SEARCH · Search USGS

Results for “Journal of Marine Sciences”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

137 records · Page 8Linked to original sources

USGS2013

Salmon-mediated nutrient flux in selected streams of the Columbia River basin, USA

Salmon provide an important resource subsidy and linkage between marine and land-based ecosystems. This flow of energy and nutrients is not uni-directional (i.e., upstream only); in addition to passive nutrient export via stream flow, juvenile emigrants actively export nutrients from freshwater environments. In some cases, nutrient export can exceed import. We evaluated nutrient fluxes in streams across central Idaho, USA using Chinook salmon (Oncorhynchus tshawytscha) adult escapement and juvenile production data from 1998 to 2008. We found in the majority of stream-years evaluated, adults imported more nutrients than progeny exported; however, in 3% of the years, juveniles exported more nutrients than their parents imported. On average, juvenile emigrants exported 22 ± 3% of the nitrogen and 30 ± 4% of the phosphorus their parents imported. This relationship was density dependent and nonlinear; during periods of low adult abundance juveniles were larger and exported up to 194% and 268% of parental nitrogen and phosphorus inputs, respectively. We highlight minimum escapement thresholds that appear to 1) maintain consistently positive net nutrient flux and 2) reduce the average proportional rate of export across study streams. Our results suggest a state-shift occurs when adult spawner abundance falls below a threshold to a point where the probability of juvenile nutrient exports exceeding adult imports becomes increasingly likely.

Idaho↗

USGS2010

EAARL coastal topography-western Florida, post-Hurricane Charley, 2004: seamless (bare earth and submerged.

Project Description These remotely sensed, geographically referenced elevation measurements of lidar-derived seamless (bare-earth and submerged) topography were produced as a collaborative effort between the U.S. Geological Survey (USGS), Coastal and Marine Geology Program (CMGP), St. Petersburg, FL, and the National Aeronautics and Space Administration (NASA), Wallops Flight Facility, VA. This project provides highly detailed and accurate datasets of a portion of the western Florida coastline beachface, acquired post-Hurricane Charley on August 17 and 18, 2004. The datasets are made available for use as a management tool to research scientists and natural-resource managers. An innovative airborne lidar instrument originally developed at the NASA Wallops Flight Facility, and known as the Experimental Advanced Airborne Research Lidar (EAARL), was used during data acquisition. The EAARL system is a raster-scanning, waveform-resolving, green-wavelength (532-nanometer) lidar designed to map near-shore bathymetry, topography, and vegetation structure simultaneously. The EAARL sensor suite includes the raster-scanning, water-penetrating full-waveform adaptive lidar, a down-looking red-green-blue (RGB) digital camera, a high-resolution multispectral color infrared (CIR) camera, two precision dual-frequency kinematic carrier-phase GPS receivers, and an integrated miniature digital inertial measurement unit, which provide for sub-meter georeferencing of each laser sample. The nominal EAARL platform is a twin-engine Cessna 310 aircraft, but the instrument may be deployed on a range of light aircraft. A single pilot, a lidar operator, and a data analyst constitute the crew for most survey operations. This sensor has the potential to make significant contributions in measuring sub-aerial and submarine coastal topography within cross-environmental surveys. Elevation measurements were collected over the survey area using the EAARL system, and the resulting data were then processed using the Airborne Lidar Processing System (ALPS), a custom-built processing system developed in a NASA-USGS collaboration. ALPS supports the exploration and processing of lidar data in an interactive or batch mode. Modules for presurvey flight-line definition, flight-path plotting, lidar raster and waveform investigation, and digital camera image playback have been developed. Processing algorithms have been developed to extract the range to the first and last significant return within each waveform. ALPS is used routinely to create maps that represent submerged or sub-aerial topography. Specialized filtering algorithms have been implemented to determine the 'bare earth' under vegetation. For more information about similar projects, please visit the Decision Support for Coastal Science and Management website. Selected References Brock, J.C., Wright, C.W., Sallenger, A.H., Krabill, W.B., and Swift, R.N., 2002, Basis and methods of NASA airborne topographic mapper Lidar surveys for coastal studies: Journal of Coastal Research, v. 18, no. 1, p. 1-13. Crane, Michael, Clayton, Tonya, Raabe, Ellen, Stoker, Jason, Handley, Larry, Bawden, Gerald, Morgan, Karen, and Queija, Vivian, 2004, Report of the U.S. Geological Survey Lidar workshop sponsored by the Land Remote Sensing Program and held in St. Petersburg, FL, November 2002: U.S. Geological Survey Open-File Report 2004-1456, 72 p. Nayegandhi, Amar, Brock, J.C., and Wright, C.W., 2009, Small-footprint, waveform-resolving Lidar estimation of submerged and sub-canopy topography in coastal environments: International Journal of Remote Sensing, v. 30, no. 4, p. 861-878. Sallenger, A.H., Wright, C.W., and Lillycrop, Jeff, 2005, Coastal impacts of the 2004 hurricanes measured with airborne Lidar; initial results: Shore and Beach, v. 73, nos. 2-3, p. 10-14. Resources Included Readme.txt File

