Following are highlights from the November issue of GEOLOGY and a summary of the science article from the November issue of GSA TODAY, published by the Geological Society of America. Stories written regarding these articles are embargoed until November 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to GEOLOGY or GSA TODAY in stories published. Please contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.
GEOLOGY
Novel paleoecology of a postextinction reef: Famennian (Late Devonian) of the Canning basin, northwestern Australia. Rachel Wood
Reefs are thought to be very sensitive to mass extinctions, recovering far more slowly than other types of communities. This paper shows that reefs can recover very quickly after such destructive events, using an example from a 350-million-year-old reef from Western Australia. Here, a totally new type of reef community appears almost immediately after the mass extinction event. This may have ramifications for the plight of modern coral reefs, which are currently under considerable threat as a result of local extinction due to society's activities.
Confocal microscopy of fluid inclusions reveals fluid-pressure histories of sediments and an unexpected origin of gas condensate. Andrew C. Aplin et al.
In the search for oil, determining how the pressure and composition of oil has changed through time is critical. The information we need to crack this problem is contained within fluid inclusions, micron scale pockets of paleofluids trapped in rock minerals. We report the results of a novel technique that finally allows this information to be tapped. The authors have exploited a biomedical technique, Confocal Laser Scanning Microscopy, to produce three-dimensional images of the inclusions. These reveal the amounts of gas and oil in the inclusions, data that can be used to model both the composition and the pressure of the paleofluids. In UK North Sea case studies, the authors use the technique to show how high fluid pressures evolved through geologic time and also that the present gas accumulations were preceded by oil just a million years ago. Of interest in oil exploration: where has the displaced oil gone?
High concentrations of greenhouse gases and polar stratospheric clouds: A possible solution to high-latitude faunal migration at the latest Paleocene thermal maximum. Robert B. Peters and Lisa Cirbus Sloan
This paper documents a climate modeling study that investigates the role of high concentrations of atmospheric methane and polar clouds in explaining the high-latitude faunal migrations that occurred approximately 55 million years ago. The model results indicate that such high-latitude migrations, which would have required year-round above-freezing conditions for many years, may have been facilitated by high atmospheric methane concentrations and the presence of polar stratospheric clouds.
Have distal impact ejecta changed through geologic time? Bruce M. Simonson and Paul Harnik
The vast majority of impact craters on the moon are over 2.5 billion years old. Craters must have been forming on Earth’s surface at the same time (a period known as the Archean), but almost all of them have long since been eradicated by erosion and tectonic deformation. Nevertheless, a record of impacts has been discovered among Archean sedimentary strata in the form of thin layers rich in distinctive sand-size spherules of former silicate melt. Around ten spherule-rich layers that range in age from ca. 2.5 to 3.4 billion years old have now been identified. All of them appear to be reworked ejecta from large impacts, and they have much in common with younger layers, especially the most extensive ejecta layer known, the Cretaceous-Tertiary boundary layer. However, there are also consistent differences; the aggregate thickness of spherules is generally 10 to 100 times greater in the Archean layers, and they display textures rarely seen in younger spherules. These contrasts may reflect progressive changes in both impactors and their target, Earth’s surface, through Earth history. A decrease in the size of impactors and increases in the depth of the Earth’s ocean and the fraction of Earth’s surface covered by continental crust through time could explain the observed differences.
Los Angeles: The most differentiated basaltic martian meteorite. Alan E. Rubin et al.
Los Angeles is a new martian meteorite. It is a type of igneous rock called basaltic shergottite, formed from volcanic lava flows on Mars. As the parent magma cooled and minerals crystallized, the remaining liquid became richer in iron, phosphorus, and other elements that do not readily partition into early crystallizing phases. Among the 16 known martian meteorites, Los Angeles is by far the most enriched in iron, phosphorus, and many trace elements, and is the most depleted in some other trace elements such as chromium. The mineralogical characteristics of pyroxene in this meteorite imply that the parent rock cooled more slowly than other basaltic shergottites and thus may have been more deeply buried. Like other martian meteorites, Los Angeles was intensely shocked (transforming the major mineral plagioclase into a rare plagioclase-composition glass called maskelynite) as it was launched into space by a meteoritic impact that blasted the surface of Mars.
Numerical simulations of glacial-valley longitudinal profile evolution. K.R. MacGregor et al.
Valley glaciers are very important in shaping mountainous landscapes. They broaden valley bottoms and produce the classic U-shaped valley cross section. In addition, glaciers can enhance local valley relief, generate multiple steps and overdeepenings of valley floors, and they cause tributary valleys to hang high above trunk valley floors. These distinctive glacial signatures, seen in virtually every mountain range that has held glaciers, result from tens to hundreds of thousands of years of subglacial erosion. This glacial erosion occurs during periods of dramatic swings in climate, which drives significant advances and retreats of alpine glaciers. We use a numerical model of glacial erosion to understand how glaciers create these unique stepped and overdeepened longitudinal profiles over many glacial cycles. Persistent, high-relief bedrock steps and overdeepenings are generated in our model runs only when we incorporate tributary glaciers. The size of the step that results from glacial erosion depends strongly on both the size of the tributary and the place that it connects to the main valley. The long-term erosion pattern in these valleys mimics closely the long-term pattern of glacier occupation of the valleys.
GSA TODAY
Dynamics of plate boundary fault systems from Basin and Range Geodetic Network (BARGEN) and geologic data. Brian Wernicke et al.
This paper presents results of 2-3 years of continuous monitoring of a new 50-station network of GPS (global positioning system) stations that were installed across the northern Basin and Range Province of the western United States. This project, funded by the NSF Continental Dynamics Program, represents a significant technological advance by documenting that continuous recording of carefully installed GPS stations can provide precise measurements (with errors possibly significantly lower than 0.5 mm per year) of tectonic displacements of station sites in the Basin and Range Province relative to the stable interior of North America. The results provide a view of the velocity field of stations that can be used to understand the extensional deformation (strain) of the Basin and Range Province. Results are also evaluated to answer important questions for continental tectonics: Is the extensional strain across the province homogeneously distributed, as is suggested by the uniform spacing and geometry of Basins and Ranges across the Province? Or is extensional strain concentrated in localized zones, as is suggested by the concentration of earthquakes, for example in the Intermountain seismic zone near Salt Lake City? The authors conclude the former - a broadly distributed deformation. By contrasting the observed GPS data with longer term geologic data, they propose a model that may reconcile complex non-linear changes in the velocity field, the apparent uniformity of the geologic pattern of strain, and the concentration of seismicity in narrow belts. According to the model, major anomalies in the velocity field result from strain pulses caused by earthquakes that occurred many decades ago, and that these pulses may influence the likelihood of earthquakes on other faults in the system. If so, combining geologic and geodetic data, as in this study, offers a potential to understand fault system behavior - especially how short term effects from one earthquake can influence patterns of stress build-up on nearby faults. These types of data are essential for developing better seismic risk assessments and linking the time tables of seismic shocks to the longer term geologic record, and vice versa.
*To view the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see http://www.geosociety.org/pubs/cattract.htm.
*Full text GEOLOGY articles and the science article from GSA TODAY are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm.
GSA Release No. 00-26
Contact: Ann Cairns
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