Field and laboratory investigations indicate a twofold division of the Tesnus formation. The lower unit is marine and consists chiefly of siliceous shales and massive low-rank gray-wackes; the clay-mineral fractions of these rocks are composed mainly of illite and a mixed-layer chlorite-vermiculite. The upper unit is predominantly nonmarine and consists of fine-grained clastic sediments, mainly bentonitic platy shales and cherty shales in which illite and montmorillonite are the clay fraction.

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