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83 records · Page 5Linked to original sources
USGS2016

Reevaluating the age of the Walden Creek Group and the kinematic evolution of the western Blue Ridge, southern Appalachians

An integrated synthesis of existing datasets (detailed geologic mapping, geochronologic, paleontologic, geophysical) with new paleontologic and geochemical investigations of rocks previously interpreted as part of the Neoproterozoic Walden Creek Group in southeastern Tennessee suggest a necessary reevaluation of the kinematics and structural architecture of the Blue Ridge Foothills. The western Blue Ridge of Tennessee, North Carolina, and Georgia is composed of numerous northwest-directed early and late Paleozoic thrust sheets, which record pronounced variation in stratigraphic/structural architecture and timing of metamorphism. The detailed spatial, temporal, and kinematic relationships of these rocks have remained controversial. Two fault blocks that are structurally isolated between the Great Smoky and Miller Cove-Greenbrier thrust sheets, here designated the Maggies Mill and Citico thrust sheets, contain Late Ordovician-Devonian conodonts and stable isotope chemostratigraphic signatures consistent with a mid-Paleozoic age. Geochemical and paleontological analyses of Walden Creek Group rocks northwest and southeast of these two thrust sheets, however, are more consistent with a Late Neoproterozoic (550–545 Ma) depositional age. Consequently, the structural juxtaposition of mid-Paleozoic rocks within a demonstrably Neoproterozoic-Cambrian succession between the Great Smoky and Miller Cove-Greenbrier thrust sheets suggests that a simple foreland-propagating thrust sequence model is not applicable in the Blue Ridge Foothills. We propose that these younger rocks were deposited landward of the Ocoee Supergroup, and were subsequently plucked from the Great Smoky fault footwall as a horse, and breached through the Great Smoky thrust sheet during Alleghanian emplacement of that structure.

North Carolina, Tennessee↗
USGS1997

Late quaternary regional geoarchaeology of Southeast Alaska Karst: A progress report

Karst systems, sea caves, and rock shelters within the coastal temperate rain forest of Alaska's Alexander Archipelago preserve important records of regional archaeology, sea level history, glacial and climatic history, and vertebrate paleontology. Two 14C AMS dates on human bone discovered in a remote cave (49-PET-408) on Prince of Wales Island document the oldest reliably dated human in Alaska to ca. 9800 B.P. A series of 14C AMS dates from cave deposits span the past 40,000 years and provide the first evidence of Pleistocene faunas from the northwest coast of North America. Other discoveries include sea caves and marine beach deposits elevated above modern sea level, extensive solution caves, and mammalian remains of species previously undocumented within the region. Records of human activity, including cave art, artifacts, and habitation sites may provide new insights into the early human colonization of the Americas. ??1997 John Wiley & Sons, Inc.

Alaska↗
USGS2002

Emsian synorogenic paleogeography of the Maine Applachians

The Acadian deformation front in the northern Appalachians of Maine and New Hampshire can now be closely located during the early Emsian (Early Devonian; 408-406 Ma). Tight correlations between paleontologically and isotopically dated rocks are possible only because of a new 408-Ma time scale tie point for the early Emsian. The deformation front lay between a belt of Lower Devonian flysch and molasse that were deposited in an Acadian foreland basin and had not yet been folded and a belt of early Emsian plutons that intruded folded Lower Devonian rocks. This plutonic belt includes the newly dated Ore Mountain gabbro (U/Pb; 406 Ma), which hosts magmatic-sulfide mineralization. Along the deformation front, a 407-Ma pluton that locally truncates Acadian folds (Katahdin) was the feeder to volcanic rocks (Traveler Rhyolite; 406-407 Ma) that are part of the foreland-basin succession involved in these same folds. The Emsian igneous rocks thus define a syncollisional magmatic province that straddled the deformation front. These findings bear on three alternative subduction geometries for the Acadian collision.

