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At least 73 records · Page 4Linked to original sources
USGS1979

Clay mineralogy, fine-grained sediment dispersal, and inferred current patterns, lower Cook Inlet and Kodiak shelf, Alaska

Because lower Cook Inlet and Kodiak shelf are being explored and developed for their petroleum resources, it is essential for environmental reasons to understand the sediment dispersal routes and current patterns. The Susitna River flows into upper Cook Inlet and is the source of clay minerals in Holocene deposits found in western lower Cook Inlet. The Copper River, in the northern Gulf of Alaska, provides clay minerals to the Kodiak shelf and southeastern lower Cook Inlet. In addition, crosion of local bedrock outcrops on the shelf produces some clays that are deposited on the Kodiak shelf. Current patterns can be inferred from the clay-mineral distribution pattern. This is true even if the clay-size fraction is a minor sediment component, and in areas where coarse-grained relict deposits occur. Some potential dangers from offshore petroleum development include: (1) rapid and complete mixing of Cook Inlet waters, (2) adsorption of pollutants by clay deposited in quiet bays, and (3) ion-exchange and adsorption of chemical pollutants on clays that are part of the suspended sediment load in lower Cook Inlet.

Alaska↗
USGS1980

Identification of bedforms in lower Cook Inlet, Alaska

The seafloor of the central part of lower Cook Inlet, Alaska, is characterized by the presence of different sizes and types of bedforms. The bedforms in the sandy sediments include straight-crested to sinuous to lunate ripples, small, medium, and large sand waves, sand ridges, sand ribbons, and sand patches. In addition, rocky and pebbly seafloor has been identified. The water depth ranges from 25 to 120 m, and surface currents average 3.8 kt (2 m/s). Bottom currents have been measured at as much as 42 cm/s at 1 m above bottom. Underwater television observations have shown that the rate of sand transport is lower than expected because small amounts of clay and organic matter appear to inhibit remobilization. Only during the last 1 to 2 h of ebb and flood stages of spring tides, and during storms, does significant transport occur. Comparison of data from high-resolution seismic profiling systems, side-scan sonar, bottom television and camera, and bottom sampling shows that bottom and bedform interpretations based solely on sonographs can be in error. Measuring the length of ‘acoustic shadows’ on sonographs to obtain bedform heights gives dimensions that are too large by factors of 3–7. Bottom television investigations revealed that the troughs between small sand waves are flat and carpeted by shell fragments. Such coarse material has a high acoustic reflectance that is not related to slope or height and can lead to false interpretations on bedform dimensions. Our observations have shown that small sand waves commonly superimposed on larger ones are slightly higher than those present on flat hard bottom but are still less than calculated from acoustic shadows. Where the bottom is rather smooth or contains elevations small enough to be masked by bathymetric ‘noise’ caused by the pitching of the vessel, sonographs typically show either small sand waves, sand ribbons, sand patches, rocks, or smooth bottom. The smooth-bottom category can vary widely from ripples to gravelly or shelly or to small rocks with biological overgrowth as verified by television observations. Our observations have clearly demonstrated the need for an integrated multi-scale observation and sampling program in order to classify the bottom characteristics and to provide quantitative data for transport calculations.

Alaska↗
USGS1980

Single and superimposed bedforms: A synthesis of San Francisco Bay and flume observations

