Abstract: Occurrences of Tertiary Porosity in Carbonate Rocks

Alonzo D. Jacka

Although porosity in carbonate rocks is considered to be primary or secondary, numerous subsurface occurrences of tertiary (third order) voids have been noted in Texas in middle Permian carbonate rocks of the Permian basin and in Lower Cretaceous (Edwards Group) carbonate rocks of the Maverick basin. Tertiary voids also have been recognized in outcrops of Permian carbonate rocks of the Permian basin and in Lower Cretaceous (Edwards) carbonate rocks of central Texas. Third-order voids commonly have remained open although primary and secondary voids have been filled by cements.

Tertiary voids record the following paragenesis. Sparry or felted lath anhydrite replaced limestones or dolostones after mineralogic stabilization and after primary and secondary voids had been partially to completely filled by cements. Borders of replacement anhydrite are characterized by stairstep morphology consisting of blocky rectangular extensions into the carbonate matrix; the margins appear cloudy because of inclusions of unreplaced carbonate material. Replacement anhydrite subsequently was dissolved by fresh groundwater and third-order stairstep molds were created.

In the Permian and Cretaceous, formation of tertiary voids is associated with depositional and diagenetic cycles which reflect eustatic sea-level fluctuations and attendant climatic changes.

Tertiary voids have an excellent chance of being preserved because emplacement and dissolution of anhydrite occurred after mineralogic stabilization of carbonate materials had been completed, and solution and solution-precipitation reactions involving carbonate minerals had been essentially terminated or greatly retarded.

In the Maverick basin numerous examples of quaternary (fourth order) voids were recognized. These record a second episode of anhydrite emplacement and dissolution.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma