ARCHEAN (2.7 GA) GLACIOMARINE SEDIMENTATION, WESTERN DHARWAR CRATON, SOUTHERN INDIA

This study delineates a new glacial event at 2.7 Ga in the basal conglomerates (diamictites) of the Chitradurga Group, Dharwar Supergroup, on the Dharwar Craton of southern India. The key aspect of this study is the identification of abundant dropstones in a fine-grained, laminated matrix .

The main unit in this ongoing study is the Talya Conglomerate. A 543-m-thick sub-vertical measured section revealed 15 diamictites from 1 to 57 m thick, separated by sandstone and mudstone units from 2 to 87 m thick. These units in the lower portion of the section are dominantly sandstones, whereas those in the upper portion are dominantly mudstones, thus defining a generally fining-upward sequence. Walther’s Law indicates that the muddier portion was deposited on the older, sandier portion as the sea further inundated the landmass, perhaps due to a glacial retreat.

We interpret that the Talya was deposited in a marine basin, with the clasts (boulders to dominant cobbles and pebbles) being ice-rafted detritus (IRD) deposited from icebergs that were calving from tongues of melting tidewater glaciers, or from ice shelves. Whether the glaciers were mountain glaciers or part of an ice sheet is speculative. The clasts (dropstones) were dropped onto a seafloor covered with fine-grained, laminated sediment deposited by plumes and bottom currents. Rare faceted and striated clasts are present.

We interpret the lower member of the Kaldurga Conglomerate, located 50-75 km to the southwest of the Talya exposures as Talya equivalent, because the diamicites of both are dominated by quartzite IRD. The upper Kaldurga contains an abundance of granitic basement detritus; basement uplift and erosion may be the result of isostatic rebound caused by glacial melting or to tectonism related to westward subduction.

The glaciers were located west and southwest of the diamictites on a landmass that included the Bababudan Group, which consists dominantly of quartzites and mafic volcanics on a basement of granitic/gneissic rocks that was unroofed as erosion continued.

Because the diamictites are present in several schist belts (i.e., greenstone belts), they can be used as a regional correlation tool. They are present over an area of more than 10,000 km²; this area was much larger prior to isoclinal folding.