Hummocky cross-stratification, tropical hurricanes, and intense winter storms

Most previous workers have inferred a storm origin for hummocky cross-stratification, which typically occurs in shallow-marine deposits. On the modern Earth, the only storms capable of profoundly affecting shallow-marine depositional environments are severe tropical cyclones (hurricanes) and mid-latitude winter wave cyclones (intense winter storms). This paper examines the palaeogeographic distribution (including palaeolatitude and palaeogeographic setting) of 107 occurrences of hummocky cross-stratification, ranging in age from the Proterozoic to Recent. In each of these stratigraphic units, both palaeolatitude and palaeogeography are consistent with a direct storm influence (associated with the passage of hurricanes or winter storms directly over the site of deposition). This palaeogeographic evidence lends support to the inferred storm origin for hummocky cross-stratification; further, the distribution of the structure suggests that most occurrences (73%) were generated by tropical hurricanes, the remaining 27% being generated by intense mid-latitude winter storms. The preferential generation of hummocky cross-stratification by hurricanes is consistent with: (1) the known differences in the nature of the bottom flows generated by the two major storm types, and (2) the inferred nature of the flows which form hummocky cross-stratification. Hurricanes couple less effectively with the water column than do intense winter storms. Due to this ineffective coupling, hurricane-generated bottom flows tend to be oscillatory-or multidirectional-dominant, with only minor unidirectional components of motion. In contrast, intense winter storms generally do couple effectively with the water column, generating bottom flows which possess a dominant or significant unidirectional component. Most previous workers have suggested that hummocky cross-stratification forms under oscillatory- or multidirectional-dominant flow; thus, it is conceptually reasonable that the vast majority of ancient occurrences of hummocky cross-stratification were probably hurricane-generated, as suggested by the aforementioned palaeogeographic distribution. The Proterozoic, Palaeozoic, Neogene, and Quaternary were times when global climate was similar to that of today. The distribution of hummocky cross-stratification deposited during these times suggests that both hurricanes and intense winter storms occupied latitudinal belts during these times which were essentially identical to those occupied by their modern counterparts. The Mesozoic and Palaeogene were non-glacial times when global climate was much warmer than that of today. The distribution of hummocky cross-stratification deposited during this interval suggests that hurricanes occurred more frequently at higher latitudes during non-glacial times than they do at present. The possibility of a broadened hurricane belt during the Mesozoic and Palaeogene is consistent with climatic considerations. A limited number of Mesozoic and Palaeogene rock units containing hummocky cross-stratification were deposited in palaeogeographic settings that preclude a direct hurricane influence; these examples were deposited in the middle latitudes, suggesting that intense winter storms continued to form hummocky cross-stratification in the middle latitudes during these much warmer times. Some previous workers have suggested that tsunamis may be capable of generating hummocky cross-stratification. The palaeogeographic distribution of the structure does not support an origin due to tsunamis. Lacustrine examples of hummocky cross-stratification reported herein are the first known non-marine occurrences; they suggest that storm effects strongly influence the sedimentary record of some lakes.     

Sustainable Development: The Economic and Environmental Case for Policy Reform

Abstract. Acceptance of the concept of sustainable development as a long term goal has shifted the nature of the debate about the environment and its relation to development. Previously, emphasis was on the adverse effects of economic development on the environment. It is now argued that a degraded and deteriorating environment is a threat to economic development. However, if the promise of sustainability is to be fully explored some fundamental changes must be made. The most important changes concern: first, the way ecological factors are integrated into economic decision making; second, the design of domestic policies; third, the emergence of environmental change as a force in international relations; and, fourth, the need to restructure the international economy and global political arrangements. Canada and New Zealand provide examples of efforts to tackle these issues, to halt environmental destruction and resource depletion, and to meet global development needs.     

