||December 21, 2000
GSA Release No. 00-31
January Geology and GSA Today Media Highlights
Following are media highlights from the January issue of GEOLOGY and
a summary of the science article from the January issue of GSA TODAY, published
by the Geological Society of America. Stories written regarding these articles
are embargoed until January 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
- Palynologically calibrated vertebrate record from North Dakota consistent
with abrupt dinosaur extinction at the Cretaceous-Tertiary boundary
Dean A. Pearson et al.
- Dinosaur extinction has been a very controversial subject since the publication
of the asteroid impact hypothesis in 1980. This paper presents detailed evidence
of the occurrence of dinosaur, bird, mammal, fish, amphibian, lizard, pterosaur,
and crocodilian fossils relative to the K-T boundary in North Dakota. Dinosaurs
are common in the youngest localities that preserve vertebrate fossils. This evidence
is compatible with the idea that the extinction of the dinosaurs was caused by
an extraterrestrial impact.
- Ocean stagnation and end-Permian anoxia
Roberta M. Hotinski et al.
- This paper presents results of a computer modeling study to assess the possible
role of ocean chemistry and circulation in the end-Permian mass extinction. The
cause of this extinction remains a mystery, but ocean chemistry is a leading candidate.
Several authors have proposed that warm climatic conditions inhibited ocean mixing
prior to the extinction, creating deep ocean waters devoid of oxygen that had
lethal effects when brought into contact with the shallow marine environment.
We find that warmth causes simulated ocean circulation that is reduced relative
to a Permian scenario with a near-modern temperature distribution, but that circulation
is not sluggish or stagnant. Oxygen levels, on the other hand, are indeed driven
to extremely low levels, but by a combination of lowered oxygen content of ventilating
waters and proportionally higher export of organic matter from high-latitudes,
rather than by a decrease in physical mixing. Our results suggest that high-latitude
warmth in the Late Permian could have contributed to the creation of a lethal
deep water reservoir, but that this unusual deep water chemistry did not require
stagnation of ocean circulation.
- Cratered cobbles in Triassic Bundsandstein conglomerates in northeastern
Spain: An indicator of shock deformation in the vicinity of large impacts
Kord Ernston et al.
- This study reports the discovery of a new indicator of shock-wave effects
from large impacts on Earth. Quartzite cobbles in outcrops of Lower Triassic conglomerates
from the northeastern Spain display unusual features, including circular craters
with central mounds surrounded by radial fractures. The conglomerates are also
marked by intense fracturing down to microscopic scale. These features have traditionally
been attributed to tectonic compression and pressure dissolution at cobble contacts.
Cross sections through the cratered cobbles, however, reveal pervasive internal
fracturing, detached segments along concave spall fractures, and zones marked
y quartz grains with planar deformation features indicative of shock metamorphism.
The authors produced similar features in impact experiments performed on artificial
conglomerates. This supports the idea that these features were produced by deformation
and spallation related to shock-wave propagation through the inhomogeneous deposits.
The conglomerates are found in the vicinity of two mid-Tertiary impact structures
in Spain the Azuara structure and the proposed Rubielos de la CÚrida structure
and the authors propose that such shock features in conglomerates provide a new,
easily recognizable regional impact signature.
- Modulation and amplication of climatic changes in the Northern Hemisphere
by the Indian summer monsoon during the past 80 k.y.
H.R. Kudrass et al.
- The monsoon of southeast Asia strongly affects the climate of the Northern
Hemisphere. Change in salinity in a 80,000 year-long record from sediments in
the northern Bay of Bengal document rapid and great changes of the intensity of
the summer monsoon that are coeval with climatic changes recorded in the Greenland
ice cores. A conceptual model depicts the great influence the monsoonal cycles
have by changing the atmospheric concentrations of water vapor.
- GPS estimate of relative motion between the Caribbean and South American
plates, and geologic implications for Trinidad and Venezuela
John C. Weber et al.
- This new study uses satellite surveying technology, the GPS (Global Positioning
System), to precisely measure the relative motion between the Caribbean and South
American plates. Unlike the motion between Earth's other plates, which is generally
well-resolved, Caribbean-South American plate motion is very poorly resolved in
previous studies that use traditional geologic methods. The new GPS result shows
that the Caribbean plate slides eastward at 20 mm/yr along northern South America.
This motion is taken up in a jerky fashion along the east-trending El Pilar fault
in Venezuela, which has a long record of generating moderate to large earthquakes.
