetition of such flooding can result halite crust without detrital mud parting. Also during a major storm flooding stage when muddy floodwaters inundated the pan ,mud layers presumably were deposited in the shallow ephemeral lake. This results to form alternating layers of halite and mud. This sequence record the deposition of halite in a salt pan in the playa center followed by retrogradation of mud flats over the halite beds. The presence of hopper halite within the mud matrix is a result of fluctuation of brine during the wet and dry period, similar to fluctuations of brine level in Bristol dry lake (Fayazi, 1991). The pink to red color of the halite is due to impregnated by iron oxides between the salt. The black to greenish color of halite especially in the surface layer results impurities of fine -grained detrital sediments.Clay - sized sediments were transported into the playa lake in two ways; 1-Clay in low salinity water is suspended by density stratification and can be transported over wide areas before it flocculates and slowly descended to the bottom (Novorka, 1982). Floodwaters move only clay-sized material out from the shoreline; silt is deposited in deltas near the shore. 2-Aeolian dust storms are an alternative mechanism for transporting silt -and clay sized material for long distance. Fine to medium sand sediments mixtured with salt crystals is derived from aeolian sands in the west of the area. Sand - sized sediments capped by mud most likely reflects a rapid fall aeolian sands in a shallow temporally lake. The change in turn from sand, black mud upward into the salt unit marks the transition from a sand flat, mud- dominated playa to a salt pan. The lack of plant roots reflects that the Playa Lake was poorly vegetated. This feature is indicative of a semi -arid to arid climain the region (Amini, 1997). 6. Conclusions To interpret the history and evolution of the lake, it may be insufficient to present a model only with these data. However, with investigation of a 4o m core and up to 2m some trenches and pits of the sediments it can be described sedimentation episodes to some extent in this basin. The material filling the basin is the result of a complex interplay of varying evaporation/ precipitation ratio, quantity and chemistry of groundwater inflow and surface runoff, and drainage basin characteristics.The stratigraphy sequences indicate, in a general way, considerably differing depositional and hydrological conditions and fluctuating water chemistry. The deposition of intermittent of sand and clay (sand flat and mud flat) suggest that the basin was influenced repeatedly by flooding and desiccation. As a result, during the floods it has been a permanent lake without change in water table and fine- grained sediments deposited in the basin for a long time. In contrast the sand layers most likely derived from aeolian sands during dessication periods for a short time. With increasing aridity, the lake gradually became shallower and more restricted and the only salt pan formed. The salt pan was formed during the latest retrogadation in this playa lake. The absence of a salt unit except the surface layer implies that hypersaline playa conditions probably have been existed only in the latest sedimentation period in this basin. Based on surface and subsurface field data a depositional model for the Gavkhoni Playa Lake has been suggested. 7. Refferences Amini, A. (1997). Provenance and depositional environment of the upper red formation, Central zone, Iran. Ph.D. Thesis, University of Manchester. Fayazi, F. (1991). Evaporates of the Howze Soltan lake basin. Ph.D. Thesis, University of East Anglia. Krinsley, D.B. (1970). A geomorphological and paleoclimatological study of the playa of Iran. U.S. Government printing office Washington D.C. 20, 402. Lowenstein, T.K. and Haride, L.A. (1985). Criteria for the recongnition of salt-pan evaporates. Sedimentology, 32, 627-644. Lugli, S.B, Schereiber, C. and Triberti, B. (1999). Giant polygonal in the realmonte mine (Agrigento, sicily): Evidence for the desiccation of a messinian halite basin. J.S.R Vol 69, NO.3, 764-771. Novorka, S. (1982). Depositional environments of marine- dominated bedded halite Permian San Andres formation, Texas, Sedimentology, 34, 1029-1054. |