TRACING CLASTIC DELIVERY TO THE PERMIAN DELAWARE BASIN USING DETRITAL ZIRCON GEOCHRONOLOGY

The Delaware basin formed one of the westernmost basins in Permian equatorial Pangaea and contains a thick Middle Permian (Guadalupian) siliciclastic fill (Delaware Mountain Group) long inferred to reflect eolian transport into a marine setting. Provenance of these voluminous clastics has been a source of speculation. Proposed sources nearly span the compass, from northwesterly to (clockwise) southwesterly, although many have posited a generally northerly/northeasterly source emanating from the remnant uplifts of the Ancestral Rocky Mountains.

To clarify the source, and thereby reconstructions of paleogeography and paleodispersal, four samples from the Delaware Mountain Group and correlatives were analyzed for detrital zircon provenance. Approximately 100 zircons from each sample were analyzed by LA-ICP-MS (laser ablation multicollector ICP-MS) at the University of Arizona Geochronology Laboratory for U-Pb age dates. The age spectra are statistically indistinguishable, although older and younger samples show a shift in dominant age groups. Three age groups, 1300-920 Ma (Grenville), 790-515 Ma (Neoproterozoic) and 495-300 Ma (Paleozoic), account for nearly 70-80% of the grains in all samples. Subordinate age populations include 1585-1300 Ma (anorogenic granites of the western U.S.), 1825-1595 Ma (Yavapai-Mazatzal), and Archean/Paleoproterozoic grains. In general, the detrital zircon spectra are more similar to Mesozoic than Paleozoic strata of the western U.S.; notably, the Yavapai-Mazatzal ages characteristic of the Ancestral Rocky Mountains (ARM) provenance in older (Permo-Pennsylvanian) samples are greatly diminished in these Middle Permian units, suggesting a provenance shift characterized in part by loss of the ARM source by Middle Permian time, earlier than previously suggested. The data are consistent with possible sources in the Appalachian-Ouachita orogenic system, as well as various terranes accreted along present-day Mexico by Permian time.