- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
- Abstract
- Affiliation
- All
- Authors
- Book Series
- DOI
- EISBN
- EISSN
- Full Text
- GeoRef ID
- ISBN
- ISSN
- Issue
- Keyword (GeoRef Descriptor)
- Meeting Information
- Report #
- Title
- Volume
NARROW
GeoRef Subject
-
all geography including DSDP/ODP Sites and Legs
-
Asia
-
Indian Peninsula
-
Pakistan
-
North-West Frontier Pakistan
-
Peshawar Pakistan (1)
-
-
-
-
-
Canada
-
Western Canada
-
Alberta (2)
-
British Columbia (2)
-
Canadian Cordillera (1)
-
Canadian Rocky Mountains (2)
-
-
-
Coast Ranges (1)
-
Green Mountains (1)
-
Mexico
-
Sonora Mexico (1)
-
-
North America
-
Appalachians
-
Blue Ridge Mountains (1)
-
Northern Appalachians (1)
-
Piedmont (1)
-
Southern Appalachians (2)
-
-
Basin and Range Province (1)
-
Eastern Overthrust Belt (1)
-
North American Cordillera
-
Canadian Cordillera (1)
-
-
Pedregosa Basin (1)
-
Rocky Mountains
-
Canadian Rocky Mountains (2)
-
U. S. Rocky Mountains (1)
-
-
-
Red Dog Mine (1)
-
Sierra Nevada (1)
-
United States
-
Alabama
-
Chilton County Alabama (1)
-
Clay County Alabama (1)
-
Coosa County Alabama (1)
-
Shelby County Alabama (1)
-
Talladega County Alabama (1)
-
-
Alaska
-
Baird Mountains Quadrangle (1)
-
Brooks Range
-
Endicott Mountains (1)
-
-
Seward Peninsula (1)
-
Sleetmute Quadrangle (1)
-
-
Allegheny Front (1)
-
Arizona (1)
-
Blue Ridge Mountains (1)
-
California
-
Calaveras Fault (1)
-
Central California (1)
-
Mariposa County California (1)
-
Melones Fault (1)
-
San Luis Obispo County California (1)
-
-
Colorado Plateau (1)
-
Idaho
-
Bonner County Idaho (1)
-
Boundary County Idaho (1)
-
-
Montana
-
Flathead County Montana (1)
-
Lincoln County Montana (1)
-
-
Nevada
-
Elko County Nevada
-
Independence Mountains (1)
-
-
Lander County Nevada (1)
-
Roberts Mountains Allochthon (2)
-
Shoshone Mountains (1)
-
-
Talladega Front (1)
-
U. S. Rocky Mountains (1)
-
Vermont
-
Rutland County Vermont (1)
-
Windsor County Vermont (1)
-
-
Virginia
-
Giles County Virginia (1)
-
-
-
-
commodities
-
energy sources (2)
-
metal ores
-
gold ores (1)
-
-
mineral deposits, genesis (1)
-
petroleum
-
natural gas (2)
-
-
-
elements, isotopes
-
isotope ratios (1)
-
isotopes
-
stable isotopes
-
Sr-87/Sr-86 (1)
-
-
-
metals
-
alkaline earth metals
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
-
-
fossils
-
Graptolithina (1)
-
ichnofossils (1)
-
Invertebrata
-
Archaeocyatha (1)
-
Brachiopoda (2)
-
Bryozoa (1)
-
Mollusca
-
Bivalvia (1)
-
-
Porifera (1)
-
Protista
-
Foraminifera (1)
-
Radiolaria (5)
-
-
-
microfossils
-
Conodonta (18)
-
-
-
geochronology methods
-
Rb/Sr (1)
-
Sr/Sr (1)
-
U/Pb (2)
-
-
geologic age
-
Mesozoic
-
Franciscan Complex (1)
-
Great Valley Sequence (1)
-
Jurassic
-
Lower Jurassic
-
Hettangian (1)
-
lower Liassic (1)
-
middle Liassic (1)
-
Pliensbachian (1)
-
Sinemurian (1)
-
Toarcian (1)
-
upper Liassic (1)
-
-
Upper Jurassic
-
Oxfordian (1)
-
-
-
Triassic
-
Lower Triassic (1)
-
Upper Triassic
-
Carnian (1)
-
Norian (1)
-
-
-
-
Paleozoic
-
Cambrian
-
Lower Cambrian
-
Shady Dolomite (1)
-
-
-
Carboniferous
-
Lower Carboniferous
-
Dinantian (2)
-
-
Mississippian
-
Lower Mississippian
-
Kinderhookian
-
Banff Formation (1)
-
-
Osagian (1)
-
Tournaisian (1)
-
-
Upper Mississippian
-
Meramecian (1)
-
-
-
Pennsylvanian (1)
-
-
Devonian
-
Lower Devonian
-
Pragian (1)
-
-
Middle Devonian
-
Givetian (1)
-
-
Upper Devonian
-
Famennian (2)
-
Frasnian
-
upper Frasnian (1)
-
-
Palliser Formation (1)
-
-
-
Exshaw Formation (1)
-
Hanson Creek Formation (1)
-
Knox Group (1)
-
Lisburne Group (1)
-
lower Paleozoic (1)
-
Ordovician
-
Lower Ordovician (1)
-
Middle Ordovician (1)
-
-
Permian (1)
-
Silurian
-
Middle Silurian
-
Roberts Mountains Formation (1)
-
-
-
Talladega Group (1)
