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The Lower Cretaceous sequence of western Alaska—demise of the Koyukuk terrane?
Warm-water Tcherskidium fauna (Brachiopoda) in the Late Ordovician Northern Hemisphere of Laurentia and peri-Laurentia
Middle Ordovician (Whiterockian) gastropods from central Sonora, Mexico: affinities with Laurentia and the Precordillera
Late Devonian magmatism and clastic deposition in the upper Earn Group (central Yukon, Canada) mark the transition from passive to active margin along western Laurentia
Late Ordovician brachiopods from east-central Alaska, northwestern margin of Laurentia
The First Stringocephalid Brachiopods in the Upper Givetian of Selennyakh Range (Northeast Asia) and their Paleobiogeographical Significance
Alaskodiscus , a New Name for The Ordovician Bellerophontoidean Gastropod Alaskadiscus Rohr, Frýda and Blodgett, 2003
Biodiversity, biogeography and phylogeography of Ordovician rhynchonelliform brachiopods
Abstract The phylogeographical evolution and the consequent changing distribution and diversity of rhynchonelliform brachiopods through the Ordovician are linked to the dynamic palaeogeography of the period. The Early Ordovician (Tremadocian and Floian) is characterized by globally low-diversity faunas with local biodiversity epicentres, notably on the South China Palaeoplate; low-latitude porambonitoid-dominated faunas with early plectambonitoid and clitambonitoid representatives, as well as high-latitude assemblages mostly dominated by orthoids, can be recognized, but many taxa are rooted in Late Cambrian stocks. The Early Ordovician displays a steady increase in rhynchonelliformean biodiversity, which was mostly driven by the increasing success of the Porambonitoidea and Orthoidea, but the billingsellids and early plectambonitoids also contributed to this expansion. During the Early to Mid Ordovician (Dapingian–Darriwilian), marine life experienced an unprecedented hike in diversity at the species, genus and family levels that firmly installed the suspension-feeding benthos as the main component of the Palaeozoic fauna. However, this may have occurred in response to an early Darriwilian annihilation of existing clades, some of which had been most successful during the Early Ordovician. New clades rapidly expanded. The continents were widely dispersed together with a large number of microcontinents and volcanic arcs related to intense magmatic and tectonic activity. Climates were warm and sea-levels were high. Pivotal to the entire diversification is the role of gamma (inter-provincial) diversity and by implication the spread of the continents and frequency of island arcs and microcontinents. The phylogeographical analysis demonstrates that this new palaeogeographical configuration was particularly well explored and utilized by the strophomenides, especially the Plectambonitoidea, which radiated rapidly during this interval. The porambonitoids, on the other hand, were still in recovery following the early Darriwilian extinctions. Orthides remained dominant, particularly at high latitudes. Biodiversity epicentres were located on most of the larger palaeoplates, as well as within the Iapetus Ocean. Provincial patterns were disrupted during the Sandbian and early Katian with the migration of many elements of the benthos into deeper-water regimes, enjoying a more cosmopolitan distribution. Later Katian faunas exhibit a partition between carbonate and clastic environments. During the latest Katian, biogeographical patterns were disrupted by polewards migrations of warm-water taxa in response to the changing climate; possibly as a consequence of low-latitude cradles being developed in, for instance, carbonate reef settings. Many clades were well established with especially the strophomenides beginning to outnumber the previously successful orthides, although this process had already begun, regionally, in the mid to late Darriwilian. At the same time, atrypoid and pentameroid clades also began to radiate in low-latitude faunas, anticipating their dominance in Silurian faunas. The Hirnantian was marked by severe extinctions particularly across orthide-strophomenide clades within the context of few, but well-defined, climatically controlled provincial belts. Supplementary material: The individual localities and a reference list for the data sources are provided at: http://www.geolsoc.org.uk/SUP18667
Abstract The biogeographical distribution of Ordovician and Silurian gastropods, monoplacophorans and mimospirids has been analysed on a generic level. The dataset contains 334 genera and 2769 species, yielding 1231 records of genera with 2274 occurrences worldwide. There is a bias towards eastern Laurentia, Baltica and Perunica records. Some 53.1% of the records are Ordovician. The study demonstrates that these molluscs are well suited to being used to improve understanding of Ordovician and Silurian biogeographical provinciality. Specific points are that: a Lower Ordovician assemblage is evident in Laurentia; the fauna of the Argentinean Precordillera is Laurentian until the Darriwilian, when taxa are shared with North China; Late Silurian gastropods from the Alexander terrane (SE Alaska) are unknown in Laurentia, but support a rift origin of this terrane from NE Siberia; Perunica, Ibero-Armorica and Morocco cluster together throughout the Ordovician but Perunica and Morocco are closer; Darriwilian–Sandbian deep-water Bohemian taxa occur in Baltica; a Laurentian–Baltica proximity is unsupported until the Silurian; Siberia clusters with North China and eastern Laurentia during the Tremadocian–Darriwilian; during the Gorstian–Pridoli Siberia clusters with the Farewell and Alexander terranes; North China may have been close to Laurentia and the Argentinean margin of Gondwana; and the affinity of Tarim taxa is problematic.
