ژنز میکرولندفرم‌های پهنه‌های گلی و نمکی دریاچه ارومیه

دریاچه ارومیه در شمال باختر ایران به عنوان یکی از بزرگترین دریاچه های فوق اشباع نمک جهان، به دلیل عوامل انسانی و طبیعی در معرض کاهش شدید تراز آب و خشک شدن قرار دارد. کاهش شدید تراز آب در دو دهه اخیر و رخنمون یافتن بستر دریاچه به صورت پهنه های گلی، گلینمکی و نمکی در بخش های بزرگی از دریاچه منجر به تغییر محیط رسوبی از یک محیط دریاچه ای دائمی به یک محیط رسوبی پلایایی تحت تاثیر تغییرات فصلی شده است. ظهور و گسترش میکرولندفرم های محیط های پلایایی در بستر خشک شده دریاچه موید تغییر در محیط رسوبی دریاچه و روند رسوبگذاری است. مهم ترین میکرولندفرم های پهنه های گلی دریاچه شامل ترک های گلی، زمین های پف کرده با ساخت تی پی و دیاپیرهای گلی می باشند. میکرولندفرم ها در پهنه های نمکی گسترش و تنوع بیشتری دارند و شامل ساخت های انحلالی نمک در مقیاس میکروسکوپی و ماکروسکوپی، رشد بلورهای هالیت و سیلویت در اشکال و اندازه های مختلف، هالیت های شعاعی، ریپل های نامتقارن نمکی، پلی گون های نمکی، قارچ های نمکی، پشته های نمکی، سواحل و سدهای نمکی، ساخت های نعل اسبی نمکی، حفره ها و غارهای نمکی، پهنه های گسیختگی نمک و فرش های جلبکی می باشند. رخنمون یافتن پهنه های نمکی در بخش های بزرگی از بستر دریاچه و نیز برداشت نمک برای استفاده های صنعتی می تواند منجر به حمل ذرات منفصل نمک به مناطق مسکونی و کشاورزی در زمان طوفان های بادی و گرد و غبار گشته و عوارض زیست محیطی برای ساکنین اطراف دریاچه به همراه داشته باشد.

Thegenesis of micro-landforms of muddy and salty plains of Urmia Lake

IntroductionUrmia Lake is one of the wordscale large hypersaline lakes with about 5500 square kilometers in the TurkishIranian Plateau (NW Iran). The lake surrounded by mountains with a mean elevation of about 2000 meters and the highest peak of 4811 meters. The lake charge with 28 permanent and ephemeral rivers. As a result of anthropogenic and natural reasons, the lake exposed to the rapid water level fall and fades. The Urmia Lake experienced rapid water level fall with over 6 meters in the last two decades, which lead to the exposure of the large part of the lake. Consequently, this event changed the permanent hypersaline lake sedimentary environment to the sessional changes depended temporary (playa) sedimentary environment. In the last 20 years, the lake floor exposed in the form of Sandy beach, muddy, muddysalty and salty plains. The appearance of the microlandforms of playa environments in the exposed lake floor is in concordance with changes in the sedimentary environment.Materials and methodsThis study was done based on several field studies on Urmia Lake during 2010, 2011, 2014, 2015, 2018 and 2019. Filed works are done in the different parts of the lake during different seasons, different lake water levels, and different weather conditions. The effect of different weather conditions including temperature, evaporation, wind speed and direction, and amount of freshwater discharge by rivers on dissolution and deposition of the evaporate minerals and forming of the different microlandforms on the lake floor were considered.Results and discussionThe sandy beaches mostly exposed in the lake coasts around the ldquo;Zanbil and Eslami rdquo; volcanic complexes. The main microlandform on the sandy beaches of the lakes is asymmetric ripple marks. In the sandy beaches, the sand grains are mostly coated grains with clastic minerals dominantly pyroxene and volcanic glasses in the core and chemicalbiochemical aragonite and halite in the rim. The muddy plains are mostly exposed between the sandy beaches and the salt plains. The mud plains in the rivers deltas have seen in the form of the swamp with everglade vegetation. The main microlandforms on the muddy plains are mud cracks, teepee structures, and mud diapers. Mud cracks with polygonal shapes are ranging between a few centimeters up to a meter in size. The tee/pee and mud diapers are common structures in the muddysalty plains and mostly forms due to the hypersaline water capillary. Compare to the sandy and muddy plains, the microlandforms on the salty plains are more variable in type and shape. The most common microlandforms in the salty plains are micro and macro scale salt dissolution structures, deposition of evaporate minerals such as sylvite and halite minerals in different size, shape and crystallography, radial halite cylinders, asymmetric salt ripple marks, salt polygons, salt mushroom, salt mounds, salty beach and barrier, small scale salty pores and caves, salt rapture, organic matters, and algal mats. The landforms of salt plains generally classify into dissolution and depositional structures. These landforms are the most sensitive structures of the Urmia Lake and rapidly reorganized with slight changes in the lake water condition. The salt dissolution structures form due to salt dissolution by fresh meteoric and river waters. These structures show channel shape in the contact place of salt plain and freshwater river. Irregular polygons and groove shapes form due to meteoric water dissolution. The evaporate minerals mostly deposit in cubic and cauliflower shapes in the Urmia Lake floor and hopper and sheet shapes on the water surface which is later falling down on the lake floor. Sylvite minerals are bigger than halite minerals and forms slowly in the dissolution holes where the lake brine has been trapped. Both halite and sylvite are the common evaporate minerals in the Urmia Lake. The radial halite is forming inside the cylindrical dissolution holes in the salt plain where the halite minerals growing from the rim to the core of the holes and forming the radial halite structure. Salt mounds build up in meter to several meters in size with an oval and rounded shape and mostly they grow in dominant wind direction. The high of the mounds from the lake floor is ranging from few centimeters to a few meters. Salty beaches normally are in elongated and Horseshoe structure shape and build up between saltymuddy plain and salty plain where the waves are broken down. The smallscale salt caves with Stalactite, stalagmite and salt column are mostly located in the cliffy costs of lake islands. Ruptured salt zone on the western coast of the northern part of the lake is a special phenomenon that probably formed due to salt viscosity or even activity of fault in the lake. Algal mat and microbial organic materials are two different types of autogenic organic matter in the lake. In addition, some organic materials such as different types of insects carried into the lake by the wind from the agriculture lands around the lake. Shahid Kalantary High way as a manmade dike separated lake into the northern and the southern parts. Due to the huge weight of the used materials in the highway, the lake green color muddymarly unconsolidated viscous sediment scapes out in both northern and southern margins of the highway and made the synusicwave shape. In addition, some water saltwater spring formed along the high way.ConclusionRapid water level fall of Urmia Lake leads to exposure of a large area of the lake floor as sandy beaches, muddy, muddysalty and salty plains with a variety of dissolution and depositional microlandforms in different shape and size. In the Urmia Lake, the microlandforms are unstable structures and reorganizing under seasonal variation of physicochemical conditions of the lake.