||September 21, 2001
GSA Release No. 01-40
BOULDER, Colo. - The Geological Society of America's October issue of GEOLOGY
contains a number of newsworthy items. Highlights from GEOLOGY and a summary of
the science article for the October GSA TODAY are provided below. Please discuss
articles of interest with the authors before publishing stories on their work,
and please make reference to GEOLOGY or GSA TODAY in stories published. Contact
Ann Cairns for copies of articles
and for additional information or assistance.
- Benthic foraminifera at the Cretaceous-Tertiary boundary around the Gulf of Mexico.
Laia Alegret et al. Departamento de Ciencias de la Tierra, Universidad de Zaragoza, 50009 Zaragoza, Spain. Pages 891-894.
- Many scientists agree that 65 Ma a large asteroid hit Earth on the Yucatan Peninsula, causing a global mass extinction. However, the ecological effects of this impact are not well known. The authors studied the shells of unicellular, bottom-dwelling organisms, sampled from seven sections located close to the site of impact in northeastern Mexico. Using the faunal compositions of these organisms, the authors estimated that the depths of deposition in the sections ranged from ~500 m in the north to ~2000 m in the south. The impact resulted in the deposition of material that slumped from shallow water into the deep basin. The bottom-dwelling organisms in the deep oceans did not suffer a major extinction, as did similar organisms at shallower depths. Temporary faunal changes after the impact event may have resulted from the mass extinction of surface-water primary producers (unicellular algae), which provided food to the deep-sea faunas. The movement of masses of sediment over the ocean floor may have had additional effects on the fauna.
- Pleistocene glass in the Australian desert: The case for an impact origin.
Peter W. Haines et al. School of Earth Sciences, University of Tasmania, GPO Box 252-97, Hobart, Tasmania 7001, Australia. Pages 899-902.
- Enigmatic, glassy melt rock (dubbed "Edeowie glass") has been found over a 30-km-wide area in the southern Australian desert and has been argon dated at ca. 600-800 Ka. The material is quite distinct from volcanic rocks, but is chemically very similar to the local soil from which it appears to have been derived. While lightning can fuse soil, none of the many samples examined have the physical characteristics of lightning-produced fulgurites, and it would be very hard to explain why such an extreme abundance of fulgurites is spatially confined. The authors propose that fusion was in some way related to the impact and explosion of an extraterrestrial body (asteroid or comet). Could radiant heat from an atmospheric explosion or the passage of a large object through the atmosphere raise surface temperatures to fusion point? This seems necessary because some of the glassy material comprises flat sheets that appear to have formed in place at the surface, rather than being flung from a distant source. One line of evidence in favor of the impact hypothesis is the rare presence of shocked rock fragments found embedded in a few samples of the melt. These fragments resemble shocked materials from known meteorite craters; however, an enduring problem is the lack of any obvious crater in the area.
- Continental island from the Upper Silurian (Ludfordian Stage) of Inner Mongolia: Implications for eustasy and paleogeography.
Markes E. Johnson et al. Department of Geosciences, Williams College, Williamstown, Massachusetts 01267, USA. Pages 955-958.
- One of the perennial debates in the geosciences concerns the long-term history of sea-level change through time. Is there a consistent record of global change or do regional mountain-building events hopelessly distort or cancel out any overall trends in worldwide patterns? Two qualifications must be met before this debate can be approached with respect to any particular time and place. The local rocks must be accurately dated, and there must exist some unambiguous method of gauging the rise or fall in water depth. This paper focuses on the Upper Silurian period, ca. 421 Ma. Sea level is marked directly by a rocky shoreline around a small Silurian island preserved in north-central Inner Mongolia, China. The island, which is named Bater island after a nearby landmark, works like a "dipstick" against which the local rise in sea level may be measured by shore sediments that slowly buried the island. In this case, a minimum rise in sea level of 30 m was necessary to completely drown the ancient island. This calibration fits well with a global sea-level curve previously proposed for the Silurian (Johnson, 1996). The contrasting coastal deposits present on Bater island are evidence of a windward (rough-water) shore and a leeward (calm-water) shore. This differentiation and the present orientation of the Silurian island suggest that the continental shelf on which the island is fixed has rotated ~90° out of phase with the prevailing northeasterly trade winds in the Northern Hemisphere. Thus, the geology of Bater island also tells us much about the original paleogeography of the parent Sino-Korean plate (or North China block).
- When did hominids first leave Africa?: New high-resolution magnetostratigraphy from the Erk-el-Ahmar Formation, Israel.
Hagai Ron, Geophysical Institute of Israel, Lod 71100, Israel, and Shaul Levi, College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA. Pages 887-890.
- New paleomagnetic data suggest an age of 1.7-2.0 m.y. for the Erk-el-Ahmar Formation, Israel, one of the oldest hominid sites outside Africa, where Oldowan tools and land artifacts were excavated. These results support the hypothesis that Early Paleolithic hominids migrating from Africa to Eurasia traversed the Levantine Corridor.
- Evidence for reduced quartz-cementation rates in oil-filled sandstones.
Ann M.E. Marchand et al. Department of Geology and Geophysics, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK. Pages 915-918.