Emsian (Late Early Devonian) Sponges from West-Central and South-Central Alaska
Stratigraphic framework and estuarine depositional environments of the Miocene Bear Lake Formation, Bristol Bay Basin, Alaska: Onshore equivalents to potential reservoir strata in a frontier gas-rich basin
Newly recognized cyclomedusoid fossils in the Antelope Mountain Quartzite confirm that it is latest Neoproterozoic (Ediacaran) in age. Biogeographic affinities of the cyclomedusoid fossils suggest that the Yreka subterrane and its close associate, the Trinity subterrane, formed after the breakup of Rodinia in an ocean basin bordering Australia, northern Canada, Siberia, and Baltica. Reevaluating biogeographic, geological, and paleomagnetic evidence in the context of this starting point, the Yreka subterrane and Trinity subterrane may have been located at either 7°N or 7°S latitude ca. 580–570 Ma, but were not necessarily close to Laurentia. Continental detrital zircons (3.2–1.3 Ga) in the Antelope Mountain Quartzite most likely came from Australia or Siberia rather than Laurentia. The Yreka subterrane and Trinity subterrane record ∼180 m.y. of active margin events somewhere in Panthalassa (Proto-Pacific Ocean). Paleozoic biogeographic data, paleomagnetism, and regional relationships indicate that Yreka subterrane and Trinity subterrane were located throughout the early Paleozoic in the part of Panthalassa surrounded by Australia, NW Laurentia, Siberia, China, Baltica, and the Uralian terranes. By the mid-Devonian they were located at 31°N or 31°S in a somewhat isolated location, probably in a Northern Hemisphere oceanic plateau or island chain well outboard of other tectonic elements, and by the Permian they were almost completely isolated from other tectonic elements. The Yreka subterrane, as part of the Klamath superterrane, was not native to North America and did not accrete to it until the Early Cretaceous.
Silurian rocks in Alaska have been identified in 12 accreted terranes and in the Tatonduk-Nation River area of east-central Alaska, which represents part of autochthonous North America. Most of the terranes are in situ or structurally imbricated portions of the North American (or Siberian) continental margin. An exception is the Alexander terrane of southeastern Alaska, which originated as an offshore island arc. Discontinuously exposed and (or) highly altered sequences have precluded detailed investigations of Silurian rocks in most parts of Alaska, but reconnaissance-level studies reveal that graptolitic shales of turbidite or hemipelagic origin record deep-water or “shale out” conditions west or north of the ancient continental margin of North America. Platform carbonates are also exposed in many areas and are particularly well represented in southwestern (Nixon Fork subterrane of Farewell terrane) and southeastern (Alexander terrane) Alaska, indicating that much of Alaska resided close to the paleoequator in the Silurian. Subtidal stromatolite reefs in southwestern and southeastern Alaska that are similar to those in Salair and the Ural Mountains, Russia, indicate a paleobiogeographic connection between these two parts of Alaska, Siberia, and eastern Baltica via the Uralian Seaway in the Late Silurian. Deposition of vast accumulations of red beds and other siliciclastic rocks beginning in the Late Silurian suggests that parts of Alaska may have been affected by late stages in Cale-donian orogenesis and (or) early stages in the Ellesmere orogeny during formation of the Laurussian landmass.
Gastropods are described from Ludlow-age strata of the Heceta Limestone on Prince of Wales Island, southeast Alaska. They are part of a diverse megabenthic fauna of the Alexander terrane, an accreted terrane of Siberian or Uralian affinities. Heceta Limestone gastropods with Uralian affinities include Kirkospira glacialis , which closely resembles “ Pleurotomaria ” lindströmi Oehlert of Chernyshev, 1893 , Retispira cf. R. volgulica ( Chernyshev, 1893 ), and Medfracaulus turriformis ( Chernyshev, 1893 ). Medfracaulus and similar morphotypes such as Coelocaulus karlae are unknown from rocks that are unquestionably part of the North American continent (Laurentia) during Late Silurian time. Beraunia is previously known only from the Silurian of Bohemia. Pachystrophia has previously been reported only from western North American terranes (Eastern Klamath, York, and Farewell terranes) and Europe. Bathmopterus Kirk, 1928 , is resurrected and is only known from the Silurian of southeast Alaska. Newly described taxa include Hecetastoma gehrelsi n. gen. and n. sp. and Baichtalia tongassensis n. gen. and n. sp.
