Stratal order in Pennsylvanian cyclothems
Bruce H. Wilkinson, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1063, USA, et al. Pages 1068–1087.
Keywords: cyclothems, eustasy, carboniferous, parasequences, stratigraphy.
Interpretation of sedimentary sequences is commonly based on perceptions order or predictability in the recurrence of one or more rock types. This is particularly true for ca. 250 million year old successions deposited at the same time as glaciation of South America, Africa, Australia, and Antarctica. In spite of widespread acceptance of this paradigm, analysis of the classic successions in west-central Illinois (where sedimentary cyclicity was first proposed) yields no compelling evidence of high-frequency order. If these sections are at all typical of other successions, then "cyclothemic" organization, manifest as recurrent and potentially periodic accumulation, may be much less evident than is generally perceived to be the case during this important interval of Earth history.
Investigation of early Eocene water-vapor transport and paleoelevation using oxygen isotope data from geographically widespread mammal remains
Henry C. Fricke, Department of Geology, Colorado College, Colorado Springs, Colorado 80903, USA. Pages 1088–1096.
Keywords: oxygen isotopes, paleoelevation, paleohydrology, Eocene, climate.
Oxygen isotope ratios of fossil tooth enamel are used to estimate oxygen isotope ratios of water ingested by animals that lived during the Eocene. In turn, these estimates are used to study vapor transport over North America and to investigate the paleotopography of Laramide basins. Results are consistent with warmer air masses being able to transport more moisture, and hence more latent heat, to high latitudes during the Eocene. Both processes can help explain warmer time periods in polar regions at this time. Comparisons of oxygen isotope ratios between Laramide basins in Wyoming are used to estimate topographic relief between them. These estimates of ~500 meters are lower than those obtained using other stable isotope approaches.
Constraints on the petrologic structure of the subduction zone slab-mantle interface from Franciscan Complex exotic ultramafic blocks
Robert L. King, Department of Geological Sciences, University of South Carolina, Columbia, South Carolina 29208, USA, et al. Pages 1097–1109.
Keywords: fluids, Franciscan Complex, mantle, metasomatism, subduction zones, ultramafic rocks.
As oceanic crust is recycled by subduction into Earth, water and other volatiles are transferred to the mantle from the recycled crust. This water is the root cause for the explosiveness of volcanoes such as Mount St. Helens. This water carries a large number of dissolved chemical components along with it from the recycled crust. This water has a dramatically different chemistry than the mantle, so the water and mantle react to form a series of metamorphic rocks along their interface. As these processes occur very deep within Earth, most evidence for these processes is provided by experiments and models because few natural specimens are available. In our contribution, we present data for a block of mantle material brought to the surface by tectonic forces. We argue that this mantle block preserves the type of metamorphic rocks that form along the deep mantle interface. Our data support the experiments and models, but provide more data from the natural sample. Our findings suggest a new interpretation for how water from recycled crust and the mantle interact. If proven correct, our conclusions could require a readjustment of the current conception of how the chemistry of explosive volcanoes above subduction zones is produced.
Debris flows as agents of morphological heterogeneity at low-order confluences, Olympic Mountains, Washington
Lee Benda, Earth Systems Institute, 310 North Mount Shasta Boulevard, Suite 6, Mount Shasta, California 96067, USA, et al. Pages 1110–1121
Keywords: slope stability, fluvial geomorphology, debris flows, natural hazards.
Landslides and debris flows are often considered natural disasters. However, landslides and debris flows are a naturally occurring form of mass erosion in mountain landscapes and they can dominate the supply of sediment and organic materials to river systems. When viewed over short time periods, landslides and debris flows often destroy existing aquatic habitats. Over longer time periods (decades to centuries), however, landslides and debris flows may create river habitats, including floodplains, channels, gravel deposits, pools, and log jams. In particular, mass forms of erosion may contribute to physical heterogeneity in streams.
Post-Paleozoic Alluvial Gravel Transport as Evidence of Continental Tilting in the U.S. Cordillera
Paul L. Heller, et al., Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA. Pages 1122-1132.
Keywords: paleohydraulics, tectonics, dynamic topography, U.S. Cordillera, conglomerate, paleotopography, syntectonic sedimentation.
The western United States contains three thin, but remarkably widespread, alluvial conglomeratic units that record episodes of large-scale tilting of the western edge of the continent over the past 210 million years. These units are the Shinarump Conglomerate in northern Arizona and adjacent parts of Utah, Nevada, and New Mexico dated ca. 210 million years ago; gravel deposits ~120 millions years old found throughout the Rocky Mountain region; and gravel-rich parts of the Ogallala Group, ~20 million years old and younger, in western Nebraska and adjacent southeastern Wyoming. Paleoslopes of the rivers depositing these units were in the range of 1 meter per kilometer to 1 meter per 10 kilometers; however depositional thickness trends of these units are not sufficient to have generated such steep paleoslopes. Thus, long wavelength tilting of Earth's surface must have occurred to transport these gravels. Although these units were deposited adjacent to large tectonic features, the tilting occurred over wavelengths too broad to be directly generated by these features. These widespread gravel units attest to the interplay between the creation of subcontinental-scale broad topography and regional sedimentation. Hence, paleotopography as determined from calculated transport gradients of sedimentary deposits provides a means of relating constructional landforms to mantle-driven processes.
Vertebrate extinction across Permian–Triassic boundary in Karoo Basin, South Africa
Gregory J. Retallack, Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA, et al. Pages 1133–1152.
Keywords: paleosol, vertebrate, Permian–Triassic, Karoo Basin, South Africa.
Death by mountain sickness is the surprising new conclusion of a recent study of ancient soils and sediments spanning the mass extinction of the Permian-Triassic boundary some 250 million years old. Fossil soils indicate a post-apocalyptic humid greenhouse, in support of carbon isotopic evidence for massive release of methane hydrates and oxidation of that methane to create high levels of atmospheric carbon dioxide. Animals at sea level thus found themselves with a shortage of oxygen comparable to that at elevations of ~5000 m today. Reptilian survivors of the life crisis had short snouts, barrel chests, reduced lumbar vertebrae, and thickened thoracic vertebrae and neural spines like those of animals adapted to the rarified air of high altitudes. Among molluscs also, this extinction selected for muscular modern forms with active mechanisms of ventilation. Archaic wetland plants in soils already challenged for air went extinct, but some conifers and other plants of well-drained soils survived. This greatest of all life crises was the result of massive hydrocarbon pollution of the atmosphere.
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