Showing papers in "Geological Society of America Bulletin in 1961"

Distribution of the Elements in Some Major Units of the Earth's Crust

Abstract: This paper presents a table of abundances of the elements in the various major units of the Earth's lithic crust with a documentation of the sources and a discussion of the choice of units and data.

Theory of Folding of Stratified Viscoelastic Media and Its Implications in Tectonics and Orogenesis

Abstract: This paper presents an introduction to the theory of folding of stratified viscoelastic media under compression and discusses its significance in the context of tectonics and orogenesis. Simplified derivations are given for results obtained earlier by the writer as particular cases of more elaborate theories. The writer emphasizes the mechanism involved in folding. The paper begins with a discussion of the buckling of an elastic rod that is under axial compression and is restrained laterally by viscous dashpots. The analysis then proceeds to the analogous problem for an elastic and a viscous plate surrounded by a viscous medium. Results of some of the more complex problems previously analyzed by the writer are also applied and discussed. Experimental verification of the theory by model tests is presented in a companion paper (Biot, Ode, and Roever, 1961). A new feature of the present approach is the emphasis on rate phenomena and time histories in tectonic folding. In purely static problems of elastic buckling, a sharply defined wavelength is associated with the instability. By contrast, for viscoelastic media, the present theory leads to the concept of dominant wavelength and band width selectivity in analogy with the theory of electric wave filters. This is well illustrated by the gradual appearance of near-regular folds when a purely viscous layer surrounded by a viscous medium of lower viscosity is subjected to a compression in a direction tangent to the layer. The theory is applied to specific examples of geological interest. These include the case of a single layer or a superposition of layers. Previous theoretical work by the writer is applied to the discussion of folding, under the simultaneous action of gravity forces and a horizontal compression, for a single layer or a superposition of layers lying at the surface of a deep substratum of lower viscosity. The case of a continuously inhomogeneous medium under similar forces is also included. Using accepted values of rock viscosity and elastic moduli, the writer finds that the time required for significant folding to take place agrees very well with the geological time scale. Folding may occur under tectonic stresses that are small in comparison with the crushing strength of the, rock. The time history of folding depends, of; course, on initial irregularities of the layers, but9 after sufficient time the folding becomes fairly! insensitive to the magnitude and the distribution of the initial disturbances. The writer concludes ; that the viscous mechanism tends to predominate in tectonic folding. As a theoretical consequence, the wavelength of the folds will, in general, not be sensitive to the magnitude of the tectonic stresses unless gravity forces become important. The calculated wavelengths are in good agreement with the range of observed values. The point at which plastic or brittle failure occurs is found to depend primarily not on the magnitude of the strain but on deformation rates. The theory can be applied to materials with nonlinear stress-strain characteristics, and the procedure to accomplish this is briefly discussed. Certain nonlinear features resulting from the geometry of deformation are shown o t have a bearing on the regularity of the folds.

Atlantic Deep-Sea Sediment Cores

Abstract: Studies of lithology, particle-size distributions, and micropaleontology and chemical analyses of 221 Atlantic and Caribbean deep-sea cores lead to new conceptions of processes of sedimentation, rates of sediment accumulation, Pleistocene chronology, and pre-Pleistocene history of the Atlantic Basin. Anomalous layers of sand, silt, and lutite occur widely in the deep basins of the Atlantic. Evidence for deposition of these layers by turbidity currents is as follows: (1) the layers occur in submarine canyons, in deltalike features at the terminal ends of canyons, in basins and depressions, never on isolated rises; (2) they are interbedded with late Pleistocene sediments of abyssal facies; (3) they are well-sorted and commonly graded; and (4) they commonly contain organic remains of shallow-water origin. Late Pleistocene slumping of compacted Neogene sediments along the banks of the Hudson Submarine Canyon at depths exceeding 3000 m indicates deepening of the canyon by erosion by turbidity currents. Variations in the planktonic Foraminifera in 108 of the cores and extrapolation of rates of sediment accumulation determined by 37 radiocarbon dates in 10 cores show that the last period of climate comparable with the present ended about 60,000 years ago. A faunal change indicating climatic amelioration, probably corresponding to the beginning of postglacial time, occurred about 11,000 years ago. Cross-correlations by micropaleontological methods establish the continuity of the climatic record deduced from the planktonic Foraminifera. Study of variation in the Planktonic Foraminifera leads to a different Pleistocene chronology from that proposed by Emiliani (1955). Cross-correlations of faunal zones and radio-carbon dates show that rates of continuous sediment accumulation, as opposed to turbidity-current deposition, range from 0.5 cm to 274.4 cm in 1000 years, depending upon bottom configuration. Cross-correlations by means of changes in coiling direction of planktonic Foraminifera give relative rates of sediment accumulation beyond the range of the radiocarbon method of dating. Forty one of the cores contain pre-Pleistocene sediments. The oldest sediment is Upper Cretaceous. Foraminifera and discoasters indicate the ages. Absence of sediment older than Late Cretaceous and thickness, 800–1000 m, of sediment in the Atlantic Basin as determined by seismic methods suggest that a large-scale reorganization of the Atlantic Basin took place in the Mesozoic.

