A climatological study of polar stratospheric clouds (1989–1997) from LIDAR measurements over Dumont d'Urville (Antarctica)

Backscatter lidar data from the French Antarctic base in Dumont d'Urville (66.40°S, 140.01°E), including aerosol background and observations of polar stratospheric clouds (PSCs), have been collected since 1989. In the present work we present a climatological study of PSCs, using a data base consisting of almost 90 observations. The seasonal evolution of PSCs, their optical classification, and their relationship with the observation temperature were studied. The first PSC was observed on day number 175 (15 June) and the last on day number 260 (7 September). The characteristic mid‐cloud altitude decreases through the season at a rate of 2.5 km/month. Type Ia, Ib, and II PSCs — identified by their optical properties — have been observed. External mixtures of these types have also been observed. These observations have been related to the local temperature measured by radiosondes. The relationship between PSC type and the period of the winter season was also investigated. Mixed (solid and liquid) type I clouds are mostly observed at the beginning of the winter. Type II clouds are observed only during the coldest period around midwinter, although temperatures below the frost point begin earlier and persist longer than this. Type Ia, solid‐particle, clouds are observed mostly at the end of the winter.     

Holocene stratigraphy and depositional history of the Narragansett Bay System, Rhode Island, U.S.A.

ABSTRACT An extensive seismic reflection survey has been used to gain further knowledge of Holocene stratigraphy and depositional history in the Narragansett Bay System (NBS). The early Holocene stream-dissected surface beneath the NBS is interpreted as having been flooded by the Holocene sea in a manner suggested by Oldale & O'Hara's (1980) sea-level-rise curve. The sea initially is believed to have penetrated the pre-NBS East Passage trunk valley about 9000 yr BP and subsequently spread landward via the trunk valley and its branches. The Holocene sediments display stratigraphic relationships that differ spatially. At passage mouths, the basal unconformity is inferred to be covered over with some 3 m of paralic and 5 m of marine sands and silts separated, by a transgressive unconformity. In contrast the interior sequences reveal (a) a valley section up to more than 15 m thick in which the regressional unconformity is overlain by probable lower fluvial and/or estuarine sand-silt facies that commonly grades upward to an estuarine silt-clay facies and (b) an interfluve section in which a basal transgressive unconformity is blanketed by an estuarine nearshore sand-silt facies that locally may change upward to a silt-clay facies. Primarily Holocene silt-clay accretion, produced by sedimentation processes associated with net non-tidal estuarine circulation, infilled the evolving NBS. Depositional bodies, lenticular in shape and comprised of 12 m or more of sediment, developed in lowlands near Gould Island (?9000 yr BP), in Upper Narragansett Bay (?7500 yr BP), around Hope Island (?7500 yr BP) and in Mt Hope Bay (?6250 yr BP) with an average minimal sedimentation rate of 1.6–2.2 mm yr^-1. Silt-clay deposition, commonly gas-bearing, has buried the basal relief in most of the NBS upper and middle portions except for middle East Passage. A comparison of NBS sedimentation with that of Chesapeake, Delaware and Hudson Estuaries shows that the estuaries to the south have accumulated more sediment over a slightly longer period yet, with the exception of the higher rate in the Hudson Estuary, the sedimentation rates appear to be similar.     

Burial dolomitization and porosity development in a mixed carbonate-clastic sequence: an example from the Bowland Basin, northern England

Widespread dolomitization and leaching occur in the Asbian to Brigantian (Dinantian) sequence of the Bowland Basin. Within this mudrock-dominated succession, dolomite is developed in calcarenites and limestone breccia/conglomerates deposited in a carbonate slope environment (Pendleside Limestone) and also within graded quartz wackes deposited by density currents in a generally ‘starved’ basin environment (Pendleside Sandstone). The dolomitized intervals range in thickness from less than one metre to several tens of metres and have a stratabound nature. All stages of calcite cement pre-date dolomitization and calcite veins are dolomitized. Dolomite crystals replace neomorphic spar and may also contain insoluble residues that were concentrated along stylolites. Thus dolomitization was a late stage process within the carbonate diagenetic sequence. A late-stage diagenetic origin is also indicated within the sandstones, with dolomite post-dating the development of quartz overgrowths. Six main textural styles of dolomite are observed: (1) scattered; (2) mosaic; (3) subhedral to euhedral rhombic; (4) microcrystalline; (5) single crystal and (6) saddle. The style of dolomite developed is dependent on the host rock mineralogy, on whether it is space-filling or replacive and also on temperature. Chemically the dolomite varies from near stoichiometric compositions to ankeritic varieties containing up to 20 mole % FeCO₃. Generally the dolomites have isotopic compositions depleted in δ¹⁸O compared to the host limestone, with similar or lighter δ¹³C values. Initial dolomite was of the scattered type, but with progressive replacement of the host a mosaic dolostone with a sucrosic texture was produced. There was a general increase in the Fe and Mn content and reduction in δ¹⁸O ratio of the crystals during dolomitization. Leaching is restricted to partly dolomitized horizons, where calcite, feldspars, micas, clays and, to some extent, dolomite have been leached. This has produced biomouldic and vuggy secondary porosity within the carbonates, whereas in the sandstones honeycombed, corroded and floating grains associated with oversized pores occur. Porosity within both carbonates and sandstones is reduced by ferroan dolomite/ankerite cements. Field, petrographic and chemical characteristics indicate that dolomitizing solutions were predominantly derived from the enclosing mudrocks (Bowland Shales) during intermediate/deep burial. Fluid migration out of the mudrocks would have been sided by dehydration reactions and overpressure, the fluids migrating along the most permeable horizons—the coarse grained carbonates and sandstones that are now dolomitized and contain secondary porosity.     

