Western Laurentia Research Articles

Ordovician and Devonian microfacies of Sierra Agua Verde, central Sonora, Mexico: A new locality of the Nuia Province from western Laurentia

The Lower Ordovician carbonate rocks exposed in the Tuntunudé hill north of the Sierra Agua Verde, northeast of the town of Mátape, in central Sonora, consists of intraformational breccia, conglomeratic limestone, sandy limestone and limestone, which are overlain by a mixed succession of quartz sandstone and dolomitic sandstone. This succession is in turn unconformably overlain by an Upper Devonian sequence of dolomitic limestone, marl, conglomeratic limestone and sandy dolomite. The Lower Ordovician rocks contain abundant bioclasts of gastropods, brachiopods, trilobites, echinoderms, and sponges. Their microfacies show a marine medium to low energy environment, with sedimentation dominated by distal tempestites in laminar stages and intraclast debris formation during deposition. The depositional paleoenvironment corresponds to a marine middle-to-outer ramp. These strata are characterized by conodonts belonging to the Oepikodus communis and Reutterodus andinus biozones, as well as cyanobacteria incertae sedis Nuia sibirica. Devonian rocks overlying the Ordovician strata contain bioturbation and abundant bioclasts of echinoderms, brachiopods, bryozoans, gastropods, chaetetiform sponges, stromatoporoids; and also, colonial corals like Hexagonaria attenuata, Phillipsastrea hennahii and Phillipsastrea jachowiczi, monothalamous foraminifera like Vicinosphaera sp., monoserials such as Tikhinella measpis and planispiral such as Nanicella gallowayi. The microfacies show a high-energy environment in marine tropical shallow open-sea lagoonal waters, with ooids and reef building organisms.

Provenance of Paleogene Strata at Slim Buttes, South Dakota: Implications for post-Laramide Evolution of Western Laurentia

Slim Buttes is a 30 km long by 10 km wide set of buttes containing Paleogene strata in northwest South Dakota. At Reva Gap in northern Slim Buttes, Eocene-Oligocene terrestrial strata of Chadron and Brule Formations of the White River Group unconformably overlie the Paleocene Fort Union Formation. An angular unconformity separates the White River Group from overlying Oligocene and Miocene strata of the Arikaree Group. Using detrital zircon U-Pb ages, we determine the provenance of these rocks as part of a broader synthesis of post-Laramide sedimentation in the Rocky Mountains and western Great Plains. The Chadron Formation age spectrum is dominated by Cretaceous and Proterozoic grains that are interpreted to be locally recycled from the underlying Cretaceous and Paleocene strata. The Brule Formation has a maximum depositional age of ~34 Ma; Paleogene zircons dominate the age spectrum, and a wide variety of older zircons are also present. The Oligocene zircons are interpreted to have been sourced from volcanic systems in the Great Basin to the southwest, while the subsequent proportions of the zircons were derived from a variety of source areas in the Nevadaplano and Rocky Mountain areas to the southwest. Sparse amounts of Archean zircons are thought to represent the burial of Laramide uplifts throughout Wyoming at the time of Brule deposition, making for a regional paleotopography with little relief across the western interior of the United States. The Miocene-age Arikaree Group sand has a maximum depositional age of ~26 Ma and a multimodal detrital zircon age spectrum. The Arikaree Group provenance likely represents continued sourcing in the Great Basin volcanic systems and Nevadaplano, the beginnings of the re-exhumation of Laramide basement uplifts, and subsequent sediment evacuation out of the western interior and into the Gulf of Mexico to the southeast. Our findings indicate that the transport process and detrital zircon provenance signatures of these strata are decoupled, and each have their own independent evolution. The volcanic signature is primarily transported via aeolian processes (i.e. volcanic ash), and the recycled detrital zircon signature is primarily transported via fluvial processes.

The early–middle Cambrian siliciclastic tide‐dominated succession in eastern Korea

