2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 12
Presentation Time: 11:00 AM


MAZUMDER, Rajat, Department of Geology, Asutosh College, Kolkata, 700 026, India and ABBOTT, Dallas H., Lamont Doherty Earth Observatory of Columbia Univ, Oceanography Building, Rm 103A, 1000 Route 9W, Palisades, NY 10964, rajatmazumder2001@yahoo.com

The 6-8 km thick late Paleoproterozoic Chaibasa Formation, eastern India is the lower part of the Singhbhum Group, comprising rocks deformed and metamorphosed to greenschist (locally amphibolite) facies. It is characterized by inter-banding of sandstones, shales, and a heterolithic (very fine-grained sandstone/siltstone-mudstone) facies. The Chaibasa sandstone formed in a subtidal setting and the heterolithic and shale facies formed in an earthquake-prone distal offshore setting above and below the storm wave base respectively. The sediment gravity flow deposits including seismites are abundant in all the facies, but with increasing frequency within the finer facies. The Singhbhum Group of rocks overlies 2100 Ma old terrestrial (alluvial fan-fluvial) Dhanjori Formation including a Continental Flood Basalt and is overlain by 1600 Ma old mantle plume related mafic-ultramafic-felsic volcanic and volcaniclastic rocks. It has been recently proposed that mantle plumes were strengthened by large extraterrestrial impact events. Additionally, it has been speculated that impact ejecta would have been more common in Precambrian deep-sea sediments. If this is true, the ideal part of geological time to test this hypothesis is within the Precambrian volcano-sedimentary succession. To test this hypothesis, we have selected deep-sea Chaibasa shales and sandstones and underlying Dhanjori volcanic, volcaniclastic and clastic sedimentary rocks and performed magnetic susceptibility measurements. The very high magnetic susceptibility values of the Chaibasa shale samples collected from different stratigraphic levels could be indicative of extraterrestrial impact during late Paleoproterozoic time. The shales have high magnetite contents. Most of the magnetite recrystallized into octahedra, but some grains are flat. The flat grains resemble the platy magnetite found in impact layers. We also see well preserved, elongated feldspar crystals that resemble the feldspar crystals found as part of impact-generated microkrystites. These may represent microkrystites that disaggregated into masses of single feldspar crystals during intense current activity. We will compare the flat magnetite grains and well-preserved feldspar crystals to Phanerozoic examples that are from known impact layers.