Depositional environment of polymictic conglomerate of the Gadag greenstone Belt, Western Dharwar Craton, south India: An insight for Neoarchean marginal sedimentation

Abstract

Clast morphometry and sedimentary facies analysis of polymictic conglomerate of the Hiriyur Formation in the Gadag greenstone Belt, Western Dharwar Craton, India, was conducted to constrain the palaeohydraulic condition, depositional environment, and type of sedimentary basin. The conglomerate grades from clast‐supported to matrix‐supported with siltstone‐sandstone intercalations. Eight lithofacies along with five major lithofacies associations are recognized in the Majjur and Attikatti Domains of the Gadag greenstone Belt. The lithofacies architecture suggests sedimentation commenced with rapid deposition of slope‐derived debris in a proximal high‐energy alluvial fan setting followed by a braided river environment. Clast morphometry and palaeohydraulic data indicate a low palaeoslope (0.000024 m/m), and stream discharge value (1.913 m3/s) calculated from the channel fill conglomerates. Greywacke overlying the conglomerate has been interpreted as turbidites from a continental slope. The accompanying 40‐m‐thick banded iron formation and carbonate rocks also support a marine context. The lack of transitional deposits between river and deep marine settings suggests that the greywacke turbidites sequence is separated from the conglomerate deposits by substantial unconformities. This change in depositional setting could be due to a change in basin subsidence rate, tectonic rejuvenation, or major sea‐level changes.

Provenance of the conglomerate and siliciclastic rocks from the Gadag Greenstone Belt, Western Dharwar Craton, India: Implications for understanding Neoarchean basin margin sedimentation

“…In the measured section, the proportion of different clast compositions among the pebble‐ to boulder‐size clasts varies from place to place. It was observed that 30–50 volume percentages are of granitoid clasts (granite, granodiorite, TTG), 20–45 volume percentages of volcanic rocks (basalt, andesite, quartz porphyry), and 10–25 volume percentage of recycled sedimentary rocks (quartzite, BIF, argillite) (Table 1; and for details, refer Pratihari et al, 2021). Clasts are generally oblate to prolate in shape, well‐rounded to angular, with high sphericity (Figure 4c).…”

“…Conglomerates from both the Attikatti and Majjur domains are polymictic and vary from clast‐ to matrix‐supported (Figure 4a,b). The matrix‐supported conglomerate constitutes 30%–34% of the total sediment succession, while the clast‐supported conglomerate constitutes 22%–27% (Pratihari et al, 2021). The matrix‐supported conglomerate grade from pebble poor to pebble rich (nearly in rhythmic manner; Figure 4b).…”

“…The index of packing of the polymictic conglomerate was studied by counting clast to clast contacts. The clast‐supported conglomerate shows clasts having point, tangential and body contacts, which indicate that they were unaffected by post‐depositional modification, such as pressure solution (Pratihari et al, 2021). In contrast, the matrix‐supported conglomerate showed floating and concavo‐convex type contacts, which implies a solution effect during deposition.…”

“…In the study area, the PGC constitutes the basement while the Bababudan Group is missing possibly due to deep erosional incision (Krapež et al, 2020). Although the detailed stratigraphic makeup of the GGB remains uncertain because of thrust‐tectonic duplication, the presence of stromatolite‐bearing carbonate rocks, the BIF and conglomerate units containing carbonate clasts (Chadwick et al, 2003; Pratihari et al, 2021) speak for the presence of the Vanivilas Formation in this belt. The generalized supracrustal succession commences with basalt of the Ingaldhal Formation (Ugarkar et al, 2015) and 2,588 ± 10, 2,576 ± 20 Ma felsic volcanic rocks (U–Pb zircon age: Corfu & Hegde, 2020; Jayananda et al, 2013; Sarma et al, 2011).…”

“…The duplex with plunging fold and reverse strike‐slip faults and deformation was possibly caused by compressive tectonic forces in an active subduction environment (Krapež et al, 2020; Ugarkar et al, 2015). Based on available age data (see Supplementary Data; Table 1) and exposed litho‐strata in each domain, a generalized comparable lithostratigraphic section (Figure 2) is proposed, showing a complex geological history and the absence of marker horizons (Pratihari et al, 2021; Ram Mohan et al, 2014) to compare these domains.…”