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At least 991 records · Page 55Linked to original sources
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

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

Macrotidal subarctic environment of Turnagain and Knik Arms, Upper Cook Inlet, Alaska: sedimentology of the intertidal zone

An extensive sheet of silty sand crossed by tidal channels is exposed at low tide in Turnagain and Knik Arms, Alaska. Transportation and deposition of sediment composing this sheet is the result of strong tidal currents due to the maximum spring tidal range of approximately 11.4 m. In Turnagain and Knik Arms, the initiation of the floodtide is accompanied by the occurrence of a tidal bore that travels at a speed of 4 m/sec or more up the tidal channels. In intertidal environments in many parts of the world, physical processes of sedimentation are typically influenced by surface fauna; sedimentary structures in some zones are altered or obliterated by the activities of the infauna. However, in Turnagain and Knik Arms, virtually no macrofauna exist, and so the area provides an opportunity to study depositional processes and sedimentary structures without the masking effects of bioturbation. Suspended-sediment values for Turnagain and Knik Arms are greatest in the summer months and nearest low-tide stages. Ice floes form during winter months and, in combination with the freezing of intertidal sediment to several meters of depth, have a significant effect on sediment dispersal and on the deepening and increased stability of the tidal channels. The intertidal sediment is mainly very well sorted, medium to very fine sand and coarse silt, with a notable paucity of gravel, coarse sand, and clay. Sediment accumulates in sandbars oriented parallel to the axis of the estuary. Textural and facies patterns reflect decreasing current energy, first, as a function of distance from the mouth of the estuary and, second, as a function of topographic elevation on the sandbars. The intertidal deposits in Turnagain Arm contain a transitional upward-fining sequence which is analogous to point-bar and channel-bar successions described by other workers. The base of the sequence contains parallel-laminated sand that is overlain by mixed parallel- and cross-laminated silty sand overlain by cross-laminated silty sand. At the top of the sequence is a unit of organically rich parallel-laminated silt and sandy silt. This suite of sedimentary features and textures indicates changing current velocities and directions as bars emerge or are modified during ebb tide, and the vertical sequences could potentially be used to record intertidal bar erosion and aggradation by point- and channel-bar migration, a process carried out primarily by bed-load currents.

Journal of Sedimentary Petrology↗
USGS1984

Bedding types in Holocene tidal channel sequences, Knik Arm, Upper Cook Inlet, Alaska

Uplifted convoluted and horizontal to subhorizontal beds of varying thickness in intertidal silt as old as 3,280 + or - 90 yr B.P. are exposed in the banks of tidal channels of unknown depth in the intertidal zone in Knik Arm of Upper Cook Inlet. Internal discordances may occur both within convoluted beds and between convoluted and horizontal to subhorizontal beds. At the base of many convoluted beds, there is a rapid gradation upward into laminae which are severely deformed; that is, in some places, the contortions appear to have originated along a single bedding plane. Where the convoluted sequences are truncated by nearly horizontal sequences, the distortion must have resulted from syndepositional or postdepositional events prior to their burial by the overlying beds. Various forms of gravitational and tidal processes caused the deformation of the Knik Arm deposits; events triggering the movement of the sediment include any or all of the following: (1) seismic activity, (2) sediment or ice loading, (3) wave-induced liquefaction, (4) pore-pressure changes brought on by changing levels of tidewater, (5) undercutting of sediments by channel migration, and (6) freezing and thawing.

Journal of Sedimentary Petrology↗
USGS1984

Rippled scour depressions on the inner continental shelf off central California

Side-scan sonar records taken during the recent Coastal Ocean Dynamics Experiment (CODE) show elongate, shore-normal rippled depressions of low relief on the inner continental shelf off central California between Bodega Bay and Point Arena. These features extend up to 2 km from the coast into water depths of up to 65 m. The proposed mechanism for their generation is storm-generated bottom currents associated with coastal downwelling during the late fall and winter which scour the surficial fine-sand sediment and expose the coarser-sand substrate in the depressions. The zones of most intense erosion and the irregular spacing of the features may be controlled by submerged rock ledges and other prominent coastal features. The large straight-crested ripples within the depressions (heights to 40 cm; wavelengths to 1.7 m) are probably formed by large-amplitude, long-period surface waves generated by winter storms.

