The isotopic age of the Flathead Sandstone (Middle Cambrian), Montana
The isotopic age of the Flathead Sandstone (Middle Cambrian), Montana
复制标题
蒙大拿州平头砂岩（中寒武统）的同位素年龄
DOI：
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发表时间：
1969
期刊：
影响因子：
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通讯作者：
D. Brookins
中科院分区：
文献类型：
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作者：
S. Chaudhuri;D. Brookins
砂岩由厚层到薄层、分选良好、干净的石英砂和海绿石砂岩组成，顶部附近有薄互层的红色和绿色裂变页岩。用Rb-Sr全岩法测定了7个互层页岩样品的年龄，测得其年龄为555+或-18m.y。最小二乘法测得初始锶(87/86)为0.7183+或-0.001。推测自生海绿石的单一Rb-sr年龄为542m.y。采用假定的初始(海水)锶(87/86)值0.707。页岩和海绿石的年龄都与中寒武世一致，根据地质证据，平头砂岩被认为是中寒武世。在安山岩床附近采集的四个页岩样品显然已经变质，破坏了原始的(沉积)Rb-Sr平衡。
The sandstone consists of thick- to thin-bedded, well sorted, clean quartzose and glauconitic sand with thin interbedded red and green fissile shales occurring near the top. Seven samples of the interbedded shales have been dated by the Rb-Sr whole rock method with a resultant age of 555 + or - 18 m.y. and initial Sr (87/86) of 0.7183 + or - 0.001 as determined by least squares analysis. A single Rb-Sr age on presumed authigenic glauconite yields an age of 542 m.y. using an assumed initial (sea water) Sr (87/86) value of 0.707. Both the shale and the glauconite ages are consistent with a Middle Cambrian age to which the Flathead Sandstone is assigned on geologic evidence. Four samples of shales taken near an andesitic sill have evidently been metamorphosed in such a way as to destroy the original (depositional) Rb-Sr balance.