Variability of cold‐water coral mounds in a high sediment input and tidal current regime,Straits of Florida     

MARK GRASMUECK  GREGOR P. EBERLI  JOHN K. REED  KLAAS VERWER  SAM PURKIS 《Sedimentology》2012,59(4):1278-1304

Cold‐water coral mound morphology and development are thought to be controlled primarily by current regime. This study, however, reveals a general lack of correlation between prevailing bottom current direction and mound morphology (i.e. footprint shape and orientation), as well as current strength and mound size (i.e. footprint area and height). These findings are based on quantitative analyses of a high‐resolution geophysical dataset collected with an Autonomous Underwater Vehicle from three cold‐water coral mound sites at the toe of slope of Great Bahama Bank. The three sites (80 km² total) have an average of 14 mounds km^?2, indicating that the Great Bahama Bank slope is a major coral mound region. At all three sites living coral colonies are observed on the surface of the mounds, documenting active mound growth. Morphometric analysis shows that mounds at these sites vary significantly in height (1 to 83 m), area (81 to 6 00 000 m²), shape (mound aspect ratio 0·1 to 1) and orientation (mound longest axis 0 to 180°). The Autonomous Underwater Vehicle measured bottom current data depict a north–south flowing current that reverses approximately every six hours. The tidal nature of this current and its intermittent deviations during reversals are interpreted to contribute to the observed mound complexity. An additional factor contributing to the variability in mound morphometrics is the sediment deposition rate that varies among and within sites. At most locations sedimentation rate lags slightly behind mound growth rate, causing mounds to develop into large structures. Where sedimentation rates are higher than mound growth rates, sediment partially or completely buries mounds. The spatial distribution and alignment of mounds can also be related to gravity mass deposits, as indicated by geomorphological features (for example, slope failure and linear topographic highs) in the three‐dimensional bathymetry. In summary, variability in sedimentation rates, current regime and underlying topography produce extraordinarily high variability in the distribution, development and morphology of coral mounds on the Great Bahama Bank slope.  相似文献   

SHRIMP dating of magmatism in the Hitachi metamorphic terrane,Abukuma Belt,Japan: Evidence for a Cambrian volcanic arc     

MICHIO TAGIRI  DANIEL J. DUNKLEY  TATSURO ADACHI  YOSHIKUNI HIROI  C. MARK FANNING 《Island Arc》2011,20(2):259-279

Ion microprobe dating of zircon from meta‐igneous samples of the Hitachi metamorphic terrane of eastern Japan yields Cambrian magmatic ages. Tuffaceous schist from the Nishidohira Formation contains ca 510 Ma zircon, overlapping in age with hornblende gneiss from the Tamadare Formation (ca 507 Ma), and meta‐andesite (ca 507 Ma) and metaporphyry (ca 505 Ma) from the Akazawa Formation. The latter is unconformably overlain by the Carboniferous Daioin Formation, in which a granite boulder from metaconglomerate yields a magmatic age of ca 500 Ma. This date overlaps a previous estimate for granite that intrudes the Akazawa Formation. Intrusive, volcanic, and volcaniclastic lithologies are products of a Cambrian volcanic arc associated with a continental shelf, as demonstrated by the presence of arkose and conglomerate in the lowermost Nishidohira Formation. Granitic magmatism of Cambrian age is unknown elsewhere in Japan, except for a single locality in far western Japan with a similar geological context. Such magmatism is also unknown on the adjacent Asian continental margin, with the exception of the Khanka block in far northeastern China. A ‘great hiatus’ in the Paleozoic stratigraphy of the Sino–Korean block also exists in the Hitachi terrane between Cambrian volcanic arc rocks and Early Carboniferous conglomerate, and may indicate a common paleogeographic provenance.  相似文献   

Structure of a silica diagenetic transformation zone: the Gjallar Ridge,offshore Norway     

