Summer Geoscience from GSA Bulletin

(Press-News.org) Boulder, Colo., USA – GSA Bulletin papers posted online from 20 July through 14 September 2012 elaborate on geoscience from Algeria, Mexico, Spain, Turkey, Nova Scotia, Switzerland, New Mexico, and the U.S. Rocky Mountains. Topics include tectonics, mineral formation, the Moho, age dating using zircon crystals, the Code of Stratigraphic Nomenclature, atmospheric CO2, and early animal evolution.

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An exceptionally long paleoseismic record of a slow-moving fault: The Alhama de Murcia fault (Eastern Betic shear zone, Spain)
María Ortuño et al., Centro de Geociencias, Universidad Nacional Autónoma de México, Blvd. Juriquilla, 3001, 76230, Juriquilla, Querétaro, México; and RiskNat group, Depto. de Geodinàmica i Geofisica, Universitat de Barcelona, Martí i Franquès s/n, 08028 Barcelona, Spain. Posted online 20 July 2012; doi: 10.1130/B30558.1.

Slow faults are capable of producing large earthquakes, and they occasionally can be even more dangerous than the fast faults. Because of their thousand-year recurrence periods, the "silence" of these faults during historical times may result in a lack of social awareness of their seismic hazard. This problem can be reduced by the study of long paleoseismic "archive books." Maria Ortuño (UNAM, Mexico, and Univ. de Barcelona, Spain) and colleagues study the geological record of the southern termination of the Alhama de Murcia fault (southeast Iberia). Last year, this fault produced a destructive earthquake (11th May 2011) that struck the city of Lorca and left nine fatalities. The geological history shows that this earthquake (magnitude Mw = 5.1) is small compared to the 6 to 7 maximum magnitude events produced by the fault in the past ~300,000 years and that the maximum time spam between consecutive events is 29,000 years. The study of this exceptionally long period was possible thanks to the particular structural and environmental conditions of the area, which allowed the preservation of many "chapters of the geological book" and also to the use of a new protocol for dating older than usual sediments by using the luminescence signal of the mineral grains.

Growth of fissure ridge travertines from geothermal springs of Denizli Basin, western Turkey
Luigi De Filippis et al., Dipartimento di Scienze Geologiche, Università Roma Tre, Rome, Italy. Posted online 20 July 2012; doi: 10.1130/B30606.1.

Understanding past climate fluctuations as well as planning and managing of subsurface natural repositories for greenhouse gases such as CO2 are increasingly becoming important targets for academic and industrial institutions as well as national governments. For these reasons, it is important to find and study natural and reliable markers of past climate fluctuations and of natural CO2 degassing from the subsurface. Travertines deposited in geothermal areas have these characteristics. Luigi De Filippis and colleagues use a multidisciplinary approach to studying Quaternary geothermal travertines from the Denizli basin, western Turkey, and understanding their growth and the controlling factors. Their results show that the travertines, which are mound-shaped deposits as high as 25 m, mainly formed during warm and wet periods when the fluid circulation was abundant. Conversely, during cold periods, degassing and travertine deposition occurred probably in coincidence with seismic events when the shaking provoked the exsolution of CO2 dissolved in deep subsurface aquifers. These results confirm that travertines are long-term markers of climate fluctuations and, on the other hand, that their complete understanding may help in the planning and long-term managing of artificial subsurface repositories of greenhouse gases.

Contribution of crystallographic preferred orientation to seismic anisotropy across a surface analog of the continental Moho at Cabo Ortegal, Spain
Sergio Llana-Fúnez and Dennis Brown, Dept. of Earth and Ocean Sciences, University of Liverpool, 4 Brownlow Street, L69 3GP Liverpool, UK. Posted online 20 July 2012; doi: 10.1130/B30568.1.

