A further evaluation of the Rb/Sr data from Monterey Formation diatomites, CT-cherts, and quartz cherts (Brueckner and Snyder 1985) confirms that a mixing relationship exists for the diatomites and porcelanites but not for the CT- and quartz cherts. The Rb/Sr system is believed to be responding to solution-reprecipitation reactions which accompany the silica-phase transitions. Sr isotopic compositions of calcites from tar-filled fractures in dilation breccias are identical to the initial Sr ratio of the quartz chert linear array (0.70909), suggesting a synchroneity of fracturing, petroleum migration, and quartz chert formation. The Rb/Sr data are used to calculate a mid-Pliocene age for the fracturing event.

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