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Displaying 11 - 20 from the 33 total records
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Underwater Television Survey of the Atlantic Outer Continental Margin near Wilmington Canyon
Daniel J. Stanley and Peter Fenner
54 pages, 18 figures
1973 (Date of Issue: 2 August 1973)
Number 11, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.11.1
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The study summarizes the results of an underwater television survey of the sea floor at the shelfbreak and at the head of Wilmington Canyon off the Middle Atlantic States. Distributions are shown for bottom currents and sedimentary structures, suspended matter, bottom firmness, bottom lithology, shell percentage, and fauna. A number of man-made objects on the sea floor in this area is documented. The Appendix is a listing of observations made at 26 stations. These data, based on direct visual observation of the sea floor, serve to complement earlier marine geological investigations made in this region. Mapping shows that surficial outer shelf and canyon head sediments at the shelfbreak are undergoing modification by both bottom current processes and bioturbation and that at the present time the Wilmington Canyon is receiving sediment mainly from the adjacent margin. Gravel, oyster banks, and near-vertical cavernous cliffs occurring at depths between 100 and 200 m are relict features related to eustatic changes of sea level that affected the outer continental margin during the Pleistocene.

An Atlas of Volcanic Ash
Grant Heiken
101 pages, 15 figures, 33 plates, 3 tables
1974 (Date of Issue: 12 April 1974)
Number 12, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.12.1
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Volcanic ash samples collected from a variety of recent eruptions were studied, using petrography, chemical analyses, and scanning electron microscopy to characterize each ash type and to relate ash morphology to magma composition and eruption type.

The ashes are best placed into two broad genetic categories: magmatic and hydrovolcanic (phreatomagmatic). Ashes from magmatic eruptions are formed when expanding gases in the magma form a froth that loses its coherence as it approaches the ground surface. During hydrovolcanic eruptions, the magma is chilled on contact with ground or surface waters, resulting in violent steam eruptions. Within these two genetic categories, ashes from different magma types can be characterized. The "pigeon hole" classification used here is for convenience; there are eruptions which are driven by both phreatic and magmatic gases.

The morphology of ash particles from magmatic eruptions of high-viscosity magma is governed primarily by vesicle density and shape. The vitric ash particles are generally angular, vesicular pumiceous fragments, or thin vesicle wall fragments. The morphology of lithic fragments is dependent on the texture and mechanical properties of the rock units broken up during the eruption; most of the samples studied contain equant, angular to subrounded lithic fragments.

Ash particles from eruptions of low-viscosity magmas are mostly droplets; droplet shape is in part controlled by surface tension, acceleration of the droplets leaving the vent, and air friction. Shapes range from perfect spheres to a variety of twisted, elongate droplets, with smooth, fluidal surfaces.

The morphology of ash particles from hydrovolcanic eruptions is controlled by stresses within the chilled magma which result in fragmentation of the glass to form small blocky or pyramidal ash particles. Vesicle density and shape play only a minor role in determining the morphology of these ash particles.

Distinctive Properties of Turbiditic and Hemipelagic Mud Layers in the Algéro-Balearic Basin, Western Mediterranean Sea
Nicolaas A. Rupke and Daniel Jean Stanley
40 pages, 21 figures, 8 tables
1974 (Date of Issue: 10 September 1974)
Number 13, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.13.1
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Two types of mud layers alternate in dominantly muddy cores of the southern Balearic Basin. Type A muds (a few cm to over 50 cm thick), macroscopically homogeneous, occur above turbidite sands or silt laminae. Type B muds (imperceptible to about 50 cm thick), comparatively coarse due to interspersed microskeletons, occur below turbidite sands or silt laminae, and lie above type A muds. The two types are distinguished in X-radiographs on the basis of texture and sedimentary structures. Type A and B mud layers in six cores were sampled at 1 to 8 cm intervals.

Type A muds are distribution graded (upward shift of the entire size distribution to finer sizes), continuing the upward grading of the underlying sand turbidites. A granulometric change occurs at the boundary with type B muds which contain sand (to 16 percent), largely tests of forams and pteropod shells. The sand fraction of type A muds (d" 1 percent) differs from that of type B in the proportion of terrigenous constituents and in remains of pelagic forams and of pteropods. Type B muds are not graded; their grain-size distribution is uniform. They have a higher (26 to 46 percent) carbonate content than type A (16 to 39 percent). In some instances, peak-height ratios of clay minerals change across the boundary between type A and type B mud layers. It is concluded that type A muds are turbiditic (deposited instantaneously), while B muds are hemipelagic deposits.