Data Series↗

USGS1994

Deep structure beneath Lake Ontario: Crustal-scale Grenville subdivisions

Lake Ontario marine seismic data reveal major Grenville crustal subdivisions beneath central and southern Lake Ontario separated by interpreted shear zones that extend to the lower crust. A shear zone bounded transition between the Elzevir and Frontenac terranes exposed north of Lake Ontario is linked to a seismically defined shear zone beneath central Lake Ontario by prominent aeromagnetic and gravity anomalies, easterly dipping wide-angle reflections, and fractures in Paleozoic strata. We suggest the central Lake Ontario zone represents crustal-scale deformation along an Elzevir–Frontenac boundary zone that extends from outcrop to the south shore of Lake Ontario.Seismic images from Lake Ontario and the exposed western Central Metasedimentary Belt are dominated by crustal-scale shear zones and reflection geometries featuring arcuate reflections truncated at their bases by apparent east-dipping linear reflections. The images show that zones analogous to the interpreted Grenville Front Tectonic Zone are also present within the Central Metasedimentary Belt and support models of northwest-directed crustal shortening for Grenvillian deep crustal deformation beneath most of southeastern Ontario.A Precambrian basement high, the Iroquoian high, is defined by a thinning of generally horizontal Paleozoic strata over a crestal area above the basement shear zone beneath central Lake Ontario. The Iroquoian high helps explain the peninsular extension into Lake Ontario forming Prince Edward County, the occurrence of Precambrian inlier outcrops in Prince Edward County, and Paleozoic fractures forming the Clarendon–Linden structure in New York.

Lake Ontario↗

USGS2023

The Lower Cretaceous sequence of western Alaska – demise of the Koyukuk terrane?

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and basin settings and unconformity-bound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk basin where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a post-subduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes.

Alaska↗

USGS2015

Dynamic hypoxic zones in Lake Erie compress fish habitat, altering vulnerability to fishing gears

Seasonal degradation of aquatic habitats from hypoxia occurs in numerous freshwater and coastal marine systems and can result in direct mortality or displacement of fish. Yet, fishery landings from these systems are frequently unresponsive to changes in the severity and extent of hypoxia, and population-scale effects have been difficult to measure except in extreme hypoxic conditions with hypoxia-sensitive species. We investigated fine-scale temporal and spatial variability in dissolved oxygen in Lake Erie as it related to fish distribution and catch efficiencies of both active (bottom trawls) and passive (trap nets) fishing gears. Temperature and dissolved oxygen loggers placed near the edge of the hypolimnion exhibited much higher than expected variability. Hypoxic episodes of variable durations were frequently punctuated by periods of normoxia, consistent with high-frequency internal waves. High-resolution interpolations of water quality and hydroacoustic surveys suggest that fish habitat is compressed during hypoxic episodes, resulting in higher fish densities near the edges of hypoxia. At fixed locations with passive commercial fishing gear, catches with the highest values occurred when bottom waters were hypoxic for intermediate proportions of time. Proximity to hypoxia explained significant variation in bottom trawl catches, with higher catch rates near the edge of hypoxia. These results emphasize how hypoxia may elevate catch rates in various types of fishing gears, leading to a lack of association between indices of hypoxia and fishery landings. Increased catch rates of fish at the edges of hypoxia have important implications for stock assessment models that assume catchability is spatially homogeneous.

