Abstract: Contrasts in Cementation, Dissolution, and Porosity Development Between Two Lower Cretaceous Reefs of Texas

C. W. Achauer

The influence of cementation and dissolution on porosity has been investigated by comparing two reefs with significantly different diagenetic histories. Reefs of the subsurface Sligo Formation in South Texas are buried at depths of 15,000 to 20,000 ft (4,570 to 6,100 m), in a narrow belt along a shelf edge of regional extent. Cores from five wells, distributed along 225 mi (362 km) of the shelf edge, show that porosity in the reef and back-reef facies is persistently occluded by (1) radiaxial fibrous calcite, a cement whose origin appears to be related to the replacement of a synsedimentary marine cement, and/or (2) coarse calcite mosaic, a cement introduced later in the diagenetic sequence during a time of basin subsidence. Calcite cements of this type and sequence also ave been reported to be detrimental to porosity in other ancient shelf-margin reefs. Therefore, the creation of porosity in shelf-margin reefs may depend on processes (dolomitization, fracturing, dissolution) which can offset the influence of cementation.

In contrast to the Sligo, large patch reefs in the outcropping lower Glen Rose Formation in south-central Texas record a simpler diagenetic history which preserved porosity. One reef, selected for detailed study, has experienced only one phase of diagenesis, namely, early subaerial exposure which created much moldic porosity and precipitated a single generation of calcite cement. After this exposure, no additional cements were introduced, despite burial beneath a few thousand feet of younger Cretaceous marine sediments and reexposure of the reef to subaerial weathering from Miocene to Holocene time. Thus, the key to porosity preservation in the Glen Rose reef lies in understanding the paleohydrologic and geochemical conditions during burial and reexposure of the reef. Whether this can be predicted is a matter for future research.

AAPG Search and Discovery Article #90967©1977 GCAGS and GC Section SEPM 27th Annual Meeting, Austin, Texas