K---Ar age values on the clay fractions in dated shales

Details K---Ar age values on the clay fractions in dated shales K---Ar age values on the clay fractions in dated shales

Mar 1963

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1963gecoa..27..279h&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 27, Issue 3, pp.279-284

Mathematics

Logic

4

Scientific paper

Clay fractions were separated from paleontologically dated clay shales ranging from Ordovician to Recent for K---Ar measurement. The samples in most cases were less than 2 (remainder less than 4 ) and were separated by ultrasonic disaggregation and centrifugation. K---Ar age measurements show values that are from 320 to 390 m.y. for Ordovician to Devonian samples from upper New York state; about 180 m.y. for mid-Cretaceous samples from Texas; and 120 to 210 m.y. for Miocene samples from Venezuela. All Recent sediments tested, from river-mouth to pelagic, show clay fractions with an average K---Ar age in excess of 100 m.y. These values are 10-20 per cent less than the estimated true geologic age in the Paleozoic samples, but are much higher than the geologic age in the younger samples. We interpret these results to indicate that these clay fractions are composed of mixtures of (a) more ancient detrital grains with older K---Ar age values, and (b) authigenic or epigenic illite with younger K---Ar age values than the time of sedimentation. In the younger samples the effect of the former predominates; and the reverse is true in the case of the older samples. This interpretation is based on several other investigations in this laboratory in which separations into a range of size fractions always shows the coarser sizes to be older, and the finer sizes to be younger than the geologic age of the sediments. The fact that the l M d components in the older shales show ages slightly less than the age of sedimentation is believed to be due to continued development of mixed layer illitemontmorillonite during deep burial.

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