STRATIGRAPHY, PETROLOGY AND FACIES ASSOCIATIONS OF THE QUATERNARY SPIN KAREZ GROUP, HANNA-SPIN KAREZ AREA, QUETTA DISTRICT, PAKISTAN

Abstract

The Hanna-Spin Karez area which comprises the Quaternary succession of the Spin Karez Group is located adjacent to the junction of Sulaiman and Kirthar Fold-Thrust Belts (Quetta Syntaxes), southwest of the Zarghun Trough in the northwestern Balochistan, Pakistan. The area comprises over 800 m thick conformable succession of the Quaternary age, covering surface area of ~30 km2. The succession has been named as the Spin Karez Group, which are further subdivide from base to top into three distinct lithostratigraphic units as: Hanna Lake Conglomerate, Hanna Red Clays and Spin Karez Conglomerate. The Hanna Lake Conglomerate comprises 200 m thick succession of boulder-pebble conglomerate. The Hanna Red Clays comprise over 500 m thick succession of dominantly red claystone rarely interbedded with siltstone and very fine-grained sandstone. The Spin Karez Conglomerate is composed of over 100 m thick succession of well-stratified and moderately- to well-sorted cobble/pebble conglomerate with occasional sandstone/siltstone lenses. The Triassic to Pliocene age older successions from the region's north and west provided the polymictic conglomerate, boulder to pebble size limestone, sandstone, chert, and conglomerate fragments. The Spin Karez Group comprises 15 types of gravel, sandstone and mudstone facies, which were grouped into 5 facies associations. The facies associations include braided channel deposits (FA-1), sheet-flood deposits (FA-2), floodplain deposits (FA-3), marginal lacustrine deposits (FA-4) and open-water lacustrine deposits (FA-5). The Spin Karez group evolved in three phases: i) Deposition of the Hanna Lake conglomerate started with the proximal and distal braided channel systems in the northern and southern parts of the study area respectively; ii) Deposition of the Hanna Red clays deposited in shallow lake with episodic sub-aerial exposure, providing reddish colour to the mud-dominant facies; iii) The depositional basin, once again, transformed to braided channel system, of the Spin Karez conglomerate, which evolved over the shallow water lake deposits of the Hanna Red clays. The Spin Karez group was mainly controlled by continued subsidence and regional tectonics during the Quaternary period that accommodated the space for the small shallow basin (~30 km2) that stacked depositional systems.