Carbon-14 ages were determined on the carbonate sand fraction of type B layers. The ages were plotted against the total sediment thickness above the dated samples in each core. A statistically significant correlation exists. However, when the turbiditic sand and mud layers are omitted and the ages are plotted only against the combined thicknesses of the hemipelagic type B layers, an even stronger correlation is obtained. The hemipelagic rate of sedimentation during the past 16,000 years approximates 10 cm/1000 years. The frequency of turbidity current incursions at a particular core location averages 3 per 2000 years.

Mineral Sciences Investigations, 1972-1973
George S. Switzer, editor
88 pages, 29 figures, 28 tables
1975 (Date of Issue: 2 July 1975)
Number 14, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.14.1
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Thirteen short contributions from the Smithsonian's Department of Mineral Sciences for 1972 and 1973 are gathered together in this volume. Scientific contributions include new data on some mercury minerals from Terlingua, Texas; a description of dashkesanite from St. Paul's Rocks; a note on high-alumina basalt from the Aleutian Trench; descriptions of samples from the Apollo 15 and 16 lunar missions; chondrule composition of the Allende meteorite; the Pulsora meteorite and metamorphic equilibration in chondrites; the possible survival of very large meteorites that encounter the earth's surface; data on eight observed-fall chondritic meteorites; chemical analyses of two microprobe standards; and a technological note on the preparation of multiple microprobe samples. A history of mineral sciences in the Smithsonian Institution and a list of meteorites in the Smithsonian collections complete the volume.

Sands in the Alboran Sea: A Model of Input in a Deep Marine Basin
Daniel Jean Stanley, Gilbert Kelling, Juan-Antonio Vera and Harrison Sheng
51 pages, 23 figures, 8 tables
1975 (Date of Issue: 16 June 1975)
Number 15, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.15.1
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The Alboran Sea, an almost totally land-enclosed, mountain-bounded (Rif, Betic ranges) basin, lies east of Gilbraltar in the westernmost Mediterranean. A petrologic study of the sand fraction in river, river mouth, and beach samples collected on the coast of the Alboran Sea defines the composition and distribution of the principal light and heavy mineral groups along its margins. The investigation details 20 mineralogical provinces on the southern Iberian and northern Moroccan margins and the Strait of Gibraltar sector and identifies the major source terrains and fluvial and marine point sources of terrigenous sediment entering the basin.

Significant sample-to-sample changes in the proportion of mineralogical components are attributed to marine processes, particularly nearshore currents, which move sands laterally along the coast and, while so doing, modify the proportions of light and heavy mineral components. Lateral trends observed within Moroccan and Spanish mineralogical provinces provide evidence on the actual sense of nearshore sediment dispersal. Marine transport agents have a more pronounced effect on the light mineral fraction, while even unstable heavy mineral species appear to suffer less modification as a result of the transport in the marine environment. The paths followed by the sands between source terrain and final depositional site in deepwater environments are complex ones. A comparison of mineral assemblages in coastal sands and in sands in deep-sea cores shows a provenance from the Serran?a de Ronda complex in the Betic range west of M?laga. After initial deposition on the coast, these river-borne sediments are transported in a southwestward direction toward Gibraltar and then eventually are funneled downslope in a southeastward direction toward the Western Alboran Basin through the Gibraltar Canyon and submarine valley.

In geological terms, the Alboran. Sea study can serve as a model for sedimentation in one type of elongate enclosed basin bounded by regions of high relief. Although the geographic and geologic configuration of the Alboran Sea and contiguous land conforms to a multisource basin model, the transport paths of sediment since the late Quaternary have been essentially longitudinal. This longitudinal input, with filling as a result of currents primarily from the Strait of Gibraltar sector, is independent of a major delta source and is thus unlike many elongate, deep-sea basins examined in present oceans and troughs (including flysch) mapped in the ancient rock record.