AbstractThe basal Taebaek Group (Myeonsan and Myobong formations) illustrates second‐order transgression on the eastern Sino‐Korean Block during the early–middle Cambrian (Stage 3?–Wuliuan). The irregular palaeotopography of Precambrian basement led to the development of localised alluvial fans and fan deltas. As transgression continued, tidally influenced estuarine and associated shallow subtidal flats developed. Continued transgression resulted in a relatively deep subtidal environment, with postulated carbonate shoals serving as barriers. Stacking of facies indicates a general deepening‐upward trend, transitioning from terrestrial–nearshore to marine environments dominated by tidal effects. The study area subsequently experienced shallowing, leading to the emergence of tidal inlet and dune environments. Siliciclastic input was attenuated, facilitating the development of carbonate shoal complexes. The overall transgressive succession of the basal Taebaek Group aligns with similar sequences observed in the Sino‐Korean Block and western Laurentia, where the lowermost Cambrian strata which formed in various depositional environments on the uneven basement rocks were followed by extensive tide‐dominated siliciclastic successions. These successions were then overlain by epeiric carbonate platforms that developed synchronously across the area as siliciclastic input diminished with the continued transgression. This comparison demonstrates that the transgressive sequences in the Sino‐Korean Block and western Laurentia can be understood within a framework that explains how extensive Cambrian carbonate platforms initially developed due to a second‐order eustatic sea‐level rise during the Cambrian greenhouse period.

Recently Identified Mesoproterozoic Strata in South‐Central Idaho Document Late‐Stage Rifting of the Nuna Supercontinent in Western Laurentia

AbstractSedimentary basins are valuable archives of tectonic processes involved in continental rifting. The northern Rocky Mountains preserve the Belt Supergroup, one of the most complete records of Mesoproterozoic strata on Earth; however, debate remains about its tectonic origin. We investigated a recently identified package of Mesoproterozoic strata at Leaton Gulch near Challis, Idaho, using a combination of traditional and newer sedimentological tools. Results suggest that the Leaton Gulch stratigraphic section was deposited in a fluvial setting ca. 1,380–1,317 Ma, spanning the poorly documented interval between late Belt Supergroup deposition at ∼1,370 Ma and recently characterized Deer Trail Group strata that are less than 1,300 Ma. Detrital zircon age distributions from Leaton Gulch demonstrate a similar provenance signature to Missoula Group rocks of the upper Belt Supergroup; however, Leaton Gulch strata are up to ∼70 Ma younger than most prior age constraints on Belt Supergroup rocks. Regional metabentonites (interpreted as metamorphosed reworked tuffs) found within Leaton Gulch and Missoula Group strata show dominantly radiogenic εHf(t), with a range of −8 to +15, interpreted as a mix of primary mantle and remelted metasedimentary sources. Zircon trace element data of the metabentonite from Leaton Gulch suggest a 1,450–1,300 Ma geochemically consistent and moderate–high silica melt source. Collectively, the strata of Leaton Gulch record basin sedimentation during a critical window of Mesoproterozoic time. We speculate that sedimentation during late‐stage Belt Supergroup deposition thickened and stepped westward, abandoning the main Belt basin, culminating with breakup of the Nuna Supercontinent.

The metamorphic footprint of western Laurentia preserved in subducted rocks from southern Australia

AbstractPolymetamorphic metapelitic rocks in central‐west Tasmania, southern Australia, contain high‐pressure mineral assemblages that formed during Cambrian‐aged subduction and relict garnet with published Lu–Hf ages of c. 1285–1240 Ma. These garnet ages, along with published detrital zircon data from throughout western Tasmania and western North America, have been used to propose the presence of Mesoproterozoic Laurentian crust in western Tasmania. In this study, we combine zircon petrochronology with compositional information from the inclusion assemblages in relict garnet to extract Mesoproterozoic pressure–temperature data from subduction‐overprinted rocks, which effectively constitute an interpreted remnant of Laurentian crust now residing in central‐west Tasmania. The new data suggest Mesoproterozoic metamorphism involved two stages. The first event is recorded by c. 1480–1235 Ma zircon that formed in a garnet‐absent, plagioclase‐present, high‐thermal gradient environment at pressures no greater than ~5–5.5 kbar. The second event recorded by c. 1285–1240 Ma relict garnet was characterized by the development of a moderate‐pressure kyanite–plagioclase–biotite‐bearing mineral assemblage, which formed at ~8.5 kbar and ~590–680°C. These pressure–temperature constraints are attributed to extension within a deep basin system associated with the cryptic East Kootenay Orogeny in North America, which coincides with the final stages of c. 1450–1370 Ma upper Belt‐Purcell Basin sedimentation. Taking into account new detrital zircon U–Pb–Hf isotopic data from central‐west Tasmania in this study and existing zircon provenance data from throughout western Tasmania and the Belt‐Purcell Basin, our results strengthen the hypothesis of a Laurentian footprint that potentially encompasses much of western Tasmania and relates to both Nuna and Rodinian tectonism.

