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March/April Geological Society of America Bulletin media highlights

Geological Society of America

Boulder, CO - Articles are now posted in the GSA BULLETIN "In Press" area of the journal Web site ( Geology topics of interest include: evidence from New Zealand challenging the hypothesis that the Northern Hemisphere drives global climate; evidence of two catastrophic volcanic eruptions in the Hannegan Pass area of Washington state's North Cascades National Park; origins of the highly productive topsoil of America's Great Plains; and new insights into hotspots in the Hawaiian-Emperor Island seamount chain.

The middle Eocene climatic optimum event in the Contessa Highway section, Umbrian Apennines, Italy
Fabio Florindo, Istituto Nazionale di Geofisica e Vulcanologia, Paleomagnetism, Roma I-00143, Italy; et al. Pages 413-427.

Keywords: Eocene, middle Eocene climatic optimum, MECO, magnetostratigraphy, biostratigraphy, stable isotope stratigraphy, Contessa Highway section, Italy.

Recently, a distinct negative shift in oxygen isotope records at ca. 40 million years ago (designated as the middle Eocene climatic optimum) has been observed in cores from the Indian and Atlantic sectors of the Southern Ocean. It has been interpreted primarily as a temperature signal that affected both surface waters and middle bathyal deep waters. Jovane et al.'s results from the Contessa section, in the Umbrian region, central Italy, provide the first evidence of this climatic event beyond the Southern Ocean, which suggests a global response of the oceanic carbon cycle to the same forcing.

Chronology and controls of avulsion along a mixed bedrock-alluvial river
Stephen Tooth, University of Wales, Aberystwyth, Institute of Geography and Earth Sciences, Ceredigion SY23 3DB, UK; et al. Pages 452-461.

Keywords: aggradation, avulsion, bedrock, incision, meanders, luminescence dating.

Avulsion--the abandonment of a river channel for a new course--influences the long-term distribution of water and sediment on many flood plains, deltas, and alluvial fans but more field data are needed to test avulsion concepts and models. This study uses optically stimulated luminescence (OSL) dating to establish an avulsion history for the meandering Klip River, eastern Free State, South Africa. Bedrock outcrop in the channel bed indicates that the long-term sedimentation rate is negligible, but OSL ages show that five avulsions have been completed during the last 30,000 years in association with meander belt development. Field evidence suggests that an avulsion is initiated when floodwater that originates on the upstream muddy flood plain drains back to the channel through low points in levees or bank tops. This forms a new channel that in subsequent floods erodes through the flood plain to connect with the original channel farther upstream. Over time, flow and sediment is diverted along the new channel, and it gradually develops meanders whilst the original channel is abandoned. European settlement in the valley in the late 1800s initiated an ongoing avulsion; rapid erosion of a new channel is proceeding alongside abandonment of a 1-2 km reach, suggesting that avulsions are probably completed in 100-150 years. The findings support previous suggestions that sedimentation rate is a primary control on the frequency and processes of avulsion but the study emphasizes the need for further avulsion studies in settings where sedimentation rates are very low.

Volcano evolution and eruptive flux on the thick crust of the Andean Central Volcanic Zone: 40Ar/39Ar constraints from Volcán Parinacota, Chile
John Hora, University of Wisconsin-Madison, Dept. of Geology & Geophysics, Madison, WI 53706, USA; et al. Pages 343-362.

Keywords: Andes, Central Volcanic Zone, Parinacota, 40Ar/39Ar geochronology, eruptive rates.

In order to erupt, magma must ascend through Earth's crust. While several volcanoes and their eruptive rates have been studied in detail for the case of relatively thin crust, there is a lack of information for volcanic growth rates on extremely thick crust. Understanding how quickly volcanoes grow (and in this case, regrow following destruction by sector collapse) in this tectonic setting allows evaluation of the degree to which thick continental crust filters magmas passing through it. Through geologic mapping, volume estimation, and stratigraphy tied to age determinations using 40Ar/39Ar geochronology, Hora et al. describe in detail the evolution of Volcán Parinacota built on 70 km thick crust in northern Chile. Their data indicate that regrowth of this volcano following sector collapse happened rapidly over approximately the last 10,000 years, with eruption rates on par with other volcanoes on thin crust. The sixteen ages determined from lavas erupted during this period are among the youngest yet obtained using the 40Ar/39Ar method. While their data indicate that thick crust does not necessarily result in diminished eruption rates, certain magma types common in other arcs are very rare in the vicinity of Parinacota. This suggests that while evolved, water-rich magmas ascend relatively unhindered, dry basalts might be preferentially filtered.

Volcanic arc emplacement onto the southernmost Appalachian Laurentian shelf: Characteristics and constraints
James Tull, Florida State University, Department of Geological Sciences, Tallahassee, Florida 32306, USA; et al. Pages 261-274.

