Revised stratigraphy of the lower Cenozoic succession of the Greater Indus Basin in Pakistan

A refined stratigraphy for the lower Cenozoic succession of the Greater Indus Basin in Pakistan is presented. This region preserves an important East Tethyan marine succession through the Paleocene–Eocene, but its interpretation in terms of regional (tectonic) and global (climatic) effects has been inhibited by poor stratigraphy. Established dinoflagellate, nannofossil, planktonic foraminiferal and shallow benthonic foraminiferal biostratigraphical data for the Greater Indus Basin in Pakistan are collated, reinterpreted (where necessary) and correlated with the global standard chronostratigraphy and biostratigraphy of the early Palaeogene. Inter-regional stratigraphical correlations for the Upper Indus Basin and Lower Indus Basin are resolved. Age-diagnostic larger benthonic foraminifera from the Late Paleocene Lockhart Formation are illustrated. These collective biostratigraphical data provide a means of interpreting the lithostratigraphy and physical stratigraphical relationships of the Palaeogene succession in terms of the interplay between local tectonics (India–Asia collision) and global sea-level change. The timing of the Tethys closure, initial and final contact of the Indian–Asian plates, and dispersal of land mammals on the Indian Plate are discussed and correlated in the stratigraphical record of the basin.


INTRODUCTION
accreted onto the eastern margin of the Indo-Pakistan plate and probably spilled over and spread across most of the Indo-Pakistan plate (Khan & Srivastava, 2006). The Himalaya mountain chain is a direct result of this continental collision, during which the fold-and-thrust belts of western and northwestern Pakistan were initiated (Le Fort, 1996).

STRATIGRAPHY OF THE GREATER INDUS BASIN IN PAKISTAN
The rocks of lower Cenozoic age in the Greater Indus Basin in Pakistan are remarkably varied in lithology and thickness, but mainly consist of marine limestone and shale with subordinate sandstone and non-marine red beds, gypsum, anhydrite, salt and coal (Shah, 1977). Terrestrial emergence at the end of the Paleocene, followed by marine submergence in the Early Eocene (Shah, 1977), was succeeded by a short-lived regression at the close of late Early Eocene times, resulting in evaporites being deposited in the Kohat area (Nagappa, 1959). Following evaporite formation, a marine transgression at the start of the Middle Eocene affected a large area, including the western Kohat, the Lower Indus Basin, the Axial Belt and the Baluchistan Basin (Shah, 1977). During Middle and Late Eocene times different parts of Pakistan became emergent and this resulted in unconformities of varying magnitude (Shah, 1977).

THE UPPER INDUS BASIN
The evolution of the lower Cenozoic stratigraphical nomenclature for the Upper Indus Basin is given in Table 2. An integrated dinoflagellate, nannofossil, shallow benthonic and planktonic foraminiferal biostratigraphy for the Upper Indus Basin, related to standard chronostratigraphy and biostratigraphy, is presented for the first time (Fig. 3). Biostratigraphical and lithostratigraphical evidence for the age of Paleocene-Eocene stratigraphical units from various parts of the Upper Indus Basin is shown in Figure 4 against global chronostratigraphy and biostratigraphy. The stratigraphical context for these units is discussed below.  Fig. 2A). The Hangu Formation of Shah (1977) is the basal Cenozoic sedimentary unit in this region (Fig. 4). It comprises sandstone, siltstone and clays in the Kohat area, Kala Chitta and Hazara ranges (Shah, 1977), with argillaceous limestone beds in the Salt Range and also a coal-bearing horizon in the Surghar Range (Shah, 1977;Warwick et al., 1993). The formation unconformably overlies the Late Cretaceous Kawagarh Formation in most of the basin (Latif, 1976;Shah, 1977), but occasionally overlies Palaeozoic units in the Salt Range and Surghar Range (Shah, 1977). The Hangu Formation is unfossiliferous in the Kohat area, Kala Chitta and Hazara ranges (Latif, 1976;Shah, 1977;Weiss, 1993) and its chronostratigraphical position is based on regional geological context (Fig. 4). In the Salt Range, upper parts of the formation yield age-diagnostic foraminifera (Davies & Pinfold, 1937;Haque, 1956;Weiss, 1993;Ferrandez-Canadell, Köthe et al., 1988). KR, Kurram River; SD, Sulaiman Depression; SR, Sulaiman Range; 11, Mughal Kot-Toi section; 12, Zinda Pir section; 13, Rakhi Nala section; 14, Muree Brewery section. Fig. 3. Dinoflagellate, nannofossil, shallow benthonic foraminiferal and planktonic foraminiferal zones for the Paleocene-Eocene of the Upper Indus Basin. Biostratigraphical data are compiled from various sections of the Kohat Basin (Shah, 1977;Köthe et al., 1988;Weiss, 1993;Afzal et al., 2005; present study), the Salt Range (Shah, 1977;Köthe et al., 1988;Gibson, 1990;Weiss 1993;Afzal & Butt, 2000;Sameeni & Butt, 2004) and the Kala Chitta Range (Akhtar & Butt, 1999. The Upper Indus Basin biostratigraphy corresponds to the shallow benthonic Zones (SBZ) of Serra-Kiel et al.
The Patala Formation is separated by an unconformity (upper P5-P6a) from the overlying nodular limestone and marl/shale of the Margala Hill Formation in the Kala Chitta and Hazara ranges (Latif, 1970(Latif, , 1976Shah, 1977;Akhtar & Butt, 1999 (Fig. 4). In most of the Kohat area the Patala Formation is conformably overlain by greenish shales of the Panoba Formation, but in the Salt and Surghar ranges it is followed conformably by the marl/shale and limestone of the Nammal Formation (Shah, 1977) (Fig. 4).
The Panoba Formation is overlain by limestone and shale of the Shekhan Formation in the northern Kohat area, and by the Jatta Gypsum or Bahadur Khel Salt in the southwest Kohat area (Shah, 1977) (Fig. 4). Limestone and marl of the Chorgali Formation overlie the Margala Hill Formation in the Kala Chitta and Hazara ranges and Sakesar Formation in the eastern Salt Range (Shah, 1977). In the Surghar Range, limestone and marl of the Sakesar Formation conformably overlie the Nammal Formation and mark the end of marine deposition in this part of the basin, being overlain unconformably by non-marine molasse sediments of Miocene age (Shah, 1977).