-
upper Paleozoic
-
Calaveras Formation (2)
-
-
-
Precambrian
-
upper Precambrian
-
Proterozoic
-
Neoproterozoic (1)
-
-
-
-
-
igneous rocks
-
igneous rocks
-
volcanic rocks
-
pyroclastics
-
tuff (1)
-
-
-
-
-
metamorphic rocks
-
metamorphic rocks
-
marbles (1)
-
metacarbonate rocks (1)
-
metasedimentary rocks
-
metalimestone (1)
-
-
metavolcanic rocks (1)
-
schists (1)
-
-
turbidite (1)
-
-
minerals
-
phosphates
-
apatite (1)
-
-
silicates
-
orthosilicates
-
nesosilicates
-
zircon group
-
zircon (1)
-
-
-
-
-
-
Primary terms
-
absolute age (4)
-
Asia
-
Indian Peninsula
-
Pakistan
-
North-West Frontier Pakistan
-
Peshawar Pakistan (1)
-
-
-
-
-
biogeography (1)
-
Canada
-
Western Canada
-
Alberta (2)
-
British Columbia (2)
-
Canadian Cordillera (1)
-
Canadian Rocky Mountains (2)
-
-
-
deformation (2)
-
diagenesis (1)
-
economic geology (4)
-
energy sources (2)
-
faults (5)
-
folds (1)
-
geochemistry (1)
-
geochronology (2)
-
geophysical methods (1)
-
geosynclines (1)
-
Graptolithina (1)
-
ichnofossils (1)
-
igneous rocks
-
volcanic rocks
-
pyroclastics
-
tuff (1)
-
-
-
-
Invertebrata
-
Archaeocyatha (1)
-
Brachiopoda (2)
-
Bryozoa (1)
-
Mollusca
-
Bivalvia (1)
-
-
Porifera (1)
-
Protista
-
Foraminifera (1)
-
Radiolaria (5)
-
-
-
isotopes
-
stable isotopes
-
Sr-87/Sr-86 (1)
-
-
-
lava (1)
-
Mesozoic
-
Franciscan Complex (1)
-
Great Valley Sequence (1)
-
Jurassic
-
Lower Jurassic
-
Hettangian (1)
-
lower Liassic (1)
-
middle Liassic (1)
-
Pliensbachian (1)
-
Sinemurian (1)
-
Toarcian (1)
-
upper Liassic (1)
-
-
Upper Jurassic
-
Oxfordian (1)
-
-
-
Triassic
-
Lower Triassic (1)
-
Upper Triassic
-
Carnian (1)
-
Norian (1)
-
-
-
-
metal ores
-
gold ores (1)
-
-
metals
-
alkaline earth metals
-
strontium
-
Sr-87/Sr-86 (1)
-
-
-
-
metamorphic rocks
-
marbles (1)
-
metacarbonate rocks (1)
-
metasedimentary rocks
-
metalimestone (1)
-
-
metavolcanic rocks (1)
-
schists (1)
-
-
metamorphism (5)
-
metasomatism (1)
-
Mexico
-
Sonora Mexico (1)
-
-
micropaleontology (1)
-
mineral deposits, genesis (1)
-
North America
-
Appalachians
-
Blue Ridge Mountains (1)
-
Northern Appalachians (1)
-
Piedmont (1)
-
Southern Appalachians (2)
-
-
Basin and Range Province (1)
-
Eastern Overthrust Belt (1)
-
North American Cordillera
-
Canadian Cordillera (1)
-
-
Pedregosa Basin (1)
-
Rocky Mountains
-
Canadian Rocky Mountains (2)
-
U. S. Rocky Mountains (1)
-
-
-
paleoecology (1)
-
paleogeography (5)
-
Paleozoic
-
Cambrian
-
Lower Cambrian
-
Shady Dolomite (1)
-
-
-
Carboniferous
-
Lower Carboniferous
-
Dinantian (2)
-
-
Mississippian
-
Lower Mississippian
-
Kinderhookian
-
Banff Formation (1)
-
-
Osagian (1)
-
Tournaisian (1)
-
-
Upper Mississippian
-
Meramecian (1)
-
-
-
Pennsylvanian (1)
-
-
Devonian
-
Lower Devonian
-
Pragian (1)
-
-
Middle Devonian
-
Givetian (1)
-
-
Upper Devonian
-
Famennian (2)
-
Frasnian
-
upper Frasnian (1)
-
-
Palliser Formation (1)
-
-
-
Exshaw Formation (1)
-
Hanson Creek Formation (1)
-
Knox Group (1)
-
Lisburne Group (1)
-
lower Paleozoic (1)
-
Ordovician
-
Lower Ordovician (1)
-
Middle Ordovician (1)
-
-
Permian (1)
-
Silurian
-
Middle Silurian
-
Roberts Mountains Formation (1)
-
-
-
Talladega Group (1)
-
upper Paleozoic
-
Calaveras Formation (2)
-
-
-
petroleum
-
natural gas (2)
-
-
plate tectonics (2)
-
Precambrian
-
upper Precambrian
-
Proterozoic
-
Neoproterozoic (1)
-
-
-
-
sea water (1)
-
sea-level changes (1)
-
sedimentary rocks
-
carbonate rocks
-
dolostone (2)
-
-
chemically precipitated rocks
-
chert (2)
-
phosphate rocks (1)
-
-
clastic rocks
-
black shale (2)
-
conglomerate (1)
-
graywacke (1)
-
mudstone (1)
-
sandstone (1)
-
shale (1)
-
siltstone (1)
-
-
-
sedimentary structures
-
planar bedding structures
-
imbrication (1)
-
-
-
sedimentation (3)