Proposed classification of volcaniclastic sediments and rocks

Abstract: Volcaniclastic sediments and rocks are divided here into autoclastic, pyroclastic, and epiclastic types with grain-size limits the same as non-volcanic epiclastic rocks. Autoclastic rocks contain fragments that are produced within (but not usually extruded from) volcanic vents, during movement of lava flows, or by gas explosions within flows that have ceased to flow. Pyroclastic rocks contain fragments produced by volcanic explosion and extruded as discrete particles from volcanic vents. Epiclastic volcanic rocks contain fragments produced by weathering and erosion of solidified or lithified volcanic rocks of any type. Volcaniclastic types may be mixed in all proportions with each other or with nonvolcanic fragments, although these mixtures are not designated within this classification. A non-genetic category, based only upon particle size and the presence of volcanic material, is included for rocks with clasts of unknown origin.

Experimental study of shear failure in anisotropic rocks

Abstract: Preliminary work has shown that planar anisotropy (foliation) may have a marked effect on both the breaking strength and the angle of shear fracture in rocks. For rocks experimentally deformed at room temperature and under low confining pressure, curves of breaking strength versus inclination of anisotropy are concave upward and parabolic in form. Shear fractures tend to develop parallel to well-developed planar anisotropy for inclinations up to 45°–60° to the direction of maximum pressure.

Magnetic survey off the west coast of north america, 40° n. latitude to 52° n. latitude

Abstract: An extensive high-resolution magnetic survey of total field at sea level reveals unusual north-south lineations and much crustal faulting. Computations indicate that the linear pattern is due to strongly magnetized mafic rocks beneath the sediments.

Sediments of shallow portions of east china sea and south china sea

Abstract: Nearly 1000 bottom samples from the shallow portions of the East China and South China seas were studied and compared with source areas and oceanographic conditions. Sediments of the Gulf of Pohai and central Yellow Sea are fine-grained, low in calcium carbonate, and contain many unstable minerals and a moderately high percentage of organic matter. Most are contributed by the Hwangho (which carries much eroded loess), Yangtze, and many smaller rivers. The thickness of these sediments is so great as to constitute a modern zeugogeosyncline. Sediments on the inner half of the continental shelf between Shanghai and Hainan and in the Gulf of Tonkin are similar to those of the central Yellow Sea; they comprise modern detrital materials contributed to the continental shelf by many rivers, but in amounts as yet insufficient to cover the shelf everywhere. Seaward of these sediments, on the outer half of the continental shelf between Korea and Hainan, is a broad belt of coarse sandy sediment from which finer sediments are winnowed away or prevented from being deposited by the strong Kuroshio Current. The sediment contains glauconite and much calcium carbonate in the form of foraminiferal tests and broken mollusk shells but very little organic matter. Because the inorganic portion is much coarser than that nearer shore, it is believed to constitute a littoral deposit left from a Pleistocene time of glacially lowered sea level. Locally on the shelf small areas of residual sediment near rock outcrops commonly contain reworked fossils. Pieces of pumice and many small shards of volcanic glass are present in the sediments but nowhere are they abundant enough to form a dominantly volcanic sediment. The shelf sediments are similar in most respects to those on the continental shelf of California, and they clearly indicate deposition below present base levels of equilibrium. Seaward of the continental shelf, in deep water of the continental slope, the sediments are finer-grained and contain more calcium carbonate. These sediments consist of the finer-grained terrigenous material that bypasses the shelf and is deposited so slowly in the quiet deep water that Foraminifera make up a large percentage of the total sediment.

Factors Influencing Supply of Major Ions to Inland Waters, with Special Reference to the Atmosphere

Abstract: Sources of ion supply to natural inland waters include not only rocks and soils but also the atmosphere, whose significance has been underestimated. Atmospheric materials are transferred to surface waters by rain or snow, as dry fallout, or in gaseous form; the sources are the sea, land surfaces, volcanoes, products of air pollution, or organic debris. Ion supply by soil and rock weathering, which is usually more important than atmospheric supply, involves solution, oxidation reduction reactions, activity of hydrogen ions, and complex formation. Transfer from soils to waters. 246 references, 23 tables.

Magnetic survey off the west coast of north america, 32° n. latitude to 42° n. latitude

Abstract: A ship-towed magnetometer survey of an area 250–300 miles wide off the foot of the continental slope along the coast of California has revealed a narrow pattern of anomalies of about 400 gammas magnitude trending north-south for more than 500 miles. The pattern is interrupted at the known faults and elsewhere; offset of the pattern at the Murray fault suggests a right-lateral displacement of about 84 nautical miles. The anomalies are such as might be expected of slablike structures underlying the ocean floor; geological possibilities include basic lava flows, topography of the main crustal layer, and intrusion of ultrabasic material from the mantle.