Ohio↗

USGS2008

Biomedical and veterinary science can increase our understanding of coral disease

A balanced approach to coral disease investigation is critical for understanding the global decline of corals. Such an approach should involve the proper use of biomedical concepts, tools, and terminology to address confusion and promote clarity in the coral disease literature. Investigating disease in corals should follow a logical series of steps including identification of disease, systematic morphologic descriptions of lesions at the gross and cellular levels, measurement of health indices, and experiments to understand disease pathogenesis and the complex interactions between host, pathogen, and the environment. This model for disease investigation is widely accepted in the medical, veterinary and invertebrate pathology disciplines. We present standard biomedical rationale behind the detection, description, and naming of diseases and offer examples of the application of Koch's postulates to elucidate the etiology of some infectious diseases. Basic epidemiologic concepts are introduced to help investigators think systematically about the cause(s) of complex diseases. A major goal of disease investigation in corals and other organisms is to gather data that will enable the establishment of standardized case definitions to distinguish among diseases. Concepts and facts amassed from empirical studies over the centuries by medical and veterinary pathologists have standardized disease investigation and are invaluable to coral researchers because of the robust comparisons they enable; examples of these are given throughout this paper. Arguments over whether coral diseases are caused by primary versus opportunistic pathogens reflect the lack of data available to prove or refute such hypotheses and emphasize the need for coral disease investigations that focus on: characterizing the normal microbiota and physiology of the healthy host; defining ecological interactions within the microbial community associated with the host; and investigating host immunity, host-agent interactions, pathology, pathogenesis, and factors that promote the pathogenicity of the causative agent(s) of disease.

Journal of Experimental Marine Biology and Ecology↗

USGS2009

Magnetostratigraphic correlations of Permian-Triassic marine-to-terrestrial sections from China

We have studied three Permian–Triassic (PT) localities from China as part of a combined magnetostratigraphic, 40 Ar/ 39 Ar and U–Pb radioisotopic, and biostratigraphic study aimed at resolving the temporal relations between terrestrial and marine records across the Permo-Triassic boundary, as well as the rate of the biotic recovery in the Early Triassic. The studied sections from Shangsi (Sichuan Province), Langdai (Guihzou Province), and the Junggar basin (Xinjiang Province), span marine, paralic, and terrestrial PT environments, respectively. Each of these sections was logged in detail in order to place geochronologic, paleomagnetic, geochemical, conodont and palynologic samples within a common stratigraphic context. Here we present rock-magnetic, paleomagnetic and magnetostratigraphic results from the three localities. At Shangsi, northern Sichuan Province, we sampled three sections spanning Permo-Triassic marine carbonates. Magnetostratigraphic results from the three sections indicate that the composite section contains at least eight polarity chrons and that the PT boundary occurs within a normal polarity chron a short distance above the mass extinction level and a reversed-to-normal (R-N) polarity reversal. Furthermore, the onset of the Illawarra mixed interval lies below the sampled section indicating that the uppermost Permian Changhsingian and at least part of the Wuchiapingian stages postdate the end of the Kiaman Permo-Carboniferous Reversed Superchron. At Langdai, Guizhou Province, we studied magnetostratigraphy of PT paralic mudstone and carbonate sediments in two sections. The composite section spans an R-N polarity sequence. Section-mean directions pass a fold test at the 95% confidence level, and the section-mean poles are close to the mean PT pole for the South China block. Based on biostratigraphic constraints, the R-N transition recorded at Langdai is consistent with that at Shangsi and demonstrates that the PT boundary occurred within a normal polarity chron a short distance above the mass extinction level. In the southern Junggar basin, Xinjiang Province, in northwest China, we determined the magnetostratigraphy of three sections of a terrestrial sequence. Normal and reversed polarity directions are roughly antipodal, and magnetostratigraphies from the three sections are highly consistent. Combined bio- and magneto-stratigraphy used to correlate this sequence to other PT sequences suggests that the previously-proposed biostratigraphic PT boundary in the Junggar sections was most likely misplaced by earlier workers suggesting that further work is necessary to confidently place the PT boundary there.

Journal of Asian Earth Sciences↗

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↗

USGS2015

Initiation of migration and movement rates of Atlantic salmon smolts in fresh water

Timing of ocean entry is critical for marine survival of both hatchery and wild Atlantic salmon ( Salmo salar ) smolts. Management practices and barriers to migration such as dams may constrain timing of smolt migrations resulting in suboptimal performance at saltwater entry. We modeled influences of stocking location, smolt development, and environmental conditions on ( i ) initiation of migration by hatchery-reared smolts and ( ii ) movement rate of hatchery- and wild-reared Atlantic salmon smolts in the Penobscot River, Maine, USA, from 2005 through 2014 using acoustic telemetry data. We also compared movement rates in free-flowing reaches with rates in reaches with hydropower dams and head ponds. We compared movement rates before and after (1) removal of two mainstem dams and (2) construction of new powerhouses. Initiation of movement by hatchery fish was influenced by smolt development, stocking location, and environmental conditions. Smolts with the greatest gill Na + , K + -ATPase (NKA) activity initiated migration 24 h sooner than fish with the lowest gill NKA activity. Fish with the greatest cumulative thermal experience initiated migration 5 days earlier than those with lowest cumulative thermal experience. Smolts released furthest from the ocean initiated migration earlier than those released downstream, but movement rate increased by fivefold closer to the ocean, indicating behavioral trade-offs between initiation and movement rate. Dams had a strong effect on movement rate. Movement rate increased from 2.8 to 5.4 km·h −1 in reaches where dams were removed, but decreased from 2.1 to 0.1 km·h −1 in reaches where new powerhouses were constructed. Movement rate varied throughout the migratory period and was inversely related to temperature. Fish moved slower at extreme high or low discharge. Responses in fish movement rates to dam removal indicate the potential scope of recovery for these activities.

