Study of Crustal variations in the part of Central Indus Basin with the help of integrated geophysical data

Abstract

The Research Area comprises of Punjab Platform, Sulaiman Foredeep and Eastern Part of Sulaiman Fold Belt (from east to west). Seismic lines are provided by the Directorate General of Petroleum Concessions. The stratigraphic correlation of approximately 3500 km seismic data is done with the help of the well data, formation tops, Synthetics and general stratigraphy of the area. The Time and Depth sections show the subsurface crustal variations. The Structural and Stratigraphical interpretation is done in detail. The 3D subsurface Time and Depth models show the structural highs and trough areas. A research area is divided into four Zones namely, A, B , C & D on the basis of subsurface structural features. Zone A is an eastern most part of the Research area in Punjab Platform where Paleozoic sediments (Permian & Cambrian) are present. Mesozoic sediments (Cretaceous, Jurassic & Triassic) and younger sediments (Paleocene & Eocene) are missing in this zone showing the uplift of Sargodha High during Paleozoic time. An uneven distribution of Paleozoic rocks shows some thickness in east. They show thinning and almost truncation in Sulaiman Foredeep which indicate their depocenter lies in east. This also represents the time of uplift of Sargodha High that shifted depocenter westward. Seismic data shows a thick Mesozoic deposition in Sulaiman Foredeep. They give coastal onlaps on the Permo- Triassic unconformity towards Sargodha High suggesting this as an area of non- deposition. The gradual uplift of Sargodha high continues, tilting the Mesozoic strata. Towards Pezu Uplift the time of major uplift is related to a collision of Indian Plate with Eurasian Plate followed by the period of intense erosion. Below unconformity sediments show an effect of folding and an angular termination against unconformity, suggesting it as an angular unconformity in this region. Paleocene, Cretaceous, Jurassic, Triassic, Permian, Cambrian and Pre-Cambrian sediments are uplifted and eroded. Afterwards rapid and thick Eocene and post- Eocene sediments deposition took place in Sulaiman Foredeep. Zone B is an area of thick deposition of Jurassic, Triassic, Cretaceous, Paleocene & Eocene sediments, showing the westward shift of depocenter after Paleozoic age. Paleozoic sediments are thinning in this zone. Mesozoic sediments are thinning towards Punjab Platform. Zone C shows the westward thinning and almost truncation of Cambrian & Permian sediments. An ancient shelf margin of Paleocene age, is marked on various seismic sections of this zone. An eastward truncation of the Vehowa and Chitarwata Formation in terms of 3stratigraphic pinchout shows the maximum extend of Early Miocene and Oligocene strata respectively in Sulaiman Foredeep. Zone D is an eastern part of Sulaiman Fold Belt. It comprises an area of Domanda Fault and Sulaiman Basement Fault. Zone D is further divided into Northern, Central & Southern parts. Northern part comprises of a trough area of Drazinda Synclinal structure which lies between the West Sulaiman Transform Fault and Domanda Fault. The anticlinal structures of Domanda, Gulan & Savi Ragha also lie in this part. Central part consists of a Safed koh trend. Southern part consists of low relief Sakhi Sarwar, Drigri & Kotrum anticlinal structures. Flat- Ramp geometry of the Basement Fault is observed, which separates the Sulaiman Block from Punjab Platform. This could be a tear fault accommodating the differential movements of Punjab platform Block and Sulaiman Block. In Sulaiman Foredeep a prominent effect of flexuring is present in Eocene and older sediments. This is marked on east-west oriented seismic sections. This effect is related with the time of collision of Indian plate and westward resistance provided by a Sulaiman Basement Fault. A normal fault is also marked in a seismic section which cuts the over all strata. The sediments are detached and in west the anticlines are formed as a fault propagation folds. Seismic data confirms the presence of Pre- Cambrian rocks in the subsurface through out the area. The Salt Range Formation is present in Punjab Platform. There is a transition zone in west where seismic data shows presence of thick sediments of equivalent age. An ancient Paleohigh of Paleozoic age is present in Bahawalpur area. It is extending in the south. It has a deposition of Jurassic & Triassic sediments while Paleocene & Cretaceous sediments onlap on Jurassic, then Eocene sediments are deposited over it.