Environmental controls on sedimentation, habitat development, and implications for palaeobiological reconstruction in the Middle Siwalik Group, Southern Indus Basin

Abstract

The Middle Siwalik Group, located near Sehwan Town in the Northern Laki Range of the Southern Indus Basin, Sindh, Pakistan, extends for a stratigraphic thickness of approximately 1055 meters at the Kari Buthi Section (KBS). This study investigates the environmental controls on sedimentation and their role in the development of habitats within the region. The lithology of the area is composed of a range of sedimentary rocks, including Sandstone, Conglomerate, Conglomeratic Sandstone, Shale, Clay, and Mudstone. The facies analysis revealed six primary depositional facies: Conglomerate and Conglomeratic Sandstone (GT), Fine to Coarse-Grained Trough Cross-Bedded Sandstone (St), Fine to Coarse-Grained Flat-Bedded Sandstone (Sh), Shale (Fm), Mudstone (Mf), and Clay (Cf), each of which reflects distinct environmental and biological conditions during sediment deposition. Grain size distribution analysis, based on sieve data from seven representative loose sandstone samples, shows a mixture of fine to medium grains, with occasional very coarse grains. The sub-angular to sub-rounded grain shapes suggest a low-energy environment of deposition, characteristic of a braided fluvial system. Petrographic analysis conducted using a LEICA 2500p Transmitted Light Polarizing Microscope identified quartz (50-60%), feldspar (15-16%), and rock fragments (5%) as the primary constituents, with minor muscovite and biotite. This mineral composition, along with the sedimentary characteristics, indicates proximity to the sediment source, providing further evidence for the presence of a braided fluvial system. From a biological perspective, the sedimentary environment likely facilitated the development of various habitats for early biota, especially within the fine-grained deposits of mudstones and shales. These depositional settings would have provided potential substrates for microbial life and early forms of aquatic organisms, contributing to the overall habitat development within this ancient fluvial system. This study emphasizes the significant role of environmental factors-sediment supply, water energy, and biological influences in shaping both the sedimentary architecture and the habitat conditions of the Middle Siwalik Group. Understanding these interactions enhances our ability to reconstruct past ecosystems and the biological processes that governed sedimentation and habitat formation in the Southern Indus Basin.