Tidal currents with maximum depth-averaged velocities ranging up to 250 cm/sec have generated ripples, two- and three-dimensional sand waves, and upper flat beds on the floor of central San Francisco Bay. Determination of the hydraulic conditions under which the observed beds exist, indicates that the bed configuration at any point in the bay is a function of the local velocity, sediment size, and depth. The bay observations, for flows up to 85 m deep, were combined with shallow-flow observations and a single set of bed-phase boundaries was determined for the combined data. Critical shear velocities calculated for the transitions from ripples to sand waves and from sand waves to upper flat beds, in flows tens of meters deep, are within 10% of critical shear velocities observed for the same transitions in flume flows only tens of centimeters deep. Comparison of bedform sequences suggests that, for flows up to tens of meters deep, beds of 0.25–0.50 mm sand respond to increasing flow velocities by forming ripples, two-dimensional sand waves, three-dimensional sand waves, and flat beds. At any constant depth, equilibrium sand waves increase in height and migration rate as flow velocity increases. The wavelength and maximum height of both two- and three-dimensional sand waves increase with depth also, but migration rates decrease. Because the maximum size of both kinds of bedforms varies with depth, classification schemes based on size arbitrarily separate genetically similar bedforms. In the bay, in contrast to flumes, sand waves having the largest height-to-depth ratios occur in relatively coarse sand. Tidal and seasonal velocity fluctuations are interpreted to be more destructive to finer-grained sand waves, because in finer grain sizes sand waves are stable at a relatively narrow range of velocities. Ripples, sand waves, and upper and lower flat beds are commonly superimposed on larger bedforms. Small bedforms can exist in equilibrium on the larger bedforms because the large bedforms generate boundary layers in which the small bedforms are locally stable. The distribution of small bedforms superimposed on larger bedforms reflects lateral and vertical variations in shear velocity in flow over large bedforms.

California↗
USGS1980

Depositional environments of some Pleistocene coastal terrace deposits, southwestern Oregon - case history of progradational beach and dune sequence.

These deposits comprise a basal gravelly unit and 3 overlying sandy units, each with mud beds, a paleosol, or the modern soil in its uppermost part. The gravelly unit is interpreted as a progradational deposit. The main parts of the sandy units are made up of 1) a crossbedded sand facies, the dominant structure in which is medium-scale crossbedding (interpreted as the product of small eolian dunes), and 2) an irregularly bedded sand facies, which is locally pebbly and is dominated by scour-and-fill structures, interpreted as deposits of interdune ephemeral streams, ephemeral ponds, and wet to dry subaerial flats. The mud beds and paleosols represent times of temporary stabilization of the dune field.- from Author

Sedimentary Geology↗
USGS1980

Experimental study of free-surface flow instability and bedforms in shallow flows.

To test erosion and development of bed sculpture in unstable, pulsating shallow flow, 3 series of experiments were performed in a 10-m flume with flows of Reynolds Number up to 103, Froude Number up to 3.0, and slope up to 2%. In the first, the bed was rigid, in the second and third series planar beds were made from well-sorted and poorly sorted sand, respectively. Development of flow surface instability above the rigid bed showed the following stages: smooth surface; small wavelets; well-developed waves; and rough flow. The second series showed the following sequence: smooth bed; flow-aligned ridge and trough pattern; combined ridge and rhomboid pattern; and rhomboid configuration. The third series demonstrated transverse segregation of the bed material across the ridges.- from Authors

Sedimentary Geology↗
USGS1980

Debris-flow deposits of Early Miocene age, Deadman Stream, Marlborough, New Zealand.

Detailed analysis is presented of a conformable succession of conglomerates and sandstones lying between massive marine mudstones. The coarse sediments reflect deposition by a spectrum of subaqueous debris-flow mechanisms during an early pulse of tectonism that ultimately resulted in Plio-Pleistocene eversion of the Kaikoura Mountains. Sparse pebbly mudstones and rare sandy conglomerates show disoriented clasts and reflect high-viscosity flows and slurry- creep flow mechanisms. Other deposits have little mud matrix, hence appear to reflect low-viscosity flow processes. Common sorted sandstones and some conglomeratic sandstones show diffuse parallel lamination. Other conglomeratic sandstones show trough cross-bedding which we attribute to entrained bedload movement during intersurge episodes of debris flow. - from Authors

Sedimentary Geology↗
USGS1983

Heavy-mineral distribution in modern and ancient bay deposits, Willapa Bay, Washington, U.S.A.