Keywords: Appalachians, volcanic arc, Talladega belt, Hillabee, Blue Ridge.

The Appalachian orogenic belt records a series of collisions between ancient North America (Laurentia), arc terranes, microcontinents, and larger continental masses, which culminate in the assembly of the late Paleozoic supercontinent Pangea. While the tectonic evolution of the northern Appalachians includes the apparent subduction of Laurentia, or peri-Laurentian blocks, beneath an exotic island arc(s) during the Ordovician Taconic orogeny, Tull et al. show that the southernmost Appalachian Talladega belt, which formed along the Alabama continental promontory, was tectonically isolated from this collisional event. Instead, a Middle Ordovician suprasubduction metavolcanic sequence, the Hillabee Greenstone, records evidence of a Laurentian continental margin arc. Both geochemical analyses and field mapping suggest the bimodal Hillabee Greenstone is an accreted fragment of a previously more extensive Ordovician volcanic complex that was thrust onto the middle Paleozoic shelf along a cryptic fault during the Late Devonian or Early Carboniferous. Emplacement of these marginal arc rocks may have resulted from a collision between the Laurentian Alabama promontory and the Ouachita arc.

U-Pb detrital zircon geochronology and provenance of the late Paleoproterozoic Dubawnt Supergroup: Linking sedimentation with tectonic reworking of the western Churchill Province, Canada
Robert Rainbird (corresponding author) and W.J. Davis, Geological Survey of Canada, Continental Geoscience, Ottawa, Ontario K1A 0E9, Canada. Pages 314-328.

Keywords: U-Pb, geochronology, provenance, Paleoproterozoic, Dubawnt Supergroup, western Churchill Province.

This paper reports on the ages of zircon (ZrSiO2) sand grains extracted from sandstones in sedimentary basins of early Proterozoic (approximately 1.8-1.6 billion years old) age from northern-central Canada (western Churchill Province). The ages of the zircon grains tell us that certain continental blocks, (e.g., Superior and Slave provinces), were colliding with the western Churchill block at the same time that these sedimentary basins were forming. Similar collisions were occurring elsewhere on Earth at this time (e.g., Australia), as part of a global-scale amalgamation known as a supercontinent. Uplift and erosion of mountain belts that formed as a result of this amalgamation shed large volumes of detritus (sand and gravel) via large river systems, into the interior of the supercontinent, where it was captured in sedimentary basins like those presently exposed in the western Churchill Province of northern-central Canada. Such studies will help geoscientists to piece together this ancient supercontinent puzzle.

Protolith ages and exhumation histories of (ultra)high-pressure rocks across the Western Gneiss Region, Norway
Emily O. Walsh, Cornell College, Geology Department, Mt. Vernon, IA 52314, USA; et al. Pages 289-301.

Keywords: ultrahigh pressure, Western Gneiss Region, secondary ion mass spectrometry, 40Ar/39Ar, exhumation, laser ablation inductively coupled plasma mass spectrometry.

The mechanisms by which ultrahigh-pressure rocks are exhumed remains a geological puzzle that can only be solved by considering the behavior of the entire orogeny, including associated lower pressure rocks. This article presents a series of new U/Pb zircon, Th/Pb monazite, and 40Ar/39Ar muscovite ages from a transect spanning the Western Gneiss Region, Norway, from the orogenic core to the foreland. These data reveal older-than-predicted protolith ages; corroborate existing metamorphic ages, and depict slab geometry during exhumation.

Late Quaternary climate change, loess sedimentation, and soil profile development in the central Great Plains: A pedosedimentary model
Peter M. Jacobs, University of Wisconsin-Whitewater, Geography and Geology, Whitewater, WI 53190, USA; and Joseph A. Mason, University of Wisconsin-Madison, Dept. of Geography, Madison, WI 53706, USA. Pages 462-475.

Keywords: Great Plains, soils, stratigraphy, loess, Holocene, paleosols.

The Great Plains region is known for both its sensitivity to drought and its importance to world food production. Jacobs and Mason conclude that the thick, dark, and highly productive topsoil of the central Plains originated through dust deposition during recurring drought conditions over much of the past 9500 years. In contrast, the clay-rich subsoil represents an ancient soil surface, marking a period of unusually slow dust sedimentation under a rapidly warming and wetter-than-present climate at the end of the Pleistocene ice age. Chemical and mineral signatures were used to trace dust dispersal, from near-source sites where dust deposits are thick and well-dated, to soils across the central Great Plains.

Orbital forcing of mid-latitude Southern Hemisphere glaciation since 100 ka inferred from cosmogenic nuclide ages of moraine boulders from the Cascade Plateau, southwest New Zealand
Rupert Sutherland, GNS Science, Wellington 6009, New Zealand; et al. Pages 443-451.

