Laser ⁴⁰Ar/³⁹Ar age constraints on Miocene sequences from the Bengal basin: Implications for middle Miocene denudation of the eastern Himalayas

Abstract

[1] Petrographic, mineral-chemistry and subsurface studies reveal that orogenic sedimentation had already begun in the Bengal basin by the early Miocene. Laser 40 Ar/ 39 Ar age determinations were made for detrital muscovite grains (145 total, among 4 samples) from the lower-to-middle Miocene Bhuban Formation. The laser fusion ages range from circa 12 Ma to 516 Ma, and thus suggest derivation from a combination of sources: the Himalayas, Indo-Burman ranges and possibly the Indian shield and Tibetan plateau. Modes of circa 16 Ma, 18 Ma, 26 and 40 Ma in the age distributions of these samples are most consistent with unroofing of the Higher Himalayas since the early Miocene. Detrital micas of such an early age (16 Ma) for the Bhuban Formation are interpreted to indicate that little time elapsed between the isotopic closure of 40 Ar in the muscovite and its ultimate deposition in middle Miocene strata. The detrital ages of circa 16 and 22 Ma in this study, most prominent in the highest stratigraphic levels sampled in this study, are younger than those previously reported in the western Himalayan foreland basins. These younger detrital ages are consistent with rapid middle Miocene unroofing and erosion as has been proposed for crystalline rocks of the eastern Himalayas. The minimum 40 Ar/ 39 Ar ages for muscovite in a particular sample seem proportional to the stratigraphic level sampled, i.e., younger ages tend to occur for samples of higher stratigraphic level. These results support earlier studies indicating that detrital geochronology can be used as an effective tool in evaluating stratigraphic ages.

The Brahmaputra tale of tectonics and erosion: Early Miocene river capture in the Eastern Himalaya

“…The lower boundary of MS1 is constrained at <18 Ma by the occurrence of detrital white mica in uppermost Barail and lowermost Bhuban samples dated by Ar-Ar at 21 ± 3 (Najman et al, 2008) and 14 ± 4 Ma (Bracciali, unpublished data), respectively. This is in agreement with prominent modes of Ar-Ar white mica dates of 16-18 Ma in Bhuban samples from well penetrations (Uddin et al, 2010), in which the youngest mica grains in three Bhuban samples of increasing stratigraphic depth are dated at 12.3 ± 0.6 Ma (upper Bhuban, well Beani-Bazar-1X; cf. well stratigraphy in Reimann, 1993), 16.8 ± 0.6 Ma and 19.3 ± 0.8 (lower Bhuban, well Fenchuganj-2; cf.…”

“…Johnson and Nur Alam, 1991;Uddin and Lundberg, 1998;Uddin et al, 2010), there is considerable debate as to the degree of input from the Indo-Burman Ranges and Shillong Plateau, the timing of river capture when material from north of the suture zone first entered the basin, and the time when the Brahmaputra diverted west of the Shillong Plateau (Johnson and Nur Alam, 1991;Brookfield, 1998;Zeitler et al, 2001a;Clark et al, 2004;Cina et al, 2009;Uddin et al, 2010;Chirouze et al, 2012a).…”

“…Surma Basin schematic stratigraphy and detrital U-Pb data from this study. The stratigraphic boundaries are constrained with variable degree of confidence as detailed in main text by biostratigraphy (Lietz and Kabir, 1982;Najman et al, 2012; NN: nannoplankton zones), magnetostratigraphy (Worm et al, 1998) and detrital mineral dates (Najman et al, 2008;Uddin et al, 2010). MS: Megasequence.…”

The record of Himalayan erosion preserved in the sedimentary rocks of the Hatia Trough of the Bengal Basin and the Chittagong Hill Tracts, Bangladesh

“…Our Bengal Basin data show consistency in results with Ar‐Ar ages derived from bulk mica separated from the Surma Basin (Sylhet Trough) of coeval age, attributed to derivation from the Higher Himalaya (Rahman & Faupl, ) and also with data from the Bhuban Formation at various locations in Bangladesh analysed by Uddin et al . (), although the uncommon pre‐Cretaceous grains they record are not represented in our samples.…”

“…A Shillong Plateau source has also been proposed for the Pliocene sediments (Johnson & Nur Alam, ). Previous provenance approaches have included determination of input directions using lithofacies maps (Uddin & Lundberg, ), isopach maps constructed from well data (Uddin & Lundberg, ) and petrography, heavy mineral analysis and Ar‐Ar dating of micas (Uddin & Lundberg, , b; Rahman & Faupl, ; Uddin et al ., ). In this study, as detailed below, we combine seismic data with various isotopic techniques to achieve an integrated provenance study.…”