Welded Tuffs and Flows in the Rhyolite Plateau of Yellowstone Park, Wyoming

Abstract: The rhyohte plateau in Yellowstone Park is made up of flows and welded tuff with subsidiary rhyolite domes, basalt, and rhyolite-basalt mix-lava. The two most important units are the Yellowstone tuff, exposed over 600 square miles within the Park, and a group of younger flows, the Plateau flows, which cover 1000 square miles in the Madison, Central, and Pitchstone plateaus. The Plateau flows occupy a tectonic basin rimmed by Yellowstone tuff and older rocks in central and southwestern Yellowstone Park. Individual flows range up to 1000 feet in thickness and cover areas up to at least 100 square miles. Surficial features, including vent domes, are locally well preserved. Exposed portions of these flows are, for the most part, banded obsidian and perlite and breccias formed by incorporation of fragments of pumiceous crust in the moving flows. The Yellowstone tuff is welded to obsidian at its base and grades upward through indurated, lithoidal rhyolite to loose ash at the top of an uneroded section. Probably all the Yellowstone tuff was erupted in a single rapid series of eruptions from dispersed vents. Textural and structural evidence suggests that the Yellowstone tuff was emplaced as pyroclastic flows. A thermodynamic analysis of this type of eruption is given. Energy changes within the conduit and during emplacement are evaluated. The minimum temperature at which rhyolite glass will weld, determined experimentally, is approximately 600°C. The data of Tuttle and Bowen (1958) are used to fix the range of initial magma temperature. Welding in the Yellowstone tuff can be explained if the tuff was emplaced as pyroclastic flows and if the magma contained less than about 4 per cent H 2 O at the start of the eruption.

Horizontal displacements in the floor of the northeastern pacific ocean

Abstract: A total magnetic intensity survey in the northeastern Pacific Ocean revealed a north-south pattern of magnetic anomalies, which is cut through by the Murray, the Pioneer, and the Mendocino faults. The amount of slip along these faults is measured by fitting the magnetic anomaly pattern across the faults. The combined left-lateral displacement across the Mendocino and the Pioneer faults is 1420 km.

Symmetry Concepts in the Structural Analysis of Deformed Rocks

Abstract: Bruno Sander has proposed that the fabric symmetry of a deformed rock reflects the kinematic symmetry of its deformation. In order to place this symmetry principle on a firmer basis, the background of the symmetry theory of fabrics is here reviewed. From very general notions of symmetry as a starting point, fabric symmetry is shown to be a statistical space symmetry consisting of a point group of symmetry operations combined with arbitrary translations in all directions. Where likely restrictions are placed upon the point groups to be expected in homogeneously deformed rocks, the usual types of symmetry observed in fabrics of deformed rocks (namely, spherical, axial, orthorhombic, monoclinic, and triclinic) remain as possible types. The general derivation demonstrates that apart from pseudocrystallographic symmetries defined by some crystallographic fabric elements, no other types of fabric symmetry can be expected in homogeneously deformed rocks. Attention is drawn to the relevance to the views of Sander of Curie9s principles governing the symmetries of “cause” and “effect” in physical phenomena. The features of a deformed rock that define its fabric are found to be a three dimensionally ordered array of discontinuities in structure (lattice planes and lines in crystals, grain boundaries, foliations, lineations, folds, and so on) which may be viewed as the “effect” of deformation. These surfaces and lines of discontinuity in structure are generally sites of surfaces and lines of discontinuity in deformation—implicit in Sander9s concept of componental movements—which collectively define the “movement picture” ( Bewegungsbild ) of the deformation. An attempt is made to state more precisely the significance of a movement picture in terms of analysis of local heterogeneities and perturbations in deformation that reflect or are responsible for the development of fabric features in a statistically homogeneously deformed aggregate. The kinds of symmetry observed in tectonite fabrics are found also to be the only kinds possible in movement pictures of homogeneous deformations. Curie9s principles may now be restated in a form directly applicable in the interpretation of tectonite fabrics, thereby amplifying Sander9s principle. Without qualification, the most important principle may be restated as follows: whatever the nature of the factors contributing to a deformation may be, the symmetry that is common to them cannot be higher than the symmetry of the deformed fabric, and symmetry elements absent in this fabric must be absent in at least one of the contributing factors.

Devitrification of Natural Glass

Abstract: Many aspects of the history of natural glasses are related to the physicochemical properties of silicate melts. Glasses are thermodynamically unstable; with time and under a variety of conditions, they become crystalline. When little water is present, devitrification may be described in much the same terms as those which apply to the crystallization of silicate melts. The writer uses simple rate theory and the available experimental data to set an approximate lower limit to the time required for thermal reconstruction of natural glasses as a function of temperature. The time required for a glass at 300° C to become a felsite is estimated to be at least a million years, at 400° C, at least a few thousand years. The secular disappearance of strain birefringence in volcanic glasses at low temperatures indicates an intermediate stage in the process of devitrification. Devitrification along fractures in most volcanic glasses, however, is dependent upon water. In a number of samples, the width of the layer hydrothermally reconstructed is moderate to vanishingly small. The geologic ages of these glasses indicate that the rate of devitrification at low temperature (around 20° C) is no more than about 4–5 μ in 100 million years. Hydrothermal reconstruction is very sensitive to temperature; the depth of devitrification generally varies substantially among macroscopically unaltered samples from a single flow. In the least devitrified specimens, the depth of devitrification seems to be related to the age of the flow. Presumably, the diffusion of water controls the rate of hydrothermal reconstruction. The diffusion constant at 20° C is estimated to be 10^(−23) cm^2/sec, and pressure-bomb experiments indicate an activation energy of 30 kcal/mole. Time-temperature relationships are estimated for hydrothermal reconstruction. In contrast to thermal reconstruction, devitrification in the presence of water requires only a short time (within experimental range) at temperatures no more than about 300° C.

