Occurrence of a Likely Tuff Bed between the Middle and Upper Siwaliks, Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan

Abstract

A likely tuff bed lies along the gradational contact of the Middle and Upper Siwaliksineastern Sulaiman Range, Taunsa areaofDera GhaziKhan district, Pakistan. This tuffaceous unitis 0.5–3m thick and extends for 10 kmalong the north-south strike in the eastern limb of the Zindapir anticline. It is greyish white to white on fresh surface, fine-grained to silty at the bottom and clayey at the topandthus showsa fining upward grain-size grading. The lower part of the ashbedshows a prominent lamination defined bymegascopically visible abundant biotite,while the central and upper parts are so fine-grained that the individual minerals cannot be seen in hand sample. Unlike the lower well-laminated part, the central and upper parts are crudely laminated to apparently massive. The bulk samples analysed withX-ray diffraction consist of quartz, feldspar (plagioclase), biotite, clays, calcite and some ore mineral likely spinel, while the clay-size fractions contain illite, chlorite, biotite and probably their mixed-layered varieties. The colour, texture, presence of abundant biotite and stratigraphic position of the Taunsa tuff correlate with those reported from Potwar plateau and fromKashmir basin. However,the apparent absence of smectite fromthe XRD pattern makes the Taunsa ash bed different from both Potwar and Kashmir tuffs. The present stratigraphic positionof the tuff bed corresponds to shallow diagenetic zone, while the absence of smectite in the tuff and crystallinity of illite suggest that the tuff is probably derivedupon reworkingfrom a deeper diagenetic zone belonging to a lower stratigraphic level. The Eocene or other older pre-Siwalik units inPakistan may have or hadsome primary ashfall depositsas reported in the northwesternHimalayasof India. Thisolder volcanic ash may have been reworked to its present site of occurrencealongthe gradational contact of the Middle and the Upper Siwaliksin Taunsa area of Dera Ghazi Khan. However, the primary source of the Taunsa tuffmay belong more likely to Chagaiarc in Pakistan thanto Dacht-e-Nawar volcanic complex in Afghanistan.

Keywords

• tuff
• mineralogy
• illite
• chlorite
• biotite
• siwaliks
• sulaiman range
• pakistan