Journal of Geology↗
USGS2022

Maybe so, maybe not: Canis lepophagus at Hagerman Fossil Beds National Monument, Idaho, USA

A canid dentary is described from the Pliocene Glenns Ferry Formation at Hagerman Fossil Beds National Monument, south-central Idaho, USA. The specimen possesses traits in alliance with and measurements falling within or exceeding those of Canis lepophagus . The dentary, along with a tarsal IV (cuboid) and an exploded canine come from the base of the fossiliferous Sahara complex within the monument. Improved geochronologic control provided by new tephrochronologic mapping by the U.S. Geological Survey-National Park Service Hagerman Paleontology, Environments, and Tephrochronology Project supports an interpolated age of approximately 3.9 Ma, placing it in the early Blancan North American Land Mammal Age. It is conservatively referred to herein as Canis aff. C . lepophagus with the caveat that it is an early and robust example of that species. A smaller canid, initially assigned to Canis lepophagus and then to Canis ferox , is also known from Hagerman. Most specimens of Canis ferox , including the holotype, were recently reassigned to Eucyon ferox , but specimens from the Hagerman and Rexroad faunas were left as Canis sp. and possibly attributed to C . lepophagus. We agree that these smaller canids belong in Canis and not Eucyon but reject placing them within C . lepophagus ; we refer to them here as Hagerman-Rexroad Canis . This study confirms the presence of two approximately coyote-sized canids at Hagerman and adds to the growing list of carnivorans now known from these fossil beds.

Idaho↗
USGS1978

Dating Quaternary faults in the southwestern United States by using buried calcic paleosols

Calcareous soils are widespread on upper Pliocene to upper Pleistocene unconsolidated surflcial deposits in semiarid portions of the southwestern United States. Where these soils are related to faults, the soils may provide a means for quantitatively estimating timing and amounts of Quaternary faulting. Soil age estimates are based on the amount of pedogenic CaCO 3 (g/cm 2 -soil column) that has accumulated from soil processes, whereas rates of calcic-soil formation were calibrated by the K-Ar dating method, tephrochronology, vertebrate paleontology, and regional soil studies. The County Dump fault, west of Albuquerque, N. Mex. cuts a 500 000-yearold datum, the Llano de Albuquerque. The downdropped block of this fault contains a sequence of faulted younger deposits and intercalated calcic paleosols. Soil ages, and hence fault ages, can be calculated by measuring the total pedogenic calcium carbonate content in a section of buried paleosols and by using an independently established maximum soil formation rate of 0.35 g CaCO 3 • cm -2 • 10 -3 yr -1 for the Albuquerque area. This particular fault segment has had four discrete episodes of movement in the past 500 000 yr, the most recent of which occurred about 20000 yr B.P. Recurrence intervals on the segment are 90 000-190 000 yr, but the composite recurrence interval for all fault movements in this area may be less by several orders of magnitude. This technique of quantifying soil properties and calculating ages of relict soils can be used to estimate ages of Quaternary deposits and associated surfaces, to correlate such features over broad regions, and to analyze regional trends in calcic-soil formation.

New Mexico↗
USGS2024

Benthic foraminiferal community changes across the Miocene climatic optimum Identified by SHEBI analysis (SHE analysis for biozone identification), Calvert Cliffs, Maryland, USA

The Calvert Cliffs, MD, an iconic section of Middle Miocene strata, have been well studied both paleontologically and stratigraphically for over a century. However, few studies of the Calvert Cliffs have looked at the benthic foraminifera. This study uses SHEBI analysis (SHE analysis for biozone identification) of benthic foraminiferal assemblages to analyze community change in the Calvert and Choptank formations of the Calvert Cliffs deposited during the Miocene Climatic Optimum (MCO; 17–14.8 Ma) and the Middle Miocene Climate Transition (MMCT; 14.8–13.8 Ma). SHE analysis differs from traditional analytical methods by defining communities based on changes in diversity rather than the relative abundance of individual species. This study uses SHE analysis on a composite section of benthic foraminiferal assemblages from three vertical transects that span the MCO and MMCT. Two communities were identified from the studied strata. Community 1 was deposited during the MCO and includes incised valley fill (IVF), transgressive system tract (TST), and highstand system tract (HST) deposits. Community 2, deposited during the MMCT, is composed of samples from TST, HST, IVF, and another HST. The assemblages of community 1 are representative of an inner to middle shelf environment whereas those of community 2 are representative of an inner shelf environment. The two foraminiferal communities differentiated by SHE analysis indicate a high relative sea level in the Salisbury Embayment during the warm MCO followed by a decrease in sea level during the subsequent cooler MMCT.

