Glacial landforms on German Bank, Scotian Shelf: evidence for Late Wisconsinan ice-sheet dynamics and implications for the formation of De Geer moraines   总被引：1，自引：1，他引：1  

BRIAN J. TODD  PAGE C. VALENTINE  ODDVAR LONGVA  JOHN SHAW 《Boreas: An International Journal of Quaternary Research》2007,36(2):148-169

The extent and behaviour of the southeast margin of the Laurentide Ice Sheet in Atlantic Canada is of significance in the study of Late Wisconsinan ice sheet-ocean interactions. Multibeam sonar imagery of subglacial, ice-marginal and glaciomarine landforms on German Bank, Scotian Shelf, provides evidence of the pattern of glacial-dynamic events in the eastern Gulf of Maine. Northwest-southeast trending drumlins and megaflutes dominate northern German Bank. On southern German Bank, megaflutes of thin glacial deposits create a distinct northwest-southeast grain. Lobate regional moraines (>10km long) are concave to the northwest, up-ice direction and strike southwest-northeast, normal to the direction of ice flow. Ubiquitous, overlying De Geer moraines (<10 km long) also strike southwest-northeast. The mapped pattern of moraines implies that, shortly after the last maximum glaciation, the tidewater ice sheet began to retreat north from German Bank, forming De Geer moraines at the grounding line with at least one glacial re-advance during the general retreat. The results indicate that the Laurentide Ice Sheet extended onto the continental shelf.  相似文献   

Subtropical to temperate facies from a transition zone, mixed carbonate–siliciclastic system, Palaeogene, North Carolina, USA     

BRIAN P COFFEY  J. FRED READ† 《Sedimentology》2007,54(2):339-365

Palaeogene passive margin sediments on the US mid‐Atlantic coastal plain provide valuable insight into facies interaction and distribution on mixed carbonate–siliciclastic shelves. This study utilizes well cuttings, outcrop, core, and seismic data to document temporal and spatial variations in admixed bryozoan‐rich skeletal carbonates and sandy siliciclastic units that were deposited on a humid passive margin located in the vicinity of a major marine transition zone. This zone was situated between north‐flowing, warm waters of the ancestral Gulf Stream (carbonate dominated settings) and south‐flowing, cold waters of the ancestral Labrador Current (siliciclastic dominated settings). Some degree of mixing of carbonates and siliciclastics occurs in all facies; however, siliciclastic‐prone sediments predominate in nearshore settings, while carbonate‐prone sediments are more common in more open marine settings of the inner shelf break and deep shelf. A distinctive dual‐break shelf depositional profile originated following a major Late Cretaceous to Palaeocene transgression that drowned the earlier shallow platform. This profile was characterized by prominent mid‐shelf break dividing the shallow shelf from the deep shelf and a major continental shelf/slope break. Incomplete filling of available accommodation space during successive buildup of the shallow shelf preserved the topographic break on this passive margin. Storm wave base also contributed to the preservation of the dual‐break shelf geometry by beveling shallow shelf sediments and transporting them onto and seaward of the mid‐shelf break. Sediment fines in deep shelf facies were produced in place, transported downdip from the shallow shelf by storm ebb currents and boundary currents, and reworked from adjacent areas of the deep shelf by strike‐parallel boundary currents. Regional climate and boundary currents controlled whether carbonate or siliciclastic material was deposited on the shelf, with warmer waters and more humid climates favouring carbonate deposition and cooler, more arid conditions favouring glaucony and siliciclastic dominated deposition. Continuous wave and current sweeping of the shallow shelf favoured deposition of mud‐lean facies across much of the shallow shelf. Skeletal components in much of the carbonate‐rich strata formed in warm, nutrient‐rich subtropical waters, as indicated by widespread occurrences of larger benthic foraminifera and molluscan assemblages. These indicators of warm water deposition within the bryozoan‐mollusk‐rich carbonate assemblage on this shelf provide an example of a warm water bryomol assemblage; such facies generally are associated with cooler water depositional settings.  相似文献   

Sheetflow fluvial processes in a rapidly subsiding basin, Altiplano plateau, Bolivia   总被引：2，自引：0，他引：2  