California↗
USGS1985

A nomogram for interpreting slope stability of fine-grained deposits in modern and ancient-marine environments.

Design of the nomogram is based on effective stress and combines consolidation theory as applicable to depositional environments with the infinite-slope model of slope-stability analysis. The link between the two combined theories is a term representing the effective overburden stress, which may be predicted from consolidation theory and a knowledge of sedimentation rate, time, and the coefficient of consolidation. In turn, if infinite-slope conditions are assumed to exist, the effective overburden stress can be used to derive a factor of safety against static slope failure by using the angle of internal friction and the slope angle. The nomogram applies to depostitional settings in which fine-grained sediment has accumulated at a relatively constant rate upon a base that is essentially impermeable. The model further assumes that the lateral extent of sediment affected by any mass movement will be great compared to its thickness and that no outside agents (e.g., cements, gas) are influencing the section. The nomogram is applicable to static conditions (inherent stability of the slope) and certain dynamic conditions (such as earthquakes). It may be used to investigate mass movements in the geologic past as well as those in modern environments.--Modified journal abstract.

Journal of Sedimentary Petrology↗
USGS1985

Classification of deep-sea, fine-grained sediments

Most deep-sea sediments contain one or more biogenic components and one dominant nonbiogenic component, usually clay or silty clay. The authors present a descriptive classification scheme in which deep-sea, fine-grained sediments are placed within a three-components system of calcareous-biogenic, siliceous-biogenic, and nonbiogenic components. In a three-procedure the user assesses whether the dominant component is biogenic or nonbiogenic, whether the dominant biogenic component is siliceous or calcareous, and what the relative abundances of the biogenic components are within limits of 10, 25, and 50%. The terminology proposed is that commonly used by many sedimentologists, with some refinements and greater precision in the use of terms.

Journal of Sedimentary Petrology↗
USGS1985

The role of erosion by fish in shaping topography around Hudson submarine canyon.

An 800-km 2 area of rough topography around the head of Hudson Canyon off the eastern United States is attributed to erosion by tilefish ( Lopholatilus chamaeleonticeps ) and associated species of crustaceans. The rough topography has a relief of 1-10 m, occurs in water depths of 120-500 m, and has been cut into a semilithified, silty clay substrate since the onset of the Holocene transgression. Commercial fishing activity indicates that a large population of tilefish, which dig burrows in the sea floor, occupy the area of the rough topography. Average tilefish burrows are 1.6 m in diameter and 1.7 m in depth. They have a clustered, not uniform, distribution, and their average density is 2,500 per km 2 . The close match of areas of rough topography and high tilefish populations, the active burrowing of the sea floor, and the clustered distribution of the burrows suggest that the hummocky topography in this area may be the result of continuous erosion by tilefish and associated crustaceans during the Holocene. An erosion rate of 13 cm per 1,000 years is necessary to create this topography during the past 13,000 years--and 18 cm per 1,000 years if(as is more likely based on the depths at which tilefish presently are found) the erosion started 9,000 years ago.

New York↗
USGS1986

Danburite in evaporites of the Paradox basin, Utah.

Danburite (CaB 2 Si 2 O 8 ) has been found as nodules in Pennsylvanian age marine evaporites of the Paradox basin, Utah. Originally danburite had been known as a high-temperature mineral that occurs at numerous localities in igneous and metamorphic rocks. Since its discovery in water-insoluble residues from a Louisiana salt dome in 1937, it has been found in several other evaporites. The occurrence of danburite and its relation to the host rock in the Paradox basin evaporites indicates that it most likely formed by diagenetic reaction of boron-rich, high-salinity brines with constituents in the anhydrite host rock.

Journal of Sedimentary Petrology↗