Early Ordovician seamounts preserved in the Canadian Cordillera: Implications for the rift history of western Laurentia

Abstract The breakup of the supercontinent Rodinia and development of the western Laurentian rifted margin are in part recorded by Neoproterozoic to mid-Paleozoic igneous and sedimentary rock successions in the Canadian Cordillera. New bedrock mapping and volcanic facies analysis of Early Ordovician mafic rocks assigned to the Menzie Creek Formation in central Yukon allow reconstruction of the depositional environment during the volcanic eruptions, whole-rock geochemical data constrain the melting depth and crust-mantle source regions of the igneous rocks within the study area, and zircon U-Pb age studies provide determination of the precise timing of submarine eruptions. Menzie Creek Formation volcanic rocks are interlayered with continental slope strata and show lithofacies consistent with those of modern seamount systems. Representative seamount facies contain several kilometers of hyaloclastite breccia and pillow basalt with rare sedimentary rocks. Menzie Creek Formation seamounts form a linear array parallel to the Twopete fault, an ancient extensional or strike-slip fault that localized magmatism along the nascent western Laurentian margin. Zircon grains from two volcanic successions yielded high-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates of ca. 484 Ma (Tremadocian), which are interpreted as the age of eruption. Menzie Creek Formation rocks are alkali basalt and have oceanic-island basalt–like geochemical compositions. The whole-rock trace element and Nd-Hf isotope compositions are consistent with the partial melting of subcontinental lithospheric mantle at ~75–100 km depth. Post-rift, Early Ordovician seamounts in central Yukon record punctuated eruptive activity along a rift-related fault, the separation of a continental fragment from western Laurentia, or the oblique post-breakup kinematics from the counterclockwise rotation of Laurentia that facilitated local extension in the passive margin.

Precise U-Pb age models refine Neoproterozoic western Laurentian rift initiation, correlation, and Earth system changes

The upper Tonian ChUMP (Chuar-Uinta Mountains-Pahrump) strata of the southwestern U.S.A. are hypothesized to be regional correlatives and to record a time of rift basin evolution commencing at ca. 770 Ma in western Laurentia (modern-day coordinates). We test this correlation using U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on detrital zircon grains from basal units within these successions. ChUMP units yield CA-ID-TIMS maximum depositional ages (MDA) between 775 and 766 Ma: the Chuar Group of AZ has an MDA of 770.1 ± 0.5 Ma (n = 1) and an additional young zircon mode at 775.7 ± 0.3 Ma (n = 11); the Uinta Mountain Group of northern UT has an MDA of 766.3 ± 0.5 Ma (n = 5) and contains a second young mode at 775.1 ± 0.7 Ma (n = 3); and the basal Horse Thief Springs Formation of the middle Pahrump Group CA has an MDA of 775.4 ± 0.7 Ma (n = 3). The ca. 775 and 770 Ma grains are interpreted to be from zircon-bearing mafic sources related to the 770–778 Ma Gunbarrel Large Igneous Province of Yukon and NW U.S.A. The 766 Ma population was either derived from the Mt Rogers complex of eastern Laurentia or could have come from conjugate margins that were in the process of rifting away, such as Tasmania.The CA-ID-TIMS dates on the Chuar Group in Grand Canyon anchor a Bayesian age model for evaluating late Tonian Earth systems. Faster sediment accumulation rates (80 + 150/-44 m/My) in the lower Chuar Group are consistent with the inception of an extensional basin related to Rodinia breakup; slower rates in the upper Chuar Group (25 + 12/-5 m/My) record are associated with relatively deeper water sedimentation and concomitant organic carbon burial during marine transgression. The model also constrains the timing of several biological events recorded in the Chuar Group, including eukaryovorous predation (>767 Ma), the first appearance of vase-shaped microfossils (∼741 Ma), and the ranges of Cerebrosphaera globosa (=C. buickii; 800–743 Ma) and Lanulatisphaera laufeldii.(766–740 Ma), both proposed as possible marine index fossils for late Tonian time. Finally, the model can also be used to search for stratigraphic evidence of a purported glaciation at ca. 751 Ma.