Petrological and Geochemical Evidence for Granitoid Formation: The Waldoboro Pluton Complex, Maine

The Acadian-age Waldoboro Pluton Complex (WPC), mid-coastalMaine, consists of seven granitoid units surrounded by migmatitic,peraluminous gneisses and schists (predominantly Bucksport orSebascodegan Formation). The complex (area >340 km²) cross-cutsthe westward-vergent St. George thrust fault, which may markthe boundary between the Avalon and Gander composite terranes.Field and petrologic data indicate in situ formation of theperaluminous, syntectonic granitoids: contacts with Bucksportparagneisses are transitional and concordant; abundant country-rockenclaves show evidence for melting; restitic garnet, biotite,and plagioclase in the granitoids are identical in compositionto garnet, biotite, and plagioclase in the country rock. Chemicalvariations among the main granitoid phases (gneissic granite,granite, and leucogranite) reflect varying degrees of melt-restiteunmixing. Major and trace elements define mixing trends betweenrefractory Bucksport lithologies and leucogranites which approximatemelt compositions. Petrographic and whole-rock chemical dataare consistent with restitic plagioclase, garnet, biotite, tourmaline,zircon, apatite, sphene, and an accessory phase such as monazite.Quantitative major-oxide mass-balance models indicate that gneissicgranite represents a mixture of 55% melt–45% restite whereasgranite represents a mixture of 76% melt–24% restite.Melt-restite proportions calculated from trace element dataagree with those calculated from major oxide data for the gneissicgranite, but are different (85% melt–15% restite) fromthose calculated from major oxide data for the granite. Thisis attributed to inhomogeneous distribution of minor phasesand the effects of metasomatism. High K₂O, Rb, Ba, Cs, Li, B,K/Rb, K/Ba, Rb/Sr, and Th/U along the eastern mylonitic marginand elsewhere within the WPC reflect post-solidification metasomaticprocesses. Intrusion of mafic magmas during uplift after crustal thickeningappears to have caused high-temperature metamorphism and anatexisof Bucksport country rocks at relatively low pressure (0.4 GPa).Dehydration melting of muscovite to produce magmas saturatedor nearly saturated with H₂O explains the formation of migmatitesin the vicinity of the WPC. Formation of granites by 50–60%fluid-absent melting of Bucksport source rocks containing 20%biotite requires that fusion occurred at T860C and consumedall of the biotite in the source rock. Phase equilibrium dataand estimated temperatures of formation provide evidence thatthe granitoids formed at T<860C, whereas petrographic dataindicate that not all biotite in the source rock was consumedduring anatexis. Therefore, the WPC granitoids could have formedby fluid-absent melting if the source rocks contained >20%biotite (the maximum amount observed). However, it is also possiblethat influx of aqueous fluid before or during anatexis allowedproduction of relatively large volumes of melt at T<860C.Available data do not allow these possibilities to be rigorouslytested. The WPC granitoids have many characteristics of S-type granitesand preserve a chemical and mineralogical record of their sourcerocks, indicating that granites can image their sources evenin tectonically complex regions.     

Anorthosites and Related Megacrystic Units in the Evolution of Archean Crust

Archean anorthositic complexes occur in essentially all Archeancratons and contain large equidimensional plagioclase crystals(up to 30 cm. diam.) with highly calcic compositions (An₈₀ toAn₉₀) but are not readily amenable to determination of theirparent melt compositions. However, insight into petrogenesisof the complexes is provided by megacrysts of plagioclase thatare identical to those in the complexes and occur in many Archeanflows, sills, and dikes whose matrices display REE and fractionationpatterns that indicate tholeiitic trends and are compatiblewith prior subtraction of plagioclase during earlier evolutionof the melts. Included blocks of anorthosite and megacrystswith very thin rims that approach the more sodic compositionof lathy plagioclase in the matrices indicate an earlier stageof cryst formation under different conditions of crystallizationthan the matrices. The megacrystic units occur both in greenstonebelts that have oceanic affinities and stable cratonic dikeswarms that have continental affinities. Both major and traceelement contents of the matrices of the megacrystic units differbetween greenstone and cratonic dike environments; the dikesbeing higher in Si0₂, TiO₂ FeO, Na₂, K₂O, and light REEs butlower in Al₂O₃ and CaO. The matrices of both environments followseparate but parallel tholeiitic fractionation with high Fe-enrichmenttrends similar to Skaergaard liquids suggesting relatively lowvolatiles and fo₂. Experimental data and projections in CMAFspace suggest a multistage petrogenesis involving a relativelyhigh-pressure fractionation of olivine and/or orthopyroxenefrom a primitive mafic melt followed by ascension of the fractionated,less-dense melt, probably in several pulses, to a low-pressurechamber, probably at 1 to 2 kb. The depressurization accompaniedby cooling could easily place the melt composition in the plagioclasefield and significantly below the liquidus resulting in severalcrystallization cycles of plagioclase in the low pressure chamber.The melts would crystallize as anorthositic complexes and periodicallyexpel pulses that would form the observed megacrystic flows,sills, and dikes.     