Maryland↗
USGS2002

Stratigraphy and paleontology of Lower Permian rocks north of Cananea, northern Sonora, Mexico

Lower Permian carbonate and overlying red bed clastic rocks are present in a 2 km2 stratigraphic window in the vicinity of Rancho La Cueva, Santa Cruz sheet (scale 1:50,000), northern Sonora, Mexico. This exposure lies unconformably beneath predominantly intermediate Upper Cretaceous volcanics yielding 40Ar/39Ar ages of 73.4?? 0.18 and 71.1 ?? 0.35 Ma. The lower part of the Permian succession consists of light- to medium-gray colored limestones of the Colina Limestone, with a minimum thickness of 235 m. Sedimentary features suggest shallow water, slightly restricted depositional environments. Although lacking observable fossils for the most part, two intervals of richly fossiliferous, silicified shell beds are present near the base and top of the Colina Limestone. The lower fauna consist mostly of gastropods and bivalves. The presence of a new microdomatid gastropod species. Glyptospira sonorensis n. sp., close to Glytospira arelela Plas, suggests a late Wolfcampian age for this horizon. The upper fauna are predominantly molluscan dominated (gastropods and bivalves), but some brachiopods (productids and the rhynchonellid genus Pontisia) are also present. Gastropod genera include Bellerophon, Warthia, Euomphalus (represented by the species, Euomphalus kaibabensis Chronic), Baylea, Worthenia, Naticopsis, Goniasma, Kinishbia, Cibecuia, and Glyptospira. The gastropods suggest a Leonardian (late Early Permian) age for this horizon, and many of the species have previously been recorded from the Supai Group and Kaibab Formation of northern and central Arizona. The Colina Limestone is conformably overlain by 11.2 m of light-gray lime mudstone and dolostone, assigned here to the Epitaph Dolomite, which in turn is succeeded by 58.8 m of red-colored sandstone and gray lime mudstone, assigned here to the Scherrer Formation. This Lower Permian succession is significant because it further strengthens the stratigraphic ties of southeastern Arizona rocks with those of northern Sonora and confirms the presence of North American cratonal stratigraphy in the northern part of the state of Sonora, Mexico. Published by Elsevier Science Ltd.

Journal of South American Earth Sciences↗
USGS1997

Stratigraphic and structural implications of conodont and detrital zircon U-Pb ages from metamorphic rocks of the Coldfoot terrane, Brooks Range, Alaska

