The ages of many of the metasedimentary formations exposed in the Himalayan foothills that surround the Peshawar Basin have long been the subject of debate. The Manki, Shahkot, Utch Khattak, Shekhai, Dakhner, Darwaza, and Hissartang Formations and Inzari Limestone of the Attock-Cherat and Gandghar Ranges, the Khyber Limestone and Ali Masjid Formation of the Khyber region, and the Ambar, Tanawal, and Gandaf Formations of the northern Peshawar basin have been assigned inferred ages based largely on lithologic correlation. Early workers correlated these rocks with unmetamorphosed Paleozoic and Mesozoic rocks of similar lithology exposed to the south (e.g. Calkins et al., 1975). However, they were not aware of the significance of the intervening Panjal-Khairabad fault, a thrust fault with large displacement that is largely concealed beneath Cenozoic basin fill. Later studies (e.g. Pogue et al., 1992) described stratigraphic contrasts across this fault that are similar to those observed on either side of the Main Central thrust (MCT) of India and Nepal.
An upper age limit of Early Ordovician on the Ambar Formation and stratigraphically lower units was established through the discovery of fossils in the overlying Misri Banda Quartzite (Pogue et al., 1992) and radiometric dating of the Mansehra Granite (LeFort et al., 1980) which intrudes the Tanawal Formation. Rocks of the Attock-Cherat and Gandghar Range have been considered to be older than these units by most workers even though stratigraphic relationships are obscured by faulting and basin fill. Most recent workers infer a Proterozoic age for the majority of the rocks of the Gandghar and Attock-Cherat Ranges based on the presence of pronounced unconformities at the base of dated Paleozoic rocks and Late Proterozoic whole rock Rb-Sr model age dates on the Hazara Formation near Abbottabad (Crawford and Davies, 1975). These dates were interpreted by Baig et al. (1988) to represent the provenance age of Hazara Formation clastic material. Outcrops of the Hazara Formation however, are confined to the south side of the Panjal-Khairabad thrust and are never in depositional contact with the inferred Proterozoic rocks of the Gandghar or Attock-Cherat Ranges.
Until recently, it has not been possible to further constrain the ages of the undated Peshawar Basin formations. The only associated igneous rocks are ubiquitous diabase dikes of Permian age and the only fossils reported have been stromatolites. Since many of the undated units are carbonates, we concluded that it might be possible to collect samples suitable for stable strontium isotope analysis. Initial seawater ratios are best preserved in unmetamorphosed pure limestone, so an effort was made to sample the purest limestone from each undated formation at its southernmost exposure. All of the sampled formations lie in the hanging wall of the Panjal-Khairabad thrust. The metamorphic grade of rocks within this tectonic block increases dramatically northward.
As expected, many of the samples appeared to have acquired excess 87Sr from silicates during metamorphism and were thus too altered to permit correlation with published seawater curves. However, five samples from the Attock-Cherat and Gandghar Ranges have 87Sr/86Sr ratios below 0.7065 indicating a minimum age of approximately 750 Ma. A ratio of 0.7074 for one sample from the Inzari Limestone in the southern tectonic block of the Attock-Cherat Range rules out the correlation of this unit with Paleozoic rocks exposed near Nowhera proposed by Pogue et al. (1999). This ratio gives an ambiguous age as it is shared by Late Proterozoic, Late Permian, and Mesozoic carbonates. We favor a Late Proterozoic age for the Inzari Limestone based on the absence of fossils which are generally common in the Permian and Mesozoic carbonates of northern Pakistan. Samples of the Khyber Limestone from near the Khyber Pass road yielded ratios as low as 0.7064 which indicates an unambiguous Proterozoic age.
These new data have important implications for the stratigraphy of northern Pakistan. All rocks stratigraphically below the Shekhai Formation, including the widely exposed Manki Formation, are at least 750 Ma in age. A Proterozoic age is also confirmed for the thick Khyber Limestone which crops out extensively in the Khyber region. Similarities in both age and lithology strongly imply a direct correlation between the Khyber Limestone and the Shahkot, Utch Khattak, and Shekhai Formations of the Attock-Cherat Range. Based on these constraints, the Precambrian/Cambrian boundary must lie somewhere between the Shekhai Formation and the Misri Banda Quartzite within the stratigraphic interval represented by the Ambar and Tanawal Formations. The Proterozoic rocks of the Peshawar Basin region are probable correlatives with the upper part of the Vindhyan Supergroup of penninsular India. The new age information also indicates a correlation of the Peshawar Basin Proterozoic section with recently dated protoliths for the Greater Himalayan sequence exposed in the hanging wall of the MCT in Nepal (Parrish and Hodges, 1996).
References Cited
Baig, M.S., Lawrence, R.D., and Snee, L.W., 1988, Evidence for Late Precambrian to Early Cambrian orogeny in Northwest Himalaya, Pakistan: Geological Magazine, v. 125, p. 83-86.
Calkins, J.A., Offield, T.W., Abdullah, S.K.M., and Ali, S.T., 1975, Geology of the southern Himalaya in Hazara, Pakistan and adjacent areas: Geological Survey of America Professional Paper 716-C.
Crawford, A.R. and Davies, R.G., 1975, Ages of pre-Mesozoic formations of the Lesser Himalaya, Hazara district, northern Pakistan: Geological Magazine, v.12, p. 509-514.
Le Fort, P., Francois, D., and Sonet, J., 1980, The "Lesser Himalayan" cordierite granite belt, typology and age of the pluton of Manserah (Pakistan): Geological Bulletin Peshawar University, v. 13, p. 51-61.
Parrish, R.R., and Hodges, K.V., 1996, Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya: Geological Society of America Bulletin, v.108, p. 904-911.
Pogue, K.R., Wardlaw, B.R., Harris, A.G., and Hussain, A., 1992, Paleozoic stratigraphy of the Peshawar basin, Pakistan: correlations and implications: Geological Society of America Bulletin, v. 104, p. 915-927.
Pogue, K.R., Hylland, M.D., Yeats, R.S., Khattak, W.U., and Hussain, A, 1999, Stratigraphic and structural framework of Himalayan foothills, northern Pakistan, in Macfarlane, A., Sorkhabi, R.B., and Quade, J., eds., Himalaya and Tibet: Mountain Roots to Mountain Tops: Geological Society of America Special Paper 328, p. 257-274.