Abstract
The Sylhet Trough, a subbasin of the Bengal Basin in northeastern Bangladesh, is bordered by two orogens, the Himalaya to the north and the Indo–Burman Ranges to the east. Detrital white micas from six sandstone samples of the Neogene subsurface Surma Group were analysed by the 40Ar/ 39Ar method. The ages of the individual steps in the Ar/Ar release diagrams range from 25 up to 92 Ma. These ages represent cooling ages rather than formation ages because they originate from large mica flakes. The low-temperature step Ar/Ar ages of detrital white mica, with plateau-type character, are diagnostic for the youngest cooling events and cluster between 25 and 36 Ma, except one age of 51 Ma. The medium- and high-temperature steps of the Ar-release diagrams are characterized by staircase-type pattern. These various age discordances are interpreted as a mixture of micas with different ages. The 40Ar/ 39Ar results point to heterogeneous ages of the source region, which seems to be typical of the early stages of a rapidly uplifting collision orogen. The narrow age range of the low-temperature steps can be correlated with early cooling ages known from the High Himalayan Crystalline basement. The Ar/Ar data indicate that the major sediment input in the Sylhet Trough during sedimentation of the Surma Group has been derived from the rising Himalaya orogen.
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