The continental Moho is one of the major features of Earth's lithosphere. It separates the crust from the mantle and is defined as a change in P-wave velocity from less than 7.5 km/s to greater than 8 km/s. This increase in velocity is caused by a change in the physical properties of the mostly mafic granulite lower crustal rocks to the olivine-dominated mantle mafic rocks. In Cabo Ortegal, northern Spain, a rare exposure of the continental Moho outcrops, providing an excellent opportunity to study these rocks and to establish the relationships between the different lithologies that can make up the Moho. Sergio Llana-Fúnez and Dennis Brown have calculated the P-wave and S-wave velocities of all of the major mineral phases in each lithology by crystal-preferred orientation analyses using electron back-scatter imaging. This allows them to calculate the P- and S-wave velocity for each lithology, to tell the contribution of each phase to its total velocity, and its contribution to seismic anisotropy. They also found that metamorphic reactions in the lower crustal rocks, especially those that produce the low density, low velocity, and weakly anisotropic mineral plagioclase, can play an important role in the seismic velocity structure of the Moho.

36Cl exposure dating if paleoearthquakes in the Eastern Mediterranean: First results from the western Anatolian Extensional Province, Manisa fault zone, Turkey
Naki Akçar et al., University of Bern, Institute of Geological Sciences, Baltzerstrasse 1-3, 3012 Bern, Switzerland. Posted online 20 July 2012; doi: 10.1130/B30614.1.

One of the most seismically active places in the world is the Eastern Mediterranean and Middle East, where societies experienced at least 150 large earthquakes (generally M >6) during the last 2,500 years. The 2,500-year time span of seismic activity is reconstructed by Naki Akçar and colleagues based on historical and instrumental data. However, this is a short time compared to the whole Quaternary (the past 2.58 million years) and beyond, and earthquake chronology is mostly unknown. Indeed, reliable long term earthquake models are required for the assessment of the vulnerability of the urban areas to earthquake disaster and for considerations of disaster prevention. Therefore, Akçar and colleagues applied cosmogenic 36Cl technique to determine the seismic history and slip rate of the Mugırtepe limestone fault plane in the western Anatolia beyond historical data. Surface faulting at Mugırtepe started no later than around 13.7 thousand years ago, and seismic activity had its maximum around 8.5 thousand years ago, during which most of the faulting was completed. Such studies in the region are needed for deeper and better understanding of the nature of such high versus low seismic activity periods. Nonetheless, these results indicate periods of enhanced tectonic activity during the late Pleistocene.

Systems paleobiology
Andrew H. Knoll, Dept. of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138, USA. Posted online 10 Aug. 2012; doi: 10.1130/B30685.1.

Systems paleobiology seeks to interpret the history of life within the framework of Earth's environmental history, using physiology as the conceptual bridge between paleontological and geochemical data sets. In some cases, physiological performance can be estimated directly and quantitatively from fossils—this is commonly the case for vascular plant remains. In other instances, statistical inferences about physiology can be made on the basis of phylogenetic relationships. The systems approach also provides a template for evaluating the habitability of other planets, not least of which is the ancient surface of Mars.

Anatomy and tectono-sedimentary evolution of a rift-related detachment system: The example of the Err detachment (central Alps, SE Switzerland)
Emmanuel Masini et al., IPGS/CNRS, DyLBas team; 1, rue Blessig F-67084 Strasbourg Cedex, France. Posted online 10 Aug. 2012; doi: 10.1130/B30557.1.

This paper by Emmanuel Masini and colleagues focuses on the description of the geology of the deepest domain between continental and oceanic domains, called "distal rifted margins." These domains are created during the tectonic extension of the continental lithosphere before the rupturing of this later, leading to oceanic seafloor spreading. Present-day examples can be offshore North and South America and offshore Europe or Africa as the Atlantic distal rifted margin domains. These domains are being targeted by industry for oil/gas investigations but remain poorly understood because of the abyssal water depth. That is the reason Masini and colleagues describe a Jurassic fossil analogue preserved in the Swiss Alps.

Detrital zircon geochronology from the Cambrian-Ordovician Bliss Sandstone, New Mexico: Evidence for contrasting Grenville-age and Cambrian sources on opposite sides of the Transcontinental Arch
Jeffrey M. Amato and Greg H. Mack, Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Posted online 10 Aug. 2012; doi: 10.1130/B30657.1.