Source,timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis

Haapaniemi, A.I., Scourse, J.D., Peck, V.L., Kennedy, H., Kennedy, P., Hemming, S.R., Furze, M.F.A., Pieńkowski, A.J., Austin, W.E.N., Walden, J., Wadsworth, E. & Hall, I.R. 2010: Source, timing, frequency and flux of ice‐rafted detritus to the Northeast Atlantic margin, 30–12 ka: testing the Heinrich precursor hypothesis. Boreas, Vol. 39, pp. 576–591. 10.1111/j.1502‐3885.2010.00141.x. ISSN 0300‐9483. Increased fluxes of ice‐rafted detritus (IRD) from European ice sheets have been documented some 1000–1500 years before the arrival of Laurentide Ice Sheet (LIS)‐sourced IRD during Heinrich (H) events. These early fluxes have become known as ‘precursor events’, and it has been suggested that they have mechanistic significance in the propagation of H events. Here we present a re‐analysis of one of the main cores used to generate the precursor concept, OMEX‐2K from the Goban Spur covering the last 30 ka, in order to identify whether the British–Irish Ice Sheet (BIIS) IRD fluxes occur only as precursors before H layers. IRD characterization and planktonic foraminiferal δ¹⁸O measurements constrained by a new age model have enabled the generation of a continuous record of IRD sources, timing, frequency and flux, and of local contemporary hydrographic conditions. The evidence indicates that BIIS IRD precursors are not uniquely, or mechanistically, linked to H events, but are part of the pervasive millennial‐scale cyclicity. Our results support an LIS source for the IRD comprising H layers, but the ambient glacial sections are dominated by assemblages typical of the Irish Sea Ice Stream. Light isotope excursions associated with H events are interpreted as resulting from the melting of the BIIS, with ice‐sheet destabilization attributed to eustatic jumps generated by LIS discharge during H events. This positive‐feedback mechanism probably caused similar responses in all circum‐Atlantic ice‐sheet margins, and the resulting gross freshwater flux contributed to the perturbation of the Atlantic Meridional Overturning Circulation during H events.     

Architecture of a coarse‐grained channel–levée system: the Rosario Formation,Baja California,Mexico

Seafloor images of coarse‐grained submarine channel–levée systems commonly reveal complex braid‐plain patterns of low‐amplitude bedforms and zones of apparent bypass; however, mechanisms of channel evolution and the resultant channel‐fill architecture are poorly understood. At Playa Esqueleto the lateral relationships between various elements of a deep‐marine slope channel system are well‐exposed. Specifically, the transition from gravel‐dominated axial thalwegs to laterally persistent marginal sandstones and isolated gravel‐filled scours is revealed. Marginal sandstones pass into a monotonous thin‐bedded succession which built to form relatively low‐relief levées bounding the channel belt; in turn, the levées onlap the canyon walls. Three orders of confinement were important during the evolution of the channel system: (i) first‐order confinement was provided by the erosional canyon which confined the entire system; (ii) confined levées built of turbidite sandstones and mudstones formed the second‐order confinement, and it is demonstrated that these built from overspill at thalweg margins; and (iii) third‐order confinement describes the erosional confinement of coarse‐grained thalwegs and scours. Finer‐grained sediment was transported in suspension and largely was unaffected by topography at the scale of individual thalwegs. Facies and clast analyses of conglomerate overlying channel‐marginal scours reveal that they were deposited by composite gravity flows, which were non‐cohesive, grain‐dominant debris flows with more fluidal cores. These flows were capable of basal erosion but were strongly depositional; frictional freezing at flow margins built gravel levées, while the core maintained a more fluidal transport regime. The resultant architecture consists of matrix‐rich, poorly sorted levées bounding better‐sorted, traction‐dominated cores. The planform geometry is interpreted to have consisted of a low‐sinuosity gravel braid‐plain built by accretion around mid‐channel and bank‐attached bars. This part of the system may be analogous to fluvial systems; however, the finer‐grained sediment load formed thick suspension clouds, probably several orders of magnitude thicker than the relief of braid‐plain topography and therefore controlled by the levées and canyon wall confinement.     