-
stratigraphy (8)
-
structural geology (3)
-
tectonics (6)
-
United States
-
Alabama
-
Chilton County Alabama (1)
-
Clay County Alabama (1)
-
Coosa County Alabama (1)
-
Shelby County Alabama (1)
-
Talladega County Alabama (1)
-
-
Alaska
-
Baird Mountains Quadrangle (1)
-
Brooks Range
-
Endicott Mountains (1)
-
-
Seward Peninsula (1)
-
Sleetmute Quadrangle (1)
-
-
Allegheny Front (1)
-
Arizona (1)
-
Blue Ridge Mountains (1)
-
California
-
Calaveras Fault (1)
-
Central California (1)
-
Mariposa County California (1)
-
Melones Fault (1)
-
San Luis Obispo County California (1)
-
-
Colorado Plateau (1)
-
Idaho
-
Bonner County Idaho (1)
-
Boundary County Idaho (1)
-
-
Montana
-
Flathead County Montana (1)
-
Lincoln County Montana (1)
-
-
Nevada
-
Elko County Nevada
-
Independence Mountains (1)
-
-
Lander County Nevada (1)
-
Roberts Mountains Allochthon (2)
-
Shoshone Mountains (1)
-
-
Talladega Front (1)
-
U. S. Rocky Mountains (1)
-
Vermont
-
Rutland County Vermont (1)
-
Windsor County Vermont (1)
-
-
Virginia
-
Giles County Virginia (1)
-
-
-
-
rock formations
-
San Miguel Formation (1)
-
Toro Formation (1)
-
-
sedimentary rocks
-
sedimentary rocks
-
carbonate rocks
-
dolostone (2)
-
-
chemically precipitated rocks
-
chert (2)
-
phosphate rocks (1)
-
-
clastic rocks
-
black shale (2)
-
conglomerate (1)
-
graywacke (1)
-
mudstone (1)
-
sandstone (1)
-
shale (1)
-
siltstone (1)
-
-
-
siliciclastics (1)
-
turbidite (1)
-
-
sedimentary structures
-
sedimentary structures
-
planar bedding structures
-
imbrication (1)
-
-
-
-
sediments
-
siliciclastics (1)
-
turbidite (1)
-
Calibration of a conodont apatite-based Ordovician 87 Sr/ 86 Sr curve to biostratigraphy and geochronology: Implications for stratigraphic resolution
Carbonate rocks of the Seward Peninsula, Alaska: Their correlation and paleogeographic significance
Paleozoic carbonate strata deposited in shallow platform to off-platform settings occur across the Seward Peninsula and range from unmetamorphosed Ordovician–Devonian(?) rocks of the York succession in the west to highly deformed and metamorphosed Cambrian–Devonian units of the Nome Complex in the east. Faunal and lithologic correlations indicate that early Paleozoic strata in the two areas formed as part of a single carbonate platform. The York succession makes up part of the York terrane and consists of Ordovician, lesser Silurian, and limited, possibly Devonian rocks. Shallow-water facies predominate, but subordinate graptolitic shale and calcareous turbidites accumulated in deeper water, intraplatform basin environments, chiefly during the Middle Ordovician. Lower Ordovician strata are mainly lime mudstone and peloid-intraclast grainstone deposited in a deepening upward regime; noncarbonate detritus is abundant in lower parts of the section. Upper Ordovician and Silurian rocks include carbonate mudstone, skeletal wackestone, and coral-stromatoporoid biostromes that are commonly dolomitic and accumulated in warm, shallow to very shallow settings with locally restricted circulation. The rest of the York terrane is mainly Ordovician and older, variously deformed and metamorphosed carbonate and siliciclastic rocks intruded by early Cambrian (and younger?) metagabbros. Older (Neoproterozoic–Cambrian) parts of these units are chiefly turbidites and may have been basement for the carbonate platform facies of the York succession; younger, shallow- and deep-water strata likely represent previously unrecognized parts of the York succession and its offshore equivalents. Intensely deformed and altered Mississippian carbonate strata crop out in a small area at the western edge of the terrane. Metacarbonate rocks form all or part of several units within the