U-Pb DATING OF TRIASSIC PALEOSOL CALCITE IN THE NEW 
HAVEN ARKOSE, HARTFORD BASIN, CONNECTICUT

WANG, Zhaosheng.
RASBURY, E. T.
HANSON, Gil N..
MEYERS, William J.
		All at: Department of Department of Earth and Space
	 		Sciences, State University of New York at Stony
			Brook, Stony Brook, NY  11794-2100

We are applying U-Pb dating of paleosol carbonate at paleo-exposure 
surfaces to date the time of sedimentation in rapidly deposited sections.  
Soil carbonate is a good candidate for U-Pb dating because it (1) can be 
rapidly identified geochemically and petrographically; (2) consists of 
low-Mg calcite which is stable during diagenesis; and (3) is enriched in 
U relative to Pb.
	In order to test whether high precision ages would be possible, 
multiple aliquots of a ca. 100 gram sample were dated independently by 
Mr. Wang and Ms. Rasbury using different chemistries, different size 
samples, and different isotopically enriched spikes.  This sample is U-
enriched pure pedogenic micritic calcite in a sheet crack about 2000 
meters below the Triassic-Jurassic boundary.  We have calculated the 
ages using two different isochron plots for the two separate aliquots of 
the same sample. The uncertainty in the 238U/206Pb - 207Pb/206Pb 
isochron (suggested by Rasbury et al., 1996) is about half that of the 
238U/204Pb - 206Pb/204Pb isochron (Table 1) because the 207Pb/206Pb 
ratio can be measured more precisely than the 206Pb/204Pb ration and 
there is less variation in the 207Pb/206Pb initial ratio.  The ages are in 
good agreement with the published Triassic - Jurassic boundary ages 
(Table 2).  With uncertainty of less than 3 Ma using the 
238U/206Pb - 207Pb/206Pb isochrons for dating the time of sedimentation 
this method of dating has high potential for calibrating the geological 
time scale.