Experimental Verification of the Theory of Folding of Stratified Viscoelastic Media

Abstract: An experimental check has been obtained for the stability theory of stratified viscoelastic media in compression. Model tests have been conducted for both an elastic layer and a viscous layer embedded in a viscous medium and subject to a compression parallel with the layer. The appearance of the folds and the measured wavelengths are in good agreement with the theoretical predictions. For a better interpretation of the tests, the writers present a theoretical evaluation of the time history of deformation for a layer whose folding develops from a given initial departure from perfect flatness. The calculated folding of the layer at various intervals is plotted for different values of the significant parameter.

Radiocarbon Dating of Late Quaternary Deposits, South Louisiana

Abstract: The late Quaternary deposits of southern and offshore Louisiana record a complete cycle of sea-level fluctuation which is associated with major changes in the volume of ice on the continents since the beginning of the last glacial stage. In order to date the major events of this cycle, 122 samples from both the surface and subsurface have been analyzed by the radiocarbon method. A eustatic curve based on the age determinations of these samples supports previous estimates from geological data that the sea during the early part of the cycle fell to a position at least 450 feet below its present level. The lowest sample on this curve, which was deposited when sea level stood at −440 feet, shows that this fall took place more than 35,000 years ago. The rise of the sea during the middle part of the cycle occurred in two successive stages. The first stage is marked by a rise from more than −440 to −250 feet, about 200 feet, before 35,000 years ago, followed by a long stillstand. This period of stillstand terminated about 18,500 years ago with the beginning of the second-stage rise which brought the sea to its present position about 5000 years ago. Sea level during the last part of the cycle has remained unchanged to the present. The eustatic curve implies that the ice sheets of the last major glacial stage not only reached their maximum extension but had begun to retreat before 35,000 years ago. Furthermore, it indicates that the final stage of retreat began about 18,500 years ago and ended about 5000 years ago. This latter estimate corresponds closely to dates obtained from glaciated areas and deep-ocean sediments for the final stage of ice withdrawal. No general agreement exists between the eustatic data and other estimates on the age of the maximum glaciation and the beginning of ice retreat.

Classification and a Scheme for the Identification of Layer Silicates

Abstract: The classification for the layer silicates (including most of the clay minerals) proposed here is based on structural studies of numerous investigators on natural materials. In many instances, however, the limits of chemical composition have been determined by research on synthetic layer silicates. The layer lattice silicates can be subdivided by the successive application of three criteria: (1) height of fundamental repeat unit or “thickness of layer”; (2) gross composition, whether diocta-hedral or trioctahedral, and ionic content of layers; (3) stacking sequence of layers and degree of orderliness of stacking. The relationship between compositional and structural divisions is clearly indicated in compositional diagrams. These show that some structurally similar families may be widely separated in composition (kaolinite and septechlorite), whereas others — e.g., phengites (micas) and beidellites—are not far apart with respect to bulk composition. A scheme of successive operations that will permit the identification of layer lattice silicates singly or in simple mixtures is outlined, particularly for the nonspecialist. The essential apparatus includes only that for powder X-ray diffraction and a small laboratory furnace.

Petrology of three volcanic suites, Umnak and Bogoslof Islands, Aleutian Islands, Alaska

Abstract: This paper tentatively relates the comparative petrology and chemistry of three volcanic suites on Umnak and Bogoslof islands in the eastern part of the Aleutian Island arc to tectonic position with respect to the axis of the arc. Umnak is a large geologically complex island on the Aleutian Ridge; Bogoslof, 22 miles north of Umnak, is the top of a Recent, largely submarine volcano that rises about 5000 feet from the floor of the Bering Sea basin. Southwestern Umnak is underlain by a pre-Quaternary basement of low-grade metamorphic rocks and post-Oligocene mediosilicic plutonic rocks, which are overlain by two Quaternary andesitic stratovolcanoes. Northeastern Umnak is underlain by a late Tertiary(?) and Quaternary basaltic shield volcano with a large central caldera. The lavas exposed at Bogoslof in 1947 consisted largely of historically dated volcanic domes ranging in composition from hornblende andesite to hornblende basalt. Fractional crystallization was probably the dominant process in the formation of quantitatively minor andesite and rhyolite masses associated with the basaltic shield volcano of northeastern Umnak. An extensive blanket of welded andesitic agglomerate and subordinate rhyodacite ash associated with the caldera-forming eruption of the basaltic shield volcano probably owes its composition to a combination of processes, including fractional crystallization, mixing with remelted basalt wall rock, and possibly assimilation of sialic rock. Most volcanic rocks of southwestern Umnak are similar in composition to the underlying plutonic rocks now exposed. The relatively high chromium and nickel in the southwestern Umnak quartz diorite and in its extrusive equivalent, the hypersthene-bearing labradorite andesite, indicate that these rocks probably did not originate from fractional crystallization of less siliceous rock. The presence of hypersthene in the andesites of southwestern Umnak is probably related to their high aluminum content ultimately derived by assimilation of aluminous sedimentary rocks. A similar origin is postulated for the hypersthene in the High Cascade lavas. The slightly alkalic lavas of Bogoslof, typified by hornblende, became progressively less siliceous from 1796 to 1927, but the magmatic diversification in the underlying chamber may have been well advanced prior to 1796. The petrochemical differences in the three volcanic suites of Umnak and Bogoslof are probably related to tectonic position with respect to the Aleutian Ridge. The hypersthenic aluminum-rich lavas of southwestern Umnak were probably generated at relatively shallow depth from pre-existing plutonic rocks of similar composition. The position of the andesitic volcanoes on a high-standing, possibly upwarped erosion surface is in accord with Benioff9s proposal that the maximum heat would be generated in the crust at the zone of maximum bending across a volcanic arc. The dominantly tensional features associated with the basaltic shield volcano on northeastern Umnak are in accord with its position on the north flank or inner side of the arc, where a zone of tension may prevail. The slightly alkalic rocks of Bogoslof Island are derived possibly from a tension fracture normal to the arc in the Bering Sea basin.

