In mid-Feburary/early March 2020, I was lucky enough to visit the Singhbhum Craton in north-western India as part of an excursion by the DFG-funded SPP1833 program. In comparison to other Archean cratons such as the Pilbara, Western Australia and Kaapvaal, South Africa, it is much less studied. Little did we know that this would be the last international trip for some time.
The Singhbhum Craton in northeastern India is one of the four major Archean cratons that comprise the Indian Shield. It is one of the few Precambrian terranes that record periods of sedimentation from the Paleoarchean to Neoproterozoic. Our field trip started in Kolkata, West Bengal and first headed due west to Jamshedpur. The southernmost point of the trip was the Daitari Greenstone Belt.
Day 1: Arriving in Kolkata
It would be a disappointment to arrive in India for the first time and not experience the culture shock. Landing in Kolkata at 06:30 AM was suitably manic. Firstly, it was not possible to get Indian Rupees in Germany and, as it turned out, not possible at Kolkata airport either! The morning rush hour in Kolkata is also pretty hectic.
Day 2: Kolkata to Jamshedpud
Day 2 was largely driving. Although we only drove around 300 km from Kolkata to Jamshedpur, there is no Autobahn in India. Even the main motorway is filled with cows, people, goats and pretty much anything else you wouldn’t expect to find on a motorway.
Day 3: Jamshedpud and around
The geology begin in ernest on Day 3 with a visit to a mafic dike formed within the Singhbhum granite. Igneous dikes typically form when magma rises upwards into fractures or fissures present in the overlying rock. This implies that the dike is always younger than the host rock age of the basement rocks; in this case, U-Pb dating of zircon minerals from the granites suggest an age of approximately 3.3 Ga, but the age of the dike is not known. The scale of dikes range from centimetres to kilometres; in this case, a few 100 m.
Day 4: Tua Dungri
The highlight of day 4 was the rare komatiitic rocks exposed at Tua Dungri. Komatiitics are ultramafic ancient lavas that are unusually rich in magnesium and are rare, but commonly found in Archean terranes. Their formation is thought to have been associated with the higher temperatures in the early Archean mantle. Modern basaltic eruptions, e.g. those on Hawaii, typically erupt at around 12oo °C, but komatiitic eruptions are thought to have been in excess of 1600 °C.
Day 5: Dhalbhumgarh
Day 5 took us to an abandaoned WW2 air base. The aeroplanes were long since gone, but the excellent outcrop of the Chaibasa Formation remains. This outcrop comprised thinly-bedded layers of sandstone and siltstone interpreted to have been deposited on a shallow marine shelf. The formation appears to have been deposited rapidly with ample sediment supply. Such rapid burial likely assisted in preserving the remarkable sedimentary features.
It has been speculated that some of the features in the Chaibasa Formation represent microbial structures, i.e. stromatolites. An alternative explanation is that these structures are penecontemporaneous folds, which are formed by inorganic processes through the deformation of individual cross-beds. These features are generally localised, e.g. a single bed, and may be found adajcent to undeformed beds. Examples include:
- Load structures: irregular bulges at the interface between sand and mud layers caused by gravity
- Ball-and-pillow structures: isolated, small round structures caused by physical movements (e.g. seismic activity) of unconsolidated sediments
- Convolute bedding
- Slump structures
Considering the abundant evidence for soft-sediment deformation processes in the Chaibasa Formation, the proposed stromatolite structures (image above, right) may just be examples of convolute bedding. This occurs frequently in deltaic environments whereby the sediment is water-saturated and easily mobilised. The mud-rich shale layers deform easily but the sandstone layers show competence to form stromatolitic structures. Such a process may have been initiated by minor seismic activiity.
Day 6: Dimna Lake Section
The Dhalbhum Formation outcrops at the shoreline of Dimna Lake. The mean of the youngest zircon grains is ~1.8 Ma corresponding to the mid-Paleoproterozoic. Soft layers of volcanic ash are interbedded with sandstone and shale layers. The sequence is around 50 m thick and beds dip steeply (80 deg). A short walk from the lake brings you to the pillow basalts of the Dalma Formation, suggesting that this area experienced extensive volcanic activity in the Paleoproterozoic.
End of part one. Part two in progress.