blueschistand greenschist-facies Nome Complex. The Layered sequence includes mafic metaigneous rocks and associated calcareous metaturbidites of Ordovician age as well as shallow-water Silurian dolostones. Scattered metacarbonate rocks are chiefly Cambrian, Ordovician, Silurian, and Devonian dolostones that formed in shallow, warmwater settings with locally restricted circulation and marbles of less constrained Paleozoic age. Carbonate metaturbidites occur on the northeast and southeast coasts and yield mainly Silurian and lesser Ordovician and Devonian conodonts; the northern succession also includes debris flows with meter-scale clasts and an argillite interval with Late Ordovician graptolites and lenses of radiolarian chert. Mafic igneous rocks at least partly of Early Devonian age are common in the southern succession. Carbonate rocks on Seward Peninsula experienced a range of deformational and thermal histories equivalent to those documented in the Brooks Range. Conodont color alteration indices (CAIs) from Seward Peninsula, like those from the Brooks Range, define distinct thermal provinces that likely reflect structural burial. Penetratively deformed high-pressure metamorphic rocks of the Nome Complex (CAIs ≥5) correspond to rocks of the Schist belt in the southern Brooks Range; both record subduction during early stages of the Jurassic–Cretaceous Brooks Range orogeny. Weakly metamorphosed to unmetamorphosed strata of the York terrane (CAIs mainly 2–5), like Brooks Range rocks in the Central belt and structural allochthons to the north, experienced moderate to shallow burial during the main phase of the Brooks Range orogeny. The nature of the contact between the York terrane and the Nome Complex is uncertain; it may be a thrust fault, an extensional surface, or a thrust fault later reactivated as an extensional fault. Lithofacies and biofacies data indicate that, in spite of their divergent Mesozoic histories, rocks of the York terrane and protoliths of the Nome Complex formed as part of the same lower Paleozoic carbonate platform. Stratigraphies in both areas feature Lower Ordovician and mid-Silurian shallow-water deposits with some deeper water facies of late Early to Middle Ordovician age. Most significantly, Ordovician conodont faunas in both successions contain a characteristic, distinctive mixture of Laurentian and Siberian-Alaskan endemic forms. Lithologic and faunal resemblances also link Seward Peninsula platform strata with coeval successions in the Brooks Range and in interior Alaska (Farewell and White Mountains terranes) and imply that all of these rocks were once part of a single carbonate platform situated between Laurentia, Siberia, and Baltica. Little is known about the basement on which Alaskan platform strata formed, and correlations between Cambrian and older rocks in these areas remain tentative. Similarities between strata and fossils in northern and interior Alaska are strongest during the Ordovician, and diminish by Middle Devonian; correlations between Seward Peninsula and Brooks Range rocks, however, extend into the Carboniferous. Ordovician mafic volcanism in the Nome Complex and the White Mountains terrane could reflect a rifting episode that began to separate platform rocks of the interior from those of Arctic Alaska. Lower Paleozoic off-platform successions on Seward Peninsula also correlate well with equivalent sections in northern and interior Alaska, and have some similarities with strata in southeast Alaska (Alexander terrane). Silurian (mainly Wenlock–Ludlow) mass flow deposits derived at least in part from a carbonate source overlie condensed graptolitic shales in most of these successions; this coeval influx of calcareous detritus suggests a common tectonic cause.
