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At least 19 recordsLinked to original sources
USGS1987

Petrologic and experimental evidence for the etching of garnets by organic acids in the upper Jurassic Morrision Formation, northwestern New Mexico

The upper Jurassic Morrison Formation in the southern part of the San Juan Basin contains uniquely etched detrital garnets characterized by smooth, crystallographically controlled faceted surfaces. The etched garnets occur in relatively homogeneous Morrison sandstones in discrete stratigraphic zones that are bounded above and below by sandstones bearing unetched garnets. Diagenetic alterations associated with etched garnets include etched staurolite, skeletal plagioclase, regularly interstratified illite-smectite, iron-rich chlorite, ankerite, albite, diagenetically altered (organic-poor) uranium ore, and extensive secondary porosity. Etching of garnets and partial to complete dissolution of other aluminosilicate minerals were caused by high concentrations of organic acids generated during the maturation of epigenetic organic matter (predominantly type-III kerogen) in the Morrison Formation. The presence of authigenic phases that form near 100 degrees C indicates that temperatures were high enough during diagenesis to cause the thermal degradation of kerogen. The subsequent release of reactive organic acids promoted a variety of water-rock reactions. This hypothesis was corroborated by experiments in which dicarboxylic acid solutions created facets on garnets identical to those developed on naturally etched Morrison garnets. The excellent complexing ability of organic acid anions accounts for the characteristic lack of alteration products on etched surfaces.

Journal of Sedimentary Petrology↗
USGS1987

Mineralogy and petrology of the Cretaceous- Tertiary boundary clay bed and adjacent clay-rich rocks, Raton Basin, New Mexico and Colorado.

The Cretaceous-Tertiary (K-T) boundary is preserved in a sequence of coal-bearing, fluvial rocks in the lower part of the Raton Formation (Late Cretaceous and Paleocene) at several sites in the east-central part of the Raton Basin. The K-T boundary occurs at the top of a kaolinitic claystone layer, commonly referred to as the "boundary clay layer," in an interval of coal and carbonaceous shale. The boundary is defined by the disappearance of certain fossil-pollen taxa. The boundary clay layer also contains shocked quartz grains and abundance anomalies of iridium, chromium, and other elements. Each of these characteristics support the hypothesis of an asteroid impact at the end of the Cretaceous. Small goyazite spheres were also found in the boundary clay bed; however, their origin is uncertain. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and thin-section petrography have shown that the boundary clay bed exhibits unique mineralogic and petrologic characteristics not possessed by other clay-rich beds in coal-bearing rocks of the Raton Basin. The boundary clay bed is composed primarily of well-crystallized kaolinite but also contains subordinate, randomly interstratified, mixed-layer illite/smectite, both of which appear to have been formed by the alteration of vitreous material in a coal-swamp environment. The bulk mineralogy of the boundary clay bed is similar to that of other kaolinite-rich clay beds (tonsteins) that altered in coal swamps from airfall volcanic ash. The alteration process and diagenetic products of the host material in the boundary clay bed, however, differed from those of the tonsteins, resulting in a unique petrographic fabric. The boundary clay bed is characterized by an overall imbricate fabric that may reflect an original vitreous material that included shards, bubbles, and spheres. The original fallout material of the boundary clay bed first altered to "cabbage-like" microspherules, indicating a halloysite or allophane precursor, and then to well-crystallized kaolinite; the well-crystallized kaolinite is the diagenetic stable phase for these rocks at their maximum burial conditions. In comparison, the vitreous components of the tonsteins were altered directly to coarse vermicular and platey kaolinite. Alteration of the original fallout material in the boundary bed was probably rapid, leading to hydrated, disordered, fine-grained clay minerals that formed into curled spherical or tubular shapes. The factors that produced these unique fabrics in the boundary clay bed existed at all K-T sites in the Raton Basin and were much different from those that formed the coarse platey and vermicular fabrics of ordinary tonsteins. The distinctive character of the boundary clay may be due to the unique nature and chemistry of the fallout material and (or) physical or chemical conditions existing shortly after its deposition.

