Stratigraphy and Depositional Environment of Lower San Andres Formation in Subsurface and Equivalent Outcrops: Chaves, Lincoln, and Roosevelt Counties, New Mexico

Abstract

This paper establishes correlations from subsurface to outcrop and develops a depositional model for the lower San Andres Formation in New Mexico. Five cyclic carbonate-evaporite sequences, numbered consecutively upward as units 1, 2, 3, 4, and 5, were identified from measurement of approximately 1,750 ft (534 m) of section in the Hondo Canyon area of Lincoln County and examination of approximately 400 ft (122 m) of core from six wells in the Levelland-Slaughter trend of Chaves and Roosevelt Counties. An idealized sequence consists of open-marine, restricted subtidal, intertidal, and supratidal facies capped by anhydrite (in the subsurface) or an evaporite dissolution zone (in outcrop). Lateral facies changes in outcrop indicate that the Pedernal uplift provided a locus f r emergent conditions at low relative sea level stands through the deposition of unit 3. Units 4 and 5 are incomplete, highly variable sequences. Lateral facies changes are not present in outcrop, indicating that the Pedernal uplift exerted little influence on the deposition of these units. Preserved anhydrite cores capping P1 and P2 sequences (equivalent to units 4 and 5) range up to 15 ft (5 m) thick, are relatively pure, commonly lack underlying intertidal/supratidal carbonates, and contain some gypsum ghosts, indicating that part of the anhydrite was deposited subaqueously. We present a depositional model involving both subaqueous (salina) and subaerial (sabkha) deposition of the anhydrite. The model accounts for the purity and thickness of the evaporite caps, the rarity of subaerial exposure features, variations in the nature of the dolomite/anhydrite transition, and the accumulation and preservation of thick subaqueous evaporites north of the study area.