Carbonate Margin, Slope, and Basin Facies of the Lisburne Group (Carboniferous-Permian) in Northern Alaska
Abstract The Lisburne Group (Carboniferous-Permian) consists of a carbonate platform that extends for >1000 km across northern Alaska, and diverse margin, slope, and basin facies that contain world-class deposits of Zn and Ba, notable phosphorites, and petroleum source rocks. Lithologic, paleontologic, isotopic, geochemical, and seismic data gathered from outcrop and subsurface studies during the past 20 years allow us to delineate the distribution, composition, and age of the off-platform facies, and to better understand the physical and chemical conditions under which they formed. The southern edge of the Lisburne platform changed from a gently sloping, homoclinal ramp in the east to a tectonically complex, distally steepened margin in the west that was partly bisected by the extensional Kuna Basin (~200 by 600 km). Carbonate turbidites, black mudrocks, and radiolarian chert accumulated in this basin; turbidites were generated mainly during times of eustatic rise in the late Early and middle Late Mississippian. Interbedded black mudrocks (up to 20 wt% total organic carbon), granular and nodular phosphorite (up to 37 wt% P 2 O 5 ), and fine-grained limestone rich in radiolarians and sponge spicules formed along basin margins during the middle Late Mississippian in response to a nutrient-rich, upwelling regime. Detrital zircons from a turbidite sample in the western Kuna Basin have mainly Neoproterozoic through early Paleozoic U-Pb ages (~900-400 Ma), with subordinate populations of Mesoproterozoic and late Paleoproterozoic grains. This age distribution is similar to that found in slightly older rocks along the northern and western margins of the basin. It also resembles age distributions reported from Carboniferous and older strata elsewhere in northwestern Alaska and on Wrangel Island. Geochemical and isotopic data indicate that suboxic, denitrifying conditions prevailed in the Kuna Basin and along its margins. High V/Mo, Cr/Mo, and Re/Mo ratios (all marine fractions [MF]) and low MnO contents (<0.01 wt%) characterize Lisburne black mudrocks. Low Qmf/Vmf ratios (mostly 0.8-4.0) suggest moderately to strongly denitrifying conditions in suboxic bottom waters during siliciclastic and phosphorite sedimentation. Elevated to high Mo contents (31-135 ppm) in some samples are consistent with seasonal to intermittent sulfidic conditions in bottom waters, developed mainly along the basin margin. High d 15 N values (6-120) imply that the waters supplying nutrients to primary producers in the photic zone had a history of denitrification either in the water column or in underlying sediments. Demise of the Lisburne platform was diachronous and reflects tectonic, eustatic, and environmental drivers. Southwestern, south-central, and northwestern parts of the platform drowned during the Late Mississippian, coincident with Zn and Ba metallogenesis within the Kuna Basin and phosphogenesis along basin margins. This drowning was temporary (except in the southwest) and likely due to eutrophication associated with upwelling and sea-level rise enhanced by regional extension, which allowed suboxic, denitrifying waters to form on platform margins. Final drowning in the southcentral area occurred in the Early Pennsylvanian and also may have been linked to regional extension. In the northwest, platform sedimentation persisted into the Permian; its demise there appears to have been due to increased siliciclastic input. Climatic cooling may have produced additional stress on parts of the Lisburne platform biota during Pennsylvanian and Permian times.
