Palaeogeography, palaeohydraulics and palaeoclimate of the Mio–Pliocene Siwalik Group, eastern India

Abstract

South–southwestward palaeocurrent swerved to east–southeast and then broadly to southeast over the transition from alluvial fan to axial channel and then to the flood plain in the Mio–Pliocene foreland system within which the Siwalik Group depositd in Darjeeling-Jalpaiguri Districts, eastern India. Palaeocurrent pattern is found to be multi-modal on the fans, virtually unimodal on the axial channel zone and again multi-modal, more profoundly, on the flood plain. Coarse siliciclastic mass-flows were progressively eliminated and gave way to predominant bed-load transport downfan and the axial river, and then to suspension-load dominance in fine siliciclastics on the flood plain. Distal flood plain lacustrine sediment included most of the coals and the entire bulk of the dolomitic limestone. Further resolution in palaeogeography within the frame of aforementioned foursome facies associations is elicited in twenty-six distinctive facies altogether. Critical evaluation of chemical indices (CIA, CIW, ICV, PIA, as well as Rb/Sr ratio) for weathering and depleted δ18O values indicate a high precipitation rate. The contention is further corroborated by the high discharge rate calculated from cross-set thicknesses within the main channel deposits. Reconciliation of various relevant data sets collected or calculated from all known worksites along the entire 2000 km-long exposure belt of the Siwaliks along the Himalayan foothills reveal confluence of two tributaries, one from the west and the other from the east, close to the present study area before escaping onto the Indian plains. Channel parameters, channel-belt width and discharge thus attained maxima in the present study area. The precipitation rate and temperature increased eastward overall as a prelude to the modern trend in this regard. 13C enrichment indicates that the transition from C3 to C4 vegetation had already set in.

Keywords

• Mio–Pliocene Siwalik Group
• Darjeeling Himalayas
• spatial variation
• axial river reconstruction
• vegetation