Preliminary Paleomagnetic Investigations of the Manchar Formation, Gaj River Section, Kirthar Range, Pakistan
Abstract
The Manchar Formation, a Neogene molasse sequence in the Kirthar Range, consists of alternating units of sandstones and shales/siltstones/claystones, with some conglomerates. The "lower Manchar" has abundant sandstone units, the "middle Manchar'' has abundant shale units, and the "upper Manchar has abundant sandstone and conglomerate units. The "lower Manchar'' is most fossiliferous, the "middle Manchar" has almost no fossils, md the "upper Manchar" has only a few fossils. Preliminary results of the paleomagnetic studies show that the fie-grained shale/siltstone/claystone units are suitable for magnetostratigraphic studies. Paleomagnetic studies show that NRM of many of the samples is composite of a secondary component of remanent magnetization, and a stable, hopefully primary, component of remanent magnetization. AF treatment of upto 400 Oe is helpful to remove the secondary component of remanent magnetization but the stable und primary component can't be obtained with certainty by AF treatment of upto 900 Oe. On the other hand, PTD enables to remove the secondary magnetization by heating at 20O oC, and also successfully isolates the stable hopefully primary, component of remanent magnetization. Therefore, all samples were subjected to a single step thermal demagnetization at 400 oC, and stable components of remanent magnetization were obtained. These stable directions were used to establish a magnetic-polarity reversal sequence. Based on the Middle to Late Miocene faunal contents of the Manchar Formation the observed magnetic-polarity-reversal sequence is correlated with the Magnetic Polarity Time Scale. This correlation suggests that the sampled part of the Manchar Formation ranges from 15.2 Ma. to 9-10 Ma.
References
Barry, J.C., Lindslay, E.H., & Jacobs, L.L., 1982. A biostratigraphic zonation of the middle and upper Siwaliks of the Potwar Plateau of northern Pakistan. Paleo, Paleo. Palm. 37, 95-130.
Berggren, W.A., Kent, D.V., Flym, J.V., & Van Couvering, J.A., 1983. Cenozoic geochronology. In 'Geochronology and the Geologic Time Scale' (Snelling, N.J., ed.), Geol. Soc. Land. Special Publication (in press).
Blanford, W.T., 1876. On the geology of Sind. Geol. Surv. India Mem. 9, 8-22.
Blanford, W.T., 1879. 'The geology of western Sind. Ibid., Mem. 17, 1-196.
Fisher, R.A., 1953. Dispersion on a sphere. Proc. R. Soc. Lond., A-217, 295-303.
Iqbal, M.W.A., 1980. Oligo-Miocene bivalves & gastropods from Kirthar province, Lower Indus Basin, Pakistan. Geol. Surv. Pak. Recs. 51, 1-59.
Khan, M.H., 1968. The dating and correlation of the Nari and the Gaj Formations. Geol. Bull. Univ. Punjab 7, 57-65.
Pascoe, C.E., 1964. A manual of the geology of India and Burma, Vol. III. Indian Government Press, Calcutta, 1-886.
Raza, S.M., Barry, J.C., Meyer, G., and Martin, L., 1984. Preliminary report on the geology and vertebrate fauna of the Miocene Manchar Formation, Sind, Pakistan, (in press).
Shah, S.M.I. (ed.), 1977. Stratigraphy of Pakistan. Geol. Surv. Pak, Mem. 12, 1-138.
Vredenburg, E.W., 1906. The classification of Tertiary system in Sind with reference to the zonal distribution of the Eocene Echinoidea described by Duncan and Sladen. India Geol. Surv. Recs. 34, 172-198.
Watson, G.S., 1953. A test of randomness of directions. Geophys. Suppl. 7, 160-161.
