Evaluation of mud crack geometry in sedimentary facies of Holocene fine grain sediments (Case Study; Voshmgir dam reservoirs in Gorgan)

Intoduction
Mud cracks are primary polygonal structures that are formed by extension strains during drying period. Major factor in the formation of cracks are humidity and temperature. Quality and quantity of clay minerals, topoghraphy, sediments size, depth of cracks, water salinity and bioturbation are also important in crack,s distribution patterns. Studies have shown that cracks are lattice type in gentle slope areas and rectangular type in elevated slopes. Crack,s width and length, components size, and fractal characteristics are environmental factors in cracks distribution patterns. Fifty samples were collected from four studied regions of Voshmgir dam reservoir, north of Gorgan (S1-S4). Geometrical characteristics of cracks were determined. Samples analyized for granoulometry aspect, using hydrometry method, liquid limit identifying and plastic limit dentifying in the lab. After identification of mud cracks and processing, geometrical parameters such as drawing polygonal geometrical shapes between cracks, number of polygonal shapes between cracks, polygonal shapes area and their internal angels and angels between crossed cracks were measured and analysed statistically in four studied regions and other regions. In general, photos were taken in 45 spots in 4 studied regions from mud cracks and suitable scale were dedicated to the photos in order to determine geometrical parameters. Then, using image,s analysis software, geometrical parameters of each crack were measured and the average were computed Discussion and Textural characteristics of samples collected from four regions in Voshmgir dam reservoirs have shown that sediments are classified in clay and silty clay zones in triangular textural diagram and CL clay in unified classification of soils and were formed in diagenetic phase. Clay mineralogy of sediments include 10% cholorite, 40% illite, 30 % vermiculite and 20% kaolinite. Clay minerals of samples were originated in east of Golestan loesses.
From geometric point of view, cracks had an average of 6.7 sides, 10 cm length of each sides and 307 cm2 area. Mean internal angle was 125 ° and total internal angles 836 °. From superficial aspect, there is a good correlation between studied mud cracks polygones and an ordered polygon. Heterogeneity in angles relations was due to curve forming in sides crossed areas after forming cracks which were related to crack age so the older the cracks become, the angel between sides is more similar to a curve.
Comparing mean geometric parameters in four studied regions have shown that first generation cracks are different from each other in geometric aspects. Due to F.distribution, P.Value and means diagram, cracks parameters and polygons in S1, S2 and S3 areas were the same but S4 characteristic parameters especially total internal angle and each sides length have meaningful difference to the other regions due to low plastic limit and high depth of sediments in S4 area.
Studies of angels between mud cracks have shown that there are two patterns of mud cracks. Some of them are T- shaped in which the crossed angle is approximately 90 °. The other group, by creating hexagonal shapes, are Y- shaped and the angle between them are approximately 120 °. Studied samples are more similar to Y- shaped pattern with a mean angle of 114 °. Keywords: Mud crack; Fine grain facies; Geometric; Voshmgir Dam reservoir; Golestan. References
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