One way to reconstruct the paths of ancient rivers and locations of ancient mountains is to focus on the ages of zircon crystals in sand deposited along beaches by rivers. This study by Jeffrey Amato and Greg Mack investigates the beach sands of the Cambrian (about 500 million years old) Bliss Sandstone, which is exposed throughout south-central and southwestern New Mexico, USA. In the center of the study area lies a granite pluton that formed 510 million years ago and was exhumed relatively soon after its emplacement. To the east of this granite, there are no 510-million-year-old zircons; however, to the west there are abundant zircons of this age. This suggests that the ocean current carried sediment from east to west. Similar age sandstones from the Grand Canyon area also do not have any 510-million-year-old zircons, indicating the presence of a barrier to deposition between New Mexico and northwestern Arizona. This barrier is was the Transcontinental Arch, which traversed the western U.S. from southwestern Arizona northeastward into Canada. Zircon ages from Cambrian sandstones provide valuable insight into depositional patterns and the locations of ancient topographic highs along the continental shoreline during the Cambrian.

The Toarcian oceanic anoxic event in the Western Saharan Atlas, Algeria (North African paleomargin): Role of anoxia and productivity
M. Reolid et al., Departamento Geología, Universidad Jaén, 23071 Jaén, Spain. Posted online 10 Aug. 2012; doi: 10.1130/B30585.1.

M. Reolid and colleagues study the Toarcian Oceanic Anoxic Event, one important episode of marine mass extinction during the Early Jurassic, through benthic foraminiferal assemblages, geochemical proxies (paleoproductivity, redox, and detrital proxies), and C and O stable isotopes. These materials are located in the Saharan Atlas (Algeria) and represent the north paleomargin of Gondwana. In the time intervals just before than the anoxic event, the diversity of the foraminiferal assemblages decreased and opportunists were progressively more abundant, indicating any unfavorable context for benthic organisms. The anoxic event is characterized in this sector by an abrupt environmental change, evidenced by the extinction of foraminifera, enrichment in redox sensitive elements and organic matter, increase in detrital and paleoproductivity proxies, and fluctuations of delta-13C and delta-18O. An anoxic or strongly dysoxic event happened coincident with increasing eolian detrital input and paleoproductivity influenced by climatic change. The return to normal oxic conditions is marked by the slow recovery of benthic foraminiferal assemblages dominated by opportunistic forms. The geochemical proxies for redox conditions and paleoproductivity come back to values previous to the anoxic event. The Toarcian anoxic event occurred in a generalized transgression and warming that may have favored water stratification and confinement of bottom waters.

Early Aptian algal bloom in a neritic proto–North Atlantic setting: Harbinger of global change related to OAE 1a?
S. Huck et al., ETH Zurich, Geological Institute, Sonneggstrasse 5, CH-8092 Zürich, Switzerland. Posted online 10 Aug. 2012; doi: 10.1130/B30587.1.

The mid-Cretaceous greenhouse world was characterized by major biotic perturbations including the Early Aptian carbonate platform breakdown in the subtropical northern Tethyan Ocean (25 degrees N) prior to the onset of oceanic anoxic event 1a (OAE 1a). The well-studied OAE 1a is expressed as global deposition of organic-rich black-shales in pelagic basins and coincides with global perturbations of the carbon-cycle. This study by S. Huck and colleagues deals with a punctuated and pronounced Early Aptian bloom of the enigmatic algae Lithocodium aggregatum, forming several peculiar decimeter-high bioconstructions along the proto-North Atlantic margin in Portugal. Similar to southern and central Tethyan shallow-water carbonate platforms, where pervasive growth of microbial communities characterizes short-lived Lower Aptian stratigraphic intervals, Lithocodium replaced the oligotrophic platform community (corals, bivalves) in the proto-North Atlantic setting. Microfacies analysis suggests that Lithocodium flourished during relative sea-level highstand under elevated nutrient levels. High-resolution carbon-isotope stratigraphy clearly argues for Lithocodium as a biotic harbinger of environmental change in the run-up of OAE 1a, which is assumed to be related to the emplacement of the Ontong Java large igneous province. The subsequent carbonate platform break-down, as observed in the studied sections, coincides with the Early Aptian carbonate platform breakdown in the Northern Tethyan realm.