Late Quaternary Sedimentation and Stratigraphy in the Strait of Sicily
Andres Maldonado and Daniel Jean Stanley
73 pages, 39 figures, 5 tables
1976 (Date of Issue: 3 August 1976)
Number 16, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.16.1
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The Strait of Sicily, a broad, elongate, topographically complex platform in the central Mediterranean, separates the deep Ionian Basin from the Alg?ro-Balearic and Tyrrhenian basins to the west. A detailed core analysis shows that the late Quaternary sections in the different sectors of the Strait are distinct from those in the deep Mediterranean basins. Strait lithofacies are characteristically uniform, highly bioturbated, and contain significant amounts of coarse calcareous sediment. Five major sediment types (coarse calcareous sand, sand- to silt-size sediment, ash, mud, and sapropel) are grouped into natural vertical successions termed sequences. The three major sequences defined in the Strait are upward-coarsening and upward-fining, uniform, and turbiditic (including both mud and sand-silt turbidites); sapropel sequences are recovered in cores on the Ionian slope east of the Strait.

The direct relation between sediment type, lateral lithofacies distribution, water depth, and structural displacement is demonstrated. For example, the proportion of turbiditic mud increases while that of hemipelagic mud and bioturbated strata decreases with depth. The effects of regional Quaternary events, particularly climatic changes and eustatic sea level oscillations, are well recorded in cores collected in shallow platform and neritic-bathyal environments; here the upper sediment sequences are truncated and fining- and coarsening-upward sequences, which include coarse calcareous sand layers interbedded with mud and sandy lutite, prevail. In contrast, well stratified units comprising sand (including gravity flow units and volcanic ash) alternating with hemipelagic and turbiditic mud form the surficial deposits in the deep (>1000 m) elongate Linosa, Pantelleria, and Malta basins. Homogeneous bioturbated light olive gray to dusty yellow muddy sequences predominate in the intermediate depth neritic-bathyal environments.

Stratigraphic correlation of cores based on carbon-14 analyses shows that individual units or sequences are not correlatable across the Strait or even within small basins, although it is possible to recognize a general vertical succession of depositional patterns. Sedimentation rates generally decrease with increasing depth. Rates in the deep basins have been relatively uniform from the late Quaternary to the present, while upper (Holocene) sequences in the shallow platform and neritic-bathyal environments have been truncated. Correlation of reflectors on high-resolution subbottom profiles indicates that faulting in many sectors of the Strait is of recent or subrecent origin and that the vertical displacement rate is locally in excess of the average sedimentation rate (i.e., greater than 20 cm per 1000 years).

The absence of sapropel layers in the Strait basins indicates that these depressions remained ventilated during periods when anaerobic conditions prevailed in the deep basins in the eastern and central Mediterranean. An early Holocene paleooceanographic model depicting a possible reversal of currents in the Strait of Sicily region is postulated.

Occurrence, Distribution, and Age of Australian Tektites
R. O. Chalmers, E. P. Henderson and Brian Mason
46 pages, 17 figures, 10 tables
1976 (Date of Issue: 9 September 1976)
Number 17, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.17.1
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Extensive field work has shown that the Australian strewnfield is less extensive than previously thought, being essentially restricted to the region south of latitudes 24? to 25?S. The few australites found north of this region probably represent specimens transported by man. Throughout much of the desert interior australites are weathering out of a late Pleistocene or early Recent horizon in a well-consolidated calcareous red sandy aeolianite; field evidence indicates that in most places they are found essentially where they fell, or stream erosion and sheet wash has transported them short distances and concentrated them in claypans and playas. Distribution within the strewnfield is irregular and can be ascribed to: (1) original nonuniform fall; (2) burial by recent deposition; (3) removal by erosion. Australites (excluding the doubtful HNa/K type) show a continuous range of composition from 80% to 66% SiO2 with related variations in other major constituents, which is reflected in the range of specific gravities (2.36-2.52) and refractive indices (1.493-1.529). The composition range is not uniform over the strewnfield, the high-silica australites being concentrated along a northwest trending band extending from western Victoria to the Lake Eyre region. Other noteworthy features are: (1) a variation in the average size of australites from place to place, those on the Nullarbor Plain being notably smaller (average < 1 gram) than those of other regions (average 3-5 grams); (2) the occurrence of many large australites (> 100 grams) in the southwestern part of Western Australia.