MARK T. IRELAND  RICHARD J. DAVIES  NEIL R. GOULTY  DANIEL CARRUTHERS 《Sedimentology》2011,58(2):424-441

Three‐dimensional seismic data from the Gjallar Ridge were used together with X‐ray diffraction data, scientific boreholes and wireline logs to analyse the seismic structure of a silica diagenetic transformation zone. The following features were identified: (i) an interval some 150 to 300 m thick that contains anomalously high‐amplitude reflections; (ii) a strong reflection event at the top of this interval which cross‐cuts stratigraphy, interpreted as the transformation boundary between opal‐A‐rich and opal‐CT‐rich sediment; (iii) amplitude variations on stratigraphic reflections within the interval, attributed to variations in the proportions of opal‐A and opal‐CT; and (iv) a second, deeper, cross‐cutting reflection event within the interval, interpreted as the transformation boundary between opal‐CT‐rich and quartz‐rich sediment. The base of the interval containing the anomalously high‐amplitude reflections is interpreted as a stratigraphic reflection demarcating the base of the silica‐rich strata. On a stratigraphic reflection within the interval of high amplitudes, roughly circular regions of anomalously high amplitude with diameters of 0·8 to 2·5 km are separated by lower amplitude regions. This pattern is similar to the cells previously identified at the opal‐A to opal‐CT transformation boundary and probably results from more complete transformation of opal‐A to opal‐CT. All of these observations provide the first recognition from seismic data that silica diagenetic transformations are not always narrow boundaries represented by single cross‐cutting seismic reflection events, as implied previously, but can be heterogeneous and hundreds of metres in thickness, as observed at outcrop.  相似文献   

Application of perturbation theory to the stability analysis of realistic atmospheric flows     

MARK D. BORGES  PRASHANT D. SARDESHMUKH 《地球，A辑:动力气象学与海洋学》1997,49(3):321-336

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The Late-Devensian proglacial Lake Humber: new evidence from littoral deposits at Ferrybridge, Yorkshire, England     

MARK D. BATEMAN  PAUL C. BUCKLAND  BRIAN CHASE  CHARLES D. FREDERICK  GEOFF D. GAUNT 《Boreas: An International Journal of Quaternary Research》2008,37(2):195-210

Proglacial Lake Humber is of UK national significance in terms of landscape drainage and development of the British Ice Sheet (BIS) during Marine Isotope Stage 2 (MIS 2), yet it is poorly understood in terms of its dynamics and timing. Sands and gravels exposed at Ferrybridge, West Yorkshire, UK, are interpreted as part of the Upper Littoral sands and gravels related to a high-level Lake Humber, which inundated the Humber Basin to ∼30 m OD during MIS 2. Excavations exposed well-rounded gravels of local origin extending downslope from the 27.5 m OD contour and interbedded sands and fine gravels, which are interpreted as the coarse littoral deposits and nearshore associated deposits. A sample from the distal sands returned an Optically Stimulated Luminescence age of 16.6±1.2 kyr, providing the first direct age for the high-level lake and for when North Sea Basin ice must have blocked the Humber Gap. An underlying sequence included a diamicton dated to after 23.3 ±1.5 kyr and before 20.5±1.2 kyr, indicating that the Late Devensian ice reached at least 15 km south of the Escrick Moraine prior to the high-level lake. Previous to both the high-level lake and this ice advance, loess found at the two sites investigated indicates a long period of loess deposition earlier in MIS 2. These new data for the history of Lake Humber are discussed in the context of ice-marginal oscillations in both the Vale of York and the North Sea Basin.  相似文献   

Summer Visitors to Auckland's Regional Parks: Characteristics, Motivations, Information Sources and Activities   总被引：1，自引：0，他引：1  

COLIN M. HARRIS  MARK B. ORAMS 《New Zealand geographer》1990,46(2):60-74

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Flow patterns, sedimentation and deposit architecture under a hydraulic jump on a non-eroding bed: defining hydraulic-jump unit bars     

ROBERT G. MACDONALD  JAN ALEXANDER  JOHN C. BACON  MARK J. COOKER† 《Sedimentology》2009,56(5):1346-1367

This paper presents results from two flume runs of an ongoing series examining flow structure, sediment transport and deposition in hydraulic jumps. It concludes in the presentation of a model for the development of sedimentary architecture, considered characteristic of a hydraulic jump over a non-eroding bed. In Run 1, a hydraulic jump was formed in sediment-free water over the solid plane sloping flume floor. Ultrasonic Doppler velocity profilers recorded the flow structure within the hydraulic jump in fine detail. Run 2 had identical initial flow conditions and a near-steady addition of sand, which formed beds with two distinct characteristics: a laterally extensive, basal, wedge-shaped massive sand bed overlain by cross-laminated sand beds. Each cross-laminated bed recorded the initiation and growth of a single surface feature, here defined as a hydraulic-jump unit bar . A small massive sand mound formed on the flume floor and grew upstream and downstream without migrating to form a unit bar. In the upstream portion of the unit bar, sand finer than the bulk load formed a set of laminae dipping upstream. This set passed downstream through the small volume of massive sand into a foreset, which was initially relatively coarse-grained and became finer-grained downstream. This downstream-fining coincided with cessation of the growth of the upstream-dipping cross-set. At intervals, a new bed feature developed above and upstream of the preceding hydraulic-jump unit bar and grew in the same way, with the foreset climbing the older unit bar. The composite architecture of the superimposed unit bars formed a fanning, climbing coset above the massive wedge, defined as one unit: a hydraulic-jump bar complex .  相似文献   

