Biostratigraphy of Kirthar Formation (Middle to Late Eocene), Sulaiman Basin, Pakistan

Abstract

   A foraminiferal (planktonic and larger) biostratigraphic investigation based on the 70 samples of the Kirthar Formation from five surface sections (one complete and three selective) in the eastern margin of the Sulaiman Range and one in subsurface of Mari Gas Field (southern Sulaiman Basin) were investigated. These new findings together with the re-evaluation of the previous biostratigraphic information, where needed, were synthesized to develop a chronostratigraphy of the Kirthar strata. The present study revealed that in most of the previous studies, the lower lithologic boundary ofthe Kirthar strata carried on varying at different stratigraphic levels, additionally a flexible taxonomical definition of some of the larger foraminiferal species was adopted which resulted contradicting age ofthe Kirthar Formation. Our investigation for a unified lithology of Kirthar Formation (sensu strato Shah, 1977) reflected that it range in age from the Middle to Late Eocene in the eastern part of the Sulaiman Range. Eight planktonic foraminiferal zones from P10 to P16/17 have been identified and compared with larger foraminiferal and calcareous nannoplankton zonations. The base of the Kirthar Formation (the base of Habib Rahi Limestone) on the shallow part of the shelf (inner shelf) has been found to lie within the larger foraminiferal Assilina spira abrardi Zone whereas in the deeper part of the basin (middle to outer shelf) it lies within upper part of zone P10 to ?P11. The regressive unit “Domanda Shale member” representing restricted marine and brackish water environments is P11 in age. Pir Koh Limestone and Marl member indicate that the basin attained a maximum depth (outer shelf to upper slope) during P12 to P13 time. Drazinda Shale member as a whole represents an inner to middle shelf environments during P14 to P16/17 time. Five correlative time lines emerged which showed almost isochronous time/facies relationship throughout the study area. However a slight degree of northward time transgressive nature of the Pir Koh Limestone has been observed. Based on the faunal association, two minor hiatuses, one at the base of Kirthar (between zones P9 and P10) and second between Pir Koh Limestone and Pir Koh Marl (between zones P13 and P14) are recognized. For the Habib Rahi Limestone and Pir Koh Limestone and Shale member our data shows a good time/sequence type consistency with the global sequences of Haq et aI., (1987) whereas a time/sequence type discrepancy exists for the Domanda Shale member.