Climatic significance of natric horizons in Permian (Asselian) palaeosols of north-central Kansas, USA

Columnar structured horizons have been recognized in ancient coastal palaeosols of several Lower Permian (Asselian) stratigraphic units of north-central Kansas. These strongly developed columnar, polygonal-shaped peds are characteristic of sodium-influenced (natric) argillic horizons, and are commonly indicative of semi-arid to arid environments. Evaporite features above and below these palaeosols support the conclusion for a dry palaeoclimate. The columnar peds are typically 3–15 cm in diameter and exhibit domed tops. Fine clay fills the cracks between the columnar peds, and is generally of a darker colour than the peds. Each natric horizon has a low value and chroma colour, apparently the result of carbonate accumulation. The natric horizons in these Permian palaeosols appear to have been partially influenced by sodium-rich groundwaters. Root traces and root moulds are found between peds in all natric horizons, indicating plant succession after columnar ped formation. These sodium-influenced palaeosol profiles occur as part of a spectrum of palaeosol types that indicate cyclical climate change associated with glacioeustatic sea-level fluctuations.     

Plio-Pleistocene sedimentation in coastal grabens,north-east Rhodes,Greece

The stratigraphy of marine Plio-Pleistocene sediments from northeast Rhodes (Dodecanese islands, Greece) is revised in the light of facies mapping and the recognition of three major transgression–regression cycles. Before late Pliocene submergence, metamorphosed Mesozoic limestones formed a high-relief karstic landscape. During subsequent transgressions, subsiding basins with spectacular cliffed and bioeroded margins were infilled by a mosaic of carbonate-dominated sediments, sourced by high rates of carbonate productivity on narrow shelves rimming the basins and their drowning islands. Periods of relative sea-level fall superimposed a complex series of ‘fossil’ coastal geomorphological features such as cliffs, abrasion platforms, surf caves, notches, boulder beaches and palaeokarsts. These created the rugged present day topography of the island's northeast coast. Three main lithostratigraphic units, the Kritika, Rhodes and Lindos Acropolis Formations, are defined. The Rhodes and Lindos Acropolis Formations are subdivided into mappable facies groups rather than members, as the lithofacies are strongly diachronous. The Kritika and Rhodes Formations were each subaerially exposed and partly eroded before re-transgression. Although there is a general pattern of lithological succession, details vary across the study area and some facies groups are restricted in development, indicating differences in tectonic behaviour and palaeo-geomorphology between neighbouring basins. A preliminary process-response model is presented for the sedimentation history of the Rhodes and Lindos Acropolis Formations, using sedimentological, palaeontological, palaeoecological and ichnological data.     

Depositional framework and controls on mixed carbonate-siliciclastic gravity flows: Pennsylvanian-Permian shelf to basin transect, south-western Great Basin, USA

Mixed carbonate-siliciclastic sediment gravity flow deposits of Late Pennsylvanian to Early Permian age are exposed in the Death Valley - Owens Valley region of east-central California. The Mexican Spring unit constitutes the upper part of the Keeler Canyon Formation and is characterized by turbidites, debris flow deposits and megabreccias, all of mixed carbonate-siliciclastic composition. The mixed composition of the Keeler Canyon Formation provides an opportunity to link facies architecture to controls on depositional system development. Depositional relationships indicate that the deposits represent a non-channellized base of slope carbonate apron system with inner, outer and basinal facies associations. These gravity flow deposits are characterized by repeated stacked, small scale (<15 m) coarsening and thickening upward cycles with superimposed medium scale (>100 m) coarsening and thickening upward cycles. Contemporaneous outer shelf and upper slope deposits of the Tippipah Limestone are exposed at Syncline Ridge on the Nevada Test Site. The deposits consist of carbonate buildups directly overlain by cross bedded, quartz-rich sandstone and conglomerate which filled channels that traversed across the previously existing carbonate shelf. Detritus was transported to the west, down the upper slope by gully systems that fed the temporally persistent base of slope apron of the upper part of the Keeler Canyon Formation. This style of deposition differs from point-sourced siliciclastic submarine fan depositional systems. However, the Keeler Canyon system has lithofacies similar to some sandy siliciclastic turbidite systems, such as the delta-fed submarine ramp facies model, which is a line-sourced, shelf-fed system that is not supply limited. The mixed clastic apron systems of the Keeler Canyon Formation differ from classical carbonate aprons in that the former is characterized by an abundance of sedimentary cycles. Controls on the development of these cycles and of the facies distribution may have resulted from changes in type and rate of sediment supply, relative sea level changes and/or tectonic events. Interpretation of the data is focused on relative changes in sea level as the most significant control on development of the depositional system. Relative sea level changes serve two important functions: (1) they provide a mechanism for bringing coarse siliciclastic and bioclastic grains together on the outer shelf, and (2) shelf margin collapse may be initiated during relative lowstands allowing for transport of the sediment to the deep basin and development of deep basinal cycles. Therefore, an abundance of mixed clastic gravity flow deposits such as these in the rock record may be an indicator of periods of high frequency changes in relative sea level, which is a characteristic of Late Palaeozoic sea level history.     