CONSTRUCTING,VISUALIZING, AND ANALYZING A DIGITAL FOOTPRINT*   总被引：1，自引：0，他引：1  

STEPHEN D. WEAVER  MARK GAHEGAN 《Geographical review》2007,97(3):324-350

ABSTRACT. Herein, we discuss the desire for new technology, the need for security, and the right to privacy; in doing so, we argue that each of these concerns comprises an important, tripartite debate. To highlight the complexities in this problem, we define our notion of a “digital footprint” and introduce Big Foot—specialized software created for the research described here to facilitate visualization and exploration of the data that comprise Stephen Weaver's personal digital footprint. Using Big Foot we demonstrate how multiple digital personae can be created from the data that constitute one unique digital footprint and provide a methodology for understanding the good and bad impacts that new technologies may have on future societies. One of the primary arguments of this work is that the debate—though not formally recognized—is currently before contemporary society and must receive sufficient attention.  相似文献   

Erosional and depositional characteristics of regional overwash deposits caused by multiple hurricanes   总被引：7，自引：0，他引：7  

PING WANG  MARK H. HORWITZ† 《Sedimentology》2007,54(3):545-564

Regional-scale washover deposits along the Florida Gulf and Atlantic coasts induced by multiple hurricanes in 2004 and 2005 were studied through coring, trenching, ground-penetrating radar imaging, aerial photography, and prestorm and poststorm beach-profile surveys. Erosional and depositional characteristics in different barrier-island sub-environments, including dune field, interior wetland and back-barrier bay were examined. Over the eroded dune fields, the washover deposits are characterized by an extensive horizontal basal erosional surface truncating the old dune deposits and horizontal to slightly landward-dipping stratification. Over the marshes in the barrier-island interior, the washover deposits are characterized by steep tabular bedding, with no erosion at the bottom. Overwash into the back-barrier bay produced the thickest deposits characterized by steep, prograding sigmoidal bedding. No significant erosional feature was observed at the bottom. Washover deposits within the dense interior mangrove swamp demonstrate both normal and reversed graded bedding. The washover deposits caused by hurricanes Frances (2004) and Jeanne (2004) along the southern Florida Atlantic coast barrier islands are substantially different from those along the northern Florida barrier islands caused by Ivan (2004) and Dennis (2005) in terms of regional extension, erosional features and sedimentary structures. These differences are controlled by different overall barrier-island morphology, vegetation type and density, and sediment properties. The homogeneity of sediment along the northern Florida coast makes distinguishing between washover deposits from Ivan and Dennis difficult. In contrast, along the Atlantic coast barrier islands, the two overwash events, as demonstrated by two phases of graded bedding of the bimodal sediments, are easily distinguishable.  相似文献   

Exceptionally long, narrow drumlins formed in subglacial cavities, North Dakota     

JOHN P. BLUEMLE  MARK L. LORD  NATHAN T. HUNKE 《Boreas: An International Journal of Quaternary Research》1993,22(1):15-24

An array of about 200 exceptionally long and narrow drumlins occurs in north-central North Dakota. The most prominent of the drumlins is 27 km long, and their average length is between 2 km and 3 km. The drumlin field is closely associated with, and occurs immediately up-glacier from, an extensive area of ice-thrust topography. Most of the drumlin ridges have small glacier-thrust masses at their heads. Internal structures are complex, but typically include large numbers of gravity faults, which dip away from the center of the drumlin ridges. Materials contained in the drumlins were transported from higher pressure areas beneath the glacier, inward toward lower pressure areas (cavities) by flowing, thrusting, squeezing and other processes. The close and systematic association of the long drumlins with nearby areas of ice-thrust topography indicates a genetic tie; conditions that caused the glacier to form large ice thrusts also contributed to drumlin formation. Major factors involved in the formation of the drumlins were high porewater pressures in interbedded permeable and impermeable materials beneath a thin, swiftly flowing glacier, and the presence of areas of frozen ground near the margin of the glacier.  相似文献