Lithologic transition and bed thickness periodicities in turbidite successions of the Antola Formation, Northern Apennines, Italy

The Antola Formation of Upper Cretaceous age crops out extensively in the Northern Apennines and consists of graded units of calcareous sandstones, sandstones, marlstones, and shales. It can be subdivided into the Cerreto, Antola Marlstone, Bruggi, and S. Donato Members on the basis of bed thicknesses and percentage of shales. Although the whole formation is interpreted as a deep-sea basin plain deposit, the members constitute lateral facies subdivisions which range from proximal, thick-bedded turbidities that show a prevalence of thinning upward cycles in bed thicknesses to distal turbidites that show predominantly thickening upward cycles and have a high percentage of shale. Repetitive patterns in the lithological sequence of the turbidite association are generally distinctive and are satisfactorily described as first order Markov chains. Only the Antola Marlstone Member has an additional second order Markov property. Imaginary eigenvalues of the transition probability matrices of all but the Bruggi Member demonstrate a strong cyclic character in the lithologic ordering within the formation. The behaviour of the Antola Marlstone and of the Bruggi may reflect the influence of a secondary ophiolitic intra-basinal source of clastics that contributed sandy turbidites and olistostromes. Systematic long-term variations in the sequence of bed thickness development in some sections of the Antola Formation are often subtle and equivocal, and pose special problems in interpretation. Fourier analysis was applied to the task of partitioning fundamental wavelengths from “background noise” introduced by essentially random depositional processes. In all members there is (1) strong short-term wavelength of two to three beds indicative of alternating thin and thick beds and judged to be typical of turbidite sequences; (2) an intermediate wavelength ranging from about five beds (proximal facies), eight beds (distal) to nine beds (very distal), which have both thinning and thickening upward trends, interpreted respectively as valley fill due to shifting talwegs of low density turbidity currents, and to progradational, flat turbidite lobes; (3) a poorly defined long-term wave-length of from thirty to greater than sixty beds that may be related to an unspecified trend in the evolution of the sedimentary basin. Phase angles associated with the coniputed power spectra give indications as to the asymmetry (thickening or thinning upward) or symmetry of the representative units.     

Geochemical Evidence for Subduction Fluxes, Mantle Melting and Fractional Crystallization Beneath the South Sandwich Island Arc

The volcanoes of the South Sandwich island arc follow threedistinct series: low-K tholeiitic (followed by Zavodovski, Candlemas,Vindication, Montagu and Bristol), tholeiitic (followed by Visokoi,Saunders and Bellinghausen) and calcalkaline (followed by Leskov,Freehand and part of Cook and Thule). Flux calculations indicatethat the percentage contribution of the subduction componentto the mantle source of all three series varies from undetectable(e.g. Zr) through small (e.g. Nd=20%) and moderate (e.g. La,Ce, Sr=50–80%) to dominant (e.g. Pb, K, Ba, Rb, Cs >90%)with little change along the arc. Isotope systematics (Pb, Nd,Sr) show that this subduction component obtains a greater contributionfrom altered oceanic crust than from pelagic sediment. Elementsfor which the subduction contribution is small show that themantle is already depleted relative to N-MORB mantle (equivalentto loss of an 2•5% melt fraction) before melting beneaththe arc. After addition of the subduction component, dynamicmelting of this depleted mantle then causes the variations inK that distinguish the three series. The estimated degree ofpartial melting (20%) is slightly greater than that beneathocean ridges, though geothermometry suggests that the primarymagma temperature (1225C) is similar to that of primary MORB.About half of the melting may be attributed to volatile addition,and half to decompression. Dynamic melting involving three-dimensional,two-phase flow may be needed to explain fully the inter-islandvariations. KEY WORDS: geochemistry; petrology; fluxes; melting; subduction ^*Corresponding author     

Basal Waitemata Group lithofacies: rapid subsidence in an early Miocene interarc basin, New Zealand

The abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early Miocene Waitemata basin. Basal shallow marine strata (Kawau Subgroup) accumulated upon a highly dissected surface that overlies deformed Mesozoic metagreywacke. The early Miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands. Kawau Subgroup lithofacies, which include pocket beach, shallow shelf and base-of-cliff talus deposits, reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried. The response to continued rapid subsidence and transgression in Waitemata basin was a decrease in the supply of coarse clastic sediment. Beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes. The first bathyal turbidite facies, which abruptly overlie the shallow-water Kawau Subgroup, include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs. When this local supply of sediment had been exhausted, coarse sediment starvation ensued and bathyal muds accumulated. With the resumption of sediment supply and gradual burial of submarine bedrock relief, submarine fans coalesced and increased in lateral extent. Subsidence of the Waitemata basin to bathyal depths is thought to have occurred in less than a million years. From the above hypothesis, a general model of sedimentation is proposed.     

