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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)


Journal of Sedimentary Petrology
Vol. 39 (1969)No. 4. (December), Pages 1297-1301

Origin of Matrix in Ordovician Greywackes, Berwyn Hills, North Wales

P. J. Brenchley


Some Ordovician volcanic greywackes have 40-60 percent matrix which appears to have a diagenetic origin. The greywacke matrix differs from diagenetically altered muds in adjacent argillaceous beds. Where sand grains in the greywacke have been partly replaced by calcite and have a calcite cement, matrix is not present: instead, the rock is a well-sorted sandstone. It is concluded that the matrix formed by the diagenesis of volcanic grains, but that where an early calcite cement was precipitated no such diagenesis occurred.

The greywackes are not turbidites, but are generally laminated and the associated sedimentary structures indicate shallow water deposition.

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