UPb 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 117942100
We are applying UPb dating of paleosol carbonate at paleoexposure
surfaces to date the time of sedimentation in rapidly deposited sections.
Soil carbonate is a good candidate for UPb dating because it (1) can be
rapidly identified geochemically and petrographically; (2) consists of
lowMg 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 TriassicJurassic 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.
Table 1. UPb ages of paleosol calcite from
New Haven Arkose
Analyst 
238U/204Pb  206Pb/204Pb ages 
238U/206Pb  207Pb/206Pb ages 
Rasbury 
213+/ 5 Ma (2 sigma) (9 points) 
212+/ 3 Ma (2 sigma) (9 points) 

212+/ 13 Ma (2 sigma) (10 points) 
215+/ 10 Ma (2 sigma) (10 points) 
Wang 
213+/ 5 Ma (2 sigma) 
212+/ 3 Ma (2 sigma) 
Table 2. Published Triassic  Jurassic
boundary ages
Authors 
Triassic  Jurassic Boundary Ages 
Harland et al. 1990 
208 +/ 16 Ma (2 sigma) 
Odin and Letolle 1982 
204 +/ 4 Ma (2 sigma) 
Webb 1981 
200 +/ 5 Ma (2 sigma) 
Seidemann 1989 
190 +/ 5 Ma (2 sigma) 
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