Late quaternary sea level: a discussion

Abstract: Recent literature and new evidence on the position of sea level during the late Quaternary are reviewed critically. Some evidence suggests a glacial interstadial and stand of sea level at about − 8 fathoms 30,000 years ago. Sea level then dropped to perhaps −65 fathoms. The rapid rise of sea level and warming of climate covered a period of time from 15,000 to 7,000 B.P. (Before Present) and consisted of a period of climatic and sea-level fluctuations rather than a single abrupt warming as proposed for 11,000 B.P.

Dispersal Centers of Paleozoic and Later Clastics of the Upper Mississippi Valley and Adjacent Areas

Abstract: Three major dispersal centers of the Paleozoic, Cretaceous, and Tertiary elastics of the upper Mississippi Valley and adjacent areas were located through integrated study of petrography, directional structures, facies maps, and regional stratigraphic relations. In pre-Mississippian time, nearly all the elastics originally were derived from the Precambrian rocks centered around the Lake Superior region of the Canadian Shield. Relatively modest streams supplied sands and muds and recycling on a stable craton produced mature and supermature sandstones. The second and third major dispersal centers resulted from Appalachian orogenic activity. Although some contributions from the Lake Superior region and Canadian Shield continued, the tectonic borderlands of the northern Appalachian mountains were the chief dispersal center of Mississippian and Pennsylvanian elastics. In post-Pennsylvanian time, this center shifted to the east of the southern Appalachian mountains. The post-Devonian clastic sediments were transported to the shallow marine shelves, coastal plains, and small deltas of the craton by a series of large, recurring drainage systems. Despite major shifts in dispersal centers, the slope of the craton in the upper Mississippi Valley and adjacent areas has persisted to the south and southwest throughout Paleozoic, Mesozoic, and Tertiary time. Inherent in the foregoing regional results are some problems of general interest. These include sedimentary differentiation, intrastratal solution, and the relationships between paleoslope, regional unconformities, and cross-bedding directions.

Algodones Dunes of Southeastern California

Abstract: The Algodones dune belt, which is 40 miles long in a northwesterly direction, 3–6 miles wide, and has individual dunes 200-300 feet high, lies along the southeastern border of the Cahuilla Basin, a structural depression in southeastern California. Much of the basin is below sea level and includes Imperial and Coachella valleys. The lowest portion of the basin is now occupied by the Salton Sea. Most detritus enters the basin from the high mountains to the west and northwest; little material is contributed by the lower, drier desert ranges to the east. The dunes appear unsystematic from the ground, but as seen from the air they consist of long northwest-trending ridges on the west which individually curve eastward and disappear in a complex of prominent south-facing slip faces. In the central and southern part of the dune area some of the slip faces are 200–300 feet high and overlook large flat-floored, sand-free depressions which are interpreted as exposed parts of the desert floor over which a succession of large, closely spaced complex barchans is advancing. Present conditions favor destruction of the depressions by encroachment of sand in the form of small linear ridges and barchan dunes from the transverse dunes. Color and degree of rounding of the sand grains indicate that the dunes are now less active than in the past and that the amount of sand is not increasing significantly. The dunes were probably produced by wind transport of sand inshore from the beaches of Lake Cahuilla, a much larger forerunner of the Salton Sea. The large volume of material in these ancient beaches indicates vigorous wave and current action along the northeastern shore of the lake.

