FACIES DISTRIBUTION, DEPOSITIONAL ENVIRONMENTS, PROVENANCE AND RESERVOIR CHARACTERS OF UPPER CRETACEOUS SUCCESSION KIRTHAR FOLD BELT PAKISTAN

Abstract

Excellent exposures of Upper Cretaceous succession (Campanian-Maastrichtian; Mughal Kot and Pab formations) in the north-south trending Kirthar Fold Belt, Pakistan are studied in detail. The succession is 7 m to 467 m thick in the study area and is comprised of fine to coarse, thin to thick-bedded sandstone with subordinate mudstones and marls. The succession was deposited on west (northwest)-facing passive margin of the Indian Plate. Twelve facies are identified and grouped into nine facies associations, which exhibit that they were formed in two partly coeval depositional systems: the Northern Depositional System and Southern Depositional System. The Northern Depositional System consists of shoreface (upper shelf), shelfal delta lobes (middle shelf) and outer shelf ramp (lower shelf) facies association, formed on a storm and flood dominated, low gradient clastic shelf of Mutti type shelf delta lobes. The Southern Depositional System is characterized by fluviodeltaic deposits in the southeast (proximal) and deep water turbidite sandstones in the northwest, formed in channel-levee and lobes complex within deep slope and basin floor settings. In the Southern system, the Mughal Kot Formation is comprised of basin floor lobes, channel filled sand bodies and base of slope mud rich lobes, whereas, the Pab Formation is comprised of submarine slope fan, channels and levee deposits. The succession was deposited during regression phase as indicated by shallowing upward trend which is evidenced from thickening upward cycles, grain size, bed thickness increase and shallow marine Ranikot Group deposited over the succession. Physiography and tectonic character of Indian Passive margin during its drifting towards north deduce its regional distribution, vertical & lateral sequences and style of sandstone bodies both in northern and southern depositional systems. Sandstones composition and petrography of these two systems are also significantly different. The sediments were supplied to the shallow marine deposits in Northern Depositional System from thermally uplifted Indian shield in the east as evidenced from persistent westward paleocurrent directions. Deep marine turbidite sands were sourced by north-northwest directed density currents. Uppermost parts of the Upper Cretaceous succession in Southern Depositional System contain an appreciable amount of volcanic fragments, which were most probably caused by Deccan Trap volcanism in south-southeast to the studied area. Low K2O/Al2O3 ratio in mudstones, high values of CIA (Chemical Index of Alteration) and SiO2 – Al2O3+K2O+Na2O diagram suggest the initial feldspar deficiency was caused by intense chemical weathering due to warm humid paleoclimatic conditions in source area. Further reduction of feldspar was caused by long transport distance and most effectively by diagenetic dissolution, alteration and replacement. The sandstones have undergone intense and complex diagenetic changes due to framework composition of sandstones, burial depth and thrusting of Bela Ophiolites. The unstable grains like feldspar and lithic volcanic fragments were dissolved considerably and altered to a variety of clay minerals. Compaction, authigenic cementation, dissolution and grain fracturing are important diagenetic events identified. Calcite, quartz, clay minerals and iron oxide are the common authigenic cements. Dissolution and alteration of feldspar and volcanic lithic fragments and pressure solution were the main sources of quartz cements. Mechanical compaction, authigenic cements like calcite and quartz reduced the primary porosity of the sandstones, whereas, dissolution of feldspar and volcanic grains have enhanced and produced secondary porosity up to 15.53% (average 2.77 to 10.61%). Chlorite coating has prevented the quartz cementation, so some microporosity was preserved. Some microporosity in interbooklets of kaolinite is observed.