Sedimentology of plio-pleistocene late orogenic deposits associated with intraplate subduction-the Upper Siwalik Subgroup of a part of Panjab Sub-Himalaya, India

Abstract

This study provides a detailed sedimentologic documentation of the lithofacies, cyclicity, sandbody geometry, palaeoflow variability and depositional models of the youngest late orogenic deposits of the Siwalik molasse. These deposits represent one of the important examples of sedimentation in the zone of intraplate subduction of the Himalayan collision belt. Lithofacies analysis has resulted in the recognition of four conglomerate facies (massive, matrix-supported conglomerate; crudely stratified conglomerate; trough cross-stratified conglomerate; planar cross-stratified conglomerate), five sandstone facies (erosional scour and fill; pebbly sandstone; medium- to coarse-grained sandstone; fine- to medium-grained ripple drift laminated sandstone; bioturbated calcareous fine-grained sandstone), and six mudstone facies (parallel-laminated bioturbated mudstone; reddish brown massive mudstone; variegated mudstone; bluish-grey to violet mudstone; buff-coloured mudstone; pebbly mudstone). In addition, hard calcareous layers have been recognised as a separate facies state. The deduced cyclic sequences of the Tatrot and Pinjor Formations of the Upper Siwalik Subgroup show fining-upward characters from CS (coarse sandstone) to MS (mudstone) through FS (fine sandstone) and SS (alternations of sandstone and siltstone). The sandbodies of the Tatrot and Pinjor Formations reveal a sheet-like geometry and an internal dominance of inter-storey scour surfaces. The presence of intraformational debris, mud drapes, reactivation surfaces and a general fining upward of grain size indicates fluctuating hydrodynamic conditions with frequent channel abandonment and re-occupation, resulting in net vertical aggradation. The dominant style of sedimentation both in the channel and overbank facies is through vertical aggradation. Data on the palaeoflow variability indicates that the Upper Siwalik Subgroup can be subdivided into four and three sediment-vector sequences in the Khetpurali and Ghaggar sections, respectively. The palaeodrainage pattern in the Khetpurali section reveals a major influence of northerly drainage twice in the stratigraphic column. The high percentage of overbank deposits in individual cycles, the lack of vegetal debris, organic-poor palaeosol units, and abundant syngenetic CaCO₃ activity warrant comparison with the modern anastomosed deposits described from arid Central Australia. Some intervals of the sequences are comparable to the sublinear, subparallel drainages with local anastomosis of the modern piedmont zone of the Himalaya. The higher parts of the stratigraphic column indicate proximal braided stream deposits and finally proximal alluvial fan deposits. Syntectonic activity has controlled the deposition of the megasequences which are a response to episodicity in the activity of basin margin-basin margin or basin margin-intrabasin structures. The vertical stacking of the Siwalik megasequences appears to be episodic in character.