Abstract

The degree of dolomitization in the reef facies of the Capitan Formation (Permian, Delaware Basin, West Texas-New Mexico) was controlled by the amount of early marine cementation, which in turn was controlled by local depositional factors. Work on the Capitan forereef facies indicates dolomitization by refluxing penecontemporaneous backreef brines. The brines passed through the reef facies causing variable dolomitization.

In the Guadalupe Mountains, the Capitan reef facies is well known for the extensive marine cementation which occluded most primary porosity and inhibited dolomitization. The rigid reef, however, fractured as it prograded over the compacting forereef facies. These fractures acted as channels for descending brines as evidenced by the dolomite halos commonly found on them. The general lack of marine cementation in the Gulf PDB-04 research core (and in the Glass Mountains) allowed free passage to brines throughout the Capitan reef facies and resulted in its almost complete dolomitization.

If marine cementation controlled dolomite distribution, the question remains what controlled marine cementation? The Capitan reef in the Guadalupe Mountains is characterized by moderate progradation (5-7 km) and extensive marine cementation. Elsewhere, however, the Capitan prograded 2-3 times further and generally lacks marine cementation. Apparently, rapid progradation results in rapid burial and short residence times for sediment near the sea floor and minimizes marine cementation. Therefore, although the Capitan of the Guadalupe Mountains has been described as a “catch-up” carbonate margin, elsewhere it can be better described as a “keep-up” margin. Probable causes for the varying progradation rates around the Delaware Basin include variations in siliciclastic input, in carbonate production and in subsidence rates.