The Stability of Sodalite in a Synthetic Syenite plus Aqueous Chloride Fluid System

Natural feldspathoidal syenites may be approximated by assemblagescontaining some or all of the phases sodalite, nepheline, oneor two alkali feldspars, and aqueous chloride fluid in the systemNaAISi₃O₈-KAISi₃O₈-NaAISiO₄-KAISiO₄-NaCI-KCI-H₂O. The stabilityof sodalite in these assemblages was studied in the range 500–700°C and 600–2000 bars fluid pressure. Sodalite appears to be a stable phase on the vapor-saturatedliquidus in this system over a wide range of pressure. At or near the vapor-saturated liquidus minimum in this system,three distinct types of sodalite-bearing syenite can crystallize.Nepheline-sodalite-one alkali feldspar rocks, nepheline-sodalite-twoalkali feldspars rocks and sodalite-analcime-bearing rocks crystallizebelow 1600 bars, between 1600 and 2750 bars and above 2750 barsfluid pressure, respectively. The effects of closed-system cooling on the assemblage sodalite-nepheline-twoalkali feldspars-aqueous fluid are different and distinguishablefrom the effects of metasomatism. Closed-system cooling resultsin replacement of K-feldspar by albite, feldspathoids remainingnearly unchanged, while metasomatism generally results in sismultaneousenrichment or impoverishment in sodalite plus K-feldspar.     

SELECTION OF SEISMIC PICKING BASED UPON THE DIP,MOVE-OUT AND AMPLITUDE OF EACH EVENT*

The general use of continuous CDP seismic lines means that the neighbourhood of each depth point can be considered according to two sets of co-ordinates: — the co-ordinates of the abscissa on the seismic lines and the reflection time, which allow investigation of the local dip of coherent events; — the co-ordinates of the reflection time and the move out, which allow investigations of the average velocity for these same events. The combination of both investigations for each coherent event is a very convenient operation for the seismologist, and can provide some useful interpretation aids such as velocity profiles, or information on the interval velocities. The principles which we use consist in the examination of a large range of possibilities, of which only the most coherent are chosen. Some examples are shown.     

Neogene non-marine sedimentation and tectonics in small pull-apart basins of the San Andreas fault system, Sonoma County, California

The San Andreas fault system in northern California forms an 80–90 km wide zone of right-lateral shear. Extensional tectonism within this broad shear zone is indicated by both Neogene silicic volcanic rocks that gradually young in the direction of shear propagation to the north-west and by numerous Neogene faultbounded structural basins filled with thick non-marine sequences. The Little Sulphur Creek basins, three well-exposed 1·5–2 km wide pull apart basins within this shear system, have sedimentation patterns analogous to those of much larger pull-apart basins. They were formed and subsequently deformed by east-west extension and by north-west to south-east-orientated right-slip concurrently with basin filling. Palaeocurrent and maximum-clast size data indicate both lateral sediment transport from fault-bounded basin margins and longitudinal transport down the basin axes. The basins are filled primarily with coarse alluvial-fan and streamflow deposits derived from a surrounding igneous, sedimentary, and metamorphic provenance. Two of the basins contain basin-plain-type lacustrine turbidites that grade laterally into distal alluvial fan, fan-delta, and sublacustrine delta deposits. Talus deposits along the south-west margin of the basins contain megabreccia indicative of active uplift. Structures indicative of dewatering, liquefaction, and slumping suggest penecontemporaneous tectonism.     