Analysis of heavy-mineral distribution in modern sediments of Willapa Bay, Washington, indicates a dominance of two mineralogic assemblages, one with approximately equivalent amounts of hornblende, orthopyroxene and clinopyroxene, the other dominated by clinopyroxene. The hornblende-orthopyroxene-clinopyroxene suite is derived from the Columbia River, which discharges into the ocean a short distance south of the bay. The clinopyroxene suite is restricted in modern sediments to sands in rivers flowing into the bay from the east. The heavy-mineral distributions suggest that sand discharged from the Columbia River, borne north by longshore transport, and carried into the bay by tidal currents accounts for most of the sand within the interior of Willapa Bay. Three heavy-mineral assemblages are present in the surrounding Pleistocene deposits; two of these are identical to the modern assemblages described above. These heavy-mineral assemblages reflect the relative influence of tidal and fluvial processes on the Late Pleistocene deposits; their relative influences are consistent with those inferred on the basis of sedimentary structures and stratigraphic relations in about two-thirds of the samples examined. The anomalies can be explained by recycling of sand from older deposits. The persistence of the two heavy-mineral assemblages suggests that the pattern of estuarine sedimentation in Late Pleistocene deposits closely resembled that of the modern bay. The third heavy-mineral suite, dominated by epidote, occurs in a few older Pleistocene units. On the north side of the bay, the association of this suite with southwest-directed foresets in crossbedded gravel indicates derivation from the northeast, perhaps from an area of glacial outwash. The presence of this suite in ancient estuarine sands exposed on the east side of the bay suggests that input from this northerly source may have intermittently dominated bay deposition in the past.

Washington↗
USGS1984

Fluvial sedimentation on a quivering craton: Influence of slight crustal movements on fluvial processes, upper Jurassic Morrison formation, western Colorado Plateau

One of the most important challenges facing the fluvial sedimentologist is identification of processes outside the stream channel that influence deposition of fluvial sediments. Detailed studies in the lower sequence of the Salt Wash Member (Morrison Formation, Upper Jurassic) demonstrate that crustal deformation at the site of deposition may considerably influence braided-stream processes. Late Jurassic crustal movements in the western part of the Colorado Plateau are interpreted largely from thickness variations and facies distribution, but other features such as vertical repetition of facies, coincidence with at least parts of present-day folds, and the geographic distribution of bedding parameters measured in the fluvial deposits, are also used as corroborating evidence of syndepositional tectonism. These features indicate that several of the large uplifts and basins in the region as well as some of the smaller folds within them were actively moving during deposition of the lower sequence. Tectonic activity altered the stream gradients, which in turn governed sinuosity, flow regime, energy levels, and sediment distribution. Cross-bedding studies indicate that reduced gradients within downwarped areas led to slight increases in sinuosity of the braided-stream channels and of the small sub-channels within them. The lowered gradients apparently resulted in a decrease in the depth of the channels and allowed the streams to flood more readily, producing abundant upper-flow regime horizontal laminations in the channel deposits. In addition, greater quantities of sediment containing higher proportions of sand were deposited in downwarped areas than in positive localities. The inability of the streams to transport bed load through downwarped areas indicates loss of stream energy. However, an increase in the quantity of upper-flow regime horizontal laminations in the same downwarped areas suggests that an increase in flow regime is not necessarily accompanied by an increase in energy levels, at least in regions of slight tectonic activity where the local configuration of the stream channels may change appreciably. Strata presently dip less than 2° throughout most of the region, and this relatively small amount of deformation reflects the combined effects of Late Jurassic, Cretaceous and Tertiary tectonism. This demonstrates that the amount of structural deformation at the site of deposition may appear to be insignificant, yet it can cause appreciable changes in the nature of braided-stream deposits.

Arizona, Utah↗
USGS1984

Depositional environments and paleogeography of the Upper Miocene Wassuk Group, west-central Nevada

Fluvial and lacustrine deposits of the Miocene Wassuk Group, exposed in Coal Valley, west-central Nevada, are divided into five lithofacies: (1) diatomite, claystone, siltstone, and carbonaceous siltstone deposited in a lake with paludal conditions at the margin; (2) upward-coarsening sequences of sandstone deposited on a delta and fan-delta; (3) channel-form sandstone deposited on a distal braided alluvial plain; (4) clast-supported conglomerate deposited on a proxial braided alluvial plain or distal alluvial fan; and (5) matrix-supported conglomerate deposited on a distal to middle alluvial fan. Petrographic analysis records an upsection change from a predominantly andesitic to a predominantly plutonic provenance. This change, combined with the overall upward-coarsening of the Wassuk Group and the great thickness (2400 m) of the sequence, suggests active uplift and rapid subsidence during deposition of the group. Facies relationships and paleocurrent directions indicate source areas to the south, southeast and west of Coal Valley. The Miocene Wassuk Group was deposited in an intra-arc basin with penecontemporaneous volcanism and tectonic activity. Syndepositional faulting at the southern margin of Coal Valley between 13 and 11 m.y. ago suggests an early episode of northeast-southwest extension prior to the onset of east-west basin and range extension.

