Responses of non-marine deposits to fluctuating base level and their implications in the Precambrian sequence building: Late Neoproterozoic Girbhakar Sandstone, Rajasthan, India

Abstract

The distinctive sedimentation patterns of the late Neoproterozoic Girbhakar Sandstone were investigated within the context of a Precambrian sedimentation system devoid of vegetation. This comprehensive study utilized an integrated approach, incorporating facies analysis, fluvial architectural elements analysis, and palaeocurrent analysis, to offer detailed insights into the controlling factors of sedimentology and depositional settings within the studied interval. Process-based facies analysis revealed thirteen distinct facies types organized into five associations, uncovering a diverse range of depositional palaeoenvironments — from alluvial fan and braided fluvial to transitional marine settings. An alluvial fan, dominated by debris flow and multiple ephemeral channels at the basin margin, emerged initially. The stratigraphic architecture of this alluvial fan and the associated ephemeral fluvial system was primarily influenced by water table fluctuations in arid to semiarid climatic conditions. A transition to a semi-perennial to perennial braided fluvial system occurred as the depositional slope decreased. However, the stratigraphic architecture of these fluvial systems exhibited distinctive variations in response to fluctuating base level rise. Notably, the basal segment, characterized by an unconformity below and a granular lag succeeded by wave-agitated sandstone above, represented the lowstand systems tract (LST). During the early LST, channel belts prograded at the northern distal end of upstream aggradational semi-perennial braided systems in response to the transgression of Girbhakar Sea from the north, aligning with the late Neoproterozoic Indian plate configuration. The coarsening upward deposits suggest a potential classification as a braid-delta, formed by a gradual rise in base level, low epeiric gradient, and episodic flashy discharges, which facilitate the downstream development of this deposit. During the late LST, the evolution of perennial fluvial systems resulted in braid-plain deposits throughout the study area, with a transitional marine unit at the downstream site. An accelerated rate of base level rise facilitated marine reworking, leading to the deposition of transitional marine sediments. The final phase of base level rise marked the cessation of terrestrial deposition, culminating in the submergence of the site underneath the sea. The study highlights the unique responses of alluvial sediments to diverse controlling factors along the deposition downslope. This enhances insights into Neoproterozoic alluvial sedimentation dynamics and the resulting stratigraphic architecture in time and space from a broad perspective.