New constraints on using paleosols to reconstruct atmospheric pCO2
Jennifer M. Cotton and Nathan D. Sheldon, Dept. of Earth and Environmental Sciences, University of Michigan, 2534 C.C. Little Building, 1100 N. University Ave., Ann Arbor, Michigan 48109-1005, USA. Posted online 10 Aug. 2012; doi: 10.1130/B30607.1.

The main control of the Earth's temperature over geologic time is the concentration of atmospheric CO2, however, the exact relationship between CO2 and temperature is not fully understood. In order to constrain climate sensitivity and better predict future climate change, we must first reconstruct more precisely atmospheric pCO2 estimates in the geologic past. The pedogenic carbonate paleobarometer is the most widely applicable method of reconstructing past atmospheric pCO2, but one of its key variables, the concentration of respired CO2 in the soil, is not well constrained. Jennifer Cotton and Nathan Sheldon have developed a new proxy for soil respired CO2 using its relationship with mean annual precipitation to reduce the uncertainty in the pedogenic carbonate paleobarometer. This new proxy was first developed through a literature review of soil respired CO2 measurements and climatic data, and was then validated using isotopic analysis of pedogenic carbonates collected from modern soils. Using this new soil respired CO2 proxy, Cotton and Sheldon have revised previous estimates of atmospheric pCO2 using the pedogenic carbonate paleobarometer and calculate estimates that are in better agreement other independent proxies and model results than previous reconstructions. These revised estimates also support CO2-temperature coupling in the late Miocene as well as the late Permian.

Detrital zircon geochronology from the Cambrian-Ordovician Bliss Sandstone, New Mexico: Evidence for contrasting Grenville-age and Cambrian sources on opposite sides of the Transcontinental Arch
Jeffrey M. Amato and Greg H. Mack, Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Posted online 10 Aug. 2012; doi: 10.1130/B30657.1.

One way to reconstruct the paths of ancient rivers and locations of ancient mountains is to focus on the ages of zircon crystals in sand deposited along beaches by rivers. This study investigates the beach sands of the Cambrian (about 500 million years old) Bliss Sandstone, which is exposed throughout south-central and southwestern New Mexico, USA. In the center of the study area lies a granite pluton that formed 510 million years ago and was exhumed relatively soon after its emplacement. To the east of this granite, there are no 510-million-year-old zircons; however, to the west there are abundant zircons of this age. This suggests that the ocean current carried sediment from east to west. Similar age sandstones from the Grand Canyon area also do not have any 510-million-year-old zircons, indicating the presence of a barrier to deposition between New Mexico and northwestern Arizona. This barrier is was the Transcontinental Arch, which traversed the western U.S. from southwestern Arizona northeastward into Canada. Zircon ages from Cambrian sandstones provide valuable insight into depositional patterns and the locations of ancient topographic highs along the continental shoreline during the Cambrian.

Probing for Proterozoic and Archean crust in the northern U.S. Cordillera with inherited zircon from the Idaho batholith
Richard M. Gaschnig et al., Dept. of Geology, University of Maryland, College Park, MD 20742, USA. Posted online 14 Sept. 2012; doi: 10.1130/B30583.1.

The Idaho batholith is a large mass of granite in the northern U.S. Rocky Mountains. While the batholith was formed from magmas that crystallized between 100 to 50 Ma, it was built on preexisting Precambrian (a little younger than 542 million years old) crust, and Richard M. Gaschnig and colleagues investigate this earlier Precambrian history by determining the ages of accidentally preserved portions of the mineral zircon. These Precambrian inherited zircon components reveal the presence of Archean crust (about 2.5 to 3.2 billion years old) with Neoproterozoic elements (about 670 million years old) beneath the southernmost portion of the batholith. Inherited zircons from the northern portion of the batholith show greater complexity and appear to be derived from melted Precambrian sedimentary rocks but do not appear to provide unambiguous information on the basement on which these sediments were deposited, beyond providing a minimum age of ~1.5 billion years.

