Hypogene Processes of the Gypsum Beds in Sangaw Sinkholes, Kurdistan Region, NE-Iraq

Abstract

The Sangaw region is located at the western part of Zagros orogenic belt at the boundary between Low and High Folded Zones, Sulaimani governorate in Kurdistan region. The area characterized by low amplitude folds that are trending northwest southeast and arranged in en echelon pattern. The exposed formations are Eocene Pila Spi (limestone), middle Miocene Fat`ha(lagoon) and Upper Fars (clastics) formations. Many large and small sinkholes are found around Ashdagh anticline; some of them about 50 m in diameter and about 30 m deep. Some are developed into complicated cave systems with collaps blocks and breccias in addition to narrow passages and fissures. The largest of them is located directly to the west of Darzilla village at the southeastern plunge of Ashdagh anticline. The sinkholes occur in Fat`ha and in the Pila Spi Formations. The walls of the sinkholes are covered by secondary gypsum, sulfur, bitumen and secondary calcite. Inside the cave collapse, breccias and blocks with lensoidal stratified clayey sediments as weathering product could be seen. The water is acidic (pH=4) inside the caves and discharges as large spring (200L/S) with white milky color; it is called in the local Kurdish language, “Awa Spi “which means white stream. The weathering of the carbonate rocks is intense inside the cave and appears as honeycombs and rills mark which have very rough surface with dull color. The sinkholes were produced from the dissolution of thick gypsum and limestone beds. The origin of these caves has been proposed to be hypogenic speleogenesis due to the presence of gypsum and bitumen. These materials with the aid of bacteria enrich the water with H2S which aciditfies the water and precipitates the sulfur and secondary gypsum on the cave wall. The formation of H2SO4 by oxidaton of H2S is the main reason that aid the sinkhole hypogene generation in Sangaw area. A realistic model is drawn to interpret and connect the following:1- The stratigraphy and structure of the area encourage the generation of underground stagnant pond suitable for reacting with the emanating H2S necessary for the hypogene generation of the sinkholes and precipitation of secondary native sulfur and gypsum.2- dissolution of gypsum and its reduction by bacteria. 3- upward migration of bitumen from nearby oil traps(hydrocarbon accumulation).