The Control of Ground–Water Occurrence by Lithofacies in the Guadalupian Reef Complex near Carlsbad, New Mexico

Abstract

The occurrence and movement of water in the Guadalupian reef complex of southeastern New Mexico is controlled primarily by lithofacies. Major Guadalupian lithofacies which affect water occurrence are: (1) a basin facies of very low permeability which confines ground water, (2) a reef-zone facies of very high permeability, (3) a shelf-carbonate facies ranging from very high permeability near the reef zone to low permeability near the shelf-evaporite facies, and (4) a shelf-evaporite facies of moderate permeability. Major Johnson Springs and Indian Big Springs occur at the contact of the shelf-evaporite and shelf-carbonate facies because of a large decrease of permeability in the latter facies. The greatest amount of solution and highest permeabilities in the shelf-carbonate facies occur along joints in coarser-textured carbonates rather than in fine-grained carbonates, and in the calcareous rather than in the dolomitic subfacies. A zone of exceedingly high permeability, extending from the reef-zone facies into the calcareous subfacies of shelf-carbonates, forms the Limestone aquifer, and a zone of much lower permeability, occupying most of the shelf-carbonate and shelf-evaporite facies, forms the Shelf aquifers. The boundaries of the Limestone aquifer and Shelf aquifers extend across formational and time boundaries but conform to major lithologic facies. Water in the Shelf aquifers takes two divergent paths; in the San Andres Formation it moves northeastward into the Roswell basin, whereas in the stratigraphic interval from the Tansill Formation through the Grayburg Formation it moves eastward into the Limestone aquifer. The Limestone aquifer which parallels the Reef Escarpment is about 20 miles long and 5 to 6 miles wide. Downward movement of water in the aquifer is prevented by underlying massive sandstone of the Cherry Canyon Formation, and lateral movement eastward is prevented by impermeable anhydrite of the Castile Formation. The poorly permeable and impermeable barriers divert water to move in a northeasterly direction parallel to the Reef Escarpment and to the axis of the Capitan reef zone. The ultimate discharge area for most water in the Limestone aquifer is Carlsbad Springs, located north of Carlsbad, New Mexico. Knowledge of hydrology of the Guadalupian reef complex has aided geologic analyses in the subsurface location of lithofacies and in understanding the geomorphic history of the Guadalupe Fault Block. The presence of potable water west of the Pecos River in Guadalupian rocks and highly mineralized water east of the river in similar rocks suggests that the Guadalupe and Sacramento Fault Blocks were uplifted and trenched by the Pecos. The circulation of water through the Limestone aquifer has caused extensive solution and caverns, of which Carlsbad Caverns are a part. Shallow oil deposits occur in some anticlinal structures east of the Pecos River, but they do not occur in similar structures west of the river because circulating ground water from the west has flushed out the oil deposits and highly mineralized water.