The Heceta Formation of southeastern Alaska (Alexander terrane) comprises a 3000-m-thick limestone-siliciclastic deposit of Early–Late Silurian age. The limestones record the first widespread evidence of carbonate platform development in this ancient island arc. Interbedded polymictic conglomerates represent interruption in platform evolution during onset of the Klakas orogeny, an arc-continent collisional event that occurred in the Late Silurian–Early Devonian. Conglomerates grade upward into finer-grained siliciclastics capped by shallow-marine limestones in sequences that are 200–300 m thick. Clasts range in diameter from 2 to 30 cm, are subangular to well rounded, poorly to moderately sorted, and densely packed in disorganized, poorly stratified beds. Most of the clasts are volcanic (basaltic-andesitic), but limestone clasts predominate in some sections; rare fragments of volcaniclastic, plutonic, and indeterminate rocks also occur. Clast compositions match the lithology of rocks in the underlying Heceta and Descon formations, and sedimentary attributes indicate redeposition of recycled material by debris flows and rivers in a coastal alluvial fan complex. This evidence—together with affinities of marine fossils, paleomagnetic and detrital zircon data, associated Old Red Sandstone-like facies, and coincidence in timing of tectonism—suggests the Klakas orogeny was a Caledonide event that is manifest in Alaska's Alexander terrane.
Devonian brachiopods of southwesternmost Laurentia: Biogeographic affinities and tectonic significance
Three brachiopod faunas discussed herein record different depositional and tectonic settings along the southwestern margin of Laurentia (North America) during Devonian time. Depositional settings include inner continental shelf (Cerros de Los Murciélagos), medial continental shelf (Rancho Placeritos), and offshelf continental rise (Rancho Los Chinos). Ages of Devonian brachiopod faunas include middle Early (Pragian) at Rancho Placeritos in west-central Sonora, late Middle (Givetian) at Cerros de Los Murciélagos in northwestern Sonora, and late Late (Famennian) at Rancho Los Chinos in central Sonora. The brachiopods of these three faunas, as well as the gastropod Orecopia , are easily recognized in outcrop and thus are useful for local and regional correlations. Pragian brachiopods dominated by Acrospirifer and Meristella in the “San Miguel Formation” at Rancho Placeritos represent the widespread Appohimchi Subprovince of eastern and southern Laurentia. Conodonts of the early to middle Pragian sulcatus to kindlei Zones associated with the brachiopods confirm the ages indicated by the brachiopod fauna and provide additional information on the depositional setting of the Devonian strata. Biostratigraphic distribution of the Appohimchi brachiopod fauna indicates continuous Early Devonian shelf deposition along the entire southern margin of Laurentia. The largely emergent southwest-trending Transcontinental arch apparently formed a barrier preventing migration and mixing of many genera and species of brachiopods from the southern shelf of Laurentia in northern Mexico to the western shelf (Cordilleran miogeocline) in the western United States. Middle Devonian Stringocephalus brachiopods and Late Devonian Orecopia gastropods in the “Los Murciélagos Formation” in northwest Sonora represent the southwesternmost occurrence of these genera in North America and date the host rocks as Givetian and Frasnian, respectively. Rhynchonelloid brachiopods ( Dzieduszyckia sonora ) and associated worm tubes in the Los Pozos Formation of the Sonora allochthon in central Sonora are also found in strati-form-barite facies in the upper Upper Devonian (Famennian) part of the Slaven Chert in the Roberts Mountains allochthon (upper plate) of central and western Nevada. Although these brachiopods and worm tubes occur in similar depositional settings along the margin of Laurentia in Mexico, they occur in allochthons that exhibit different tectonic styles and times of emplacement. Thus, the allochthons containing the brachiopods and worm tubes in Sonora and Nevada are parts of separate orogenic belts and have different geographic settings and tectonic histories. Devonian facies belts and faunas in northern Mexico indicate a continuous continental shelf along the entire southern margin of Laurentia. These data, in addition to the continuity of the late Paleozoic Ouachita-Marathon-Sonora orogen across northern Mexico, contradict the early Late Jurassic Mojave-Sonora megashear as a viable hypothesis for large-magnitude offset (600–1100 km) of Proterozoic through Middle Jurassic rocks from California to Sonora.
