AGE OF BOUSE FORMATION ESTIMATED BY AUTHIGENIC 10BE/9BE DATING

Bouse Formation is one of the key elements for reconstructing the evolution of the Colorado River. We applied authigenic ¹⁰Be and ¹⁰Be/⁹Be dating to estimate the age of deposition of Bouse Formation at Cibola, AZ. Authigenic ¹⁰Be and ¹⁰Be/⁹Be dating are based on the assumption that the initial ratio of ¹⁰Be/⁹Be as well as ¹⁰Be decayed with time which passed since deposition, with a completely closed system. Due to the variance of ¹⁰Be production manipulated by changing magnetic field intensity, the ratio of authigenic ¹⁰Be/⁹Be is reported to be the better proxy than pure atoms of authigenic ¹⁰Be for dating of marine pelagic sediments. The total 25 samples collected from a 2.3-m-thick section near the base of Bouse mudstsone were analyzed for this study. The ¹⁰Be abundances range from 6.28X10⁶ – 1.44X10⁷ and the ratio of ¹⁰Be/⁹Be are bracketed by 1.57X10^-10 and 3.06X10^-10. The corresponding values for the surface samples are 1.4X10⁷ and 2.39X10^-9. The ages based on the two proxies center around 2.3-3.9 Ma and 4.1-5.4 Ma with slight increase with depth but the authigenic ¹⁰Be/⁹Be shows better correlation with depth. The age range yielded from authigenic ¹⁰Be/⁹Be dating is generally consistent with independent age constraints for the Bouse Formation, suggesting that the ¹⁰Be/⁹Be ratio is a better indicator of depositional age. The difference in age between bottom and top suggests that the Bouse Formation was deposited in relatively fast (less than 1.5 Ma) fashion and the low variation in authigenic ⁹Be (0.63 ± 0.06 ppm) through the section documents that the provenance was relatively constant. Thus this study demonstrates that the ratio of authigenic ¹⁰Be/⁹Be would be the promising proxy for dating of fine-grained terrestrial sediments as well as marine ones.