Gypsum karstification in the Middle Miocene Fatha Formation, Mosul area, northern Iraq

Karstified Middle Miocene sediments are widely exposed in northern Iraq particularly in the area surrounding the city of Mosul. The unit is dominated by gypsum and exposed in thirteen anticlinal structures within the investigated area of about 1600 square kilometers. Synclines, though containing the same sequence, are not karstified due to a Quaternary cover. Karst features were located from air photos: Over 4000 were recorded, the smallest detectable being two meters in diameter. The majority are sinkholes (dolines), developed in gypsum and manifested in the overlying collapsing limestone. They are singular, in lines or clusters. Shafts and karren are fewer in number and are usually developed in uncovered gypsum. Sinkholes are visibly located along fractures and at fracture intersections over gently inclined limestone beds overlying the gypsum. Two karst systems were identified, an active and recent system characteristic of all the anticlinal structures and an older (Pleistocene) fossil karst system characteristic of Alan, Ishkaft, Albu Saif and Hammam structures. The fossil karst system is preserved on remnant elevated old land surfaces and produces characteristic tight undulations in the limestone due to collapse inwards in sinkholes and elongated tunnels formed along a series of sinkholes. The fracture study of anticlinal structures reveals that the mean fracture density per area ranges between 4 and 8 (km/km2) and shows a unimodal character for most of the structures. However the distribution of karst in relation to fractures is bimodal for at least half of the structures with mean values ranging from 4.5 to 11 (km/km2). The fractures in the anticlines are thought to have formed due to folding but some are associated with major lineaments cross cutting the structures, which is reflected in the bimodality and the crude unimodal fracture/karst distribution. Karst features are related to the general fracture pattern but are more localized in densely fractured areas. Karst areas were also found to correlate with lower slope gradient and lower drainage density.