Provenance and Basin Tectonics of Oligocene-miocene Sequences of the Bengal Basin, Bangladesh

Abstract

Oligocene and Miocene strata of the Bengal basin, Bangladesh, show significant changes in sediment composition and record sediment-input history from different sources adjacent to the basin. Oligocene Barail sands (Qt70F2L28) are quartzolithic with rare to no feldspars. Dominance of quartz, presence of some lithic fragments and paucity of feldspar suggest erosion from a low-relief area under strong chemical weathering. The source area could have been the Indian craton. Minor contribution from the Indo-Burman ranges can not also be ruled out. Miocene Surma sandstones (Qt59F23L18) are quartzolithic to quartzofeldspathic and have comparatively high feldspar and lithic contents. This change may represent the initiation of uplift and erosion of the Himalayas.

Oligocene heavy minerals are dominated by opaque and ultra-stable minerals, whereas the Miocene sediments show fewer opaque grains and contain garnets. Textural and mineralogical maturity of the heavy minerals along with the presence of chrome spinel in the Oligocene sediments suggests a relatively short transport distance and a proximal source, which could be the Rajmahal Trap (located adjacent to the northwestern part of the basin). However, presence of epidote, garnet, and aluminum-silicate minerals (sillimanite) in Lower Miocene units indicate the possibility of input from an orogenic source from the eastern Himalayas and the Indo-Burman ranges. Detrital garnets in Miocene sediments are mostly almandine-rich with varied pyrope and low grossular content. These garnets indicate provenance from medium- to low-grade regionally metamorphosed rocks of the Higher Himalayas and high-grade metamorphic rocks of the Indian craton. Chrome spinel data of Oligocene sediments show that the spinels have high chromium content (average 42%) and wide range in TiO2 wt% (0.3 to 3%). These spinels might have been derived from the Alpine-type ophiolites in the Indo-Burman ranges and the Rajmahal Trap. Tourmaline and calcic amphibole data from the Bengal basin also indicate a dominantly low-grade and metasedimentary provenance with minor contribution from granitoid and pegmatitic rocks.

High sedimentation rates, extremely low permeability of confining units, and very high compressibility are the main reasons for overpressure in Miocene strata at three of the studied structures (Titas, Bakhrabar, and Rashidpur). However, a combination of factors, including strong horizontal deviatoric regional stress from the advancing Indo-Burman ranges and uplift produced by fault propagation folding and concomitant shale diapirism are responsible for overpressure in the Sitakund structure of eastern fold belt. Absence of overpressure in the Oligocene strata in the Bengal basin indicates apparent lack of any significant orogeny in the east.

The transition from the Oligocene to Miocene strata records changes in sediment composition and input from additional sources to the basin depocenters. The basin received sediments mostly from the Indian craton during the Oligocene time with minor input from the Rajmahal Trap to the west and distant orogenic belts to the east. At the beginning of the Miocene time, the Bengal basin moved proximal to the encroaching Himalayan and the Indo-Burman orogenic belts, and started receiving sediments from the north and the east.