Cambrian–Ordovician Sedimentary Rocks of Alaska
Abstract Cambrian-Lower Ordovician carbonate rocks that likely formed as part of the Laurentian continental margin, and may thus have been part of the Cambrian-Ordovician great American carbonate bank, occur in east-central Alaska in the Nation Arch area. These strata accumulated on the southwestern margin (present-day coordinates) of the Yukon stable block, a broad area of early Paleozoic carbonate platform deposition in the northern Yukon Territory, and constitute two successions. The first consists of approximately 900 m (∼2950 ft) of shallow-water limestone and dolostone that are in part silicified, laminated, oolitic, and pisolitic, and make up the lower member of the Jones Ridge Limestone. Conodonts, trilobites, archaeo-cyathids, and brachiopods indicate an age of Early Cambrian to early Early Ordovician (Tremadoc; Ibexian) and have Laurentian biogeographic affinities. Upper Ordovician bio-clastic limestone (the upper member of the Jones Ridge Limestone) unconformably overlies these strata. A roughly coeval, but somewhat deeper water, succession crops out near the Jones Ridge Limestone and consists of, in ascending order, the Funnel Creek Limestone, Adams Argillite, and Hillard Limestone. The Funnel Creek (15-400 m [50-1310 ft] thick) is mainly nonfossilif-erous, extensively silicified, commonly oolitic limestone and dolostone and is assumed to be Lower Cambrian in age. It is overlain by argillite, siltstone, cross-laminated quartzite, and oolitic to sandy limestone of the Adams Argillite (90-180 m [295-550 ft] thick). This unit contains the trace fossil Oldhamia and Lower Cambrian archaeocyathids and trilobites that have Siberian affinities. The Hillard (30-150 m [100-490 ft] thick) is chiefly limestone, with local ooids, edgewise and boulder conglomerate, and phosphatic horizons, and likely formed in a platform-margin setting. Trilobites and brachiopods from this unit are Early Cambrian to earliest Ordovician in age and have mainly Laurentian affinities. Slope and/or basinal rocks of the Road River Formation that are as old as Early Ordovician (early middle Arenig; Ibexian) unconformably overlie the Hillard Limestone. Abrupt facies transitions between the two Nation Arch area carbonate successions may reflect relatively steep paleoslopes and/or telescoping of facies by imbricate thrust faults. Carbonate strata of Cambrian–Ordovician age are also found north of the Nation Arch area in the Porcupine terrane. These rocks have been little studied, and their precise Stratigraphic succession and paleogeographic setting are uncertain. The few fossil collections indicate mainly Laurentian affinities and include Cambrian(?) trilobites and Lower and Middle Ordovician conodonts. Lower Paleozoic strata of the Porcupine terrane probably formed at or near the northwestern edge (present-day coordinates) of the Yukon stable block. Cambrian–Ordovician carbonate strata occur widely in northern Alaska (parts of the Arctic Alaska, York, and Seward terranes) and interior Alaska (Farewell terrane). These rocks share distinctive lithologic and faunal features and were deposited in a range of shallow-shelf to basinal environments. Carbonate platform successions in northern and interior Alaska include fossils of both Laurentian and Siberian biotic provinces and may have formed on a single crustal fragment that rifted away from the Siberian craton during the late Proterozoic. These Alaskan strata were most likely in faunal exchange with, but not physically attached to, the great American carbonate bank. Lower–Middle Ordovician carbonate and siliciclastic rocks are also found in the White Mountains, Livengood, and Ruby terranes of interior Alaska, the Alexander terrane in southeastern Alaska, and the Goodnews terrane in southwestern Alaska. These successions were likely not attached to Laurentia during their deposition, although some authors have proposed Laurentian origins for the White Mountains and Livengood terranes. Little detailed information is available on the resource potential of Cambrian–Ordovician successions in Alaska. Most have low porosity and are too thermally mature to be prospective for oil and gas, although a few units in east-central and northern Alaska may have some potential as petroleum source and reservoir rocks. Strata of this age have potential for metallic mineral resources; strata-bound Zn-Pb ± Ag occurrences are known in the Funnel Creek Limestone in east-central Alaska, as well as several units of possible Cambrian and/or Ordovician age in northern and interior Alaska.
