Clastic Source, Depositional Environment and Sequence Stratigraphy of Mixed Carbonate-Clastic Sequences: Inferences from Facies Analysis and Petrography of Oligocene Nari Formation from the Hundi Anticline, Karachi Embayment, Indus Basin, Pakistan

The present study seeks to constrain the clastic source, depositional environment and sequence stratigraphy of the mixed carbonate-clastic sequences of the Oligocene Nari Formation exposed in the Hundi anticline of Karachi Embayment, Indus Basin, Pakistan, with the help of petrographic and microfacies studies. Six different lithofacies were identified during the field observations, which include arenaceous limestone, calcareous sandstone, argillaceous sandstone, laterite/oxidized sandstone, arenaceous shale and calcareous mudstone. Seven microfacies of packed biomicrite, sparse biomicrite, Sub-arkose sandstone and quartz-arenite are identified on the basis of petrographic analysis. The minerals identified during the petrographic analysis indicate that clasts present in the studied samples are supplied by the Indian Craton. Nari Formation in the studied section show a general progradational facies pattern, deposited in a variety of environments from shallow-marine outer ramp to fluvial environment. Two depositional sequences with eight phases of regression and seven transgression phases are interpreted during sequence stratigraphic analysis. The progradational facies pattern of mixed carbonate-clastic rocks is interpreted to be deposited in the continually shrinking realm of Neo-Tethys as a result of plate collision and mountain building process of Himalaya and associated fold belts in the Indian Sub-continent. This study will bring new perspectives that can be used to refine the interpretations of sedimentary processes in mixed carbonate-clastic environments in the region thereby advancing our understanding to geological history of the study area.