ABSTRACT 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. 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.

The exceptional transport distance of long-runout landslides requires a mechanism for reduced frictional resistance to sliding. Here, we use zircons in the frictional wear products generated during emplacement of the Sevier gravity slide (southwest Utah, USA) to identify how the source of material evolves with transport distance and discuss how changes in frictional strength are reflected in this data set. Across the ∼38 km runout distance of the slide, basal wear products have unique zircon age distributions, or tectonic chronofacies, which capture changes in material sources and indicate poor mixing across the structure. Over much of this distance, basal material forms by breakdown of slide blocks, with little input from the underlying substrate. This suggests the basal slide plane has low frictional strength, buffering the substrate from deformation. We also observe a decrease in the mean age of zircons within the basal layer with increasing transport distance as abrasive wear is localized at the base of the overlying block during slip. Toward the distal portion of the slide, the amount of substrate zircons in the basal layer increases, consistent with greater frictional coupling during deceleration. Tying the unique tectonic provenance recorded by zircons within the basal layer of the Sevier gravity slide to larger deformation styles, we argue that the observed spatial evolution in frictional strength is consistent with widespread fluid pressurization.

The Bear Valley Formation (Fm.) is a distinctive eolian sandstone interbedded with thick volcanic rocks of the Marysvale volcanic field of southwest Utah, the southern part of which failed during eruptive activity along three mega-scale gravity slides. The formation is as thick as 300 m and extends over an area of >2,500 km 2 in the Black Mountains and Markagunt Plateau. The Bear Valley Fm. is composed of tuffaceous sandstone interbedded with tuff, conglomerate, and polymict volcanic mudflow breccias. The sandstone beds are lithic arenite and lithic wacke that occur as massive beds with large-scale cross bedding. The Bear Valley Fm. occurs in the upper plate of the Markagunt gravity slide and is in both the upper and lower plates of the Black Mountains gravity slide. We used laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to acquire U/Pb dates of detrital zircons (N = 3, n = 346) from the autochthonous Bear Valley Fm. at Kane Spring and Jako Wash in the Black Mountains and the allochthonous Bear Valley at Sandy Wash in the central Markagunt Plateau. All samples are dominated by Oligocene zircons with maximum likelihood ages for deposition ranging from 23.6 to 24.0 Ma. The western-most sample from Jako Wash also preserves a slightly older group of zircons, indicating derivation from either the underlying Wah Wah Springs Fm. or another unit erupted from the Indian Peak caldera complex to the west. Thus, the upper Bear Valley Fm. was deposited within ~400 kyr before the emplacement of the Markagunt gravity slide at 23 Ma, reflecting accelerated uplift of the northern Marysvale complex that ultimately resulted in collapse and slide emplacement.

The Marysvale volcanic field in southwestern Utah hosts three large-volume gravity slides: the Sevier (SGS), the Markagunt (MGS), and the Black Mountains (BGS). The gravity slides are composed of lahar deposits, lava flows, and ash-flow tuffs erupted from former stratovolcanoes and other vents during the Oligocene and Miocene. The ash-flow tuffs are prime targets for dating to constrain the age of the gravity slides because some ash-flow tuffs are deformed within the slides, whereas others are undeformed and cap the slides. Furthermore, the gravity slides produced pseudotachylyte during slide motion, a direct indicator for the timing of each slide. This work provides new 40Ar/39Ar dates for several ash-flow tuffs and pseudotachylyte for the SGS, along with U/Pb zircon dates for one deformed tuff and alluvium near the slide plane. Results show that the slide was emplaced at 25.25 ± 0.05 Ma and was immediately followed by the eruption of the Antimony Tuff at 25.19 ± 0.02 Ma. The model presented here suggests that the intrusion of magma related to the Antimony Tuff acted as a triggering mechanism for the slide and that slide movement itself led to decompression melting and eruption of the Antimony Tuff. This sequence of events occurred on a geologically rapid timescale and may have been virtually instantaneous.