Unsolved problems include: (1) the inconsistency between geological age (7000-20,000 years BP) and K-Ar and fission track ages (700,000-860,000 years); (2) the relationship, if any, between australites and the  microtektites in Indian Ocean sediments; and (3) the source region of the australite material.

A Catalog of the Type Specimens in the Mineral Collection, National Museum of Natural History
Arthur Roe and John S. White, Jr.
43 pages
1976 (Date of Issue: 22 November 1976)
Number 18, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.18.1
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A complete list, arranged alphabetically by mineral name, of the type specimens in the mineral collection of the National Museum of Natural History. These are the actual specimens that were used in defining new mineral species. For each species the catalog number, locality, literature citation for the original description, source of specimens, and date of ccession are given.

Mineral Sciences Investigations 1974-1975
Brian Mason, editor
125 pages, 48 figures, 37 tables
1977 (Date of Issue: 9 March 1977)
Number 19, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.19.1
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Nine short contributions from the Smithsonian's Department of Mineral Sciences for 1974 and 1975 are gathered together in this volume. These contributions comprise an account of the composition of garnet xenocrysts from three kimberlite pipes in Arizona and New Mexico; a catalog of major element chemistry of abyssal volcanic glasses, and the application of these data to determine magma compositions; descriptions of the Harleton (Texas), St. Mary's County (Maryland), and Ras Tanura (Saudi Arabia) chondritic meteorites; a comparative study of eight chondrite meteorites from India and Pakistan; geochemical data on separated components of the Allende carbonaceous chondrite; and a mineralogical and chemical study of silicate inclusions in the El Taco mass of the Campo del Cielo iron meteorite.

Catalonian, Eastern Betic, and Balearic Margins: Structural Types and Geologically Recent Foundering of the Western Mediterranean Basin
Daniel Jean Stanley, Henri Got, Neil H. Kenyon, Andre; Monaco and Yehezkiel Weiler
67 pages, 33 figures
1976 (Date of Issue: 20 September 1976)
Number 20, Smithsonian Contributions to the Earth Sciences
DOI: 10.5479/si.00810274.20.1
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A high-resolution seismic study of the Catalonian, eastern Betic, and Balearic regions in the western Mediterranean emphasizes the importance of large-scale post-Miocene vertical displacement of upper Miocene and overlying unconsolidated sediment sequences. The structural configuration of the Pliocene and Quaternary series observed on subbottom profiles indicates that the geologically recent margins in these sectors have subsided along preexisting (Oligocene or older) as well as more recent (post-Miocene) tectonic trends. The post-Miocene movements are not necessarily synchronous and marked differences of structural styles are identified. The origin of the three major types of margins - abrupt, intermediate (or steplike), and progressive - is closely related to tectonic trends on land. Subsidence of the sub-Pliocene seafloor on the order of 1500 m is estimated on the basis of seismic profiles presented here.

The abrupt margin type, exemplified by the Emile Baudot and Mazzaron escarpments, occurs in areas where the edge of the basin parallels the major structures on land and where vertical displacement has developed in a relatively restricted structural zone. Intermediate margins, such as the sector southeast of the Betic chain, are localized in areas where two major structural trends converge (NE-SW and NW-SE fractures predominate), and show a steplike (growth fault) displacement landward. Progressive margins, such as off Catalonia, display a flexing of the Pliocene and Quaternary cover and are related to Pliocene foundering followed by more gentle subsidence from the upper Pliocene to the present. This type of margin occurs seaward of Tertiary basins where older (Hercynian to Miocene) tectonic trends have been reactivated. Seismic evidence indicates that submarine canyon development on these different margins is associated with the post-Miocene tectonics as well as with Quaternary eustatic events.

The Balearic Rise, a large continental block detached from the southern part of the Balearic Platform, foundered largely in post-Miocene time, but this feature has not yet completely subsided to the level of the Alg?ro-Balearic Basin plain. It is possible that large continental blocks beyond the base of slopes that are presently buried by a thick Pliocene-Quaternary cover may have a structural origin analogous to that of the Rise. Margin formation and the evolution of the western Mediterranean Basin bear some similarities to the structural development of rift zones.

Displaying 11 - 20 from the 33 total records