DETRITAL ZIRCONS IN UPPER TRIASSIC STRATA OF THE LOWER CHINLE AND DOCKUM GROUPS, NEW MEXICO AND TEXAS

Upper Triassic sandstones of the lower Chinle Group in northeast New Mexico (Tres Lagunas Member of Santa Rosa Formation at Santa Rosa) and the lower Dockum Group in northwest Texas (Santa Rosa Formation of the Canadian River valley) contain somewhat different U-Pb age populations of detrital zircons. Both are highly quartzose sandstones (>90% quartz). U-Pb analyses were obtained for individual detrital zircon grains using Laser Ablation – ICPMS with a beam diameter of 50 microns. A total of 100 Tres Lagunas grains were analyzed, but five analyses are discounted because of poor analytical precision or age discordance >20%. The Tres Lagunas sample contains a large (~50%) Grenville-age zircon population (970-1300 Ma; diffuse peak at 1110-1225 Ma) derived ultimately from the flank of the Appalachian-Ouachita orogen but possibly recycled through turbidites within the Ouachita system or pre-Triassic strata of the Ouachita foreland. Pennsylvanian sandstones of the Haymond Formation in the Marathon segment of the Ouachita orogenic belt contain zircons of ages that closely match Tres Lagunas zircons. The most abundant (~25%) non-Grenville zircons in the Tres Lagunas sample are Paleozoic to Neoproterozoic (240-820 Ma; sharp peak at 460 Ma), which could derive mainly from the core of the Appalachian-Ouachita orogen with possible recycling through the flank of the Ouachita system. Appalachian and Ouachita detritus could have been transported to New Mexico by Late Triassic streams flowing along the Ouachita foreland. Subordinate older Proterozoic Tres Lagunas grains are populations (1320-1460 Ma, ~10%; 1580-1740 Ma, ~10%) suggesting ultimate derivation from southwest Laurentia on the craton side of the Ouachita foreland belt, but likely recycled through Paleozoic strata. The Dockum sample from a previous study contains a prominent spike of Cambrian age zircons derived from the Amarillo-Wichita uplift to the north, as confirmed by local paleocurrent trends indicating sediment transport toward the southwest. Cambrian zircons in the Tres Lagunas sample are subordinate and masked by abundant Paleozoic-Neoproterozoic zircon grains of varied ages.