Middle America Trench: Topography and Structure

Abstract: From 1952 to 1959, during nine expeditions of the Scripps Institution of Oceanography and one of the U. S. Navy Electronics Laboratory, research vessels recorded 31,950 miles of echo-sounding traverses in and adjacent to the Middle America Trench, which extends from the Islas Tres Marias off western Mexico to the Cocos Ridge southwest of Costa Rica. The Middle America Trench is continuous at depths greater than 2400 fathoms (4400 m) for 1260 miles, except off Manzanillo and Zihuatanejo, Mexico, where submarine mountains lie in the trench. It is deeper than 3000 fathoms (5500 m) for 380 miles as the Guatemala Deep. Northwest of Acapulco it is generally U-shaped in cross section, with a steeper shoreward flank and a flat bottom suggesting sedimentary fill. From Acapulco southeast to the west side of the Gulf of Tehuantepec, the trench shoals, in a series of basins, to 2700 fathoms (5000 m). To the southeast it widens and deepens abruptly to a maximum 3500 fathoms (6400 m) off western Guatemala, then shoals gradually to merge into the sea floor off Costa Rica. The southeast segment is also asymmetrical in cross section but is V-shaped with irregular bottom. A northeast-trending band of ridge-and-trough topography, 60 miles wide, separates the 1800- to 1900-fathom sea floor outside the trench off southern Mexico from the 2100- to 2200-fathom Guatemala Basin. This zone has been traced from several hundred miles offshore to an intersection with the trench near the west side of the Gulf of Tehuantepec. Seismic-refraction studies reported in an accompanying paper (Shor and Fisher, 1961) were employed in determining the trench structure. Three refraction stations were taken along the axis of the trench west of Acapulco and two along its axis off Guatemala and El Salvador. Another station was shot on the shelf and one 60 miles seaward of the trench off Guatemala. Thick sediments were found in the Tres Marias Basin off Manzanillo and at the shelf station off Guatemala. Arrivals from rock with compressional wave velocity of 4–6 km/sec were observed at the Tres Marias Basin and Guatemala shelf stations. Off Guatemala, on a section normal to the trench, the depth below sea level to the M discontinuity is interpreted from these seismic data as about 9 km (Pacific Basin), 10 km (outer ridge), 16 km (trench), and 17 km (shelf). Below the sea floor the crust thickens from 5–7 to 10–17 km along this section. The M discontinuity is deeper and the crust below the sediments thicker under the two southern stations than under the two central trench stations. The mantle is deeper under the Tres Marias Basin, where thick (1½ km) sediments are found, than under the central stations. The Gulf of Tehuantepec marks a major change in trench configuration and possibly in age. Northwest of Tehuantepec the flat trench bottom developed in most places suggests a greater age. Southeast of the gulf the deep V-shaped trench, with thicker crustal layers but very little fill, borders a volcanically active coast. The zone of ridge-and-trough topography trending southwest from Tehuantepec may be another evidence of this boundary.

Lava temperatures in the 1959 kilauea eruption and cooling lake

Abstract: The 1959 summit eruption of Kilauea Volcano, Hawaii, filled the crater of Kilauea Iki with a lake of lava 365 feet deep. Temperatures of the erupting basalt ranged between 1060° and 1190°C. Temperatures down a 12.7-foot-deep hole, drilled into the crust of the lake 5 months after cessation of eruptive activity, agree with calculated temperatures based on the heat equation. The cooling effect of rainfall is pronounced only in the upper 3½ feet of the crust.

Squantum "Tillite", Massachusetts—Evidence of Glaciation or Subaqueous Mass Movements?

Abstract: The writer interprets the Boston Bay group as partially intertongued lithofacies of coarse conglomerate and associated volcanic rocks (Roxbury conglomerate) and finely laminated siltstone and mudstone (Cambridge “slate”). He regards the Squantum “tillite” as multiple discontinuous lenses of unsorted pebbly to bouldery mudstone of various ages interstratified within this complex. Although fossil evidence is inconclusive, stratigraphic relationships and clastic compositions suggest that these deposits were formed during middle Paleozoic diastrophism. The writer does not consider the Squantum as a temporal equivalent to southern hemisphere Permo-Carboniferous glaciation, as advocates of a glacial origin had supposed. Lack of glacial pavement and comparison of composition, graded bedding, gross stratigraphy, and tectonic setting with known submarine slide and mass-flow deposits render the glacial interpretation suspect. Origin by gravity movement of rapidly deposited, volcanic-rich sediments and periodic resedimentation by turbidity currents in a Paleozoic eugeosynclinal belt is a more plausible alternative. Very poor sorting, striated clasts, faceting, and even some rafted erratic fragments in fine, laminated mudstones can arise from mechanisms other than glaciation. A preserved, extensive, grooved and polished pavement overlain by poorly sorted, till-like material—particularly if nonmarine—is the most compelling glacial evidence. Very large erratic boulders are suggestive of ice movement, as are abundant rafted erratic fragments in fine muds, as in the Gowganda formation of Ontario. Independent biologic or isotopic cold-temperature indicators are sorely needed to strengthen glacial interpretations. General stratigraphic relationships and tectonic setting serve as important factors in judging probability of alternate interpretations, particularly in geosynclinal sequences. From these criteria, the writer judges that only the Permo-Carboniferous glaciation in certain parts of the southern hemisphere is firmly established. The Gowganda and some other Precambrian deposits are very likely glacial, but most postulated examples must be re-evaluated.

Mathematical Models of Slope Development

Abstract: Various theoretical postulates regarding the origin of degradational slopes have been investigated. These postulates represent specific physical conditions which can be expressed in the form of nonlinear hyperbolic partial differential equations. A number of such differential equations, corresponding to various possible physical conditions, has been integrated numerically, and the results of the computations are presented. Thus one obtains a number of slope profiles upon which the observational scientists may draw. In every case, a comparison between theoretical and observed slope profiles will ascertain the physical conditions that produced the latter.