Mappability, stratigraphic variation, and diagenetic problems in sedimentary map unit definition and field mapping
Loren A. Raymond et al., Appalachian State University, Boone, NC 28608 and Peridotite Publishers, 3327 Cypress Way, Santa Rosa, CA 95405, USA. Posted online 14 Sept. 2012; doi: 10.1130/B30621.1.

Geologists who engage in traditional field mapping of sedimentary rocks frequently find the need to address one or more of three kinds of stratigraphic mapping problems. These are (1) that some formations named in the 20th century do not meet the thickness and mappability requirements of the Code of Stratigraphic Nomenclature; (2) that thickness and rock-type ratios vary over small to large distances, making it difficult to apply the names of formations previously defined outside the area being mapped to local formations within the new map area; and (3) that considerable difficulty is experienced in locating the tops and bottoms of formations (the contacts), in cases where formations are defined and recognized on the basis of post-depositional changes in rock character (i.e., diagenetic changes). Authors Loren A. Raymond and colleagues recommend (1) flexibility in the use of defining characteristics when applying the names of previously defined formations to rocks in new locales, and (2) rigorous adherence to the stratigraphic code in naming new formations.

The seismic architecture and geometry of grounding-zone wedges formed at the marine margins of past ice sheets
J.A. Dowdeswell and E.M.G. Fugelli, Scott Polar Research Institute, University of Cambridge, Cambridge CB2 1ER, UK. Posted online 14 Sept. 2012; doi: 10.1130/B30628.1.

Grounding-zone wedges (GZW) are bodies of sediment that form when ice sheets halt for periods of decades to centuries during their retreat across Arctic and Antarctic continental shelves during deglaciation. Sediment builds up at the grounding-zone through delivery of soft, deforming basal debris from fast-flowing ice streams. GZW can be recognized in the geological record through their characteristic asymmetry in ice-flow direction, with steeper ice-distal sides. Typical GZW are about 5-20 km long and 50-100 m thick, with a lateral width of several tens of kilometers. These dimensions are controlled by sediment flux, still-stand duration, cavity shape, and ice-stream width. Low-angle ice-shelf cavity roofs immediately beyond the grounding zone probably restrict vertical accommodation space, preventing formation of steep banks or ridges. GZW are mainly transparent or chaotic seismically, probably resulting from delivery of unsorted debris. Channels are present within some GZW; meltwater flow is under high pressure, and V-shaped incisions suggest high-energy flow. Where ice-sheet mass loss is dominated by meltwater runoff, GZW probably contain more sorted sediment than those from high-latitudes. The presence of GZW on polar continental shelves indicates episodic rather than catastrophic ice-sheet retreat. GZW are a mechanism for ice-shelf stabilization because wedge growth counteracts collapse induced by ice-sheet thinning and sea-level rise.

Probing for Proterozoic and Archean crust in the northern U.S. Cordillera with inherited zircon from the Idaho batholith
Richard M. Gaschnig et al., Dept. of Geology, University of Maryland, College Park, MD 20742, USA. Posted online 14 Sept. 2012; doi: 10.1130/B30583.1.

The Idaho batholith is a large mass of granite in the northern U.S. Rocky Mountains. While the batholith was formed from magmas that crystallized between 100 to 50 Ma, it was built on preexisting Precambrian (a little younger than 542 million years old) crust, and Richard M. Gaschnig and colleagues investigate this earlier Precambrian history by determining the ages of accidentally preserved portions of the mineral zircon. These Precambrian inherited zircon components reveal the presence of Archean crust (about 2.5 to 3.2 billion years old) with Neoproterozoic elements (about 670 million years old) beneath the southernmost portion of the batholith. Inherited zircons from the northern portion of the batholith show greater complexity and appear to be derived from melted Precambrian sedimentary rocks but do not appear to provide unambiguous information on the basement on which these sediments were deposited, beyond providing a minimum age of ~1.5 billion years.

