Seismic stratigraphy and the sedimentation history in the Laxmi Basin of the eastern Arabian Sea:Constraints from IODP Expedition 355

Detailed interpretation of seismic stratigraphic sequences in the Laxmi Basin of the eastern Arabian Sea are presented in this study using closely spaced high resolution multi-channel seismic(MCS)data.Our stratigraphic interpretation is further corroborated using recent drilling results in the Laxmi Basin,derived from the long sediment cores collected during International Ocean Discovery Program(IODP)Expedition 355.Integrated core-log interpretation discussed in the present study,offer important insights about the lithostratigraphic variations in this region.Analyses of multi-channel seismic reflection data reveal five depositional sequences(ranging from Paleocene to Recent)that led to the development of this marginal basin since the Cenozoic period.Regional igneous basement is successfully imaged,which was also validated by deep sea coring during the IODP Expedition 355.In the present study,we primarily focus on the post-rift sedimentation in the Laxmi Basin and its possible mechanisms.Our detailed interpretation in the prevailing tectonic framework of the basin suggests that near-shelf oldest volcaniclastic sedimentation immediately overlying the acoustic basement is linked to the onset of India-Madagascar and India-Seychelles rifting activities during the Late Cretaceous period.Eventually,during the Early-Mid to Late Miocene,the basin received maximum sedimentation dominantly through an extensive mass transport mechanism implying possible large-scale deformation on the Indian shelf.Subsequent sediment input to the basin appears to have been fed variably via the Indus Fan as well as coastal discharge from the Indian mainland.The total sediment thickness in the Laxmi Basin ranges from 1.1 to 3.5 km.New stratigraphic information and sediment isopach maps presented here provide vital information about syn-and post-rift sedimentation pattern in the region and their long term tectonic implications.

Geochemistry of core sediments from the southeastern Bay of Bengal:Inferences on weathering and early diagenetic changes

A sediment core(ABP24/05),collected at a water depth of 3520 m from the southeastern Bay of Bengal was studied to determine the change in chemical weathering during the last glacial to deglacial periods and the factors of sedimentary environment which controlled earliest diagenetic changes in the sediment after its deposition.High ratios of K/Rb,Ti/Al and Zr/Rb during~45 to~18 cal kyr B.P.in the core sediments may be attributed to the stronger physical erosion and turbidity currents activity during this period.This might have brought a higher quantity of unaltered minerals to the study area.Low ratios of K/Rb,Zr/Rb,and Ti/Al and increase of SiO_(2)/TiO_(2),Rb/Al and Cs/Al from~18 cal kyr B.P.to present may be indicating an increase in the rate of chemical weathering during this period.The time of increased chemical weathering in the study area is consistent with deglaciation warming in the tropical Indian Ocean and strengthening of river runoff into the Andaman Sea.Climate change during the interglacial period by increased solar insolation thereby strengthened the summer monsoon which might have led to intensified chemical weathering in the source region since~18 cal kyr B.P.The low organic carbon(OC),high Mn/Al,Fe/Al and the Mn-oxides minerals precipitation indicate prevailing of oxic conditions during~11 cal kyr B.P.in the core sediments,which is contradictory to suboxic conditions developed in the deep ocean sediments in the western Bay of Bengal and the equatorial Indian Ocean.The low terrigenous influx and export of less OC to the bottom sediments might have created a favorable condition for the formation of Mn-oxides in the study area during Holocene.