THE LOWER INDUS BASIN
The early Palaeogene sediments of the Lower Indus Basin were deposited on a broad shelf area of the passive continental margin of the Indo-Pakistan Plate (Bannert, 1992). The history of stratigraphical nomenclature for the Lower Indus Basin is given in Table 3 and the biostratigraphical framework in Figures 5 and 6. A regional stratigraphical correlation with the Upper Indus Basin is given in Figure 7. Early Palaeogene marine sediments are well exposed across the basin (Fig. 2B). The context for the different stratigraphical units from key sections is discussed below.

The Sulaiman Range and Kirthar Range
The Sulaiman Range forms a lobate structure in the northern part of the Lower Indus Basin, while the Kirthar Range forms a north-south linear feature in the southern region (Figs 1, 2B). The succession in the Sulaiman and Kirthar ranges has been studied since the nineteenth century.
The Dungan Formation of Kazmi (1995) marks the basal lithological unit of the lower Tertiary and unconformably overlies Late Cretaceous units in most of the Lower Indus Basin. It equates to the Khadro, Bara, Lakhra and Dungan formations of Shah (1977). The lowermost sandstone, siltstone and shale portion (Khadro Formation of Shah, 1977) of the formation is widely developed in the Kirthar Range, but rare or absent in the Sulaiman Range (e.g. Rakhi Nala). It has yielded planktonic foraminifera of Zone P1 (Nagappa, 1959) (Figs 5, 6). The overlying sandstone/siltstone unit (Bara Formation of Shah, 1977) of the lower Dungan Formation is widely distributed in the Kirthar Range, but rare in the Sulaiman Range. It lacks age-diagnostic fossils (Shah, 1977;Afzal, 1996;Wakefield & Monteil, 2002). The upper Dungan Formation (the Lakhra and Dungan formations of Shah (1977) and Bara and Lakhra members of Wakefield & Monteil (2002)) is dominantly limestone and shale, and is well developed in the Sulaiman and Kirthar ranges. Many biostratigraphically important larger benthonic foraminifera from the formation include Miscellanea miscella, Discocyclina ranikotensis, D. dispansa, Lockhartia haimei, Alveolina sp., Ranikothalia nuttalli and Assilina dandotica (in Shah, 1977;Weiss, 1993;Akhtar & Butt, 2000;Wakefield & Monteil, 2002), which suggest an age of late Thanetian to early Ilerdian. The nannofossil Zones NP4, NP7 and NP9 (Köthe et al., 1988) and planktonic foraminiferal Zones P7 (Afzal, 1996) and P3-P7 (Jones, 1997;Warraich et al., 2000) further support a Middle Paleocene-Early Eocene age. The upper contact of the formation with the overlying Ghazij Formation has been interpreted as conformable in most of the basin (Shah, 1977(Shah, , 1990Kazmi, 1995); however, Warraich et al. (2000) reported Zone P6-lower P7? to be missing, with a conglomeratic bed between these formations in the northwestern Sulaiman Range (Rakhi Nala and Zinda Pir areas), suggesting this relationship to be unconformable (Figs 6, 7).
