News Release

Fossil landscapes in New England

New Geology articles posted online ahead of print on Oct. 23, 2015

Peer-Reviewed Publication

Geological Society of America

Boulder, Colo., USA - Maine's Katahdin and New Hampshire's Mount Washington stand as the highest peaks over New England, but over the past several million years, both were repeatedly buried by continental ice sheets advancing southward from Canada. Using atom-counting measurements of the very rare isotopes carbon-14, beryllium-10, and aluminum-26, Paul Bierman and colleagues determined that the glacial ice that once covered the peaks was frozen to the rock below, unable to accomplish much if any erosion.

Thus, the landscapes on these summits are fossil -- preserved from before the latest glacial advance about 30,000 years ago. The contrast in erosion between stable fossil summit landscapes and adjacent valleys deeply eroded by glacial ice likely contributes to the development and maintenance of northern Appalachian topography and gives us the spectacular landscapes we enjoy today.

FEATURED ARTICLE Cold-based Laurentide ice covered New England's highest summits during the Last Glacial Maximum
Paul R. Bierman et al., Dept. of Geology, University of Vermont, Burlington, Vermont 05405-1758, USA. This paper is online at http://dx.doi.org/10.1130/G37225.1. For more information, contact Professor Paul Bierman, University of Vermont, pbierman@uvm.edu.

Other recently posted GEOLOGY articles are highlighted below:


Tsunami propagation over a wide, shallow continental shelf caused by the Storegga slide, southeastern North Sea, Denmark
M. Fruergaard et al., Morphodynamique Continentale et Côtière, University of Caen Lower Normandy, UMR CNRS 6143, 2-4 rue des Tilleuls, FR-14000 Caen, France. This paper is online at http://dx.doi.org/10.1130/G37151.1.

About 8,150 years ago an enormous submarine landslide of the Norwegian shelf generated a series of tsunami waves in the Norwegian Sea and in the North Sea. Sediment deposited by the Storegga Slide tsunami have been identified along the coasts of Greenland, Norway, Faroe Islands, Shetland Islands, Scotland, and the northernmost coasts of England but never along the southeastern coasts of the North Sea. It has been assumed that the wide, shallow continental shelf of the North Sea dissipated and attenuated most of the energy of the tsunami before impacting the coast. Furthermore, any depositional records of the tsunami in the southeastern North Sea will be buried several meters below present-day sea level because this area has experienced a long-term relative sea-level rise that has drowned the former coastlines. In this paper, Mikkel Fruergaard and colleagues provide the first direct evidence for the Storegga Slide tsunami in the southeastern North Sea by using detailed analysis of sediment cores. The analyzed data demonstrate that the tsunami run-up height at the impacted coast was between 1.5 and 5.5 m and the findings may suggest that the entire coastline of the southeastern North Sea was impacted by the tsunami.


Paleomagnetic evidence for ~4000 km of crustal shortening across the 1 Ga Grenville orogen of North America
Henry C. Halls, Dept. of Chemical and Physical Sciences, University of Toronto Mississauga, 3359 Mississauga Road, Mississauga, Ontario L5L 1C6, Canada. This paper is online at http://dx.doi.org/10.1130/G37188.1.

The Grenville Province, a belt of deformed and metamorphosed rocks, follows the North American eastern seaboard. It arises from collision, about a billion years ago, of two continents: Laurentia and ancestral North American and Amazonia, part of South America. It represents an older, deeply eroded equivalent of the Himalayan mountain range, raised by ongoing collision between India and Asia. Across the Himalayas ~2000 km of crustal shortening has already occurred over the last 130 million years. For Laurentia, between 1100 and 950 million years ago, paleomagnetic measurements give the path of Earth's geomagnetic pole, attributed to movement of Laurentia. The Grenville comprises a stack of southeast-dipping faults that have transported overlying rocks northwestwards. Within this stack a major thrust marks a ~90 degree change in paleomagnetic direction. Rocks to the southeast, above this thrust, yield a paleomagnetic pole plotting ~4000 km northwest of that with similar age from Laurentia, suggesting their northwestward transportation of ~4000 km. The Grenville Province is considered part of Laurentia, so Laurentian crust has been shortened by this amount. If crustal shortening of several thousand kilometers typifies continental collisions, continental configurations reconstructed before these collisions should allow for the change in shape and size of affected continents.


