EVIDENCE FROM U-PB AGES OF DETRITAL ZIRCONS FOR DERIVATION OF SAND IN THE CHINLE-DOCKUM FLUVIAL SYSTEM AND THE MARINE AULD LANG SYNE GROUP FROM THE INCIPIENT GULF OF MEXICO RIFT SYSTEM

Concordant and slightly discordant U-Pb ages for individual detrital zircon (DZ) grains in Upper Triassic sandstones deposited along the principal paleoriver courses of the Chinle-Dockum fluvial system on the High Plains (N=3 samples; n=267 grains) and Colorado Plateau (N=7 samples; n=631 grains), and within the Auld Lang Syne backarc marine basin at the fluvial terminus in the Great Basin (N=3 samples; n=282 grains), reflect major recycling of clastic detritus from the Ouachita orogenic belt where uplifted along the northern shoulder of the incipient Gulf of Mexico rift system in Texas. A composite grain population (N=13 samples; n=1180 grains) displays a dominant broad age peak at 1100 Ma (Grenvillian) and less bulky Neoproterozoic (630, 550 Ma) and Paleozoic (475, 445, 400 Ma) age peaks inferred jointly to document derivation of zircon grains from Ouachita turbidites and Texas Mesoproterozoic basement. Subordinate contributions of detritus from Precambrian basement of southwest Laurentia (compound 1800-1665 and unitary 1440 Ma age peaks), and from Permian-Triassic and Late Triassic (Norian) arc assemblages of northeastern Mexico (composite 240 Ma age peak) are also present in the sandstones. Sandstones from southerly and northerly tributaries of the Chinle-Dockum trunk paleorivers contain different age populations of detrital zircons that reflect derivation respectively from Laurentian basement lying south of the Colorado Plateau and from Cambrian granite in the Amarillo-Wichita uplift of the Texas panhandle and adjacent Oklahoma. The DZ data imply that master Chinle-Dockum paleodrainages (an older Shinarump paleoriver and a younger Cottonwood paleovalley), trending ESE to WNW from Texas to Nevada, flowed along the axis of a backarc trough (formed by dynamic subsidence behind the incipient Cordilleran magmatic arc of SW USA and NW Mexico) from headwaters in the pre-Gulf of Mexico rift uplift in Texas to the sediment sump formed by the marine Auld Lang Syne backarc basin of western Nevada into which volcaniclastic detritus from the nearby Cordilleran arc to the west was also delivered. The regional relations of Chinle-Dockum and Auld Lang Syne depositional systems highlight the guiding control that tectonic events along craton margins exert on sediment dispersal across craton surfaces.