Maine↗

USGS2019

India National Gas Hydrate Program Expedition-02: Operational and technical summary

The India National Gas Hydrate Program is being steered by the government of India's Ministry of Petroleum and Natural Gas (MoPNG) with participation of Directorate General of Hydrocarbons (DGH), Oil and Natural Gas Corporation Limited (ONGC), and the National Oil Companies and Research Institutes of India. The India National Gas Hydrate Program Expedition 01 (NGHP-01) established the presence of gas hydrate in the Krishna Godavari (KG) and Mahanadi Basins and in the offshore area of the Andaman Sea Basin. However, the gas hydrates discovered during NGHP-01 were mainly distributed as fracture-filling material in fine-grained clay-rich sediments. The India National Gas Hydrate Program Expedition 02 (NGHP-02) was carried out with an objective to discover gas hydrate in sand-rich sediment along the eastern offshore margin of India. ONGC planned and executed NGHP-02 on the behalf of the MoPNG. NGHP-02 started on March 3, 2015 and was completed on July 28, 2015 (total 147 days) using the Japanese scientific Drilling Vessel Chikyu (D/V Chikyu). During NGHP-02, 42 holes at 25 sites were drilled, cored, and/or surveyed with downhole logging tools. These sites were located in four areas along the eastern margin of India and formally named Area A (Mahanadi Basin, three sites), Area B (northern part of the KG-Basin, twelve sites), Area C (central part of the KG-Basin, six sites), and Area E (southern part to the KG-Basin, four sites). All 25 sites established during NGHP-02 were first drilled and logged with logging-while-drilling (LWD) tools and an additional 17 holes were then drilled and/or cored with conventional coring tools (HPCS/ESCS) or pressure coring tools (PCTB). Wireline logging was conducted in 10 holes and formation tests using a dual packer Modular Formation Dynamics Tester (MDT) tool were carried out in two holes. The onboard science team used the laboratory facilities on the D/V Chikyu to examine and analyse the physical properties, geochemistry, and sedimentology of all the cores collected during the expedition. Core samples were also analysed in additional post-expedition shore-based studies conducted in numerous domestic and international gas hydrate research laboratories. The NGHP-02 sediment cores were archived at the National Gas Hydrate Core Repository in Mumbai (India), which is associated with the ONGC Gas Hydrate Research and Technology Centre (GHRTC). The necessary data for characterizing the occurrence of gas hydrate, such as interstitial water chlorinities, core-derived gas chemistry, physical and sedimentological properties, thermal images of the recovered cores, pressure core and downhole measured logging data (LWD and/or conventional wireline log data), were obtained from most of the drill sites established during NGHP-02. Almost all the drill sites yielded evidence for the occurrence of gas hydrate; however, the inferred in situ concentration of gas hydrate varied substantially from site to site. For the most part, the interpretation of downhole logging data, core thermal images, interstitial water analyses, and pressure core images from the sites established during NGHP-02 indicate that the occurrence of concentrated gas hydrate is mostly associated with coarser grained (sand-rich) sediments. This paper presents the operational and technical summary of NGHP-02. NGHP-02 started on March 3, 2015 and was completed on July 28, 2015 (total 147 days) using the Japanese scientific Drilling Vessel Chikyu (D/V Chikyu). During NGHP-02, 42 holes at 25 sites were drilled, cored, and/or surveyed with downhole logging tools. These sites were located in four areas along the eastern margin of India and formally named Area A (Mahanadi Basin, three sites), Area B (northern part of the KG-Basin, twelve sites), Area C (central part of the KG-Basin, six sites), and Area E (southern part to the KG-Basin, four sites). All 25 sites established during NGHP-02 were first drilled and logged with logging-while-drilling (LWD) to

Journal of Marine and Petroleum Geology↗

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↗