OXYGEN-ISOTOPE EVIDENCE FOR 3RD-ORDER (MY-SCALE) GLACIO-EUSTATIC SEA-LEVEL CHANGE IN MIDDLE PENNSYLVANIAN CYCLIC CARBONATES OF CENTRAL NEW MEXICO

Evidence of high-frequency (4th-5th-order) sea-level change in Pennsylvanian marine deposits has been well documented for 70+ years with the main line of evidence coming from widespread, cyclically arranged deep- through shallow-marine/nonmarine deposits. In this study, in addition to high-frequency cycles, we document 3rd-order (1-5 My) glacio-eustatic sea-level changes in Middle Pennsylvanian carbonates using d18O values from conodont apatite.

The Desmoinesian Gray Mesa Formation of central New Mexico (290 m) is characterized by ~75 fifth-order cycles (1-8 m), which are bundled into five 3rd-order sequences (40-80 m). The subtidal cycles are composed of calcareous mudstone or thin-bedded wackestone overlain by thick-bedded skeletal wacke/packstone. Half of the subtidal cycle tops display discolored mottling and pedogenic calcrete indicating prolonged subaerial exposure. Third-order sequences are composed of thicker-than-average deep subtidal cycles overlain by thinner-than-average shallow subtidal cycles.

d18O values from conodonts collected at the base of successive 5th-order cycles through portions of three 3rd-order sequences record 1-2‰ negative shifts during the transgressive systems tract and maximum flooding, and a 1‰ positive shift during highstands. These relationships with facies and isotopic values clearly support a paleoclimatic origin (glacio-eustasy and seawater temperature changes) for the 3rd-order sea-level changes. Using the Quaternary as a guide, the Pennsylvanian d18O shifts suggest amplitudes of sea-level rise/fall on the order of 75-150 m and tropical seawater temperature changes of about 2-4°C. These are minimum values because the full extent of 3rd-order sea-level rise and fall are not recorded at this inner shelf locality. These magnitudes of change are similar to those observed in the Pleistocene-Holocene but are occurring over My-time scales. We interpret that these 3rd-order glacio-eustatic sea-level changes are controlled by long-period orbital variations.