A large-scale meandering submarine canyon: outcrop example from the late Proterozoic Adelaide Geosyncline, South Australia

The late Proterozoic Adelaide Geosyncline, along with overlying Cambrian strata, comprises a thick sequence of sediments and sparse volcanics which accumulated in a major rift and passive margin setting. During late syn-rift or early post-rift phases, large volumes of terrigenous and carbonate sediments of the late Proterozoic Umberatana and Wilpena Groups and Cambrian Hawker Group filled the rift. Submarine canyon development was related to at least four of these depositional cycles, the most notable of which resulted in incision and subsequent filling of the major (several kilometres in width and up to 1.5 km deep) submarine canyons by the Wonoka Formation. The Wonoka Formation canyons are not obviously fault controlled. They are interpreted to have been eroded by turbidity currents during a relative low-stand of sea-level. They were subsequently filled by a fining-upwards suite of sediments which reflects subsequent relative rise of sea-level and carbonate platform development. Ultimately the canyon complex was buried by north-westerly progradation of overlying fluvial and slope sequences (Billy Springs Beds and possibly correlative upper Pound Subgroup). It is considered likely that more distal elements of this prograding clastic wedge provided the necessary material for canyon erosion, prior to canyon filling and ultimate burial by what may have been elements of the same depositional cycle. It is considered possible that the series of isolated outcrops of canyon cross-sections within the Wonoka Formation are sections of a single canyon thalweg developed within a considerably broader zone of slope degradation. If this interpretation is correct, then the gorge-like Patsy Springs Canyon lies in more proximal regions of the basin-slope, whereas 40 km to the north-east the lower slope is cut by the Fortress Hill Canyon Complex. Palaeocurrent analyses of channel-fill turbidites within the canyons imply that the Fortress Hill Complex is in fact the outcropping western edge of a sinuous, incised canyon thalweg. The Wonoka Formation canyons, containing basal sedimentary breccias but only minor conglomerates, are considered typical of passive margin canyon development. They are contrasted with the generally highly conglomeratic channel-fills observed in outcropping Tertiary and Cretaceous examples of active margin canyons and upper fan valleys.     

Terrigenous sand in Labrador Sea hemipelagic sediments and paleoglacial events on Baffin Island over the last 100,00 years

Eight Labrador Sea piston cores with faunal and ash-zone stratigraphies correlated to deep-sea oxygen isotope stages were used to compute Labrador Sea terrigenous sand input rates (mg/cm²/1000 years) during the last 100,000 years. Sources of the sand in Labrador Sea cores are likely to be ice-rafting, turbid glacial meltwater inflow or deflation and wind erosion of unvegetated landscapes in the wake of retreating continental ice sheets. High levels of sand input to the Labrador Sea are therefore undoubtedly glacier-related while low levels of sand input are not. Comparison of the history of Labrador Sea sand input with the chronology of glacial and non-glacial events on Baffin Island reveals that the era of highest sand input rates, the isotopic stage 5a/4 transition, closely coincided with an episode of early Foxe glacier advance to tidewater (Ayr Lake Stade) along the outer coast of Baffin Island ca. 80,000 B.P. to 60,000 B.P. The period of lowest Labrador Sea sand input rates, late isotopic stage 3 to the present, largely corresponds to a major disconformity in the raised marine and glacigenic sediments on Baffin Island, but includes also the late Foxe/early Holocene Cockburn glacial advance (which did not reach the outer coast of the island) and the modern glacial minimum. Labrador Sea and central-subpolar North Atlantic sand input histories are reciprocally related over the last 80,000 years. Accelerated sand input in the Labrador Sea during times of reduced sand input in the North Atlantic implies: (1) major early Wisconsin glacier expansion in the circum Labrador Sea/Baffin Bay region and/or; (2) a surface circulation pattern in the North Atlantic which inhibited iceberg melting there while delivering icebergs and relatively warm surface water into the Labrador Sea. Conversely, reduced sand input in the Labrador Sea during times of accelerated sand input in the North Atlantic implies: (1) late Wisconsin glacier recession in the circum Labrador Sea/Baffin Bay region and/or; (2) a circulation pattern which carries icebergs southward and eastward away from the Labrador Sea. These implications are discussed in the light of paleoceanographic evidence for three periods - 80,000 B.P. to 57,000 B.P.; 25,000 B.P. to 13,000 B.P.; and 13,000 B.P. to 9800 B.P     

