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At least 37 records · Page 2Linked to original sources
USGS1981

Geochemical evidence for modern sediment accumulation on the continental shelf off southern New England

An area of fine-grained sediment approximately 170 km x 74 km in size, located in water depths between 60 m and 150 m, south of Martha's Vineyard, Mass., is a site of modern sediment deposition. The 14 C ages systematically increase with sediment depth from about 1,300 years B.P. at the surface to 8,000-10,000 years B.P. at the depth of maximum core penetration. The old age for the surface sediments probably results from a combination of deposition of old carbon and faunal mixing. In the finest sediments, the sedimentation rates were approximately 130 cm/1,000 yrs when deposition began and have decreased to about 25 cm/1,000 yrs. The decreasing sedimentation rate reflects a diminishing source of fine sediments, which presumably came from the Georges Bank and Nantucket Shoals area. Inventories of excess 210 Pb in undisturbed cores average 70 dpm/cm 2 (disintegrations per minute per square centimeter), more than two times higher than the flux of 210 Pb from the atmosphere and from 226 Ra decay in the overlying water. This additional influx of 210 Pb either must be with new fine-grained sediment material or from solutions that are stripped of their 210 Pb by particulates in the bottom nepheloid layer. Stable Pb concentrations in surface sediments are about 28 ppm, as much as two times higher than concentrations at depth. The high accumulation rates, 210 Pb inventories, and trace-metal profiles imply that this area is a modern sink for fine-grained sediments and for pollutants associated with particulate matter in the water column. To our knowledge, this is the only site of present-day natural deposition on the Continental Shelf off the eastern United States, exclusive of the Gulf of Maine. Because the net currents on the outer half of this Continental Shelf flow from northeast to southwest, this fine-grained deposit may receive its sediments and possible contaminants from the Nantucket Shoals and Georges Bank regions.

Massachusetts↗
USGS1981

Authigenic kaolinite and associated pyrite in chalk of the Cretaceous Niobrara Formation, Eastern Colorado

Cores from the Smoky Hill Chalk Member of the Cretaceous Niobrara Formation have several zones containing authigenic kaolinite as spherical, moldic, polycrystalline aggregates that occur within single or multichambered foraminiferal tests and are commonly associated with framboidal pyrite. Such kaolinite is inferred to result from volcanic ash deposited during chalk sedimentation. Shortly after burial, a colloidal aluminous gel or solution formed from the unstable ash and moved into organic-rich foraminiferal tests, where sulfate-reducing bacteria created a favorable microenvironment for the simultaneous crystallization of kaolinite and pyrite.

Journal of Sedimentary Petrology↗
USGS1981

Depositional environments of the Pennsylvanian Pottsville Formation in the Black Warrior basin of Alabama

The Pennsylvanian Pottsville Formation of the Black Warrior basin in Alabama comprises as much as 3000 m of shale, sandstone, and coal. The boundary between the informal units of the lower Pottsville and the upper Pottsville is the base of the Black Creek coal group, in the middle part of the section. Lower Pottsville strata include orthoquartzitic sandstone, shale, and coal interpreted as having been deposited in a barrier/back-barrier setting. Upper Pottsville strata consist of lithic arenite, shale, coal, and minor amounts of orthoquartzite, and are interpreted as representing a lateral gradation from lower delta plain to barrier bar. Previous studies indicated northward sediment dispersal. The change in sedimentation patterns in the middle part of the Pottsville suggests introduction into the basin of sediments from an eastern source during Pottsville deposition.

Journal of Sedimentary Petrology↗
USGS1981

Reservoir properties of submarine- fan facies: Great Valley sequence, California

Submarine-fan sandstones of the Great Valley sequence west of the Sacramento Valley, California, have low porosities and permeabilities (64 samples averaged 10.1% porosity and 0.87 millidarcies permeability). However, petrography and scanning electron microscope studies indicate that most sands in almost all submarine fan environments are originally porous and permeable. Thin turbidite sandstones deposited in areas dominated by shale in outer-fan, basin-plain, and overbank environments are cemented mainly by calcite; shale dewatering is inferred to contribute to rapid cementation early in the burial process. Sands deposited in inner- and middle-fan channels within interchannel and fan-fringe environments that contain only thin shale beds have small percentages of intergranular matrix or cement. The original porosity is substantially reduced mechanically at shallow depths and by pressure solution at deeper levels. Permeability decreases systematically with increasing age of the rocks, presumably as a result of increasing burial depths. Computer-run stepwise regression analyses show that the porosity is inversely related to the percentage of calcite cement. Such parameters as the contents of quartz, feldspar, and unstable rock fragments have no correlative effect on either porosity or permeability. The results reported here indicate original porosity and permeability can be high in deep-water submarine fans and that fan environments dominated by sand (with high sand/shale ratios) are more likely to retain higher porosity and permeability to greater depths than sand interbedded with thick shale sequences.

Journal of Sedimentary Petrology↗
USGS1982

Cyclic deposits and hummocky cross-stratification of probable storm origins in Upper Cretaceous rocks of the Cape Sebastian area, southwestern Oregon

Cyclic deposits containing hummocky cross-stratification occur in the upper part of the Cape Sebastian Sandstone of Bourgeois (1980), a shallow marine transgressive sandstone of Late Cetaceous age on the southern Oregon coast. The cycles average 1.6 m in thickness and consist, where complete, of a lower hummocky cross-stratified sandstone, a middle planar and ripple bedded sandstone with a shale bed in its middle part, and an upper bioturbated sandstone. Noteworthy features of the hummocky cross-stratification include the presence of depositional domes in addition to scoured depressions, the absence of significant bedform migration, and the presence of a small proportion of dip angles greater than the angle of repose (>34 degrees ) in addition to the large proportion of low (<15 degrees ) dip angles. The lower, stratified, fining-upward part of the cycle (up to the top of the shale bed) is interpreted as having accumulated under conditions of initially great but gradually decreasing current velocity and deposition rate. The currents probably had a strong oscillatory component, and the depositional event is inferred to have been a storm. The part of the planar- and ripple-bedded sandstone above the shale bed was probably deposited during relatively fair weather after the storm but before re-establishment of a normal benthic fauna. The bioturbated sandstone is interpreted to have been deposited during fair weather or during minor storms separated by long intervals of fair weather.

Journal of Sedimentary Petrology↗
USGS1982

Geochemical indices of fine sediment transport, northwest Gulf of Mexico

The 210 Pb distribution, the clay mineralogy distribution, and the distribution of three trace metals, barium, lead, and manganese, in the sediments of the south Texas shelf are related to the dynamics of the sedimentary transport process. 210 Pb, whose concentration is time dependent, defines three loci of recent sediment accumulations. In addition, the variation of 210 Pb activity at the sediment-water interface delineates areas of terrestrial sedimentation from hemipelagic sedimentation. The clay mineralogy composition of the bottom and suspended sediments assists in defining the origin of the persistent nepheloid layer and bottom sediment. Barium, a major element used in drilling mud, tags sediment movement from areas of hydrocarbon exploration. Lead concentrations, anthropogenically introduced from urban areas, tag the sediment derived from the metropolitan complexes of coastal Texas. Manganese, because of diagenic mobilization, is concentrated in areas of very slow sediment accumulation. The distribution of these geochemical properties of the sediment are in direct response to the sediment regime of the shelf. Based on this data, a model of sediment transport and deposition which relates currents, wind, tides, sediment flux, and precipitation has been formulated. This model differs from the "advective" transport or convergent current schemes previously proposed for this shelf.

Journal of Sedimentary Petrology↗
USGS1982

Dynamic ice-wallow relief of northern Alaska's nearshore

Contour maps with 0.5-m depth interval were prepared for a small area seaward of Reindeer Island, a barrier island in the Beaufort Sea, Alaska, by repeated surveys with very accurate navigation and very close trackline spacing. The maps reveal numerous closed depressions and mounds about 50 to 100 m in diameter and 2 to 3 m in relief, presumably related to grounded ice floes common in the area year round. Some of the features were obliterated over the course of three seasons while new ones formed. Although the depressions resemble kettles, they are formed by very different mechanisms. We believe that these bedforms represent erosion and deposition caused by: a) intensified flow around stationary ice floes serving as obstacles and b) pulsating currents generated by vertical oscillations or rocking motions of grounded floes in a seaway. Because sediment transport occurs around the ice, not where it directly touches the sea floor, the depressions are much larger than the base of the acting floes. Ice-wallow bedforms, although not found everywhere, are characteristic of arctic nearshore regions with non-cohesive sediments, and most likely occur in other ice-stressed coastal environments in differing degrees. The bedforms studied here are highly active and must be considered in planning nearshore construction activities.