New paleontologic and isotopic data from the Emma Creek and Marion Creek schists of the Coldfoot terrane, Arctic Alaska superterrane, central Brooks Range, suggest Devonian and possibly younger ages of deposition for their sedimentary protoliths. Conodonts from marble of the Emma Creek schist, intruded by a roughly 392 Ma orthogneiss, are late Lochkovian (early Early Devonian, between about 408 and 396 Ma) and Silurian to Devonian at two other locations. Spherical to oblong detrital zircons from quartz-mica schist of the overlying Marion Creek schist yield mostly discordant U—Pb data suggestive of provenance ages of 3.0, 2.0–1.8, and 1.5–1.4 Ga; however, several euhedral grains of zircon from Marion Creek quartz-mica schist have concordant U—Pb ages from 370 to 360 Ma. The Marion Creek schist in our study area therefore is at least 26 m.y. younger than the Emma Creek schist. The age data imply that the protolith of the Emma Creek schist is age correlative with Devonian carbonate rocks in the Hammond and North Slope terranes, whereas the Marion Creek schist is age correlative with Upper Devonian and Lower Mississippian clastic sedimentary rocks of the Endicott Group in the Endicott Mountains terrane and shale and carbonate units in the De Long Mountains and Sheenjek River terranes. Consequently, tectonic models restoring the entire Coldfoot terrane beneath partly or wholly coeval rocks of the Hammond, Endicott Mountains, De Long Mountains, and Sheenjek River terranes of the Arctic Alaska superterrane require revision. Alternative reconstructions, including restoration of the Coldfoot terrane inboard of the Endicott Mountains terrane or outboard of the De Long Mountains and Sheenjek River terranes are plausible but require either larger amounts of shortening than previously suggested or indicate problematic facies relations.

Journal of Geophysical Research B: Solid Earth↗
USGS1994

Lake-level history of Lake Michigan for the past 12,000 years: the record from deep lacustrine sediments

Collection and analysis of an extensive set of seismic-reflection profiles and cores from southern Lake Michigan have provided new data that document the history of the lake basin for the past 12,000 years. Analyses of the seismic data, together with radiocarbon dating, magnetic, sedimentologic, isotopic, and paleontologic studies of core samples, have allowed us to reconstruct lake-level changes during this recent part of the lake's history. The post-glacial history of lake-level changes in the Lake Michigan basin begins about 11.2 ka with the fall from the high Calumet level, caused by the retreat of the Two Rivers glacier, which had blocked the northern outlet of the lake. This lake-level fall was temporarily reversed by a major influx of water from glacial Lake Agassiz (about 10.6 ka), during which deposition of the distinctive gray Wilmette Bed of the Lake Michigan Formation interrupted deposition of red glaciolacustrine sediment. Lake level then continued to fall, culminating in the opening of the North Bay outlet at about 10.3 ka. During the resulting Chippewa low phase, lake level was about 80 m lower than it is today in the southern basin of Lake Michigan. The rise of the early Holocene lake level, controlled primarily by isostatic rebound of the North Bay outlet, resulted in a prominent, planar, transgressive unconformity that eroded most of the shoreline features below present lake level. Superimposed on this overall rise in lake level, a second influx of water from Lake Agassiz temporarily raised lake levels an unknown amount about 9.1 ka. At about 7 ka, lake level may have fallen below the level of the outlet because of sharply drier climate. Sometime between 6 and 5 ka, the character of the lake changed dramatically, probably due mostly to climatic causes, becoming highly undersaturated with respect to calcium carbonate and returning primary control of lake level to the isostatically rising North Bay outlet. Post-Nipissing (about 5 ka) lake level has fallen about 6 m due to erosion of the Port Huron outlet, a trend around which occurred relatively small (± ∼2 m), short-term fluctuations controlled mainly by climatic changes. These cyclic fluctuations are reflected in the sed-imentological and sediment-magnetic properties of the sediments.

Indiana, Illinois, Michigan, Wisconsin↗
USGS2013

The major-ion composition of Cenozoic seawater: the past 36 million years from fluid inclusions in marine halite

