--> Abstract: Contrasts in Cementation, Dissolution, and Porosity Development Between Two Early Cretaceous Reefs of Texas, by C. W. Achauer; #90968 (1977).

Datapages, Inc.Print this page

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

C. W. Achauer

The influence of cementation and dissolution on porosity is demonstrated by comparison of two reefs with significantly different diagenetic histories. Reefs of the subsurface Sligo Formation in South Texas are buried to depths of 15,000 to 20,000 ft (4,572 to 6,096 m) in a narrow belt along a shelf edge of regional extent. Cores from six wells show that porosity in the reefs is occluded persistently by (1) radiaxial fibrous calcite, a cement which formed early in the diagenetic sequence and whose origin is controversial; and/or (2) coarse calcite mosaic, a cement introduced later in the diagenetic sequence during a time of basin deformation. Calcite cements of this type and sequence also have been reported to be detrimental to porosity in other ancient shelf-margin reefs. Thus, 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 undergone only one phase of diagenesis, that of 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 recent time. Thus the key to understanding porosity preservation in the Glen Rose reef lies in the understanding of the paleohydrologic and geochemical conditions during burial and reexposure of the reef.< P>

AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC