Carbon and oxygen isotope evidence for high-frequency (10(4)-10(5) yr) and My-scale glacio-eustasy in Middle Pennsylvanian cyclic carbonates (Gray Mesa Formation), central New Mexico

摘要

We combine cyclo- and sequence stratigraphy along with whole rock delta C-13 and conodont apatite delta O-18 analysis to document high-frequency (10(4)-10(5)yr) and My-scale sea-level changes for the Middle Pennsylvanian (Desmoinesian or Moscovian) Gray Mesa Formation of central New Mexico. Approximately 75 subtidal cycles (1-8 m) are grouped into 4 1/2 My-scale depositional sequences (40-80 m). About 50% of the cycles show evidence of prolonged subaerial exposure at cycle tops with the development of calcretes, diagenetic mottling, and regolith intraclasts. High-resolution delta C-13 analysis of whole rock limestones across nine of the cycles indicates that the cycle tops were diagenetically altered by isotopically light, meteoric fluids during sea-level fall and lowstand. These delta C-13 trends support the interpretation that high-frequency sea-level changes were responsible for cycle development. Conodont apatite delta O-18 values from sampled cycles indicate that the high-frequency sea-level changes were driven by glacio-eustasy combined with changes in surface seawater temperature (SST). delta O-18 values from conodont apatite, spanning parts of three depositional sequences indicate that My-scale glacio-eustasy and/or SS'T changes controlled sequence development delta O-18 shifts indicate that the magnitudes of 10(4)-10(5) yr glacioeustasy were between similar to 55 and 170(+) m combined with tropical SST changes of similar to 1.5 degrees-6 degrees C. Calculated My-scale glacio-eustatic oscillations were between similar to 60 and 140 m, with SST changes of <3.5 degrees C. The most plausible driver for the My-scale paleoclimate changes is long-period obliquity (similar to 1.2 My) variations. These calculated high-frequency, glacio-eustatic values are similar or greater than Pleistocene values, and lie within the range estimated for other Middle Pennsylvanian successions using a variety of independent eustatic proxies. The similarity in range of magnitudes between high-frequency and My-scale sea-level changes combined with the large differences in magnitudes between individual high-frequency sea-level oscillations helps explain the lack of systematic cycle-stacking patterns within these Pennsylvanian icehouse sequences.