Showing papers in "Annual Review of Earth and Planetary Sciences in 1985"

Patterns of alluvial rivers

Abstract: The pattern (planform) of a river can be considered at vastly different scales, depending upon both the size of the river and the part of the fluvial system that is under consideration (Figure 1). For example, in the broadest sense, river patterns comprise a drainage network (dendritic, parallel, trellis, etc; Figure lA). The type of pattern is of interest to geomorphologists and geologists who interpret geologic conditions from aerial photographs. At another scale a river reach (which in Figure lB is meandering) is of interest to the geomorphologist who is interested in what that pattern reveals about river history and behavior, and to the engineer who is charged with maintaining navigation and preventing major instability. When a single meander is examined (Figure 1 C), the hydraulics offtow, the sediment transport, and the potential for bank erosion are of concern. In addition, the sedimentologist is interested in the distribution of sediment within the bend, bed forms within the channel (Figure lD), and sedimentary structures (Figure IE), which also establish a component of roughness for the hydraulic engineer. Finally, the individual grains (Figure IF) provide geologic information on the sediment sources, the nature of sediment loads, and the feasibility of dredging for gravel. There is an interaction of hydrology, hydraulics, geology, and geomorphology at all scales, which emphasizes the point that the fluvial system as a whole cannot be ignored, even though only a component of the system is to be studied. In this review only the patterns or planforms of alluvial rivers are discussed, although it is apparent that the hydrologic and sediment yield characteristics of the drainage basin (Figure lA), as well as its geologic history, cannot be ignored in the explanation of the pattern of any river

The magma ocean concept and lunar evolution

Abstract: The model of lunar evolution in which the anorthositic plagioclase-rich oldest crust of the moon is formed over a period of 300 Myr or less by crystallization as it floats on a global ocean of magma tens or hundreds of km thick is examined in a review of petrological and theoretical studies. Consideration is given to the classification of lunar rocks, the evidence for primordial deep global differentiation, constraints on the depth of the molten zone, the effects of pressure on mineral stability relationships, mainly-liquid vs mainly-magmifer ocean models, and the evidence for multiple ancient differentiation episodes. A synthesis of the model of primordial differentiation and its aftereffects is presented, and the generalization of the model to the earth and to Mars, Mercury, Venus, and the asteroids is discussed.

Cosmic Dust: Collection and Research

Abstract: The term "cosmic dust" as used here refers to particulate material that exists or has existed in the interplanetary medium as bodies smaller than 1 mm. The particles can be collected both in space and in the terrestrial environment, and they are a valuable resource of meteoritic material. The dust samples are complementary to the traditional meteorites that were much larger meteoroids in space. Because of their size, the analysis of collected dust particles is more difficult and limited than studies of meteorites. Nevertheless, collected dust samples are being vigorously investigated for two fundamentally important reasons. The first is that the most friable extraterrestrial materials can only be collected in the form of dust. Highly fragile materials cannot survive hyper velocity entry into the atmosphere in chunks as large as conventional meteorites. Most cometary meteoroids are known to be fragile, and dust collection is the only Earth­ based technique for obtaining typical cometary solids. The second reason why dust is important is that it is very abundant. Meteorites are exceedingly rare, but cosmic dust is so common that quite literally every footstep a person takes contacts a fragment of cosmic dust. Dust is so abundant that, with appropriate techniques, it can be recovered from historical deposits in deep-sea sediments and collected in real time in space and in the stratosphere. Stratigraphic layers in sediments record a continuous history of the terrestrial accretion of space debris that extends beyond 108 yr ago. This record can be searched for temporal changes in the meteoroid complex, such as might occur during fluctuations in the number of comets in the inner solar system or during passage of the solar system through an interstellar cloud. Effects of isolated events, such as the terrestrial impact of a large body, are also contained in sediments. The flux

The Midcontinent Rift System

Abstract: One of the most prominent features on gravity and aeromagnetic maps of the United States is a series of major, generally linear anomalies that extend from central Kansas to Lake Superior and then turn southward into central Michigan (Figures 1, 2). This system of anomalies clearly reflects major geological features, and as such it must represent a very significant episode in the history of the North American continental lithosphere. The geologic features associated with these anomalies, particularly axial basins filled with basalt and immature clastic rocks along with evidence of crustal extension, indicate that the episode was probably one of incipient rifting.