Journal of Sedimentary Petrology↗
USGS1982

Ice rafting of fine-grained sediment, a sorting and transport mechanism, Beaufort Sea, Alaska

The presence of turbid, sediment-rich fast ice in the Arctic is a major factor affecting transport of fine-grained sediment. Turbid ice was found to be present in a zone 10 to 20 km wide along the coast of the Beaufort Sea. Sediment concentrations observed in cores taken in seasonal fast ice ranged from 3 to more than 1,600 g/m 3 . Finely disseminated silt and clay-sized particles dominated, discoloring the upper segments of the cores in a layer up to 1 m thick. In almost all of the cores the upper-most 10 cm of ice was relatively less turbid, and the relief at the base of the turbid layer was 10 to 20 cm over distances of 1 to 2 m. Observers have documented the widespread, sporadic occurrence of sediment-rich fast ice in both the Beaufort and Bering Seas. The known sources and transport relations of suspended sediment did not explain this distribution of turbid ice. The occurrence of sediment in only the upper part of the seasonal fast ice indicates that sediment-rich ice forms early during ice growth. The most likely mechanism for the formation of sediment-laden ice and preferential sorting of fine sediments requires resuspension of nearshore bottom sediment during storms, accompanied by formation of frazil ice and subsequent lateral advection before the fast ice is stabilized. We estimate that the sediment incorporated in the Beaufort ice canopy formed a significant proportion of the seasonal influx of terrigenous fine-grained sediment. The dominance of fine-grained sediment suggests that in the Arctic and sub-Arctic these size fractions may be ice rafted in greater volumes than the coarse fraction of traditionally recognized ice-rafted sediment. The incorporation and transport mechanism may be a significant factor in the transport of fine-grained sediments in the Arctic basin.

Journal of Sedimentary Petrology↗
USGS1982

Modern shallow-water graded sand layers from storm surges, Bering Shelf: a mimic of Bouma sequences and turbidite systems

A sequence of graded sand layers, interbedded with mud, extends offshore over 100 km from the Yukon Delta shoreline across the flat, shallow (<20 m depth) epicontinental shelf of the northern Bering Sea, Alaska. Proximal graded sand beds on the delta-front platform near the shoreline are coarser (2-3phi ), thicker (10 to 20 cm), and contain more complete vertical sequences of sedimentary structures than distal beds. The inshore graded vertical sequence of structures from the base to the top of individual sand layers includes plane-parallel lamination (S b ), cross lamination (S c ), plane-parallel lamination (S d ), and mud (S e ) analogous to the Bouma T (sub a-e) turbidite sequence. Structures vary between interchannel platform deposits with complete S b -S e sequences and channel-floor sands that are all trough cross-laminated. Distally, storm-sand layers change to thin (1 to 5 cm) silt beds that contain flat and ripple-drift lamination (S (sub c-e,de) ), are commonly bioturbated, and are associated with shell and pebble lags from storm-wave reworking. The sequence of graded sands appears to be related to the major storm surges that occur every several years. The major storms increase the average 10-m water depth in southern Norton Sound as much as 5 m and cause fluctuations in pore pressure from wave cyclic loading that may liquefy the upper 2 to 3 m of sediment. Storm-associated bottom currents, possibly dominated by rapidly waning ebb flow, transport the liquefied inshore sand far offshore (> 100 kin). Such shallow-water graded layers off lobate deltas may be distinguished from similar deep-water turbidites by: 1) the predominance of trough cross-lamination, perhaps resulting from wave oscillation effects, in the proximal part of the system, and 2) gradation to common shallow marine fossils, bioturbation, and storm lag layers in distal areas.