Two episodes of late Mesoproterozoic magmatism in the southwestern Yangtze Block, China, and their association with Grenville-aged orogenic cycles

We report three sets of newly identified late Mesoproterozoic mafic intrusions in the southwestern Yangtze Block, China. Secondary ion mass spectrometry (SIMS) baddeleyite and zircon U-Pb analyses yield crystallization ages of ca. 1.15 Ga for the Boka dolerites, ca. 1.07 Ga for the Lanniping gabbros, and ca. 1.06 Ga for the Tong’an mafic dikes. These are tholeiitic to alkaline rocks with enriched mid-ocean-ridge basalt (E-MORB)−like trace-element patterns and superchondritic εNd(t) values, indicating a partial melting origin from the asthenospheric mantle. These results, along with existing data, define two episodes of late Mesoproterozoic, rift-related mafic magmatism in the southwestern Yangtze Block: an early phase at 1.17−1.14 Ga and a late phase at 1.07−1.02 Ga. Initiation of the two episodes of extensional events in the southwestern Yangtze Block coincided with high-grade Grenville-aged metamorphism in Hainan Island and possible early collision of Cathaysia with the southwestern Yangtze Block, indicating an impactogen model. Therefore, we interpret the observed two episodes to have resulted from local collision between the southwestern Yangtze Block and the Cathaysia Block (including Hainan Island), which was possibly a part of western Laurentia during the assembly of Rodinia.

One billion years of tectonism at the Paleoproterozoic interface of North and South Australia

The Mount Woods Domain, in the northeastern Gawler Craton, occupies a tectonically important location in Proterozoic Australia, yet there is very little published U–Pb geochronology data from this region to underpin tectonic models. New LA-ICP-MS U–Pb monazite and detrital zircon geochronology reveal Archean to Paleoproterozoic basement in the central Mount Woods Domain, comprising metasedimentary rocks and garnet-bearing granite with protolith ages of c. 2550–2400 Ma and metasedimentary rocks deposited after c. 1855 Ma. The southern Mount Woods Domain contains younger metasedimentary sequences deposited after 1750 Ma. Metamorphic monazite and zircon geochronology combined with phase equilibria modelling show the rocks of the central Mount Woods Domain were metamorphosed to granulite facies between 1700 and 1670 Ma, reaching pressure and temperature conditions of 4.8–5.3 kbar and 800–840 °C. Monazite geochronology from samples located along major shear zones and in the westernmost Mount Woods Domain record amphibolite facies metamorphism and reworking at 1570–1550 Ma, with a further phase of shear zone activity along the northern margin of the Mount Woods Domain at c. 1480 Ma. Laser ablation inductively coupled plasma triple quadrupole mass spectrometry (LA-ICP-QQQ-MS) Rb–Sr biotite ages from across the Mount Woods Domain range between 1480 and 1390 Ma. The protracted geological history in the Mount Woods Domain from c. 2500–1400 Ma provides a piercing point linking different regions of Proterozoic Australia and western Laurentia during the tenure of the Nuna supercontinent.

The Antler orogeny reconsidered and implications for late Paleozoic tectonics of western Laurentia

Abstract Mississippian rocks of the Antler foreland basin in Nevada (western United States) record its evolution in detail, clarifying the timing and tectonic significance of the Antler orogeny. Synorogenic clastic sediments mark the initiation of tectonism; they arrived in Nevada from the north or northwest in late Early Mississippian (Osagean). An orogenic highland formed in northern Nevada in late Middle Mississippian (Meramecian), producing a regional angular unconformity and terminating deposition in the Antler foreland basin. Unconformably overlying Upper Mississippian (Chesterian) rocks provide tight age control for the redefined “Antler orogeny” sensu stricto. Palinspastic restoration of Sr isotopic data reveals a projecting step in the Laurentian margin in northern Nevada. The Antler orogeny resulted from late Middle Mississippian sinistral-oblique convergence at this step. Continuing oblique convergence produced the subsequent late Paleozoic basins, structures, and unconformities in northern Nevada. The Roberts Mountains allochthon is translated material from farther north emplaced onto the margin. Its basal bounding fault, long mapped as the “Roberts Mountains thrust,” is in fact not a single continuous fault, motion on it was not synchronous, and displacement on many fault segments mapped as the Roberts Mountains thrust postdate the late Middle Mississippian. We therefore propose that the term “Roberts Mountains thrust” be abandoned. This new paradigm not only explains anomalous characteristics of the Antler orogeny but also illustrates criteria for recognizing oblique convergence at other ancient plate margins.