Limnology, Sedimentation, and Microorganisms of the Salton Sea, California

Abstract: The Salton Sea, originally a fresh-water lake formed by the flood of the Colorado River in 1905, is located in the center of the Colorado Desert of California. By 1929, the water of the lake had become almost as saline as ocean water because of intense evaporation and solution of salts present on the floor of the basin before the flood. In recent years, in spite of the high rate of evaporation, excessive runoff from irrigation water has maintained and even raised the level of the lake. Evaluation of the water budget has made it possible to anticipate future changes in the chemical composition of the water— i.e., an increase in the amount of sulfates is predicted for the future. Measurements with a current cross and studies of salinity distribution indicate a counter-clockwise current pattern in the Salton Sea. Sands, silts, and clays are deposited in that order from the shore line to the center of the lake. The water content, amount of calcium carbonate, and textural characteristics indicate that most of the sediments of the Salton Sea were derived from the suspended load of the Colorado River, whereas the mineralogical study suggests that some of the sediments have a local origin. Two entirely different methods of computing the rate of sedimentation and of calculating the amount of calcium carbonate in the sediments give results of the same order of magnitude. The large amount of calcium carbonate is partly due to precipitation and partly to transportation by Colorado River water. Foraminifera are the most abundant microorganisms in the lake. Several ecological factors influence their distribution; among these factors, chlorinity and temperature are not important, depth may be important by its effect on other factors, but the variation in the pH of the sediments is significant. The living foraminiferal assemblage is due to accidental introduction, but the natural effect of environmental conditions has resulted in the creation of a dwarf fauna and many malformed individuals. In contrast to conditions in the ocean, abundance of species decreases offshore; however, as in the ocean, the number of living Foraminifera increases greatly after a bloom of phytoplankton. The average productivity of the Salton Sea is greater than that of the oceans. Conclusions derived from this study were applied to sediments deposited in the same basin during the past and made it possible to suggest a late Pliocene or early Pleistocene age for the Borrego formation and a Pleistocene age for the Brawley formation.

Structural Geology of the Beartooth Mountains, Montana and Wyoming

Abstract: The Beartooth Mountains—an 80 by 40 mile elevated crystal block of Precambrian crystalline rocks—are in the Middle Rocky Mountains east of the great north-south belt of overthrusts. The Beartooth Mountains are bounded on the east by the Bighorn Basin (average structural relief, 15,000 feet); on the northeast and north by the Nye-Bowler lineament (along which movement has been dominantly left-lateral); and on the north by the Crazy Mountain syncline (average structural relief of at least 10,000 feet). The southwest side (Yellowstone-Absaroka Mountains) is mostly covered by thick Tertiary volcanic rocks. Detailed mapping, supplemented by results of earlier workers, provides an integrated geologic picture of the tectonic development of the block. Much of the tectonic history is recorded in the structures developed in the Paleozoic and Mesozoic sedimentary rocks exposed around the edge of the mountains. The sedimentary rocks played a relatively passive role in the dynamic rise of the block during Laramide time, as they draped, folded, broke, and slid along the edge of the block. A thrust plane(s) dipping west-southwest is the major structural feature between the Clarks Fork of the Yellowstone and Nye, Montana. Near Red Lodge numerous tear faults displace the upper plate of the thrust as much as 10,000 feet. From Nye to Livingston, Montana, the major bounding structure is a north-northeast-dipping thrust plane(s) terminating in several left-lateral tear faults. Near Gardiner a high-angle thrust plane dipping northeast bounds part of the southwest Beartooth. Several structural mechanisms apparently operated during deformation: (1) Uplifting, tilting, and depressing of large crustal blocks. These commonly form ramps from the Beartooth massif to adjacent basins; during uplift smaller blocks were rotated in the opposite sense. (2) Vertical raising of the northwest and southeast corners. Here, the general lack of horizontal displacement relative to the adjacent basins limits the total horizontal displacement of the block. (3) Thrust faulting. Most of the frontal thrusts show moderate dips. Imbrication is developed particularly at the unconfined northeast corner. The major thrust steepens with structural depth to the south. The horizontal thrust displacement is not directly measurable at most places but is limited by the lack of horizontal movement at the two fixed corners of the block. (4) Tear faulting. Several major tear faults (displacements up to 10,000 feet) bound large keystone-shaped blocks near Red Lodge. (5) Lateral shearing. During the late stage of uplift, horizontal movement toward the unconfined Red Lodge corner created major lateral shears, associated with imbrication, along the north and east sides, with folding along vertical axes and consequent shortening of the sedimentary section. Tectonic development is attributed to (1) horizontal compressive forces, responsible for early crustal block movements and subcrustal plastic shifting, and (2) vertical forces of fluid pressures from the continued transfer of subcrustal material which probably helped raise the block. Uplift ceased when one or both of these forces terminated.

Paleotemperature Analysis of the Plio-Pleistocene Section at Le Castella, Calabria, Southern Italy