Complex magma mixing, mingling, and withdrawal associated with an intra-Plinian ignimbrite eruption at a large silicic caldera volcano: Los Humeros of central Mexico
Gerardo Carrasco-Núñez et al., Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Boulevard Juriquilla 3001, Querétaro, Qro. 76230, Mexico. Posted online 14 Sept. 2012; doi: 10.1130/B30501.1.

Several highly explosive volcanoes in central Mexico have erupted cataclysmically in pre-historic times. The largest, Los Humeros, is still hot, and electricity is being generated from it (geothermal power). But what happens under such potentially dangerous volcanoes immediately before a large eruption? What goes on in the underground plumbing system, where incandescent, mostly liquid rock and gases ultimately cause a cataclysmic explosion? An international team of volcanologists, including first author on this paper Gerardo Carrasco-Núñez, has been finding out by looking at the pumice ejected from the volcano a hundred thousand years ago. Liquid rock thrust explosively from the volcano chills in the air, freezing the chemical story of what happened. It reveals that immediately before the eruption, several different hot liquid magmas came into contact with each other, transferring heat and chemicals. But this happened too quickly for the liquids to completely mix together. The eruption may have been triggered by the arrival of new dense magma from a deep source. Dense (andesite) magma and lighter (rhyodacite) magma seem to have interacted, possibly within hot, partly molten bodies made of crystals and liquid rock. Strangely, the chemistry of the pumice yields no record of former rhyolite magma known to have erupted previously from the volcano.

Cambrian–Ordovician acritarchs in the Meguma terrane, Nova Scotia, Canada: Resolution of early Paleozoic stratigraphy and implications for paleogeography
C.E. White et al., Dept. of Earth and Environmental Science, Acadia University, Wolfville, Nova Scotia B4P 2R6. Canada. Posted online 14 Sept. 2012; doi: 10.1130/B30638.1.

Tiny fossils are preserved in some rocks now exposed on land in southern Nova Scotia, Canada. These rocks formed from sediments deposited on the deep ocean floor about 540 to 475 million years ago. By comparing the organic remains in these Nova Scotian rocks to those in rocks of similar age elsewhere in the world, we can better understand the early evolution of animals and the oceans in which they lived. We can also better understand where the deep ocean floor now exposed in Nova Scotia was located relative to the continents of that time and make interpretations about how southern Nova Scotia travelled to its present-day location. For example, similarities in age to rocks now located in North Wales suggest links between that area and southern Nova Scotia 500 million years ago.

Mappability, stratigraphic variation, and diagenetic problems in sedimentary map unit definition and field mapping
Loren A. Raymond et al., Appalachian State University, Boone, NC 28608, USA; and Peridotite Publishers, 3327 Cypress Way, Santa Rosa, CA 95405, USA. Posted online 14 Sept. 2012; doi: 10.1130/B30621.1.

Geologists who engage in traditional field mapping of sedimentary rocks frequently find the need to address one or more of three kinds of stratigraphic mapping problems. These are (1) that some formations named in the 20th century do not meet the thickness and mappability requirements of the Code of Stratigraphic Nomenclature; (2) that thickness and rock-type ratios vary over small to large distances, making it difficult to apply the names of formations previously defined outside the area being mapped to local formations within the new map area; and (3) that considerable difficulty is experienced in locating the tops and bottoms of formations (the contacts), in cases where formations are defined and recognized on the basis of post-depositional changes in rock character (i.e., diagenetic changes). Authors Loren A. Raymond and colleagues recommend (1) flexibility in the use of defining characteristics when applying the names of previously defined formations to rocks in new locales, and (2) rigorous adherence to the stratigraphic code in naming new formations.

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