Journal of Sedimentary Petrology↗
USGS1978

Factors controlling heavy-mineral variations on the South Texas outer continental shelf, Gulf of Mexico

Heavy-mineral distribution on the outer continental shelf off the southern coast of Texas shows regional variability induced by provenance and local variability reflecting genetic differences in sea-floor sediments. Q-mode factor analysis showed that three suites of heavy minerals are present. The southern ancestral Rio Grande delta sediments contain a distinct opaque-pyroxene-garnet suite, whereas the northern ancestral Brazes-Colorado delta sediments contain a tourmaline-green hornblende suite. An interdelta region contains a heavy mineral suite that is mixed owing to contributions from both ancestral deltas. Analysis of variance, correlation, and regression methods indicate that heavy-mineral variations in each sedimentary province have been significantly influenced by hydraulic fractionation by size, shape, and density, and by a selective chemical decomposition of unstable minerals. These factors have operated at varying degrees on the relict, palimpsest, and modern sediment populations of the sedimentary provinces since the end of Pleistocene time. The differential effects of these processes on the sediments have resulted in local variations that contribute to the total mineralogical variability. A comparison of the heavy mineral suites of the modern and relict Rio Grande delta sediments, which are derived from a common provenance, also shows mineral variations resulting from factors other than provenance.

Texas↗
USGS1978

Estimating pore and cement volumes in thin section

Point count estimates of pore, grain and cement volumes from thin sections are inaccurate, often by more than 100 percent, even though they may be surprisingly precise (reproducibility + or - 3 percent). Errors are produced by: 1) inclusion of submicroscopic pore space within solid volume and 2) edge effects caused by grain curvature within a 30-micron thick thin section. Submicroscopic porosity may be measured by various physical tests or may be visually estimated from scanning electron micrographs. Edge error takes the form of an envelope around grains and increases with decreasing grain size and sorting, increasing grain irregularity and tighter grain packing. Cements are greatly involved in edge error because of their position at grain peripheries and their generally small grain size. Edge error is minimized by methods which reduce the thickness of the sample viewed during point counting. Methods which effectively reduce thickness include use of ultra-thin thin sections or acetate peels, point counting in reflected light, or carefully focusing and counting on the upper surface of the thin section.

Journal of Sedimentary Petrology↗
USGS1979

Inverse grading and hydraulic equivalence in grain-flow deposits

Inversely graded grain-flow deposits are characterized by a hydraulic equivalence that differs from that based on settling velocities or entrainment. Dispersive equivalence, derived from the dispersive pressure hypothesis on how inverse grading develops, was found to agree reasonably well with observed relationships between grain sizes and densities in grain-flow deposits. Furthermore, observed relationships in deposits formed in subaerial and subaqueous environments were found to be independent of fluid density as is required by dispersive equivalence. The results suggest that dispersive pressure controls the development of the inverse grading common to beach foreshore laminations, slip-face foreset strata, the basal parts of some coarse-grained turbidites, and other diverse deposits.

Journal of Sedimentary Petrology↗
USGS1979

Fresh-water cementation of a 1,000-year-old oolite

Calcite cementation of aragonite ooid sand is producing oolite on Joulters Cays, Bahamas. During the last 1,000 years, calcite cement has formed at an average rate of between 27 and 55 cm 3 /m 3 /yr and is derived from dissolution of ooid aragonite in fresh water. The dissolution-reprecipitation of carbonate minerals in the aquifer results in ground waters of unusually high Sr content. Sea water and mixtures of fresh and sea water appear to inhibit cementation. A pronounced cement fabric change occurs across the water table and has produced an obvious petrographic record of fresh-water diagenesis. Above the water table, cement is typically near grain contact positions, where water is held by capillarity; below the water table, cement is more randomly distributed around grains. At the water table a transition zone, 1 meter thick, marks the boundary between cement textures. No porosity reduction is associated with cementation; calcite cement precipitation is apparently compensated by an equal or greater amount of aragonite dissolution in the interval undergoing cementation. Permeability is more variable above the water table than below it, reflecting early channelling of flow patterns in the vadose zone. Effective permeability below the water table is one to two orders of magnitude higher than above the water table because of entrained gas in the vadose zone. This permeability difference promotes preservation of unstable minerals above the water table and continued diagenetic alteration below the water table.

