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.