Structural control of Cambrian paleotopography and patterns of transgression in western Laurentia

Abstract The Cambrian transgression across the Great Unconformity produced one of the largest expansions of shallow marine habitats and associated diversification of marine invertebrate faunas in Earth history. However, identification of the underlying controls on the pattern of transgression of Cambrian seas has been hampered by imprecise or inaccurate age assignments for many formations. Recovery of an Ehmaniella Zone trilobite fauna from the Lodore Formation in northwestern Colorado (United States) revises the age of this unit to be significantly older, specifically middle Miaolingian (upper Wuliuan). This expands the established distribution of thick Miaolingian deposits of the northern Rocky Mountains to within 90 km of a broad region of central Colorado where Miaolingian strata are missing and Furongian successions rest directly on basement. The boundary between these two regions marks the position of an ~200 km east-west offset within the generally north-south–trending Cambrian paleoshoreline of western North America. The offset is co-located with a Precambrian continental suture zone (Cheyenne belt) at the northern Yavapai terrane margin and is directly east of an offset of similar magnitude and latitude in the early to middle Paleozoic shelf edge in Nevada. We thus posit that Precambrian deep-seated crustal-scale features controlled the Cambrian paleotopography of western Laurentia, strongly influencing the patterns of Cambrian transgression and structure of the shallow marine ecosystem established during continental submergence. These continental-scale structural elements remained a major control on marine paleogeography for >200 m.y. into the late Paleozoic.

Large detrital zircon data set investigation and provenance mapping: Local versus regional and continental sediment sources before, during, and after Ancestral Rocky Mountain deformation

The increasing number and size of detrital geochronology data sets offer new opportunities for increased accuracy and resolution of sediment routing models. However, the new opportunities come coupled with challenges in large data integration and visualization. We address these challenges by outlining two novel approaches that aid in analyzing and interpreting large detrital geochronology data sets: (1) combination of bottom-up and top-down detrital zircon source modeling, and (2) sediment provenance mapping. Combining source-modeling methods provides guidance in identifying empirical detrital zircon sources and determining source proportions. Provenance mapping integrates source proportions from modeling results and complimentary geologic data (e.g., paleocurrents, paleogeography, and stratal thickness maps) to extrapolate provenance information through areas with sparse or ambiguous data, thus mitigating issues of data distribution heterogeneity. Sediment provenance maps also provide a synoptic view of data that, along with detrital zircon source modeling, aids in circumventing lengthy descriptions of individual age modes for data sets containing hundreds of samples, which can obscure underlying trends in the data. We apply this approach to late Paleozoic−early Mesozoic strata, using 329 published and new U-Pb detrital zircon samples, and document five sediment-routing episodes in the core zone of intraplate deformation in western Laurentia (i.e., the Ancestral Rocky Mountains (ARM)). The transitions between these episodes are defined by changes in sediment source distribution, which are illustrated by provenance maps that show (1) the degree and extent of ARM basin isolation from transcontinental sediment sources and (2) ARM-driven changes in transcontinental sediment routing systems. We map possible sediment pathways of distally derived sediment around the ARM core, illustrating that ARM uplifts diverted transcontinental systems around areas of intense intraplate deformation. Further, the evolution of sediment routing in western Laurentia before, during, and after ARM deformation provides an example of the interaction between transcontinental sediment routing and intraplate deformation.

Mesoproterozoic basins (Yukon, Canada) in the evolution of supercontinent Columbia

Geochemical and geochronological data from the Pinguicula Group and unit PR1 of the lower Fifteenmile Group (Yukon, Canada) provide information on sediment provenance and timing of break-up of supercontinent Columbia and seaway development on Laurentia’s northwestern margin. The older unit PR1, in the Coal Creek inlier, has a near-unimodal detrital zircon population with an age of 1499 ± 3 Ma. The Pinguicula Group detrital zircon data, in the Wernecke and Hart River inliers, display a polymodal detrital zircon population with a maximum age of <1322 ± 23 Ma. Using detrital zircon signatures, Sm–Nd isotopic data, and C-isotopic signatures, lithostratigraphic correlations between the Pinguicula Group in the Wernecke and Hart River inliers are confirmed, whereas the Pinguicula Group and unit PR1 are no longer considered correlative. The zircon population in unit PR1 requires a proximal source, but sources of this age are not known in western Laurentia. Based on detrital zircon and Sm–Nd data, sediment in unit PR1 was derived from the Mt. Isa inlier in northeastern Australia sometime after 1460 Ma. Unit PR1 correlates with older Mesoproterozoic successions, including the Belt-Purcell, that were deposited during break-up of supercontinent Columbia, and contain sediment from Australia and the Mawson continent. Mesoproterozoic successions deposited after 1.45 Ga, including the Missoula Group, lack North American Magmatic Gap (NAMG)-aged zircon and instead record sediment provenance from southern Laurentia, as north Australia and the Mawson continent rifted from Laurentia’s western margin. The Pinguicula Group has few NAMG-aged grains that were probably recycled from older Mesoproterozoic basins.