Solid-State Nuclear Magnetic Resonance Spectroscopy of Minerals

Abstract: Nuclear magnetic resonance (NMR) spectroscopy has been a useful tool for examining the structure of molecules in solution since the 1950s. It has only been since the late 1970s and early 1980s, though, that developments in very high field superconducting magnets and magic-angle sample-spinning (MASS) have made NMR routinely useful for examining the structure of solids. In this paper, we briefly review the theory of MASS NMR spectroscopy and some of the recent experimental results for minerals. N MR spectroscopy examines the local structural environment of atoms, to at most the third and fourth nearest neighbors. For crystalline silicates, silicon-29 and aluminum-27 MASS NMR have already proven useful in examining the validity of the aluminum avoidance principle and the extent of Al(4)/Si order/disorder, in determining the extent of polymerization, in determining the number of crystallographically distinct silicon sites present, in estimating bond strength sums and Si-O-(Si, AI) bond angles, and in detecting the presence of Al(4) and Al(6) and estimating Al(4)/Si and Al(6)/Al(4) ratios. As the NMR behavior of more nuclides (certainly including boron-H, nitrogen-IS, oxygen-17, fluorine-19, sodium-23, magnesium-25, and phosphorus-31) becomes better understood, we feel that this method will become a powerful tool for examining the local structural environment of a wide variety of species in a broad range of

Oxidation Status of the Mantle: Past and Present

Abstract: It is important for a number of reasons that we obtain a thorough understanding of the present oxidation status of the mantle. Suband supersolidus phase relationships, the nature of degassed volatile species, electrical conductivity, diffusivity, and mechanical behavior arc some of the properties that are a function of this parameter. Of further interest is the fact that the present oxidation state must in some way reflect the complex processes that have occurred since the formation of the proto-mantle some 4.5 Gyr ago. These processes include the redox characteristics of accreted material, heterogeneous equilibria prevailing during the growth of the Earth (such as those involved in core formation), convective cycling in the mantle, subduction of oxidized surface layers of the Earth and mixing in the mantle, incorporation of impacted planetesimals, infiltration by melts and fluids, and selective volatile losses and gains. It would be fair to characterize the current range of opinions on the oxidation status of the upper mantle as diverse, partly as a result of discrepancies between theoretical and experimental approaches to the problem, and partly because of differences in philosophical viewpoints concerning the major redox controls. Granted this divergence of opinion for the upper mantle, it must also be recognized that knowledge of the redox state(s) of the greater part of the mantle to the boundary with the core is rudimcntary. There does appear, however, to be some intriguing evidence not only that the present upper mantle is heterogeneous with respect to oxidation state, but also that a secular trend toward oxidation of the subcontinental mantle portion of the lithosphere has taken place.

Evolution of the Archean Continental Crust

Abstract: Some 20 years of plate tectonic theory, combined with new insights into the fine structure of the lithosphere, the application of multielement geo­ chemical and isotopic studies, paleomagnetism, and geophysical modeling of mantle processes, have profoundly influenced present thinking on the origin and evolution of the Earth's early continental crust; previously, our knowledge of the continental crust was based almost exclusively on field geological observations. Although there is now general agreement on how the Earth worked for the last 200 m.y. because of observable evidence in the oceans and continents (e.g. Bird 1980, Condie 1982), it has proved difficult to extend this history into more ancient times in view of the lost oceanic record and the ambiguity and complexity of the pre-Mesozoic rock relationships in the continents (Dewey 1982). However, preserved characteristic rock as­ semblages uniquely identifying modern-type Wilson-cycle processes (i.e. opening and closure of oceans underlain by oceanic crust) have now been recognized in continental terranes as old as �900 m.y., and these assemblages provide strong evidence for the conclusion that the present global tectonic regime has governed the evolution of the lithosphere at least since the late Precambrian (Kroner 1977, 1981a,b; Goodwin 1981). Profound disagreement on the older crustal history, however, prevails to the present day, since typical features characterizing Phanerozoic accretionary terranes (such as obducted ophiolites, blueschists, and Franciscan-type melanges) have not been found in more ancient regions. Thus, two types of evolutionary models have been developed. One type postulates uniformitarian development back to the earliest Archean (Burke

The geological record of insects

Abstract: Insects constitute the largest class of the phylum Arthropoda. Like other members of the phylum, they have an exoskeleton and are metamerically segmented. The insect head consists of six fused segments; the thorax three segments, each with a pair of segmented legs; and the abdomen eleven segments, which may be reduced or modified in some specialized species. Most insects have two pairs of wings attached to the second and third thoracic segments, but some have only one pair, and others have secondarily lost both pairs. Wings are important in the classification of insects at both the ordinal and familial levels, and especially so for fossil specimens. The cuticular nature of the wings helps their preservation as fossils, even in situations where the rest of the insect has been decomposed or otherwise destroyed. Insects are basically terrestrial, and they respire by a series of tracheal tubes that open laterally on the thorax and abdomen. Nevertheless, many orders of insects have become secondarily adapted to freshwater habitats, either as immature stages or as adults, or both. In these instances, the tracheae terminate in gills that allow oxygen in solution in the water to diffuse into the insect's respiratory system. Most insects become fossils by falling into a body of water, where they may be entombed in silts or other fine sediments and eventually be preserved, if not destroyed by various scavengers and detritivores. Chitin and protein, primary constituents of the insect exoskeleton, are rapidly destroyed by a variety of microorganisms, and insects must generally undergo immediate burial under anaerobic conditions for fossilization to take place. Hence, most insects are fossilized under only very precise

