Aspects of the Cenozoic stratigraphy of the Northern Sulaiman Ranges, Pakistan

The results of micropalaeontological, nannopalaeontological and palynological analyses of Cenozoic sections from the Northern Sulaiman Ranges in Pakistan are discussed. They are in keeping with previously published results (though in some cases significantly more refined). They enable placement of most of the sampled lithostratigraphic units in a global bio- and sequence-stratigraphic framework. Limestones appear to be associated with essentially transgressive global sequences, shales with essentially regressive sequences. Palaeoenvironmental interpretations indicate a range of depositional settings from continental through shallow marine to basinal. A number of shallow marine carbonate platform sub-environments are recognized over the Palaeocene–Eocene section


INTRODUCTION
BP has recently undertaken a field survey of part of the Northern Sulaiman Ranges in Pakistan (Fig. 1). The primary objective of the survey was to log selected sections and to collect samples for geochemical, sedimentological and palaeontological analysis in order to identify any resource (source, reservoir and seal) potential. The secondary objective was to obtain enough structural geological information to construct a cross-section through the mountain-front to constrain appropriate source maturation and migration models. Structural aspects are discussed by Banks & Warburtom (1986) and Jadoon et al. (1 994) Samples were made available for palaeontological analysis from the Dunghan, Ghazij, Baska, Kirthar and Chitanvata Formations and the Siwalik Group of the Zam Tower, Zor Shahr, Domanda, Toi River and Baska sections (Fig. 1).
Kirthar Formations (Fig. 2), which, by analogy with the Potwar Basin to the northeast (Brown & Dey, 1975;Khan et al., 1986) were perceived to have the most resource potential.
Micropalaeontological analyses were undertaken by the author (calcareous algae and foraminifera) and John Athersuch (ostracods), nannopalaeontologica1 analyses by Ted Finch and Julie Pearce, and palynological analyses by Bob Ravn, Paul Ventris and John Williams.

RESULTS AND DISCUSSION
Reults are summarized on Fig. 3, and discussed in more detail (in ascending stratigraphic order) below.
Palaeoenvironmental interpretation -The absence of marine palynomorphs provides negative evidence of a non-marine palaeoenvironment.
The Baska Formation appears to be associated with the essentially regressive global third-order (?eustatically mediated) sequence TA3.1 of Haq et al. (1987).
Palaeoenvironmental interpretation -Microfaunal evidence points to a low-energy back-bank lagoon characterized by Miliolida. The presence of sea-grasses is indicated by the presence of epiphytic discorbids, the influence of the open ocean by rare globigerinids. The depth of depositional environment is difficult to determine, because it would have been determined not by the foraminifera themselves but by the photosynthetic algal symbionts to which they acted as host. Modem Archaias aduncus, Cyclorbiculina compressa and Peneroplis proteus contain photosynthetic green algae which restrict them to between @-Ism (e.g. Murray, 1973;Reiss & Hottinger, 1984;Hallock, 1988).
The nummulitid populations; in the studied samples are characteristically dominated either by large B-forms or by small A-forms. Aigner (1982Aigner ( , 1983Aigner ( , 1985 has argued that this is a function of hydrodynamic sorting, and that B-form dominated populations are characteristic of autochthonous and A-form dominated populations of allochthonous accumulations. This has been disputed by various others authors (e.g. Wells, 1986). However, it seems reasonable to interpret A-form dominated assemblages associated with planktonic foraminifera as allochthonous. The latter fall in the 'toe-of-slope' field of a ternary plot of foraminiferal 'morphogroups' (Fig. 4). These assemblages are believed to have been transported along with fine clastic material on account of their high internal porosity and low threshold. Present-day populations of the nummulitid Operculi,na bartschi have been reported similarly transported into deep water off New Caledonia (Debenay, 1988).
Palaeoenvironmental interpretation -Micro faunal evidence points to a low-energy back-bank lagoon characterized by Miliolida. The depth of depositional environment is difficult to determine, because it would have been determined not by the foraminifera themselves but by the photosynthetic algal symbionts to which they acted as host. Modern Alveolinella quoii contains photosynthetic B-3 diatoms which restrict it to between 15 and 75 m, with optimum development between 30 and 50m (e.g. Reiss & Hottinger, 1984;Murray, 1973;Hallock, 1988).
Palaeoenvironmental interpretation -Microfaunal evidence points to a range of carbonate platform and associated basinal sub-environments.
Palaeoenvironmental interpretation -Micro faunal evidence points to a range of carbonate platform and associated basinal sub-environments.

Chitarwata Formation
The dominant lithology was mudstone. Palynological analyses were undertaken on two samples (166 and 173). Results. The in situ palynoflora is impoverished, and included only rare freshwater algae (Pediastrum) and terrestrially derived pollen (alder (Alms), hazel (?Corylus), hemlock (Tsuga), pine (Pinus) and sundry flowering-plant (angiosperm) and grass ( G r a m i n e ae ) p o 11 e n ) a n d s p o r e s ( D elt o idosp o r a , Laevigatosporites, Verrucosisporites). Discussion. Stratigraphy -No conclusions can be drawn with regard to age. Regional evidence indicates Oligocene.
Palaeoenvironmental interpretation -Palynological evidence points toward a continental environment characterized by a forested (?montane) hinterland. Published information indicates a shallow marine environment for the essentially age-equivalent Muree Formation (Bossart & Ottiger, 1989). These formations collectively can be regarded as representing a diachronous foreland basin megasequence younging from northeast to southwest.

Siwalik Group
The dominant lithogy was mudstone. Palynological analyses were undertaken on two samples (159 and 163), both of which were barren. No conclusions can be drawn with regard to age or depositional environment. Published information indicates a Neogene-Pleistogene age and a range of continental depositional environments (see, for instance, Gill, 1953).

CONCLUSIONS
The results of palaeontological analyses of samples from sections from the Northern Sulaiman Ranges in Pakistan are in keeping with previously published information (though in some cases, including that of the prospective Kirthar Formation, significantly more refined). They enable placement of most of the sampled lithostratigraphic units in a global bio-and sequence-stratigraphic framework. The limestones appear to be associated with essentially transgressive global sequences, the shales w) th essentially regressive sequences.
Palaeoenvironmental intinterpretations indicate a range of depositional settings from continental through shallow marine to basinal. A number of shallow marine carbonate platform subenvironments are recognized over the Palaeocene-Eocene section (essentially back-bank lagoon, bank and fore-bank).

Manuscript received August 1994
Manuscript accepted September 1994