Keywords: glacial deposits, moraines, climate, Milankovitch, Last Glacial Maximum, cosmogenic elements, beryllium.

Periodic changes in Earth's orbit are now well established as drivers of long-term global climate change, but the influence that each hemisphere may have on the other has been a subject of debate. The last global ice age reached its maximum around 20 thousand years ago, when summer sunshine was at a minimum in the Northern Hemisphere--leading to the hypothesis that the Northern Hemisphere drives global climate. However, recent work in temperate parts of the Southern Hemisphere suggest an onset of the ice age maximum there as early as 30 thousand years ago, when local summer sunshine was at a minimum. The results presented by Sutherland et al. support an early onset of glacial conditions in New Zealand and show that previous glacial advances at 60 and 80 thousand years ago occurred during the two previous Southern Hemisphere summer-sunshine minima. It seems that local changes in summer sunshine, caused by changes in Earth's orbit, have been an important driver of Southern Hemisphere climate.

Mode and tempo of the Paleocene-Eocene thermal maximum in an expanded section from the Venetian pre-Alps
Luca Giusberti, University of Padova, Department of Geology, Paleontology and Geophysics, Padova 35137, Italy; et al. Pages 391-412.

Keywords: Paleocene-Eocene thermal maximum, carbon isotope record, central western Tethys, stratigraphy, geochemistry, mineralogy, chronology, silicate pump.

A unique global warming event occurred about 55 million years ago. Previous works suggest that the event started abruptly with a 5°C warming, which lasted for nearly 100,000 years. It took another 100,000 years to recover from this warming before the temperature conditions on Earth were back to "normal" again, or rather, what was normal 54-55 million years ago. This warming event, commonly referred to as the Paleocene-Eocene Thermal Maximum, or PETM for short, resulted in, for example, huge changes in ocean chemistry and a mass extinction of a group of sand-grain sized bottom-living organisms (benthic foraminifera). The geological hallmark of the PETM event is a strong and complex carbon isotope excursion, considered to have been caused by a massive (more than 4000 gigatons) and rapid addition of light carbon (carbon-12) into the oceans and atmosphere at this time. Because the PETM may represent an analog of the present accelerated emission of greenhouse gases caused by humans, it has attracted much interest both with respect to data collecting and modeling efforts. It has been pointed out by many specialists that a more comprehensive understanding of the carbon isotope excursion event requires the study of numerous high quality sedimentary records from a range of different latitudes and water depths. Giusberti et al.'s integrated study of a new section (Forada section), located near the city of Belluno north of Venice in northern Italy, provides one such much-needed record. This record provides important insight into the much-debated problem of the duration of the carbon isotope excursion. The Paleocene-Eocene boundary sediments preserved in the continuously deposited Forada section show an exceptionally expanded (3.3 m) interval containing the main carbon isotope excursion event (105,000 ± 10,000 years) and an unusually clearly defined "recovery" interval (126,000 ± 12,000 years). The data obtained from Forada suggest that the source of the light carbon ceased to be added to the ocean-atmosphere system exactly at the transition between the top of the main carbon isotope excursion event and the base of the recovery interval.

Diagenesis, sediment strength, and pore collapse in sediment approaching the Nankai Trough subduction zone
Glenn A. Spinelli, New Mexico Tech, Earth & Environmental Science Department, Socorro, NM 87801, U.S.A.; et al. Pages 377-390.

Keywords: diagenesis, opal, cement, subduction, Ocean Drilling Program.

Spinelli et al. have determined that a very small amount of opal acts as a cement that strengthens and dramatically controls the deformation of sediment offshore southern Japan. Opal comprises less than 1% of the sediment, but it coats sediment grains and cements grain contacts. At temperatures above 55 ˚C, the opal begins to dissolve and the weight of the overlying sediment breaks the cement. The cemented sediment is approaching the Nankai Trough subduction zone off southern Japan. The anomalously strong cemented zone deforms differently as it enters the subduction zone. The cemented zone has very few fractures in it, while sediments above and below the cemented zone have numerous fractures. This has implications for the distribution of deformation and fluid flow in subduction zones. Similar cementation and anomalous sediment strength may be important at other locations, including offshore southern Alaska, northern California, and Guatemala.

Terrestrial records of a regional weathering profile at the Paleocene-Eocene boundary in the Williston Basin of North Dakota
Elizabeth R. Clechenko, University of Wisconsin-Madison, Geology and Geophysics, Madison, WI 53706, U.S.A.; et al. Pages 428-442.

Keywords: Paleocene-Eocene Thermal Maximum, Williston Basin, kaolinite, Golden Valley Formation.

