New Mexico Geological Society Annual Spring Meeting — Abstracts

Exposures of the uppermost Eocene to Oligocene (35-25 Ma) Chuska Sandstone (535 m in maximum thickness) occupy ~1000 km² of the elongate crest of the Chuska Mountains along the NE Arizona–NW New Mexico border. The Narbona Pass Member, overlying the discontinuous Deza Bluffs Member of fluvial origin and forming most of the formation, is composed of eolian sandstone representing the central remnant of the largest Cenozoic erg known from western North America, with a reconstructed areal extent of ~100,000 km² (Cather et al., 2008 GSA Bull. 120:13-33).

At Narbona (Washington) Pass near Crystal and at Buffalo Pass near Lukachukai ~50 km to the north, the petrofacies of the Narbona Pass Member is arkosic (mean detrital mode ~Qm60-F35-Lt5). Of 100 detrital-zircon (DZ) grains for which concordant or nearly concordant U-Pb ages were obtained by LA-MC-ICP-MS geochronology from the Narbona Pass Member as exposed in a roadcut beside NM Highway 134 between Crystal and Narbona Pass, 90% fall (at 1σ) within the age range of 1425-1820 Ma. Prominent age peaks at 1700 Ma and 1420 Ma are interpreted to reflect derivation of the sand principally from Yavapai-Mazatzal Precambrian basement (1700 Ma age peak) intruded by Mesoproterozoic anorogenic granitic plutons (1420 Ma age peak) in central Arizona and adjoining areas. DZ populations in lower Upper Cretaceous (Turonian) Toreva and Gallup sandstones of the Black Mesa and San Juan basins display essentially identical age peaks of comparable dimensions (P=0.55 from comparative K-S analysis of Chuska and Toreva-Gallup DZ populations), and are known from paleocurrent indicators and subregional facies relations to have been derived from the same or closely similar bedrock sources lying to the southwest of the Colorado Plateau (Dickinson and Gehrels, 2008 Am. Jour. Sci. 308:1041-1082). Reworking the Narbona Pass DZ population from Toreva-Gallup sandstones is not a feasible interpretation, however, because the Cretaceous sandstones are distinctly more quartzose and less feldspathic (~Qm75-F20-L5).

Weathering and sediment transport during reworking could not be expected to enhance feldspar content and produce the arkosic sand of the Narbona Pass Member. We instead infer fluvial transport of sand northward from Precambrian source rocks, followed by deflation of alluvial plains to blow sand eastward into the Chuska erg by paleowinds having the mean azimuth recorded by cross-bedding in Chuska eolianites. Our DZ data bearing on the derivation of the arkosic sand in the Narbona Pass Member do not apply to southern Chuska outcrops near Whiskey Lake where more quartzose detrital modes (~Qm80- F15-Lt5) closely resemble sand of uncertain provenance forming the underlying Deza Bluffs Member of fluvial origin, nor to volcaniclastic sands (mean detrital mode ~Qm10-F25-Lt65) exposed at the southern fringe of the reconstructed Chuska erg along the northern flank of the Mogollon–Datil volcanic field, nor to sand along the eastern fringe of the Chuska erg where quartzolithic Isleta sands (mean detrital mode ~Qm60-F20-Lt20) from the subsurface of the Rio Grande rift closely resemble sand aggregates in the underlying Galisteo Formation and overlying Zia Formation of presumably local origin. Petrographic data thus imply areal variations in sand provenance within the Chuska erg, and in pathways for sand transport across the Chuska erg, but our DZ data reveal the provenance of voluminous arkosic sand in the central Chuska erg.