Eustatic and tectonic controls on cyclic sediment accumulation patterns in lower-middle Pennsylvanian strata of the Orogrande Basin, New Mexico
Thomas J. Algeo, James L. Wilson, and Kyger C. Lohmann
The Lower-Middle Pennsylvanian Gobbler Formation and equivalent units of southern New Mexico is a 250-450-m-thick succession of shallow-marine sediments composed, in part, of 20-25 carbonate shoaling cycles. Cycle tops show evidence of subaerial exposure and meteoric diagenesis, including karstic solution pits, brecciation, cemented and oxidized crusts, and isotopic depletion of carbon in the rock matrix. Basinwide correlatability of cycles, exposure of cycle tops in both shelfal and basinal sequences, and average periodicities of 100,000-200,000 yrs implicate high-frequency, high-amplitude Carboniferous glacio-eustatic sea-level fluctuations as the primary control on cycle development. Cycle sets comprising five to seven cycles that alternate between shale- and carbonate-rich compositions record a ca. 1.0 Ma oscillation that represents a longer-period custatic or tectonic cycle. These Lower-Middle Pennsylvanian strata were deposited during initial subsidence of the Orogrande Basin, a north-trending intracratonic trough formed during the Ouachita-Marathon orogeny. The broad western (Robledo) ramp was tectonically stable, while the narrow eastern (Sacramento) shelf exhibited considerable relief along a fault-bounded shelf margin and intrashelf graben. Structural features controlled sedimentation patterns on the eastern shelf, which is characterized by rapid lateral and vertical facies changes, variable cycle thicknesses, shelf-margin progradation, and development of phylloid-algal bioherms. Cycle thicknesses vary systematically across the basin from 6 m on the western ramp to 9 m on the eastern shelf, reflecting differential rates of subsidence.
- Algeo, Thomas J.; Wilson, James L.; Lohmann, Kyger C., 1991, Eustatic and tectonic controls on cyclic sediment accumulation patterns in lower-middle Pennsylvanian strata of the Orogrande Basin, New Mexico, in: Geology of the Sierra Blanca, Sacramento and Capitan Ranges, New Mexico, Barker, James M.; Kues, Barry S.; Austin, George S.; Lucas, Spencer, G., New Mexico Geological Society, Guidebook, 42nd Field Conference, pp. 203-212.