THE APPALACHIAN­ OUACHITA CONNECTION: Paleozoic Orogenic Belt at the Southern Margin of North America

Abstract: Late Paleozoic orogenic belt structures exposed in the Appalachian Mountains of Alabama and in the Ouachita Mountains of Arkansas plunge from opposite directions beneath postorogenic Mesozoic-Cenozoic strata in the Mississippi Embayment of the Gulf Coastal Plain (Figure 1). The Paleozoic stratigraphic sequence and details of structural style in the Ouachita outcrops contrast strongly with those in the Appalachian outcrops. Furthermore, straight-line projection of structural strike from the outcrops does not lead to a simple connection of structures beneath the Coastal Plain. Concealed by the Coastal Plain cover are answers to questions such as the following: Are the orogenic belts continuous, discontinuous, offset, or intersecting? How are along-strike changes in stratigraphy and structural style expressed? What is the sequence of tectonic evolution? Early attempts to interpret the subsurface relationships between the Appalachian and Ouachita structures were of necessity based on the first few scattered wells that were drilled through Coastal Plain strata into Paleozoic rocks. Subsequent studies have enjoyed the availability of progressively more numerous deep wells and a growing accumulation of geophysical data. Various stages in the evolution of thought were outlined by King (1950, 1961, 1975) and by Thomas (1973, 1976). This review summarizes available subsurface data in a pre-Mesozoic paleogeologic

The geological significance of the geoid

Abstract: data are being used to study terrestrial phenomena at scales from tens of thousands of kilometers down to hundreds or even tens of kilometers. This paper focuses on the interpretation of global gravity in the form of the geoid and concentrates on what has already been learned from it about the structure of the Earth at long-to-intermed iate wavelengths. The longest wavelengths of the gravity field are dominated by density distributions that must be supported by very large-scale convective processes. I attempt here to document the claim that these density anomalies are both deep and old. At slightly shorter length scales, the geoidal anomalies give very useful constraints on possible subduction processes and lead to the conclusion that density contrasts in the subducting slabs must be regionally partially compensated and supported from below. At yet shorter length scales, satellite altimetry results give insight into the thermal and mechanical structure of the oceanic litho­ sphere, seeming to favor plate over thermal boundary layer models of cooling. At the shortest length scales, the location and mode of compen­ sation of individual seamounts can be studied.

Downhole Geophysical Logging

Abstract: Downhole geophysical logging is the method of making geophysical measurements in drill holes and interpreting these measurements for evaluation of geological properties of the subsurface. The downhole measurements can be made with great spatial resolution with a single sonde, containing both source and receivers, which are separated by 10 cm to tens of meters. Measurements can be repeated every few centimeters as the sonde moves upward or downward in the borehole. This technique is called well logging. It is also possible to make downhole measurements with the sensor in the borehole at depth and the source at the surface. When the sensor package is moved to a different depth, the source signal is repeated. When these measurements are made with seismic waves, the process is called vertical seismic profiling (VSP). In VSP the surface source can be any appropriate system that would generate a seismic pulse (dynamite, air gun) or a sinusoidal wave (vibroseis), and the downhole sensor can be a hydrophone or borehole geophone. Vertical seismic profiling is a rapidly expanding technique, and some of its applications are described in a series of recent books (Gal'perin 1974, Hardage 1983, Balch & Lee 1984, Toksoz & Stewart 1984). Another method of downhole measurement is possible when closely spaced drill holes are available, with the source placed in one drill hole and the detector (receiver) in another. This technique, called cross-

Direct Tem Imaging of Complex Structures and Defects in Silicates

Abstract: Seventy years ago, the X-ray diffraction determination of the NaCI crystal structure by W. H. and W. L. Bragg ushered in a new age in the science of mineralogy. Over the following 50 years, the structures of most minerals were determined, and the ideal crystal structures of most of the important rock-forming minerals have now been refined to high levels of precision. It is perhaps only in the last 20 years or so that the importance of the nonideal, nonperiodic aspects of mineral structures has been recognized. These nonperiodic features, or defects, in many cases can control important properties of a mineral (such as color, mechanical properties, and intracrystalline diffusion rates), and they may play important roles in solid­ state reaction processes. In the present paper, I limit the discussion to one­ and two-dimensional defects, which are sometimes called extended defects. While X-ray and neutron diffraction are ideal tools for the determination of periodic structures with relatively small unit cells, their application in studies of defects is limited to the determination of average defect densities in highly disordered materials. One powerful method that can be used to investigate the structures of extended defects in minerals is transmission electron microscopy (TEM). Although the transmission electron micro­ scope was developed during the 1930s, the intensive application ofTEM to the study of defects in minerals did not occur until the early 1970s, when a number of important investigations, many of them on lunar minerals, were performed.