Joined: June 2006
Isochron analysis of Rb-Sr, Ar-Kr
About halfway down the page:
|Larson, et al. (1994, p. 266-267) found a correlation between the K-Ar dates and weight percent K2O in the samples. Samples with anomalously high K2O values are associated with younger dates so they proposed a perfectly reasonable (and testable! explanation for the bad K-Ar dates from the Cardenas Basalt:|
The explanation that seems most consistent with the data is that the progressive decrease in the dates is the result of increased loss of Ar associated with preferential burial alteration of those flows containing the higher contents of K2O. The more felsic the flow, the greater its viscosity, and the greater the content of mesostasis material containing large quantities of K2O and, therefore, the greater the likelihood of Ar loss during burial metamorphism.
Therefore it's clear that Woodmorappe misquoted McKee and Noble and was very selective in the presentation of data to support his claims. A full examination of the data shows the reliability of the Rb-Sr method for dating the Cardenas Basalt and a testable explanation for the argon loss and unsuitability of the Cardenas for K-Ar dating methods.
This publication has more direct answers to the geomorphology questions, and why the date analysis is difficult, but much closer to the middle answer of 1100 Ma:
|"Tectonic inferences from the ca. 1255–1100 Ma Unkar Group and Nankoweap Formation, Grand Canyon: Intracratonic deformation and basin formation during protracted Grenville orogenesis" J. Michael Timmons, et al|
Geological Society of America Bulletin, vol.117, no.11-12, pp.1573-1595, Dec 2005