The Ghazij Formation, as recognized here, corresponds to the Ghazij and Laki formations of Shah (1977), the Laki Formation of Wakefield & Monteil (2002) and the Ghazij Group of Shah (1990) and Kazmi (1995). It is dominantly shale with subordinate claystone, sandstone, limestone, coal and conglomerate. The formation is well developed in the Sulaiman Range and parts of the Kirthar Range (Shah, 1977). Early biostratigraphical ages determined from rich occurrences of larger benthonic foraminifera, e.g. Assilina leymeriei, A. pustulosa, Orbitolites complanatus, Nummulites globulus, etc. (equivalent to Zones SBZ8-SBZ13) (Eames, 1952;Nagappa, 1959) were later confirmed by Weiss (1993) and Wakefield & Monteil (2002). Planktonic foraminiferal biostratigraphical ages were first investigated by Latif (1964) and Samantha (1973) and later by Afzal (1996), who supported an age range of Zones P7-P9. This assignment has recently been confirmed by the detailed work of Wakefield & Monteil (2002) and Warraich & Nishi (2003), who reported a continuous record of planktonic Zones P7 to P10? (Figs 5, 6). Planktonic foraminiferal studies also show a gap spanning upper P10? to P11 in the upper part of the Ghazij Formation to the lower part of the Kirthar Formation in the western Sulaiman Range (Warraich & Nishi, 2003) (Figs 5, 6). However, Köthe et al. (1988) reported dinoflagellate Zone Pak-DIX (equivalent to nannofossil Zones NP12-lower NP14) from the upper part of the formation and Pak-DX to Pak-DXI (equivalent to upper NP14-NP19/20) from the overlying Kirthar Formation of Shah (1977) (Figs 5, 6). These results suggest a conformable relationship between the Ghazij and Kirthar formations in the western Lower Indus Basin; however, this relationship is unconformable in the eastern Lower Indus Basin with a c. 2 million-year hiatus, with Zone P11 absent (Wakefield & Monteil, 2002) (Figs 6, 7).
The Kirthar Formation consists of limestone and shale with minor marl (Shah, 1977). The formation is widely distributed in the Sulaiman-Kirthar ranges and is richly fossiliferous with many age-diagnostic fossils (Shah, 1977). Based on the foraminiferal records of the Hunting Survey Corporation (1960), Shah (1977) assigned a broad stratigraphical range of Ypresian-Priabonian. However, other foraminiferal studies have given an age of late Lutetian-early Priabonian based on occurrences of planktonic foraminiferal species indicative of Zone P14 (Latif, 1964) and of Zones P12-P13 and P15-P17 (Samantha, 1973). Warraich & Nishi (2003) and Wakefield & Monteil (2002) recently established the presence of a continuous record of Zones P12 to P15? (Figs 6, 7). The lower part of the Kirthar Formation is rich in larger benthonic foraminifera, including Assilina spinosa, A. exponens, A. cancellata, Nummulites beaumonti and Discocyclina sowerbyi, equivalent to Zones SBZ13-SBZ18 (Eames, 1952;Nagappa, 1959;Weiss, 1993), suggesting a shallow-marine environment, which may account for the gap in the planktonic foraminiferal records. The Kirthar Formation is mostly overlain by Miocene-Pliocene age molasse sediments of the Siwalik Group (Shah, 1977).

