Palaeoenvironments, diagenesis, and geochemical studies of the Dungan Formation (Palaeocene), eastern Sulaiman Range, Pakistan.

The Sulaiman Range is located on the northwest margin of the Indian Plate. It forms the middle part of the Indus basin, and has been termed the Sulaiman sub-basin. The Indus basin is filled with a thick sequence of Mesozoic and Cenozoic sediments. The Dungan Formation is Palaeocene in age and comprises a 98 m thick predominantly carbonate sequence which extends about 200 km from north to south along the Sulaiman Range. The Formation is characterised by different microfacies. Petrographic studies revealed the following microfacies; peritidal carbonate, inner lagoon, oolitic, outer lagoon, rhodolith platform, nummulitic, larger foraminiferal, and planktonic foraminiferal. All these facies can be interpreted broadly within a homoclinal, windward carbonate ramp model. The cementation history has been investigated by petrography of stained thin sections, cathodoluminescence microscopy, XRD, electron microprobe, SEM (in back scattered mode), XRF, and carbon and oxygen stable isotope analysis to reveal trace element contents and their variation in the diagenetic cements. The diagenesis of the formation starts from the marine phreatic environment, characterised by micritization and peloidal cements. These cements are of high Mg-calcite, and low in Fe and Mn. They do not show luminescence. The active fresh water phreatic zone may involve leaching in the zone of solution, neomorphism, isopachous bladed cement around the oolites and foraminifera, syntaxial overgrowth cement on echinoderm fragments, and interlocking crystals of equant calcite that coarsen towards the pore centres (drusy cement). The neomorphic spars having irregular boundaries and are cloudy and patchy. They are also low in Fe and Mn, and non-luminescent. The isopachous bladed cements are low in Mg and high in Fe and Mn and show orange luminescence. The syntaxial overgrowth cements are low Mg-calcite. They show bright zoned luminescence, indicating different zones of Fe and Mn concentration. The drusy cement is also low Mg-calcite, and shows dull to orange luminescence indicating the high concentrations of Fe and Mn as compared to marine cement. The oxygen isotopic values of this cement type are low when compared to the whole rock. Vug and mould porosities are developed in the freshwater vadose zone, which are filled by sparry calcite cement. The shallow burial environment is marked by compaction of skeletal grains, stylolites and fractures. The fractures show two generations. The early fracture fills cements are non ferroan and show bright zoned luminescence. The later fracture fills cements are ferroan and non-luminescent. The oxygen isotope values of non-ferroan cement filling fractures are more negative when compared with that of ferroan cement filling fractures, which may be formed in shallow burial environments by formation water.