International Collaborations In Geodesy Advance Geoscience Research

UNAVCO supports geoscience research at 113 US academic Member institutions,and another 104 Associate Member institutions include international universities,laboratories,observatories,academies of science,and a museum.This diverse membership shares UNAVCO’s purpose at home and abroad,giving UNAVCO global reach in advancing geodesy.Since the mid-1980s,modern geodesy has evolved into a cutting-edge,multi-faceted toolbox with remarkably diverse research and real-world applications,including studies and observation or forecasting of solid-Earth hazards,the dynamics of the atmosphere,climate,near-Earth space environment,and of key environmental parameters such as water storage,soil moisture,and seaand lake-level changes.UNAVCO operates facilities on behalf of the U.S.National Science Foundation to support investigators who use geodetic tools across all of these Earth and atmospheric domains.UNAVCO has built a number of large dense regional networks of GPS stations,including the Earth Scope Plate Boundary Observatory in North America,the COCONetCaribbean network,TLALOCNet in Mexico,GNET in Greenland,and ANET in Antarctica.Going forward,UNAVCO plans to federate the Plate Boundary Observatory(USA),TLALOCNet(Mexico),and COCONet(Caribbean)GPS networks as the Network of the Americas,with upgrades to state-of-the-art,multi-sensor,multi-GNSS observations.While UNAVCO community scientists actively engage in using space and terrestrial geodetic techniques to study geodynamics at all scales,this proliferation of continuous networks is the basis for a suite of recent contributions that focus on improved daily positioning to sense Earth’s elastic response and other perturbations to loading by atmospheric and surface water,oceans,and ice.Day-to-day and sub-daily variations in the GPS vertical and horizontal correlate to increasingly well-understood short-term mass variability,such as monsoonal flooding in Bangladesh,sub-daily changes in tidal loading at continent scales,day-to-day surface water and ice storage in the western U.S.,variations in the rate of GIA in Greenland across a variety of scales,and improved understanding of the inter-annual variation in sea level rise due to changes in terrestrial water storage.     

Threshold of sediment motion under unidirectional currents

Carefully selected data for the threshold of sediment movement under unidirectional flow conditions have been utilized to re-examine the various empirical curves that are commonly employed to predict this threshold. After a review of the existing data, we employed only that data obtained from open channel flumes with parallel sidewalls where flows were uniform and steady over flattened beds of unigranular, rounded sediments. Without these restrictions, an unmanageable amount of scatter is introduced. This selected data is used to develop a modified Shields-type threshold diagram that extends the limits of the original diagram by three orders of magnitude in the grain-Reynolds number. The equally general but more easily employed Yalin diagram for sediment threshold is also examined. Although the Shields and Yalin diagrams are general in that they apply to a wide range of different liquids, in both cases somewhat different curves are obtained for threshold under air than for the liquids. The often used empirical curves of the friction velocity u_*, the velocity 100 cm above the bed u₁₀₀, the bottom stress θ_t, and Shields’ relative stress θ_t, all versus the grain diameter D, are limited in their ranges of application to certain combinations of grain density, fluid density, fluid viscosity and gravity. These conditions must be selected before the curves are generated from either the more general Shields or Yalin curves. For example, on the basis of the data selected for use in this paper, empirical threshold relationships for quartz density material in water are where the velocity u₁₀₀ measured 100 cm above the sediment bed is given in cm/sec and the grain diameter D is in cm. The limitations on any of the threshold relationships are severe. These limitations should be properly understood so that the empirical curves and relationships are not improperly employed.