Joulters Cays↗
USGS1980

Comparison of bed form variance spectra within a meander bend during flood and average discharge

Time series analysis of streambed elevation in a meander bend along the Congaree River was used to determine the changes in bed form population succeeding a 16-year flood event. Bed forms observed during the flood event had a significantly greater total height variance than bed forms observed at the same location one week later. Variance spectra were computed for a 595 m longitudinal profile. The data indicate that: a) the bed form variance for the flood record is significantly greater for all wavelengths from 5 to 30 m; b) no well-demarcated bed form classes were present during the survey times, pointing to the possible existence of a continuum of bed form sizes rather than well-defined classes; and c) bed forms produced by the flood discharge were rapidly altered as the stage returned toward average level.

Journal of Sedimentary Petrology↗
USGS1980

The provenance of rutile.

Most coarse detrital rutile is derived from high-grade metamorphic rocks. Contrary to a conventional assumption, independent rutile grains are particularly rare in igneous rocks except alkalic rocks. The use of rutile in the ZTR (zircon-tourmaline-rutile) index of mineralogic maturity is only partially valid, owing to its restricted provenance.

Journal of Sedimentary Petrology↗
USGS1980

Opaque minerals as aids in distinguishing between source and sorting effects on beach sand mineralogy in southwestern Oregon

Both source area and wave sorting affect the heavy-mineral distributions of beach sands. Although source and sorting effects are difficult to distinguish, they can be separated on four Oregon beaches by studying the magnetic fraction of the sand. Prominent headlands bound the four beaches, which are located on the southwestern Oregon coast. On three beaches the percentage of magnetite in the sand from the upper swash zone consistently increases toward the north end of each beach, apparently owing to selective sorting during littoral transport. However, the percentages of chromium and titanium in the magnetite are generally independent of the relative location of the samples along the beach and therefore independent of sorting effects. Each beach appears to be characterized by a fairly distinct range of Ti/Cr in the magnetic fraction, and the range differs from beach to beach. The magnetite percentage and the Ti/Cr range suggest that sorting and source effects can be distinguished on beaches rich in "black" sands, and that sand transport around headlands in this area is not extensive enough to mask source differences. The results of this study suggest that sorting and source effects can be distinguished wherever there are chemical or mineralogical differences in a restricted density fraction.

Journal of Sedimentary Petrology↗
USGS1980

Paleoenvironment of the New Albany Shale Group ( Devonian- Mississippian) of Illinois

The distribution of lithofacies in the New Albany Shale Group of Illinois was determined by wave energy, bottom oxygenation, and bottom topography in a deep water stratified anoxic basin. A transect from the margin to the center of the Illinois Basin reveals a complete transition from high energy, aerobic, shallow-water environments to quiet, anaerobic, deep-water environments. Shallow areas at the margin of the basin are characterized by rapid facies transitions over short distances. High energy, very shallow conditions are recorded by oolitic-skeletal grainstones and packstones with abundant brachiopods, crinoids, trilobites, and other calcified marine invertebrates. Bioturbation did not destroy primary sedimentary structures in these facies. Offshore, less agitated areas are represented by highly bioturbated carbonate wackestones, argillaceous quartz siltstones, and greenish-gray mudstones. Calcified invertebrates are generally rare in these facies, indicating deposition in dysaerobic conditions. Basinward, slope areas are characterized by olive-gray to black, weakly bioturbated shales commonly interbedded with thickly laminated black shales. Trace fossils, including Zoophycos, Chondrites , and Planolites , are abundant along the bases of the olive-gray beds. In areas where the anaerobic/dysaerobic boundary intersected the bottom slope, slight fluctuations of the position of the boundary resulted in thin interbedding of olive-gray and black shales and laterally persistent interfingering of the two lithologies. Anaerobic conditions prevailed during most of New Albany time in the deepest areas of the basin, and finely laminated, undisturbed, pelagic black shales were deposited.