Nevada↗
USGS1984

Oligocene tectonics and sedimentation, California

During the Oligocene epoch, California was marked by extensive nonmarine sedimentation, in contrast to its pre-Oligocene and post-Oligocene depositional history. The Oligocene continental deposits are especially widespread in southern California and fill a number of small and generally partly restricted basins. Fluvial facies in many basins prograded over previously deposited lower Tertiary turbidites. Volcanism, from widespread centers, was associated with the nonmarine sedimentation. However, some basins remained marine and a few contain Oligocene turbidites and pelagic sediments deposited at bathyal depths. The Oligocene redbeds of California do not form a post-orogenic molasse sequence comparable to the Old Red Sandstone or Alpine molasse. They are synorogenic and record local uplift of basins and surrounding source areas. Late Cretaceous to contemporary orogenesis in California has been generally characterized by the formation of small restricted basins of variable depth adjacent to small upland areas in response to strike-slip faulting. Deposition of Oligocene redbeds was associated with climatic change from warm and humid to cold and semiarid, and a global lowering of sea level. Oligocene tectonism occurred during the transition from subduction of the Farallon Plate to initiation of the modern San Andreas transform system. However, the major influence that caused uplift, formation of fault-bounded basins, and extensive redbed deposition, especially in southern California, was the approach of the Pacific—Farallon spreading ridge to the western margin of California.

California↗
USGS1986

Carbonate apron models: Alternatives to the submarine fan model for paleoenvironmental analysis and hydrocarbon exploration

Sediment gravity flow deposition along the deep-water flanks of carbonate platforms typically does not produce submarine fans. Rather, wedge-shaped carbonate aprons develop parallel to the adjacent shelf/slope break. The major difference between submarine fans and carbonate aprons is a point source with channelized sedimentation on fans, versus a line source with sheet-flow sedimentation on aprons. Two types of carbonate aprons may develop. Along relatively gentle (< 4°) platform-margin slopes, aprons form immediately adjacent to the shallow-water platform and are referred to as carbonate slope aprons . Along relatively steep (4–15°) platform margin slopes, redeposited limestones accumulate in a base-of-slope setting, by-passing an upper slope via a multitude of small submarine canyons, and are referred to as carbonate base-of-slope aprons . Both apron types are further subdivided into inner and outer facies belts. Inner apron sediments consist of thick, mud-supported conglomerates and megabreccias (Facies F) as well as thick, coarse-grained turbidites (Facies A) interbedded with subordinate amounts of fine-grained, peri-platform ooze (Facies G). Outer apron sediments consist of thinner, grain-supported conglomerates and turbidites (Facies A) as well as classical turbidites (Facies C) with recognizable Bouma divisions, interbedded with approximately equal proportions of peri-platform ooze (Facies G). Seaward, aprons grade laterally into basinal facies of thin, base-cut-out carbonate turbidites (Facies D) that are subordinate to peri-platform oozes (Facies G). Carbonate base-of-slope aprons grade shelfward into an upper slope facies of fine-grained peri-platform ooze (Facies G) cut by numerous small canyons that are filled with coarse debris, as well as intraformational truncation surfaces which result from submarine sliding. In contrast, slope aprons grade shelfward immediately into shoal-water, platform-margin facies without an intervening by-pass slope. The two carbonate apron models presented here offer alternatives to the submarine-fan model for paleoenvironmental analysis and hydrocarbon exploration for mass-transported carbonate facies.

