Bain Diamicite: Lithology, Age and Origin
Abstract
The Bain diamictite is a part of the Plio-Pleistocene Siwalik Group of northwest Pakistan. The type section is exposed in the Bhittani Range on the south edge of the Kurram river drainage basin near its confluence with the Indus. A counterpart section, reported here for the first time, occurs in the Shinghar Range north of the Kurram River. The unit in both areas is an unsorted, un-stratified deposit that has heterogeneous lithology, including significantly large volcanic clasts and basal ash stringer. Cooling or drying cracks occur on the upper surface in at least one locality. Clast lithologies indicate a transport direction from the highlands of Afghanistan in the WNW. Previous workers have considered the Bain diamictite to be a Mite, but the low elevation of about 500m and lack of possible glacial source area prior to main Himalayan orogenesis rule this out. Instead a debris- flow or volcanic-labar origin from Dasht-i-Nawar caldera 300-400km away in Afghanistan is most likely. The probable volume of 30x109m3 of the deposit is compatible with most probable pre-caldera volcano topography and post-eruption caldera size, but is still perhaps the world's largest known lahar.
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