Abstract Cambrian and Ordovician shelf, platform, and basin rocks are present in Sonora, México, and southern Arizona and were deposited on the southwestern continental margin of North America (Laurentia). Cambrian and Ordovician rocks in Sonora, México, are mostly exposed in scattered outcrops in the northern half of the state. Their discontinuous nature results from extensive Quaternary and Tertiary surficial cover, from Tertiary and Mesozoic granitic batholiths in western Sonora, and from widespread Tertiary volcanic deposits in the Sierra Madre Occidental in eastern Sonora. Cambrian and Ordovician shelf rocks were deposited as part of the southern Laurentian miogeocline on the southwestern continental margin of North America. Lower Cambrian shelf units in Sonora consist mainly of quartzite, siltstone, and silty limestone; limestone increases upward in the sequence. Middle Cambrian shelf rocks consist mostly of limestone, dolostone, and siltstone. Upper Cambrian shelf rocks are sparse in Sonora; where present, they consist chiefly of siltstone and minor limestone. Cambrian shelf rocks display subtle facies changes from west to east across Sonora. In northwestern Sonora, these rocks attain their maximum thickness and may represent the Early Cambrian shelf margin. At the Sierra Agua Verde section, 110 km (68 mi) east of Hermosillo, these rocks thin, have greater proportions of clastic material, and were probably deposited in an inner-shelf setting. A major unconformity is present near the base of the Cambrian in Sonora and is similar to the Sauk I unconformity in the Wood Canyon Formation in Nevada and California. The top of the Cambrian is transitional with overlying Ordovician strata. Cambrian cratonic platform rocks are exposed in northern Sonora and southern Arizona and include the Middle Cambrian Bolsa Quartzite and Middle and Upper Cambrian Abrigo Limestone. The most complete sections of Ordovician shelf rocks in Sonora are 50 km (31 mi) northwest of Hermosillo. In these sections, the Lower Ordovician is characterized by intraclastic limestone, siltstone, shale, and chert. The Middle Ordovician is mostly silty limestone and quartzite, and the Upper Ordovician is cherty limestone and some argillaceous limestone. A major disconformity separates the Middle Ordovician quartzite from the overlying Upper Ordovician carbonate rocks and is similar to the disconformity between the Middle and Upper Ordovician Eureka Quartzite and Upper Ordovician Ely Springs Dolomite in Nevada and California. In parts of northwestern Sonora, Ordovician rocks are disconformably overlain by Upper Silurian rocks. Northeastward in Sonora and Arizona, toward the craton, Ordovician rocks are progressively truncated by a major onlap unconformity and are overlain by Devonian rocks. Except in local areas, Ordovician rocks are generally absent in cratonic platform sequences in northern Sonora and southern Arizona.
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.
The restricted Gemuk Group: A Triassic to Lower Cretaceous succession in southwestern Alaska
New data from an Upper Triassic to Lower Cretaceous deep marine succession—the herein reinstated and restricted Gemuk Group—provide a vital piece of the puzzle for unraveling southwestern Alaska's tectonic history. First defined by Cady et al. in 1955 , the Gemuk Group soon became a regional catchall unit that ended up as part of at least four different terranes. In this paper we provide the first new data in nearly half a century from the Gemuk Group in the original type area in Taylor Mountains quadrangle and from contiguous rocks to the north in Sleetmute quadrangle. Discontinuous exposure, hints of complex structure, the reconnaissance level of our mapping, and spotty age constraints together permit definition of only a rough stratigraphy. The restricted Gemuk Group is at least 2250 m thick, and could easily be at least twice as thick. The age range of the restricted Gemuk Group is tightened on the basis of ten radiolarian ages, two new bivalve ages, one conodont age, two U-Pb zircon ages on tuff, and U-Pb ages of 110 detrital zircons from two sandstones. The Triassic part of the restricted Gemuk Group, which consists of intermediate pillow lavas interbedded with siltstone, chert, and rare limestone, produced radiolarians, bivalves, and conodonts of Carnian and Norian ages. The Jurassic part appears to be mostly siltstone and chert, and yielded radiolarians of Hettangian-Sinemurian, Pliensbachian-Toarcian, and