BRIAN A. HAMPTON  BRIAN K. HORTON† 《Sedimentology》2007,54(5):1121-1148

Although facies models of braided, meandering and anastomosing rivers have provided the cornerstones of fluvial sedimentology for several decades, the depositional processes and external controls on sheetflow fluvial systems remain poorly understood. Sheetflow fluvial systems represent a volumetrically significant part of the non‐marine sedimentary record and documented here are the lithofacies, depositional processes and possible roles of rapid subsidence and arid climate in generating a sheetflow‐dominated fluvial system in the Cenozoic hinterland of the central Andes. A 6500 m thick succession comprising the Late Eocene–Oligocene Potoco Formation is exposed continuously for >100 km along the eastern limb of the Corque syncline in the high Altiplano plateau of Bolivia. Fluvial sandstone and mudstone units were deposited over an extensive region (>10 000 km²) with remarkably few incised channels or stacked‐channel complexes. The Potoco succession provides an exceptional example of rapid production of accommodation sustained over a prolonged period of time in a non‐marine setting (>0·45 mm year⁻¹ for 14 Myr). The lower ≈4000 m of the succession coarsens upward and consists of fine‐grained to medium‐grained sandstone, mudstone and gypsum deposits with palaeocurrent indicators demonstrating eastward transport. The upper 2500 m also coarsens upward, but contains mostly fine‐grained to medium‐grained sandstone that exhibits westward palaeoflow. Three facies associations were identified from the Potoco Formation and are interpreted to represent different depositional environments in a sheetflow‐dominated system. (i) Playa lake deposits confined to the lower 750 m are composed of interbedded gypsum, gypsiferous mudstone and sandstone. (ii) Floodplain deposits occur throughout the succession and include laterally extensive (>200 m) laminated to massive mudstone and horizontally stratified and ripple cross‐stratified sandstone. Pedogenic alteration and root casts are common. (iii) Poorly confined channel and unconfined sheet sandstone deposits include laterally continuous beds (50 to >200 m) that are defined primarily by horizontally stratified and ripple cross‐stratified sandstone encased in mudstone‐rich floodplain deposits. The ubiquitous thin‐sheet geometry and spatial distribution of individual facies within channel sandstone and floodplain deposits suggest that confined to unconfined, episodic (flash) flood events were the primary mode of deposition. The laterally extensive deposition and possible distributary nature of this sheetflow‐dominated system are attributed to fluvial fan conditions in an arid to semi‐arid, possibly seasonal, environment. High rates of sediment accumulation and tectonic subsidence during early Andean orogenesis may have favoured the development and long‐term maintenance of a sheetflow system rather than a braided, meandering or anastomosing fluvial style. It is suggested here that rapidly produced accommodation space and a relatively arid, seasonal climate are critical conditions promoting the generation of sheetflow‐dominated fluvial systems.  相似文献   

Spherular modern dolomite from the Coorong area, South Australia   总被引：1，自引：0，他引：1  

C. C. VON DER  BORCH J. B. JONES 《Sedimentology》1976,23(4):587-591

Scanning electron micrographs show that the youngest and apparently least altered of the Coorong dolomite is in the form of spherular bodies about 0.2–1.0 μm in size which themselves are composed of spherules about 100 nm in diameter. Older and more lithified sediments show sharply defined dolomite crystals suggesting an origin as primary dolomite spherules followed by aggregation and diagenetic alteration to well crystallized dolomites.  相似文献   

Seasonal variation of hydrocarbons in the free troposphere at mid-latitudes     

P. LIGHTMAN  A. S. KALLEND  A. R. W. MARSH  B. M. R. JONES  S. A. PENKETT 《Tellus. Series B, Chemical and physical meteorology》1990,42(5):408-422

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Aspects of the large-scale tropical atmospheric circulation     

BRIAN HOSKINS  RICHARD NEALE  MARK RODWELL  GUI-YING YANG 《Tellus. Series B, Chemical and physical meteorology》1999,51(1):33-44

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Tomography: A window on the role of sulfur in the structure of micrometeorites     

Susan TAYLOR  Keith W. JONES  Gregory F. HERZOG  Claire E. HORNIG 《Meteoritics & planetary science》2011,46(10):1498-1509

Abstract– To determine the role played by sulfides in the formation of vesicles and FeNi metal beads, we mapped the locations and tabulated the numbers of sulfides, metal beads, and vesicles in 1583 sectioned micrometeorites (MMs) using conventional microscopy and in 190 whole MMs using synchrotron computed microtomography (SCMT). Both the section and the SCMT images show that sulfides melt, coalesce, and migrate to the MMs surface. The decomposition of sulfides may occur during all these stages. Given the sulfide morphologies and compositions that we see in section, we think the breakdown of Ni sulfides produces the FeNi beads. The SCMT images show that metal beads are common in melted MMs, >50% have them. Vesicles in porphyritic and scoriaceous MMs are also probably formed as sulfides decompose. Not only do sulfides abut the vesicles but also the temperatures at which sulfides decompose overlap those at which MM surfaces first melt and temporarily seal, suggesting that S gases could produce most of these vesicles. As the vesicle shapes and patterns of distribution differ among MM classes, tomography can be used to nondestructively screen for specific types of MMs. Tomography is a powerful tool for visualizing the three‐dimensional distribution of metal beads, sulfides, mean densities, and vesicles in MMs.  相似文献   

