Mathematics – Logic
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v42f..02k&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V42F-02
Mathematics
Logic
1035 Geochronology, 5480 Volcanism (8450), 8105 Continental Margins And Sedimentary Basins, 8450 Planetary Volcanism (5480), 9320 Asia
Scientific paper
The Rajahmundry Traps of eastern peninsular India, often considered to be outliers of the Deccan Traps, occupy ~35 km2 centered on the Krishna-Godavari Basin and extending offshore in the sub-surface. Onshore exposures average 60m in thickness, including a laterally continuous sedimentary interlayer of laterite, limestone and shale ( ~2m thick, total) separating `upper' flows from `lower' flows. 40Ar/39Ar CO2 laser incremental heating analysis of twelve plagioclase separates from Rajahmundry Trap basalts reveal an age of ~64.6 Ma for the entire sequence based on the FCs standard at 28.02 Ma. Flows chosen for dating include 8 sites spanning both the `upper' and `lower' flow sequences. Paleontological studies of sediments adjacent to the basalt at depth in Krishna-Godavari Basin, e.g. Jaiprakash et al. (1993), suggest that the period of time covered by the two flows and intertrappean sediments is up to ca. 6 myr. Dates obtained for this study, however, show that ages for both upper and lower flows are indistinguishable within 2σ error from one another, and span ~2 Ma at most, pointing to a substantial hiatus in the sedimentary record at the top of the upper basalt flows. Extremely high Ca/K ratios (up to ~400) in several samples limits precision due to error propagation attending the large correction necessary for reactor produced 36Ar from Ca. However, plateau ages as precise as 64.8 +/- 0.4 and 65.5 +/- 0.8 from above and below (respectively, 2σ errors) the sedimentary interlayer have been obtained. Samples with both high and low Ca/K ratios confirm rapid eruption of the entire Rajahmundry Trap sequence. A petrogenetic link between these basalts and the Deccan Trap basalts (the remains of which lie over 300 km from the nearest exposure of Rajahmundry Trap) has been suggested but has yet to be substantiated. These new data clearly place the eruption of the Rajahmundry Traps temporally close to the K-T boundary, coincident with late stage Deccan volcanism and within ca. 1 Ma of initiation of main stage Deccan volcanism in e.g. the Western Ghats. Paleomagnetic studies of Vandamme and Courtillot (1992) revealed a reverse to normal magnetization polarity sequence within the Rajahmundry Traps, supporting a temporal connection with waning of Deccan Trap volcanism, consistent with our results. This now unequivocal temporal connection between the Rajahmundry Traps and Deccan Traps may suggest a much larger area of melt generation beneath the Indian sub continent than previously estimated, or may be the result of extensive overland flow distances (e.g. Baksi, et al., 1994). The latter possibility would require significant paleovalleys to be maintained even after serving as conduits for, and presumably being filled by several individual flows, evidence of which is lacking. Recent reconstructions modeling the evolution of the Krishna-Godavari Basin and catchments clearly show that then, as now, this basin drains much of the Indian Peninsula, including the main Deccan Traps at their time of eruption (Halkett, et al., 2001). Overland flow of Deccan Trap volcanism thus remains a plausible origin for the Rajahmundry Traps.
Halkett Angus
Knight Kim B.
Renne Paul R.
White Neil
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