SEDIMENTATION MODEL FOR THE QUATERNARY DEPOSITS OF SIMLA HILLS, NORTHWESTERN HIMALAYA, INDIA

Quaternary deposits of Simla hills are represented by the youngest horizon of the Upper Tretiary sequence-the Siwalik Group(Middle Miocene to Pleistocene) named as Upper Siwalik Boulder Conglomerate. During this period,the newly up heaved Lower Tertiaries in Simla were drained by the southerly flowing rivers of Koshalia, Ghaggar and Markanda which transported the materials from the positive region and debouched at the foot of the mountain.This resulted into the deposition of the Upper Siwalik Boulder Conglomerate as alluvial fans in the foreland basin carved out to the South of the Himalaya. Later, the adjacent fans coalesced together to constitute a thick, extensive, more or less continuous sedimentation unit.

The stratigraphic columns have been prepared for 20 different localities selected along the rivers Koshalia (5), Ghaggar (8) and Markanda (7). The various sedimentation units identified in these sections have been subjected to textural and palaeocurrent analyses. Alluvial fans of Upper Siwalik Boulder Conglomerate exposed in the Simla Hills exhibit varying lithology and texture, both vertically as well as laterally, indicating thereby more than one mode of deposition. Two primary modes of sedimentation characteristic of alluvial fan environment have been recognized in these sections. The onset of conglomerate deposition started with ‘Water-laid sediments’ followed by upward changes into ‘Debris-flow deposits’.

The basal part of most of the conglomerate horizons consists of sheets of sand, silt and gravel. The conglomerate horizons of varying thickness are interbedded with pale-brown beds and lenses of clay/silt/sand. These indicate a ‘water laid’ deposition by a network of braided distributary channels. The upper part of the stratigraphic columns of the Upper Siwalik Boulder Conglomerate comprises poorly sorted, pebble to boulder-sized fragments (some of them measuring more than 256 mm) with lesser amount of clay/silt lenses. Such deposits characterize proximal alluvial fan conditions. The shift in the palaeocurrent direction from southerly to southwesterly is attributed to the Neotectonic activity at the foot of the Himalayan chain.