Investigation Of Evaporate Deposits On Gavkhoni Playa Lake

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.