Investigation Of Evaporate Deposits On Gavkhoni Playa Lake

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.