Hypervelocity capture of meteoritic particles in nonsilica aerogels     

Steven M. JONES  George FLYNN 《Meteoritics & planetary science》2011,46(9):1253-1264

Abstract– The Stardust mission captured particles from the comet 81P/Wild 2 in gradient density silica aerogel and returned the collected samples to earth in 2006. The analyses of these particles have revealed several new insights into the formation of our solar system. However, since the aerogel used as the capture material was silica, the elemental analyses of the silica‐rich particles were made more complicated in certain ways due to the mixing of the silicon of the particles and that of the aerogel. By using a nonsilica aerogel, future elemental analyses of silica–rich particles captured in aerogel could be made more straightforward. Resorcinol/formaldehyde (RF), alumina, and zirconia aerogels were impact tested with meteoritic fragments and the captured fragments were mapped with synchrotron‐based X‐ray microprobe (XRM) and the particles were analyzed with X‐ray fluorescence (XRF). The resorcinol/formaldehyde aerogel proved to be the best capture material, in that it could be keystoned and XRF could be used to locate and analyze particles that were less than 10 μm.  相似文献   

Feldspar in type 4–6 ordinary chondrites: Metamorphic processing on the H and LL chondrite parent bodies     

Hollis A. KOVACH  Rhian H. JONES 《Meteoritics & planetary science》2010,45(2):246-264

Abstract– We have carried out a study of feldspar compositions in a suite of H and LL ordinary chondrites, of petrologic types 4, 5, and 6, in order to examine the process of recrystallization and equilibration of feldspar as the degree of metamorphism increases. In the H chondrites, there is little variation in feldspar compositions among the petrologic types, suggesting that homogenization of chondrule mesostasis, from which feldspar is presumed to have crystallized, occurred before feldspar crystallization began. The LL chondrites we studied are more complex. In Bjurböle (L/LL4), plagioclase in individual relict chondrules has distinct compositions, with a range of An/Ab ratios and low Or contents. This heterogeneity is most likely attributable to original compositional heterogeneity among chondrule mesostases: localized recrystallization of mesostasis must have occurred before diffusional equilibration took place. In Tuxtuac (LL5), the An/Ab ratio of plagioclase is more homogeneous, and plagioclase includes a significant Or component. In addition, we observe what appears to be exsolution of K‐feldspar from albitic host grains. In Saint Séverin (LL6), the An/Ab ratio of plagioclase is homogeneous, but plagioclase compositions show a range of Or contents, corresponding to a patchy distribution of K in individual feldspar grains. The observations in these LL chondrites are difficult to interpret with a simple model of progressive equilibration with increasing petrologic type. We suggest that the current criteria for assigning petrologic types are poorly defined: it is possible that the assigned petrologic types of these chondrites do not correlate with their peak temperatures. We propose that feldspar compositions might record conditions during the heating stage of metamorphism, and that the early stages of metamorphism may have occurred in the presence of fluids, rather than under the dry conditions that are commonly assumed.  相似文献   

Middle and late Cenomanian oceanic anoxic events in shallow and deeper shelf environments of western Morocco     

BRIAN GERTSCH  THIERRY ADATTE  GERTA KELLER  ABDEL AZIZ A.M. TANTAWY  ZSOLT BERNER  HAYDON P. MORT  DOMINIK FLEITMANN 《Sedimentology》2010,57(6):1430-1462

The response of shallow‐water sequences to oceanic anoxic event 2 and mid‐Cenomanian events 1a and 1b was investigated along the west African margin of Morocco north of Agadir (Azazoul) and correlated with the deep‐water sequence of the Tarfaya Basin (Mohammed Beach) based on biostratigraphy, mineralogy, phosphorus and stable isotopes. In the deeper Mohammed Beach section results show double peaks in δ¹³C_org for mid‐Cenomanian events 1a and 1b (Rotalipora reicheli biozone, lower CC10a biozone), the characteristic oceanic anoxic event 2 δ¹³C excursion (Rotalipora cushmani extinction, top of CC10a biozone) and laminated (anoxic) black shale. In the shallow environment north of Agadir, a fluctuating sea‐level associated with dysoxic, brackish and mesotrophic conditions prevailed during the middle to late Cenomanian, as indicated by oyster biostromes, nannofossils, planktonic and benthonic foraminiferal assemblages. Anoxic conditions characteristic of oceanic anoxic event 2 (for example, laminated black shales) did not reach into shallow‐water environments until the maximum transgression of the early Turonian. Climate conditions decoupled along the western margin of Morocco between mid‐Cenomanian event 1b and the Cenomanian–Turonian boundary, as also observed in eastern Tethys. North of Agadir alternating humid and dry seasonal conditions prevailed, whereas in the Tarfaya Basin the climate was dry and seasonal. This climatic decoupling can be attributed to variations in the Intertropical Convergence Zone and in the intensity of the north‐east trade winds in tropical areas.  相似文献