Devonian brachiopods from Northeastern Washington: Evidence for a non-allochthonous terrane and Late Devonian biogeographic update
There are several isolated outcrops (outliers?) of Devonian rocks in the Pacific Northwest (Washington and Oregon), USA. A locality in northeastern Washington, Limestone Hill, is considered in detail, and other small outcrops in northwest Washington and central Oregon are discussed. Limestone Hill is a Paleozoic outlier. The locality has Ordovician and Silurian (Llandovery and Wenlock) strata, Lower Devonian (Lochstone conglomerate, and Upper Devonian (Frasnian) carbonates with fossils. It has long been known that the area has many allochthons, and it has been assumed that Limestone Hill represents lithologies deposited much farther west. More recent data suggest that Limestone Hill is parautochthonous. Several brachiopod taxa, previously unknown from the Frasnian portion of Limestone Hill, have been found recently and are described herein. The brachiopod faunule consists of Emanuella sp., “ Allanella ” engelmanni , Cyrtina sp., Thomasaria sp., and an athyridid. These brachiopods appear to be like coeval faunas in Idaho, Montana, Utah, and Nevada, although more species assignments must be made. Frasnian brachiopods are in serious need of updates, as Famennian miospore and acritarch data suggest significant basin restriction and reduced seaway connectivity, with at least ephemerally extensive land areas with ubiquitous land plant taxa.
Paleobiogeographic affinities of Emsian (late Early Devonian) gastropods from Farewell terrane (west-central Alaska)
The vast majority of Emsian gastropods from Limestone Mountain, Medfra B-4 quadrangle, west-central Alaska (Farewell terrane) belong to species with lecithotrophic larval strategy. The present data show that there is no significant difference in the paleobiogeographic distribution of Emsian gastropod genera with lecithotrophic and planktotrophic larval strategies. Numerical analysis of the faunal affinities of the Emsian gastropod fauna from the Farewell terrane reveals that this terrane has much stronger faunal connections to regions like Variscan Europe, eastern Australia, and the Alexander terrane of southeast Alaska than to cratonic North America (Laurentia). The Canadian Arctic Islands is the only region of cratonic North America (Laurentia) that shows significant faunal affinities to the Emsian gastropod faunas of the Farewell terrane. The analysis also indicates a close faunal link between the Farewell and Alexander terranes. Published paleontological and geological data suggest that the Farewell and Alexander terranes represents tectonic entities that have been rifted away from the Siberia, Baltica, or the paleo-Pacific margin of Australia. The results of the present numerical analysis are not in conflict with any of these possibilities. However, the principle of spatial continuity of the wandering path prefers Siberia as the most probable “parental” paleocontinent for the derivation of both the Farewell and Alexander terranes.
U-Pb isotopic dating of detrital zircons from a conglomeratic barite sandstone in the Sonora allochthon and a calciclastic sandstone in the Mina México foredeep of the Minas de Barita area reveals two main age groups in the Upper Devonian part of the Los Pozos Formation, 1.73–1.65 Ga and 1.44–1.42 Ga; and three main age groups in the Lower Permian part of the Mina México Formation, 1.93–1.91 Ga, 1.45–1.42 Ga, and 1.1–1.0 Ga. Small numbers of zircons with ages of 2.72–2.65 Ga, 1.30–1.24 Ga, ca. 2.46 Ga, ca. 1.83 Ga, and ca. 0.53 Ga are also present in the Los Pozos sandstone. Detrital zircons ranging in age from 1.73 to 1.65 Ga are considered to have been derived from the Yavapai, Mojave, and Mazatzal Provinces and their transition zones of the southwestern United States and northwestern Mexico. The 1.45–1.30 Ga detrital zircons were probably derived from scattered granite bodies within the Mojave and Mazatzal basement rocks in the southwestern United States and northwestern Mexico, and possibly from the Southern and Eastern Granite-Rhyolite Provinces of the southern United States. The 1.24–1.0 Ga detrital zircons are believed to have been derived from the Grenville (Llano) Province to the east and northeast or from Grenville-age intrusions or anatectites to the north. Several detrital zircon ages ranging from 2.72 to 1.91 Ga were probably derived originally from the Archean Wyoming Province and Early Paleoproterozoic rocks of the Lake Superior region. These older detrital zircons most likely have been recycled one or more times into the Paleozoic sandstones of central Sonora. The 0.53 Ga zircon is believed to have been derived from a Lower Cambrian granitoid or metamorphic rock northeast of central Sonora, possibly in New Mexico and Colorado, or Oklahoma. Detrital zircon geochronology suggests that most of the detritus in both samples was derived from Laurentia to the north, whereas some detritus in the Permian synorogenic foredeep sequence was derived from the evolving accretionary wedge to the south. Compositional and sedimentological differences between the continental-rise Los Pozos conglomeratic barite sandstone and the foredeep Mina México calciclastic sandstone imply different depositional and tectonic settings.