Oxfordian ages. Two tuffs near the Jurassic-Cretaceous boundary record nearby arc volcanism: one at 146 Ma is interbedded with red and green siltstone, and a second at ca. 137 Ma is interbedded with graywacke turbidites. Graywacke appears to be the dominant rock type in the Lower Cretaceous part of the restricted Gemuk Group. Detrital zircon analyses were performed on two sandstone samples using SHRIMP. One sandstone yielded a dominant age cluster of 133–180 Ma; the oldest grain is only 316 Ma. The second sample is dominated by zircons of 130–154 Ma; the oldest grain is 292 Ma. The youngest zircons are probably not much older than the sandstone itself. Point counts of restricted Gemuk Group sandstones yield average ratios of 24/29/47 for Q/F/L, 15/83/2 for Ls/Lv/Lm, and 41/48/11 for Qm/P/K. In the field, sandstones of the restricted Gemuk Group are not easily distinguished from sandstones of the overlying Upper Cretaceous turbidite-dominated Kuskokwim Group. Petrographically, however, the restricted Gemuk Group has modal K-feldspar, whereas the Kuskokwim Group generally does not (average Qm/P/K of 64/36/0). Some K-feldspar-bearing graywacke that was previously mapped as Kuskokwim Group ( Cady et al., 1955 ) is here reassigned to the restricted Gemuk Group. Major- and trace-element geochemistry of shales from the restricted Gemuk Group and the Kuskokwim Group show distinct differences. The chemical index of alteration (CIA) is distinctly higherforshales of the Kuskokwim Group than forthose of the restricted Gemuk Group, suggesting more intense weathering during deposition of the Kuskokwim Group. The restricted Gemuk Group represents an estimated 90–100 m.y. of deep-water sedimentation, first accompanied by submarine volcanism and later by nearby explosive arc activity. Two hypotheses are presented for the tectonic setting. One model that needs additional testing is that the restricted Gemuk Group consists of imbricated oceanic plate stratigraphy. Based on available information, our preferred model is that it was deposited in a back-arc, intra-arc, or forearc basin that was subsequently deformed. The terrane affinity of the restricted Gemuk Group is uncertain. The rocks of this area were formerly assigned to the Hagemeister subterrane of the Togiak terrane—a Late Triassic to Early Cretaceous arc—but our data show this to be a poor match. None of the other possibilities (e.g., Nukluk and Tikchik subterranes of the Goodnews terrane) is viable; hence, the terrane subdivision and distribution in southwestern Alaska may need to be revisited. The geologic history revealed by our study of the restricted Gemuk Group gives us a solid toehold in unraveling the Mesozoic paleogeography of this part of the northern Cordillera.
Late Devonian Alamo Impact, southern Nevada, USA: Evidence of size, marine site, and widespread effects
The early Late Devonian (early Frasnian) Alamo Impact targeted an oceanic, off-platform site in southern Nevada, excavating a crater with a final diameter of 44–65 km. The original crater is now dismembered and buried beneath younger rocks. Consequently, its size and site must be deduced through multiple converging lines of geological and paleontological evidence. Previous and new evidence includes the catastrophically emplaced Alamo Breccia, tsunamites, shock-metamorphosed quartz grains, carbonate accretionary lapilli, an iridium anomaly, sub-Breccia clastic injection, deep-water Breccia channels, and ejecta material. We now demonstrate, on the basis of conodont microfossils in carbonate ejecta clasts within lapillistone blocks and widely distributed shocked-quartz and lithic-clast ejecta within the upper part of the Breccia, that the Alamo Impact excavated down at least into Upper Cambrian strata, at a depth of 1.7 km, and possibly into the underlying Proterozoic–Lower Cambrian Prospect Mountain Quartzite, ∼2.5 km beneath the Late Devonian seafloor. Distal tsunamites and probable ejecta are now documented as far north as Devils Gate, northern Nevada, and as far northeast as the Confusion Range, western Utah. A charcoal-bearing, early Frasnian estuarine deposit in the Bighorn Mountains, Wyoming, may provide the first evidence of an Alamo Impact fallout-generated forest fire. Our new data further document the widespread effects and size of the Alamo Impact, and constrain the likely present position of the tectonically transported crater to an area between the Timpahute and Hot Creek Ranges, southern Nevada.