Sedimentary Geology↗
USGS1988

Suspended sediment transport under estuarine tidal channel conditions

A modified version of the GEOPROBE tripod has been used to monitor flow conditions and suspended sediment distribution in the bottom boundary layer of a tidal channel within San Francisco Bay, California. Measurements were made every 15 minutes over three successive tidal cycles. They included mean velocity profiles from four electromagnetic current meters within 1 m of the seabed; mean suspended sediment concentration profiles from seven miniature nephelometers operated within 1 m of the seabed; near-bottom pressure fluctuations; vertical temperature gradient; and bottom photographs. Additionally, suspended sediment was sampled from four levels within 1 m of the seabed three times during each successive flood and ebb cycle. While the instrument was deployed, STD-nephelometer measurements were made throughout the water column, water samples were collected each 1–2 hours, and bottom sediment was sampled at the deployment site. From these measurements, estimates were made of particle settling velocity ( w s ) from size distributions of the suspended sediment, friction velocity (U * U &#x2217; "> ) from the velocity profiles, and reference concentration ( C a ) was measured at z = 20 cm. These parameters were used in the suspended sediment distribution equations to evaluate their ability to predict the observed suspended sediment profiles. Three suspended sediment particle conditions were evaluated: (1) individual particle size in the 4–11 φ (62.5-0.5 μm) range with the reference concentration C a at z = 20 cm ( C φ ), (2) individual particle size in the 4–6 φ size range, flocs representing the 7–11 φ size range with the reference concentration C a at z = 20 cm ( C f ), and (3) individual particle size in the 4–6 φ size range, flocs representing the 7–11 φ size range with the reference concentration predicted as a function of the bed sediment size distribution and the square of the excess shear stress. In addition, computations of particle flux were made in order to show vertical variations in horizontal mass flux for varying flow conditions.

California↗
USGS1988

Paleolimnology of Lake Tubutulik, an iron-meromictic Eocene Lake, eastern Seward Peninsula, Alaska

Sideritic lacustrine mudstone was found in drill core from a uranium deposit in the Death Valley area in the eastern part of the Seward Peninsula, Alaska. The precursor sediments for this rock were deposited in an unusual “iron-meromictic” Eocene lake, herein named Lake Tubutulik, which occupied part of the Boulder Creek basin, a structural graben that is probably a southern extension of the larger Death Valley basin. The Boulder Creek basin is bounded on the west by granite of the Late Cretaceous Darby Pluton, on the east by Precambrian to Paleozoic metasedimentary rocks. The lake basin was formed by basaltic flows that dammed the river valley of the ancestral Tubutulik River in early Eocene time. Lake Tubutulik contained a nearshore facies of fine-grained organic mud and an offshore facies of laminated sideritic mud. The offshore (profundal) laminated mudstone consists of alternating layers of authigenic siderite and detrital layers containing mostly quartz and clay minerals. Both lacustrine facies contain turbidities. The lacustrine sediments graded laterally into an onshore facies of colluvial and fluvial sandstone, paludal mudstone, and coal. The ancient lake apparently occupied a small deep basin in a tectonically active area of high relief. Meromixus was probably stabilized by reduced iron and bicarbonate dissolved in the monimolimnion. The intensity of meromixus decreased as the lake became shallower from sediment filling. The source of the iron, abundant in the monimolimnion of Lake Tubutulik, was probably the Eocene basalt. Based on carbon isotope analysis of the siderite, the dissolved bicarbonate in the profundal facies was largely inorganic. Sideritic carbon in one sample from the onshore paludal facies has an isotopic signature ( δ 13 C = +16.9) consistent with residual carbon formed during methanogenic fermentation.