- Oil and gas are held deep below ground in small pores between grains of sand. When quartz-rich sands are buried and heated, the pores between the sand grains become smaller as the growth of microscopic crystals, such as quartz, gradually cements the loose sand into a hard rock. To get oil out of the ground efficiently and cheaply, it is necessary to predict the amount of pore space and its spatial distribution within a sandstone oil reservoir. The authors' study examines an oil field in the British North Sea, where the oil-filled part of the sandstone has twice as much pore volume as the underlying water-filled part. The authors found that there is hardly any quartz cement in the oil-filled sandstone; by contrast the water-filled sandstone has abundant quartz cement, which has filled up most of the pores. They measured the temperatures at which the quartz formed and tested ancient water in which the quartz cement crystals grew, and compared the results to computer modeling of the burial of the sandstone. The authors found that oil had migrated into the sandstone at cool temperatures, before the quartz cement had started to grow, and had then gradually continued to fill the oil field over 40 m.y., while quartz cement continued to grow in the water-filled pores but not in the oil-filled pores. As a result, some oil fields contain twice as much oil as would be expected, and it would be most efficient to drill additional boreholes into the oil, not into the water, as has previously been supposed.
- Modeling heterogeneous stretching during episodic or steady rifting of the continental lithosphere.
Genene Mulugeta and Woldai Ghebreab, Hans Ramberg Tectonic Laboratory, Department of Earth Sciences, Uppsala University, Villavägen 16, SE-752 36, Uppsala, Sweden. Pages 895-898.
- It is widely recognized that the process by which a continent breaks and opens up to form an ocean is preceeded by stretching and rifting of the lithosphere. One of the critical problems surrounding rifting and the formation of continental margins is understanding how the extension is accommodated in the lithospheric column. In this paper, the authors use analogue models to address the modes of normal faulting and isostatic adjustment that take place during stretching and rifting of the continental lithosphere. In particular they explore the response of the continental lithosphere to steady or episodic extension. The strength profile of a rheologically stratified lithosphere is simulated using a homogeneous frictional material (sand) to represent the brittle upper crust and various nonlinear bouncing putties to represent the lower crust and mantle layers. When the continental lithosphere moves steadily, the rifting and subsidence in the softer central rift zone is accommodated by formation of marginal grabens and a panel of domino-style normal faults. In contrast, when the continental lithosphere moves episodically, graben-and-horst-style normal faults develop, with less subsidence in the central rift zone. This is due to a counterflow of ductile material from the margins to the central rift zone. The model results are applied to the western margin of the southern Red Sea to provide clues concerning its extensional history.
- Immense vent complex marks flood-basalt eruption in a wet, failed rift: Coombs Hills, Antarctica.
J.D.L. White and M.K. McClintock, Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand. Pages 935-938.
- Fluid basalt lavas are commonly effused as lava flows or in fire fountains, with recent eruptions of Kilauea providing well-studied examples. Where basalt mixes with water near the surface, however, far more explosive phreatomagmatic eruptions result. These eruptions quarry deep into the ground, ejecting shattered country rock together with fine basaltic ash. Flood basalt eruptions are immense outpourings of lava that are not active today. In Antarctica, Southern Africa, and elsewhere, studies of flood basalt provinces have revealed phreatomagmatic rocks underlying the flood basalts. The rocks at Coombs Hills, Antarctica, represent the first recognized source volcano for these widespread phreatomagmatic deposits. The authors infer that the volcano is of a previously unknown type, one specific to wet flood-basalt eruption sites.
- Geology and venting characteristics of the Mothra hydrothermal field, Endeavour segment, Juan de Fuca Ridge.
Deborah S. Kelley et al. School of Oceanography, P.O. Box 357940, University of Washington, Seattle, Washington 98195, USA. Pages 959-962.
- The Endeavour segment of the Juan de Fuca Ridge hosts the most vigorously venting hydrothermal system known in the world's oceans. Five active hydrothermal fields are located along the central 10-km section of the Endeavour segment. This paper describes the largest venting site on the segment, the Mothra hydrothermal field, which extends 500 m in length and hosts at least five actively venting sulfide complexes. The complexes are composed of multiple, steep-sided sulfide pinnacles, which rise up to 20 m above the seafloor. Most of the sulfide structures are awash in diffusely venting fluids (30-200 °C) that support rich and diverse macrofaunal and microbial communities. Isolated black smoker chimneys vent 302 °C fluids.
- Footprint of the Expanded West Antarctic Ice Sheet: Ice Stream History and Behavior
John B. Anderson, et al. Department of Earth Science, Rice University.
- The future behavior of the West Antarctic Ice Sheet is of crucial importance for humankind. Understanding its past behavior and identifying controls of its behavior are essential for reconstructing present and future processes. This group's work delineating and interpreting the submerged features carved and deposited by the ice sheet at its maximum extent clearly shows the importance of the ice streams and the extent to which their behavior is controlled by the substrate over which they move. The paper provides a clear and beautifully illustrated story of what is known about ice stream behavior and how it has been pieced together; it is sure to be widely read and very likely to change what is taught in many introductory and upper level earth science courses; perhaps even this semester!
*To view abstracts for the current issues of GEOLOGY and GSA TODAY, go to www.gsajournals.org.
To obtain a complimentary copy of any GEOLOGY article, contact Ann