In sandstones of the Great Valley Sequence of Mesozoic age at Cache Creek, post-depositional albitization of plagioclase and chloritization of biotite are widespread in noncalcareous rocks, but uncommon in rocks with calcareous cement. The degree of alteration increases systematically with age and inferred depth of burial in Upper Cretaceous strata, and is uniformly great in Lower Cretaceous strata that were buried from 20,000 to 30,000 feet. Laumontite is characteristic of thoroughly altered Lower Cretaceous rocks. Other metamorphic assemblages may be widespread at higher and lower horizons.

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