The Mesoproterozoic (~1470 Ma) Wolf River batholith (WRB) is exposed over 6500 km2, encompassing 11 plutons that crosscut the Archean Marshfield and Proterozoic Penokean terranes. As the WRB is the classically defined anorogenic batholith, to test this hypothesis, seven igneous phases were analyzed using anisotropy of magnetic susceptibility (AMS), as a proxy for magmatic flow during intrusion, and the samples recorded a sub-horizontal emplacement in six different orientations. Paleopoles from six of eight igneous samples preserve a wide variety of sub-vertical orientations with two reversed and four normal polarities. The synorogenic Baldwin Conglomerate is the youngest rock (<1460 Ga) associated with WRB. Magnetic fabrics are horizontal, but multidomain and paleopole signatures, where interpretable, are sub-vertical. The North American APWP places middle Laurentia at low-latitude during Geon 14, and all our paleopoles are sub-vertical, not sub-horizontal, again suggesting post-intrusion deformation. Moreover, the McCauley gneiss (1886 Ma; U-Pb zircon), Rib Mountain Quartzite (1750 Ma MDA; U-Pb zircon, n = 150), Dells of the Eau Claire rhyolite (1483 Ma; U-Pb zircon, 1469 Ma; monazites-in-garnet), and Baldwin conglomerate (1460 Ma MDA; U-Pb zircons, n = 150) are sub-vertical inliers (xenoliths) in the igneous suite; the Proterozoic Wausau turbidite (1850 Ma MDA; U-Pb zircon, n = 150) was intruded by the WRB and dips 25°W. Here, we present a reinterpretation of the WRB as a deformed synorogenic rather than an anorogenic intrusion.

ABSTRACT The Black Hills of South Dakota are the easternmost basement-cored uplift in the Laramide foreland. Uplift occurred during the Latest Cretaceous-Eocene and was coeval with the emplacement of suite of alkaline and peralkaline hypabyssal intrusions. These intrusions were emplaced into Proterozoic metasedimentary rocks and Phanerozoic cratonic strata. Laramide exhumation stripped the Black Hills down to the Precambrian core. This study presents newly dated 54 Ma age peak zircon data of the Mountain Meadow Formation, the lower strata of the White River Group of South Dakota, to the many dates of these and adjacent rocks recently established recently by the authors The Eocene-Oligocene White River Group strata are unconformably overlain by the Arikareean Group of Oligocene-Miocene age. The Oligocene Mountain Meadow Formation is a post-orogenic gravel that consists of locally-derived clasts and rests at high elevations indicating that the Black Hills were at least partially buried following the Laramide Orogeny. Here, we present detrital zircon data (LA-ICPMS at the Arizona Laserchron Center) for the matrix of the Mountain Meadow Formation near Deadwood, South Dakota (n=143). Here, the Mountain Meadow is a crudely stratified, 75 m succession of cobble-boulder gravel that rests on Precambrian basement and early Paleozoic strata. About two-thirds of the zircons are Paleogene (41-63 Ma) in age, with no distinct peaks evident. These were likely derived locally from the Paleogene intrusive rocks. The balance of the zircon age spectrum is dominated by Precambrian grains, again with no prominent age peaks evident. The Mesoproterozoic grains were likely sourced locally from the underlying upper Cambrian Deadwood Formation. The Archean grains could be derived from the Deadwood as well, or perhaps they were derived from Paleoproterozoic basement rocks. Thus, the Mountain Meadow Formation here appears to be all locally derived, with transport distances of less than 10 km likely. This contrasts with coeval strata that cap the Bighorn uplift to the west, and that occur in the adjacent Great Plains and Powder River Basin, each of which have distal, westerly sediment source areas.

The Washington County Quartzite (WCQ) in southeastern Iowa is the southernmost occurrence of the Geon 17 “Baraboo Interval” quartzites in the Laurentian midcontinent region. Three drill holes encountered poorly sorted quartzite and phyllite, likely deposited in a braided fluvial or deltaic environment near the Laurentian continental margin on the Columbia supercontinent. 100 new LA-ICPMS detrital zircon U-Pb ages from the WCQ show a prominent 1.78 age peak, representing local Yavapai-aged basement, a secondary peak at 1.8-1.9 Ga representing a distal Penokean source, and a minor <2.5 Ga peak derived from distal sources in the Superior Province. Multidimensional scaling of other Baraboo Interval quartzites and potential sources show that the WCQ is indistinguishable from the lower interval of the Baraboo Quartzite. Cumulative distribution plots also reflect principal source areas derived by erosion of underlying Yavapai-aged crust and distally derived Penokean and older sources from the southern Superior Province. New whole rock geochemical data from the WCQ and potential granitic sources from northwestern Iowa are compared and show high (>80) chemical index of alteration (CIA) values, indicating similar weathering intensities to other Baraboo Interval localities. The WCQ likely serves as the down slope equivalent during initial Baraboo deposition.

