The Formation Of The Karachi Trough
Sindh is situated in the lower part of the Indus Basin which is bounded by higher folded belt on the west on the east of this fold belt the area is covered by alluvium deposits. Major diastrophism occurred associated with the Himalayan Orogeny in Pro-Pleistocene time which is rise to the tectonic pulses generating compressional forces due to which the present ranges are formed. The rocks exposed are almost sedimentary and ranges are age from Cretaceous to Recent (Farshori, 1972).
During the tectonic disturbances in Lower Indus Basin, there was a development of marginal Sag basin, also the Karachi area was moving eastward away from Baluchistan during Oligocene and Miocene causing the Karachi trough to open. The area under study lies on the south-western corner of the Karachi. Karachi trough is an embayment opening up into the Arabian Sea. The Trough is characterized by thick Early Cretaceous sediments and also marks the last stages of marine sedimentation. It contains a large number of narrow chains like anticlines. The most interesting feature of Karachi Trough is the reportedly continued deposition across the Cretaceous/Tertiary (K/T) boundary where in Korata Shales were deposited (Kadri, 1995).
According to Bender and Raza (1995) the geological structural setup of Karachi region belongs to the Southern Indian Basin which is a result of the rifting during Triassic and oblique collision of the Indo-Pakistan plate with the Afghan blocks during the Late-Cretaceous, and by post collision deformation during Neogene and Quaternary periods. Tectonically, the counter clockwise movement of Indian plate during Eocene time, after its collision with Eurasian plate, has resulted in the formation of Karachi Embayment. It has remained as a trough in the recent geological time. It was followed by structural deformation in the late Pleistocene to middle Pleistocene related to the Himalayan Orogeny (Bender and Raza, 1995).
Karachi embayment is an embayment opening up into the Arabian Sea. The Trough is characterized by thick Early Cretaceous sediments and also marks the last stages of marine sedimentation. This zone is the southward continuation of the Kirthar fold belt. It is largely covered by Miocene to Recent sediments which form well defined broad SS W trending folds. The southern part of the embayment is still receiving sediments from different sources. It is comprised of a series of parallel to sub parallel short, narrow, accurate (convex to east) enechelon ridges and wide, dome-shaped anticlinal hills.
It contains large number of narrow chain like anticlines (Kadri, 1995). The Malir River and Lyari River is the main stream draining this region. Structurally, the city of Karachi lies approximately 150 km east of the triple junction between the Arabian, Indian, and Asian plates (Apel et al. 2006). A thrust-and-fold belt extending northward parallel to the transform fault separating India from Asia, and the Kachchh fault system trending westward toward the city. Karachi is a part of a major synclinorium stretching from Ranpathani River in the east to Mehar and Mol Jabals in the north and sea coast in the south (Hamid et al., 2013).
A number of geological structures as anticlines are present in the synclinorium which are Pipri, Gulistan-e- Johar,Pir Mangho and Cape Monze. The anticlinal structures have been complicated by several faults (Hasnain et al., 1987). Malir River (MR), Liary River (LR) and Korangi Conglomerate (KG) on the down side of the probable Malir River fault (MR). This is perhaps part of a fault zone (thick wavy red line) that forms this tectonic boundary which runs west to east from the SW tip of this map to Thatta and beyond. Right lateral movement and drag along this tectonic boundary - caused by eastward creep of Karachi Arc (red arrow) (Sarwar et al., 2013).