Inelastic surface deformation during the 2013 Mw 7.7 Balochistan, Pakistan, earthquake
A. Vallage et al., Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS, 1 Rue Jussieu, F-75005 Paris, France. This paper is online at http://dx.doi.org/10.1130/G37290.1.

The A.D. 2013 Mw 7.7 Balochistan, Pakistan, earthquake was imaged both with high-resolution optical satellite images and radar satellite imagery. Correlating the optical images acquired before and after the event, we derived the horizontal co-seismic displacement with an unprecedented 2.5 m ground resolution, used latter to determine the detailed slip-distribution at the ground surface, along the 200 km-long rupture. In parallel, using sub-metric optical images we mapped in great details, down to decimeter-scale, the entire ground rupture pattern. These original observations point to the high complexity of the ground ruptures with very peculiar distribution of the deformation along some sections of the rupture. In fact, in several places we find evidence for the amounts of slip at the fault tip that exceed by far motion predicted by classic elastic models. Drastic change in fault geometry promoted by rupture propagating in unconsolidated sediments, when getting to the surface, might be responsible for such large inelastic deformation. Hence, our new ability to look at earthquake deformation patterns at high resolution at the fault, because it allows quantifying deformation distributed directly around the main rupture, brings to reconsider measurements limited only to principal rupture and any subsequent mechanical inferences about rupture processes.


Salt precipitation in magmatic-hydrothermal systems associated with upper crustal plutons
Pilar Lecumberri-Sanchez et al., Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, CH-8092 Zurich, Switzerland. This paper is online at http://dx.doi.org/10.1130/G37163.1.

Salt precipitation due to evaporation in sedimentary environments has been extensively studied. In contrast, the formation of solid salt by boiling hydrothermal fluids at deeper levels in Earth's upper crust is less clear. Fluids exsolved from magmas in the upper crust are commonly saline and undergo boiling, producing high-salinity brines. Theoretical modeling suggests that during ascent, boiling fluids may become salt-saturated and precipitate halite. However, direct evidence of salt precipitation in magmatic-hydrothermal systems has been elusive, likely owing to re-dissolution of salt by later fluid circulation. Here, Pilar Lecumberri-Sanchez and colleagues present evidence that salt precipitation is a common phenomenon in upper-crustal magmatic-hydrothermal systems worldwide, suggesting that salt precipitation is a natural consequence of hydrothermal fluid evolution in many systems. Salt, as one of the major components in geologic fluids, can affect rock porosity/permeability and thus fluid circulation; the mechanical properties of the rock; temporary storage of halogens (which are otherwise incompatible in most geologic materials); and metal transport and deposition at the temperature-pressure conditions of economic mineral deposit formation.


Protection of phototrophic iron(II)-oxidizing bacteria from UV irradiation by biogenic iron(III) minerals: Implications for early Archean banded iron formation
Tina Gauger et al., Geomicrobiology, Center for Applied Geosciences, University of Tübingen, 72074 Tü-bingen, Germany, http://dx.doi.org/10.1130/G37095.1.

The lack of a protective ozone layer on the O2-free Archean Earth (4.0 to 2.5 billion years ago) made the shallow photic zone of ancient oceans a harsh environment since harmful ultraviolet (UV) radiation reached Earths' surface almost unattenuated. Consequently, primitive photosynthetic bacteria thriving in these light-influenced environments would have needed protection to prevent UV-induced damage. In this study, Gauger and colleagues show that phototrophic Fe(II)-oxidizing (so-called photoferrotrophic) bacteria that use Fe(II) and light to grow are able to produce their own UV screen in form of nanometer-sized Fe(III) minerals. These Fe(III) minerals absorb light in the UV range, but still transmit light that is used for photosynthesis. Such a self-made sunscreen would have enabled similar primitive photoferrotrophic bacteria to live in shallow water environments of the ancient oceans strengthening their possible role in the deposition of Archean banded iron formations (BIFs), the most important supplies for iron on Earth.