DETERMINATION OF LATERAL INHOMOGENEITIES IN REFLECTION SEISMICS BY INVERSION OF TRAVELTIME RESIDUALS*

Lateral inhomogeneities generate fluctuations in the traveltime of seismic waves. By evaluation of these traveltime fluctuations from different source and receiver positions, lateral inhomogeneities can be located using a pseudo inverse matrix method (Aki, Christoffersson and Husebye 1977). The formulation of the problem is possible for transmitted waves as well as for reflected and refracted waves. In reflection seismics this method is of importance, if no reflections from the inhomogeneities themselves, but only reflections from lower boundaries can be observed. The basic assumptions for the mathematical formulation are (1) the average velocities and depths of the reflecting horizons are known already from standard processing methods, and (2) the traveltime residuals are due to lateral velocity changes between different reflectors or between reflectors and the surface. The area of the earth to be considered is divided into layers and the layers into rectangular blocks. The parallel displacement of a ray after passing a disturbed block is neglected, only the traveltime residual is taken into account. In this paper the method and its application to data obtained with two-dimensional models are described.     

Late Miocene Halimeda alga-microbial segment reefs in the marginal Mediterranean Sorbas Basin, Spain

A ～6 Ma Messinian (late Miocene) Bioherm Unit on the southern slope of the Sorbas Basin, SE Spain, contains numerous biotically diverse lensoid patch reefs that formed on a shelf to basin slope during a cycle of relative sea-level change. Halimeda reefs are the largest and most complex of the patch reefs and are divisible into core, cap, and flank facies. On the upper and midslope they are up to 40 m thick and 400 m long. They become smaller downslope. The core consists of jumbled Halimeda segments, released by spontaneous disaggregation of the alga. The segments were stabilized close to their sites of growth and rapidly lithified by micritic and peloidal microbial crusts. Residual cavities were further veneered by isopachous marine cements. Flank facies, consisting of bedded packstones to rudstones, form wedge-shaped units lateral to the mounds. Cap facies consist of bioclastic calcarenites/calcirudites and microbial carbonates. Synsedimentary lithification assisted rapid accretion and inhibited off-mound export of sediment. Allochthonous reef-derived blocks on the mid-slope reflect penecontemporaneous rigidity of the Halimeda bioherms. Proximal Porites coral frame patch reefs associated with calcarenites were located near the shelf margin during the initial lowstand stage. Halimeda segment reefs associated with calcarenites and silty marls developed on the midslope and bivalve-bryozoan-serpulid reefs formed on the lower slope in silty marls with occasional turbidites. During the transgressive stage, coral patch reefs near the shelfbreak were overgrown by Halimeda. During highstand progradation, cap facies spread basinward as a sheet connecting many of the midslope patch reefs. These ancient analogues differ from most modern Halimeda reefs in being discrete laterally restricted patch reefs, surrounded by marly sediment, and located on a slope. They are, however, broadly comparable in biota, thickness, and depositional depth. Intense early lithification by microbial crusts and marine cements is an important feature of these Messinian segment reefs. It has not been reported from modern examples.     

Early Cambrian braid-delta deposits, MacKenzie Mountains, north-western Canada

Latest Neoproterozoic to earliest Cambrian strata in north-western Canada provide an example of a pre-vegetation braid-delta depositional system. Depositional environments represented in the succession include braided fluvial and braid-delta distributary channels, aeolian dune fields and interdistributary lagoons/bays, as well as mouth bar, beach to shoreface, and prodelta to distal shelf settings. Three formations have been investigated: the Ingta Formation formed in wave-dominated nearshore to offshore shelf environments with little or no apparent deltaic influence, whereas the overlying Backbone Ranges and Vampire formations contain an extensive record of braid-delta deposits ranging from braidplain to distal prodelta facies. On the braid-plain, river channels reached widths of up to several kilometres. Such channels terminated seaward in braid deltas that showed some shoreline protuberance and were characterized by fluvial-dominated mouth-bar deposition with lesser wave influence; wave-dominated deltaic successions are rare in the succession. Interdeltaic areas were characterized by wave-dominated prograding shorelines. Interdistributary lagoons probably formed primarily in abandoned distributary channels. Delta-front/prodelta deposits are silt-rich and contain abundant soft-sediment deformation, including slumps. The deposits in these formations illustrate the significantly different nature of sedimentation prior to the advent of land plants. This is illustrated in the dominance of braided fluvial deposition and of silt-rich offshore facies that may have resulted from enhanced aeolian transport of loess. The non-actualistic effects of limited bioturbation and extensive microbial binding apparently exerted relatively little control on the distribution of facies. However, the absence of extensive bioturbation is manifest in pristine preservation of primary sedimentary structures, while the hypothesized latest Proterozoic-earliest Cambrian decline in microbial binding may be reflected in the upward increase in the abundance of sole marks in the succession.