This paper describes a regional weathering profile preserved in continental strata of the Golden Valley Formation in western North Dakota (Williston Basin). Combined palynostratigraphic and chemostratigraphic evidence suggest that this episode of continental weathering occurred during an ancient (ca. 55 Ma) global warming event referred to as the Paleocene-Eocene Thermal Maximum, or PETM. Clay mineral assemblages within these weathered strata are dominated by thermodynamically stable kaolinte, which lends credence to the hypothesis that continental weathering intensified during the greenhouse climatic conditions of the PETM.

Geology and complex collapse mechanisms of the 3.72 Ma Hannegan caldera, North Cascades, Washington, USA
David Tucker, Western Washington University, Geology, Bellingham, WA 98225, U.S.A.; et al. Pages 329-342.

Keywords: Hannegan, caldera, volcanology, geochronology, collapse mechanism, trap door.

Geologic research in the Hannegan Pass area of North Cascades National Park has determined that eroded rock deposits over 3000 feet thick were left by two catastrophic volcanic eruptions around 3.72 million years ago. The rocks, about 44 miles east of Bellingham, Washington, are hardened volcanic ash enclosed in a roughly elliptical collapse area measuring 4.8 by 2 miles (8 by 3.5 kilometers). The study's authors estimate that over 33 cubic miles of ash were erupted in the cataclysmic blasts. Airborne ash and seering incandescent clouds of gas-propelled ash and rocks flowed over and buried the landscape, certainly obliterating all life for tens of miles around the volcano. Each eruption resulted in collapse of the surface in a manner similar to the formation of the far younger Crater Lake caldera in southern Oregon. In between eruption of the two calderas, a lake formed in the blasted crater. In the time since eruption of the thick piles of ash, erosion has stripped a layer of rock at least a half mile thick from the Cascade landscape. Volcanic activity has migrated from the Hannegan caldera southeastward to Mount Baker in the succeeding millennia. The Hannegan caldera is only the sixth of these colossal volcanoes recognized from the Cascades. It's discovery serves as a reminder that relatively quiet, snow-covered cones are not the only type of volcano known to erupt in the Cascade Mountains.

Combined U-Pb geochronology and Hf isotope geochemistry of detrital zircons from early Paleozoic sedimentary rocks, Ellsworth-Whitmore Mountains block, Antarctica
M.J. Flowerdew, British Antarctic Survey, Geological Sciences Division, Cambridge, Cambridgeshire CB3 0ET, U.K.; et al. Pages 275-288.

Keywords: Antarctica, absolute age, U-Pb, Hf, zircon provenance.

A common approach used to determine which rocks were eroded and deposited as sediment in ancient sedimentary basins is to study grains of zircon, a mineral that survives erosion, sediment transport, deposition, and transformation into sedimentary rock. The ability to determine the age of zircons means a spectrum of ages for tens of individual grains within a sedimentary rock can be matched with rock ages within potential source regions, and thus the most likely source region can be determined. For Paleozoic sedimentary rocks that were deposited at the margin of the supercontinent of Gondwana (comprising parts of South America, Antarctica, Australia and New Zealand), the detrital age spectrums are remarkably similar along the length of the margin, over a distance of thousands of kilometers, so confident characterization of the source is not possible. But, with the ability to measure the hafnium isotope composition of a zircon grain in addition to its age, a more detailed geological fingerprint of the source region is revealed. Paleozoic-aged sedimentary rocks of the Ellsworth Mountains, Antarctica, have similar detrital zircon age populations throughout the sequence; however, the additional hafnium data reveal that the sediment source has changed through the evolution of the basin: older sedimentary rock has a source region that was proximal to the basin, being derived from rock similar to that which underlies the basin, whereas the younger parts of the sequence have a source that is more distal, predominantly from within southern Africa and parts of South America, rather than East Antarctica. The inference is that source regions from within west Gondwana and east Gondwana can be better distinguished with the additional hafnium data. Moreover, this technique can be used to correlate sedimentary rock units. Analyses from two-isolated rock exposures south and west of the Ellsworth Mountains suggest they correlate with the older parts of the Ellsworth Mountains sequence, whereas another locality apparently has a different source, located within East Antarctica. These results suggest that such an approach to provenance analysis can better identify the source region, substantiate changes of sediment provenance up-stratigraphy, and can also be used as a broad correlation tool.

The Bend: Origin and significance
Rex H. Pilger, Landmark Graphics, R&D, Highlands Ranch, CO 80129, U.S.A. Pages 314-328.

Keywords: plate tectonics, hotspots, Hawaii, plumes, kinematics, plate reconstructions.

Is there another way to look at hotspots, especially the Hawaiian-Emperor island-seamount chain? Rex Pilger shows how sets of hotspots may be "embedded" in the upper mantle and move together. These sets thereby define a few large stable reference frames he calls "mesoplates." A key to this inference is the distinctive "bend" in the chain and its correspondence with motion paths of the Pacific and adjacent continental plates over the past 75 million years.


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