Brown, Alton A.; Loucks, Robert G.
The anhydrite - dolomite facies change in the Seven Rivers Formation (SRF) exposed on outcrop can be interpreted as depositional or dissolution. To reduce ambiguity resulting from Cenozoic outcrop dissolution, the facies change was examined in the subsurface of eastern Eddy County, New Mexico.
North of the facies boundary, over 80% of the SRF section is anhydrite with thin dolomites and siltstones. Over a narrow zone approximately 2 km wide, anhydrite abundance drops to about 20% anhydrite as dolomite thickens. By 4 km south of the start of anhydrite thinning, all of the SRF is tight (1 - 4% porosity) dolomite with thin siltstone beds. The dolomites in the transition zone have high, irregular gamma ray (GR) related to high uranium. The high-GR dolomite grades southwards into low-GR dolomite with higher porosity.
The SRF thins by about 100 ft in the transition zone, with the most abrupt thinning near the northern end of the facies transition. The abrupt SRF thinning is equal to the thinning of net SRF anhydrite. Farther south within the transition zone, SRF thinning is irregular, and thinner anhydrite is compensated by thicker dolomite. Net SRF thinning is compensated mainly by thickening in the Yates Formation.
The SRF depositional anhydrite-dolomite facies change is interpreted as an originally gradational facies change that maintained a relatively constant position on the shelf interior. SRF sulfate thickness was modified during Yates deposition. Permian subaerial exposure, especially along the Yates B surface, could have caused meteoric, shallow-subsurface sulfate dissolution selectively within the carbonate facies, making the overall facies transition more abrupt.
Although the shelf margin during SRF deposition shows significant seaward progradation, the anhydrite-dolomite transition shows no evidence for seaward progradation. SRF shelf-interior processes are apparently decoupled from shelf margin processes. The same lack of seaward progradation is evident in the Yates anhydrite-dolomite facies transition. Major sea-level events (such as SRF-Yates boundary) reset the position of shelf-interior facies boundaries, and these boundaries remain rather static between major sea-level events.
The outcrop Seven Rivers facies change thus represents a gradational facies change modified first by Permian meteoric dissolution throughout SE New Mexico and then by Cenozoic dissolution near the outcrop belt.
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013