DEPOSITIONAL HISTORY OF THE MESOPROTEROZOIC CHHATTISGARH BASIN, CENTRAL INDIA: CONSTRAINTS FROM NEW SHRIMP ZIRCON AGES

Chhattisgarh Basin, one of the so-called Purana basins in central India, contains unmetamorphosed and mostly undeformed sedimentary rocks that have, for years, been thought to be latest Mesoproterozoic to Neoproterozoic in age (~?1100-550 Ma). Earlier, we showed, on the basis of SHRIMP U-Pb analysis of magmatic zircons, that the Sukhda rhyolitic tuff, near the top of the succession, is about 1010 Ma, indicating that the upper part of the basin fill is Mesoproterozoic. Here we report the results of further SHRIMP analysis of zircons from a tuff near the base of the succession that indicate that the basin is almost entirely Mesoproterozoic. We also present data for detrital zircon suites in sandstones at several stratigraphic levels that shed light on provenance. Zircons from a tuff unit in the Singhora Group, near the base of the section in the western part of the basin, yielded ²⁰⁷Pb/²⁰⁶Pb ages of 2663+/-8, 2489+/-12, 1783+/-14, ca. 2050, ca. 2350, 2480, 1510+/-18, and 1405+/-9 Ma. Although most of the zircons are clearly detrital grains or xenocrysts, we conclude that the Singhora tuff is not older than 1405+/-9 Ma, the age of the youngest grain dated. Das et al. (2009) obtained an age of ~1500 Ma (EMPA; zircon and monazite) for the same tuff. SHRIMP U-Pb data, however, more concisely constrain the maximum age of the Singhora tuff. Thus, the basin mostly filled between ca. 1400 and 1000 Ma and preserves an important Mesoproterozoic sedimentary record, including platform and ramp carbonates. Detrital zircons from related clastic rocks occurring between these tuffs show restricted ²⁰⁷Pb/²⁰⁶Pb age ranges:

(1) Rehatikol Formation (arkose and conglomerate; Singhora Group) resting upon crystalline basement, yielded ages between 2177 and 2535 Ma with a major peak at 2473 Ma.

(2) Lohardih Formation, arkose resting on basement in the eastern part of the basin, yielded ages between 2422 and 2487 Ma, with a strong peak at 2462 Ma.

(3) Kansapathar sandstone, near the middle of the Chhattisgarh succession, yielded ages between 2463-2488 Ma, with a peak at 2484 Ma.

Ca. 2380-2480 Ma granites and ca. 2528-2636 Ma gneisses dominate the cratonic basement of the basin and are evidently the major provenance of the detrital zircons.