Journal of Sedimentary Petrology↗
USGS1980

Littoral transport in the surf zone elucidated by an Eulerian sediment tracer

An Eulerian, or time integration, sand tracer experiment was designed and carried out in the surf zone near Pt. Mugu, California on April 19, 1972. Data indicate that conditions of stationarity and finite boundaries required for proper application of Eulerian tracer theory exist for short time periods in the surf zone. Grain counts suggest time required for tracer sand to attain equilibrium concentration is on the order of 30-60 minutes. Grain counts also indicate transport (discharge) was strongly dependent upon grain size, with the maximum rate occurring in the size 2.5-2.75phi (0.18-0.15 mm) decreasing to both finer and coarser sizes. The measured instantaneous transport was at the annual rate of 2.4 x 10 6 m 3 /yr.

Journal of Sedimentary Petrology↗
USGS1980

Thickness change involved in the peat-to- coal transformation for a bituminous coal of Cretaceous age in central Utah

The ratio of the thicknesses of a layer of peat and the coal bed formed from that peat has been calculated for a bituminous coal bed in central Utah. The method used involves comparison of the thickness of peat eroded by a laterally migrating channel system with the thickness of coal now absent from the outcrop. The peat:coal thickness ratio calculated by this method is approximately 11:1.

Journal of Sedimentary Petrology↗
USGS1981

Origin of chert grains and a halite- silcrete bed in the Cambrian and Ordovician Whitehall Formation of eastern New York State

A chert bed in the Whitehall Formation (Cambrian and Ordovician) of eastern New York State is strikingly similar in petrography and inferred origin to Australian and South African silcretes. The chert in the Whitehall, like its Australian and South African counterparts, occurs along an erosion surface that formed subarially, and it contains colloform chalcedony and abundant ferruginous minerals. This chert also contains pseudomorphs and ghosts of halite. Silica precipitated from a solution that became enriched in electrolytes as a result of dissolving halite. Sand-size chert grains in the Whitehall are petrographically like the Whitehall silcrete and are probably grains of reworked silcrete.

Journal of Sedimentary Petrology↗
USGS1981

Progradational sequences in Miocene shoreline deposits, southeastern Caliente Range, California

An exceptionally well exposed marine-nonmarine transition in middle Miocene strata exists in the southeastern Caliente Range, California. About 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m (where predominantly nonmarine) to more than 2500 m (where predominantly marine). Paleogreographic evidence in basalt flows near the top of the succession and in overlying fluvial deposists indicates that these middle Miocene strata were deposited across a north-northwest trending shoreline. A complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units. Individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface. The gravel is overlain by structureless siltstone or fine-grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes. These rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation. Crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of failry long-period surface waves. The bedded fine sandstone is sharply overlain by a crossbedded coarse sandstone facies that is interpreted as a combined offshore bar-rip channel-surf zone assemblage. Cross-strata dip dominantly offshore, suggesting substantial deposition from rip currents. A secondary, shore=parallel mode of cross-strata direction suggests longshore currents produced by surface waves from the northwest. The crossbedded coarse-grained sandstone grades upward into planar-bedded medium-grained sandstone that is interpreted as a beach foreshore. This facies grades upward through structureless medium-grained sandstone into nonmarine or lagoonal red and green mudstone of the Caliente Formation. The middle Miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically; the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m.y. The average duration of the transgressive-regressive cycles, a few tens of thousands of years, together with their distribution in groups of three or four in the lower two-thirds of the succession, is consistent with the pattern of long-term climatic cycles produced by periodicity of the earth's solar orbit and may be related to eustatic sea level changes attendant to the development of the Antarctic ice cap. Changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may hav been precursors to the onset of movement along the San Andreas fault in this area 12-14 m.y. ago.

California↗
USGS1981

Swash mark and grain flow

Swash marks composed entirely of coarse sand are commonly found on coarse-sand beaches. These swash marks are 10 to 30 centimeters in width and a few millimeters to one centimeter in height. Previous observations, mostly on finer-sand beaches, indicate swash marks are seldom over a few millimeters in height and are commonly composed of material readily floated by surface tension (e.g., mica flakes and shell fragments). Swash marks composed of coarse sand have both fining seaward and fining with depth trends in grain size. Apparently, the leading margin of a wave upwash drives a highly concentrated flow of grains in which both grain size and grain velocity decrease with depth. Therefore, large grains are transported at greater velocities than are smaller grains. Thus, at the maximum advance of an upwash, a swash mark is deposited which has the observed fining seaward and fining with depth trends in grain size.

Journal of Sedimentary Petrology↗
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↗