Fluid inclusions from ten Cenozoic (Eocene-Miocene) marine halites are used to quantify the major-ion composition (Mg 2+ , Ca 2+ , K + , Na + , SO 4 2− , and Cl − ) of seawater over the past 36 My. Criteria used to determine a seawater origin of the halites include: (1) stratigraphic, sedimentologic, and paleontologic observations; (2) Br − in halite; (3) δ 34 S of sulfate minerals; (4) 87 Sr/ 86 Sr of carbonates and sulfates; and (5) fluid inclusion brine compositions and evaporation paths, which must overlap from geographically separated basins of the same age to confirm a “global” seawater chemical signal. Changes in the major-ion chemistry of Cenozoic seawater record the end of a systematic, long term (>150 My) shift from the Ca 2+ -rich, Mg 2+ - and SO 4 2− -poor seawater of the Mesozoic (“CaCl 2 seas”) to the “MgSO 4 seas” (with higher Mg 2+ and SO 4 2− >Ca 2+ ) of the Cenozoic. The major ion composition of Cenozoic seawater is calculated for the Eocene-Oligocene (36-34 Ma), Serravallian-Tortonian (13.5-11.8 Ma) and the Messinian (6-5 Ma), assuming chlorinity (565 mmolal), salinity, and the K + concentration (11 mmolal) are constant and the same as in modern seawater. Fluid inclusions from Cenozoic marine halites show that the concentrations of Mg 2+ and SO 4 2− have increased in seawater over the past 36 My and the concentration of Ca 2+ has decreased. Mg 2+ concentrations increased from 36 mmolal in Eocene-Oligocene seawater (36-34 Ma) to 55 mmolal in modern seawater. The Mg 2+ /Ca 2+ ratio of seawater has risen from ∼2.3 at the end of the Eocene, to 3.4 and 4.0, respectively, at 13.5 to 11.8 Ma and 6 to 5 Ma, and to 5 in modern seawater. Eocene-Oligocene seawater (36-34 Ma) has estimated ranges of SO 4 2− = 14–23 mmolal and Ca 2+ = 11–20 mmolal. If the (Ca 2+ )(SO 4 2− ) product is assumed to be the same as in modern seawater (∼300 mmolal 2 ), Eocene-Oligocene seawater had Ca 2+ ∼16 mmolal and SO 4 2− ∼19 mmolal. The same estimates of Ca 2+ and SO 4 2− for Serravallian-Tortonian seawater (13.5-11.8 Ma) are SO 4 2− = 19–27 mmolal and Ca 2+ = 8–16 mmolal and SO 4 2− ∼24 mmolal and Ca 2+ ∼ 13 mmolal if the (Ca 2+ )(SO 4 2− ) product is equal to that in modern seawater. Messinian seawater has an estimated range of SO 4 2− ∼21–29 mmolal and Ca 2+ ∼7–15 mmolal with SO 4 2− ∼26 mmolal and Ca 2+ ∼12 mmolal assuming the (Ca 2+ )(SO 4 2− ) product is equal to that in modern seawater. Regardless of the estimation procedure, SO 4 2− shows progressively increasing concentrations from 36 Ma to the present values, which are the highest of the Cenozoic.

American Journal of Science↗
USGS2001

The geology of a part of Acadia and the nature of the Acadian orogeny across Central and Eastern Maine

