Abstract
<jats:title>ABSTRACT</jats:title> <jats:p>The rise of the late Paleozoic Ancestral Rocky Mountains orogen, an assemblage of fault-bounded basement uplifts, peripheral basement arches lacking documented flanking faults, and adjacent sedimentary basins, created an intricate array of continental and marine depositional settings through which sediment was transported and deposited. A dataset of 22 new U-Pb detrital zircon (DZ) samples, including 12 with Hf isotopic values, combined with 100 previously published samples from the Morrowan–late Wolfcampian Ancestral Rocky Mountain orogen, six samples of pre–Ancestral Rocky Mountains Mississippian strata, and 48 samples of post–Ancestral Rocky Mountains uppermost Wolfcampian–Guadalupian strata, helps to refine proposed models of sediment routing within and around the orogen. The combined dataset provides insight on relations of locally sourced sediment and detritus derived from distant sources. We define here two broad classes of DZ chronofacies, an intra–Ancestral Rocky Mountains chronofacies and an extra–Ancestral Rocky Mountains chronofacies, which discriminate locally derived sediment from clastic detritus mixed from distant source areas. Each chronofacies varies in detail: Intra–Ancestral Rocky Mountains chronofacies variants permit delineation of local basement sources that shed fluvial and alluvial sediment to nearby depositional sites, whereas extra–Ancestral Rocky Mountains chronofacies record varying proportions of zircon grain ages derived from most known basement and recycled sources in Laurentia and Gondwana. Transport histories of extra–Ancestral Rocky Mountains chronofacies usually include an eolian component. The intra–Ancestral Rocky Mountains chronofacies consists of several age distributions with one or two prominent age modes, most commonly at ca. 1700 ± 20 Ma and ca. 1430 ± 11 Ma, present in fluvial and shallow-marine strata near core Ancestral Rocky Mountains uplifts. The initial intra–Ancestral Rocky Mountains chronofacies in Morrowan strata records previously exposed basement on the Transcontinental arch rather than unroofing of incipient Ancestral Rocky Mountains uplifts. The contrasting extra–Ancestral Rocky Mountains chronofacies consists of multimodal age distributions that vary in detail but include Archean, Proterozoic, and Paleozoic age modes. This chronofacies, universal in deposits interpreted as eolian dust and sand, accompanied encroachment of eolian depositional systems into the Ancestral Rocky Mountains region as early as Atokan time. It became widespread in early Permian time with growing aridification of western Pangea, likely due to northward drift of the Ancestral Rocky Mountains region in combination with increased climatic continentality resulting from the amalgamation of Laurentia and Pangea.</jats:p> <jats:p>A series of sediment-dispersal maps illustrates changing patterns of fluvial and eolian networks as Pennsylvanian fluvial systems gave way to the extensive ergs of early Permian (Wolfcampian–Leonardian) time. Incipient Pennsylvanian ergs developed in coastal settings where southwestward littoral transport introduced extra–Ancestral Rocky Mountains chronofacies to shorelines of western Laurentia. In Late Pennsylvanian to early Wolfcampian time, dune systems expanded southward, preceded by a broad leeward loess blanket, across a former fluvial plain. Leonardian expansion of two temporally distinct ergs was fed by convergence of sediment derived from two geographic sources. A northwestern source included the marine margin and ephemeral wadi systems draining relict Ancestral Rocky Mountains uplifts, whereas a northeastern source consisted of sand deflated southwestward from an intermittent transcontinental fluvial system. The younger Leonardian erg deposits buried several former Ancestral Rocky Mountains uplifts. The subsidence mechanism that preserved the voluminous sand of this eolian convergence event remains debated, but it was likely a result of dynamic subsidence augmented by an isostatic response to the sediment load itself. During Leonardian time, the Delaware basin in the southeastern part of the Ancestral Rocky Mountains province received mainly dust-caliber eolian sediment from both Laurentian and Gondwanan sources. The coarse silt and very fine sand, driven southwest by zonal circulation, was deposited in both shelfal and deep-marine settings. A local western source consisting of Cambrian intrusive rocks in southern New Mexico supplied coarse-grained sediment to some slope and basin deposits of the western Delaware basin.</jats:p>