Magnesium isotope evidence for a recycled origin of cratonic eclogites
Shui-Jiong Wang et al., Isotope Laboratory, Dept. of Earth and Space Sciences, University of Washington, Seattle, Washington 98195-1310, USA. This article is online at http://dx.doi.org/10.1130/G37259.1.

Eclogite xenoliths brought up by kimberlite magma have garnered significant interest because that a significant proportion of kimberlite-born diamonds has grown in an eclogitic medium. However, debate continues as to whether all eclogite xenoliths are originated from recycling of ancient oceanic crust or they represent high-pressure mantle cumulates. Non-traditional stable Mg isotopes provide insights on their origins, because Mg isotopes are strongly fractionated during low-temperature surficial processes but display very limited variation during high-temperature igneous differentiation. The extremely large Mg isotope fractionations observed in the xenolithic eclogites from the Koidu kimberlite complex, Sierra Leone, West Africa, suggest that the low-MgO eclogites are derived from recycled altered oceanic crust, and the high-MgO eclogites represent the reaction product of Mg-Fe exchange between low-MgO eclogites and surrounding peridotite in the lithospheric mantle. Therefore, the recycling of oceanic crust increase the Mg isotope heterogeneity of the mantle, and the xenolithic eclogites preserve a record of Mg isotopic compositions produced by low-pressure, surficial isotope fractionations in the mantle.


Formation of Mississippi Valley-type deposits linked to hydrocarbon generation in extensional tectonic settings: Evidence from the Jabali Zn-Pb-(Ag) deposit (Yemen)
Jörg Ostendorf et al., Dept. of Mineralogy, Technische Universität Bergakademie Freiberg, Brennhaus-gasse 14, Freiberg D-09599, Germany, http://dx.doi.org/10.1130/G37112.1.

Mississippi Valley-type (MVT) deposits are major sources for zinc and lead to the global economy. The origin of MVT deposits is linked to the migration of warm, salt-rich brines from marine sedimentary basins -- the source of Pb and Zn -- into marginal marine carbonate rocks where metal deposits form. The deposits are often found in proximity to hydrocarbon reservoirs, but the relationship between hydrocarbon generation and ore formation usually remains tentative. The most widely accepted model for MVT deposit formation relates ore formation to compressional tectonics and uplift, with brines driven by gravity. In this article, Jörg Ostendorf and colleagues present a model for the formation of the Jabali MVT-deposit in central Yemen, which challenges the prevailing paradigm for MVT deposit formation. Based on an isotopic rubidium-strontium age of 144.0 plus or minus 4.3 Ma for sphalerite (zinc sulfide), the authors illustrate that ore formation coincided with hydrocarbon migration during a period of extensional tectonics in the Late Jurassic-Early Cretaceous.


Eocene atmospheric CO2 from the nahcolite proxy
Elliot A. Jagniecki et al., Dept. of Geological Sciences and Environmental Studies, Binghamton University, Binghamton, New York 13902, USA. This paper is online at http://dx.doi.org/10.1130/G36886.1.

Estimates of the atmospheric concentration of CO2, [CO2]atm, for the "hothouse" climate of the early Eocene climatic optimum (EECO) vary for different proxies. Extensive beds of the mineral nahcolite (NaHCO3) in evaporite deposits of the Green River Formation, Piceance Creek Basin, Colorado, USA, previously established [CO2]atm for the EECO to be >1125 ppm by volume (ppm). Here, we present experimental data that revise the sodium carbonate mineral equilibria as a function of [CO2] and temperature. Co-precipitation of nahcolite and halite (NaCl) now establishes a well-constrained lower [CO2]atm limit of 680 ppm for the EECO. Paleotemperature estimates from leaf fossils and fluid inclusions in halite suggest an upper limit for [CO2]atm in the EECO from the nahcolite proxy of ~1260 ppm. These data support a causal connection between elevated [CO2]atm and early Eocene global warmth, but at significantly lower [CO2]atm than previously thought, which suggests that ancient climates on Earth may have been more sensitive to a doubling of [CO2]atm than is currently assumed.

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GEOLOGY articles published online ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary articles by contacting Kea Giles at the address above.

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