Distributary channels, sand lobes, and mesotopography of Navy Submarine Fan, California Borderland, with applications to ancient fan sediments

The deep-tow instrument package of Scripps Institution of Oceanography provides a unique opportunity to delineate small-scale features of a size comparable to those features usually described from ancient deep-sea fan deposits. On Navy Fan, the deep-tow side-scanning sonar readily detected steep channel walls and steps and terraces within channels. The most striking features observed in side-scan are large crescentic depressions commonly occurring in groups. These appear to be large scours or flutes carved by turbidity currents. Four distinct acoustic facies were mapped on the basis of qualitative assessment of reflectivity of 4 kHz reflection profiles. There is a distinct increase in depth of acoustic penetration, number of sub-bottom reflectors, and reflector continuity from the upper fan-valley to the lower fan. These changes are accompanied by a decrease in surface relief. Navy Fan is made up of three active sectors. The active upper fan is dominated by a single channel with prominent levees that decrease in height downstream. The active mid-fan region or suprafan is where sand is deposited. Well defined distributary channels with steps, terraces, and other mesotopography terminate in depositional lobes. Interchannel areas are rough, containing giant scours as well as other relief. The active lower fan accumulates mud and silt and is without resolvable surface morphology. The morphological features seen on Navy Fan other than levees, interchannel areas, and lobes are principally erosional. The distributary channels are up to 0.5 km wide and 5–15 m deep. Such features, because of their large size and low relief, are rarely completely exposed or easily detectable in ancient rock sequences. Some flute-shaped scours are larger than channels in cross section but many are 5-30 m across and 1-2 m deep. If observed in ancient rocks transverse to palaeo-current direction, they would perhaps be indistinguishable from channels. Surface sediment distribution combined with fan morphology can be used to relate modern sediments to facies models for ancient fan sediments. Gravel and sand occur in the upper valley, massive sand beds in the mid-fan distributary channels, classical complete Bouma sequences on depositional lobes, incomplete Bouma sequences (lacking division a) on the lower mid-fan, and Bouma sequence with lenticular shape or other limited extent on mid-fan interchannel areas and on levees.     

The variability of critical shear stress, friction angle, and grain protrusion in water-worked sediments

The erodibility of a grain on a rough bed is controlled by, among other factors, its relative projection above the mean bed, its exposure relative to upstream grains, and its friction angle. Here we report direct measurements of friction angles, grain projection and exposure, and small-scale topographic structure on a variety of water-worked mixed-grain sediment surfaces. Using a simple analytical model of the force balance on individual grains, we calculate the distribution of critical shear stress for idealized spherical grains on the measured bed topography. The friction angle, projection, and exposure of single grain sizes vary widely from point to point within a given bed surface; the variability within a single surface often exceeds the difference between the mean values of disparate surfaces. As a result, the critical shear stress for a given grain size on a sediment surface is characterized by a probability distribution, rather than a single value. On a given bed, the crtitical shear stress distributions of different grain sizes have similar lower bounds, but above their lower tails they diverge rapidly, with smaller grains having substantially higher median critical shear stresses. Large numbers of fines, trapp.ed within pockets on the bed or shielded by upstream grains, are effectively lost to the flow. Our calculations suggest that critical shear stress, as conventionally measured, is defined by the most erodible grains, entrained during transient shear stress excursions associated with the turbulent flow; this implies a physical basis for the indeterminacy of initial motion. These observations suggest that transport rate/shear stress relationships may be controlled, in part, by the increasing numbers of grains that become available for entrainment as mean shear stress increases. They also suggest that bed textures and grain size distributions may be controlled, within the constraints of an imposed shear stress and sediment supply regime, by the influence of each size fraction on the erodibility of other grain sizes present on the bed.     