Geochemistry of carbonate cements in the Sag River and Shublik Formations (Triassic/Jurassic), North Slope, Alaska: implications for the geochemical evolution of formation waters

Carbonate cements (calcite, siderite, dolomite, and ankerite) formed throughout the diagenetic history of the Sag River and Shublik Formations. The trace element and isotopic geochemistry of these cements varies as a function of the timing of precipitation. Earliest calcites, formed prior to significant compaction of the sediment, are relatively enriched in Mg (up to 4·4 mol%), and have ⁸⁷Sr/⁸⁶Sr values (mean = 0·707898) compatible with the original marine pore waters. Later calcites are relatively Fe-rich (up to 5·0 mol%) and are characterized by increasing ⁸⁷Sr/⁸⁶Sr values (up to 0·712823) and Sr content with decreasing age. The Fe content of zoned siderite and dolomite/ankerite rhombs increases towards the outside of the rhombs (i.e. increasing Fe content with decreasing age). These geochemical variations appear principally to result from changes in pore-water chemistry during diagenesis. The increase in ⁸⁷Sr/⁸⁶ Sr and Sr content of the cements is most likely due to interaction between pore waters and ⁸⁷ Sr-rich clay and possibly feldspar in Ellesmerian mudrocks (whole rock ⁸⁷Sr/⁸⁶ Sr signatures for the mudrocks are > 0·716). Pore-water Fe²⁺ concentration was probably controlled by diagenetic alterations involving Fe-bearing minerals (e.g. pyrite precipitation). A reconnaissance examination of carbonate cements in the overlying Kingak Shale indicates that similar alterations occurred in the Kingak. The low δ¹⁸ O value of some calcite cements (-11·96% PDB) suggests that an influx of meteoric water may have occurred in the mid-Neocomian, though the low value could also result from an abnormally high geothermal gradient associated with mid-Neocomian rifting.     

SOME ASPECTS OF THE INTERACTION OF BLOWING SNOW WITH THE ATMOSPHERIC BOUNDARY LAYER

Several possible effects of blowing snow on the atmospheric boundary layer are investigated, mostly within the general framework of the Prairie Blowing Snow Model (PBSM). The processes of snow saltation and suspension are first described. Variations to the drift density profile are tested and the effects of stratification and density variation calculations are evaluated. Despite high density gradients of blowing snow, stratification effects on turbulence and the velocity profiles can generally be neglected. However, with saltating or suspended snow in a constant shear stress layer, part of the shear stress is carried by the particles. A highly simplified, single-phase approach, based on the density variation of the air–snow mixture coupled to a simple turbulent stress–strain relationship, is used to illustrate this. Sublimation rates in a column of blowing snow are calculated using the PBSM and results are compared with those obtained with a modified formulation which incorporates a spectrum of sublimating particles of varying sizes at each height in a steady-state surface boundary layer and different specifications of the ventilation velocity.     

VELOCITY–DISCHARGE RELATIONSHIPS DERIVED FROM DYE TRACER EXPERIMENTS IN GLACIAL MELTWATERS: IMPLICATIONS FOR SUBGLACIAL FLOW CONDITIONS

Repeated dye tracer tests were undertaken from individual moulins at Haut Glacier d'Arolla, Switzerland, over a number of diurnal discharge cycles during the summers of 1989–1991. It was hoped to use the concepts of at-a-station hydraulic geometry to infer flow conditions in subglacial channels from the form of the velocity–discharge relationships derived from these tests. The results, however, displayed both clockwise and anticlockwise velocity–discharge hysteresis, in addition to the simple power function relationship assumed in the hydraulic geometry approach. Clockwise hysteresis seems to indicate that a moulin drains into a small tributary channel rather than directly into an arterial channel, and that discharges in the two channels vary out of phase with each other. Anticlockwise hysteresis is accompanied by strong diurnal variations in the value of dispersivity derived from the dye breakthrough curve, and is best explained by hydraulic damming of moulins or sub/englacial passageways. Despite the complex velocity–discharge relationships observed, some indication of subglacial flow conditions may be obtained if tributary channels comprise only a small fraction of the drainage path and power function velocity–discharge relationships are derived from dye injections conducted during periods when the supraglacial discharge entering the moulin and the bulk discharge vary in phase. Analyses based on this premise suggest that both open and closed channel flow occur beneath Haut Glacier d'Arolla, and that flow conditions are highly variable at and between sites.     

D) UNITED Nations

Synopsis

In areas of accentuated relief, some of the basic assumptions made in the use of standard methods of assessing areal mean rainfall are often untenable. It is shown in this paper, that, not only does topography affect the actual rainfall distribution, but that the areal variability, measured as the correlation between any two points, is also dependent on the relief. Two methods are used to show this. Once method compares the areal variability of a flat area to one of accentuated relief, while the second method relates areal variability to topographic factors using a multiple regression technique.

The conclusions reached are then used for three purposed. The first is to develop a method of ascribing objectively areas or points to a particular raingauge, taking into account the nature of the terrain. The second is to establish a procedure for estimating the rainfall at ungauged points, by taking into account the rainfall at a selected nearby rainguage and the topographic situation of the points, and the third purpose is to provide means of establishing a correction factor to be applied to a raingauge reading in order that the reading may more accurately represent the area ascribed to it.