ISOTOPIC VARIABILITY IN BACKREEF STRATA OF THE PERMIAN REEF COMPLEX, MCKITTRICK CANYON, WEST TEXAS

Isotopic variability across carbonate platforms reflects the dynamics of carbon cycling and circulation at local, basinal, and global scales. However, resolving processes at each of these scales requires a full understanding of isotopic variability across bathymetric profiles as well as through time. The broad Middle Permian Capitan shelf is an ideal setting to resolve spatial and temporal isotopic variability at a critical time in Earth history—during the demise of Late Paleozoic glaciation and prior to the Late Permian mass extinctions. In the Guadalupe Mountains, excellent stratigraphic control allows the detailed correlation of sections ~3 km down-dip, from the pisolite shoal to the immediate backreef. Preliminary results from the Seven Rivers 2 high frequency sequence (HFS) in McKittrick Canyon document ~3.5‰ variability in δ¹³C and δ¹⁸O from microdrilled calcites of marine cements, brachiopods, and packstone matrix. The textures of sampled calcites are exceptionally well preserved. Radial and radiaxial fibrous cements retain sweeping extinctions and compromise grain boundaries, brachiopods retain shell structure, and all calcites are low in Fe and Mn.

Isotopic values vary considerably both down-dip and stratigraphically. At the HFS-scale, chemostratigraphic trends from different sections do not covary in sections sampled at ~ 5-10 m intervals. However, higher-resolution sampling from individual sections suggests that δ¹³C and δ¹⁸O values may fluctuate systematically within individual m-scale cycles or cycle sets. Preliminary results suggest that δ¹³C and δ¹⁸O values are systematically enriched ~2‰ near cycle tops. This enrichment is probably not related to diagenesis because meteoric diagenesis tends to deplete δ¹³C and δ¹⁸O values, not enrich them, and isotopic values do not correlate with the degree of textural preservation or of luminescence under CL. Rather, we hypothesize that global or basin-scale dynamics might be responsible for δ¹³C and δ¹⁸O trends. Future work will test this hypothesis through high-resolution sampling from multiple sections of one HFS in both McKittrick and Slaughter canyons.