THE ILLINOIS AULACOGEN AND THE RIFTING OF THE ARGENTINE PRECORDILLERA

William A. Thomas made many important contributions to the understanding of sedimentation and tectonics of orogenic belts, continental margins, and cratonic basins. Among his most notable and favorite, we believe, was his recognition that the Argentine Precordillera (i.e. Cuyania terrane) rifted from the southern continental margin of Laurentia during the late Neoproterozoic and early Cambrian. Cuyania subsequently accreted to Gondwana during the Ordovician. The rifting of Cuyania is associated with two failed rifts, the WNW-trending Oklahoma Aulacogen and the NNE-trending Reelfoot Rift, which underlies the Illinois Basin. Over the past decade, deep drilling of five wells associated with geologics CO₂ storage in basal Cambrian strata of the Illinois Basin has revealed a northern extension of the Reelfoot Rift that is now referred to as Illinois Aulacogen. Termination depths of four of these wells occur in the Precambrian basement, revealing granite or rhyolite ages of 1650, 1476, 1467, and 1376 Ma. Basal Cambrian sandstones in each of the five wells include a suite of late Neoproterozoic to Cambrian detrital zircons (age peak of ~532 Ma). A 526.03 +/- 1.09 Ma basalt dike intrudes Cambrian sandstone in the Wabash well, constrain the timing of Cuyania rifting from Laurentia. In addition to the Cambrian zircons, the rift was filled by proximally-sourced sediment from the local Mazatzal and Midcontinent Granite-Rhyolite terranes, forming a unique detrital zircon chronofacies. As the rift filled and the local source areas adjacent to the rift were buried, distally derived sediment from the Archean Superior and late Mesoproterozoic Grenville provinces became dominant in the late Cambrian arenites of the Laurentian midcontinent. After accretion to Gondwana, the Cuyanian lithosphere continued to define the style and location of Central Andean deformation and sediment sourcing during compressional cycles in the Paleozoic and Cenozoic, and an extensional cycle during the late Paleozoic and Mesozoic.