ABSTRACT

In southwestern New Mexico, Lower Permian (Wolfcampian) rocks grade southward from nonmarine siliciclastics (Abo and Earp Formations) to marine carbonates (Hueco and Horquilla Formations). A transitional zone between siliciclastic and carbonate facies trends east-northeast across southwestern New Mexico and consists of 64 to 186 m of cyclically interbedded siliciclastic and carbonate rocks, which were deposited in tidal-flat and shallow-marine environments. Shallow-marine facies include fossiliferous limestone and olive-gray shale. Tidal-flat facies consist of 1) ripple-laminated sandstone, which was deposited on intertidal sandflats near mean low tide, 2) mixed sandstone-shale, which was deposited on an intertidal flat shoreward of the ripple-laminated sandstone facies, and 3) nodula shale, which is characterized by pedogenic calcareous nodules and was deposited in a supratidal setting. The intertidal facies are truncated by or grade laterally into rare channel sandstones, which represent tidal-creek or estuarine facies. In addition to siliciclastic tidal-flat deposits, a few beds of laminated carbonate also were deposited in the intertidal zone.

Vertical sequence analysis aids in delineating three types of depositional cycles. Asymmetrical cycles display the vertical sequence: basal fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--mixed sandstone-shale--nodular shale, and record shoreline progradation. The asymmetrical cycle is always overlain by fossiliferous limestone, which indicates a major transgression that inhibited siliciclastic sedimentation. A common symmetrical cycle consists of fossiliferous limestone--olive-gray shale--ripple-laminated sandstone--olive-gray shale--fossiliferous limestone, and indicates systematic seaward and landward migration of facies zones associated with small-scale sea-level changes. A less common symmetrical cycle involves laminated carbonate--fossiliferous limestone-- aminated carbonate. Cyclic sedimentation in Abo-Hueco transitional strata is most likely the result of glacial eustatic sea-level fluctuations.