Depositional environments of Campanian–Maastrichtian successions in the Kirthar Fold Belt, southwest Pakistan: Tectonic influences on late cretaceous sedimentation across the Indian passive margin

Abstract

The creation of a complex submarine morphology on the passive margin of the Indo-Pakistan continental plate during later stages of its northward drift, just prior to collision with Eurasia, is clearly demonstrated by the nature, architecture and regional distribution of sandstone bodies within the Campanian–Maastrichtian successions (Mughal Kot and Pab formations), exposed within the north–south trending Kirthar Fold Belt of southwest Pakistan. These successions range in thickness from 7 m to more than 500 m and are dominated by fine to coarse, sometimes pebbly, thin to thick-bedded sandstones with subordinate mudstones and marls, all of which were deposited on the north-facing passive margin of the Indo-Pakistan Plate. Twelve facies have been identified in these sequences and these have been grouped into nine Facies Associations, which were formed within two broadly coeval depositional systems that occupied the contiguous Southern Kirthar and Central Kirthar sub-basins. It is argued here that these sub-basins and their fill-sequences developed mainly in response to major, tectonically induced changes in both the morphology of this sector of the palaeo-margin and in the sediment-supply systems. The more proximal sequences in the east and southeast sectors of the Southern Kirthar sub-basin are dominated by fluviodeltaic deposits while more distal areas to the northwest are characterized by turbidite sandstone bodies formed in channel-levee and lobe complexes within slope and basin floor settings of a relatively confined deep marine trough. In this Southern Kirthar sub-basin, the Mughal Kot Formation comprises basin floor lobes, submarine channel-fill sand bodies and base of slope mud-rich lobes, whereas the succeeding Pab Formation reflects overall progradation of the system, being dominated by submarine slope fan lobes, channels and levee deposits. The Central Kirthar sub-basin displays a broadly east-to-west transition from shoreface (inner shelf), through shelfal delta lobes (middle shelf) and deeper shelf/ramp (outer shelf) facies associations, all formed on a gently inclined, storm- and flood-dominated clastic ramp. Synsedimentary normal faults observed in this study were controlled the depositional architecture and creation of closed deep marine southern Kirthar sub-basin during Late Cretaceous.