Fold-and-thrust belts and associated basins: a perspective on their structure, sedimentation, and dynamics

Fold-and-thrust belts are structural features that accommodate upper-crustal shortening by the growth of a series of thrust faults and folds. Recent studies show that a better understanding of the structure and sedimentation styles of fold-and-thrust belts and their associated basins can provide crucial insights for improved interpretations of the evolution of ancient and modern convergent margins and the mechanisms of intracontinental deformation. To achieve a more comprehensive understanding of the development of contractional orogenic belts, this thematic collection gathers contributions that explore different types of fold-and-thrust belts at various scales around the world, via different approaches including theory development, structural and stratigraphic observations from the field, geophysical analyses, and numerical modelling. Case studies include the northern margin of the Tibetan plateau and Pamir region, the Timanian and Caledonian orogenies in northern Norway, orogenic belts in western Laurentia, and the Andes of western South America. These studies reemphasize the importance of integrating broad datasets when documenting the distribution, geometry, and kinematics of structures in fold-and-thrust belts and their associated basins, including field-based structural observations, provenance, low-temperature thermochronologic, geomorphologic, and subsurface data, and analog and numerical models. This thematic collection aims to encourage further efforts for comparative studies of the fold-and-thrust belts around the world and proposes interdisciplinary research to address outstanding questions in the study of contractional orogens. Thematic collection: This article is part of the Fold-and-thrust belts collection available at: https://www.lyellcollection.org/topic/collections/fold-and-thrust-belts-and-associated-basins

Novel age constraints for the onset of the Steptoean Positive Isotopic Carbon Excursion (SPICE) and the late Cambrian time scale using high-precision U-Pb detrital zircon ages

AbstractThe Steptoean Positive Isotopic Carbon Excursion (SPICE) is a prominent +4–5‰ shift in the Cambrian δ13C record used for global chronostratigraphic correlation. The onset of this excursion is traditionally placed at the base of the Pterocephaliid trilobite biomere (base of the Furongian Series). Recent studies have documented local controls on the expression of the SPICE and emphasize the need for chronostratigraphic standards for these complex biogeochemical signals. We build upon prior work in western Laurentia by integrating δ13C and biostratigraphy with high-precision isotope dilution U-Pb detrital zircon maximum depositional ages that are coincident with the onset, peak, and falling limb of the SPICE. Our study provides the first useful numerical age constraint for the onset of the SPICE and the Laurentian trilobite biozones and requires revision of the late Cambrian geologic time scale boundaries by several million years.

Demarcation of Early Pennsylvanian paleovalleys in depozones of the Appalachian foreland-basin system based on detrital-zircon U-Pb and Hf analysis

Abstract Detrital-zircon (DZ) U-Pb data show that Appalachian-affiliated sediment was transported to western Laurentia by the Carboniferous, yet additional DZ U-Pb data from the eastern United States suggest that sediment-routing systems were oriented south toward the Ouachita deepwater sink. Within this context, this study presents DZ U-Pb ages from the Lower Pennsylvanian Caseyville Formation of Illinois, and U-Pb ages and εHf values from the coeval Pottsville Formation of Alabama as well as sandstone petrographic data from the Caseyville Formation, the Pottsville Formation, and the Jackfork Group of the Ouachita Basin to document provenance, delineate drainage divides in the Appalachian foreland-basin system, and comment on the unlikelihood of transcontinental sediment routing from the eastern United States to western United States at this time. Two DZ U-Pb age distributions from quartz arenite sandstones of the Caseyville Formation display prominent ca. 1250–950 Ma, 1550–1300 Ma, 1800–1600 Ma, and 3500–3000 Ma ages, consistent with ultimate derivation from Grenville, Midcontinent granite–rhyolite, Yavapai–Mazatzal, and Superior provinces, as well as minor contributions from ca. 500–400 Ma and 2000–1800 Ma grains. Two DZ U-Pb age distributions from sublitharenite sandstones of the Pottsville Formation display prominent ca. 500–400 Ma, 1250–950 Ma, 1550–1300 Ma, and 1800–1600 Ma ages, consistent with ultimate derivation from Appalachian, Grenville, Midcontinent granite–rhyolite, and Yavapai–Mazatzal provinces, as well as minor contributions from ca. 2000–1800 Ma and 3500–3000 Ma grains. The Pottsville Formation samples demonstrate a greater percentage of Appalachian and Grenville ages relative to the Caseyville Formation samples, whereas the Caseyville Formation samples have elevated Yavapai–Mazatzal and Superior percentages relative to the Pottsville. We interpret these differences to suggest parallel fluvial systems in the foredeep and back-bulge depozones of the Appalachian foreland-basin system. Like DZ studies of modern deep-sea fans that demonstrate an affinity to feeder fluvial systems, this study demonstrates fidelity between endmember segments of ancient fluvial-to-deepwater systems. Multidimensional scaling (MDS) analysis shows that DZ samples from the Pottsville and Caseyville formations cluster with deepwater Jackfork Group samples, and we infer a source-to-sink relationship from these two distinct source areas to the Ouachita terminal sink. One example of large-scale inclined strata thickness from the Caseyville Formation also suggests a drainage basin area of > 105 km2. Contextualized with these observations, we suggest that the foredeep and backbulge depozones of the Appalachian foreland-basin system steered distinct Early Pennsylvanian rivers across emergent continental shelves during periods of low sea-level, which discharged to distinct slope canyons and sourced > 100-km-long deep-sea fans. Clearly circumscribed, southward- or southwestward-oriented paleodrainage areas provide a template of the Appalachian foreland-basin system, and as such the central and southern Appalachians were an unlikely source for the Appalachian signature observed in the western United States at this time.

