Cyclic Sedimentation in the Mixed Siliciclastic-Carbonate Abo-Hueco Transitional Zone (Lower Permian), Southwestern New Mexico

Abstract

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.