BIOFACIES, PALEOENVIRONMENT ENVIRONMENT AND STRATIGRAPHIC DISTRIBUTION OF EOCENE CARBONATES IN HAZARA BASIN, LESSER HIMALAYAS, PAKISTAN

The Eocene shallow marine carbonates of Hazara region in Pakistan belongs to an important part of geological history, yet very little attention has been given from the scientists. Five stratigraphic sections have been selected in Hazara basin of Eocene Margalla Hill Limestone for the current study with emphasis on lithostratigraphy, biostratigraphy, depositional environment, sequence stratigraphic setup and diagenesis. A total of eight lithological units are identified across all the sections. Petrographic studies revealed that the biogenic communities are characterized by numerous larger benthic foraminiferal assemblages including Nummulites, Assilina, Discocyclina, Lockharita, Alveolina, Operculina, Ranikothalia and rare planktonic foraminiferas. The different concerns about morphological features, progression of small and larger benthic foraminifera including size, appearance, diversity and occurrence during the Eocene are analyzed. Based on the fossils assemblages mainly larger benthic foraminifera, carbonate grains and fabric, a total of nine microfacies (MF1-MF9) were identified. Numerous variations in microfacies based sea-level curve have shown alternating transgressive and regressive system tracts. The biotic assemblages of larger benthic foraminiferas, carbonate grains component indicate that the depositional setting from deep marine towards the shallower environment reflects a coarsening upward cycle during the deposition of the Margalla Hill Limestone. Based on the outcrop data along with the petrographic studies, a detailed paragenetic sequence is also established which indicates that the Margalla Hill Limestone has undergone various diagenetic alterations which include bioturbation, micritization, cementation, mechanical and chemical compaction, and dolomitization.

In conclusion the Margalla Hill Limestone is deposited in shallow marine homoclinal ramp settings, comprised of larger benthic foraminiferal assemblages showing alternating transgressive and regressive system tracts and affected by various diagenetic changes.