Fine-scale relief related to Late Holocene channel shifting within the floor of the upper Redondo Fan, offshore Southern California

Erosional and depositional bedforms have been imaged at outcrop scale in the upper Redondo Fan, in the San Pedro Basin of offshore Southern California in ≥600 m water depths, using an Autonomous Underwater Vehicle developed by the Monterey Bay Aquarium Research Institute. The Autonomous Underwater Vehicle is equipped with multibeam and chirp sub-bottom sonars. Sampling and photographic images using the Monterey Bay Aquarium Research Institute Remotely Operated Vehicle Tiburon provide groundtruth for the Autonomous Underwater Vehicle survey. The 0·3 m vertical and 1·5 m lateral bathymetric resolution and 0·1 m sub-bottom profile resolution provide unprecedented detail of bedform morphology and structure. Multiple channels within the Redondo Fan have been active at different times during the Late Holocene (0 to 3000 yr bp ). The currently active channel extending from Redondo Canyon makes an abrupt 90° turn at the canyon mouth before resuming a south-easterly course along the east side of the Redondo Fan. This channel is floored by sand and characterized by small steps generally <1 m in relief, spaced 10 to 80 m in the down-channel direction. A broader channel complex lies along the western side of the fan valley that was last active more than 850 years ago. Two distinct trains of large scours, with widths ranging from tens to a few hundred metres and depths of 20 m, occur on the floor of the western channel complex, which has a thin mud drape. If observed in cross-section only, these large scours would probably be misidentified as the thalweg of an active channel.     

Comparative scanning electron microscopy study of oriented till blocks, glacial grains and Devonian sands in Estonia and Latvia

Samples of Middle Devonian (Eifelian age; 387–380 Ma) indurated and non‐cemented sandstone were compared with Pleistocene basal tills in Estonia and Latvia to test a hypothesis that glacial SEM (scanning electron microscopy) microtextures are distinctly different from those produced in a fluvial depositional environment. The deposits of Middle Devonian Aruküla Stage were emplaced in a continental water basin close to sea level and well away from any glacial source. Therefore, the SEM microtextures on quartz grains from the Aruküla Stage should show mainly the effect of stream transport. The basal tills are of Late Weichselian age deposited as ground moraine directly over the sandstone. Additional glaciofluvial and glaciolacustrine samples were included with the tills to determine whether glacial and fluvial‐lacustrine transport could be differentiated by the SEM microtextures. Samples of oriented blocks of till from a limited number of sites were studied without pretreatment to determine whether sand clast orientation could provide a method for determining glacial flow vectors. While there are some microtextural similarities between grains from glacial and glaciofluvial‐lacustrine depositional environments, the vast majority of grains from till deposits (50%–60%) are faceted, sharp edged, angular to subangular, and comprised of numerous and distinct microfeatures including abraded surfaces over microfractures, deep linear and curved troughs (striations), step features, and a preponderance of conchoidal and linear microfractures. Glaciofluvial and lacustrine grains contain abundant abrasion features and v‐shaped percussion cracks that make them very distinct from glacial grains. Fluvial transport produces primarily rounded grains, well abraded, with v‐shaped percussion scars dominating. Thus, it is possible to use microtextural differences between the three sample suites to identify particular depositional environments. Oriented till blocks provide information on sand clast orientation. Although carbonate coatings often obscure sand clasts in untreated blocks, it is possible to determine some microfabric information that can be useful in determining flow direction of the ice.