The zone of Acadian collision between the Medial New England and Composite Avalon terranes is well preserved in Maine. A transect from northwest (Rome) to southeast (Camden) crosses the eastern part of Medial New England comprising the Central Maine basin, Liberty-Orrington thrust sheet, and Fredericton trough, and the western part of Composite Avalon, including the Graham Lake, Clarry Hill, and Clam Cove thrust sheets. U-Pb geochronology of events before, during, and after the Acadian orogeny helps elucidate the nature and distribution of tectonostratigraphic belts in this zone and the timing of some Acadian events in the Northern Appalachians. The Central Maine basin consists of sedimentary and volcanic rocks of Middle Ordovician (∼470 to ∼460 Ma) age overlain with probable conformity by latest Ordovician(?) through earliest Devonian marine rift and flysch sedimentary rocks; these are intruded by weakly to undeformed plutonic rocks of Early and Middle Devonian age (∼399–378 Ma). The Fredericton trough consists of Early Silurian gray pelite and sandstone to earliest Late Silurian calcareous turbidite, deformed and variably metamorphosed prior to the emplacement of Late Silurian (∼422 Ma) and Early to Late Devonian (∼418 to ∼368 Ma) plutons. The Liberty-Orrington thrust sheet consists of Cambrian(?)-Ordovician (>∼474 to ∼469 Ma and younger) clastic sedimentary and volcanic rocks intruded by highly deformed Late Silurian (∼424 to ∼422 Ma) and Devonian (∼418 to ∼389 Ma) plutons, possibly metamorphosed in Late Silurian time (prior to ∼417 Ma), and metamorphosed to amphibolite facies in Early to Middle Devonian time (∼400 to ∼381 Ma). The Graham Lake thrust sheet contains possible Precambrian rocks, Cambrian sedimentary rocks with a volcanic unit dated at ∼503 Ma, and Ordovician rocks with possible Caradocian Old World fossils, metamorphosed and deformed in Silurian time and intruded by mildly to undeformed Late Silurian (∼421 Ma) and Late Devonian (∼371 to ∼368 Ma) plutons. The Clarry Hill thrust sheet consists of poorly studied, highly metamorphosed Cambrian (?) rocks. The Clam Cove thrust sheet contains highly deformed Precambrian limestone, shale, sandstone, and conglomerate, metamorphosed to epidote amphibolite facies and intruded by a mildly deformed pluton dated at ∼421 Ma. Metamorphism, deformation, and voluminous intrusive igneous activity of Silurian age are common to both the most southeastern parts of Medial New England and the thrust sheets of Composite Avalon. In contrast to Medial New England, the thrust sheets of Composite Avalon show only modest effects of Devonian deformation and metamorphism. Regional stratigraphic relations, paleontologic findings, and U-Pb geochronology suggest that the Graham Lake, Clarry Hill, and Clam Cove thrust sheets are far-traveled allochthons that were widely separated from Medial New England in the Silurian. One hundred nine (109) new U-Pb analyses of zircon, monazite, and sphene from 25 samples of metamorphosed, stratified, and intrusive igneous rocks are used to decipher the history of events along the transect. In our view, many of the structures within Medial New England were formed during the closing of the back-arc ocean during the waning stages of the Taconian orogeny. These features include the precursors to the upright folds found in Silurian sedimentary rocks, the west-facing Liberty-Orrington thrust sheet, and the Silurian plutons of Medial New England. Acadian tectonic features, including flysch sedimentation, igneous activity, deformation with nappe emplacement, and metamorphism record the progressive loading of Medial New England by a stack of thrust nappes emplaced in latest Silurian to Middle Devonian time. The Acadian orogeny is a prolonged event, lasting from earliest Late Silurian to the Late Devonian, whose evolution involved: (1) convergence between Medial New England and Composite Avalon along an east-dipping subduction zone from earliest Late Silurian to Early Devonian time; (2) collision and concurrent delamination of lithospheric mantle beneath Medial New England in Early Devonian time resulting in deformation, high-grade metamorphism, and intrusive igneous activity in the most eastern part of Medial New England and the western parts of Composite Avalon; (3) Early to Middle Devonian northwest-migrating penetrative deformations of the Acadian Main stage, including northwest-directed thrusting and recumbent folding followed by tightening of folds possibly produced in the waning stages of the Taconian orogeny and forming folded wave trains of isoclinal folds (∼419 to ∼404 Ma), (4) asymmetrical folds produced by east-west shortening (∼399 to ∼380 Ma); and (5) final westward emplacement of Composite Avalon thrust sheets onto Medial New England in Late Devonian time (∼380 to ∼371 Ma). The present boundary between rocks of Medial New England and Composite Avalon at the surface is the Sennebec Pond fault, a high-angle fault that cuts the Graham Lake and Clarry Hill thrusts, and is intruded by the Mt. Waldo pluton (∼371), one of several Late Devonian plutons (∼371 to ∼367 Ma) that mark the end of the Acadian orogeny in coastal Maine. The thrust sheets of Composite Avalon carried rocks of peri-Gondwanan affinity and with possible Old World fauna many kilometers to the west over Medial New England, thus potentially hiding the original suture at depth under the Gulf of Maine.

Maine↗