Deccan Bole Beds: Mineral magnetism, interpretation and genesis

Deccan volcanism is one of the most celebrated continental flood volcanism and has been widely studied for its time constrains, duration and K-T mass extinction. Bole beds in Deccan volcanism delineates the hiatus in successive lava flows and are spectator of the extensive volcanism. Bole beds (composed of clays and volcanic ashes) are considered as the weathering products of the older neighboring lava flows and are trapped by younger lava flow. Field occurrences shows common red color for bole beds but occurs in a wide range from brown to green colors. Different color bole beds of DVP have been studied for their Field characteristics, Magnetic susceptibility (MS), Anhysteretic remanent magnetization (ARM), Isothermal remanent magnetization (IRM) and Temperature-susceptibility (k-t) variations. A unique combination of high magnetic susceptibility (χlf), frequency dependent susceptibility (χfd %) and remanence coercivities [B0(CR)] in the red boles mark the significant presence of the finer/SP antiferromagnetic fraction explained here as oxidative baking product of the weathered basaltic sediment enriched in clay. The ultrafine hematite producing red coloration is characteristically of re-deposited pigmentary nature. The high reversibility k-t relation amongst red boles further substantiates their pre-heated nature up to ~ 700 degree Celsius and no significant addition/alteration of iron oxides after re-deposition. The [B0(CR)] for brown boles fall in the range of hard ferrimagnets (40 to 50 mT) and that for the green boles typically show softer or multi-domain (MD) nature (majority of <15mT with rare in the range of 30 to 40 mT). The presence of harder ferrimagnets [single-domain (SD) magnetites] in the brown boles proposes the authigenic (lacustrine) conditions in support of field characters. The green boles representing detrital ferrimagnets (larger/soft MD particles) is in accordance with their field occurrences of infilling, infiltration or detrital nature of deposition. The present study conducted over a variety of field occurrences within the Deccan Volcanic Province infer predominance of detrital processes governed by laterally migrating shallow channels and surface water run-off due to precipitation. Infiltration into fragmentary tops of the basalt bedrocks with baked/unbaked (red/brown bole) sediments; and the infilling into vugs and interstices is characteristics of green boles representing the less oxidative preservations. This resulted into a variety of depositional environments from ephemeral stream to floodplain ponding condtions. The study does not favour any significant pedogenesis or paleo-lateratization process representing a significant interval of non-deposition as proposed by previous work.