Age and implications of the phosphatic Birmania Formation, Rajasthan, India

Abstract

The Birmania inlier in western Rajasthan, India, contains important phosphate deposits, the depositional age of which is poorly constrained. Here we provide the first direct age constraints for the phosphate-bearing Birmania Formation using a combination of paleontological and detrital zircon U–Pb data. The occurrence of the multicellular algal fossil Wengania exquisita in phosphatic chert of the Birmania Formation suggest that it was deposited during the Ediacaran. Detrital zircon age distributions contain prominent populations of 1.7–1.9Ga grains, with subordinate younger grains that range from 650 to 980Ma. The distribution broadly resembles those from Neoproterozoic strata from both cratonic and Himalayan India but, like zircon age distributions from the Marwar Group, lacks 1.0–1.2Ga grains, which may suggest that both areas shared similar local sources. The lack of zircon grains younger than ∼650Ma is consistent with an Ediacaran depositional age because almost all Cambrian or younger strata from India have yielded Cambrian or latest Neoproterozoic age grains. These findings raise the possibility of a previously unrecognized late Neoproterozoic episode of phosphogenesis on the India craton. The presence of Wengania exquisita further supports strong palaeogeographic affinity between the India and South China during the Neoproterozoic and Cambrian.