Spatial distribution of 1.4-1.3 Ga LIPs and carbonatite-related REE deposits: Evidence for large-scale continental rifting in the Columbia (Nuna) supercontinent

Large-scale continental rifting and large igneous provinces (LIPs) have played important roles in the breakup and dispersal of supercontinents Pangea and Rodinia. Previous paleomagnetic and geological evidence suggests fragmentation of Columbia (or Nuna) supercontinent during 1.4-1.2 Ga. However, no large-scale continental rift system has been identified in Columbia during this period, although some continental rifting events between 1.6 Ga and 1.2 Ga have been reported. Here we provide a refined ca. 1.4 Ga paleogeographic reconstruction of Columbia based on previously published paleomagnetic data and geological evidence for connections between continental blocks, and consistent with spatial and temporal distributions of the 1.4-1.3 Ga LIPs and smaller intraplate mafic magmatic events interpreted as LIP fragments/remnants. Our results indicate a 1.4–1.3 Ga large-scale continental rift zone located along western Laurentia, western-northern Siberia, southeastern Baltica, western-northern West Africa, southwestern Amazonia, southern-eastern Congo/São Francisco, eastern Kalahari, northern North China and northern North Australia. This rift zone extends about 15,000 km across supercontinent Columbia, and is a main indicator and a proximal reason for its final breakup. The spatial distribution of the 1.4-1.3 Ga carbonatite-related REE deposits shows that this newly identified rift system also controlled Bayan Obo, the world's first and largest REE-Nb deposit in northern North China, and Mountain Pass, the world's second largest REE deposit in western North America. Given the scale of this rift system it can be expected to be the locus of additional carbonatite intrusions as well as other commodity types, and provides important constraints on mechanism and processes for final breakup and dispersal of Columbia supercontinent.

Extensional Setting of Hainan Island in Mesoproterozoic: Evidence from Granitic Intrusions in the Baoban Group

AbstractThe Mesoproterozoic Baoban Group is the oldest basement in Hainan Island and has played an important role in Columbia (Nuna) supercontinent reconstructions. The Mesoproterozoic granitic intrusions in the Baoban Group are the most widely‐exposed Precambrian magmatic rocks and are the key to understanding the tectonic settings of Hainan Island and its relationship with the South China Block and the Columbia supercontinent. New LA‐ICP‐MS zircon U‐Pb dating on three mylonitic granite samples from the Tuwaishan and Baoban areas yield ages ranging from 1447 Ma to 1437 Ma, representing the absolute timing of the emplacement of the granitic intrusions. Combined with previously published geochronological data for rocks from the Baoban Group and regional mafic intrusions, it is concluded that the Baoban Group formed at 1460–1430 Ma, coeval with the emplacement of the granitic and mafic intrusions. New in‐situ zircon Lu‐Hf isotope analyses for the three mylonitic granite samples yielded positive ∊Hf(t) values, ranging from +0.49 to +8.27, with model ages () ranging from 2181 Ma to 1687 Ma, suggesting that the granitic intrusions originated from a mixed source of Paleoproterozoic crust with juvenile crust. New zircon trace element data show characteristics of high Th/U values of 0.24–1.50, steep slopes from LREE to HREE and negative Pr, Eu anomalies with positive Ce, Sm anomalies, representing typical magmatic zircons formed in continental crust. Compared with available magmatic and detrital zircon ages from Precambrian rocks in the Cathaysia Block, Yangtze Block and western Laurentia, it is inferred that Hainan Island was separated from both the Cathaysia Block and the Yangtze Block, instead being connected with western Laurentia in the Columbia supercontinent. Considering the decreasing tendency of basin deposition time along the western margin of Laurentia, it is proposed that Hainan Island was located to the north or northwest of the Belt‐Purcell Supergroup, along the western margin of Laurentia, during the breakup of the Columbia supercontinent.