In neighboring Trinidad, about 14 mm/yr of Caribbean-South American plate motion
is concentrated along the Central Range fault, which is highly oblique to the
plate motion vector, and was historically, and is currently, aseismic. The Central
Range fault is thus either creeping aseismically, or locked and accumulating elastic
- Pleistocene reduction of polar ice caps: Evidence from Cariaco Basin marine
R.Z. Poore and H.J. Dowsett
- Microfossil assemblages and stable isotope measurements in high-accumulation
rate marine sediments from the Cariaco Basin, Venezuela, provide evidence for
an extended interval of warm climate and associated high sea level about 400,000
years ago during Marine Isotope Stage (MIS) 11. The study supports controversial
geologic evidence that polar ice caps have been smaller than today under the natural
range of Pleistocene interglacial climates leading to sea level rise of as much
as 20 m above modern sea level. Current sea level rise estimates of 13 to 94 cm
over the next 100 years assume polar ice caps will remain essentially stable over
century time scales. This study suggests that the current interglacial climate
will continue into the future and that any anthropogenic climate warming will
accelerate the natural process of ice cap decay resulting in increased rates of
sea level rise.
- Thermal ionization mass spectrometry U-series dating of a hominid site
near Nanjing, China
Jian-xin Zhao et al.
- High-precision and reliable dating of key hominid sites is crucial for a better
understanding of human evolution, particularly for the resolution of a long-term
debate between out-of-Africa and multiregional evolution schools regarding the
origin of modern human species. To obtain a better constraint on the origin, evolution,
and destination of Asian Homo erectus, we carried out mass spectrometric U-series
dating of speleothems from Tangshan Cave, where Nanjing Man, a typical H. erectus
morphologically correlated with Peking Man at Zhoukoudian, was discovered. Our
results, combined with ecological and paleoclimatic evidence, indicates that Nanjing
Man should be at least 580 thousand years old, or more likely lived during the
glacial oxygen isotope stage 16 (~620 thousand years before present). Such an
age estimate, which is ~270 thousand years older than previous electron-spin-resonance
(ESR) and alpha-counting U-series dates, has significant implications for the
evolution of Asian H. erectus. Tooth dentine and enamel samples from the coexisting
fossil layer yield significantly younger apparent ages, with that of the enamel
sample being only less than a quarter of the minimum age of Nanjing Man. This
suggests that U uptake history is far more complex than existing models can handle.
As a result, great care must be taken in the interpretation of ESR and U-series
dates of fossil teeth.
- Crustal structure and tectonics from the Los Angeles basin to the Mojave
Desert, southern California
G.S. Fuis et al.
- The Los Angeles region is underlain by a network of active faults, including
many that are deep and do not break Earth's surface. These hidden faults include
the previously unknown one responsible for the devastating January 1994 Northridge
earthquake, the costliest quake in U.S. history. So that structures can be built
or strengthened to withstand the quakes that are certain in the future, the Los
Angeles Region Seismic Experiment (LARSE) is locating hidden earthquake hazards
beneath the region to help scientists determine where the strongest shaking will
occur. Images of the sedimentary basin beneath the San Gabriel Valley show that
its depth reaches 3 miles, 50% more than earlier estimates. Because deeper sedimentary
basins have greater shaking potential, earthquake hazards in the San Gabriel Valley
need to be reevaluated. Another major finding is a strongly reflective zone located
deep beneath the San Gabriel Mountains. This zone begins at about 12 miles depth
near the vertical San Andreas Fault and rises in a ramp-like fashion southward
toward the Los Angeles Basin. It appears to connect to the fault system responsible
for the 1987 magnitude 5.9 Whittier Narrows earthquake, which occurred on a blind
thrust fault. This reflective zone is interpreted as a "master" blind thrust fault
that transfers stress and strain upward and southward to a network of faults in
the San Gabriel Valley and Los Angeles Basin. The LARSE data obtained so far have
provided answers that could not have been obtained in any other way. The knowledge
gained will be used to make new and existing structures in the region better able
to withstand earthquakes.
- Erosion, Himalayan geodynamics, and the geomorphology of metamorphism
Peter K. Zeitler et al.
- Using the Himalayan Mountains, this paper discusses the interaction of erosion with structural and petrological processes in the evolution of mountain belts. The Himalayas contain two unique corner zones, or syntaxes, where active metamorphic massifs are co-located with deep river gorges and some of the most extreme relief on Earth. Geologic, petrologic, and geophysical zonations around the massifs, together with the magnitude of erosional fluxes that occur in these regions, leads them to conclude that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.
*To view the complete table of contents of Geology, as well as that
of the Geological Society of America Bulletin, see the Coming
*Full text Geology articles and the science article from GSA Today
are available on the first of each month at our Online