Alaska↗
USGS1988

Synthesis of late Paleozoic and Mesozoic eolian deposits of the Western Interior of the United States

Late Paleozoic and Mesozoic eolian deposits include rock units that were deposited in ergs (eolian sand seas), erg margins and dune fields. They form an important part of Middle Pennsylvanian through Upper Jurassic sedimentary rocks across the Western Interior of the United States. These sedimentary rock units comprise approximately three dozen major eolian-bearing sequences and several smaller ones. Isopach and facies maps and accompanying cross sections indicate that most eolian units display varied geometry and complex facies relations to adjacent non-eolian rocks. Paleozoic erg deposits are widespread from Montana to Arizona and include Pennsylvanian formations (Weber, Tensleep, Casper and Quadrant Sandstones) chiefly in the Northern and Central Rocky Mountains with some deposits (Hermosa and Supai Groups) on the Colorado Plateau. Lower Permian (Wolfcampian) erg deposits (Weber, Tensleep, Casper, Minnelusa, Ingleside, Cedar Mesa, Elephant Canyon, Queantoweap and Esplanade Formations) are more widespread and thicken into the central Colorado Plateau. Middle Permian (Leonardian I) erg deposits (De Chelly and Schnebly Hill Formations) are distributed across the southern Colorado Plateau on the north edge of the Holbrook basin. Leonardian II erg deposits (Coconino and Glorieta Sandstones) are slightly more widespread on the southern Colorado Plateau. Leonardian III erg deposits formed adjacent to the Toroweap-Kaibab sea in Utah and Arizona (Coconino and White Rim Sandstones) and in north-central Colorado (Lyons Sandstone). Recognized Triassic eolian deposits include major erg deposits in the Jelm Formation of central Colorado-Wyoming and smaller eolian deposits in the Rock Point Member of the Wingate Sandstone and upper Dolores Formation, both of the Four Corners region. None of these have as yet received a modern or thorough study. Jurassic deposits of eolian origin extend from the Black Hills to the southern Cordilleran arc terrain. Lower Jurassic intervals include the Jurassic part of the Wingate Sandstone and the Navajo-Aztec-Nugget complex and coeval deposits in the arc terrain to the south and west of the Colorado Plateau. Major Middle Jurassic deposits include the Page Sandstone on the Colorado Plateau and the widespread Entrada Sandstone, Sundance Formation, and coeval deposits. Less extensive eolian deposits occur in the Carmel Formation, Temple Cap Sandstone, Romana Sandstone and Moab Tongue of the Entrada Sandstone, mostly on the central and western Colorado Plateau. Upper Jurassic eolian deposits include the Bluff Sandstone Member and Recapture Member of the Morrison Formation and Junction Creek Sandstone, all of the Four Corners region, and smaller eolian deposits in the Morrison Formation of central Wyoming and apparently coeval Unkpapa Sandstone of the Black Hills. Late Paleozoic and Mesozoic eolian deposits responded to changing climatic, tectonic and eustatic controls that are documented elsewhere in this volume. All of the eolian deposits are intricately interbedded with non-eolian deposits, including units of fluvial, lacustrine and shallow-marine origin, clearly dispelling the myth that eolian sandstones are simple sheet-like bodies. Rather, these units form some of the most complex bodies in the stratigraphic record.

Western Interior↗
USGS1988

Pennsylvanian to Jurassic eolian transportation systems in the western United States

The direction of sediment transport in eolian sandstones of Pennsylvanian to Jurassic age was interpreted from crossbedding resultants (vector means) obtained from studies of eolian rocks in the western U.S., supplemented by data from the few eolian units of eastern North America. These were compiled from the published or unpublished (theses) literature, from unpublished field data contributed by colleagues, or from measurements made for this study. In addition, new paleogeographic maps were compiled to evaluate the influence of geographic features on the atmospheric circulation patterns that are inferred from the crossbedding studies. Regionally, the crossbedding indicates northeasterly, northerly, or northwesterly winds (present coordinates) from Pennsylvanian through most of Middle Jurassic time. A rather abrupt change in wind directions occurred in late Middle Jurassic time (late part of the Callovian Age) when westerly wind patterns developed. By the Late Jurassic the winds shifted to southwesterly. Calculations of the consistency factor (vector mean strength) made from region-wide analyses of the resultants indicate fairly unidirectional winds from the Pennsylvanian through the Early Jurassic. Middle Jurassic circulation was more varied, judging from crossbedding studies in the lower part of the Entrada Sandstone. Crossbedding in Upper Jurassic eolian rocks of Wyoming and South Dakota yielded a random pattern but Upper Jurassic rocks farther south on the Colorado Plateau and adjoining areas show a return to a fairly unidirectional pattern. Comparing the resultants with their reconstructed paleogeographic setting shows surprisingly little influence of major geographic features on overall circulation patterns. However, the greatest amount of local variation occurred at or near highly indented shorelines where the temperature contrast between land and water produces local wind currents that may vary appreciably from regional circulation patterns. Although they do not cause noticeable horizontal deflections in wind patterns, small and low topographic highs appear to be able to promote the development of a dune field if a source of sand is available and if streams do not enter the growing dune field.