Devonian to Carboniferous continental-scale carbonate turnover in Western Laurentia (North America): upwelling or climate cooling?

The Devonian to Carboniferous (DC) transition coincided with a green-to-ice house climatic shift, anoxia, disappearance of lower latitude carbonate banks, and turnover from warm-to-cool water carbonate factories. In western Laurentia, the switch to carbonate factories dominated by cool-water biota was contemporaneous with a tectonically driven palaeogeographic change. To investigate this depositional shift and infer the relative impact of climate vs tectonics, a continental-scale sedimentological and geochemical study was conducted on twelve stratigraphic sections of DC strata from western Canada to southern Nevada (USA). The spatial–temporal distribution of microfacies records the turnover from [i] a Famennian lime mud-rich, shallow warm-water carbonate ramp with low sedimentation rates, mesotrophic conditions and tabular geometry to [ii] Tournaisian to Viséan lime mud-depleted and grainstone dominated cool-water carbonate ramp with anomalous high sedimentation rates, oligotrophic conditions and a pronounced slope. Positive excursions of δ18Ocarb (+ 2‰ V-PDB) and δ13Ccarb (+ 4‰ V-PDB) of Lower Mississippian carbonates likely correspond to the first cooling peak of the Carboniferous-Permian icehouse climate, following carbon withdrawal during black shale deposition during the late Famennian and early Tournaisian. However, late Tournaisian return of photozoan elements and their persistence throughout the Viséan suggests that warmer surface water existed, revealing a decoupling of the lower latitude ocean and the atmosphere. Shoaling of the thermocline was likely a result of cold-water upwelling along an open coast, as the Antler orogen no longer provided an oceanic obstruction to the west. This study shows that carbonate platforms are more susceptible to regional changes than global shifts.

Detrital Zircon U-Pb Geochronology of Upper Devonian and Lower Carboniferous Strata of Western Laurentia (North America): A Record of Transition from Passive to Convergent Margin

Abstract The Late Devonian-Early Carboniferous (DC) Antler orogeny in southwestern Laurentia is contemporaneous with influx of clastic sediments, unconformities, and volcanism across much of western Laurentia (WL), suggesting the demise of the Paleozoic passive margin. However beyond the type Antler orogeny in southwestern Laurentia, the DC tectonic setting is still unclear. Westerly sediment provenance has been suggested as evidence of a convergent margin setting in a foreland basin. However, there is a gap in our understanding in central WL (Alberta and Montana) despite the fact that paleogeographic reconstructions place this area at the centre of WL. We provide detrital zircon (DZ) U-Pb geochronological data from strata in Alberta, Montana, and Nevada that are synchronous with the Antler orogeny to constrain sediment dispersal patterns and test the westerly sediment sourcing hypothesis. We show three DZ facies specific to particular geographic locations: DZ facies 1 in southern Nevada has a prominent subpopulation of early to mid-Mesoproterozoic (mode at 1430 Ma), DZ facies 2 in northeastern Nevada has a late Paleoproterozoic population (mode at 1823 Ma), and DZ facies 3 in Alberta and Montana displays Mesoproterozoic to Neoproterozoic (mode at 1036 Ma), mid-Paleozoic (mode at 411 Ma), and depositional (ca. 360-340 Ma) ages. North-south variation in DZ facies indicates that WL basins were locally sourced from various tectonic fragments having different signatures. Comparing our data with published data, we show that WL is dominated by DZ recycled from uplifted older strata with input from mid-Paleozoic arc terrane (s) to the west. Westerly sourcing is evidenced by the presence of near-depositional ages and affinities of this study’s DZ facies with strata located to the west. Our results and geological evidence from other studies suggest that the Antler orogeny triggered a depositional shift and controlled sediments dispersal in WL, signaling the demise of the Paleozoic passive margin.