Journal of Sedimentary Petrology↗
USGS1982

A rapid method for concentrating sedimentary organic matter for vitrinite reflectance analysis

The tecnique discussed in this paper utilizes crushing, high-speed blending, and ultrasonic treatment to mechanically disaggregate rock and release the sedimentary organic matter (OM) in a suitable heavy liquid. This new method can provide freeze-dried concentrated OM in approximately 8 to 24 hours (longer time is necessary for removing carbonate). Under optimal conditions, it is possible to concentrate the OM and prepare a hardened epoxy microscope slide in about 24 hours. Subsequent grinding, polishing, and drying allows microscopic examination of the organic concentrate the next day.

Journal of Sedimentary Petrology↗
USGS1983

Authigenic vivianite in Potomac River sediments: control by ferric oxy-hydroxides

Sand-size particles of vivianite (Fe 3 (PO 4 ) 2 .8H 2 O) have been identified in surficial sediments of the tidal Potomac River near a major sewage treatment plant. Vivianite is concentrated in a surface layer of coarse-sand dredge spoil, which overlies much finer sediment. Although saturation indices indicate that the pore waters of virtually all the sediments in the study area are supersaturated with respect to vivianite, it is found only in association with dredge spoil. Pore-water profiles of Fe, P, and Eh, and the size and morphology of individual grains, indicate that the vivianite is authigenic. The major control on the occurrence of the mineral is the presence or absence of amorphous ferric oxy-hydroxides, which react with pore-water phosphorus to form stable ferric hydroxy-phosphates preferentially to the formation of vivianite.--Modified journal abstract.

Journal of Sedimentary Petrology↗
USGS1983

Mechanical and chemical compaction in fine-grained shallow-water limestones

Artificial compaction of in-situ cores of sediments resulted in: 1) reduction of sediment thickness by 50 percent and more; 2) reduction of initial porosities of 65 to 75 percent to 35 to 45 percent; 3) creation of megascopic textures almost identical to many ancient lime mud- and wackestone; 4) creation of organic, wispy "stylolite-like" layers; 5) chemical compaction, evidenced by thin sections showing quartz grains piercing mollusc shells without causing fractures and SEM evidence of solutional interfitting of 1 to 4-mu m-size aragonitic carbonate grains; 6) obliteration of pellets and birdseye or fenestral voids in those sediments where early cementation was lacking; obliteration of identifiable marine grasses and vertical "root" tube voids; 8) mashing of sediment-filled circular burrows to produce ellipsoidal structures. Significant mechanical compaction resulted from pressures simulating less than 1,000 ft of burial. Increasing loads to more than 10,000 ft did not significantly increase compaction. Chemical compaction was detected only in cores compacted to pressures greater than 10,000 ft of burial. These experiments suggest that chemical compaction would begin at much shallower depths given geologic time. Experiments that caused chemical compaction lend support to the hypothesis that cement required to produce a low-porosity/low-permeability fine-grained limestone is derived internally. Dissolution, ion diffusion, and reprecipitation are the most likely processes for creating significant thicknesses of dense limestones. Continuation of chemical compaction after significant porosity reduction necessitates expulsion of connate fluids, possibly including hydrocarbons.--Modified journal abstract.

Journal of Sedimentary Petrology↗
USGS1983

Birdseyes, fenestrae, shrinkage pores, and loferites: a reevaluation

Birdseyes, birdseye limestone, fenestrae, fenestral fabric, shrinkage pores, and loferites are considered similar or synonymous when occurring in lime mudstone or syndepositional dolomite, especially in association with mudcracks and stromatolites. Compaction experiments indicate, however, that without early cementation, these vugs can be obliterated, whereas mudcracks and stromatolites are unchanged. Lumping of burrows and root tubes under the general terms birdseyes or fenestrae, such as for example tubular fenestrae, burrow fenestrae, or root-tube fenestrae, is discouraged. They should be called burrows or root tubes because the birdseyes, fenestrae, and shrinkage pores are so intimately associated with tidal flats to most geologists. Submarine cementation of pelletal and oolitic botryoidal grainstone under 5 to 6 m of water on the Bahama Banks has resulted in vugs provocatively similar to many birdseyes and fenestrae generally attributed to peritidal conditions in the literature. Birdseyes and fenestrae in ancient grainstones cannot therefore be reliably identified without consideration of the sedimentary sequence and associated sedimentary structures.--Modified journal abstract.

