METEORIC DIAGENESIS OF THE MIDDLE PENNSYLVANIAN (DESMOINESIAN) MADERA GROUP, LUCERO BASIN, NEW MEXICO: FIELD, PETROGRAPHIC AND ISOTOPIC EVIDENCE

The Middle Pennsylvanian (Desmoinesian) Madera Group (Lucero Basin, central New Mexico) is composed of 240 m of cyclic mixed carbonate-siliciclastic deposits. Alternations of deeper marine deposits (terrigenous and calcareous mudstones and wackestones) overlain by shallower marine deposits (wackestones through grainstones) define ~60 upward-shallowing cycles (subtidal cycles; 3-8 m thick). Cycles stack into 6 larger (25-50 m) sequences. Average cycle durations are ~70-140 ka; sequence durations are ~0.6-1.2 ma.

Field, petrographic, and isotopic evidence suggests that during periods of relative sea-level fall, subtidal cycle caps were subjected to subaerial exposure and meteoric diagenesis. First, cycle tops commonly display laterally persistent (up to 2 km) early diagenetic features that overprint the original rock type up to 50 cm down from cycle tops. Features include irregular dark calcite infilling vertical and horizontal cracks, oxidized burrow mottling, and in-situ breccias and conglomerates. Second, laminated inclusion-rich calcite, alveolar septal structures, and mm-scale rhizoliths are present in thin section. Third, preliminary d¹³C_carb values from closely spaced (~30 cm) samples within individual cycles show up to 5 ‰ negative shifts from bottom to top (+3‰ to -2‰ ). The shift to smaller values persists up to 50 cm down from cycle tops, suggesting alteration by isotopically light soil gas during periods of subaerial exposure. Relatively uniform negative d¹⁸O_carb values (average=-6‰ ) indicate late diagenetic overprinting.

Results to date indicate minor soil formation, small, shallow, and widely spaced rooting, minor dissolution features, and alteration of meteoric fluids only to 50 cm depth during periods of subaerial exposure; thus, suggesting an overall drier climate in central New Mexico during the Middle Pennsylvanian. Studies by previous workers on slightly younger strata (Missourian-Virgilian) indicate more extensive pedogenesis, dissolution, and rooting during periods of subaerial exposure, suggesting a relatively wetter climate in southern New Mexico during the Late Pennsylvanian. Comparison between Middle and Upper Pennsylvanian cycles in New Mexico suggests overall drier climate conditions during the Middle Pennsylvanian, which may correspond in time to peak Carboniferous-Permian glaciation.