Studies of active burial diagenesis in Frio (Oligocene) sandstones in the temperature range 90 to 160°C (burial depth, 2,500 to 4,450 m), combined with other studies at lower temperatures, show important sources and sinks for calcium. The main calcium sources are conversion of smectite to illite (40 to >150°C), pressure solution of detrital micrite (<90°C), albitization of calcic plagioclase (100 to 120°C), and to a lesser extent devitrification of acidic volcanic glass (<90°C).

The main calcium sink is cement-forming calcite. The composition of the calcite appears to reflect the various sources, thus calcite cement from the breakdown of smectite shows a late-stage iron-magnesium enrichment, whereas calcite replacing albitized plagioclase is nearly pure calcite. Another sink for calcium, previously undescribed for the Gulf Coast Tertiary, is authigenic sphene which is found at temperatures greater than 150°C.

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