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

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 .     

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

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.     

Erosional and depositional characteristics of regional overwash deposits caused by multiple hurricanes

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.     

Messinian stromatolite-thrombolite associations,Santa Pola,SE Spain: an analogue for the Palaeozoic?

Stromatolite-thrombolite associations are the dominant facies forming large portions of the Santa Pola carbonate platform (SE Spain) during deposition of the Terminal Carbonate Complex (TCC). The TCC, the last period of marine sedimentation in the Western Mediterranean associated with the Messinian Salinity Crisis, comprises a NE-SW trending thrombolite reef with occasionally interlayered stromatolite horizons and a predominantply stromatolite and oolite facies in the back-reef area. The stromatolites are mainly dome shaped, but fine-columnar or wavy-undulose forms can occur. The stromatolites form huge bioherms, extending tens to hundreds of metres. They are finely laminated with alternating layers of dolomicrite and dolomicrospar. The dolomicrite layers appear to be a primary dolomite precipitate, whereas the dolomite crystals in the dolomicrospar layers apparently formed around a meta-stable nuclei which was subsequently dissolved or degraded. The low content of sand-sized particles in the stromatolitic layers indicates formation under low-energy conditions, possibly on a tidal flat. As reported from other areas in the Western Mediterranean, deposition of the TCC at Santa Pola was apparently cyclic, whereby stromatolites generally terminate each depositional cycle. Subtidal Conophyton stromatolites, possibly the only known occurrence younger than Palaeozoic, are, however, found on the reef slope at the base of the first TCC depositional cycle. The dolomitic nature of the unadulterated stromatolitic laminations and the association of stromatolites and thrombolites as platform builders were a common feature in the Early Palaeozoic but are unusual in post-Ordovician carbonate facies. We propose that the conditions during TCC deposition were very restricted, possibly reflecting an environment similar to that of the Early Palaeozoic.     

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

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.     

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

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.     

Subglacial to proglacial sediment transition in a shallow ice-contact lake

A complete subglacial to glacial-lacustrine facies transition is described from a temporary exposure in the Lake Michigan bluffs of southeastern Wisconsin. This transition occurs where a Late Woodfordian terminal moraine intersects the bluff line and grades from basal meltout till to chaotic diamicton to rhythmites over an abrupt 90 m lateral distance. The boundary of the subglacial meltout deposits is marked by an abrupt increase in pebbles and cobbles, which cluster at specific horizons, producing an incipient stratification. Thereafter, the diamicton develops contorted flow structures with progressive segregation into coarse and fine-grained fractions, ultimately into well-stratified rhythmites.     

Conceptual model for early diagenetic chert and dolomite, Amuri Limestone Group, north-eastern South Island, New Zealand

The Late Cretaceous to Early Eocene, dominantly micritic, Amuri Limestone Group (ALG) was deposited in an approximately NW trending trough, in eastern Marlborough, New Zealand. The ALG comprises: the Mead Hill Formation; the Teredo, Lower and Middle Limestone formations; and the Upper and Lower Marl formations. Chert and dolomite are concentrated in the Mead Hill Formation, which contains five of six recognized diagenetic zones: Zone I at the base of the ALG consists almost entirely of chert; Zone II consists solely of chert and dolomite; Zone III comprises chert and limestone; Zone IV is composed of chert plus dolomite; Zone V is a chertified mudstone; and the minor amounts of chert found in the Middle Limestone Formation comprise Zone VI. With the exception of Zones IV and V, chert decreases stratigraphically upwards and away from the basin centre. All the dolomites are composed of <1 mm diameter rhombohedra in discontinuous beds and lenses. Generally Ca-rich, and non- to slightly ferroan, the dolomite contains approximately 500–900 ppm Mn and 200–400 ppm Sr. δ¹³C values average 1–2%_PDB with δ¹⁸O ratios of about -4%_PDB. Mass balance calculations indicate that the Mg²⁺ for dolomitization was derived from sea water. Sr, Fe and Mn concentrations are interpreted as indicating dolomite formation in the marine environment, with no influence from meteoric waters. The intimate association with pyrite implies dolomite formation in association with sulphate reduction, in the upper sediment column. δ¹⁸O data show that the bulk of the dolomite formed at temperatures below 50°C. All chert samples contain in excess of 90 wt% SiO₂, about 1 wt% Al₂O₃ and 1 wt% from losses on ignition. Generally all other major elements total less than 2 wt% oxide. δ¹⁸O values range from 26·8 to 29·0%_SMOW. Chert chemistry is consistent with the replacement of host carbonate and expulsion of carbonate-bound components from the site of chertification, and the effective dilution by SiO₂ of non-carbonate-bound insoluble residues. δ¹⁸O data indicate that chert formed in fluids of similar composition and temperature as the dolomite. The abundance of disseminated pyrite in cherts implies an association with sulphate reduction. Silica for chertification is thought to have initially come from dissolution of siliceous organisms. However, there is insufficient biogenic silica available to form the volumes of chert observed. It is suggested that the bulk of the silica came from SiO₂-rich pore waters generated by clay mineral reactions in the thick underlying mudstones. The ALG compacted down through these pore waters. Chert and dolomite nucleation are considered to have been penecontemporaneous. Dolomitization was initially probably the faster process, continuing as long as sulphate reduction prevailed and there was an adequate supply of Mg²⁺. The nucleation of chert, although initially slower (probably due to a relatively lower initial SiO₂ supply), continued after cessation of dolomitization to the extent of completely chertifying the dolomite intercrystalline matrix. The amount of chertification decreased progressively as SiO₂ supplies diminished, both stratigraphically upwards and away from the basin centre.     

CONSTRUCTING,VISUALIZING, AND ANALYZING A DIGITAL FOOTPRINT*

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.