We report the results of detrital zircon U-Pb analysis from the middle Cambrian Flathead Sandstone from two locations in the eastern Bighorn Mountains of Wyoming and their implications for post-rift tectonics in western Laurentia. The Flathead Sandstone U-Pb zircon age spectra shows prominent age peaks at ~1.8 Ga and minor age peaks at ~2.7 Ga. Detrital zircon samples taken across the extent of the Flathead Sandstone have a much higher proportion of Archean grains nearer to the basal unconformity and further to the west, and age spectra overall get progressively younger in samples taken higher in the section and further east and south. The 2.7 Ga signature observed in the Bighorn Mountains is largely absent from the Wyoming Province rocks and is likely derived from the Superior Province. Proterozoic zircons are sourced from the east and southeast in the Trans-Hudson and Yavapai Province rocks exposed in the Transcontinental Arch. The differences in age spectra between samples correlated to their spatial and temporal differences likely reflects subsidence along the passive margin and progressive southwestward-advancing uplift of the Transcontinental Arch during Flathead deposition. Subsidence, denudation and burial of Wyoming Province rocks and uplift of those exposed in the Transcontinental Arch led to the removal of the Wyoming Province as a source terrane by the time of Flathead Sandstone deposition in the Bighorn Mountains. The Transcontinental Arch was a significant contributor to detrital zircons across Laurentia, and concurrently acted as a barrier to sediment dispersal from more distal and younger Proterozoic terranes to the east and south into northwestern Laurentia.

The Late Paleozoic Ice Age (LPIA) was a principal control of sedimentation across Gondwana from the late Devonian through early Permian. We assess the hypothesis that glacial to interglacial transitions in western Argentina were the primary control influencing sediment routing patterns among the various Carboniferous-Permian basins in western Argentina. The Carboniferous Ansilta Formation consists of glaciomarine, nearshore, and fluvial systems deposited during the LPIA along the eastern margin of the Calingasta-Uspallata Basin in Argentina. The lower, glacially influenced succession of the Ansilta Formation records at least five glacial advances; the upper succession of consists of progradational shallow marine, deltaic, and fluvial strata. We combine 1225 new U–Pb zircon ages from six samples of the Carboniferous Ansilta Formation in the Calingasta-Uspallata Basin with 5864 U–Pb ages from 147 published samples in the detritalPy-mix forward mixture model to characterize provenance shifts. For the glacially influenced lower Ansilta Formation, sediment was derived locally from the Protoprecordillera, which was a prominent highland with alpine glaciers flowing west and east into the Calingasta-Uspallata and Paganzo basins, respectively. Thus, there was little or no connection between these two basins during Serpukhovian-Bashkirian glaciation. The fluvial/deltaic upper Ansilta had distal sediment sources in the Sierras Pampeanas. Furthermore, our results support the collapse of the Protoprecordillera topographic barrier, enabling drainage patterns connecting the Paganzo and Calingasta-Uspallata basins by late Pennsylvanian-early Permian time.

Here we present the results of detailed (1:24,000 scale) mapping of the Spanish Point 7.5 Minute Quadrangle, Wyoming, which occurs along the western flank of the Bighorn Mountains. The Quadrangle includes Archean crystalline rocks of the Wyoming Province Beartooth-Bighorn Magmatic Terrane, Paleozoic miogeoclinal clastic and carbonate strata (Flathead, Gros Ventre, Gallatin, Big Horn, Madison, and Amsden formations, and Quaternary (Pinedale Formation) glacial deposits. Isotopic age determinations (LA-ICPMS U-Pb on zircon) of the basement crystalline rocks, which occupy the higher elevations in the eastern part of the Quadrangle, were conducted at the University of Arizona Laserchron Center. Seven samples of grey foliated to massive granodiorite were analyzed, all yielding ages of about 2880–2885 Ma, which is correlative to the Lookout Mountain granodiorite to the north. The dominant basement fabric orientation is ∼N75W90. The Lookout Mountain Granodiorite was uplifted along the N-S striking, west verging Laramide Spanish Point Reverse fault, which dips steeply to the east. Paleozoic strata in the footwall are steeply inclined to overturned. Displacement along the fault is as much as 150 m. To the west, the Paleozoic strata roll into a homocline that dips ∼10 degrees to the west. The Pleistocene Pinedale Formation glacial deposits, which are as much as 50 m in thickness, occur as ground, end, and lateral moraines along the principal drainages.

New petrological, geochemical, and P-T modelling results from igneous samples clarify how carbonatite-lamprophyre magmatism, fluorite and rare earth element (REE) enrichment are petrogenetically related in southern Illinois. P-T modelling reveals that igneous rocks derive from a deep mantle carbonated source, that is consistent with trace element signatures for a fluorine-rich transition zone origin. Major element systematics suggests liquid-immiscibility with lamprophyric melts as the origin for Ca-carbonatites. Heavy REE (HREE) enrichments in Hicks Dome breccias likely formed through preferential partitioning and transport of HREE by brine-melts, exsolved from a deep carbonatite body. Brine-melts redistributed HREEs throughout the system along brecciated pathways where they reprecipitated as HREE-rich phosphate/fluorcarbonate minerals (e.g. xenotime, florencite, synchesite) in host bedrock. The diversity of igneous rocks in southern Illinois highlights the area as an excellent natural laboratory to study carbonated melt petrogenesis and evolution.