western United States↗
USGS1988

Wind directions predicted from global circulation models and wind directions determined from eolian sandstones of the western United States - A comparison

Wind directions for Middle Pennsylvanian through Jurassic time are predicted from global circulation models for the western United States. These predictions are compared with paleowind directions interpreted from eolian sandstones of Middle Pennsylvanian through Jurassic age. Predicted regional wind directions correspond with at least three-quarters of the paleowind data from the sandstones; the rest of the data may indicate problems with correlation, local effects of paleogeography on winds, and lack of resolution of the circulation models. The data and predictions suggest the following paleoclimatic developments through the time interval studied: predominance of winter subtropical high-pressure circulation in the Late Pennsylvanian; predominance of summer subtropical high-pressure circulation in the Permian; predominance of summer monsoonal circulation in the Triassic and earliest Jurassic; and, during the remainder of the Jurassic, influence of both summer subtropical and summer monsoonal circulation, with the boundary between the two systems over the western United States. This sequence of climatic changes is largely owing to paleogeographic changes, which influenced the buildup and breakdown of the monsoonal circulation, and possibly owing partly to a decrease in the global temperature gradient, which might have lessened the influence of the subtropical high-pressure circulation. The atypical humidity of Triassic time probably resulted from the monsoonal circulation created by the geography of Pangaea. This circulation is predicted to have been at a maximum in the Triassic and was likely to have been powerful enough to draw moisture along the equator from the ocean to the west.

western United States↗
USGS1988

Pin stripe lamination: A distinctive feature of modern and ancient eolian sediments

Pin stripe laminations are a distinctive feature of modern and ancient eolian sediments. In sets of eolian ripple (or translatent) strata they represent deposition of silt and very fine sand in the troughs of the advancing wind ripples. In sets of avalanche strata they probably result from the downward settling of fine sand and silt within the moving avalanche to the interface of moving and unmoving sands. Wind tunnel experiments suggest that pin stripe laminations can also form in grainfall deposits. The textural segregation associated with deposition of the fine layers in most cases leads to early cementation along and near the finest sand and silt comprising the pin stripe lamination. The pin stripe effect seen in outcrops is usually due to resistance to weathering along such cemented zones. The cementation of the pin stripe laminations can occur early in the history of diagenesis and thus may provide clues to the post-depositional history of the rock. Pin stripe laminations in many instances represent the sequestering of the small population of ultrafine sediment present in most eolian depositional systems. They may prove useful in the recognition of ancient eolian sediments.

Sedimentary Geology↗
USGS1988

Daily cycles in coastal dunes

Daily cycles of summer sea breezes produce distinctive cyclic foreset deposits in dune sands of the Texas and Oregon coasts. In both areas the winds are strong enough to transport sand only during part of the day, reach a peak during the afternoon, and vary little in direction during the period of sand transport. Cyclicity in the foreset deposits is made evident by variations in the type of sedimentary structure, the texture, and the heavy-mineral content of the sand. Some of the cyclic deposits are made up entirely of one basic type of structure, in which the character of the structure varies cyclically; for example, the angle of climb in a climbing-wind-ripple structure may vary cyclically. Other cyclic deposits are characterized by alternations of two or more structural types. Variations in the concentration of fine-grained heavy minerals, which account for the most striking cyclicity, arise mainly because of segregation on wind-rippled depositional surfaces: where the ripples climb at low angles, the coarsegrained light minerals, which accumulate preferentially on ripple crests, tend to be excluded from the local deposit. Daily cyclic deposits are thickest and best developed on small dunes and are least recognizable near the bases of large dunes.

California, Oregon, Texas↗