REGIONAL STRATIGRAPHICAL CONTEXT
The lower Cenozoic succession of the Greater Indus Basin in Pakistan is characterized by considerable changes in lithologies and fauna. Inter-regional stratigraphical correlations for the Greater Indus Basin in Pakistan are given in Figure 7 and are related to global sea-level variations and biochronostratigraphy.
The open-marine planktonic foraminifera of the lower Cenozoic successions of the Greater Indus Basin in Pakistan show abrupt changes in composition, for example there was an increase in tropical-subtropical species of the morozovellid group during P4-P5 zones followed by a decrease in morozovellids and an increase in cooler-water species of subbotinid group foraminifera during Zones P6-P7 (Afzal & Butt, 2000;Warraich et al., 2000;Warraich & Nishi, 2003). The shallow-marine benthonic foraminiferal communities of the Greater Indus Basin in Pakistan experienced a significant diversification of species near the Paleocene-Eocene boundary; Thanetian-earliest Ilerdian (= SBZ4-SBZ6?) small species, including Miscellanea, Ranikothalia and Lockhartia, were succeeded by early Ilerdian (= SBZ6-SBZ8) large species of Nummulites, Discocyclina, Alveolina and Assilina (Weiss, 1993;Akhtar & Butt, 1999, 2000Sameeni & Butt, 2004). These marine faunal changes in the region during the late Thanetian-early Ypresian may have been associated with long-term global warming events of the lower Cenozoic (Kelly et al., 1996;Zachos et al., 2001;Scheibner et al., 2005).
The Ypresian-early Lutetian (P7-P10) sediments show a shallowing-upward sequence, associated with the Ypresian-Lutetian marine transgression-regression (Haq et al., 1987) (Figs 4, 6, 7). In the northeast (Kala Chitta, Hazara, Salt and Surghar ranges), these sediments comprise carbonate-rich units (Margala Hill Formation/Nammal Formation;Shah, 1977;Akhtar & Butt, 1999;Afzal & Butt, 2000) and in the northwest (Kohat area) a mudstone/shale-rich unit (Panoba Formation; Köthe et al., 1988;Weiss, 1993) and a carbonate-rich unit (Shekhan Formation; Köthe et al., 1988;Weiss, 1993). The higher parts of the succession include evaporites (= the Bahadar Khel Salt-Jatta Gypsum; Shah, 1977) and finally the continental red bed/sandstone mammal-bearing Kuldana Formation (Gingerich, 2003) (Figs 4, 7). The mammals of the upper Subathu Formation or Kalakot Zone of India (stratigraphically coeval to the Kuldana Formation; Sahni & Jolly, 1993) are comparable with the mammals of the Kuldana Formation (Sahni & Jolly, 1993;Gingerich, 2003). The marine regression is also recognizable in the south-southwestern parts of the basin (Lower Indus Basin), where the Ghazij Formation developed gypsum-rich, coal-and mammal-bearing beds (Clyde et al., 2003) (Figs 6, 7). The mammal taxa from the Ghazij Formation indicate a pattern of decreasing endemism, increasing cosmopolitanism and increasing modernity through time (= P7-lower P9; Clyde et al., 2003). This suggests a bridging contact of the Indian plate with the Asian plate in parts of the northwestern Lower Indus Basin, which was broken up by marine deposition of limestone and shale of the upper Ghazij and lower Kirthar formations during early Lutetian time (Johnson et al., 1999). The shale and carbonates of the Sakesar Formation, a marine correlative of the Kuldana Formation in the western Salt Range and Surghar Range, is overlain by Miocene-Recent terrestrial sediments derived from the Himalaya (Shah, 1977), marking the closure of Tethys in the southeastern Upper Indus Basin (Figs 4, 7).
The late Lutetian-Priabonian regression (Haq et al., 1987) is represented by the upper Kirthar Formation in the southsouthwest and the correlative uppermost Kohat Formation in the north-northwest. This followed closure of the Tethys in the north-northwestern parts of the basin (e.g. Kohat area, Kala Chitta and Hazara ranges) (Figs 4, 6, 7). The gradual retreat of the Tethys Sea continued south-southwest through late Lutetian to Bartonian time and it finally closed in the Priabonian (P15; Warraich et al., 2000;Wakefield & Monteil, 2002). Oligocene marine sedimentation was restricted to the south of the Lower Indus Basin (Raza, 2001a), while the rest of the Greater Indus Basin in Pakistan remained a non-depositional lowland until the formation of Neogene molasse (Shah, 1977;Raza, 2001a).

CONCLUSIONS
The lower Cenozoic succession of the Greater Indus Basin in Pakistan preserves an excellent sedimentary and biotal record of the east Tethyan Sea. These provide significant stratigraphical evidence of locally and globally significant geologically important events.
The succession is dominated by shallow-marine shelf sediments intermixed with deep-marine sediments rich in stratigraphically important microbiota. Previously published stratigraphical data have been reinterpreted and many stratigraphical levels have been revised. In addition, biostratigraphically significant shallow benthonic foraminifera from the Lockhart Formation are illustrated. Inter-basinal correlations between various units and with the global standard biostratigraphy and chronostratigraphy are presented. These have enabled recognition of unconformities associated with ongoing India-Asia tectonics and global sea-level change about 55 Ma ago. The closure of Tethys was initiated from the north and northwest during early Lutetian time and was completed by the Priabonian in the south and southwest. This also implies that the Indian Plate came in contact with the Asian Plate in the north first, and later in the southwest, which resulted in the closure of the Tethys Sea and cessation of sedimentation in the basin.