Journal of Sedimentary Petrology↗
USGS1984

Accretionary lapilli in altered tuffs associated with coal beds

Accretionary lapilli have been identified in claystones (tonsteins) associated with coal beds of Late Cretaceous age in Wyoming, Utah, and New Mexico. The presence of accretionary lapilli in these tonstein partings confirms their volcanic origin. Similar concentric structures in other claystones not in coal, such as some flint clays, may also be accretionary lapilli, indicating a volcanic origin for these deposits. The lapilli are spherical aggregates of volcanic ash that form in eruption columns, where water vapor and turbulence promote accretion of layers of fine ash about coarse-grained aggregates of ash acting as nuclei. The very fine particle size of the crystalline components of the claystones containing accretionary lapilli, coupled with the relatively small size of the lapilli themselves (2-7.5 mm), suggests that the lapilli formed far downwind from the volcano through interaction with moisture from the atmosphere, rather than with magmatic or phreatic moisture near the source vent.

Journal of Sedimentary Petrology↗
USGS1984

Sedimentary structures formed in sand by surface tension on melting hailstones

Craters form when hailstones impact sand. When a hailstone melts, wet but unsaturated sand within the crater is attracted to the hailstone surface by surface tension. Shrinkage of the melting hailstone then produces one or more rings of sand within the impact crater.--Modified journal abstract.

Journal of Sedimentary Petrology↗
USGS1984

Submarine sand dunes and sedimentary environments in Oceanographer Canyon.

Observations from research submersibles in the northern part of Oceanographer Canyon reveal the presence of an extensive field of large sand dunes on the canyon floor. The dunes are medium to coarse sand, are oriented across the axis, and the largest of them are as high as 3 m and have wavelengths up to 15 m. Their asymmetry, grain size, and height suggest that they are formed by axial currents flowing up- and downcanyon and that the largest dunes require flows of at least 70 cm/sec. Shelf sand, low in silt and clay content, is transported by currents down and along the canyon walls onto the canyon floor. As the sand enters the canyon, it is mixed with immobile gravel deposits on the canyon rim; lower on the walls, the sand is mixed with silt and clay burrowed by organisms from the semiconsolidated sandy silt that underlies the canyon walls and floor. Upon reaching the canyon floor, the sand is sculpted into bed forms by currents, and the fines are winnowed out and transported out of the canyon. At present, the shelf and canyon walls are being eroded by bottom currents and burrowing organisms, whereas the canyon floor is covered by mobile sand that moves both up and down the axis in this part of the canyon.

Journal of Sedimentary Petrology↗
USGS1984

Sedimentary fluorite in a lacustrine zeolitic tuff of the Gila Conglomerate near Buckhorn, Grant County, New Mexico

Fluorite makes up 20-30% of a zeolitic tuff in a Pliocene or Pleistocene lacustrine facies of the Gila Conglomerate that has not been subjected to hydrothermal activity. The light gray zeolitic tuff is 40 cm thick and crops out over a 0.6-km 2 area about 2.5 km east of Buckhorn, New Mexico. This tuff is overlain by zeolitic mudstone that contains abundant and conspicuous Magadi-type chert. The fluorite occurs as prolate pellets and, rarely, as ooids that are mostly 0.14.3 mm in size. Broken pellets and ooids are extremely rare. Studies by X-ray diffraction and scanning electron microscopy show that the pellets and ooids consist mainly of submicrometer-size fluorite and quartz and that both minerals have poorly defined morphology. These pellets and ooids are embedded in a matrix that consists chiefly of micrometer-size mordenite and smectite and that has a vague vitroclastic texture. The pellets and ooids probably are the result of primary precipitation of fluorite and magadiite where dilute, calcium-bearing water from springs or streams mixed with the saline, alkaline lake water that had a high fluorine content. The pellets and ooids were then transported basinward and were incorporated with reworked vitric ash. During diagenesis, the magadiite of the pellets and ooids was converted to quartz, and the glass of the ash was altered to mordenite and smectite.

Journal of Sedimentary Petrology↗