SEDIMENTOLOGY AND PETROLOGY OF THE VOLCANICLASTIC ROCKS OF THE BIBAI FORMATION, ZIARAT DISTRICT, BALOCHISTAN, PAKISTAN

Abstract

The Upper Cretaceous Bibai Formation is exposed in Kach-Ziarat and Spera Ragha-Chinjun valleys and near Muslunbagh, within the western part of the Sulaiman Thrust-Fold Belt east of the Quetta Syntaxis. The formation generally comprises basic volcanic rocks, volcanic conglomerate and breccia, sandstone, mudstone and ash beds. Within the Kach-Ziarat valley it is dominantly composed of volcaniclastic sediments and rarely lava flows, while, within the Spera Ragha-Chinjun valley dominantly the in- situ basaltic volcanic rocks. The volcaniclastic succession may be categorized into various facies viz volcanic conglomerate (VC), volcanic breccia (VB), sandstone (SS), sandstone interbedded with mudstone (SSMS), mudstone (MS), limestone (LS) and lava flows (VOL), which are comparable with the facies classes A, B, C, D, E, and F of Mutti & Ricci Lucchi (1972, 1975) and Pickering et al. (1986a), indicating deposition by sediment gravity flows and slumping / soft sediment deformation. The volcanic conglomerate, VCP-VCC-SS association of the facies, their stacking pattern, erosive bases and fining-upward trend suggest deposition within a channelized complex anastomosing on a submarine fan system. The SSMS facies of sandstone interbedded with mudstone, characterized by Bouma (1962) Tabcde, Tbcde, Tcde and Tde sequences, sole marks, soft sediment deformation, pinch-and-swell and general thinning- and fining-upward trends of 2nd-order cycles, indicate deposition by turbidity currents in overbank (-levee) complex between channels. The mudstone (MS) facies, possessing occasional thin sandstone beds in lower part and profusion of shallow marine fauna in upper part, indicate deposition in lower fan / basin plane conditions and shows an overall shallowing-up of the succession. Limestone (LS) facies, interbedded with volcaniclastic facies in lower part of the formation, is very finely crystalline (bio-micritic) possessing micro-foraminifera of the globotmncana family suggest deposition during calm periods when volcanic activity had been suspended intermittently. The general south-southwestward flow of the paleocurrent pattern and litho- facies distribution in various studied sections suggest that source area was north of the Bibai Peak. Texture, composition and whole rock geochemistry of rock fragments of the volcanic conglomerate indicate that sediments were derived from a hotspot related volcanic terrain where detritus of the alkaline acidic igneous rocks was also available, from time to time, along with the major proportion of basic volcanic rocks, also of alkaline nature. Based on characters of various facies associations, their vertical and lateral organization, paleocurrent pattern and composition of detritus, we propose that the Bibai Formation comprises a special category of "channel (-levee) -overbank complex", we name it the Bibai Submarine Fan, which developed on the slope of a series of seamounts (hotspot volcanos). Litho facies and their associations clearly define the mid¬ fan, overbank (-levee) and lower-fan / basin plane components of the submarine fan. Seamounts developed on sea floor of the northwestern margin of the Indo-Pakistan Plate, which later on emerged, and provided detritus to the Bibai Submarine Fan. We suggest that the present trend of paleocurrents, generally southward, has been rotated anticlockwise along with its north and northeastward drift and anticlockwise rotation of the Indo-Pakistan Plate towards Eurasia during the Upper Cretaceous and later periods till present time. Its restoration by clockwise rotation back to its Upper Cretaceous (71.4±3.4 My) position would give its original west-northwestward paleoslope at the northwestern margin of the Indo-Pakistan Plate.