Abstract: The Plio-Pleistocene boundary has been located at the place where certain northern species of marine invertebrates first appear in the continuous Plio-Pleistocene sections of Italy. The section at Le Castella, near Crotone, southern Italy, consists of clays with diatomaceous layers and a few thin sand layers. The Plio-Pleistocene boundary there is clearly marked by the appearance of Anomalina baltica and other northern species of Foraminifera. Calabnan (lower Pleistocene) sediments 210m thick overlie the boundary, and upper Pliocene sediments 1405 m thick underlie it. The fossil fauna of the Plio-Pleistocene section consists essentially of abundant pelagic and benthonic Foraminifera, and diatoms. The benthonic microfauna indicates a depositional depth of about 500 m. There is no evidence of turbidity currents and submarine slumping. A section 167.80 m thick, including the paleontologically defined Plio-Pleistocene boundary, was logged and sampled at close stratigraphic intervals. Oxygen isotopic analyses of different species of pelagic and benthonic Foraminifera and of shell fragments of benthonic mollusks at successively higher stratigraphic positions in the section have revealed numerous temperature oscillations. Surface temperatures, probably representing summer averages, as indicated by Globigerinoides rubra and G. sacculifera, ranged from 21°C. to more than 30°C. in the late Pliocene, from 16°C. to more than 30°C. in the late Pliocene-early Pleistocene, and from 12° C. to 28° C. in the late Pleistocene. Temperatures at some depth and/or at a season other than the summer, as indicated by Globigerina bulloides and G. inflata, ranged from 20°C. to 28°C. in the late Pliocene, from 11°C. to 22°C. in the late Pliocene-early Pleistocene, and from 9°C. to 18°C. in the late Pleistocene. Temperatures given by benthonic Foraminifera ranged from 14°C. to 20°C. in the late Pliocene and from 11°C. to 22°C. in the late Pliocene-early Pleistocene (no data available for the late Pleistocene). Temperatures above 30°C. may be due, in part, to isotopic effects of the sea water. The above figures clearly indicate a major shift of the temperature ranges toward lower values from the late Pliocene to the late Pleistocene. No major temperature change seems to have occurred across the paleontologically defined Plio-Pleistocene boundary. Since the secular temperature minima of the late Pliocene are somewhat lower than present average summer temperature, an areal extension of the ice somewhat greater than that at present is suggested. Such extension probably increased at the times corresponding to the even lower temperature minima of the early Pleistocene. Major continental glaciations, however, may not have started until later, because none of the temperature minima in the section at Le Castella appear to have reached values as low as those obtaining during the glacial ages in the eastern Mediterranean. The temperature changes in the section at Le Castella are paralleled by important changes in the microfauna.

Origin of the Gulf of California

Abstract: The probable cumulative Late Cretaceous and Cenozoic right-lateral strike-slip displacement along the San Andreas fault in central California is 350 miles. The San Andreas and the allied faults into which it branches southward trend longitudinally into the Gulf of California, and the seismicity of the region indicates that the fault system follows the length of the Gulf and enters the Pacific basin south of Baja California. Crustal structure of most of the Gulf is of oceanic type, so that an origin by structural depression of continental rocks is not possible. Tectonic styles north and south of Los Angeles differ greatly. To the north, the Coast Ranges expose thick Upper Cretaceous and Cenozoic sedimentary rocks that were deposited in local basins and deformed tightly and repeatedly. To the south, in the Peninsular Ranges and Baja California, correlative rocks are thin and show little compressive deformation. The California batholith of mid-Cretaceous age and allied crystalline rocks form the basement of Baja California, southwestern Arizona, and northwestern Sonora and probably extend along the coast of mainland Mexico; the Gulf apparently bisects the crystalline belt longitudinally. These features suggest that Baja California initially lay 300 miles to the southeast, against the continental-margin bulge of Jalisco. The Gulf of California may be a pull-apart feature caused by strike-slip displacement plus up to 100 miles of cross-strike separation of the continental plate, subcontinental materials having welled up into the rift gap. The strike-slip motion has a tensional component across the continental margin south of Los Angeles but a compressional component to the north.

Middle America Trench: Seismic-Refraction Studies

Abstract: Seismic-refraction profiles in the Middle America Trench show that the main crustal layer ranges from normal oceanic thickness (about 5 km) t o half again as thick. Sediments are thicker than in the Pacific Basin. Because of the increased depth of water and sediment at all stations and thickening of the main crustal layer at some stations, the M discontinuity bows down beneath the trench. At the outer edge of the continental shelf near Guatemala, the water is replaced by sediments and rocks which may be either consolidated sediments or volcanic rocks; the M discontinuity is at little greater depth than beneath the trench.

Origin of Pennsylvanian Underclay and Related Seat Rocks

Abstract: Seat rocks, including underclay, underlie coal beds and show features such as roots, profiles similar to water-logged soils, lack of bedding, soil-like fracture, and gradation into normally bedded sedimentary rocks indicating that they were once soils. Coarse-grained seat rocks range from argillaceous to nearly pure quartz sandstone (ganister). Seat rocks composed of clay-sized particles (underclays) range from mixtures of illite, mixed-layer clay, and kaolinite to essentially pure kaolinite. Variations in the quartz or kaolinite content suggest that there are degrees of alteration in the formation of a seat rock and that all kinds of seat rock were formed by leaching of the substratum in a swamp. The increasing crystallinity of the clay minerals from plastic underclays to semiflint clay to flint clay and the corresponding increase in titanium support this conclusion. These two features suggest strongly that flint clays formed in place in an acidic swamp environment. The Olive Hill clay shows residual acidity. Seat rocks are affected by the chemical environments in the swamp before peat accumulation, during peat accumulation, and during coalification. Kaolinite is forming in present-day swamps and in wet acidic soils in Hawaii and probably formed in late Paleozoic swamps. The facts can be interpreted to show that all kinds of seat rocks could have formed in Pennsylvanian swamps and probably most did.