40Ar/39Ar multigrain dating of detrital white mica of sandstones of the Surma Group in the Sylhet Trough, Bengal Basin, Bangladesh

✍ Scribed by M.Julleh Jalalur Rahman; P. Faupl

• Publisher: Elsevier Science
• Year: 2003
• Tongue: English
• Weight: 292 KB
• Volume: 155
• Category: Article
• ISSN: 0037-0738
• DOI: 10.1016/s0037-0738(02)00188-4

✦ Synopsis

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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.